ORMDL3 (ORM1-like protein 3) is a conserved, multi-pass transmembrane protein of 153 amino acids that resides in the endoplasmic reticulum (ER) membrane. It functions as a ceramide-sensitive regulatory subunit of the serine palmitoyltransferase (SPT) complex (SPTLC1/SPTLC2/ssSPTa/ORMDL3), the enzyme that catalyzes the first and rate-limiting step in de novo sphingolipid biosynthesis. ORMDL3 acts as a negative regulator of SPT: its N-terminus physically occludes the substrate entry tunnel of SPT, and this inhibitory conformation is stabilized by ceramide binding, thereby implementing a homeostatic negative feedback loop that maintains sphingolipid levels. Structurally resolved by cryo-EM, ORMDL3 has four transmembrane helices and a cytoplasmic N-terminal ceramide-sensing region. The 17q21 locus containing ORMDL3 is the strongest and most replicated genetic risk locus for childhood asthma. Beyond sphingolipid regulation, ORMDL3 has been implicated in ER stress/UPR signaling, ER calcium homeostasis, and autophagy regulation in immune cells, though these may represent downstream consequences of altered sphingolipid metabolism rather than independent molecular functions.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0030148 sphingolipid biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: ORMDL3 is a regulatory subunit of the SPT complex that controls the rate-limiting step in sphingolipid de novo biosynthesis. The IBA annotation to sphingolipid biosynthetic process is well supported by the conserved role of ORM/ORMDL family proteins in sphingolipid metabolism from yeast to human (PMID:20182505, PMID:33558761). Reason: Core function. ORMDL3 is an integral component of the SPT complex and directly regulates sphingolipid biosynthetic flux. This is phylogenetically conserved across the ORM family. Supporting Evidence: PMID:20182505 Orm family proteins mediate sphingolipid homeostasis... we identify Orm proteins as negative regulators of sphingolipid synthesis that form a conserved complex with serine palmitoyltransferase, the first and rate-limiting enzyme in sphingolipid production. PMID:33558761 the first and rate-limiting step of sphingolipid synthesis is catalyzed by the serine palmitoyltransferase holocomplex, which consists of catalytic components (SPTLC1 and SPTLC2) and regulatory components (ssSPTa and ORMDL3). |
| GO:0090156 intracellular sphingolipid homeostasis | IBA GO_REF:0000033 | ACCEPT | Summary: ORMDL3 implements ceramide-dependent negative feedback on SPT to maintain sphingolipid homeostasis. Ceramide binding to the ORMDL3 N-terminus stabilizes an inhibitory conformation that blocks SPT substrate entry, constituting a homeostatic sensor mechanism (PMID:20182505, PMID:37308477). Reason: Core function. ORMDL3 is the ceramide-sensing subunit of the SPT complex that provides homeostatic feedback regulation of sphingolipid levels. This is the primary evolved role of the ORM/ORMDL family. Supporting Evidence: PMID:20182505 Orm proteins are dynamic negative regulators of serine palmitoyltransferase whose inhibitory activity is dependent on adequate levels of downstream sphingolipids PMID:37308477 Structural studies indicate that ceramide can induce and lock the N-terminus of ORMDL3 into an inhibitory conformation. file:human/ORMDL3/ORMDL3-deep-research-falcon.md Ceramide binds to the human SPT–ORMDL complex to signal elevated sphingolipid levels; this promotes ORMDL-mediated SPT inhibition, forming a homeostatic negative-feedback loop |
| GO:0017059 serine palmitoyltransferase complex | IBA GO_REF:0000033 | ACCEPT | Summary: ORMDL3 is a stoichiometric subunit of the SPT complex. The phylogenetic inference is consistent with conserved physical association of ORM/ORMDL proteins with SPT from yeast to human, confirmed by co-IP and cryo-EM structural studies (PMID:20182505, PMID:33558761, PMID:33558762). Reason: Core localization. Structural and biochemical data confirm ORMDL3 is an integral subunit of the SPT holoenzyme complex. Supporting Evidence: PMID:20182505 Immunoprecipitations from HEK293T cells expressing 3×Flag-tagged Ormdl3 led to a prominent Sptlc1 band detected by Western blot (Fig |
| GO:0006672 ceramide metabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: ORMDL3 regulates ceramide levels by controlling SPT activity, the rate-limiting step upstream of ceramide synthesis. This IBA annotation is phylogenetically sound, as yeast ORM proteins also regulate ceramide levels (PMID:20182505). Reason: Ceramide levels are directly controlled by ORMDL3-mediated regulation of SPT. Loss of ORM/ORMDL function leads to elevated ceramide levels in both yeast and human cells. Supporting Evidence: PMID:20182505 we found that simultaneous knock-down by RNAi of ORMDL1/2/3 in Hela cells causes an approximately three-fold increase in ceramide levels |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000120 | ACCEPT | Summary: ORMDL3 is an ER membrane-resident multi-pass transmembrane protein. This IEA is consistent with experimental data showing ER membrane localization by GFP-tagging (PMID:12093374), immunofluorescence (PMID:19819884), and cryo-EM structures showing ORMDL3 embedded in the membrane within the SPT complex (PMID:33558761, PMID:33558762). Reason: Correct and well-supported. UniProt annotation from subcellular localization data; multiple experimental lines confirm ER membrane localization. Supporting Evidence: PMID:12093374 fusion proteins of enhanced green fluorescent protein and ORMDL (EGFP-ORMDL) appeared predominantly in the perinuclear endoplasmic reticulum (ER) and, to a lesser extent, throughout the extended ER network |
| GO:0006665 sphingolipid metabolic process | IEA GO_REF:0000117 | ACCEPT | Summary: IEA annotation to sphingolipid metabolic process from ARBA. This is correct but broader than the more specific IBA annotations already present (GO:0030148 sphingolipid biosynthetic process, GO:0090156 intracellular sphingolipid homeostasis). Reason: Correct parent term. Although more specific terms are already annotated via IBA, this broader IEA annotation is not wrong and reflects the general role of ORMDL3 in sphingolipid metabolism. |
| GO:0016020 membrane | IEA GO_REF:0000002 | ACCEPT | Summary: Generic membrane annotation from InterPro domain mapping. ORMDL3 is indeed a multi-pass transmembrane protein with four transmembrane helices. Reason: Correct but very generic. More specific ER membrane annotation is also present. This is an acceptable broad IEA. |
| GO:2000303 regulation of ceramide biosynthetic process | IEA GO_REF:0000117 | ACCEPT | Summary: IEA from ARBA for regulation of ceramide biosynthetic process. This is correct but could be more specific: ORMDL3 is a negative regulator, and the more specific child term GO:1900060 (negative regulation of ceramide biosynthetic process) is already annotated via IMP. Reason: Correct at the level of specificity typical for IEA. The more precise negative regulation term (GO:1900060) is captured by the IMP annotation from PMID:25691431. |
| GO:0005515 protein binding | IPI PMID:20182505 Orm family proteins mediate sphingolipid homeostasis. | MODIFY | Summary: Physical interaction between ORMDL3 and SPTLC1 (O15269) demonstrated by co-immunoprecipitation from HEK293T cells expressing 3xFlag-tagged ORMDL3 (PMID:20182505). This interaction is functionally significant as ORMDL3 is a regulatory subunit of the SPT complex. Reason: The protein binding annotation is uninformative. The interaction with SPTLC1 reflects ORMDL3's role as a subunit of the SPT complex. A more informative annotation would be enzyme inhibitor activity (GO:0004857), reflecting ORMDL3's function as a direct inhibitor of SPT catalytic activity through physical interaction. Proposed replacements: enzyme inhibitor activity Supporting Evidence: PMID:20182505 Immunoprecipitations from HEK293T cells expressing 3×Flag-tagged Ormdl3 led to a prominent Sptlc1 band detected by Western blot (Fig |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: High-throughput binary protein interactome study (HuRI) reporting many interactions for ORMDL3. Most of these interactions (with AQP6, ARL13B, various GPCRs, channels, etc.) lack functional validation and likely represent the promiscuous detection typical of membrane protein screens. The biologically meaningful interactions are with SPT complex components, not with the diverse set of membrane proteins identified here. Reason: High-throughput interactome data. The 25+ distinct interactors from this single study include many membrane proteins with no known functional relationship to ORMDL3 (e.g., AQP6, GPR101, GPR152, ROM1). Generic protein binding from HT screens is uninformative for ORMDL3's function. Supporting Evidence: PMID:32296183 Here we present a human 'all-by-all' reference interactome map of human binary protein interactions, or 'HuRI'. With approximately 53,000 protein-protein interactions |
| GO:0005515 protein binding | IPI PMID:33558761 Structural insights into the regulation of human serine palm... | MODIFY | Summary: Cryo-EM structure of the human SPT-ORMDL3 complex at 2.6 Angstrom resolution demonstrates direct physical contact between ORMDL3 and SPTLC1 (O15269) within the SPT holoenzyme (PMID:33558761). This is structurally validated interaction, not mere binding. Reason: The protein binding term is uninformative. The structural data reveal that ORMDL3 physically occludes the SPT substrate entry tunnel, acting as an enzyme inhibitor. This should be annotated as enzyme inhibitor activity. Proposed replacements: enzyme inhibitor activity Supporting Evidence: PMID:33558761 ORMDL3 blocks the tunnel and competes with substrate binding through its amino terminus |
| GO:0005515 protein binding | IPI PMID:33558762 Structural insights into the assembly and substrate selectiv... | MODIFY | Summary: Second independent cryo-EM structure study of the SPT-ORMDL3 complex (Li et al. 2021) confirming physical interaction between ORMDL3 and SPTLC1 within the SPT holoenzyme at 3.2 Angstrom resolution. Reason: As with PMID:33558761, the generic protein binding annotation is uninformative. The structural interaction represents enzyme inhibitor activity within the SPT complex. Proposed replacements: enzyme inhibitor activity Supporting Evidence: PMID:33558762 ORMDL3 is located in the center of the complex, serving to stabilize the SPT assembly |
| GO:0005783 endoplasmic reticulum | IDA GO_REF:0000052 | ACCEPT | Summary: IDA annotation from HPA immunofluorescence data. ORMDL3 localizes to the ER, consistent with its known ER membrane topology and function as an SPT complex subunit. Reason: Correct. ER localization is the primary subcellular localization of ORMDL3, confirmed by multiple methods including GFP-fusion (PMID:12093374), immunofluorescence (PMID:19819884), and cryo-EM structures. |
| GO:0002903 negative regulation of B cell apoptotic process | IMP PMID:28747345 ORMDL3 Facilitates the Survival of Splenic B Cells via an AT... | KEEP AS NON CORE | Summary: Dang et al. 2017 showed that Ormdl3 knockout mice have increased apoptosis of splenic B cells, and that ORMDL3 facilitates B cell survival via ATF6-Beclin1 autophagy pathway. The annotation reflects a downstream phenotypic consequence in the immune system. Reason: This is a pleiotropic downstream effect likely mediated through ORMDL3's primary role in sphingolipid/ER stress regulation, rather than a direct molecular function of ORMDL3 in anti-apoptotic signaling. B cell survival effects are likely secondary to altered sphingolipid metabolism and ER stress responses. Supporting Evidence: PMID:28747345 silencing Ormdl3 in vivo significantly decreased the proportions of mature B lymphocytes and transitional 2B cells in spleen... knockdown of Ormdl3 augmented the apoptosis of total splenic cells and splenic CD19+ B cells |
| GO:0010508 positive regulation of autophagy | IDA PMID:28747345 ORMDL3 Facilitates the Survival of Splenic B Cells via an AT... | KEEP AS NON CORE | Summary: Dang et al. 2017 showed ORMDL3 promotes autophagy via the ATF6-Beclin1 pathway. ORMDL3 overexpression increased Beclin1 and LC3-II expression and increased autophagy marker levels. This likely reflects secondary effects of ORMDL3's role in ER stress modulation. Reason: Autophagy regulation by ORMDL3 appears to be an indirect consequence of its effects on ER stress/UPR signaling (ATF6 activation), which itself may stem from altered sphingolipid metabolism. This is not a core molecular function. Supporting Evidence: PMID:28747345 ORMDL3 potentially mediates the autophagy via the ATF 6-Beclin1 autophagy pathway, and it facilitates the survival of splenic B cells via promoting autophagy and suppressing apoptosis |
| GO:0010508 positive regulation of autophagy | IMP PMID:28747345 ORMDL3 Facilitates the Survival of Splenic B Cells via an AT... | KEEP AS NON CORE | Summary: Duplicate annotation for positive regulation of autophagy with IMP evidence from the same study. Ormdl3 knockout in mice showed reduced autophagy markers in splenic B cells. Reason: Same rationale as the IDA annotation above. Autophagy regulation is a downstream pleiotropic effect of ORMDL3's primary sphingolipid regulatory function, not a core molecular activity. Supporting Evidence: PMID:28747345 ORMDL3 potentially mediates the autophagy via the ATF 6-Beclin1 autophagy pathway, and it facilitates the survival of splenic B cells via promoting autophagy and suppressing apoptosis |
| GO:1900182 positive regulation of protein localization to nucleus | IDA PMID:28747345 ORMDL3 Facilitates the Survival of Splenic B Cells via an AT... | KEEP AS NON CORE | Summary: Dang et al. 2017 reported that ORMDL3 promotes nuclear localization of ATF6 (a UPR transcription factor). This likely reflects ORMDL3's effects on ER stress signaling through sphingolipid metabolism, not a direct protein transport function. Reason: The positive regulation of protein localization to nucleus annotation appears to refer to ATF6 nuclear translocation as part of the UPR. This is an indirect downstream effect of ORMDL3's sphingolipid regulatory function, not a core activity. Supporting Evidence: PMID:28747345 we uncovered a role of ORMDL3 in fine-tuning B cell development and survival, besides highlighting a potential mechanism by which ORMDL3 regulates autophagy via ATF6 pathway |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-6798743 | REMOVE | Summary: Reactome annotation placing ORMDL3 at the plasma membrane based on neutrophil degranulation pathway. ORMDL3 is an ER-resident protein with no experimental evidence for plasma membrane localization. The original discovery paper (PMID:12093374) explicitly showed no colocalization with plasma membrane markers. Reason: ORMDL3 is established as an ER-resident multi-pass transmembrane protein. Hjelmqvist et al. 2002 showed that EGFP-ORMDL3 colocalizes with the ER marker PDI but showed no overlapping signal with plasma membrane marker concanavalin A. The Reactome pathway for neutrophil degranulation may include ORMDL3 based on proteomics of secretory granule fractions, but this does not represent the functional localization of ORMDL3. Supporting Evidence: PMID:12093374 double-label fluorescence with biotin-labeled concanavalin A... showed no overlapping signal with EGFP-ORMDL at the plasmatic membrane |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-6799350 | REMOVE | Summary: Second Reactome annotation to plasma membrane from specific granule exocytosis pathway. Same issue as above. Reason: No experimental evidence supports plasma membrane localization of ORMDL3. ORMDL3 is firmly established as an ER membrane protein by GFP-fusion, immunofluorescence, and cryo-EM structural studies. |
| GO:0030667 secretory granule membrane | TAS Reactome:R-HSA-6798743 | REMOVE | Summary: Reactome annotation placing ORMDL3 in secretory granule membranes from the neutrophil degranulation pathway. While ORMDL3 may have been detected in proteomics of granule fractions, its established function and localization is in the ER as part of the SPT complex. Reason: ORMDL3's confirmed localization and functional site is the ER membrane. Detection in granule proteomic fractions may reflect ER contamination or transit through the secretory pathway, not a functional localization. |
| GO:0035579 specific granule membrane | TAS Reactome:R-HSA-6799350 | REMOVE | Summary: Reactome annotation to specific granule membrane from neutrophil-specific granule exocytosis pathway. Same concern as secretory granule membrane. Reason: No functional evidence supports ORMDL3 localization to specific granule membranes. This likely arises from proteomic detection in granule fractions that may include ER membrane contaminants. |
| GO:0017059 serine palmitoyltransferase complex | IDA PMID:25691431 ORMDL/serine palmitoyltransferase stoichiometry determines e... | ACCEPT | Summary: Siow et al. 2015 demonstrated that ORMDLs and SPT form stable complexes in human bronchial epithelial cells (HBECs) and HeLa cells, and that ORMDL is expressed in functional excess relative to SPT. Direct experimental evidence for SPT complex membership. Reason: Core localization. Direct biochemical evidence for ORMDL3 as a component of the SPT complex in human cells. Supporting Evidence: PMID:25691431 ORMDLs and SPT form stable complexes that are not increased by elevated ORMDL3 expression |
| GO:1900060 negative regulation of ceramide biosynthetic process | IMP PMID:25691431 ORMDL/serine palmitoyltransferase stoichiometry determines e... | ACCEPT | Summary: Siow et al. 2015 showed that ORMDL3 negatively regulates ceramide biosynthesis, though the relationship is complex due to ORMDL stoichiometry with SPT. At normal expression levels, ORMDL is in functional excess over SPT, meaning additional ORMDL3 does not further suppress biosynthesis. Reason: Core function. ORMDL3's role as a negative regulator of ceramide biosynthesis (via SPT inhibition) is well established. This is the most specific biological process term available for ORMDL3's primary function. Supporting Evidence: PMID:25691431 The ORM1 (Saccharomyces cerevisiae)-like proteins (ORMDLs) and their yeast orthologs, the Orms, are negative homeostatic regulators of the initiating enzyme in sphingolipid biosynthesis, serine palmitoyltransferase (SPT) |
| GO:0005783 endoplasmic reticulum | IDA PMID:12093374 ORMDL proteins are a conserved new family of endoplasmic ret... | ACCEPT | Summary: Original characterization of the ORMDL gene family by Hjelmqvist et al. 2002. GFP-ORMDL3 fusion protein colocalized with the ER marker PDI in COS-7 cells by confocal microscopy, establishing ER localization. Reason: Primary experimental evidence for ER localization of ORMDL3. This is the foundational study establishing the subcellular localization of the ORMDL protein family. Supporting Evidence: PMID:12093374 Confocal scanning microscopy using an antibody against protein-disulfide isomerase (PDI), an ER-marker protein, showed an overlapping signal in the endoplasmic reticulum... these data show that the ORMDL proteins locate at the ER membrane |
| GO:0006672 ceramide metabolic process | IMP PMID:20182505 Orm family proteins mediate sphingolipid homeostasis. | ACCEPT | Summary: Breslow et al. 2010 demonstrated that simultaneous RNAi knockdown of ORMDL1/2/3 in HeLa cells causes approximately three-fold increase in ceramide levels. This landmark study established the role of ORM/ORMDL proteins in sphingolipid homeostasis. Reason: Core function. Direct experimental evidence that ORMDL proteins regulate ceramide levels in human cells. Supporting Evidence: PMID:20182505 we found that simultaneous knock-down by RNAi of ORMDL1/2/3 in Hela cells causes an approximately three-fold increase in ceramide levels |
| GO:0017059 serine palmitoyltransferase complex | IDA PMID:20182505 Orm family proteins mediate sphingolipid homeostasis. | ACCEPT | Summary: Breslow et al. 2010 showed that Flag-tagged ORMDL3 co-immunoprecipitates with SPTLC1 from HEK293T cells, demonstrating the conserved physical association between ORMDL3 and the SPT complex in human cells. Reason: Core localization. Direct co-IP evidence for ORMDL3 as a component of the SPT complex in human cells, confirmed by subsequent cryo-EM structures. Supporting Evidence: PMID:20182505 Immunoprecipitations from HEK293T cells expressing 3×Flag-tagged Ormdl3 led to a prominent Sptlc1 band detected by Western blot (Fig |
| GO:0004857 enzyme inhibitor activity | IDA PMID:33558761 Structural insights into the regulation of human serine palm... | NEW | Summary: ORMDL3 functions as a direct inhibitor of the SPT enzyme by physically occluding the substrate entry tunnel through its N-terminus. Cryo-EM structures at 2.6 Angstrom resolution show that ORMDL3 blocks the substrate-binding tunnel and competes with substrate binding. This is the primary molecular function of ORMDL3. Reason: ORMDL3 has no MF annotation other than the uninformative protein binding. The structural and biochemical evidence clearly demonstrates that ORMDL3 functions as an enzyme inhibitor of SPT. This is the core molecular function of the protein. Supporting Evidence: PMID:33558761 ORMDL3 blocks the tunnel and competes with substrate binding through its amino terminus PMID:20182505 we identify Orm proteins as negative regulators of sphingolipid synthesis that form a conserved complex with serine palmitoyltransferase, the first and rate-limiting enzyme in sphingolipid production |
| GO:0097001 ceramide binding | IDA PMID:37308477 Ceramide sensing by human SPT-ORMDL complex for establishing... | NEW | Summary: Xie et al. 2023 demonstrated that ceramide binds to the N-terminus of ORMDL3, stabilizing an inhibitory conformation. Key residue Asn13 is critical for ceramide binding; N13A mutation disrupts ceramide responsiveness. Cryo-EM structures resolved the ceramide-bound SPT-ORMDL3 complex. Reason: Ceramide binding is a critical molecular function of ORMDL3 that enables the homeostatic feedback loop. This is structurally and mutagenetically validated and represents a genuine molecular function annotation. Supporting Evidence: PMID:37308477 purified human SPT-ORMDL complexes are inhibited by the central sphingolipid metabolite ceramide. We have solved the cryo-EM structure of the SPT-ORMDL3 complex in a ceramide-bound state. Structure-guided mutational analyses reveal the essential function of this ceramide binding site for the suppression of SPT activity. |
| GO:0090155 negative regulation of sphingolipid biosynthetic process | IMP PMID:20182505 Orm family proteins mediate sphingolipid homeostasis. | NEW | Summary: Breslow et al. 2010 established that ORM/ORMDL proteins are negative regulators of sphingolipid biosynthesis. RNAi knockdown of all three ORMDL genes in HeLa cells increased ceramide levels three-fold, demonstrating the negative regulatory role. This term captures the specific directionality of ORMDL3's regulatory function better than the more general terms currently annotated. Reason: This term specifically captures ORMDL3's role as a negative regulator of sphingolipid biosynthesis, which is more precise than the existing GO:0030148 (sphingolipid biosynthetic process) annotation. The current GO annotations lack a biological process term that specifies the negative regulatory direction. Supporting Evidence: PMID:20182505 we identify Orm proteins as negative regulators of sphingolipid synthesis that form a conserved complex with serine palmitoyltransferase |
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