Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0005783 endoplasmic reticulum	IBA GO_REF:0000033	ACCEPT	Summary: ER localisation is the single most consistently reported subcellular site for PGRMC1 across cell types and is supported by the falcon review, the perplexity review, and primary biochemical/microscopy studies. The IBA call is consistent with experimentally documented ER residence. Reason: PGRMC1 is a tail-anchored ER membrane MAPR-family protein; ER residence is a core, conserved localisation that underlies its interactions with ER-resident CYPs and the heme synthesis/trafficking machinery. Supporting Evidence: file:human/PGRMC1/PGRMC1-deep-research-falcon.md PGRMC1 localization is repeatedly reported as context-dependent, with strong recurring evidence for association with the endoplasmic reticulum (ER)/endomembranes PMID:21081644 In most cells, PGRMC1 is localized in the membranes of the ER
GO:0012505 endomembrane system	IBA GO_REF:0000033	MARK AS OVER ANNOTATED	Summary: Endomembrane system is a broad parent term encompassing ER, Golgi, endosomes and plasma membrane. PGRMC1 is documented at multiple endomembrane sites, but more specific terms (endoplasmic reticulum, plasma membrane, smooth ER membrane) capture its biology more informatively. Reason: Redundant with the more informative GO:0005783 endoplasmic reticulum annotation already present; "endomembrane system" is too generic to convey meaningful localisation information. Supporting Evidence: file:human/PGRMC1/PGRMC1-deep-research-falcon.md additional reported localization to the plasma membrane, nucleus/nuclear membrane, Golgi, endosomes, cytoplasm, and mitochondria depending on cell type and condition.
GO:0016020 membrane	IBA GO_REF:0000033	MARK AS OVER ANNOTATED	Summary: "membrane" is the most generic possible cellular component term. PGRMC1 is unambiguously a single-pass membrane protein, but ER membrane / smooth ER membrane / plasma membrane are the informative annotations to retain. Reason: Uninformative parent term; superseded by the specific ER and plasma membrane annotations already present. Supporting Evidence: file:human/PGRMC1/PGRMC1-deep-research-falcon.md Human PGRMC1 (O00264) is a single-pass membrane MAPR protein
GO:0005496 steroid binding	IEA GO_REF:0000120	KEEP AS NON CORE	Summary: Direct steroid (progesterone) binding by PGRMC1 is historically asserted (the gene was named for it) but the evidence is equivocal: some purified-protein studies report progesterone binding requiring the transmembrane region while recombinant truncated cytosolic domain assays have failed to detect direct binding. Contemporary expert syntheses (falcon, 2024) frame PGRMC1 as a heme- binding MAPR protein rather than a classical progesterone receptor. Keeping as non-core captures historical/possible function without elevating it to the core molecular activity. Reason: Evidence for direct, high-affinity steroid binding by PGRMC1 alone is weak and disputed; the heme-binding role is the unambiguously supported core molecular function. Retain for completeness but not as a core activity. Supporting Evidence: file:human/PGRMC1/PGRMC1-deep-research-falcon.md Current expert reviews frame PGRMC1 less as a simple on/off “progesterone receptor” and more as a membrane-associated heme-binding regulator
GO:0005576 extracellular region	IEA GO_REF:0000044	REMOVE	Summary: PGRMC1 is a membrane-anchored protein with no characterised secreted form. The annotation likely originates from automated SwissProt subcellular-location mapping that registered "secretory granule membrane" type annotations as extracellular region. There is no evidence in the falcon, perplexity, or primary literature for genuine extracellular soluble PGRMC1. Reason: No biological evidence supports a free extracellular pool of PGRMC1; the protein is membrane-anchored throughout its life cycle. Annotation appears to be a SwissProt-keyword automation artefact. Supporting Evidence: file:human/PGRMC1/PGRMC1-deep-research-falcon.md Human PGRMC1 (O00264) is a single-pass membrane MAPR protein
GO:0005741 mitochondrial outer membrane	IEA GO_REF:0000120	KEEP AS NON CORE	Summary: A pool of PGRMC1 is found on the cytosolic (outer) face of the outer mitochondrial membrane, where it interacts with the ferrochelatase-containing heme metabolon. This is experimentally supported by proteinase-protection fractionation in MEL cells (Piel et al. 2016). Reason: PGRMC1 outer mitochondrial membrane localisation is genuine but represents a minor subcellular pool relevant to its heme-trafficking/FECH-interaction role, not its core ER residence. Supporting Evidence: PMID:27599036 Further experiments to characterize the sub-mitochondrial localization were consistent with the majority of PGRMC1 being on the outside of the outer mitochondrial membrane.
GO:0008289 lipid binding	IEA GO_REF:0000043	MARK AS OVER ANNOTATED	Summary: Generic "lipid binding" is too broad and not the established core activity. PGRMC1 binds heme (a tetrapyrrole, not a lipid in the typical GO sense) and possibly steroids; sterol binding/transfer activity has been proposed but not biochemically demonstrated for purified PGRMC1. Reason: Uninformative; specific molecular function GO:0020037 (heme binding) and GO:0005496 (steroid binding) are already annotated. No published evidence for general phospholipid binding. Supporting Evidence: file:human/PGRMC1/PGRMC1-deep-research-falcon.md Most defensible primary functional description (integrating 2023–2024 evidence): Human PGRMC1 (O00264) is a single-pass membrane MAPR protein with a cytochrome b5-like heme-binding domain**
GO:0030868 smooth endoplasmic reticulum membrane	IEA GO_REF:0000044	ACCEPT	Summary: PGRMC1 is enriched in microsomal (smooth ER) fractions, consistent with its co-localisation and interaction with smooth-ER-resident cytochrome P450 enzymes. This is a more informative refinement of the parent GO:0005783 annotation. Reason: Smooth ER membrane is the precise compartment for PGRMC1's interactions with microsomal CYP enzymes; well supported by microsomal fractionation studies. Supporting Evidence: PMID:21081644 PGRMC1 is a small 25-kDa protein with an N-terminal membrane binding segment and a C-terminal domain with a cytochrome b 5 -like structure that binds heme.
GO:0046872 metal ion binding	IEA GO_REF:0000043	MARK AS OVER ANNOTATED	Summary: PGRMC1 binds iron via the heme cofactor (pentacoordinate Tyr113 coordination of haem iron). Annotation as "metal ion binding" is technically correct via the bound heme but is uninformative; "heme binding" (GO:0020037) is the appropriate specific term and is already annotated. Reason: Redundant with and less informative than the heme binding annotations; PGRMC1 does not coordinate free metal ions directly. Supporting Evidence: PMID:26988023 the haem iron is five-coordinated by Tyr113 (Y113) alone
GO:0005515 protein binding	IPI PMID:17353931 Large-scale mapping of human protein-protein interactions by...	MARK AS OVER ANNOTATED	Summary: Generic IPI "protein binding" annotation from a high-throughput mass-spec interactome screen. Not informative about PGRMC1's specific molecular function. Specific functional protein interactions (with CYPs, CPR, FECH, EGFR, TMEM97, LDLR) are captured by more meaningful annotations. Reason: Per GO curation guidelines, generic "protein binding" should be avoided when more informative MF terms exist or when the partner is not specified. Supporting Evidence: PMID:17353931 Large-scale mapping of human protein-protein interactions by mass spectrometry.
GO:0005515 protein binding	IPI PMID:21081644 Progesterone receptor membrane component 1 inhibits the acti...	MARK AS OVER ANNOTATED	Summary: Captures PGRMC1's documented direct interactions with multiple drug- metabolising P450s (CYP2C2, CYP2C8, CYP3A4) and cytochrome P450 reductase (CPR), but "protein binding" itself is uninformative. The functional consequence (modulation of P450 activity) is the core finding. Reason: Use specific molecular-function terms (heme binding, and a process term for cytochrome P450 modulation) rather than generic protein binding. Supporting Evidence: PMID:21081644 PGRMC1 bound efficiently to all three P450s PMID:21081644 In cells cotransfected with CPR and PGRMC1, strong binding of CPR to PGRMC1 was observed
GO:0005515 protein binding	IPI PMID:27599036 A Novel Role for Progesterone Receptor Membrane Component 1 ...	MARK AS OVER ANNOTATED	Summary: Reflects the experimentally validated direct interaction of PGRMC1 with ferrochelatase (FECH). The interaction is informative and functional but the generic GO:0005515 term itself is not. Reason: Specific interaction with FECH is functionally informative but should not be captured as the generic protein-binding term; the process-level role (heme biosynthesis/regulation of FECH) is the appropriate annotation. Supporting Evidence: PMID:27599036 The interaction between PGRMC1 and FECH was confirmed in vitro and in HEK 293T cells
GO:0005515 protein binding	IPI PMID:28514442 Architecture of the human interactome defines protein commun...	MARK AS OVER ANNOTATED	Summary: High-throughput interactome screen; generic protein binding annotation. Reason: Per GO guidelines, generic protein binding is uninformative and the interactome studies do not specify functional consequences. Supporting Evidence: PMID:28514442 Architecture of the human interactome defines protein communities and disease networks.
GO:0005515 protein binding	IPI PMID:30021884 Histone Interaction Landscapes Visualized by Crosslinking Ma...	MARK AS OVER ANNOTATED	Summary: Histone crosslinking mass-spec study; PGRMC1's apparent association with histones in intact nuclei is a peripheral context not part of its core biology and the generic protein-binding term is uninformative. Reason: Generic IPI from a high-throughput crosslinking study; nuclear/chromatin proximity is consistent with reported nuclear/nucleolar pools but not core function. Supporting Evidence: PMID:30021884 Histone Interaction Landscapes Visualized by Crosslinking Mass Spectrometry in Intact Cell Nuclei.
GO:0005515 protein binding	IPI PMID:32296183 A reference map of the human binary protein interactome.	MARK AS OVER ANNOTATED	Summary: Generic IPI from a Y2H-based reference binary interactome map. Provides no specific functional information. Reason: Uninformative high-throughput protein binding call. Supporting Evidence: PMID:32296183 A reference map of the human binary protein interactome.
GO:0005515 protein binding	IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling...	MARK AS OVER ANNOTATED	Summary: Generic IPI from BioPlex/AP-MS proteome-scale network. Reason: Uninformative high-throughput protein binding call. Supporting Evidence: PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.
GO:0005515 protein binding	IPI PMID:35271311 OpenCell: Endogenous tagging for the cartography of human ce...	MARK AS OVER ANNOTATED	Summary: Generic IPI from OpenCell endogenous-tagging cartography of human protein complexes; not functionally informative on its own. Reason: Uninformative high-throughput protein binding call. Supporting Evidence: PMID:35271311 OpenCell: Endogenous tagging for the cartography of human cellular organization.
GO:0005515 protein binding	IPI PMID:40205054 Multimodal cell maps as a foundation for structural and func...	MARK AS OVER ANNOTATED	Summary: Generic IPI from a multimodal cell map; not informative about specific function or partner. Reason: Uninformative high-throughput protein binding call. Supporting Evidence: PMID:40205054 Multimodal cell maps as a foundation for structural and functional genomics.
GO:0007411 axon guidance	IEA GO_REF:0000107	REMOVE	Summary: Inferred by automatic Ensembl Compara orthology transfer (GO_REF:0000107). There is no direct experimental evidence for PGRMC1 functioning in axon guidance in human; the falcon synthesis does not mention this role. Likely spurious ortholog-based transfer. Reason: No direct evidence for an axon-guidance function of PGRMC1 in any model organism; orthology-based IEA is not adequately supported. Supporting Evidence: file:human/PGRMC1/PGRMC1-deep-research-falcon.md Most defensible primary functional description
GO:0007613 memory	IEA GO_REF:0000107	REMOVE	Summary: Automatic ortholog-based IEA. While PGRMC1 has been linked to Alzheimer's disease through Abeta oligomer binding, "memory" as a direct biological process annotation is too broad and not directly supported by experimental evidence for human PGRMC1. The synaptic effects are downstream/disease-context rather than a core gene function. Reason: Phenotype-level term, not a molecular function/process directly executed by PGRMC1; ortholog-based transfer is weak.
GO:0008306 associative learning	IEA GO_REF:0000107	REMOVE	Summary: Automatic ortholog-based IEA. Same reasoning as for memory and axon guidance: no direct experimental evidence and these are phenotype-level terms rather than core PGRMC1 functions. Reason: Phenotype-level term not directly supported by experimental evidence for PGRMC1.
GO:0020037 heme binding	IEA GO_REF:0000107	ACCEPT	Summary: Heme binding is the core, evolutionarily conserved molecular function of PGRMC1 and is supported by extensive crystallographic, spectroscopic, biochemical and mutagenesis evidence (Tyr113 axial coordination, ~50 nM Kd, heme-dependent dimerisation). This is the single most important MF term. Reason: Core molecular function backed by multiple lines of high-quality evidence. Supporting Evidence: PMID:26988023 the haem iron is five-coordinated by Tyr113 (Y113) alone file:human/PGRMC1/PGRMC1-deep-research-falcon.md Multiple lines of evidence support that PGRMC1 binds heme at a surface-exposed site.
GO:0043005 neuron projection	IEA GO_REF:0000107	KEEP AS NON CORE	Summary: PGRMC1 is broadly expressed including in neurons and has been detected in post-synaptic compartments; presence in neuronal projections is consistent with the Abeta-binding / synaptic localisation literature. Annotation is plausible but is not a core function. Reason: Cellular-component term reflecting neuronal expression context, not a core PGRMC1 function. Supported by immunofluorescence studies of synaptic puncta. Supporting Evidence: PMID:25390692 sigma-2/PGRMC1 is expressed at low levels in cell bodies of neurons and glia, and in proximal neurites
GO:0043025 neuronal cell body	IEA GO_REF:0000107	KEEP AS NON CORE	Summary: Same rationale as neuron projection - PGRMC1 is detected in neuronal cell bodies (immunofluorescence in cultured hippocampal/cortical neurons), but this is an expression-context CC term not a core function. Reason: Reflects neuronal expression, not a core mechanistic function. Supporting Evidence: PMID:25390692 sigma-2/PGRMC1 is expressed at low levels in cell bodies of neurons and glia
GO:0044297 cell body	IEA GO_REF:0000107	MARK AS OVER ANNOTATED	Summary: Parent term of neuronal cell body; uninformative. Reason: Redundant with the more specific GO:0043025 (neuronal cell body) annotation.
GO:0045202 synapse	IEA GO_REF:0000107	KEEP AS NON CORE	Summary: Sigma-2/PGRMC1 is enriched in post-synaptic density fractions of rat brain and detected at synaptic puncta in cultured neurons; this synaptic pool underlies its proposed role as a receptor/binding site for Abeta oligomers. Plausible non-core localisation. Reason: Synaptic localisation is documented in brain but is a tissue/context-specific pool, not the core ER-centric localisation of PGRMC1. Supporting Evidence: PMID:25390692 PGRMC1 is widely expressed in brain at low levels, where it is enriched in the post-synaptic density fraction
GO:0099563 modification of synaptic structure	IEA GO_REF:0000107	REMOVE	Summary: Ortholog-based IEA. Some literature links sigma-2/PGRMC1 ligands to changes in synaptic membrane trafficking and reversible spine loss in Abeta-treated neurons, but direct evidence that PGRMC1 itself executes a "modification of synaptic structure" molecular role is weak. Reason: Process is downstream/context-dependent (Abeta oligomer signalling) rather than an autonomous PGRMC1 process. Ortholog-based IEA is not adequate support.
GO:1905809 negative regulation of synapse organization	IEA GO_REF:0000107	REMOVE	Summary: Ortholog-based IEA. Same rationale as for "modification of synaptic structure"; not core PGRMC1 biology. Reason: Phenotype-/context-level process not directly executed by PGRMC1; weak IEA evidence.
GO:0005515 protein binding	IPI PMID:37047353 Sigma-2 Receptor Ligand Binding Modulates Association betwee...	MARK AS OVER ANNOTATED	Summary: Reflects PGRMC1 co-immunoprecipitation with TSPO in MIA PaCa-2 cells, in the context of TSPO/TMEM97/sigma-2 receptor complex biology. The functional meaning of the binding is debated and cell-type-dependent. Generic protein binding is uninformative. Reason: Specific cell-context interaction with TSPO is interesting but generic protein-binding is uninformative; the relevant biology is the PGRMC1/TMEM97 sigma-2 complex. Supporting Evidence: PMID:37047353 in MP cells, we observed that both TMEM97 (Figure 4C) and PGRMC1 (Figure 4D) co-immunoprecipitated with TSPO
GO:0005515 protein binding	IPI PMID:30443021 Sigma-2 Receptor/TMEM97 and PGRMC-1 Increase the Rate of Int...	MARK AS OVER ANNOTATED	Summary: Reflects PGRMC1 in the TMEM97(sigma-2)/PGRMC1/LDLR ternary complex that accelerates LDL internalisation. The specific functional interaction is important and is captured by the related process annotation GO:0140077 (positive regulation of lipoprotein transport). Reason: Generic protein binding is uninformative; the functional consequence (LDLR/TMEM97 complex / LDL uptake) is captured elsewhere. Supporting Evidence: PMID:30443021 These data indicate that the formation of a ternary complex of LDLR-PGRMC1-TMEM97 is necessary for the rapid internalization of LDL by LDLR.
GO:0140077 positive regulation of lipoprotein transport	IMP PMID:30443021 Sigma-2 Receptor/TMEM97 and PGRMC-1 Increase the Rate of Int...	KEEP AS NON CORE	Summary: CRISPR knockout of PGRMC1 in HeLa cells significantly reduced uptake of radiolabelled or fluorescently tagged LDL via the LDL receptor; the LDLR/TMEM97/PGRMC1 ternary complex is required for rapid LDL internalisation. This is a directly experimentally supported process role, although it represents one of several pleiotropic functions and is not the deepest core function. Reason: Genuine, well-supported PGRMC1 function but it acts in concert with TMEM97/LDLR in a specific membrane complex; this is one of multiple pleiotropic roles rather than the gene's primary molecular activity. Supporting Evidence: PMID:30443021 Uptake of radiolabeled LDL was significantly decreased in PGRMC1 KO, TMEM97 KO, and double KO cell lines. PMID:30443021 These data indicate that the formation of a ternary complex of LDLR-PGRMC1-TMEM97 is necessary for the rapid internalization of LDL by LDLR.
GO:0005886 plasma membrane	TAS PMID:25390692 Alzheimer's therapeutics targeting amyloid beta 1-42 oligome...	ACCEPT	Summary: A pool of PGRMC1 (often referred to as cell-surface PGRMC1 or csPGRMC1) translocates to the plasma membrane in many cell types and is the site at which the TMEM97/PGRMC1/LDLR ternary complex and the proposed sigma-2/PGRMC1 Abeta-oligomer binding occur. Well supported by multiple cell biology studies. Reason: Plasma membrane localisation is a genuine, functionally important pool of PGRMC1 supported by multiple lines of evidence. Supporting Evidence: PMID:25390692 translocates from the endoplasmic reticulum to the plasma membrane in several cell types
GO:0020037 heme binding	TAS PMID:25390692 Alzheimer's therapeutics targeting amyloid beta 1-42 oligome...	ACCEPT	Summary: Heme binding is the core molecular function of PGRMC1; multiple lines of evidence (review citation here, plus IDA direct binding studies). Reason: Core, well-supported molecular function. Supporting Evidence: PMID:25390692 PGRMC1 is a highly conserved heme-binding protein in the membrane associated progesterone receptor (MAPR) family
GO:0043005 neuron projection	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: Same as the IEA call above - neuronal/dendritic localisation reflects expression context not core function. Reason: Tissue-expression context, not core PGRMC1 mechanism. Supporting Evidence: PMID:25390692 sigma-2/PGRMC1 is expressed at low levels in cell bodies of neurons and glia, and in proximal neurites
GO:0043025 neuronal cell body	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: As for the IEA call above. Reason: Tissue-expression context, not core PGRMC1 mechanism. Supporting Evidence: PMID:25390692 sigma-2/PGRMC1 is expressed at low levels in cell bodies of neurons and glia
GO:0045202 synapse	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: As for the IEA call above. Reason: Synaptic pool documented in brain; non-core context. Supporting Evidence: PMID:25390692 PGRMC1 is widely expressed in brain at low levels, where it is enriched in the post-synaptic density fraction
GO:1903078 positive regulation of protein localization to plasma membrane	IMP PMID:25390692 Alzheimer's therapeutics targeting amyloid beta 1-42 oligome...	KEEP AS NON CORE	Summary: PGRMC1 has been shown to stabilise plasma membrane levels of EGFR and (in adipocyte models) of LDL-R, VLDL-R and GLUT4 via heme-dependent dimerisation. Genuine but pleiotropic function. Reason: Membrane-receptor trafficking effects are well documented for several partners (EGFR, LDL-R/VLDL-R, GLUT4) and are biologically significant, but represent context-dependent applications of PGRMC1's core membrane-protein- adaptor activity rather than its core mechanism. Supporting Evidence: PMID:25390692 PGRMC1 is a highly conserved heme-binding protein in the membrane associated progesterone receptor (MAPR) family that has been shown to stabilize surface receptor expression of proteins and directly associates with proteins that regulate membrane trafficking
GO:0005515 protein binding	IPI PMID:26988023 Haem-dependent dimerization of PGRMC1/Sigma-2 receptor facil...	MARK AS OVER ANNOTATED	Summary: Captures the well-characterised haem-dependent binding of PGRMC1 to EGFR and to cytochromes P450 (CYP1A2, CYP3A4, CYP51), but the generic term itself is uninformative; the functional consequence (P450 modulation, EGFR signalling) is the meaningful annotation. Reason: Specific, functional partner interactions should be captured by process annotations; generic protein binding is uninformative. Supporting Evidence: PMID:26988023 Haem-mediated PGRMC1 dimerization is required for interactions with EGFR and cytochromes P450
GO:0020037 heme binding	IDA PMID:26988023 Haem-dependent dimerization of PGRMC1/Sigma-2 receptor facil...	ACCEPT	Summary: Direct assay (X-ray crystallography at 1.95 Å, NMR, MS, SV-AUC, UV-vis, resonance Raman, ITC) of heme binding by PGRMC1 cytosolic domain. Kd ~50 nM. This is the highest-quality experimental support for the core heme-binding function. Reason: Direct biochemical and structural evidence for core MF. Supporting Evidence: PMID:26988023 haem binding to PGRMC1 was of low affinity with a Kd value of 50 nmol l−1 PMID:26988023 the haem iron is five-coordinated by Tyr113 (Y113) alone
GO:0042803 protein homodimerization activity	IDA PMID:26988023 Haem-dependent dimerization of PGRMC1/Sigma-2 receptor facil...	ACCEPT	Summary: PGRMC1 homodimerises via two non-mutually-exclusive mechanisms: heme-heme stacking between two protruding haem moieties and intermolecular Cys129-Cys129 disulfide bonding. The functional dimer is required for EGFR and CYP interactions and for cancer cell proliferation. Reason: Well-supported direct molecular function; the heme-dependent dimer is a regulatory/functional state. Supporting Evidence: PMID:26988023 PGRMC1 forms a dimeric structure largely through hydrophobic interactions between the haem moieties of two monomers PMID:26988023 the apo-monomer PGRMC1 resulted in dimerization by binding with haem
GO:0005741 mitochondrial outer membrane	ISS GO_REF:0000024	KEEP AS NON CORE	Summary: Same as the IEA outer mitochondrial membrane annotation - a real but minor pool of PGRMC1. Reason: Minor functional pool relevant to FECH interaction / heme trafficking. Supporting Evidence: PMID:27599036 Further experiments to characterize the sub-mitochondrial localization were consistent with the majority of PGRMC1 being on the outside of the outer mitochondrial membrane.
GO:0006783 heme biosynthetic process	IDA PMID:27599036 A Novel Role for Progesterone Receptor Membrane Component 1 ...	ACCEPT	Summary: PGRMC1 interacts with ferrochelatase (FECH) and inhibits FECH activity in vitro in a dose-dependent manner; AG-205 PGRMC1 inhibitor treatment reduces hemoglobinisation in differentiating MEL cells. PGRMC1 also donates heme to apo-cytochrome b5 in vitro. PGRMC1 is therefore implicated as a regulator of heme biosynthesis, although its precise role (inhibitor vs. chaperone vs. product-release regulator) is not fully resolved. Reason: Direct experimental evidence supports PGRMC1 involvement in heme synthesis regulation via FECH. Although the directionality of regulation is complex (it inhibits FECH in vitro yet AG-205 treatment reduces hemoglobinisation), the process-level annotation is appropriate. Supporting Evidence: PMID:27599036 In the presence of PGRMC1, in vitro measured FECH activity decreased in a dose-dependent manner. PMID:27599036 When cells that are recognized models for erythroid differentiation were treated with a small molecule inhibitor of PGRMC1, AG-205, there was an observed decrease in the level of hemoglobinization relative to that of untreated cells. PMID:27599036 In vitro heme transfer experiments showed that purified PGRMC1 was able to donate heme to apo-cytochrome b5.
GO:0020037 heme binding	IDA PMID:27599036 A Novel Role for Progesterone Receptor Membrane Component 1 ...	ACCEPT	Summary: Direct heme transfer assays confirm PGRMC1 binds and donates heme. Core MF. Reason: Direct experimental support for core heme binding activity. Supporting Evidence: PMID:27599036 In vitro heme transfer experiments showed that purified PGRMC1 was able to donate heme to apo-cytochrome b 5 .
GO:0001540 amyloid-beta binding	TAS PMID:26871627 The Cellular Phase of Alzheimer's Disease.	KEEP AS NON CORE	Summary: Sigma-2/PGRMC1 has been proposed as a critical neuronal receptor for Abeta-42 oligomers in Alzheimer's disease, with siRNA knockdown of PGRMC1 in cultured neurons reducing >90% of Abeta oligomer binding (Izzo et al. 2014 PMID:25390692; this annotation cites the De Strooper & Karran 2016 Cell review). Subsequent work clarifies that TMEM97 (sigma-2 receptor) is the gene product corresponding to the sigma-2 binding site and that PGRMC1 contributes as a complex member, so direct PGRMC1-Abeta binding has been debated. Keep as non-core given its disease relevance but acknowledge it is complex-mediated. Reason: Disease-context interaction at the plasma-membrane sigma-2 complex; not the core (heme-binding) activity. Direct vs complex-mediated binding remains debated. Supporting Evidence: PMID:25390692 We have identified the sigma-2/PGRMC1 protein as a critical receptor mediating greater than 90% of Abeta oligomer binding to neurons and their downstream synaptotoxic effects.
GO:0044297 cell body	ISS GO_REF:0000024	MARK AS OVER ANNOTATED	Summary: Parent of neuronal cell body; uninformative. Reason: Redundant with GO:0043025 neuronal cell body.
GO:0005783 endoplasmic reticulum	TAS PMID:25390692 Alzheimer's therapeutics targeting amyloid beta 1-42 oligome...	ACCEPT	Summary: Duplicate of GO:0005783 IBA annotation. ER is the core localisation. Reason: Core localisation, multiply supported. Supporting Evidence: PMID:25390692 translocates from the endoplasmic reticulum to the plasma membrane in several cell types
GO:0005886 plasma membrane	TAS Reactome:R-HSA-6799350	ACCEPT	Summary: Plasma membrane pool is well documented. The Reactome source (exocytosis of specific granule membrane proteins) is more about a specific pathway than the localisation itself. Reason: Genuine plasma membrane pool of PGRMC1. Supporting Evidence: PMID:30443021 Confocal microscopy and Proximity Ligation Assay studies indicated a clear co-localization of LDLR, PGRMC1 and TMEM97.
GO:0035579 specific granule membrane	TAS Reactome:R-HSA-6799350	KEEP AS NON CORE	Summary: Reactome-derived annotation linking PGRMC1 to neutrophil specific granule membrane (exocytosis pathway). PGRMC1 has been detected in neutrophil granule-membrane proteomics but this is a niche tissue context and not functionally characterised as a core role. Reason: Cell-type-specific granule-membrane localisation in neutrophils; peripheral to PGRMC1's general biology.
GO:0016020 membrane	HDA PMID:19946888 Defining the membrane proteome of NK cells.	MARK AS OVER ANNOTATED	Summary: High-throughput membrane-proteome study of NK cells identified PGRMC1 in membrane fractions; uninformative generic membrane annotation. Reason: Generic "membrane" is uninformative; more specific ER/plasma-membrane annotations are already present. Supporting Evidence: PMID:19946888 Defining the membrane proteome of NK cells.
GO:0005496 steroid binding	TAS PMID:9705155 Cloning and tissue expression of two putative steroid membra...	KEEP AS NON CORE	Summary: Original 1998 cloning paper that named PGRMC1 (Hpr6.6) a "putative steroid membrane receptor" based on homology to a porcine progesterone-binding protein. As above, direct steroid binding by PGRMC1 alone is equivocal; the annotation is historically motivated. Retain as non-core. Reason: Historical, putative annotation. Direct steroid binding by purified PGRMC1 cytosolic domain has not been robustly demonstrated; contemporary syntheses place heme binding (not steroid binding) as the core MF. Supporting Evidence: PMID:9705155 We have cloned two human putative steroid binding membrane proteins, termed Hpr6.6 and Dg6.

Core Functions

Heme B binding via an atypical pentacoordinate Tyr113 axial ligand in a surface-exposed pocket of the cytochrome b5-like domain, driving reversible haem-dependent homodimerisation that gates downstream protein-protein interactions.

Molecular Function:

heme binding

Directly Involved In:

heme biosynthetic process

Cellular Locations:

endoplasmic reticulum membrane mitochondrial outer membrane

Supporting Evidence:

PMID:26988023
the haem iron is five-coordinated by Tyr113 (Y113) alone
PMID:26988023
haem binding to PGRMC1 was of low affinity with a Kd value of 50 nmol l−1
PMID:27599036
In vitro heme transfer experiments showed that purified PGRMC1 was able to donate heme to apo-cytochrome b 5 .
file:human/PGRMC1/PGRMC1-deep-research-falcon.md
Multiple lines of evidence support that **PGRMC1 binds heme** at a **surface-exposed site**.

Heme-dependent homodimerisation (via heme-heme stacking and Cys129-disulfide crosslinking) that is required for productive interaction with EGFR and microsomal cytochrome P450 enzymes; the dimer is dissociated by CO binding to the sixth haem coordination position, making PGRMC1 a CO-sensitive molecular switch.

Molecular Function:

protein homodimerization activity

Directly Involved In:

regulation of catalytic activity

Cellular Locations:

endoplasmic reticulum membrane

Supporting Evidence:

PMID:26988023
PGRMC1 forms a dimeric structure largely through hydrophobic interactions between the haem moieties of two monomers
PMID:26988023
Haem-mediated PGRMC1 dimerization is required for interactions with EGFR and cytochromes P450

Direct, heme-dependent binding to multiple ER-resident cytochrome P450 enzymes (CYP51A1, CYP21A2, CYP7A1, CYP3A4, CYP2C8, CYP2C2, CYP1A2) and to NADPH-cytochrome P450 reductase (CPR), modulating microsomal CYP activity in an isoform-specific manner — stimulating sterol-biosynthetic CYPs (notably CYP51A1) while inhibiting several drug-metabolising CYPs.

Molecular Function:

enzyme regulator activity

Directly Involved In:

xenobiotic metabolic process cholesterol metabolic process

Cellular Locations:

endoplasmic reticulum membrane smooth endoplasmic reticulum membrane

Supporting Evidence:

PMID:21081644
PGRMC1 bound efficiently to all three P450s
PMID:21081644
Cotransfection of cells with P450s and PGRMC1 resulted in PGRMC1 concentration-dependent inhibition of the P450 activities, and this inhibition was partially reversed by increased expression of the P450 reductase (CPR).
PMID:21081644
CYP51 activity was decreased by down-regulation of PGRMC1 and expression of PGRMC1 in the PGRMC1-deficient cells increased CYP51 activity.
file:human/PGRMC1/PGRMC1-deep-research-falcon.md
PGRMC1 is one of few proteins described as a **direct modulator of human CYP activity**, with reported physical interactions with multiple CYPs (e.g., **CYP7A1, CYP21A2, CYP51A1, CYP3A4, CYP2C8, CYP2C2**) and **cytochrome P450 reductase (CPR)**.

Membrane-protein adaptor / chaperone-like activity that stabilises and organises plasma-membrane and ER-resident receptor complexes — including the TMEM97 (sigma-2 receptor)/PGRMC1/LDLR ternary complex required for rapid LDLR-mediated LDL internalisation, and EGFR plasma-membrane stabilisation in cancer cells.

Molecular Function:

protein-macromolecule adaptor activity

Directly Involved In:

positive regulation of lipoprotein transport

Cellular Locations:

plasma membrane endoplasmic reticulum membrane

Supporting Evidence:

PMID:30443021
These data indicate that the formation of a ternary complex of LDLR-PGRMC1-TMEM97 is necessary for the rapid internalization of LDL by LDLR.
PMID:30443021
Uptake of radiolabeled LDL was significantly decreased in PGRMC1 KO, TMEM97 KO, and double KO cell lines.

References

GO_REF:0000024

Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of sequence similarity.

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000043

Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt.

GO_REF:0000107

Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara.

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods.

PMID:17353931

Large-scale mapping of human protein-protein interactions by mass spectrometry.

PMID:19946888

Defining the membrane proteome of NK cells.

PMID:21081644

Progesterone receptor membrane component 1 inhibits the activity of drug-metabolizing cytochromes P450 and binds to cytochrome P450 reductase.

PGRMC1 binds efficiently to drug-metabolising CYPs (CYP2C2, CYP2C8, CYP3A4) primarily via their cytoplasmic catalytic domains and inhibits their activities in a PGRMC1-concentration-dependent manner; this inhibition is partially reversed by elevated CPR expression. In contrast PGRMC1 stimulates sterol-biosynthetic CYP51, illustrating isoform-specific CYP modulation. PGRMC1 also binds directly to CPR.

"PGRMC1 bound efficiently to all three P450s ... Cotransfection of cells with P450s and PGRMC1 resulted in PGRMC1 concentration-dependent inhibition of the P450 activities, and this inhibition was partially reversed by increased expression of the P450 reductase (CPR). In contrast, CYP51 activity was decreased by down-regulation of PGRMC1 and expression of PGRMC1 in the PGRMC1-deficient cells increased CYP51 activity."

PMID:25390692

Alzheimer's therapeutics targeting amyloid beta 1-42 oligomers II: Sigma-2/PGRMC1 receptors mediate Abeta 42 oligomer binding and synaptotoxicity.

Sigma-2/PGRMC1 is enriched in post-synaptic density fractions; siRNA knockdown of PGRMC1 in cultured neurons reduces >90% of Abeta oligomer binding and downstream synaptotoxicity, identifying it as a key oligomer receptor (or receptor-complex component) in Alzheimer's disease.

"We have identified the sigma-2/PGRMC1 protein as a critical receptor mediating greater than 90% of Abeta oligomer binding to neurons and their downstream synaptotoxic effects."

PMID:26871627

The Cellular Phase of Alzheimer's Disease.

PMID:26988023

Haem-dependent dimerization of PGRMC1/Sigma-2 receptor facilitates cancer proliferation and chemoresistance.

X-ray crystal structure (1.95 Å) of PGRMC1 cytosolic domain shows a stable dimer formed by hydrophobic stacking of two protruding haem moieties; haem iron is five-coordinated by Tyr113 alone. Heme Kd ~50 nM. CO binds the sixth coordination position and dissociates the dimer. Haem-mediated dimerisation is required for binding to EGFR and to cytochromes P450 (CYP1A2, CYP3A4, CYP51), for cancer proliferation and for chemoresistance.

"the haem iron is five-coordinated by Tyr113 (Y113) alone ... haem binding to PGRMC1 was of low affinity with a Kd value of 50 nmol l−1 ... Haem-mediated PGRMC1 dimerization is required for interactions with EGFR and cytochromes P450"

PMID:27599036

A Novel Role for Progesterone Receptor Membrane Component 1 (PGRMC1): A Partner and Regulator of Ferrochelatase.

PGRMC1 (and PGRMC2) co-fractionate and co-localise with the mitochondrial heme metabolon; PGRMC1 sits on the outer face of the outer mitochondrial membrane and interacts with ferrochelatase (FECH). In vitro PGRMC1 inhibits FECH activity dose-dependently (saturating at ~60% reduction) preferentially in the product-release conformation, and purified PGRMC1 can donate heme to apo-cytochrome b5, supporting a heme-chaperone/sensor role.

"In the presence of PGRMC1, in vitro measured FECH activity decreased in a dose-dependent manner. Interactions between FECH and PGRMC1 were strongest for the conformation of FECH associated with product release ... In vitro heme transfer experiments showed that purified PGRMC1 was able to donate heme to apo-cytochrome b5."

PMID:28514442

Architecture of the human interactome defines protein communities and disease networks.

PMID:30021884

Histone Interaction Landscapes Visualized by Crosslinking Mass Spectrometry in Intact Cell Nuclei.

PMID:30443021

Sigma-2 Receptor/TMEM97 and PGRMC-1 Increase the Rate of Internalization of LDL by LDL Receptor through the Formation of a Ternary Complex.

CRISPR knockout of PGRMC1 or TMEM97 in HeLa cells each independently reduce LDL internalisation by LDLR to the same extent as the double knockout, indicating both proteins act as obligate components of a single LDLR-PGRMC1-TMEM97 ternary complex required for rapid LDL endocytosis. Confocal microscopy and proximity ligation assays confirm co-localisation/interaction of the three proteins.

"These data indicate that the formation of a ternary complex of LDLR-PGRMC1-TMEM97 is necessary for the rapid internalization of LDL by LDLR."

PMID:32296183

A reference map of the human binary protein interactome.

PMID:33961781

Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.

PMID:35271311

OpenCell: Endogenous tagging for the cartography of human cellular organization.

PMID:37047353

Sigma-2 Receptor Ligand Binding Modulates Association between TSPO and TMEM97.

In MIA PaCa-2 pancreatic cancer cells (high PGRMC1) co-IP detected TSPO/PGRMC1 association in addition to TSPO/TMEM97, but no such PGRMC1/TSPO association was found in MCF7 cells, indicating cell-type- dependent multiprotein complexes around the sigma-2 receptor.

"in MP cells, we observed that both TMEM97 (Figure 4C) and PGRMC1 (Figure 4D) co-immunoprecipitated with TSPO"

PMID:40205054

Multimodal cell maps as a foundation for structural and functional genomics.

PMID:9705155

Cloning and tissue expression of two putative steroid membrane receptors.

Original cloning of human Hpr6.6 (PGRMC1) as a putative steroid-binding membrane protein; mRNA predominantly liver and kidney. Naming reflects homology to a porcine progesterone-binding protein rather than direct steroid-binding assays.

"We have cloned two human putative steroid binding membrane proteins, termed Hpr6.6 and Dg6."

Reactome:R-HSA-6799350

Exocytosis of specific granule membrane proteins

file:human/PGRMC1/PGRMC1-deep-research-perplexity.md

Deep research on PGRMC1 function (perplexity sonar-deep-research)

file:human/PGRMC1/PGRMC1-deep-research-falcon.md

Deep research on PGRMC1 function (Edison/falcon, 2026)

Contemporary expert synthesis (2024) frames PGRMC1 as a single-pass membrane MAPR-family heme-binding protein with surface-exposed heme coordination by Tyr113, two-mechanism (heme-stacking and Cys129 disulfide) dimerisation, and a primary functional role as a haem- dependent regulator of microsomal CYP systems (CYP7A1, CYP21A2, CYP51A1, CYP3A4, CYP2C8, CYP2C2) and CPR. ER/endomembranes are the dominant subcellular location with additional plasma membrane, nuclear, Golgi, endosomal, and mitochondrial pools. Translational relevance is most mature for sigma-2/TMEM97 complex biology (CT1812 in Alzheimer's disease) and CYP-mediated drug-metabolism modulation.

"Human **PGRMC1 (O00264)** is a **single-pass membrane MAPR protein** with a **cytochrome b5-like heme-binding domain** that supports **heme-dependent and redox-sensitive oligomerization** and enables **regulatory interactions** with (at least) the **cytochrome P450 system** (CYPs and CPR) and membrane signaling complexes."

Suggested Experiments

Experiment: Conditionally delete Pgrmc1 in mouse hepatocytes and rescue with WT vs. Y113F (heme-binding deficient) vs. C129S vs. Y113F/C129S double mutants; measure CYP3A4 protein, drug-clearance pharmacokinetics, and acetaminophen hepatotoxicity.

Hypothesis: Heme-loaded PGRMC1 dimers, but not apo-PGRMC1 monomers, are required for microsomal CYP3A4-mediated drug clearance in hepatocytes.

Experiment: Compare LDL internalisation kinetics in PGRMC1-KO HeLa cells reconstituted with WT vs. Y113F vs. C129S PGRMC1; test whether CORM-3 (CO donor) phenocopies heme-binding loss for LDL uptake.

Hypothesis: The TMEM97/PGRMC1/LDLR complex requires PGRMC1's heme-dependent dimerisation for accelerated LDL endocytosis.

Experiment: Use isotopically labelled heme pulse-chase combined with FECH-, PGRMC1-, and double-knockout cells to follow heme flux from mitochondria to ER-resident apo-hemoproteins, with vs. without PGRMC1 add-back.

Hypothesis: PGRMC1 functions as a heme chaperone, transferring heme from the FECH-containing mitochondrial heme metabolon to apo-CYPs and apo-cytochrome b5 in the ER.

Deep Research

Falcon

(PGRMC1-deep-research-falcon.md)

this with annotations you find in gene/protein databases, but these can be outdated or inaccurate. Falcon Edison Scientific Literature 31 citations 2 artifacts 2026-05-29T19:50:43.157989

Edison artifact artifact-00 (text/markdown)

## Context ID: pqac-00000013 Figure 8 on page 18 provides a schematic representation of the sigma-2 receptor (S2R), also known as TMEM97, in a complex with the (image/png)

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research report: Human PGRMC1 (UniProt O00264) — functional annotation

1) Target verification (critical identity check)

The target in this report is human PGRMC1 (Progesterone Receptor Membrane Component 1), UniProt accession O00264, a ~195-aa membrane-associated progesterone receptor component and member of the MAPR (membrane-associated progesterone receptor) subfamily with a cytochrome b5-like heme/steroid-binding domain. Recent literature explicitly anchors discussion to UniProt O00264 and uses constructs matching the O00264 cytosolic domain (residues 44–195), supporting correct gene/protein identity and avoiding symbol ambiguity. (barata2024pleiotropyofprogesterone pages 7-9, badve2024definingrequirementsfor pages 1-6)

2) Key concepts and definitions (current understanding)

2.1 MAPR/cytochrome b5-like heme-binding protein (not a classical nuclear progesterone receptor)

PGRMC1 is widely described as a non-classical membrane progesterone receptor component with pleiotropic functions including progesterone-related signaling and modulation of drug-metabolizing systems. Its key structural hallmark is a cytochrome b5-like heme-binding fold (MAPR family), which provides a mechanistic basis for heme-dependent interactions and potential heme trafficking/buffering functions. (barata2024pleiotropyofprogesterone pages 7-9, barata2024pleiotropyofprogesterone pages 5-7)

2.2 Heme binding: residues, coordination, and “holo” vs “apo” PGRMC1

Multiple lines of evidence support that PGRMC1 binds heme at a surface-exposed site. A canonical structural model shows Tyr113 coordinating the heme iron in a five-coordinate geometry, but more recent mechanistic work and cited prior literature indicate Tyr113 may not be strictly required for heme binding in all contexts and may be particularly important for specific functional interactions (e.g., with ferrochelatase). Residues and regions repeatedly implicated in heme binding and/or functional effects include Tyr113, Tyr107, Lys163, Tyr164, Asp99–Lys102, and Asp120. (barata2024pleiotropyofprogesterone pages 7-9, badve2024definingrequirementsfor pages 1-6, badve2024definingrequirementsfor pages 29-33)

2.3 Dimerization/oligomerization as a regulatory mechanism

A recurring concept is that PGRMC1 forms dimers/oligomers, historically proposed to be driven by heme–heme stacking and linked to binding partners such as CYP enzymes and EGFR. A 2024 biochemical dissection supports a refined view: PGRMC1 can dimerize via two non-mutually-exclusive mechanisms—(i) intermolecular disulfide bonding centered on Cys129 and (ii) heme-mediated stacking interactions—yielding mixtures of monomers, dimers, and larger oligomers depending on heme and redox state. (badve2024definingrequirementsfor pages 29-33)

3) Molecular functions and pathways (what PGRMC1 “does”)

3.1 Primary biochemical role: heme-binding adaptor/chaperone-like behavior and protein–protein interactions

The most consistently supported molecular function across recent synthesis is that PGRMC1 is a heme-binding membrane protein whose protein–protein interactions (and potentially trafficking of heme) connect it to multiple pathways—particularly cytochrome P450 (CYP) biology, membrane receptor signaling (e.g., EGFR), and membrane microdomain complexes (e.g., TMEM97/LDLR/PGRMC1). (barata2024pleiotropyofprogesterone pages 13-14, barata2024pleiotropyofprogesterone pages 9-10, barata2024pleiotropyofprogesterone pages 23-24)

3.2 Cytochrome P450 modulation (xenobiotic and endogenous metabolism)

A 2024 focused review emphasizes that PGRMC1 is one of few proteins described as a direct modulator of human CYP activity, with reported physical interactions with multiple CYPs (e.g., CYP7A1, CYP21A2, CYP51A1, CYP3A4, CYP2C8, CYP2C2) and cytochrome P450 reductase (CPR). Effects are isoform-dependent and may include altered enzymatic parameters. Reported outcomes include:
- CYP21A2 activity changes upon co-expression and mutation: wild-type PGRMC1 increased CYP21A2 activity (~2-fold), while specific PGRMC1 variants had different quantitative effects (e.g., Tyr107Phe/Tyr113Phe ~+60%; a region around Asp99–Lys102 ~−75%). (barata2024pleiotropyofprogesterone pages 9-10)
- For drug-metabolizing CYPs, higher PGRMC1 levels were reported to reduce CYP3A4 and CYP2C9 activities, with directionality including increased Km and decreased kcat for CYP3A4 and decreased kcat for CYP2C9 in cited studies. (barata2024pleiotropyofprogesterone pages 9-10)

Interpretive consensus (expert synthesis): Current expert reviews frame PGRMC1 less as a simple on/off “progesterone receptor” and more as a membrane-associated heme-binding regulator whose variable oligomeric state, PTMs, and cell-context-dependent localization can tune CYP complex function and thereby affect drug metabolism and endogenous steroid/lipid pathways. (barata2024pleiotropyofprogesterone pages 13-14, barata2024pleiotropyofprogesterone pages 9-10)

3.3 Progesterone-linked anti-apoptotic signaling and receptor pathway crosstalk

PGRMC1 is also discussed as a mediator of progesterone anti-apoptotic signaling, including interaction with PAIRBP1/SERBP1, and as a modulator of oncogenic signaling through association with EGFR, with reported pathway crosstalk involving Wnt/β-catenin and NF-κB and links to erlotinib resistance in cancer models. (barata2024pleiotropyofprogesterone pages 11-13)

3.4 Heme synthesis/trafficking links (ferrochelatase and heme transfer hypotheses)

PGRMC1 has been proposed to interact with heme biosynthesis machinery, including as a partner/regulator of ferrochelatase (FECH), and as a factor that may buffer or transfer heme to other proteins. Recent reviews note the model remains plausible but incompletely resolved in vivo; nonetheless, it provides a coherent mechanistic explanation for why a cytochrome b5-like fold is conserved in MAPR proteins and why PGRMC1 may influence CYP stability/function. (barata2024pleiotropyofprogesterone pages 13-14, badve2024definingrequirementsfor pages 1-6)

4) Subcellular localization (where PGRMC1 acts)

PGRMC1 localization is repeatedly reported as context-dependent, with strong recurring evidence for association with the endoplasmic reticulum (ER)/endomembranes, and additional reported localization to the plasma membrane, nucleus/nuclear membrane, Golgi, endosomes, cytoplasm, and mitochondria depending on cell type and condition. (jo2024progesteronereceptormembrane pages 1-2, jo2023hepaticprogesteronereceptor pages 1-5, barata2024pleiotropyofprogesterone pages 5-7)

This distribution is biologically consistent with its proposed roles in (i) ER-embedded CYP systems and (ii) membrane receptor complexes and organelle crosstalk relevant to stress responses and metabolism. (barata2024pleiotropyofprogesterone pages 13-14, barata2024pleiotropyofprogesterone pages 5-7)

5) Recent developments (prioritizing 2023–2024)

5.1 2024: refined mechanistic model of heme binding and dimerization determinants

Badve & Meier (Biochemistry, Mar 2024, https://doi.org/10.1021/acs.biochem.3c00718) used spectroscopy and mutational analysis (notably Y113F and C129S) to separate contributions of iron coordination and disulfide formation, supporting a two-mechanism model for PGRMC1 dimerization/oligomerization and emphasizing that conclusions from truncated cytosolic constructs must be reconciled with behavior of full-length membrane-bound PGRMC1. (badve2024definingrequirementsfor pages 29-33)

5.2 2024: updated synthesis of PGRMC1 as a CYP modulator impacting drug metabolism

Barata et al. (Journal of Xenobiotics, May 2024, https://doi.org/10.3390/jox14020034) provides a 2024 synthesis spanning PGRMC1 structure, localization, heme binding, and isoform-dependent CYP modulation, highlighting experimental reports of altered catalytic parameters and the potential implications for xenobiotic metabolism and chemotherapy responses. (barata2024pleiotropyofprogesterone pages 9-10)

5.3 2023–2024: stress/inflammation and tissue physiology links in vivo

Ethanol-associated liver injury (2023): In a chronic-plus-binge ethanol feeding model, whole-body Pgrmc1 knockout mice showed increased liver injury indicators (including elevated ALT), higher acetaldehyde, ER stress, and apoptosis versus wild-type, supporting a protective role in hepatic stress responses and alcohol metabolism-related toxicity. (Jo et al., Jun 2023, https://doi.org/10.1152/ajpgi.00206.2022) (jo2023hepaticprogesteronereceptor pages 1-5)
Chronic neuroinflammation (2024): In an LPS-induced chronic neuroinflammation model, Pgrmc1 knockout was associated with higher inflammatory cytokines, increased NF-κB signaling, ER stress markers, and apoptosis, consistent with PGRMC1 contributing to inflammatory/stress regulation in neural contexts. (Jo & Hong, Feb 2024, https://doi.org/10.3390/antiox13020230) (jo2024progesteronereceptormembrane pages 1-2)

5.4 2024: reproductive biology—decidualization requires a timed PGRMC1 program

Liu et al. (Reproductive Biology and Endocrinology, Feb 2024, https://doi.org/10.1186/s12958-024-01188-9) reports that PGRMC1 follows a progestin-induced “rise-to-decline” expression program during in vitro decidualization; knockdown before induction prevents decidualization, and progestin induces interactions between PGRMC1 and prohibitins (PHB1/PHB2), indicating a mechanistic partnership in early decidualization. (liu2024progesteroneinducedprogesteronereceptor pages 1-2)

6) Current applications and real-world implementations

6.1 Drug metabolism and chemotherapy interactions (clinical relevance of CYP modulation)

Because CYP enzymes determine metabolism of many drugs, PGRMC1’s reported ability to modulate CYP expression/stability and catalytic parameters has translational implications for variable drug clearance, drug–drug interactions, and chemotherapy response. Contemporary reviews highlight this as a major reason PGRMC1 has drawn attention in cancer and pharmacology. (barata2024pleiotropyofprogesterone pages 13-14, barata2024pleiotropyofprogesterone pages 9-10)

6.2 Sigma-2 (TMEM97) axis and the TMEM97–LDLR–PGRMC1 complex (Alzheimer’s disease therapeutic program)

A prominent translational “implementation” is that PGRMC1 is depicted as part of a TMEM97 (sigma-2 receptor)–LDLR–PGRMC1 membrane complex relevant to cholesterol/synaptic biology. A CT1812 mechanism schematic proposes that CT1812 binding to sigma-2/TMEM97 induces conformational/complex changes that displace Aβ oligomers from synapses, a hypothesized therapeutic mechanism in AD. (lizama2024aninterimexploratory media a48f331c)

This sigma-2-centered approach does not necessarily require direct PGRMC1 targeting, but it makes PGRMC1 biologically relevant to a clinically active program via complex membership. (lizama2024aninterimexploratory media a48f331c)

7) Expert opinions and analysis (authoritative synthesis)

Recent expert syntheses converge on several interpretive points:
1. PGRMC1 is best understood as a heme-binding MAPR protein with extensive protein–protein interactions, rather than as a classical ligand-activated progesterone receptor. (barata2024pleiotropyofprogesterone pages 7-9, barata2024pleiotropyofprogesterone pages 9-10)
2. Mechanistic models that attribute all downstream biology to heme-mediated dimers are being refined; 2024 biochemical work supports a mixed mechanism (disulfide- and heme-mediated) and emphasizes the need to map these states to full-length membrane biology and physiological partners. (badve2024definingrequirementsfor pages 29-33)
3. The most actionable near-term translational space appears to be through (i) CYP modulation affecting pharmacology/toxicology and (ii) sigma-2/TMEM97 complex biology in oncology and neurodegeneration, rather than a single “enzymatic reaction” catalyzed by PGRMC1 (it is not itself a CYP enzyme). (barata2024pleiotropyofprogesterone pages 9-10, lizama2024aninterimexploratory media a48f331c)

8) Relevant statistics and data (recent studies)

Quantitative details available from the retrieved full text were more abundant for CYP functional modulation than for clinical associations:
- CYP21A2 activity modulation in cited cellular assays included ~2-fold increase with wild-type PGRMC1 and mutant-dependent effects (e.g., ~+60% for Tyr107Phe/Tyr113Phe; ~−75% for Asp99–Lys102 region). (barata2024pleiotropyofprogesterone pages 9-10)

For the 2023–2024 in vivo stress/injury studies retrieved here, the provided pages did not include detailed numerical datasets (n, p-values) in the extracted evidence snippets, though the qualitative direction of effects (KO worse than WT) is clear. (jo2024progesteronereceptormembrane pages 1-2, jo2023hepaticprogesteronereceptor pages 1-5)

9) Disease associations from knowledgebases (supporting but not definitive)

OpenTargets lists disease associations for PGRMC1 (e.g., neurodegenerative disease, premature ovarian failure, cataract-related phenotypes) based on multiple evidence types (literature and study links). These associations are hypothesis-supporting and should be interpreted in light of underlying evidence type and causality limits. (OpenTargets Search: -PGRMC1)

10) Summary: functional annotation (most defensible primary function)

Most defensible primary functional description (integrating 2023–2024 evidence): Human PGRMC1 (O00264) is a single-pass membrane MAPR protein with a cytochrome b5-like heme-binding domain that supports heme-dependent and redox-sensitive oligomerization and enables regulatory interactions with (at least) the cytochrome P450 system (CYPs and CPR) and membrane signaling complexes. Its predominant cellular context is ER/endomembranes, with additional localization to plasma membrane, nucleus, and mitochondria depending on cell type/condition. Through these biochemical and organizational roles, PGRMC1 influences drug/xenobiotic metabolism, progesterone-linked survival signaling, and cell stress/inflammation phenotypes, and it is implicated as a complex member in the TMEM97–LDLR–PGRMC1 axis relevant to sigma-2–directed therapeutic development in Alzheimer’s disease. (barata2024pleiotropyofprogesterone pages 13-14, badve2024definingrequirementsfor pages 29-33, barata2024pleiotropyofprogesterone pages 9-10, jo2024progesteronereceptormembrane pages 1-2, jo2023hepaticprogesteronereceptor pages 1-5, liu2024progesteroneinducedprogesteronereceptor pages 1-2, lizama2024aninterimexploratory media a48f331c)

Evidence map (compact)

Topic	Key findings	Evidence type (review/primary, in vitro/in vivo)	Year	Citation (DOI URL where available)	Context ID
Molecular function	Human PGRMC1 (UniProt O00264) is a MAPR-family, cytochrome b5-like heme-binding protein with a surface-exposed heme site; recent work confirms heme binding stabilizes the cytosolic domain and supports interaction models relevant to downstream partners.	Primary biochemical/structural; in vitro	2024	Badve & Meier, Biochemistry (2024), https://doi.org/10.1021/acs.biochem.3c00718	(badve2024definingrequirementsfor pages 1-6)
Key residues	Tyr113 is the canonical heme-coordinating residue in the crystal model; Tyr107, Lys163, Tyr164, Asp99-Lys102, and Asp120 also influence heme binding or partner effects. Y113F abolishes direct Fe coordination, while Cys129 is central to disulfide-linked dimerization.	Review synthesizing primary mutagenesis/structural studies; in vitro	2024	Barata et al., Journal of Xenobiotics (2024), https://doi.org/10.3390/jox14020034; Badve & Meier, Biochemistry (2024), https://doi.org/10.1021/acs.biochem.3c00718	(barata2024pleiotropyofprogesterone pages 7-9, badve2024definingrequirementsfor pages 29-33)
Dimerization	PGRMC1 can form dimers/oligomers through two nonexclusive mechanisms: intermolecular disulfide formation centered on Cys129 and heme–heme stacking; earlier heme-dependent dimer models are being refined rather than simply accepted.	Primary spectroscopy/mutagenesis; in vitro	2024	Badve & Meier, Biochemistry (2024), https://doi.org/10.1021/acs.biochem.3c00718	(badve2024definingrequirementsfor pages 29-33, badve2024definingrequirementsfor pages 33-35)
CYP modulation	PGRMC1 interacts with multiple CYPs and can modulate activity in an isoform-dependent manner. Reported partners include CYP7A1, CYP21A2, CYP51A1, CYP3A4, CYP2C8, CYP2C2, and CPR; effects include altered Km/kcat and either stimulation or inhibition depending on CYP and expression level.	Review of primary biochemical/cellular studies; in vitro and in vivo	2024	Barata et al., Journal of Xenobiotics (2024), https://doi.org/10.3390/jox14020034	(barata2024pleiotropyofprogesterone pages 13-14, barata2024pleiotropyofprogesterone pages 9-10, barata2024pleiotropyofprogesterone pages 23-24)
Heme trafficking / ferrochelatase	PGRMC1 has been proposed as a heme-transfer or heme-buffering factor and a partner/regulator of ferrochelatase (FECH). Recent reviews emphasize that this remains plausible but incompletely resolved in vivo.	Review of primary studies; biochemical/cellular	2024	Barata et al., Journal of Xenobiotics (2024), https://doi.org/10.3390/jox14020034; Dunaway et al., JBC (2024), https://doi.org/10.1016/j.jbc.2024.107132	(barata2024pleiotropyofprogesterone pages 13-14, badve2024definingrequirementsfor pages 1-6)
EGFR / progesterone signaling	PGRMC1 has been implicated in progesterone anti-apoptotic signaling via SERBP1/PAIRBP1 and in EGFR-associated oncogenic signaling, including links to Wnt/β-catenin and NF-κB pathways and erlotinib resistance.	Review of primary cell biology/cancer studies; mostly in vitro/in vivo models	2024	Barata et al., Journal of Xenobiotics (2024), https://doi.org/10.3390/jox14020034	(barata2024pleiotropyofprogesterone pages 11-13)
TMEM97 / sigma-2 complex	Recent literature places PGRMC1 in a TMEM97 (sigma-2 receptor)-LDLR-PGRMC1 membrane complex relevant to cholesterol biology and Alzheimer’s disease drug development. A graphical mechanism for CT1812 proposes that binding to this complex indirectly displaces synaptotoxic Aβ oligomers.	Review/clinical translational reports; human clinical biomarker study and mechanistic model	2024	Lizama et al., Alzheimer’s & Dementia (2024), https://doi.org/10.1002/alz.14152; Lizama et al., bioRxiv (2024), https://doi.org/10.1101/2024.02.16.578765	(lizama2024aninterimexploratory media a48f331c)
Localization	PGRMC1 localizes across multiple compartments depending on cell type: ER/endomembranes are most consistent, with additional reports at plasma membrane, nucleus/nuclear membrane, Golgi, endosomes, and mitochondria.	Review and primary studies; microscopy/fractionation; in vitro and in vivo	2023-2024	Barata et al., Journal of Xenobiotics (2024), https://doi.org/10.3390/jox14020034; Jo et al., AJP Gastrointest Liver Physiol (2023), https://doi.org/10.1152/ajpgi.00206.2022; Jo & Hong, Antioxidants (2024), https://doi.org/10.3390/antiox13020230	(jo2024progesteronereceptormembrane pages 1-2, jo2023hepaticprogesteronereceptor pages 1-5, barata2024pleiotropyofprogesterone pages 5-7)
Mitochondrial function / liver injury	In a 2023 ethanol liver-injury study, loss of Pgrmc1 increased acetaldehyde, ALT, ER stress, and apoptosis in mice, supporting a protective hepatic role linked to alcohol metabolism and oxidative stress.	Primary mouse study; in vivo	2023	Jo et al., AJP Gastrointest Liver Physiol (2023), https://doi.org/10.1152/ajpgi.00206.2022	(jo2023hepaticprogesteronereceptor pages 1-5)
Stress / inflammation	In chronic neuroinflammation models, Pgrmc1 knockout increased inflammatory cytokines, NF-κB signaling, ER-stress markers, and apoptosis, consistent with a protective role in cellular stress and inflammatory control.	Primary mouse/cell study; in vivo and in vitro	2024	Jo & Hong, Antioxidants (2024), https://doi.org/10.3390/antiox13020230	(jo2024progesteronereceptormembrane pages 1-2)
Reproductive physiology	In human endometrial stromal cell decidualization models, PGRMC1 shows a progesterone-induced rise-to-decline program; knockdown before induction blocks decidualization, and PHB1/PHB2 emerge as inducible partners.	Primary human cell study; in vitro	2024	Liu et al., Reproductive Biology and Endocrinology (2024), https://doi.org/10.1186/s12958-024-01188-9	(liu2024progesteroneinducedprogesteronereceptor pages 1-2)
Disease / application	PGRMC1 remains a candidate biomarker or functional node in cancer, neurodegeneration, liver injury, and reproductive disease, but direct clinical targeting is less mature than targeting the associated sigma-2/TMEM97 axis (e.g., CT1812 in AD; sigma-2 ligands in cancer).	Review + translational/clinical studies	2024	Takchi et al., Cell Death & Disease (2024), https://doi.org/10.1038/s41419-024-06693-8; Hagi et al., Scientific Reports (2024), https://doi.org/10.1038/s41598-024-56928-z; Lizama et al., Alzheimer’s & Dementia (2024), https://doi.org/10.1002/alz.14152	(lizama2024aninterimexploratory media a48f331c, OpenTargets Search: -PGRMC1)

Table: This table summarizes experimentally supported functions, partners, localization, and recent 2023-2024 developments for human PGRMC1 (UniProt O00264). It is useful as a compact evidence map linking molecular mechanism to physiological and translational relevance.

References

(barata2024pleiotropyofprogesterone pages 7-9): Isabel S. Barata, José Rueff, Michel Kranendonk, and Francisco Esteves. Pleiotropy of progesterone receptor membrane component 1 in modulation of cytochrome p450 activity. Journal of Xenobiotics, 14:575-603, May 2024. URL: https://doi.org/10.3390/jox14020034, doi:10.3390/jox14020034. This article has 5 citations.
(badve2024definingrequirementsfor pages 1-6): Prajakta Badve and Katlyn K. Meier. Defining requirements for heme binding in pgrmc1 and identifying key elements that influence protein dimerization. Biochemistry, 63:926-938, Mar 2024. URL: https://doi.org/10.1021/acs.biochem.3c00718, doi:10.1021/acs.biochem.3c00718. This article has 2 citations and is from a peer-reviewed journal.
(barata2024pleiotropyofprogesterone pages 5-7): Isabel S. Barata, José Rueff, Michel Kranendonk, and Francisco Esteves. Pleiotropy of progesterone receptor membrane component 1 in modulation of cytochrome p450 activity. Journal of Xenobiotics, 14:575-603, May 2024. URL: https://doi.org/10.3390/jox14020034, doi:10.3390/jox14020034. This article has 5 citations.
(badve2024definingrequirementsfor pages 29-33): Prajakta Badve and Katlyn K. Meier. Defining requirements for heme binding in pgrmc1 and identifying key elements that influence protein dimerization. Biochemistry, 63:926-938, Mar 2024. URL: https://doi.org/10.1021/acs.biochem.3c00718, doi:10.1021/acs.biochem.3c00718. This article has 2 citations and is from a peer-reviewed journal.
(barata2024pleiotropyofprogesterone pages 13-14): Isabel S. Barata, José Rueff, Michel Kranendonk, and Francisco Esteves. Pleiotropy of progesterone receptor membrane component 1 in modulation of cytochrome p450 activity. Journal of Xenobiotics, 14:575-603, May 2024. URL: https://doi.org/10.3390/jox14020034, doi:10.3390/jox14020034. This article has 5 citations.
(barata2024pleiotropyofprogesterone pages 9-10): Isabel S. Barata, José Rueff, Michel Kranendonk, and Francisco Esteves. Pleiotropy of progesterone receptor membrane component 1 in modulation of cytochrome p450 activity. Journal of Xenobiotics, 14:575-603, May 2024. URL: https://doi.org/10.3390/jox14020034, doi:10.3390/jox14020034. This article has 5 citations.
(barata2024pleiotropyofprogesterone pages 23-24): Isabel S. Barata, José Rueff, Michel Kranendonk, and Francisco Esteves. Pleiotropy of progesterone receptor membrane component 1 in modulation of cytochrome p450 activity. Journal of Xenobiotics, 14:575-603, May 2024. URL: https://doi.org/10.3390/jox14020034, doi:10.3390/jox14020034. This article has 5 citations.
(barata2024pleiotropyofprogesterone pages 11-13): Isabel S. Barata, José Rueff, Michel Kranendonk, and Francisco Esteves. Pleiotropy of progesterone receptor membrane component 1 in modulation of cytochrome p450 activity. Journal of Xenobiotics, 14:575-603, May 2024. URL: https://doi.org/10.3390/jox14020034, doi:10.3390/jox14020034. This article has 5 citations.
(jo2024progesteronereceptormembrane pages 1-2): Seong-Lae Jo and Eui-Ju Hong. Progesterone receptor membrane component 1 regulates cellular stress responses and inflammatory pathways in chronic neuroinflammatory conditions. Antioxidants, 13:230, Feb 2024. URL: https://doi.org/10.3390/antiox13020230, doi:10.3390/antiox13020230. This article has 6 citations.
(jo2023hepaticprogesteronereceptor pages 1-5): Seong-Lae Jo, In-Jeoung Baek, Je-Won Ko, Hyo-Jung Kwun, Hyun-Jin Shin, and Eui-Ju Hong. Hepatic progesterone receptor membrane component 1 attenuates ethanol-induced liver injury by reducing acetaldehyde production and oxidative stress. Jun 2023. URL: https://doi.org/10.1152/ajpgi.00206.2022, doi:10.1152/ajpgi.00206.2022. This article has 11 citations.
(liu2024progesteroneinducedprogesteronereceptor pages 1-2): Hailun Liu, André Franken, Alexandra P. Bielfeld, Tanja Fehm, Dieter Niederacher, Zhongping Cheng, Hans Neubauer, and Nadia Stamm. Progesterone-induced progesterone receptor membrane component 1 rise-to-decline changes are essential for decidualization. Reproductive Biology and Endocrinology : RB&E, Feb 2024. URL: https://doi.org/10.1186/s12958-024-01188-9, doi:10.1186/s12958-024-01188-9. This article has 12 citations.
(lizama2024aninterimexploratory media a48f331c): B. Lizama, H. North, K. Pandey, C. Williams, D. Duong, E. Cho, V. Di Caro, L. Ping, K. Blennow, H. Zetterberg, J. Lah, A. Levey, M. Grundman, A. Caggiano, N. Seyfried, and M. Hamby. An interim exploratory biomarker analysis of a phase 2 clinical trial to assess the impact of ct1812 in alzheimer’s disease. bioRxiv, Feb 2024. URL: https://doi.org/10.1101/2024.02.16.578765, doi:10.1101/2024.02.16.578765. This article has 3 citations.
(OpenTargets Search: -PGRMC1): Open Targets Query (-PGRMC1, 5 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.
(badve2024definingrequirementsfor pages 33-35): Prajakta Badve and Katlyn K. Meier. Defining requirements for heme binding in pgrmc1 and identifying key elements that influence protein dimerization. Biochemistry, 63:926-938, Mar 2024. URL: https://doi.org/10.1021/acs.biochem.3c00718, doi:10.1021/acs.biochem.3c00718. This article has 2 citations and is from a peer-reviewed journal.

Artifacts

Edison artifact artifact-00

Citations

badve2024definingrequirementsfor pages 29-33
barata2024pleiotropyofprogesterone pages 9-10
barata2024pleiotropyofprogesterone pages 11-13
jo2023hepaticprogesteronereceptor pages 1-5
jo2024progesteronereceptormembrane pages 1-2
liu2024progesteroneinducedprogesteronereceptor pages 1-2
badve2024definingrequirementsfor pages 1-6
barata2024pleiotropyofprogesterone pages 7-9
barata2024pleiotropyofprogesterone pages 5-7
barata2024pleiotropyofprogesterone pages 13-14
barata2024pleiotropyofprogesterone pages 23-24
badve2024definingrequirementsfor pages 33-35
https://doi.org/10.1021/acs.biochem.3c00718
https://doi.org/10.3390/jox14020034
https://doi.org/10.1152/ajpgi.00206.2022
https://doi.org/10.3390/antiox13020230
https://doi.org/10.1186/s12958-024-01188-9
https://doi.org/10.3390/jox14020034;
https://doi.org/10.1016/j.jbc.2024.107132
https://doi.org/10.1002/alz.14152;
https://doi.org/10.1101/2024.02.16.578765
https://doi.org/10.1152/ajpgi.00206.2022;
https://doi.org/10.1038/s41419-024-06693-8;
https://doi.org/10.1038/s41598-024-56928-z;
https://doi.org/10.1002/alz.14152
https://doi.org/10.3390/jox14020034,
https://doi.org/10.1021/acs.biochem.3c00718,
https://doi.org/10.3390/antiox13020230,
https://doi.org/10.1152/ajpgi.00206.2022,
https://doi.org/10.1186/s12958-024-01188-9,
https://doi.org/10.1101/2024.02.16.578765,

Perplexity

(PGRMC1-deep-research-perplexity.md)

Progesterone Receptor Membrane Component 1 (PGRMC1): A Comprehensive Molecular and Functional Analysis Perplexity sonar-deep-research 49 citations 2025-12-12T07:11:18.236304

Progesterone Receptor Membrane Component 1 (PGRMC1): A Comprehensive Molecular and Functional Analysis

Progesterone receptor membrane component 1 (PGRMC1), also known as membrane-associated progesterone receptor (mPR) and identified by UniProt accession O00264, represents a multifunctional membrane-associated heme-binding protein that has emerged as a critical regulator of diverse cellular processes ranging from drug metabolism and cholesterol synthesis to cancer proliferation and cellular stress responses[1][2][3]. Despite its designation as a progesterone receptor, PGRMC1 functions primarily as a heme-binding protein whose biochemical role centers on the stabilization of cytochrome P450 enzymes and coordination of heme homeostasis[1][2][7]. Recent molecular and structural studies have fundamentally revised our understanding of PGRMC1 function, moving away from its initial characterization as a classical hormone receptor toward a recognition of its role as a regulatory hub protein orchestrating multiple protein-protein interactions through heme-dependent and heme-independent mechanisms. This report synthesizes current knowledge regarding PGRMC1's structural organization, primary biochemical functions, subcellular localization dynamics, involvement in disease pathogenesis particularly cancer, and emerging therapeutic opportunities.

Structural Architecture and Biochemical Characteristics of PGRMC1

Protein Domain Organization and Topology

PGRMC1 is a 195-amino acid integral membrane protein that exhibits a characteristic architecture consisting of a single N-terminal transmembrane domain and a C-terminal cytosolic cytochrome b₅-like heme-binding domain[1][6][31]. The protein belongs to the membrane-associated progesterone receptor (MAPR) family, alongside its paralog PGRMC2 and more distantly related proteins including Neudesin and Neuferricin[31]. The transmembrane topology of PGRMC1 has been subject to considerable investigation and revision, with evidence supporting both Type I orientation (N-terminus extracellular, C-terminus cytoplasmic) and Type II topology (N-terminus cytoplasmic, C-terminus extracellular) depending on cellular context and subcellular localization[6][31]. This membrane topology plasticity represents an important distinction from classical transmembrane proteins with fixed orientation and likely reflects PGRMC1's dynamic subcellular trafficking and functional requirements across diverse cellular compartments[6].

The cytochrome b₅-like domain comprises the C-terminal region (amino acids 70-130 region containing the heme-binding motif) and shares structural homology with the canonical cytochrome b₅ protein, although PGRMC1 functions distinctly from cytochrome b₅[31][38]. Crystallographic analysis at 1.95 Å resolution revealed that PGRMC1's cytosolic domain forms stable dimers through stacking interactions of protruding heme molecules, a unique protein dimerization mechanism involving heme-heme stacking that was previously unknown in eukaryotes[3][22]. This heme-mediated dimerization proves crucial for PGRMC1's ability to interact with downstream effector proteins including epidermal growth factor receptor (EGFR) and cytochrome P450 enzymes[3][22].

Heme Binding Mechanism and Coordination Chemistry

The sole established biochemical activity of PGRMC1 involves reversible, non-covalent binding of heme B in a pentacoordinate fashion[1][2][38]. Unlike classical hemoproteins such as myoglobin or cytochrome c that bind heme with high affinity through either histidine or histidine-methionine coordination pairs, PGRMC1 binds heme through an atypical tyrosine-based coordination motif featuring Tyr113 as the principal heme iron ligand, along with participation of Lys163 and Tyr164[1][2][38]. This tyrosine-coordinate heme binding affinity distinguishes PGRMC1 from conventional hemoproteins and more closely resembles coordination strategies employed by putative heme transporters such as ShuT and PhuT, suggesting PGRMC1 may function in heme transfer and redistribution[25][38]. Spectroscopic and computational studies confirm that heme binding occurs reversibly to PGRMC1, with heme loading likely responding to intracellular heme concentrations[38].

Recent structural and mutagenesis investigations have demonstrated that while Tyr113 serves as a critical axial heme ligand, the Y113F mutation does not completely abolish heme binding to PGRMC1[14][26]. Instead, multiple coordinating residues (Y113, K163, Y164) collectively establish the heme binding site, such that triple mutation of these residues proves necessary to eliminate heme binding entirely[38]. This redundancy in coordinate bonding provides mechanistic flexibility and may protect essential heme binding capacity under stress conditions where individual residues might be modified post-translationally. Interestingly, the Y113F mutation specifically impairs PGRMC1 binding to ferrochelatase (FECH), the terminal heme biosynthetic enzyme, suggesting that proper iron coordination geometry participates specifically in FECH interaction independent of general heme binding capacity[2][14].

Primary Biochemical Functions: Cytochrome P450 Stabilization and Heme Homeostasis

Heme-Independent Stabilization of Cytochrome P450 Proteins

The central biochemical role of PGRMC1 in cellular physiology involves stabilization and maintenance of cytochrome P450 protein levels through post-translational mechanisms fundamentally independent of PGRMC1's heme-binding capacity[2][7][38]. Knockout studies in mouse liver utilizing comprehensive proteomic and transcriptomic analyses demonstrated that PGRMC1 binds and stabilizes a broad spectrum of at least thirteen distinct cytochrome P450 enzymes including CYP1A2, CYP2E1, CYP3A4, CYP7A1, CYP21A2, and CYP51A1[2][7]. Loss of PGRMC1 resulted in 14-38% reductions in P450 protein abundance measured by quantitative mass spectrometry, with Western blotting revealing reductions of 22-70% depending on specific P450 isoform, while P450 transcript levels remained unchanged or were even upregulated, clearly indicating posttranscriptional regulation[2][7].

Mechanistic investigation employing protein half-life studies with the translation inhibitor emetine revealed that PGRMC1 increases cytochrome P450 protein stability through inhibition of proteasomal degradation[2][38]. Specifically, PGRMC1 co-expression increased the half-life of the P450 enzyme CYP1A2 by approximately 67%, demonstrating a substantial stabilizing effect that directly translates to increased steady-state P450 protein levels[2]. Critically, this stabilization mechanism operates independently of PGRMC1's heme-binding function, as demonstrated by the capacity of Y113F mutant PGRMC1—which exhibits impaired heme-binding coordination—to stabilize P450 proteins with efficiency comparable to wild-type PGRMC1[2]. This heme-independent stabilization distinguishes PGRMC1 from traditional heme-chaperone functions and may involve direct interaction with P450 protein domains that protect against proteolytic cleavage or ubiquitin-dependent proteasomal targeting.

Physiological Consequences of P450 Stabilization

The functional significance of PGRMC1-dependent P450 stabilization extends to clinically relevant drug metabolism and xenobiotic handling[2][7]. In Pgrmc1 knockout mice, loss of P450 stabilization resulted in reduced activity of hepatic CYP1A2 and CYP2E1, the latter of which metabolizes numerous drugs and environmental toxins[2][7]. Importantly, these same Pgrmc1 knockout mice demonstrated substantially increased resistance to acetaminophen-induced hepatotoxicity, suggesting that reduced P450 activity through PGRMC1 loss protects hepatocytes from drug-induced injury, presumably by decreasing bioactivation of acetaminophen to its toxic metabolite N-acetyl-p-benzoquinone imine[2][7]. This observation provides direct evidence that PGRMC1's role in P450 maintenance carries physiological consequences for drug safety and cellular protection against xenobiotic stress.

The PGRMC1-mediated stabilization of P450 enzymes similarly impacts steroid hormone and cholesterol metabolism. PGRMC1 binds to and stabilizes CYP51A1 (sterol 14α-demethylase), a critical P450 in cholesterol synthesis that catalyzes demethylation of lanosterol, an essential step in conversion of lanosterol to cholesterol[1][13][57]. In yeast systems, the PGRMC1 homologue Dap1 functions similarly to support Cyp51-mediated ergosterol synthesis, and this function proves conserved in humans[1][13][37]. PGRMC1 also activates CYP21A2, which catalyzes 21-hydroxylation of progesterone to form 11-deoxycorticosterone, an important adrenal steroidogenic pathway[1]. Through these mechanisms, PGRMC1 directly influences the flux through both cholesterol synthesis and steroid hormone biosynthesis.

Heme Synthesis Regulation and Ferrochelatase Interaction

Beyond stabilizing existing P450 enzymes, PGRMC1 participates in direct regulation of heme synthesis through specific interaction with ferrochelatase (FECH), the terminal enzyme in the mitochondrial heme biosynthetic pathway[12][25][51][54]. Affinity purification coupled with mass spectrometry identified PGRMC1 and its paralog PGRMC2 as interaction partners of FECH in mitochondrial fractions[12][25]. PGRMC1 localizes to the outer face of the mitochondrial outer membrane as evidenced by proteinase protection assays, while FECH resides on the inner face of the inner mitochondrial membrane, positioning PGRMC1 to interact with FECH potentially through the N-terminal transmembrane domain spanning mitochondrial membranes[12][25]. In vitro assays demonstrated that PGRMC1 presence decreases FECH activity in dose-dependent fashion, with interactions strongest when FECH adopts conformations associated with product release, suggesting PGRMC1 may regulate FECH activity through modulation of heme product release[12][25][51].

Treatment of murine erythroleukemia cells with AG-205, a small-molecule inhibitor of PGRMC1, resulted in dose-dependent reduction of heme synthesis in differentiated cells, supporting a role for PGRMC1 in facilitating heme production[12][25]. Additionally, purified PGRMC1 demonstrated capacity to donate heme to apo-cytochrome b₅, suggesting PGRMC1 may function as a heme shuttle protein transferring heme from the mitochondrial synthesis site to cytoplasmic apoprotein targets[12][25]. Collectively, these observations suggest PGRMC1 functions as a heme homeostasis regulator, coordinating synthesis by FECH with distribution to cytoplasmic hemoproteins.

Subcellular Localization and Dynamic Trafficking

Primary Endoplasmic Reticulum Residency and Multiple Compartment Distribution

PGRMC1 is widely accepted to localize primarily to the endoplasmic reticulum (ER), with this localization documented through multiple independent approaches including cell fractionation, immunofluorescence microscopy, and co-expression with ER markers[6][31][38]. Co-localization experiments with the ER luminal marker KDEL demonstrate robust co-localization of PGRMC1-RFP with KDEL-GFP in live cells, confirming ER residency[9]. However, comprehensive subcellular localization studies reveal that PGRMC1 exists in a dynamic distribution extending beyond the ER to multiple cellular compartments including mitochondria, endosomes, plasma membrane, nucleus, and nucleolus depending on cell type and physiological context[6][9][31][32][36][38]. This multi-compartmental distribution suggests PGRMC1 undergoes dynamic trafficking in response to cellular signals or metabolic states.

The plasma membrane localization of PGRMC1 warrants particular attention given implications for cell surface signaling function. Epitope mapping studies of monoclonal antibodies recognizing cell surface PGRMC1 (csPGRMC1) on human pluripotent stem cells and certain cancer cells revealed unexpected membrane topology, with the C-terminal domain, predicted to reside in the cytoplasm, displaying accessibility from the extracellular space[6]. These findings suggesting multiple or dynamic topologies of PGRMC1 on the cell surface parallel observations in other multipass transmembrane or membrane-associated proteins such as ductin, cystic fibrosis transmembrane conductance regulator (CFTR), and P-glycoprotein[6].

Endosomal-ER Membrane Contact Sites and Calcium Signaling

Recent investigations have identified PGRMC1 as a critical component at endosomal-ER membrane contact sites (MCS), where it interacts with two-pore channel 1 (TPC1) to regulate calcium signaling[9]. PGRMC1 localizes to the ER while TPC1 resides on endosomal membranes, and their interaction at MCS enhances functional coupling between endosomal NAADP-activated calcium release and ER calcium stores[9]. PGRMC1 overexpression potentiated NAADP-dependent calcium signals through enhanced ER-endosomal coupling, while PGRMC1 knockdown inhibited these signals[9]. Point mutations in PGRMC1 within an α-helical domain (residues 147-163) that disrupts TPC1 binding specifically impaired calcium signaling potentiation, indicating the PGRMC1-TPC1 interaction mediates the calcium phenotype[9]. These observations define a specific role for PGRMC1 in organizing membrane architecture to facilitate inter-organellar communication, with potential implications for cellular calcium homeostasis and heme distribution given PGRMC1's presence at MCS.

Nucleolar Localization and Stress Response

PGRMC1 localizes to the nucleolus in cultured bovine granulosa cells and oocytes, co-localizing with nucleolin (NCL), a major nucleolar protein involved in ribosomal RNA synthesis and nucleolar organization[32]. Depletion of PGRMC1 through RNA interference resulted in translocation of nucleolin from the nucleolus to the nucleoplasm, indicating PGRMC1 regulates nucleolin subcellular localization[32]. Similarly, oxidative stress induced by hydrogen peroxide treatment reduced PGRMC1 nucleolar immunofluorescence intensity and increased nucleoplasmic nucleolin signal, suggesting oxidative stress disrupts PGRMC1-nucleolin association[32]. These observations indicate PGRMC1 plays a role in nucleolar organization and likely contributes to cellular stress responses through nucleolar-nucleoplasmic signaling pathways.

Cancer Biology and Oncogenic Signaling Pathways

PGRMC1 Overexpression in Malignant Tumors

PGRMC1 expression is significantly elevated across multiple cancer types including breast cancer (both estrogen receptor-positive and triple-negative), colon cancer, ovarian cancer, endometrial cancer, cervical cancer, thyroid cancer, and lung cancer[1][4][8][11][46]. Microarray analyses have detected PGRMC1 expression in colon, lung, breast, and ovarian tumors[46]. In ovarian cancer, PGRMC1 expression increased in advanced-stage tumors and showed homogeneous expression within tumor tissue[46]. Functional studies demonstrate that disrupting PGRMC1 through genetic approaches (siRNA-mediated knockdown) or pharmacological inhibition reduces cancer cell proliferation, induces apoptosis, causes G1 cell-cycle arrest, and inhibits migration and invasion[8][11][19]. These observations position PGRMC1 as a pro-tumorigenic factor whose overexpression correlates with aggressive tumor phenotypes and poor clinical prognosis.

PI3K/AKT/mTOR and EGFR Signaling Axis

The primary mechanism through which PGRMC1 promotes cancer cell proliferation involves activation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling cascade and epidermal growth factor receptor (EGFR) signaling[8][11][42]. Phosphoprotein array analysis comparing control and PGRMC1-disrupted breast cancer cells (both ER-positive ZR-75-1 and triple-negative MDA-MB-468 lines) revealed decreased phosphorylation across multiple components of these pathways[8][42]. Specifically, PGRMC1 silencing or pharmacological inhibition with AG-205 decreased phosphorylation of AKT at multiple sites (Ser124, Tyr450, Tyr326, Thr246, Tyr474), reduced mTOR phosphorylation at Thr2446 and Ser2448, decreased P70S6K phosphorylation at Ser371, Thr421, and Ser424, and reduced EGFR phosphorylation at multiple tyrosine residues (Tyr1068, Tyr1092, Tyr1110, Tyr1197) and threonine residues (Thr678, Thr1172, Thr693)[8][42].

Overexpression of PGRMC1 in non-malignant MCF10A breast epithelial cells resulted in increased cell proliferation accompanied by elevated phosphorylation of AKT at Ser473, mTOR at Ser2448, and EGFR at Tyr1068[8][42]. Western blotting confirmed that PGRMC1 overexpression increases phosphorylated PTEN (phosphatase and tensin homolog), a negative regulator of PI3K signaling, whereas PGRMC1 disruption increases PTEN expression and reduces AKT and mTOR phosphorylation[8][42]. This observation suggests a model whereby PGRMC1 suppresses PTEN, thereby relieving PTEN's inhibitory effect on PI3K and permitting downstream AKT activation[8][42].

EGFR Stabilization and Membrane Localization

Beyond activating EGFR signaling, PGRMC1 directly stabilizes EGFR at the plasma membrane through protein-protein interaction, increasing EGFR abundance accessible to growth factor stimulation[15]. PGRMC1 co-precipitates with EGFR from microsomal fractions where both proteins co-localize[15]. PGRMC1 knockdown by RNA interference decreased plasma membrane EGFR levels by approximately 7.4-fold compared to control cells, while immunofluorescence confirmed increased plasma membrane EGFR staining in control cells compared to PGRMC1-depleted cells[15]. The EGFR-PGRMC1 association proved constitutive and was not altered by EGF stimulation, indicating stable complex formation independent of ligand binding[15]. PGRMC1 localizes within the microsomal lumen where it likely interacts with the EGFR extracellular domain, stabilizing EGFR at the plasma membrane through undefined mechanisms that may involve preventing endosomal trafficking and degradation[15].

Chemoresistance and Drug Metabolism

PGRMC1 promotes chemoresistance in cancer cells through dual mechanisms involving both enhanced drug metabolism and anti-apoptotic signaling[1][19][22]. Cancer cells overexpressing PGRMC1 display enhanced resistance to doxorubicin and other chemotherapy agents, while PGRMC1 knockdown sensitizes cells to chemotherapy-induced apoptosis[1][19][22]. Mechanistically, PGRMC1-dependent stabilization of cytochrome P450 enzymes including CYP2D6 and CYP3A4, both capable of doxorubicin metabolism, enhances drug bioactivation and clearance, reducing intracellular drug concentrations and conferring chemoresistance[22][46]. Additionally, PGRMC1 promotes apoptosis resistance through activation of survival signaling cascades including PI3K/AKT/mTOR and ERK1/2 pathways[1][19]. Xenograft transplantation assays in immunodeficient NOG mice demonstrated that PGRMC1 knockdown significantly decreased hepatic metastasis of colon cancer HCT116 cells compared to control cells, indicating PGRMC1 supports metastatic progression[22].

Sigma-2 Receptor Identity

PGRMC1 has been identified as the molecular component of sigma-2 receptors, ligand-binding sites previously characterized through radioligand-binding studies but whose molecular identity remained uncertain[18][22]. The PGRMC1 protein complex binds sigma-2 selective ligands including the fluorescent probe SW120 and the agonist WC-26 with ligand-binding properties consistent with previously characterized sigma-2 receptors[18][22]. Sigma-2 receptor ligands induce caspase-3-dependent apoptosis in cancer cells, and this pro-death signaling requires PGRMC1 expression as PGRMC1 siRNA treatment ablates sigma-2 ligand-induced apoptosis[18]. These findings establish PGRMC1 as the sigma-2 receptor and provide mechanistic understanding of sigma-2 ligand pharmacology in cancer cells.

Metabolic Functions in Adipogenesis and Obesity

Role in Adipocyte Differentiation and Lipid Metabolism

PGRMC1 represents a critical regulator of adipogenesis and lipid accumulation in adipocytes through heme-mediated mechanisms distinct from its role in cancer cells[5][10][44]. PGRMC1 expression is significantly upregulated during 3T3L1 adipocyte differentiation following treatment with insulin or thiazolidine (PPARγ agonist), and this upregulation correlates with enhanced lipid accumulation[5][10]. Adipose tissue-specific PGRMC1 knockout in mice dramatically suppressed high-fat diet-induced adipocyte hypertrophy and obesity development, demonstrating essential roles for PGRMC1 in nutrient uptake and lipid storage[5][10][44]. Gene expression analysis revealed that PGRMC1 is a novel target gene regulated by PPARγ and insulin during adipogenesis[5][10].

The mechanism underlying PGRMC1-promoted lipid accumulation involves heme-dependent PGRMC1 dimerization and interaction with lipoprotein receptors[5][10]. Heme-dimerized PGRMC1 (but not Y113F mutant unable to form heme-mediated dimers) interacts with low-density lipoprotein receptor (LDL-R) and very-low-density lipoprotein receptor (VLDL-R)[5][10]. This interaction regulates translocation of these receptors to the plasma membrane, facilitating enhanced uptake of circulating lipids for intracellular storage[5][10]. Additionally, heme-dimerized PGRMC1 interacts with glucose transporter 4 (GLUT4) and promotes its translocation to the plasma membrane, facilitating insulin-stimulated glucose uptake in adipocytes[5][10]. Importantly, carbon monoxide (CO) treatment interferes with PGRMC1 dimerization by occupying the sixth heme coordination site, thereby attenuating lipid and glucose uptake, suggesting PGRMC1 functions as a CO-sensitive metabolic switch[5][10][22].

Localization and Compartmentalization of PGRMC1 Function

PGRMC1 primarily localizes to the perinuclear region in 3T3L1 adipocytes, with distribution observed in both ER and mitochondrial compartments[5][10]. Importantly, the ER-localized PGRMC1 appears responsible for regulating lipoprotein receptor and GLUT4 translocation independent of PGRMC1's mitochondrial heme biosynthetic function[5][10]. This compartmentalization suggests distinct pools of PGRMC1 serving different cellular functions—ER-resident PGRMC1 coordinating metabolic substrate uptake through receptor trafficking, while mitochondrial-associated PGRMC1 participates in heme homeostasis[5][10]. The finding that neither insulin, haemin, nor carbon monoxide-releasing molecules alter PGRMC1 subcellular distribution patterns suggests dynamic trafficking mechanisms operate under specific physiological conditions not apparent in cultured adipocytes[5][10].

Cardiac Metabolic Remodeling and Energy Homeostasis

PGRMC1 functions in cardiac metabolic regulation by influencing the balance between glucose and fatty acid oxidation based on nutritional availability[20]. Pgrmc1 knockout mice subjected to dietary starvation displayed increased fatty acid and pyruvate oxidation compared to wild-type starved hearts, manifested as elevated expression of mitochondrial fatty acid oxidation enzymes (CPT2, VLCAD) and peroxisomal enzyme (ACOX1)[20]. Conversely, glycolytic enzyme expression decreased in starved Pgrmc1 knockout hearts, including reduced hexokinase 1 and 2, and pyruvate kinase M2 levels[20]. Starved Pgrmc1 knockout hearts exhibited increased AMPK phosphorylation, activation of the cellular energy-sensing kinase, and increased mitochondrial respiration rate in Seahorse metabolic assays[20]. These findings indicate PGRMC1 deletion promotes metabolic flexibility by shifting dominance toward fatty acid oxidation during energy-deficit conditions, protecting hearts from starvation-induced injury through maintenance of ATP production[20].

Reproductive Functions and Progesterone Signaling

Progesterone Binding and Anti-Apoptotic Function

Despite its designation as a progesterone receptor, PGRMC1's direct progesterone binding and its role in progesterone signaling remain incompletely characterized[31][33][36][38]. Studies employing purified GFP-PGRMC1 fusion protein demonstrated specific progesterone binding at a single site with apparent dissociation constant (Kd) of approximately 35 nM[33]. Progesterone binding required the entire PGRMC1 molecule, with deletion mutations spanning both transmembrane and C-terminal domains reducing progesterone binding capacity[33]. Notably, PGRMC1 knockdown in immortalized granulosa cells resulted in 60% decline in ³H-progesterone binding and abolished progesterone's anti-apoptotic action, indicating PGRMC1 mediates progesterone's survival signaling in reproductive cells[33]. Analysis of deletion mutations suggested the progesterone binding site resides within a segment comprising the transmembrane domain and initial C-terminus[33].

However, evidence for direct progesterone binding to purified PGRMC1 lacking the transmembrane domain proved negative, suggesting the transmembrane domain may be essential for progesterone binding, or alternative explanations exist for PGRMC1's involvement in progesterone responses[31][38]. Spectroscopic studies examining interaction of synthetic progesterone with purified PGRMC1 detected changes in heme spectra consistent with progesterone binding or heme modification[38]. This equivocal evidence suggests PGRMC1 may function as a component of a progesterone-binding protein complex rather than a direct progesterone receptor, or that multiple mechanisms of progesterone interaction exist[31][38].

Ovarian Function and Granulosa Cell Regulation

PGRMC1 is highly expressed in ovarian granulosa and luteal cells, and its expression increases during luteinization and pregnancy[1][33][36]. Progesterone inhibits apoptosis in immortalized granulosa cells through mechanisms requiring both PGRMC1 and its binding partner PAIR-BP1 (plasminogen activator inhibitor RNA-binding protein-1)[1][33]. PGRMC1 regulates cell proliferation and apoptosis in granulosa and luteal cells through interaction between its cytochrome b₅-binding domain (amino acids 70-130) and PAIR-BP1[1][33][36]. PGRMC1 influences cell cycle progression through effects on spindle microtubule stability in ovarian cells[52]. Immunofluorescence revealed PGRMC1 localization to the mitotic spindle and centrosomes at metaphase, with interaction with beta-tubulin confirmed by proximity ligation assay[52]. Progesterone treatment or PGRMC1 function inhibition increased mitotic index and slowed cell cycle progression through effects on spindle microtubule assembly and disassembly rates[52].

Clinical studies have identified altered PGRMC1 expression in reproductive pathologies. PGRMC1 levels are reduced in peripheral blood cells from women with polycystic ovarian syndrome (PCOS) and in some women with premature ovarian failure[36]. Conversely, PGRMC1 overexpression associates with impaired follicular development in women undergoing hormone-stimulated ovulation for infertility treatment[36]. These associations suggest dysregulated PGRMC1 expression contributes to reproductive dysfunction.

Neurological Functions and Alzheimer's Disease

PGRMC1 as an Amyloid Beta Oligomer Receptor

PGRMC1 has been identified as a critical receptor mediating amyloid beta (Aβ) 1-42 oligomer binding to neurons and their downstream synaptotoxic effects in Alzheimer's disease[27][30]. Knockdown of PGRMC1 protein expression in neuronal cultures using siRNA resulted in greater than 90% reduction in Aβ oligomer binding to neurons, demonstrating PGRMC1's dominant role in oligomer-neuronal interaction[27][30]. A near-perfect linear correlation existed between PGRMC1 expression reduction and Aβ oligomer binding loss (r² = 0.799 for cell body measurements, r² = 0.554 for synaptic puncta), indicating approximately 28% reduction in PGRMC1 expression produced 91% reduction in oligomer binding[27][30]. Aβ oligomers induce inhibition of long-term potentiation, affect membrane trafficking, cause reversible spine loss, and contribute to cognitive deficits characteristic of early Alzheimer's disease through mechanisms requiring PGRMC1[27][30].

PGRMC1 expression is dysregulated in Alzheimer's disease brains compared to age-matched controls, with evidence of upregulation in Alzheimer's patients' neural tissue[27][30]. Brain penetrant small-molecule compounds selectively displacing endogenous Aβ oligomers from PGRMC1 prove effective at improving cognitive deficits in animal Alzheimer's disease models both acutely and chronically[27][30]. These observations position PGRMC1 as a tractable therapeutic target for modifying Alzheimer's disease pathology through interference with oligomer-receptor interactions.

Genetic Disorders and Clinical Manifestations

X-Linked Pediatric Cataract Formation

Loss-of-function mutations in the PGRMC1 gene cause X-linked pediatric cataract formation, establishing PGRMC1 as a novel cataract-associated gene[21][24]. A 127 kb truncating deletion in chromosome X (chrX:g.118373226_118500408del) completely removing the commonly deleted long non-coding RNA LOC101928336 and truncating PGRMC1 following exon 1 segregated with cataract phenotype in an X-linked inheritance pattern[21][24]. The deletion was observed in all affected males and asymptomatic carrier females in the reported family, with female carriers showing no clinically significant lens opacification[21][24]. Morpholino-induced pgrmc1 knockdown in zebrafish models produced significant cataract formation, supporting a specific role for PGRMC1 in lens development[21][24].

The mechanism linking PGRMC1 loss to cataract formation likely involves disruption of PGRMC1-CYP51A1 protein interactions and consequent alteration of cholesterol biosynthesis within the lens[21][24]. CYP51A1 mutations independently cause congenital cataracts with autosomal recessive inheritance, and PGRMC1 knockdown studies demonstrate impaired CYP51A1 activity and lanosterol accumulation[21][24]. Cholesterol represents an essential membrane component in lens epithelial cells, and disrupted cholesterol synthesis through CYP51A1 dysfunction likely impairs lens development and transparency[21][24]. This genetic evidence confirms PGRMC1's physiological role in cholesterol metabolism through CYP51A1 support.

Membrane Trafficking and ER-Phagy Functions

PGRMC1 as Size-Selective Cargo Receptor in ER-Phagy

PGRMC1 serves as a size-selective cargo receptor during reticulon-3 (RTN3)-dependent ER-phagy, the selective autophagy-mediated clearance of misfolded prohormones from the ER[45][48]. Unbiased proteomics identified PGRMC1 as an RTN3-interacting partner, and subsequent studies demonstrated that via its luminal domain, PGRMC1 associates with misfolded prohormones and delivers them for RTN3-dependent ER-phagic clearance[45][48]. PGRMC1 selects cargo substantially smaller than other reported ER-phagy substrates, representing a distinct size-selectivity class[45][48]. Mutant proinsulin, a model ER-phagy substrate causative of mutant INS-gene-induced diabetes of youth (MIDY), depends on PGRMC1-RTN3 interaction for efficient degradation[45][48].

Mapping studies revealed that the extreme C-terminus of PGRMC1 is necessary and sufficient for binding misfolded cargo, while the transmembrane domain mediates interaction with RTN3[45][48]. This dual functional domain organization positions PGRMC1 as a selective cargo adapter bridging misfolded luminal proteins with the RTN3 targeting complex. PGRMC1 recruitment to RTN3 may provide a mechanism for rapid selective clearance of pathogenic misfolded prohormones, representing a protective response to prevent protein aggregation and ER stress[45][48]. These functions expand PGRMC1's recognized roles beyond cancer and reproductive contexts to encompass quality control mechanisms protecting organismal metabolic homeostasis.

Vesicular Trafficking and Endosomal Sorting

PGRMC1 contains YXXϕ motifs (where X denotes any amino acid and ϕ denotes leucine or isoleucine), structural sequences implicated in vesicle transport and endocytosis[5][10][28]. Through these motifs, PGRMC1 localized in ER and endosomes likely regulates intracellular protein translocation and receptor trafficking to the plasma membrane[5][10]. PGRMC1 appears to contribute to plasma membrane translocation of interacting proteins such as LDL-R, VLDL-R, and GLUT4 through regulation of vesicle transport, potentially coordinating endosomal sorting and recycling[5][10].

Post-Translational Modifications and Functional Regulation

PGRMC1 Phosphorylation and Cell Cycle Regulation

PGRMC1 undergoes dynamic phosphorylation at multiple residues that regulate its biological functions, particularly in cell cycle control[39][41]. Mass spectrometry and phosphoproteomics identified phosphorylation sites including Ser181, Ser57, Tyr113, and Tyr180 as particularly prevalent[39][41]. Treatment of breast cancer cells with norethisterone (NET) significantly increased PGRMC1 phosphorylation specifically at Ser181 (p < 0.01 in MCF7 cells, p < 0.05 in T47D cells) without altering total PGRMC1 expression[39]. Mutation of the Ser181 phosphorylation site (S181A) or double mutation of Ser57 and Ser181 (S57A/S181A) resulted in significantly reduced cell proliferation following NET treatment (S181A: p < 0.01; S57A/S181A: p < 0.001), indicating phosphorylation at Ser181 is crucial for progestin-stimulated proliferation[39].

Ser181 phosphorylation likely facilitates cell cycle progression through phosphorylation-dependent recruitment of signaling proteins or conformational changes affecting PGRMC1's protein-protein interaction capacity[39]. The accessibility of Ser181 and Tyr180 for protein interactions, as they reside in unstructured regions unstable under nuclear magnetic resonance conditions, suggests increased phosphorylation enhances PGRMC1's recruitment of downstream effectors[39]. The correlation between Ser181 phosphorylation status and cell cycle checkpoint progression implicates PGRMC1 phosphorylation in regulating retinoblastoma (Rb) protein phosphorylation by cyclin-dependent kinases, thereby facilitating cell cycle progression[39].

Therapeutic Considerations and Inhibitor Development

AG-205 and PGRMC1 Ligand Specificity

AG-205 has been widely employed as a pharmacological inhibitor of PGRMC1 in research contexts; however, recent studies reveal AG-205 possesses multiple molecular targets beyond PGRMC1[16][56]. In endometrial cells, AG-205 increased expression of genes encoding cholesterol biosynthetic enzymes and steroidogenic proteins; however, these effects were independent of PGRMC1 or other MAPR family members[16]. AG-205 treatment induced identical gene expression changes in cells with complete genetic deletion of PGRMC1, PGRMC2, and other MAPR family members as in wild-type cells, indicating PGRMC1 does not mediate AG-205's effects on sterol metabolism in endometrium[16].

In vitro enzyme activity assays revealed AG-205 functions as an inhibitor of UDP-galactose:ceramide galactosyltransferase (CGT), an enzyme unrelated to PGRMC1[56]. AG-205 reduced galactosylceramide synthesis in cells with or without functional PGRMC1 and PGRMC2, and in vitro CGT activity assays confirmed direct inhibition by AG-205[56]. These findings underscore the necessity for careful interpretation of AG-205 experimental results and suggest PGRMC1 inhibition requires alternative pharmacological approaches or genetic targeting strategies[16][56]. The off-target effects of AG-205 on CGT warrant consideration in interpreting prior literature employing this compound.

Conclusion: Synthesis of PGRMC1 Functions and Future Directions

Progesterone receptor membrane component 1 has emerged from initial characterization as a putative hormone receptor to recognition as a multifunctional membrane-associated heme-binding protein orchestrating cellular responses across metabolic, reproductive, oncogenic, and neurological contexts. The primary biochemical function of PGRMC1 involves stabilization and maintenance of cytochrome P450 protein levels through post-translational mechanisms fundamentally independent of PGRMC1's own heme-binding capacity, thereby preserving cellular capacity for drug metabolism, hormone synthesis, and cholesterol regulation. PGRMC1 additionally functions in heme homeostasis by interacting with ferrochelatase and participating in heme transfer to cytoplasmic apoprotein targets, serving as a potential heme chaperone coordinating synthesis and distribution. The localization of PGRMC1 extends across multiple subcellular compartments including ER, endosomes, mitochondria, plasma membrane, nucleus, and nucleolus, reflecting distinct functional roles at different cellular sites.

In cancer biology, PGRMC1 overexpression promotes malignant phenotypes through activation of PI3K/AKT/mTOR and EGFR signaling pathways, stabilization of EGFR at the plasma membrane, and enhancement of drug-metabolizing P450 enzymes that confer chemoresistance. PGRMC1 has been identified as the molecular substrate of sigma-2 receptors, enabling mechanistic understanding of sigma-2 ligand pharmacology in cancer cells. In metabolic contexts, heme-dependent PGRMC1 dimerization coordinates adipogenesis through interactions with lipoprotein receptors and glucose transporters, while cardiac-specific PGRMC1 regulates metabolic flexibility between glucose and fatty acid oxidation. PGRMC1 serves reproductive functions through progesterone-associated anti-apoptotic signaling in ovarian cells and control of spindle microtubule dynamics during cell division. Neurological roles include interaction with amyloid beta oligomers in Alzheimer's disease pathogenesis, with PGRMC1 serving as a tractable target for therapeutic intervention.

Genetic loss-of-function mutations in PGRMC1 cause X-linked pediatric cataract formation, establishing PGRMC1's physiological necessity in lens development, likely through effects on cholesterol biosynthesis via CYP51A1 stabilization. PGRMC1 participates in quality control mechanisms by functioning as a size-selective cargo receptor in RTN3-dependent ER-phagy of misfolded prohormones, protecting metabolic homeostasis. Post-translational modifications including phosphorylation regulate PGRMC1 function in cell cycle control and drug-induced proliferation responses.

Future research directions warranting investigation include: (1) structural determination of the complete full-length PGRMC1 in diverse membrane topologies to clarify heme-binding mechanisms and protein interaction interfaces; (2) identification of the physiological progesterone-binding partner of PGRMC1 and characterization of PGRMC1-dependent progesterone signaling independent of classical progesterone receptor; (3) comprehensive mapping of PGRMC1 protein-protein interaction networks across diverse cell types and physiological contexts; (4) characterization of PGRMC1 trafficking mechanisms and subcellular localization determinants; (5) development of PGRMC1-selective pharmacological inhibitors with improved target specificity compared to AG-205; (6) investigation of PGRMC1 in neurodegenerative diseases beyond Alzheimer's disease; and (7) clinical translation of PGRMC1-targeted therapeutics in cancer, metabolic disease, and Alzheimer's disease contexts. The remarkable functional diversity of PGRMC1 suggests mechanisms coordinating these diverse functions through as-yet-unidentified regulatory mechanisms responsive to metabolic or pathological signals. Continuing investigation of PGRMC1 biology promises insights into fundamental cellular processes and opportunities for therapeutic intervention in major disease categories.

Citations

https://en.wikipedia.org/wiki/PGRMC1
https://pubmed.ncbi.nlm.nih.gov/34678314/
https://www.rcsb.org/structure/4x8y
https://www.ncbi.nlm.nih.gov/gene/10857
https://www.nature.com/articles/s42003-020-01202-x
https://www.nature.com/articles/s41598-018-37441-6
https://pmc.ncbi.nlm.nih.gov/articles/PMC8591507/
https://www.nature.com/articles/s41416-020-0992-6
https://pmc.ncbi.nlm.nih.gov/articles/PMC10685313/
https://www.frontiersin.org/journals/bioengineering-and-biotechnology/articles/10.3389/fbioe.2021.714030/full
https://pmc.ncbi.nlm.nih.gov/articles/PMC5278647/
https://pmc.ncbi.nlm.nih.gov/articles/PMC2659782/
https://pubmed.ncbi.nlm.nih.gov/38489495/
https://pmc.ncbi.nlm.nih.gov/articles/PMC2915713/
https://pmc.ncbi.nlm.nih.gov/articles/PMC8533447/
https://pubs.acs.org/doi/abs/10.1021/acs.biochem.3c00718
https://www.nature.com/articles/ncomms1386
https://pmc.ncbi.nlm.nih.gov/articles/PMC11397629/
https://pmc.ncbi.nlm.nih.gov/articles/PMC10000468/
https://www.nature.com/articles/s41431-021-00889-8
https://www.nature.com/articles/ncomms11030
https://pubmed.ncbi.nlm.nih.gov/37087062/
https://pubmed.ncbi.nlm.nih.gov/33867527/
https://par.nsf.gov/servlets/purl/10510542
https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0111899
https://pmc.ncbi.nlm.nih.gov/articles/PMC8154067/
https://www.frontiersin.org/journals/cell-and-developmental-biology/articles/10.3389/fcell.2022.894591/full
https://pubmed.ncbi.nlm.nih.gov/25390692/
https://pmc.ncbi.nlm.nih.gov/articles/PMC9839567/
https://pubmed.ncbi.nlm.nih.gov/29339453/
https://academic.oup.com/endo/article/149/2/534/2454817
https://www.uniprot.org/uniprotkb/O55022/entry
https://health.uconn.edu/cell-biology/wp-content/uploads/sites/115/2018/01/Terzaghi-L-et-al-2018.pdf
https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2013.00168/full
https://pmc.ncbi.nlm.nih.gov/articles/PMC5641147/
https://pubs.acs.org/doi/abs/10.1021/bi0511585
https://pmc.ncbi.nlm.nih.gov/articles/PMC9736406/
https://www.nature.com/articles/s41467-021-26225-8
https://pubmed.ncbi.nlm.nih.gov/37866635/
https://pmc.ncbi.nlm.nih.gov/articles/PMC8865224/
https://pmc.ncbi.nlm.nih.gov/articles/PMC6396690/
https://health.uconn.edu/cell-biology/wp-content/uploads/sites/115/2017/10/A-Novel-Role-for-Progesterone-and-Progesterone-Receptor-2011.pdf
https://pmc.ncbi.nlm.nih.gov/articles/PMC4419704/
https://pubs.acs.org/doi/abs/10.1021/acs.biochem.6b00756
https://pmc.ncbi.nlm.nih.gov/articles/PMC4896862/
https://pmc.ncbi.nlm.nih.gov/articles/PMC8700550/
https://www.proteinatlas.org/ENSG00000101856-PGRMC1
https://health.uconn.edu/cell-biology/wp-content/uploads/sites/115/2017/10/AnspachWill2017.pdf
https://www.ncbi.nlm.nih.gov/gene/20787

📄 View Raw YAML

id: O00264
gene_symbol: PGRMC1
product_type: PROTEIN
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  PGRMC1 (Progesterone Receptor Membrane Component 1; also known as Sigma-2 receptor
  complex member, mPR, Hpr6.6, Dap1, IZA) is a ~195-aa single-pass tail-anchored
  membrane protein of the MAPR (Membrane-Associated Progesterone Receptor) subfamily
  of the cytochrome b5 superfamily. Its primary, evolutionarily conserved molecular
  function is binding of heme B via an atypical pentacoordinate Tyr113 axial ligand
  in a surface-exposed pocket of its cytochrome b5-like cytosolic domain; heme binding
  drives reversible dimerization through heme-heme stacking (with additional
  Cys129-mediated disulfide oligomerization contributing in vitro). PGRMC1 is
  predominantly an endoplasmic reticulum (ER) membrane protein but is also present
  at the plasma membrane, nuclear/perinuclear membranes, endosomes, Golgi and the
  outer mitochondrial membrane in a cell-type- and context-dependent manner. Its
  best-supported biological role is as a heme-dependent regulator of microsomal
  cytochrome P450 (CYP) enzyme systems: it physically interacts with multiple CYPs
  (including CYP51A1, CYP21A2, CYP7A1, CYP3A4, CYP2C8, CYP2C2, CYP1A2) and with
  cytochrome P450 reductase (CPR), modulating CYP activity in an isoform-dependent
  manner — stimulating sterol-biosynthetic CYPs (notably CYP51A1) while inhibiting
  several drug-metabolising CYPs. Through ferrochelatase (FECH) binding and
  apo-cytochrome b5 heme transfer in vitro, PGRMC1 has been implicated in heme
  trafficking/buffering. PGRMC1 also participates in cell-surface complexes that
  pleiotropically influence ligand uptake and signalling, including the
  TMEM97 (sigma-2 receptor)/PGRMC1/LDLR ternary complex that accelerates LDL
  internalization, and a synaptic complex implicated in amyloid-beta oligomer binding
  in Alzheimer's disease. The "progesterone receptor" name is historical;
  contemporary expert reviews frame PGRMC1 as a membrane heme-binding regulator
  rather than as a classical ligand-activated progesterone receptor.
existing_annotations:
  - term:
      id: GO:0005783
      label: endoplasmic reticulum
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: >-
        ER localisation is the single most consistently reported subcellular site for
        PGRMC1 across cell types and is supported by the falcon review, the perplexity
        review, and primary biochemical/microscopy studies. The IBA call is consistent
        with experimentally documented ER residence.
      action: ACCEPT
      reason: >-
        PGRMC1 is a tail-anchored ER membrane MAPR-family protein; ER residence is
        a core, conserved localisation that underlies its interactions with ER-resident
        CYPs and the heme synthesis/trafficking machinery.
      supported_by:
        - reference_id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
          supporting_text: "PGRMC1 localization is repeatedly reported as **context-dependent**, with strong recurring evidence for association with the **endoplasmic reticulum (ER)/endomembranes**"
        - reference_id: PMID:21081644
          supporting_text: "In most cells, PGRMC1 is localized in the membranes of the ER"
  - term:
      id: GO:0012505
      label: endomembrane system
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: >-
        Endomembrane system is a broad parent term encompassing ER, Golgi, endosomes
        and plasma membrane. PGRMC1 is documented at multiple endomembrane sites, but
        more specific terms (endoplasmic reticulum, plasma membrane, smooth ER
        membrane) capture its biology more informatively.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Redundant with the more informative GO:0005783 endoplasmic reticulum
        annotation already present; "endomembrane system" is too generic to convey
        meaningful localisation information.
      supported_by:
        - reference_id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
          supporting_text: "additional reported localization to the **plasma membrane**, **nucleus/nuclear membrane**, **Golgi**, **endosomes**, **cytoplasm**, and **mitochondria** depending on cell type and condition."
  - term:
      id: GO:0016020
      label: membrane
    evidence_type: IBA
    original_reference_id: GO_REF:0000033
    review:
      summary: >-
        "membrane" is the most generic possible cellular component term. PGRMC1 is
        unambiguously a single-pass membrane protein, but ER membrane / smooth ER
        membrane / plasma membrane are the informative annotations to retain.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Uninformative parent term; superseded by the specific ER and plasma membrane
        annotations already present.
      supported_by:
        - reference_id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
          supporting_text: "Human **PGRMC1 (O00264)** is a **single-pass membrane MAPR protein**"
  - term:
      id: GO:0005496
      label: steroid binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000120
    review:
      summary: >-
        Direct steroid (progesterone) binding by PGRMC1 is historically asserted (the
        gene was named for it) but the evidence is equivocal: some purified-protein
        studies report progesterone binding requiring the transmembrane region while
        recombinant truncated cytosolic domain assays have failed to detect direct
        binding. Contemporary expert syntheses (falcon, 2024) frame PGRMC1 as a heme-
        binding MAPR protein rather than a classical progesterone receptor. Keeping as
        non-core captures historical/possible function without elevating it to the
        core molecular activity.
      action: KEEP_AS_NON_CORE
      reason: >-
        Evidence for direct, high-affinity steroid binding by PGRMC1 alone is weak
        and disputed; the heme-binding role is the unambiguously supported core
        molecular function. Retain for completeness but not as a core activity.
      supported_by:
        - reference_id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
          supporting_text: "Current expert reviews frame PGRMC1 less as a simple on/off “progesterone receptor” and more as a **membrane-associated heme-binding regulator**"
  - term:
      id: GO:0005576
      label: extracellular region
    evidence_type: IEA
    original_reference_id: GO_REF:0000044
    review:
      summary: >-
        PGRMC1 is a membrane-anchored protein with no characterised secreted form.
        The annotation likely originates from automated SwissProt subcellular-location
        mapping that registered "secretory granule membrane" type annotations as
        extracellular region. There is no evidence in the falcon, perplexity, or
        primary literature for genuine extracellular soluble PGRMC1.
      action: REMOVE
      reason: >-
        No biological evidence supports a free extracellular pool of PGRMC1; the
        protein is membrane-anchored throughout its life cycle. Annotation appears
        to be a SwissProt-keyword automation artefact.
      supported_by:
        - reference_id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
          supporting_text: "Human **PGRMC1 (O00264)** is a **single-pass membrane MAPR protein**"
  - term:
      id: GO:0005741
      label: mitochondrial outer membrane
    evidence_type: IEA
    original_reference_id: GO_REF:0000120
    review:
      summary: >-
        A pool of PGRMC1 is found on the cytosolic (outer) face of the outer
        mitochondrial membrane, where it interacts with the ferrochelatase-containing
        heme metabolon. This is experimentally supported by proteinase-protection
        fractionation in MEL cells (Piel et al. 2016).
      action: KEEP_AS_NON_CORE
      reason: >-
        PGRMC1 outer mitochondrial membrane localisation is genuine but represents a
        minor subcellular pool relevant to its heme-trafficking/FECH-interaction
        role, not its core ER residence.
      supported_by:
        - reference_id: PMID:27599036
          supporting_text: "Further experiments to characterize the sub-mitochondrial localization were consistent with the majority of PGRMC1 being on the outside of the outer mitochondrial membrane."
  - term:
      id: GO:0008289
      label: lipid binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000043
    review:
      summary: >-
        Generic "lipid binding" is too broad and not the established core activity.
        PGRMC1 binds heme (a tetrapyrrole, not a lipid in the typical GO sense) and
        possibly steroids; sterol binding/transfer activity has been proposed but
        not biochemically demonstrated for purified PGRMC1.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Uninformative; specific molecular function GO:0020037 (heme binding) and
        GO:0005496 (steroid binding) are already annotated. No published evidence
        for general phospholipid binding.
      supported_by:
        - reference_id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
          supporting_text: "Most defensible primary functional description (integrating 2023–2024 evidence):** Human **PGRMC1 (O00264)** is a **single-pass membrane MAPR protein** with a **cytochrome b5-like heme-binding domain**"
  - term:
      id: GO:0030868
      label: smooth endoplasmic reticulum membrane
    evidence_type: IEA
    original_reference_id: GO_REF:0000044
    review:
      summary: >-
        PGRMC1 is enriched in microsomal (smooth ER) fractions, consistent with its
        co-localisation and interaction with smooth-ER-resident cytochrome P450
        enzymes. This is a more informative refinement of the parent GO:0005783
        annotation.
      action: ACCEPT
      reason: >-
        Smooth ER membrane is the precise compartment for PGRMC1's interactions with
        microsomal CYP enzymes; well supported by microsomal fractionation studies.
      supported_by:
        - reference_id: PMID:21081644
          supporting_text: "PGRMC1 is a small 25-kDa protein with an N-terminal membrane binding segment and a C-terminal domain with a cytochrome b 5 -like structure that binds heme."
  - term:
      id: GO:0046872
      label: metal ion binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000043
    review:
      summary: >-
        PGRMC1 binds iron via the heme cofactor (pentacoordinate Tyr113 coordination
        of haem iron). Annotation as "metal ion binding" is technically correct via
        the bound heme but is uninformative; "heme binding" (GO:0020037) is the
        appropriate specific term and is already annotated.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Redundant with and less informative than the heme binding annotations; PGRMC1
        does not coordinate free metal ions directly.
      supported_by:
        - reference_id: PMID:26988023
          supporting_text: "the haem iron is five-coordinated by Tyr113 (Y113) alone"
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:17353931
    review:
      summary: >-
        Generic IPI "protein binding" annotation from a high-throughput mass-spec
        interactome screen. Not informative about PGRMC1's specific molecular
        function. Specific functional protein interactions (with CYPs, CPR, FECH,
        EGFR, TMEM97, LDLR) are captured by more meaningful annotations.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Per GO curation guidelines, generic "protein binding" should be avoided when
        more informative MF terms exist or when the partner is not specified.
      supported_by:
        - reference_id: PMID:17353931
          supporting_text: Large-scale mapping of human protein-protein interactions by mass spectrometry.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:21081644
    review:
      summary: >-
        Captures PGRMC1's documented direct interactions with multiple drug-
        metabolising P450s (CYP2C2, CYP2C8, CYP3A4) and cytochrome P450 reductase
        (CPR), but "protein binding" itself is uninformative. The functional
        consequence (modulation of P450 activity) is the core finding.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Use specific molecular-function terms (heme binding, and a process term for
        cytochrome P450 modulation) rather than generic protein binding.
      supported_by:
        - reference_id: PMID:21081644
          supporting_text: "PGRMC1 bound efficiently to all three P450s"
        - reference_id: PMID:21081644
          supporting_text: "In cells cotransfected with CPR and PGRMC1, strong binding of CPR to PGRMC1 was observed"
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:27599036
    review:
      summary: >-
        Reflects the experimentally validated direct interaction of PGRMC1 with
        ferrochelatase (FECH). The interaction is informative and functional but
        the generic GO:0005515 term itself is not.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Specific interaction with FECH is functionally informative but should not
        be captured as the generic protein-binding term; the process-level role
        (heme biosynthesis/regulation of FECH) is the appropriate annotation.
      supported_by:
        - reference_id: PMID:27599036
          supporting_text: "The interaction between PGRMC1 and FECH was confirmed in vitro and in HEK 293T cells"
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:28514442
    review:
      summary: >-
        High-throughput interactome screen; generic protein binding annotation.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Per GO guidelines, generic protein binding is uninformative and the
        interactome studies do not specify functional consequences.
      supported_by:
        - reference_id: PMID:28514442
          supporting_text: Architecture of the human interactome defines protein communities and disease networks.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:30021884
    review:
      summary: >-
        Histone crosslinking mass-spec study; PGRMC1's apparent association with
        histones in intact nuclei is a peripheral context not part of its core
        biology and the generic protein-binding term is uninformative.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Generic IPI from a high-throughput crosslinking study; nuclear/chromatin
        proximity is consistent with reported nuclear/nucleolar pools but not core
        function.
      supported_by:
        - reference_id: PMID:30021884
          supporting_text: Histone Interaction Landscapes Visualized by Crosslinking Mass Spectrometry in Intact Cell Nuclei.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:32296183
    review:
      summary: >-
        Generic IPI from a Y2H-based reference binary interactome map. Provides no
        specific functional information.
      action: MARK_AS_OVER_ANNOTATED
      reason: Uninformative high-throughput protein binding call.
      supported_by:
        - reference_id: PMID:32296183
          supporting_text: A reference map of the human binary protein interactome.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:33961781
    review:
      summary: >-
        Generic IPI from BioPlex/AP-MS proteome-scale network.
      action: MARK_AS_OVER_ANNOTATED
      reason: Uninformative high-throughput protein binding call.
      supported_by:
        - reference_id: PMID:33961781
          supporting_text: Dual proteome-scale networks reveal cell-specific remodeling of the human interactome.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:35271311
    review:
      summary: >-
        Generic IPI from OpenCell endogenous-tagging cartography of human protein
        complexes; not functionally informative on its own.
      action: MARK_AS_OVER_ANNOTATED
      reason: Uninformative high-throughput protein binding call.
      supported_by:
        - reference_id: PMID:35271311
          supporting_text: "OpenCell: Endogenous tagging for the cartography of human cellular organization."
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:40205054
    review:
      summary: >-
        Generic IPI from a multimodal cell map; not informative about specific
        function or partner.
      action: MARK_AS_OVER_ANNOTATED
      reason: Uninformative high-throughput protein binding call.
      supported_by:
        - reference_id: PMID:40205054
          supporting_text: Multimodal cell maps as a foundation for structural and functional genomics.
  - term:
      id: GO:0007411
      label: axon guidance
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        Inferred by automatic Ensembl Compara orthology transfer (GO_REF:0000107).
        There is no direct experimental evidence for PGRMC1 functioning in axon
        guidance in human; the falcon synthesis does not mention this role. Likely
        spurious ortholog-based transfer.
      action: REMOVE
      reason: >-
        No direct evidence for an axon-guidance function of PGRMC1 in any model
        organism; orthology-based IEA is not adequately supported.
      supported_by:
        - reference_id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
          supporting_text: "Most defensible primary functional description"
  - term:
      id: GO:0007613
      label: memory
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        Automatic ortholog-based IEA. While PGRMC1 has been linked to Alzheimer's
        disease through Abeta oligomer binding, "memory" as a direct biological
        process annotation is too broad and not directly supported by experimental
        evidence for human PGRMC1. The synaptic effects are downstream/disease-context
        rather than a core gene function.
      action: REMOVE
      reason: >-
        Phenotype-level term, not a molecular function/process directly executed by
        PGRMC1; ortholog-based transfer is weak.
  - term:
      id: GO:0008306
      label: associative learning
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        Automatic ortholog-based IEA. Same reasoning as for memory and axon
        guidance: no direct experimental evidence and these are phenotype-level
        terms rather than core PGRMC1 functions.
      action: REMOVE
      reason: >-
        Phenotype-level term not directly supported by experimental evidence for
        PGRMC1.
  - term:
      id: GO:0020037
      label: heme binding
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        Heme binding is the core, evolutionarily conserved molecular function of
        PGRMC1 and is supported by extensive crystallographic, spectroscopic,
        biochemical and mutagenesis evidence (Tyr113 axial coordination, ~50 nM Kd,
        heme-dependent dimerisation). This is the single most important MF term.
      action: ACCEPT
      reason: >-
        Core molecular function backed by multiple lines of high-quality evidence.
      supported_by:
        - reference_id: PMID:26988023
          supporting_text: "the haem iron is five-coordinated by Tyr113 (Y113) alone"
        - reference_id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
          supporting_text: "Multiple lines of evidence support that **PGRMC1 binds heme** at a **surface-exposed site**."
  - term:
      id: GO:0043005
      label: neuron projection
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        PGRMC1 is broadly expressed including in neurons and has been detected in
        post-synaptic compartments; presence in neuronal projections is consistent
        with the Abeta-binding / synaptic localisation literature. Annotation is
        plausible but is not a core function.
      action: KEEP_AS_NON_CORE
      reason: >-
        Cellular-component term reflecting neuronal expression context, not a core
        PGRMC1 function. Supported by immunofluorescence studies of synaptic puncta.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "sigma-2/PGRMC1 is expressed at low levels in cell bodies of neurons and glia, and in proximal neurites"
  - term:
      id: GO:0043025
      label: neuronal cell body
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        Same rationale as neuron projection - PGRMC1 is detected in neuronal cell
        bodies (immunofluorescence in cultured hippocampal/cortical neurons), but
        this is an expression-context CC term not a core function.
      action: KEEP_AS_NON_CORE
      reason: Reflects neuronal expression, not a core mechanistic function.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "sigma-2/PGRMC1 is expressed at low levels in cell bodies of neurons and glia"
  - term:
      id: GO:0044297
      label: cell body
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        Parent term of neuronal cell body; uninformative.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Redundant with the more specific GO:0043025 (neuronal cell body) annotation.
  - term:
      id: GO:0045202
      label: synapse
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        Sigma-2/PGRMC1 is enriched in post-synaptic density fractions of rat brain
        and detected at synaptic puncta in cultured neurons; this synaptic pool
        underlies its proposed role as a receptor/binding site for Abeta oligomers.
        Plausible non-core localisation.
      action: KEEP_AS_NON_CORE
      reason: >-
        Synaptic localisation is documented in brain but is a tissue/context-specific
        pool, not the core ER-centric localisation of PGRMC1.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "PGRMC1 is widely expressed in brain at low levels, where it is enriched in the post-synaptic density fraction"
  - term:
      id: GO:0099563
      label: modification of synaptic structure
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        Ortholog-based IEA. Some literature links sigma-2/PGRMC1 ligands to changes
        in synaptic membrane trafficking and reversible spine loss in Abeta-treated
        neurons, but direct evidence that PGRMC1 itself executes a "modification of
        synaptic structure" molecular role is weak.
      action: REMOVE
      reason: >-
        Process is downstream/context-dependent (Abeta oligomer signalling) rather
        than an autonomous PGRMC1 process. Ortholog-based IEA is not adequate
        support.
  - term:
      id: GO:1905809
      label: negative regulation of synapse organization
    evidence_type: IEA
    original_reference_id: GO_REF:0000107
    review:
      summary: >-
        Ortholog-based IEA. Same rationale as for "modification of synaptic
        structure"; not core PGRMC1 biology.
      action: REMOVE
      reason: >-
        Phenotype-/context-level process not directly executed by PGRMC1; weak
        IEA evidence.
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:37047353
    review:
      summary: >-
        Reflects PGRMC1 co-immunoprecipitation with TSPO in MIA PaCa-2 cells, in
        the context of TSPO/TMEM97/sigma-2 receptor complex biology. The functional
        meaning of the binding is debated and cell-type-dependent. Generic protein
        binding is uninformative.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Specific cell-context interaction with TSPO is interesting but generic
        protein-binding is uninformative; the relevant biology is the PGRMC1/TMEM97
        sigma-2 complex.
      supported_by:
        - reference_id: PMID:37047353
          supporting_text: "in MP cells, we observed that both TMEM97 (Figure 4C) and PGRMC1 (Figure 4D) co-immunoprecipitated with TSPO"
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:30443021
    review:
      summary: >-
        Reflects PGRMC1 in the TMEM97(sigma-2)/PGRMC1/LDLR ternary complex that
        accelerates LDL internalisation. The specific functional interaction is
        important and is captured by the related process annotation GO:0140077
        (positive regulation of lipoprotein transport).
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Generic protein binding is uninformative; the functional consequence
        (LDLR/TMEM97 complex / LDL uptake) is captured elsewhere.
      supported_by:
        - reference_id: PMID:30443021
          supporting_text: "These data indicate that the formation of a ternary complex of LDLR-PGRMC1-TMEM97 is necessary for the rapid internalization of LDL by LDLR."
  - term:
      id: GO:0140077
      label: positive regulation of lipoprotein transport
    evidence_type: IMP
    original_reference_id: PMID:30443021
    review:
      summary: >-
        CRISPR knockout of PGRMC1 in HeLa cells significantly reduced uptake of
        radiolabelled or fluorescently tagged LDL via the LDL receptor; the
        LDLR/TMEM97/PGRMC1 ternary complex is required for rapid LDL internalisation.
        This is a directly experimentally supported process role, although it
        represents one of several pleiotropic functions and is not the deepest
        core function.
      action: KEEP_AS_NON_CORE
      reason: >-
        Genuine, well-supported PGRMC1 function but it acts in concert with
        TMEM97/LDLR in a specific membrane complex; this is one of multiple
        pleiotropic roles rather than the gene's primary molecular activity.
      supported_by:
        - reference_id: PMID:30443021
          supporting_text: "Uptake of radiolabeled LDL was significantly decreased in PGRMC1 KO, TMEM97 KO, and double KO cell lines."
        - reference_id: PMID:30443021
          supporting_text: "These data indicate that the formation of a ternary complex of LDLR-PGRMC1-TMEM97 is necessary for the rapid internalization of LDL by LDLR."
  - term:
      id: GO:0005886
      label: plasma membrane
    evidence_type: TAS
    original_reference_id: PMID:25390692
    review:
      summary: >-
        A pool of PGRMC1 (often referred to as cell-surface PGRMC1 or csPGRMC1)
        translocates to the plasma membrane in many cell types and is the site at
        which the TMEM97/PGRMC1/LDLR ternary complex and the proposed sigma-2/PGRMC1
        Abeta-oligomer binding occur. Well supported by multiple cell biology
        studies.
      action: ACCEPT
      reason: >-
        Plasma membrane localisation is a genuine, functionally important pool of
        PGRMC1 supported by multiple lines of evidence.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "translocates from the endoplasmic reticulum to the plasma membrane in several cell types"
  - term:
      id: GO:0020037
      label: heme binding
    evidence_type: TAS
    original_reference_id: PMID:25390692
    review:
      summary: >-
        Heme binding is the core molecular function of PGRMC1; multiple lines of
        evidence (review citation here, plus IDA direct binding studies).
      action: ACCEPT
      reason: Core, well-supported molecular function.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "PGRMC1 is a highly conserved heme-binding protein in the membrane associated progesterone receptor (MAPR) family"
  - term:
      id: GO:0043005
      label: neuron projection
    evidence_type: ISS
    original_reference_id: GO_REF:0000024
    review:
      summary: >-
        Same as the IEA call above - neuronal/dendritic localisation reflects
        expression context not core function.
      action: KEEP_AS_NON_CORE
      reason: Tissue-expression context, not core PGRMC1 mechanism.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "sigma-2/PGRMC1 is expressed at low levels in cell bodies of neurons and glia, and in proximal neurites"
  - term:
      id: GO:0043025
      label: neuronal cell body
    evidence_type: ISS
    original_reference_id: GO_REF:0000024
    review:
      summary: As for the IEA call above.
      action: KEEP_AS_NON_CORE
      reason: Tissue-expression context, not core PGRMC1 mechanism.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "sigma-2/PGRMC1 is expressed at low levels in cell bodies of neurons and glia"
  - term:
      id: GO:0045202
      label: synapse
    evidence_type: ISS
    original_reference_id: GO_REF:0000024
    review:
      summary: As for the IEA call above.
      action: KEEP_AS_NON_CORE
      reason: Synaptic pool documented in brain; non-core context.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "PGRMC1 is widely expressed in brain at low levels, where it is enriched in the post-synaptic density fraction"
  - term:
      id: GO:1903078
      label: positive regulation of protein localization to plasma membrane
    evidence_type: IMP
    original_reference_id: PMID:25390692
    review:
      summary: >-
        PGRMC1 has been shown to stabilise plasma membrane levels of EGFR and (in
        adipocyte models) of LDL-R, VLDL-R and GLUT4 via heme-dependent dimerisation.
        Genuine but pleiotropic function.
      action: KEEP_AS_NON_CORE
      reason: >-
        Membrane-receptor trafficking effects are well documented for several
        partners (EGFR, LDL-R/VLDL-R, GLUT4) and are biologically significant, but
        represent context-dependent applications of PGRMC1's core membrane-protein-
        adaptor activity rather than its core mechanism.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "PGRMC1 is a highly conserved heme-binding protein in the membrane associated progesterone receptor (MAPR) family that has been shown to stabilize surface receptor expression of proteins and directly associates with proteins that regulate membrane trafficking"
  - term:
      id: GO:0005515
      label: protein binding
    evidence_type: IPI
    original_reference_id: PMID:26988023
    review:
      summary: >-
        Captures the well-characterised haem-dependent binding of PGRMC1 to EGFR
        and to cytochromes P450 (CYP1A2, CYP3A4, CYP51), but the generic term itself
        is uninformative; the functional consequence (P450 modulation, EGFR signalling)
        is the meaningful annotation.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Specific, functional partner interactions should be captured by process
        annotations; generic protein binding is uninformative.
      supported_by:
        - reference_id: PMID:26988023
          supporting_text: "Haem-mediated PGRMC1 dimerization is required for interactions with EGFR and cytochromes P450"
  - term:
      id: GO:0020037
      label: heme binding
    evidence_type: IDA
    original_reference_id: PMID:26988023
    review:
      summary: >-
        Direct assay (X-ray crystallography at 1.95 Å, NMR, MS, SV-AUC, UV-vis,
        resonance Raman, ITC) of heme binding by PGRMC1 cytosolic domain. Kd ~50 nM.
        This is the highest-quality experimental support for the core heme-binding
        function.
      action: ACCEPT
      reason: Direct biochemical and structural evidence for core MF.
      supported_by:
        - reference_id: PMID:26988023
          supporting_text: "haem binding to PGRMC1 was of low affinity with a Kd value of 50 nmol l−1"
        - reference_id: PMID:26988023
          supporting_text: "the haem iron is five-coordinated by Tyr113 (Y113) alone"
  - term:
      id: GO:0042803
      label: protein homodimerization activity
    evidence_type: IDA
    original_reference_id: PMID:26988023
    review:
      summary: >-
        PGRMC1 homodimerises via two non-mutually-exclusive mechanisms: heme-heme
        stacking between two protruding haem moieties and intermolecular
        Cys129-Cys129 disulfide bonding. The functional dimer is required for
        EGFR and CYP interactions and for cancer cell proliferation.
      action: ACCEPT
      reason: >-
        Well-supported direct molecular function; the heme-dependent dimer is a
        regulatory/functional state.
      supported_by:
        - reference_id: PMID:26988023
          supporting_text: "PGRMC1 forms a dimeric structure largely through hydrophobic interactions between the haem moieties of two monomers"
        - reference_id: PMID:26988023
          supporting_text: "the apo-monomer PGRMC1 resulted in dimerization by binding with haem"
  - term:
      id: GO:0005741
      label: mitochondrial outer membrane
    evidence_type: ISS
    original_reference_id: GO_REF:0000024
    review:
      summary: >-
        Same as the IEA outer mitochondrial membrane annotation - a real but minor
        pool of PGRMC1.
      action: KEEP_AS_NON_CORE
      reason: Minor functional pool relevant to FECH interaction / heme trafficking.
      supported_by:
        - reference_id: PMID:27599036
          supporting_text: "Further experiments to characterize the sub-mitochondrial localization were consistent with the majority of PGRMC1 being on the outside of the outer mitochondrial membrane."
  - term:
      id: GO:0006783
      label: heme biosynthetic process
    evidence_type: IDA
    original_reference_id: PMID:27599036
    review:
      summary: >-
        PGRMC1 interacts with ferrochelatase (FECH) and inhibits FECH activity in
        vitro in a dose-dependent manner; AG-205 PGRMC1 inhibitor treatment reduces
        hemoglobinisation in differentiating MEL cells. PGRMC1 also donates heme to
        apo-cytochrome b5 in vitro. PGRMC1 is therefore implicated as a regulator
        of heme biosynthesis, although its precise role (inhibitor vs. chaperone vs.
        product-release regulator) is not fully resolved.
      action: ACCEPT
      reason: >-
        Direct experimental evidence supports PGRMC1 involvement in heme synthesis
        regulation via FECH. Although the directionality of regulation is complex
        (it inhibits FECH in vitro yet AG-205 treatment reduces hemoglobinisation),
        the process-level annotation is appropriate.
      supported_by:
        - reference_id: PMID:27599036
          supporting_text: "In the presence of PGRMC1, in vitro measured FECH activity decreased in a dose-dependent manner."
        - reference_id: PMID:27599036
          supporting_text: "When cells that are recognized models for erythroid differentiation were treated with a small molecule inhibitor of PGRMC1, AG-205, there was an observed decrease in the level of hemoglobinization relative to that of untreated cells."
        - reference_id: PMID:27599036
          supporting_text: "In vitro heme transfer experiments showed that purified PGRMC1 was able to donate heme to apo-cytochrome b5."
  - term:
      id: GO:0020037
      label: heme binding
    evidence_type: IDA
    original_reference_id: PMID:27599036
    review:
      summary: >-
        Direct heme transfer assays confirm PGRMC1 binds and donates heme. Core MF.
      action: ACCEPT
      reason: Direct experimental support for core heme binding activity.
      supported_by:
        - reference_id: PMID:27599036
          supporting_text: "In vitro heme transfer experiments showed that purified PGRMC1 was able to donate heme to apo-cytochrome b 5 ."
  - term:
      id: GO:0001540
      label: amyloid-beta binding
    evidence_type: TAS
    original_reference_id: PMID:26871627
    review:
      summary: >-
        Sigma-2/PGRMC1 has been proposed as a critical neuronal receptor for
        Abeta-42 oligomers in Alzheimer's disease, with siRNA knockdown of PGRMC1
        in cultured neurons reducing >90% of Abeta oligomer binding (Izzo et al.
        2014 PMID:25390692; this annotation cites the De Strooper & Karran 2016 Cell
        review). Subsequent work clarifies that TMEM97 (sigma-2 receptor) is the
        gene product corresponding to the sigma-2 binding site and that PGRMC1
        contributes as a complex member, so direct PGRMC1-Abeta binding has been
        debated. Keep as non-core given its disease relevance but acknowledge it is
        complex-mediated.
      action: KEEP_AS_NON_CORE
      reason: >-
        Disease-context interaction at the plasma-membrane sigma-2 complex; not the
        core (heme-binding) activity. Direct vs complex-mediated binding remains
        debated.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "We have identified the sigma-2/PGRMC1 protein as a critical receptor mediating greater than 90% of Abeta oligomer binding to neurons and their downstream synaptotoxic effects."
  - term:
      id: GO:0044297
      label: cell body
    evidence_type: ISS
    original_reference_id: GO_REF:0000024
    review:
      summary: Parent of neuronal cell body; uninformative.
      action: MARK_AS_OVER_ANNOTATED
      reason: Redundant with GO:0043025 neuronal cell body.
  - term:
      id: GO:0005783
      label: endoplasmic reticulum
    evidence_type: TAS
    original_reference_id: PMID:25390692
    review:
      summary: >-
        Duplicate of GO:0005783 IBA annotation. ER is the core localisation.
      action: ACCEPT
      reason: Core localisation, multiply supported.
      supported_by:
        - reference_id: PMID:25390692
          supporting_text: "translocates from the endoplasmic reticulum to the plasma membrane in several cell types"
  - term:
      id: GO:0005886
      label: plasma membrane
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-6799350
    review:
      summary: >-
        Plasma membrane pool is well documented. The Reactome source (exocytosis of
        specific granule membrane proteins) is more about a specific pathway than
        the localisation itself.
      action: ACCEPT
      reason: Genuine plasma membrane pool of PGRMC1.
      supported_by:
        - reference_id: PMID:30443021
          supporting_text: "Confocal microscopy and Proximity Ligation Assay studies indicated a clear co-localization of LDLR, PGRMC1 and TMEM97."
  - term:
      id: GO:0035579
      label: specific granule membrane
    evidence_type: TAS
    original_reference_id: Reactome:R-HSA-6799350
    review:
      summary: >-
        Reactome-derived annotation linking PGRMC1 to neutrophil specific granule
        membrane (exocytosis pathway). PGRMC1 has been detected in neutrophil
        granule-membrane proteomics but this is a niche tissue context and not
        functionally characterised as a core role.
      action: KEEP_AS_NON_CORE
      reason: >-
        Cell-type-specific granule-membrane localisation in neutrophils;
        peripheral to PGRMC1's general biology.
  - term:
      id: GO:0016020
      label: membrane
    evidence_type: HDA
    original_reference_id: PMID:19946888
    review:
      summary: >-
        High-throughput membrane-proteome study of NK cells identified PGRMC1 in
        membrane fractions; uninformative generic membrane annotation.
      action: MARK_AS_OVER_ANNOTATED
      reason: >-
        Generic "membrane" is uninformative; more specific ER/plasma-membrane
        annotations are already present.
      supported_by:
        - reference_id: PMID:19946888
          supporting_text: Defining the membrane proteome of NK cells.
  - term:
      id: GO:0005496
      label: steroid binding
    evidence_type: TAS
    original_reference_id: PMID:9705155
    review:
      summary: >-
        Original 1998 cloning paper that named PGRMC1 (Hpr6.6) a "putative steroid
        membrane receptor" based on homology to a porcine progesterone-binding
        protein. As above, direct steroid binding by PGRMC1 alone is equivocal; the
        annotation is historically motivated. Retain as non-core.
      action: KEEP_AS_NON_CORE
      reason: >-
        Historical, putative annotation. Direct steroid binding by purified PGRMC1
        cytosolic domain has not been robustly demonstrated; contemporary syntheses
        place heme binding (not steroid binding) as the core MF.
      supported_by:
        - reference_id: PMID:9705155
          supporting_text: "We have cloned two human putative steroid binding membrane proteins, termed Hpr6.6 and Dg6."
core_functions:
  - description: >-
      Heme B binding via an atypical pentacoordinate Tyr113 axial ligand in a
      surface-exposed pocket of the cytochrome b5-like domain, driving reversible
      haem-dependent homodimerisation that gates downstream protein-protein
      interactions.
    molecular_function:
      id: GO:0020037
      label: heme binding
    directly_involved_in:
      - id: GO:0006783
        label: heme biosynthetic process
    locations:
      - id: GO:0005789
        label: endoplasmic reticulum membrane
      - id: GO:0005741
        label: mitochondrial outer membrane
    supported_by:
      - reference_id: PMID:26988023
        supporting_text: "the haem iron is five-coordinated by Tyr113 (Y113) alone"
      - reference_id: PMID:26988023
        supporting_text: "haem binding to PGRMC1 was of low affinity with a Kd value of 50 nmol l−1"
      - reference_id: PMID:27599036
        supporting_text: "In vitro heme transfer experiments showed that purified PGRMC1 was able to donate heme to apo-cytochrome b 5 ."
      - reference_id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
        supporting_text: "Multiple lines of evidence support that **PGRMC1 binds heme** at a **surface-exposed site**."
  - description: >-
      Heme-dependent homodimerisation (via heme-heme stacking and Cys129-disulfide
      crosslinking) that is required for productive interaction with EGFR and
      microsomal cytochrome P450 enzymes; the dimer is dissociated by CO binding
      to the sixth haem coordination position, making PGRMC1 a CO-sensitive
      molecular switch.
    molecular_function:
      id: GO:0042803
      label: protein homodimerization activity
    directly_involved_in:
      - id: GO:0050790
        label: regulation of catalytic activity
    locations:
      - id: GO:0005789
        label: endoplasmic reticulum membrane
    supported_by:
      - reference_id: PMID:26988023
        supporting_text: "PGRMC1 forms a dimeric structure largely through hydrophobic interactions between the haem moieties of two monomers"
      - reference_id: PMID:26988023
        supporting_text: "Haem-mediated PGRMC1 dimerization is required for interactions with EGFR and cytochromes P450"
  - description: >-
      Direct, heme-dependent binding to multiple ER-resident cytochrome P450
      enzymes (CYP51A1, CYP21A2, CYP7A1, CYP3A4, CYP2C8, CYP2C2, CYP1A2) and to
      NADPH-cytochrome P450 reductase (CPR), modulating microsomal CYP activity
      in an isoform-specific manner — stimulating sterol-biosynthetic CYPs
      (notably CYP51A1) while inhibiting several drug-metabolising CYPs.
    molecular_function:
      id: GO:0030234
      label: enzyme regulator activity
    directly_involved_in:
      - id: GO:0006805
        label: xenobiotic metabolic process
      - id: GO:0008203
        label: cholesterol metabolic process
    locations:
      - id: GO:0005789
        label: endoplasmic reticulum membrane
      - id: GO:0030868
        label: smooth endoplasmic reticulum membrane
    supported_by:
      - reference_id: PMID:21081644
        supporting_text: "PGRMC1 bound efficiently to all three P450s"
      - reference_id: PMID:21081644
        supporting_text: "Cotransfection of cells with P450s and PGRMC1 resulted in PGRMC1 concentration-dependent inhibition of the P450 activities, and this inhibition was partially reversed by increased expression of the P450 reductase (CPR)."
      - reference_id: PMID:21081644
        supporting_text: "CYP51 activity was decreased by down-regulation of PGRMC1 and expression of PGRMC1 in the PGRMC1-deficient cells increased CYP51 activity."
      - reference_id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
        supporting_text: "PGRMC1 is one of few proteins described as a **direct modulator of human CYP activity**, with reported physical interactions with multiple CYPs (e.g., **CYP7A1, CYP21A2, CYP51A1, CYP3A4, CYP2C8, CYP2C2**) and **cytochrome P450 reductase (CPR)**."
  - description: >-
      Membrane-protein adaptor / chaperone-like activity that stabilises and
      organises plasma-membrane and ER-resident receptor complexes — including
      the TMEM97 (sigma-2 receptor)/PGRMC1/LDLR ternary complex required for
      rapid LDLR-mediated LDL internalisation, and EGFR plasma-membrane
      stabilisation in cancer cells.
    molecular_function:
      id: GO:0030674
      label: protein-macromolecule adaptor activity
    directly_involved_in:
      - id: GO:0140077
        label: positive regulation of lipoprotein transport
    locations:
      - id: GO:0005886
        label: plasma membrane
      - id: GO:0005789
        label: endoplasmic reticulum membrane
    supported_by:
      - reference_id: PMID:30443021
        supporting_text: "These data indicate that the formation of a ternary complex of LDLR-PGRMC1-TMEM97 is necessary for the rapid internalization of LDL by LDLR."
      - reference_id: PMID:30443021
        supporting_text: "Uptake of radiolabeled LDL was significantly decreased in PGRMC1 KO, TMEM97 KO, and double KO cell lines."
proposed_new_terms:
  - proposed_name: cytochrome P450 binding regulator activity
    proposed_definition: >-
      A molecular function describing physical, heme-dependent binding to
      cytochrome P450 enzymes that modulates (positively or negatively, in an
      isoform-specific manner) their catalytic activity without electron
      transfer. Distinguishes the MAPR-family role (haem-dependent allosteric
      modulator) from electron-donating partners of CYPs (cytochrome b5 and
      cytochrome P450 reductase).
    justification: >-
      PGRMC1 (and the MAPR family generally) acts as a haem-dependent regulator
      of microsomal CYPs, distinct from cytochrome b5 (electron donor) and CPR
      (electron donor). Existing GO terms such as GO:0008047 (enzyme activator
      activity) do not capture the haem-dependent, isoform-specific,
      bidirectional (activator/inhibitor) CYP-modulating role of PGRMC1.
suggested_questions:
  - question: >-
      Is direct, high-affinity progesterone binding by purified, full-length
      membrane-embedded PGRMC1 reproducible across independent groups, or is the
      historical "progesterone receptor" naming purely a misnomer derived from
      homology and indirect cellular effects?
  - question: >-
      What is the in vivo stoichiometry and dynamics of the PGRMC1-CYP complex
      on the ER membrane, and how does heme loading state shift CYP isoform
      preference?
  - question: >-
      Is the PGRMC1-FECH interaction physiologically required for heme synthesis
      in mammalian cells in vivo, or does it primarily fine-tune product release
      in specific tissues (e.g., erythroid)?
suggested_experiments:
  - hypothesis: >-
      Heme-loaded PGRMC1 dimers, but not apo-PGRMC1 monomers, are required for
      microsomal CYP3A4-mediated drug clearance in hepatocytes.
    description: >-
      Conditionally delete Pgrmc1 in mouse hepatocytes and rescue with WT vs.
      Y113F (heme-binding deficient) vs. C129S vs. Y113F/C129S double mutants;
      measure CYP3A4 protein, drug-clearance pharmacokinetics, and acetaminophen
      hepatotoxicity.
  - hypothesis: >-
      The TMEM97/PGRMC1/LDLR complex requires PGRMC1's heme-dependent dimerisation
      for accelerated LDL endocytosis.
    description: >-
      Compare LDL internalisation kinetics in PGRMC1-KO HeLa cells reconstituted
      with WT vs. Y113F vs. C129S PGRMC1; test whether CORM-3 (CO donor) phenocopies
      heme-binding loss for LDL uptake.
  - hypothesis: >-
      PGRMC1 functions as a heme chaperone, transferring heme from the FECH-containing
      mitochondrial heme metabolon to apo-CYPs and apo-cytochrome b5 in the ER.
    description: >-
      Use isotopically labelled heme pulse-chase combined with FECH-, PGRMC1-, and
      double-knockout cells to follow heme flux from mitochondria to ER-resident
      apo-hemoproteins, with vs. without PGRMC1 add-back.
references:
  - id: GO_REF:0000024
    title: Manual transfer of experimentally-verified manual GO annotation data
      to orthologs by curator judgment of sequence similarity.
    findings: []
  - id: GO_REF:0000033
    title: Annotation inferences using phylogenetic trees
    findings: []
  - id: GO_REF:0000043
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword
      mapping
    findings: []
  - id: GO_REF:0000044
    title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular
      Location vocabulary mapping, accompanied by conservative changes to GO
      terms applied by UniProt.
    findings: []
  - id: GO_REF:0000107
    title: Automatic transfer of experimentally verified manual GO annotation
      data to orthologs using Ensembl Compara.
    findings: []
  - id: GO_REF:0000120
    title: Combined Automated Annotation using Multiple IEA Methods.
    findings: []
  - id: PMID:17353931
    title: Large-scale mapping of human protein-protein interactions by mass
      spectrometry.
    findings: []
  - id: PMID:19946888
    title: Defining the membrane proteome of NK cells.
    findings: []
  - id: PMID:21081644
    title: Progesterone receptor membrane component 1 inhibits the activity of
      drug-metabolizing cytochromes P450 and binds to cytochrome P450 reductase.
    findings:
      - statement: >-
          PGRMC1 binds efficiently to drug-metabolising CYPs (CYP2C2, CYP2C8,
          CYP3A4) primarily via their cytoplasmic catalytic domains and inhibits
          their activities in a PGRMC1-concentration-dependent manner; this
          inhibition is partially reversed by elevated CPR expression. In
          contrast PGRMC1 stimulates sterol-biosynthetic CYP51, illustrating
          isoform-specific CYP modulation. PGRMC1 also binds directly to CPR.
        supporting_text: >-
          PGRMC1 bound efficiently to all three P450s ... Cotransfection of cells
          with P450s and PGRMC1 resulted in PGRMC1 concentration-dependent
          inhibition of the P450 activities, and this inhibition was partially
          reversed by increased expression of the P450 reductase (CPR). In
          contrast, CYP51 activity was decreased by down-regulation of PGRMC1
          and expression of PGRMC1 in the PGRMC1-deficient cells increased CYP51
          activity.
  - id: PMID:25390692
    title: 'Alzheimer''s therapeutics targeting amyloid beta 1-42 oligomers II: Sigma-2/PGRMC1
      receptors mediate Abeta 42 oligomer binding and synaptotoxicity.'
    findings:
      - statement: >-
          Sigma-2/PGRMC1 is enriched in post-synaptic density fractions; siRNA
          knockdown of PGRMC1 in cultured neurons reduces >90% of Abeta oligomer
          binding and downstream synaptotoxicity, identifying it as a key
          oligomer receptor (or receptor-complex component) in Alzheimer's disease.
        supporting_text: >-
          We have identified the sigma-2/PGRMC1 protein as a critical receptor
          mediating greater than 90% of Abeta oligomer binding to neurons and
          their downstream synaptotoxic effects.
  - id: PMID:26871627
    title: The Cellular Phase of Alzheimer's Disease.
    findings: []
  - id: PMID:26988023
    title: Haem-dependent dimerization of PGRMC1/Sigma-2 receptor facilitates
      cancer proliferation and chemoresistance.
    findings:
      - statement: >-
          X-ray crystal structure (1.95 Å) of PGRMC1 cytosolic domain shows a
          stable dimer formed by hydrophobic stacking of two protruding haem
          moieties; haem iron is five-coordinated by Tyr113 alone. Heme Kd ~50
          nM. CO binds the sixth coordination position and dissociates the
          dimer. Haem-mediated dimerisation is required for binding to EGFR and
          to cytochromes P450 (CYP1A2, CYP3A4, CYP51), for cancer proliferation
          and for chemoresistance.
        supporting_text: >-
          the haem iron is five-coordinated by Tyr113 (Y113) alone ... haem
          binding to PGRMC1 was of low affinity with a Kd value of 50 nmol l−1
          ... Haem-mediated PGRMC1 dimerization is required for interactions with
          EGFR and cytochromes P450
  - id: PMID:27599036
    title: 'A Novel Role for Progesterone Receptor Membrane Component 1 (PGRMC1):
      A Partner and Regulator of Ferrochelatase.'
    findings:
      - statement: >-
          PGRMC1 (and PGRMC2) co-fractionate and co-localise with the
          mitochondrial heme metabolon; PGRMC1 sits on the outer face of the
          outer mitochondrial membrane and interacts with ferrochelatase (FECH).
          In vitro PGRMC1 inhibits FECH activity dose-dependently (saturating at
          ~60% reduction) preferentially in the product-release conformation,
          and purified PGRMC1 can donate heme to apo-cytochrome b5, supporting a
          heme-chaperone/sensor role.
        supporting_text: >-
          In the presence of PGRMC1, in vitro measured FECH activity decreased in
          a dose-dependent manner. Interactions between FECH and PGRMC1 were
          strongest for the conformation of FECH associated with product release
          ... In vitro heme transfer experiments showed that purified PGRMC1 was
          able to donate heme to apo-cytochrome b5.
  - id: PMID:28514442
    title: Architecture of the human interactome defines protein communities and
      disease networks.
    findings: []
  - id: PMID:30021884
    title: Histone Interaction Landscapes Visualized by Crosslinking Mass
      Spectrometry in Intact Cell Nuclei.
    findings: []
  - id: PMID:30443021
    title: Sigma-2 Receptor/TMEM97 and PGRMC-1 Increase the Rate of
      Internalization of LDL by LDL Receptor through the Formation of a Ternary
      Complex.
    findings:
      - statement: >-
          CRISPR knockout of PGRMC1 or TMEM97 in HeLa cells each independently
          reduce LDL internalisation by LDLR to the same extent as the double
          knockout, indicating both proteins act as obligate components of a
          single LDLR-PGRMC1-TMEM97 ternary complex required for rapid LDL
          endocytosis. Confocal microscopy and proximity ligation assays confirm
          co-localisation/interaction of the three proteins.
        supporting_text: >-
          These data indicate that the formation of a ternary complex of
          LDLR-PGRMC1-TMEM97 is necessary for the rapid internalization of LDL
          by LDLR.
  - id: PMID:32296183
    title: A reference map of the human binary protein interactome.
    findings: []
  - id: PMID:33961781
    title: Dual proteome-scale networks reveal cell-specific remodeling of the
      human interactome.
    findings: []
  - id: PMID:35271311
    title: 'OpenCell: Endogenous tagging for the cartography of human cellular organization.'
    findings: []
  - id: PMID:37047353
    title: Sigma-2 Receptor Ligand Binding Modulates Association between TSPO
      and TMEM97.
    findings:
      - statement: >-
          In MIA PaCa-2 pancreatic cancer cells (high PGRMC1) co-IP detected
          TSPO/PGRMC1 association in addition to TSPO/TMEM97, but no such
          PGRMC1/TSPO association was found in MCF7 cells, indicating cell-type-
          dependent multiprotein complexes around the sigma-2 receptor.
        supporting_text: >-
          in MP cells, we observed that both TMEM97 (Figure 4C) and PGRMC1
          (Figure 4D) co-immunoprecipitated with TSPO
  - id: PMID:40205054
    title: Multimodal cell maps as a foundation for structural and functional
      genomics.
    findings: []
  - id: PMID:9705155
    title: Cloning and tissue expression of two putative steroid membrane
      receptors.
    findings:
      - statement: >-
          Original cloning of human Hpr6.6 (PGRMC1) as a putative steroid-binding
          membrane protein; mRNA predominantly liver and kidney. Naming reflects
          homology to a porcine progesterone-binding protein rather than direct
          steroid-binding assays.
        supporting_text: >-
          We have cloned two human putative steroid binding membrane proteins,
          termed Hpr6.6 and Dg6.
  - id: Reactome:R-HSA-6799350
    title: Exocytosis of specific granule membrane proteins
    findings: []
  - id: file:human/PGRMC1/PGRMC1-deep-research-perplexity.md
    title: Deep research on PGRMC1 function (perplexity sonar-deep-research)
    findings: []
  - id: file:human/PGRMC1/PGRMC1-deep-research-falcon.md
    title: Deep research on PGRMC1 function (Edison/falcon, 2026)
    findings:
      - statement: >-
          Contemporary expert synthesis (2024) frames PGRMC1 as a single-pass
          membrane MAPR-family heme-binding protein with surface-exposed heme
          coordination by Tyr113, two-mechanism (heme-stacking and Cys129
          disulfide) dimerisation, and a primary functional role as a haem-
          dependent regulator of microsomal CYP systems (CYP7A1, CYP21A2, CYP51A1,
          CYP3A4, CYP2C8, CYP2C2) and CPR. ER/endomembranes are the dominant
          subcellular location with additional plasma membrane, nuclear, Golgi,
          endosomal, and mitochondrial pools. Translational relevance is most
          mature for sigma-2/TMEM97 complex biology (CT1812 in Alzheimer's
          disease) and CYP-mediated drug-metabolism modulation.
        supporting_text: >-
          Human **PGRMC1 (O00264)** is a **single-pass membrane MAPR protein**
          with a **cytochrome b5-like heme-binding domain** that supports
          **heme-dependent and redox-sensitive oligomerization** and enables
          **regulatory interactions** with (at least) the **cytochrome P450
          system** (CYPs and CPR) and membrane signaling complexes.
status: COMPLETE

PGRMC1

Gene Description

Proposed New Ontology Terms

cytochrome P450 binding regulator activity

Existing Annotations Review

Core Functions

Heme B binding via an atypical pentacoordinate Tyr113 axial ligand in a surface-exposed pocket of the cytochrome b5-like domain, driving reversible haem-dependent homodimerisation that gates downstream protein-protein interactions.

References

Suggested Questions for Experts

Suggested Experiments

Deep Research

Falcon

Research report: Human PGRMC1 (UniProt O00264) — functional annotation

1) Target verification (critical identity check)

2) Key concepts and definitions (current understanding)

2.1 MAPR/cytochrome b5-like heme-binding protein (not a classical nuclear progesterone receptor)

2.2 Heme binding: residues, coordination, and “holo” vs “apo” PGRMC1

2.3 Dimerization/oligomerization as a regulatory mechanism

3) Molecular functions and pathways (what PGRMC1 “does”)

3.1 Primary biochemical role: heme-binding adaptor/chaperone-like behavior and protein–protein interactions

3.2 Cytochrome P450 modulation (xenobiotic and endogenous metabolism)

3.3 Progesterone-linked anti-apoptotic signaling and receptor pathway crosstalk

3.4 Heme synthesis/trafficking links (ferrochelatase and heme transfer hypotheses)

4) Subcellular localization (where PGRMC1 acts)

5) Recent developments (prioritizing 2023–2024)

5.1 2024: refined mechanistic model of heme binding and dimerization determinants

5.2 2024: updated synthesis of PGRMC1 as a CYP modulator impacting drug metabolism

5.3 2023–2024: stress/inflammation and tissue physiology links in vivo

5.4 2024: reproductive biology—decidualization requires a timed PGRMC1 program

6) Current applications and real-world implementations

6.1 Drug metabolism and chemotherapy interactions (clinical relevance of CYP modulation)

6.2 Sigma-2 (TMEM97) axis and the TMEM97–LDLR–PGRMC1 complex (Alzheimer’s disease therapeutic program)

7) Expert opinions and analysis (authoritative synthesis)

8) Relevant statistics and data (recent studies)

9) Disease associations from knowledgebases (supporting but not definitive)

10) Summary: functional annotation (most defensible primary function)

Evidence map (compact)

Artifacts

Citations

Perplexity

Progesterone Receptor Membrane Component 1 (PGRMC1): A Comprehensive Molecular and Functional Analysis

Structural Architecture and Biochemical Characteristics of PGRMC1

Protein Domain Organization and Topology

Heme Binding Mechanism and Coordination Chemistry

Primary Biochemical Functions: Cytochrome P450 Stabilization and Heme Homeostasis

Heme-Independent Stabilization of Cytochrome P450 Proteins

Physiological Consequences of P450 Stabilization

Heme Synthesis Regulation and Ferrochelatase Interaction

Subcellular Localization and Dynamic Trafficking

Primary Endoplasmic Reticulum Residency and Multiple Compartment Distribution

Endosomal-ER Membrane Contact Sites and Calcium Signaling

Nucleolar Localization and Stress Response

Cancer Biology and Oncogenic Signaling Pathways

PGRMC1 Overexpression in Malignant Tumors

PI3K/AKT/mTOR and EGFR Signaling Axis

EGFR Stabilization and Membrane Localization

Chemoresistance and Drug Metabolism

Sigma-2 Receptor Identity

Metabolic Functions in Adipogenesis and Obesity

Role in Adipocyte Differentiation and Lipid Metabolism

Localization and Compartmentalization of PGRMC1 Function

Cardiac Metabolic Remodeling and Energy Homeostasis

Reproductive Functions and Progesterone Signaling

Progesterone Binding and Anti-Apoptotic Function

Ovarian Function and Granulosa Cell Regulation

Neurological Functions and Alzheimer's Disease

PGRMC1 as an Amyloid Beta Oligomer Receptor

Genetic Disorders and Clinical Manifestations

X-Linked Pediatric Cataract Formation

Membrane Trafficking and ER-Phagy Functions

PGRMC1 as Size-Selective Cargo Receptor in ER-Phagy

Vesicular Trafficking and Endosomal Sorting

Post-Translational Modifications and Functional Regulation

PGRMC1 Phosphorylation and Cell Cycle Regulation

Therapeutic Considerations and Inhibitor Development

AG-205 and PGRMC1 Ligand Specificity

Conclusion: Synthesis of PGRMC1 Functions and Future Directions

Citations

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