GRAMD1A (also known as Aster-A) encodes an ER-anchored sterol transfer protein that mediates nonvesicular transport of accessible cholesterol from the plasma membrane (PM) to the endoplasmic reticulum (ER) at ER-PM membrane contact sites. The protein contains an N-terminal GRAM domain that functions as a coincidence detector for accessible PM cholesterol and anionic phospholipids (particularly phosphatidylserine), a central VASt/StART-like (ASTER) sterol-binding domain that binds and transfers cholesterol, and a C-terminal single-pass ER transmembrane helix for ER anchoring. GRAMD1A is recruited to ER-PM contact sites upon increases in accessible PM cholesterol and transfers this cholesterol to the ER, thereby supplying the ER with cholesterol for SREBP-2 feedback suppression and esterification by ACAT. Recent work also demonstrates GRAMD1A functions at ER-Golgi contacts to export excess Golgi cholesterol to the ER. GRAMD1A forms homo- and heteromeric complexes with paralogs GRAMD1B and GRAMD1C. It is broadly expressed with notable brain expression.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0005886
plasma membrane
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: GRAMD1A localizes to ER-PM membrane contact sites where its GRAM domain binds the plasma membrane in response to elevated accessible cholesterol. PMID:29469807 demonstrates that GRAMD1a-eGFP localizes to ER-PM contact sites in human cells, with the GRAM domain mediating PM association. The annotation is appropriate as the protein functionally contacts the PM at membrane contact sites.
Reason: GRAMD1A is an ER-anchored protein that localizes to ER-PM contact sites. The IBA annotation is phylogenetically supported and consistent with experimental data showing GRAMD1a recruitment to PM upon cholesterol loading [PMID:29469807]. While not a PM-resident protein per se, it does localize to PM-contacting sites and binds PM lipids via its GRAM domain.
Supporting Evidence:
PMID:29469807
Our analysis of the human family members, GRAMD1a and GRAMD2a, demonstrates that they are ER-PM MCS proteins, which mark separate regions of the plasma membrane (PM) and perform distinct functions in vivo.
file:human/GRAMD1A/GRAMD1A-deep-research-falcon.md
model: Edison Scientific Literature
|
|
GO:0005789
endoplasmic reticulum membrane
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: GRAMD1A is an ER membrane integral protein anchored via its C-terminal single-pass transmembrane helix. This is the primary steady-state localization of the protein.
Reason: Core localization well-supported by multiple lines of evidence including phylogenetic inference (IBA), UniProt annotation, and experimental data [PMID:29469807]. The C-terminal transmembrane helix (residues 610-630) anchors GRAMD1A in the ER membrane.
Supporting Evidence:
PMID:29469807
GRAMD1a-eGFP with mCherry-Sec61beta and lyn-mCherry Z-stack, sample images are displayed in Figure 1C
|
|
GO:0015485
cholesterol binding
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: GRAMD1A binds cholesterol through its VASt/StART-like (ASTER) domain. The domain contains a hydrophobic cavity that binds one sterol molecule and mediates intermembrane sterol transfer. This is a core molecular function of the protein.
Reason: Cholesterol binding is the primary molecular function of GRAMD1A, mediated by its VASt/StART-like domain. The IBA annotation is phylogenetically robust and consistent with structural and biochemical studies on the Aster family [PMID:30220461].
Supporting Evidence:
PMID:30220461
three ER-resident proteins (Aster-A, -B, -C) that bind cholesterol and facilitate its removal from the plasma membrane
|
|
GO:0032366
intracellular sterol transport
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: GRAMD1A mediates nonvesicular transport of cholesterol from PM to ER. This is the core biological process annotation for the gene.
Reason: Intracellular sterol transport is the primary biological function of GRAMD1A. The IBA annotation captures the core function accurately. More specific child term GO:0032367 (intracellular cholesterol transport) would also be appropriate.
Supporting Evidence:
PMID:31724953
endoplasmic reticulum (ER)-anchored lipid transfer proteins, the GRAMD1s, sense and transport accessible PM cholesterol to the ER
|
|
GO:0120020
cholesterol transfer activity
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: GRAMD1A possesses cholesterol transfer activity mediated by its VASt/StART-like domain, enabling nonvesicular transfer of cholesterol between membranes at contact sites.
Reason: Cholesterol transfer activity is the core molecular function of GRAMD1A. The IBA annotation is accurate and represents the enzymatic/transporter activity of the protein.
Supporting Evidence:
PMID:30220461
Aster Proteins Facilitate Nonvesicular Plasma Membrane to ER Cholesterol Transport in Mammalian Cells
|
|
GO:0140268
endoplasmic reticulum-plasma membrane contact site
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: GRAMD1A localizes to ER-PM contact sites where it functions in cholesterol transport. The GRAM domain senses accessible PM cholesterol and recruits the protein to ER-PM contacts.
Reason: ER-PM contact site localization is central to GRAMD1A function. The protein is recruited to these sites upon increases in accessible PM cholesterol, where it mediates sterol transfer. This is a core cellular component annotation.
Supporting Evidence:
PMID:29469807
GRAMD1a marks distinct ER-PM contacts in a PI(4,5)P2-independent manner
|
|
GO:0005776
autophagosome
|
IEA
GO_REF:0000044 |
REMOVE |
Summary: This annotation derives from UniProt subcellular location vocabulary mapping based on PMID:31222192 which reports GRAMD1A accumulates at sites of autophagosome initiation. However, GRAMD1A is not an autophagosome-resident protein; rather its cholesterol transfer function affects autophagosome biogenesis. This is an over-annotation that conflates functional effect with true localization.
Reason: The annotation to autophagosome as a cellular component is misleading. GRAMD1A is an ER-anchored protein that transiently localizes to ER-PM and ER-Golgi contact sites. PMID:31222192 shows GRAMD1A affects autophagosome biogenesis through its cholesterol transfer function, but this does not mean GRAMD1A is a constituent of autophagosomes. The core function of GRAMD1A is nonvesicular cholesterol transport at membrane contact sites, and any autophagy effects are downstream consequences of cholesterol redistribution.
|
|
GO:0005789
endoplasmic reticulum membrane
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: Duplicate annotation for ER membrane localization via automated combined IEA methods. This is consistent with the IBA annotation and experimental evidence.
Reason: Correct localization. Duplicate of the IBA annotation but acceptable as it represents independent computational evidence supporting ER membrane localization.
|
|
GO:0005886
plasma membrane
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: Duplicate annotation for PM association via automated combined IEA methods. GRAMD1A contacts the PM at ER-PM membrane contact sites through its GRAM domain.
Reason: While GRAMD1A is not a PM-resident protein, it does functionally associate with the PM at membrane contact sites. This annotation is acceptable as it captures the PM-binding function via the GRAM domain.
|
|
GO:0006869
lipid transport
|
IEA
GO_REF:0000043 |
ACCEPT |
Summary: Broad parent term annotation derived from UniProt keyword mapping. GRAMD1A does function in lipid transport, specifically sterol/cholesterol transport.
Reason: This is a correct but overly broad annotation. GRAMD1A specifically transports sterols/ cholesterol. The more specific child terms (intracellular sterol transport, intracellular cholesterol transport) are preferable, but this parent term annotation is not incorrect.
|
|
GO:0006914
autophagy
|
IEA
GO_REF:0000043 |
MARK AS OVER ANNOTATED |
Summary: This annotation derives from UniProt keyword mapping based on PMID:31222192. While that paper reports GRAMD1A affects autophagosome biogenesis, this is a secondary consequence of GRAMD1A's cholesterol transport function, not its primary role. Autophagy is not the core function of GRAMD1A.
Reason: GRAMD1A's primary function is nonvesicular cholesterol transport at ER-PM and ER-Golgi membrane contact sites. The autophagy connection reported in PMID:31222192 appears to be a downstream consequence of cholesterol redistribution affecting autophagosome biogenesis, rather than a direct role in autophagy machinery. This annotation risks misrepresenting the gene's function. The core Aster family papers [PMID:30220461, PMID:31724953, PMID:37735529] make no mention of autophagy - they focus entirely on cholesterol transport and SREBP regulation. This annotation should be reconsidered or at minimum marked as non-core.
|
|
GO:0008289
lipid binding
|
IEA
GO_REF:0000043 |
ACCEPT |
Summary: Broad parent term annotation for lipid binding derived from UniProt keyword mapping. GRAMD1A binds cholesterol via its VASt domain and phosphatidylserine/anionic lipids via its GRAM domain.
Reason: Correct but broad. GRAMD1A does bind lipids - specifically cholesterol (VASt domain) and phosphatidylserine (GRAM domain). More specific terms like cholesterol binding (GO:0015485) are preferable, but this parent term is accurate.
|
|
GO:0031410
cytoplasmic vesicle
|
IEA
GO_REF:0000043 |
REMOVE |
Summary: This annotation derives from UniProt keyword mapping, likely from the autophagosome connection. However, GRAMD1A is not a vesicular protein - it mediates nonvesicular cholesterol transport at membrane contact sites.
Reason: This annotation is misleading. GRAMD1A is an ER-anchored membrane contact site protein that specifically functions in NONvesicular cholesterol transport. It is not associated with cytoplasmic vesicles in its normal function. This annotation likely derives from the autophagosome keyword, which itself is an over-annotation.
|
|
GO:0044232
organelle membrane contact site
|
IEA
GO_REF:0000117 |
ACCEPT |
Summary: GRAMD1A localizes to organelle membrane contact sites, specifically ER-PM and ER-Golgi contact sites, where it functions in cholesterol transport.
Reason: Accurate annotation. GRAMD1A functions at membrane contact sites. This parent term encompasses both ER-PM contact sites (primary) and ER-Golgi contact sites (recently established role). The more specific term GO:0140268 is also annotated.
Supporting Evidence:
PMID:37735529
GRAMD1s transport excess cholesterol from the Golgi to the ER, thereby preventing its build-up
|
|
GO:0005515
protein binding
|
IPI
PMID:25416956 A proteome-scale map of the human interactome network. |
REMOVE |
Summary: High-throughput interactome study detecting binary protein-protein interactions. The term 'protein binding' is too generic to be informative about molecular function.
Reason: Per curation guidelines, 'protein binding' does not convey meaningful functional information. GRAMD1A does interact with proteins (forms complexes with GRAMD1B/C, interacts with MAL per IntAct), but the generic protein binding term should be avoided in favor of more specific interaction terms where possible. This high-throughput data does not illuminate the specific functional context of interactions.
Supporting Evidence:
PMID:25416956
A proteome-scale map of the human interactome network.
|
|
GO:0005515
protein binding
|
IPI
PMID:32296183 A reference map of the human binary protein interactome. |
REMOVE |
Summary: Duplicate protein binding annotation from a second high-throughput interactome study (HuRI).
Reason: Same reasoning as above - 'protein binding' is too vague to be informative. GRAMD1A does form homo- and heteromeric complexes with GRAMD1B/C via transmembrane and luminal regions, but this generic annotation does not capture that specificity.
Supporting Evidence:
PMID:32296183
Apr 8. A reference map of the human binary protein interactome.
|
|
GO:0015485
cholesterol binding
|
IEA
GO_REF:0000107 |
ACCEPT |
Summary: Automated transfer annotation for cholesterol binding via Ensembl Compara. Consistent with other annotations and the known function of the VASt/ASTER domain.
Reason: Correct annotation. Duplicate evidence for cholesterol binding, which is a core molecular function of GRAMD1A mediated by its VASt domain.
|
|
GO:0032367
intracellular cholesterol transport
|
IEA
GO_REF:0000107 |
ACCEPT |
Summary: More specific than GO:0032366 (intracellular sterol transport). GRAMD1A specifically transports cholesterol from PM to ER and from Golgi to ER.
Reason: Excellent annotation - more specific than the parent term and accurately captures the core biological process of GRAMD1A. This is the primary function of the protein.
|
|
GO:0120020
cholesterol transfer activity
|
IEA
GO_REF:0000107 |
ACCEPT |
Summary: Automated transfer annotation for cholesterol transfer activity. Consistent with IBA annotation and biochemical evidence.
Reason: Correct core molecular function annotation. Duplicate evidence supporting the cholesterol transfer activity of the VASt domain.
|
|
GO:0140268
endoplasmic reticulum-plasma membrane contact site
|
IEA
GO_REF:0000107 |
ACCEPT |
Summary: Automated transfer annotation for ER-PM contact site localization. Consistent with IBA annotation and experimental evidence.
Reason: Correct core cellular component annotation. ER-PM contact site localization is central to GRAMD1A function.
|
|
GO:0032367
intracellular cholesterol transport
|
ISS
GO_REF:0000024 |
ACCEPT |
Summary: Manual transfer annotation based on sequence similarity. This is the core biological process of GRAMD1A.
Reason: Correct and specific biological process annotation. Intracellular cholesterol transport (PM to ER, Golgi to ER) is the primary function of GRAMD1A.
|
|
GO:0005886
plasma membrane
|
IDA
GO_REF:0000052 |
ACCEPT |
Summary: IDA annotation based on curation of immunofluorescence data showing PM localization.
Reason: Experimental evidence supporting PM association. GRAMD1A contacts the PM at ER-PM membrane contact sites via its GRAM domain.
|
|
GO:0005886
plasma membrane
|
IDA
PMID:29469807 GRAM domain proteins specialize functionally distinct ER-PM ... |
ACCEPT |
Summary: Direct experimental evidence from Besprozvannaya et al. 2018 showing GRAMD1a localizes to ER-PM contact sites and contacts the plasma membrane.
Reason: High-quality experimental evidence. PMID:29469807 demonstrates that GRAMD1a-eGFP co-localizes with PM markers at ER-PM contact sites.
Supporting Evidence:
PMID:29469807
GRAMD1a and GRAMD2a localize to ER-PM contact sites
|
|
GO:0015485
cholesterol binding
|
ISS
GO_REF:0000024 |
ACCEPT |
Summary: Manual sequence similarity transfer annotation for cholesterol binding. The VASt domain is well-characterized as a sterol-binding domain.
Reason: Correct molecular function annotation with strong evolutionary support. The VASt domain is conserved and structurally characterized for sterol binding.
|
|
GO:0120020
cholesterol transfer activity
|
ISS
GO_REF:0000024 |
ACCEPT |
Summary: Manual sequence similarity transfer annotation for cholesterol transfer activity.
Reason: Correct core molecular function annotation. The Aster family is defined by its cholesterol transfer activity mediated by the VASt domain.
|
|
GO:0140268
endoplasmic reticulum-plasma membrane contact site
|
ISS
GO_REF:0000024 |
ACCEPT |
Summary: Manual sequence similarity transfer annotation for ER-PM contact site localization.
Reason: Correct core cellular component annotation. ER-PM MCS localization is conserved across the Aster/GRAMD1 family.
|
|
GO:0005789
endoplasmic reticulum membrane
|
IDA
PMID:29469807 GRAM domain proteins specialize functionally distinct ER-PM ... |
ACCEPT |
Summary: Direct experimental evidence from Besprozvannaya et al. 2018 showing GRAMD1a co-localizes with ER marker Sec61beta.
Reason: High-quality experimental evidence demonstrating ER membrane localization.
Supporting Evidence:
PMID:29469807
GRAMD1a-eGFP with mCherry-Sec61beta and lyn-mCherry Z-stack, sample images are displayed in Figure 1C
|
|
GO:0044232
organelle membrane contact site
|
IDA
PMID:29469807 GRAM domain proteins specialize functionally distinct ER-PM ... |
ACCEPT |
Summary: Direct experimental evidence showing GRAMD1a localizes to membrane contact sites (specifically ER-PM MCS) in human cells.
Reason: High-quality experimental evidence. PMID:29469807 is a key paper establishing GRAMD1a as an ER-PM membrane contact site protein.
Supporting Evidence:
PMID:29469807
Our analysis of the human family members, GRAMD1a and GRAMD2a, demonstrates that they are ER-PM MCS proteins, which mark separate regions of the plasma membrane (PM) and perform distinct functions in vivo
|
Q: What is the relative contribution of GRAMD1A vs GRAMD1B/C to cholesterol transport in different tissues?
Q: Does GRAMD1A have tissue-specific functions beyond cholesterol transport?
Q: What regulates GRAMD1A expression and activity?
Experiment: Tissue-specific knockout studies to determine physiological roles in different organs
Hypothesis: GRAMD1A may have tissue-specific roles distinct from GRAMD1B/C
Experiment: Structural studies of full-length GRAMD1A to understand conformational changes during cholesterol transfer
Hypothesis: Cholesterol binding may induce conformational changes that facilitate transfer
Experiment: Investigation of post-translational modifications regulating GRAMD1A activity
Hypothesis: Phosphorylation or other PTMs may regulate GRAMD1A recruitment to membrane contact sites
provider: falcon
model: Edison Scientific Literature
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start_time: '2026-01-19T10:05:41.965662'
end_time: '2026-01-19T10:13:15.673218'
duration_seconds: 453.71
template_file: templates/gene_research_go_focused.md
template_variables:
organism: human
gene_id: GRAMD1A
gene_symbol: GRAMD1A
uniprot_accession: Q96CP6
protein_description: 'RecName: Full=Protein Aster-A {ECO:0000250|UniProtKB:Q8VEF1};
AltName: Full=GRAM domain-containing protein 1A {ECO:0000305};'
gene_info: Name=GRAMD1A {ECO:0000312|HGNC:HGNC:29305}; Synonyms=KIAA1533;
organism_full: Homo sapiens (Human).
protein_family: Not specified in UniProt
protein_domains: Cholesterol_transport. (IPR051482); GRAM. (IPR004182); PH-like_dom_sf.
(IPR011993); VASt. (IPR031968); GRAM (PF02893)
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BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.
DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'GRAMD1A' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information
Please provide a comprehensive research report on the gene GRAMD1A (gene ID: GRAMD1A, UniProt: Q96CP6) in human.
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.
Question: You are an expert researcher providing comprehensive, well-cited information.
Provide detailed information focusing on:
1. Key concepts and definitions with current understanding
2. Recent developments and latest research (prioritize 2023-2024 sources)
3. Current applications and real-world implementations
4. Expert opinions and analysis from authoritative sources
5. Relevant statistics and data from recent studies
Format as a comprehensive research report with proper citations. Include URLs and publication dates where available.
Always prioritize recent, authoritative sources and provide specific citations for all major claims.
BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.
DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'GRAMD1A' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information
Please provide a comprehensive research report on the gene GRAMD1A (gene ID: GRAMD1A, UniProt: Q96CP6) in human.
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.
Plan overview
We verified identity and domains, assembled recent primary and review literature (prioritizing 2023–2024), extracted mechanistic and localization evidence, integrated pathway/physiology findings (liver and intestine), assessed redundancy with paralogs, and compiled expert analyses. A compact evidence table is embedded for quick reference.
| Aspect | Key findings (1–2 sentences) | Primary sources (journal, year, URL) |
|---|---|---|
| Identity / nomenclature (GRAMD1A = Aster-A) | GRAMD1A is the gene encoding Aster-A, one of three Aster/GRAMD1 paralogs (Aster‑A/B/C) implicated in nonvesicular sterol transport. | Cell (Sandhu et al.), 2018, https://doi.org/10.1016/j.cell.2018.08.033 (sandhu2018asterproteinsfacilitate pages 1-3); eLife (Naito et al.), 2019, https://doi.org/10.7554/elife.51401 (naito2019movementofaccessible pages 2-3) |
| Domain architecture | Aster‑A contains an N-terminal GRAM lipid‑binding module, a central VASt / StART‑like (ASTER) sterol‑binding domain, and a C‑terminal single‑pass ER transmembrane helix. | Cell, 2018, https://doi.org/10.1016/j.cell.2018.08.033 (sandhu2018asterproteinsfacilitate pages 1-3); Cold Spring Harb Perspect (Kennelly & Tontonoz), 2023, https://doi.org/10.1101/cshperspect.a041263 (kennelly2023cholesteroltransportto pages 4-6) |
| ER anchoring and ER–PM contact site recruitment | Aster‑A is ER‑anchored via the C‑terminal TM helix and relocalizes to ER–PM contact sites upon increases in accessible PM cholesterol. | Cell, 2018, https://doi.org/10.1016/j.cell.2018.08.033 (sandhu2018asterproteinsfacilitate pages 1-3); eLife, 2019, https://doi.org/10.7554/elife.51401 (naito2019movementofaccessible pages 2-3) |
| GRAM domain sensing of accessible cholesterol + PS | The GRAM domain acts as a coincidence detector for accessible (unsequestered) PM cholesterol and anionic lipids (notably phosphatidylserine), triggering recruitment to contacts. | EMBO J / eLife mechanistic work (Ercan / Naito), 2019–2021, https://doi.org/10.7554/elife.51401 (naito2019movementofaccessible pages 22-24, sandhu2018theasterproteins pages 41-45) |
| Sterol binding / transfer by VASt / StART‑like domain | The central ASTER (VASt/StART‑like) domain contains a hydrophobic cavity that binds cholesterol and mediates intermembrane sterol transfer in vitro. | Cell, 2018, https://doi.org/10.1016/j.cell.2018.08.033 (sandhu2018asterproteinsfacilitate pages 1-3); Cold Spring Harb Perspect, 2023, https://doi.org/10.1101/cshperspect.a041263 (kennelly2023cholesteroltransportto pages 4-6) |
| Nonvesicular PM→ER transport and SREBP2 regulation | GRAMD1 proteins move accessible PM cholesterol to the ER; this transfer suppresses SREBP‑2 activation, and loss of GRAMD1s expands the accessible PM cholesterol pool and derepresses SREBP‑2. | eLife, 2019, https://doi.org/10.7554/elife.51401 (naito2019movementofaccessible pages 3-5); Cold Spring Harb Perspect, 2023, https://doi.org/10.1101/cshperspect.a041263 (kennelly2023cholesteroltransportto pages 4-6) |
| Golgi→ER transport control (2023) | Recent work shows GRAMD1s/Asters also export excess cholesterol from the Golgi to the ER, preventing Golgi cholesterol accumulation and chronic SREBP‑2 activation. | Nat Commun (Naito et al.), 2023, https://doi.org/10.1038/s41467-023-41213-w (naito2023regulationofcellular pages 1-2) |
| Complex formation with GRAMD1B/C | GRAMD1A can form homo‑ and heteromeric complexes with GRAMD1B/C via transmembrane regions and luminal helices, which modulate recruitment and function. | eLife, 2019, https://doi.org/10.7554/elife.51401 (naito2019movementofaccessible pages 2-3); Sandhu dissertation/analysis, 2018 (sandhu2018theasterproteins pages 90-95) |
| Tissue expression highlights | Paralogs show distinct tissue enrichments (e.g., Aster‑B high in adrenal); Aster‑A is broadly expressed with notable brain expression reported in reviews. | Cold Spring Harb Perspect, 2023, https://doi.org/10.1101/cshperspect.a041263 (kennelly2023cholesteroltransportto pages 4-6); Sandhu 2018 (sandhu2018theasterproteins pages 90-95) |
| Hepatic role in systemic lipid homeostasis (2023) | Hepatocyte Aster function is critical for nonvesicular sterol flux to the ER and systemic lipid homeostasis; liver Aster loss perturbs ER cholesterol sensing and systemic sterol handling (Nature Metabolism, 2023). | Nat Metab (Xiao et al.), 2023, https://doi.org/10.1038/s42255-022-00722-6 (discussed in reviews) (naito2023regulationofcellular pages 1-2, ferrari2023asterdependentnonvesiculartransport pages 1-3) |
| Intestinal dietary cholesterol uptake & pharmacologic inhibition (2023) | Aster‑B/C aid enterocyte PM→ER cholesterol movement downstream of NPC1L1; Aster deficiency reduces dietary cholesterol absorption and small‑molecule Aster inhibitors can blunt uptake (Science, 2023). | Science (Ferrari et al.), 2023, https://doi.org/10.1126/science.adf0966 (ferrari2023asterdependentnonvesiculartransport pages 1-3) |
| Aster‑C whole‑body balance minor (2024) | Genetic loss of Aster‑C alone produces modest effects on whole‑body cholesterol under varied diets, suggesting partial redundancy among paralogs. | Front Physiol (Banerjee et al.), 2024, https://doi.org/10.3389/fphys.2024.1371096 (ferrari2023asterdependentnonvesiculartransport pages 1-3, sandhu2018theasterproteins pages 45-49) |
Table: Compact, citable summary of GRAMD1A (Aster‑A) identity, domains, mechanism, localization, and recent (2023–2024) in vivo roles with primary-source links and context IDs for verification.
Gene/protein verification and nomenclature
- Target identity: GRAMD1A (HGNC:29305) encodes Aster-A, a human endoplasmic reticulum (ER)–resident lipid transfer protein of the Aster/GRAMD1 family (Aster-A/B/C; genes GRAMD1A/B/C). This aligns with UniProt Q96CP6 and the literature that established the Aster nomenclature (Aster-A for GRAMD1A) (sandhu2018asterproteinsfacilitate pages 1-3, naito2019movementofaccessible pages 2-3).
- Organism: Homo sapiens. All cited mechanistic studies include human cell systems or human proteins, consistent with the target (sandhu2018asterproteinsfacilitate pages 1-3, naito2019movementofaccessible pages 2-3).
- Domain architecture: Aster-A comprises an N-terminal GRAM domain, a central VASt/StART-like (often called ASTER) sterol-binding module, and a C-terminal single-pass ER transmembrane (TM) helix. The VASt/StART-like domain forms a hydrophobic cavity for sterol binding; the GRAM domain mediates cholesterol- and anionic lipid–dependent membrane engagement; the TM helix anchors the protein in the ER (sandhu2018asterproteinsfacilitate pages 1-3, kennelly2023cholesteroltransportto pages 4-6, sandhu2018theasterproteins pages 41-45).
Key concepts and definitions (current understanding)
- Function and substrate specificity: GRAMD1A (Aster-A) mediates nonvesicular transfer of “accessible” plasma membrane (PM) cholesterol to the ER at ER–PM contact sites. The transported substrate is cholesterol (and select sterols in vitro) bound within the ASTER/VASt/StART-like cavity. This transfer contributes to feedback suppression of SREBP-2 and to cholesterol esterification in the ER (naito2019movementofaccessible pages 3-5, sandhu2018asterproteinsfacilitate pages 1-3, kennelly2023cholesteroltransportto pages 4-6).
- Accessible cholesterol pool: A non-raft, unsequestered PM cholesterol fraction that can be acutely expanded (e.g., by sphingomyelin hydrolysis) and detected by cytolysin-derived probes. GRAMD1 proteins respond to transient expansions of this pool to initiate PM→ER cholesterol transfer (naito2019movementofaccessible pages 3-5, kennelly2023cholesteroltransportto pages 4-6).
- ER–PM membrane contact sites (MCS): Physical appositions where Aster-A is recruited to move cholesterol without vesicular trafficking; Aster-A marks a subset of ER–PM contacts specialized for sterol transport (sandhu2018asterproteinsfacilitate pages 1-3, naito2019movementofaccessible pages 2-3).
- Paralogs and complexes: GRAMD1A forms homo- and hetero-oligomeric complexes with GRAMD1B and GRAMD1C through TM/luminal regions, modulating recruitment and function (naito2019movementofaccessible pages 2-3, naito2019movementofaccessible pages 3-5).
Mechanism and domain-level biochemistry
- GRAM domain (sensing and recruitment): The GRAM domain functions as a coincidence detector for (i) accessible PM cholesterol and (ii) anionic phospholipids, prominently phosphatidylserine (PS). Deleting the GRAM domain abolishes cholesterol-dependent PM recruitment, while the isolated GRAM domain is necessary and sufficient for cholesterol-triggered recruitment to the PM. Structural/biochemical analyses indicate distinct, synergistic sites within GRAMD1 GRAM domains for cholesterol and PS sensing (sandhu2018theasterproteins pages 41-45, naito2019movementofaccessible pages 22-24, kennelly2023cholesteroltransportto pages 4-6).
- VASt/StART-like (ASTER) domain (binding/transfer): Aster-A’s ASTER domain is a VASt/StART-like fold with a largely enclosed sterol-binding pocket that carries one sterol and supports intermembrane transfer. Structural features and mutational analyses indicate a ligand-binding mode distinct from classical StART proteins yet optimized for sterol transfer at contacts (sandhu2018asterproteinsfacilitate pages 1-3, sandhu2018theasterproteins pages 41-45, kennelly2023cholesteroltransportto pages 4-6).
- ER anchoring and ER–PM MCS localization: The C-terminal TM helix anchors Aster-A in the ER; upon PM cholesterol loading (e.g., exogenous cholesterol or sphingomyelinase treatment), Aster-A rapidly relocalizes to ER–PM contacts in a GRAM-dependent manner. Aster-defined contacts partially overlap with, but are distinct from, ORP/E-Syt contact domains (sandhu2018asterproteinsfacilitate pages 1-3, sandhu2018theasterproteins pages 41-45).
- Transport directionality and targets: Aster-A mediates PM→ER cholesterol transfer when the PM accessible pool expands, down its chemical potential gradient, thereby supplying the ER for esterification and feedback. Emerging data also show Aster paralogs facilitate Golgi→ER export of excess cholesterol to prevent Golgi accumulation, integrating Asters into ER–Golgi and ER–PM sterol circuits (naito2019movementofaccessible pages 3-5, naito2023regulationofcellular pages 1-2).
- Pathway consequences: Loss of GRAMD1 function (single or triple knockout/knockdown) expands the accessible PM cholesterol pool, impairs PM→ER sterol flux, delays or diminishes SREBP-2 suppression by exogenous cholesterol, and reduces cholesteryl ester formation—placing Aster-A upstream of ER sterol sensing and ACAT-mediated esterification (naito2019movementofaccessible pages 3-5, sandhu2018theasterproteins pages 45-49).
Recent developments and latest research (prioritizing 2023–2024)
- ER sterol homeostasis at ER–Golgi contacts (2023): GRAMD1s (including Aster-A) cooperate with ORP9/OSBP to maintain cholesterol distribution by exporting excess Golgi cholesterol to the ER, averting Golgi sterol accumulation and chronic SREBP-2 activation (Nature Communications, 2023). This generalizes Aster function beyond ER–PM to ER–Golgi interfaces (naito2023regulationofcellular pages 1-2).
- Physiological role in liver (2023 perspective with primary data basis): Reviews highlight hepatocyte Aster-mediated PM→ER transport as critical for hepatic and systemic lipid homeostasis, integrating liver Aster action with reverse cholesterol transport and LDL uptake; these conclusions reference recent mouse genetics that establish hepatic Aster function (Cold Spring Harbor Perspectives in Biology, 2023) (kennelly2023cholesteroltransportto pages 4-6).
- Dietary cholesterol uptake in intestine (2023): Aster proteins act downstream of NPC1L1 in enterocytes to move apically delivered cholesterol from the brush border PM to the ER. Double loss of intestinal Asters (B/C) impairs cholesterol absorption and protects against diet-induced hypercholesterolemia; a small-molecule Aster inhibitor (AI-3d) reduces absorption in murine models (Science, 2023) (ferrari2023asterdependentnonvesiculartransport pages 1-3).
- Redundancy and whole-body balance (2024): Global Aster-C (GRAMD1C) deficiency causes minimal alterations in whole-body cholesterol under both low and high dietary cholesterol, implying partial redundancy among Asters and tissue-specific specialization; modest shifts in select bile acids and cortisol were observed under low-cholesterol diets (Frontiers in Physiology, 2024) (ferrari2023asterdependentnonvesiculartransport pages 1-3).
Current applications and real-world implementations
- Pharmacological modulation: Proof-of-concept Aster inhibitors can manipulate intestinal cholesterol absorption. AI-3d treatment increased accessible PM cholesterol and reduced dietary cholesterol uptake in mouse and human intestinal enteroids and in vivo, highlighting the Aster pathway as pharmacologically tractable for lipid lowering (Science, 2023) (ferrari2023asterdependentnonvesiculartransport pages 1-3).
- Research tools and biomarkers: GRAM domain behavior (cholesterol/PS dependence) and cytolysin-derived probes (e.g., ALOD4, D4(D)) are used experimentally to monitor the accessible cholesterol pool and Aster recruitment, enabling studies of sterol dynamics and SREBP feedback (naito2019movementofaccessible pages 3-5, kennelly2023cholesteroltransportto pages 4-6).
Expert opinions and authoritative analyses
- Mechanistic perspective: Authoritative reviews synthesize that Aster-A/B/C are key ER sterol transporters that couple sensing of accessible PM cholesterol to nonvesicular transfer to the ER, integrating with cellular feedback (SREBP-2) and sterol utilization (esterification, sterol metabolism) (Cold Spring Harbor Perspectives in Biology, 2023) (kennelly2023cholesteroltransportto pages 4-6).
- Physiological integration: Contemporary reviews and commentaries emphasize Aster pathways as central nodes in hepatic and intestinal cholesterol handling, with therapeutic implications for hypercholesterolemia and metabolic disease (Nature Communications, 2023; Science, 2023) (naito2023regulationofcellular pages 1-2, ferrari2023asterdependentnonvesiculartransport pages 1-3).
Relevant statistics and data from recent studies
- Cellular knockdown/knockout: In Aster-deficient cells, accessible PM cholesterol pools expand and the suppression of SREBP-2 cleavage by exogenous cholesterol is impaired; acute recruitment of GRAMD1b can reverse the accumulated accessible cholesterol, demonstrating rate-limiting control by Asters at ER–PM contacts (eLife, 2019) (naito2019movementofaccessible pages 3-5, naito2019movementofaccessible pages 22-24).
- Enterocyte physiology: Enterocyte-specific Aster loss (B/C) causes accumulation of accessible PM cholesterol, ER sterol depletion, activation of SREBP2, production of CE-depleted chylomicrons, and reduced systemic cholesterol burden in mice; pharmacologic inhibition phenocopies reduced absorption (Science, 2023) (ferrari2023asterdependentnonvesiculartransport pages 1-3).
- Golgi homeostasis: Loss of ORP9 causes Golgi cholesterol accumulation that is exacerbated by GRAMD1 depletion, with chronic SREBP-2 activation, indicating Asters are necessary to drain excess Golgi cholesterol to the ER (Nature Communications, 2023) (naito2023regulationofcellular pages 1-2).
Subcellular localization and pathway placement
- Localization: ER integral membrane protein that dynamically enriches at ER–PM contact sites in response to increased accessible PM cholesterol; acts at ER–Golgi contacts to export Golgi cholesterol under perturbation (sandhu2018asterproteinsfacilitate pages 1-3, naito2023regulationofcellular pages 1-2).
- Pathways: Nonvesicular cholesterol transport; feedback regulation of cholesterol synthesis and uptake via SREBP-2; coupling to ER esterification (ACAT) and downstream metabolic fates (naito2019movementofaccessible pages 3-5, kennelly2023cholesteroltransportto pages 4-6).
Redundancy and tissue expression
- Redundancy: GRAMD1A forms complexes with GRAMD1B/C; paralog co-expression in many cell types yields partial redundancy, whereas tissue-specific dominance (e.g., adrenal Aster-B; intestine Aster-B/C; broader/brain expression for Aster-A) leads to specialized phenotypes upon single-gene loss (naito2019movementofaccessible pages 2-3, kennelly2023cholesteroltransportto pages 4-6, ferrari2023asterdependentnonvesiculartransport pages 1-3).
- Tissue highlights: Reviews note broad Aster-A expression including brain; Aster-B is highly expressed in adrenal and supports HDL→ER transport for steroidogenesis; intestine expresses Aster-B/C for dietary uptake; liver requires Aster action for systemic homeostasis (kennelly2023cholesteroltransportto pages 4-6, sandhu2018asterproteinsfacilitate pages 1-3).
Concise mechanistic model
- Aster-A (GRAMD1A) is an ER-anchored, modular sterol transporter whose GRAM domain senses accessible PM cholesterol in a PS-dependent manner, recruiting the protein to ER–PM contacts. The ASTER/VASt-like domain binds a sterol molecule and transfers it nonvesicularly into ER membranes, supplying the ER with cholesterol for sensing (SREBP-2 suppression) and esterification. Aster-A also participates, with paralogs, in exporting excess cholesterol from the Golgi to the ER at ER–Golgi contacts. Complex formation among GRAMD1A/B/C tunes recruitment and transfer efficiency (sandhu2018asterproteinsfacilitate pages 1-3, naito2019movementofaccessible pages 3-5, naito2023regulationofcellular pages 1-2, kennelly2023cholesteroltransportto pages 4-6).
Selected primary sources with URLs and publication dates
- Sandhu et al., Aster Proteins Facilitate Nonvesicular PM→ER Cholesterol Transport in Mammalian Cells. Cell, Oct 2018. URL: https://doi.org/10.1016/j.cell.2018.08.033 (sandhu2018asterproteinsfacilitate pages 1-3)
- Naito et al., Movement of accessible plasma membrane cholesterol by the GRAMD1 complex. eLife, Nov 2019. URL: https://doi.org/10.7554/elife.51401 (naito2019movementofaccessible pages 2-3, naito2019movementofaccessible pages 3-5)
- Ercan et al., Molecular basis of accessible PM cholesterol recognition by the GRAM domain of GRAMD1b. EMBO J, Feb 2021. URL: https://doi.org/10.15252/embj.2020106524 (naito2019movementofaccessible pages 22-24)
- Kennelly & Tontonoz, Cholesterol Transport to the ER. Cold Spring Harbor Perspectives in Biology, Aug 2023. URL: https://doi.org/10.1101/cshperspect.a041263 (kennelly2023cholesteroltransportto pages 4-6)
- Naito et al., Regulation of cellular cholesterol distribution via non-vesicular transport at ER–Golgi contacts. Nature Communications, Sep 2023. URL: https://doi.org/10.1038/s41467-023-41213-w (naito2023regulationofcellular pages 1-2)
- Ferrari et al., Aster-dependent nonvesicular transport facilitates dietary cholesterol uptake. Science, Nov 2023. URL: https://doi.org/10.1126/science.adf0966 (ferrari2023asterdependentnonvesiculartransport pages 1-3)
- Banerjee et al., The nonvesicular sterol transporter Aster-C plays a minor role in whole body cholesterol balance. Frontiers in Physiology, Apr 2024. URL: https://doi.org/10.3389/fphys.2024.1371096 (ferrari2023asterdependentnonvesiculartransport pages 1-3)
Conclusion
Human GRAMD1A (Aster-A) is a core ER sterol transporter that senses accessible PM cholesterol via its GRAM domain and transfers cholesterol to the ER via its VASt/StART-like domain at ER–PM and ER–Golgi contacts. This activity couples membrane cholesterol status to ER-based sensing (SREBP-2), esterification, and organismal lipid homeostasis. Recent work in 2023–2024 extends Aster function to intestinal dietary cholesterol uptake (with pharmacologic tractability) and to Golgi cholesterol export, while genetic redundancy among paralogs modulates tissue-level phenotypes (sandhu2018asterproteinsfacilitate pages 1-3, naito2019movementofaccessible pages 3-5, naito2023regulationofcellular pages 1-2, ferrari2023asterdependentnonvesiculartransport pages 1-3, kennelly2023cholesteroltransportto pages 4-6).
References
(sandhu2018asterproteinsfacilitate pages 1-3): Jaspreet Sandhu, Shiqian Li, Louise Fairall, Simon G. Pfisterer, Jennifer E. Gurnett, Xu Xiao, Thomas A. Weston, Dipti Vashi, Alessandra Ferrari, Jose L. Orozco, Celine L. Hartman, David Strugatsky, Stephen D. Lee, Cuiwen He, Cynthia Hong, Haibo Jiang, Laurent A. Bentolila, Alberto T. Gatta, Tim P. Levine, Annie Ferng, Richard Lee, David A. Ford, Stephen G. Young, Elina Ikonen, John W.R. Schwabe, and Peter Tontonoz. Aster proteins facilitate nonvesicular plasma membrane to er cholesterol transport in mammalian cells. Cell, 175:514-529.e20, Oct 2018. URL: https://doi.org/10.1016/j.cell.2018.08.033, doi:10.1016/j.cell.2018.08.033. This article has 299 citations and is from a highest quality peer-reviewed journal.
(naito2019movementofaccessible pages 2-3): Tomoki Naito, Bilge Ercan, Logesvaran Krshnan, Alexander Triebl, Dylan Hong Zheng Koh, Fan-Yan Wei, Kazuhito Tomizawa, Federico Tesio Torta, Markus R Wenk, and Yasunori Saheki. Movement of accessible plasma membrane cholesterol by the gramd1 lipid transfer protein complex. eLife, Nov 2019. URL: https://doi.org/10.7554/elife.51401, doi:10.7554/elife.51401. This article has 174 citations and is from a domain leading peer-reviewed journal.
(kennelly2023cholesteroltransportto pages 4-6): John P. Kennelly and Peter Tontonoz. Cholesterol transport to the endoplasmic reticulum. Cold Spring Harbor perspectives in biology, 15:a041263, Aug 2023. URL: https://doi.org/10.1101/cshperspect.a041263, doi:10.1101/cshperspect.a041263. This article has 23 citations and is from a peer-reviewed journal.
(naito2019movementofaccessible pages 22-24): Tomoki Naito, Bilge Ercan, Logesvaran Krshnan, Alexander Triebl, Dylan Hong Zheng Koh, Fan-Yan Wei, Kazuhito Tomizawa, Federico Tesio Torta, Markus R Wenk, and Yasunori Saheki. Movement of accessible plasma membrane cholesterol by the gramd1 lipid transfer protein complex. eLife, Nov 2019. URL: https://doi.org/10.7554/elife.51401, doi:10.7554/elife.51401. This article has 174 citations and is from a domain leading peer-reviewed journal.
(sandhu2018theasterproteins pages 41-45): JS Sandhu. The aster proteins: key mediators of plasma membrane to er cholesterol transport. Unknown journal, 2018.
(naito2019movementofaccessible pages 3-5): Tomoki Naito, Bilge Ercan, Logesvaran Krshnan, Alexander Triebl, Dylan Hong Zheng Koh, Fan-Yan Wei, Kazuhito Tomizawa, Federico Tesio Torta, Markus R Wenk, and Yasunori Saheki. Movement of accessible plasma membrane cholesterol by the gramd1 lipid transfer protein complex. eLife, Nov 2019. URL: https://doi.org/10.7554/elife.51401, doi:10.7554/elife.51401. This article has 174 citations and is from a domain leading peer-reviewed journal.
(naito2023regulationofcellular pages 1-2): Tomoki Naito, Haoning Yang, Dylan Hong Zheng Koh, Divyanshu Mahajan, Lei Lu, and Yasunori Saheki. Regulation of cellular cholesterol distribution via non-vesicular lipid transport at er-golgi contact sites. Nature Communications, Sep 2023. URL: https://doi.org/10.1038/s41467-023-41213-w, doi:10.1038/s41467-023-41213-w. This article has 42 citations and is from a highest quality peer-reviewed journal.
(sandhu2018theasterproteins pages 90-95): JS Sandhu. The aster proteins: key mediators of plasma membrane to er cholesterol transport. Unknown journal, 2018.
(ferrari2023asterdependentnonvesiculartransport pages 1-3): Alessandra Ferrari, Emily Whang, Xu Xiao, John P. Kennelly, Beatriz Romartinez-Alonso, Julia J. Mack, Thomas Weston, Kai Chen, Youngjae Kim, Marcus J. Tol, Lara Bideyan, Alexander Nguyen, Yajing Gao, Liujuan Cui, Alexander H. Bedard, Jaspreet Sandhu, Stephen D. Lee, Louise Fairall, Kevin J. Williams, Wenxin Song, Priscilla Munguia, Robert A. Russell, Martin G. Martin, Michael E. Jung, Haibo Jiang, John W. R. Schwabe, Stephen G. Young, and Peter Tontonoz. Aster-dependent nonvesicular transport facilitates dietary cholesterol uptake. Science, Nov 2023. URL: https://doi.org/10.1126/science.adf0966, doi:10.1126/science.adf0966. This article has 46 citations and is from a highest quality peer-reviewed journal.
(sandhu2018theasterproteins pages 45-49): JS Sandhu. The aster proteins: key mediators of plasma membrane to er cholesterol transport. Unknown journal, 2018.
id: Q96CP6
gene_symbol: GRAMD1A
product_type: PROTEIN
status: COMPLETE
taxon:
id: NCBITaxon:9606
label: Homo sapiens
description: >-
GRAMD1A (also known as Aster-A) encodes an ER-anchored sterol transfer protein that
mediates
nonvesicular transport of accessible cholesterol from the plasma membrane (PM) to
the endoplasmic
reticulum (ER) at ER-PM membrane contact sites. The protein contains an N-terminal
GRAM domain
that functions as a coincidence detector for accessible PM cholesterol and anionic
phospholipids
(particularly phosphatidylserine), a central VASt/StART-like (ASTER) sterol-binding
domain that
binds and transfers cholesterol, and a C-terminal single-pass ER transmembrane helix
for ER
anchoring. GRAMD1A is recruited to ER-PM contact sites upon increases in accessible
PM
cholesterol and transfers this cholesterol to the ER, thereby supplying the ER with
cholesterol
for SREBP-2 feedback suppression and esterification by ACAT. Recent work also demonstrates
GRAMD1A functions at ER-Golgi contacts to export excess Golgi cholesterol to the
ER. GRAMD1A
forms homo- and heteromeric complexes with paralogs GRAMD1B and GRAMD1C. It is broadly
expressed
with notable brain expression.
existing_annotations:
- term:
id: GO:0005886
label: plasma membrane
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
GRAMD1A localizes to ER-PM membrane contact sites where its GRAM domain binds
the plasma
membrane in response to elevated accessible cholesterol. PMID:29469807 demonstrates
that
GRAMD1a-eGFP localizes to ER-PM contact sites in human cells, with the GRAM
domain mediating
PM association. The annotation is appropriate as the protein functionally
contacts the PM
at membrane contact sites.
action: ACCEPT
reason: >-
GRAMD1A is an ER-anchored protein that localizes to ER-PM contact sites. The
IBA annotation
is phylogenetically supported and consistent with experimental data showing
GRAMD1a
recruitment to PM upon cholesterol loading [PMID:29469807]. While not a PM-resident
protein
per se, it does localize to PM-contacting sites and binds PM lipids via its
GRAM domain.
supported_by:
- reference_id: PMID:29469807
supporting_text: >-
Our analysis of the human family members, GRAMD1a and GRAMD2a, demonstrates
that they are
ER-PM MCS proteins, which mark separate regions of the plasma membrane
(PM) and perform
distinct functions in vivo.
- reference_id: file:human/GRAMD1A/GRAMD1A-deep-research-falcon.md
supporting_text: 'model: Edison Scientific Literature'
- term:
id: GO:0005789
label: endoplasmic reticulum membrane
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
GRAMD1A is an ER membrane integral protein anchored via its C-terminal single-pass
transmembrane helix. This is the primary steady-state localization of the
protein.
action: ACCEPT
reason: >-
Core localization well-supported by multiple lines of evidence including phylogenetic
inference (IBA), UniProt annotation, and experimental data [PMID:29469807].
The C-terminal
transmembrane helix (residues 610-630) anchors GRAMD1A in the ER membrane.
supported_by:
- reference_id: PMID:29469807
supporting_text: >-
GRAMD1a-eGFP with mCherry-Sec61beta and lyn-mCherry Z-stack, sample images
are displayed in Figure 1C
- term:
id: GO:0015485
label: cholesterol binding
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
GRAMD1A binds cholesterol through its VASt/StART-like (ASTER) domain. The
domain contains
a hydrophobic cavity that binds one sterol molecule and mediates intermembrane
sterol
transfer. This is a core molecular function of the protein.
action: ACCEPT
reason: >-
Cholesterol binding is the primary molecular function of GRAMD1A, mediated
by its
VASt/StART-like domain. The IBA annotation is phylogenetically robust and
consistent with
structural and biochemical studies on the Aster family [PMID:30220461].
supported_by:
- reference_id: PMID:30220461
supporting_text: >-
three ER-resident proteins (Aster-A, -B, -C) that bind cholesterol and
facilitate its removal from the plasma membrane
- term:
id: GO:0032366
label: intracellular sterol transport
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
GRAMD1A mediates nonvesicular transport of cholesterol from PM to ER. This
is the core
biological process annotation for the gene.
action: ACCEPT
reason: >-
Intracellular sterol transport is the primary biological function of GRAMD1A.
The IBA
annotation captures the core function accurately. More specific child term
GO:0032367
(intracellular cholesterol transport) would also be appropriate.
supported_by:
- reference_id: PMID:31724953
supporting_text: >-
endoplasmic reticulum (ER)-anchored lipid transfer proteins, the GRAMD1s,
sense and transport accessible PM cholesterol to the ER
- term:
id: GO:0120020
label: cholesterol transfer activity
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
GRAMD1A possesses cholesterol transfer activity mediated by its VASt/StART-like
domain,
enabling nonvesicular transfer of cholesterol between membranes at contact
sites.
action: ACCEPT
reason: >-
Cholesterol transfer activity is the core molecular function of GRAMD1A. The
IBA annotation
is accurate and represents the enzymatic/transporter activity of the protein.
supported_by:
- reference_id: PMID:30220461
supporting_text: >-
Aster Proteins Facilitate Nonvesicular Plasma Membrane to ER Cholesterol
Transport in Mammalian Cells
- term:
id: GO:0140268
label: endoplasmic reticulum-plasma membrane contact site
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: >-
GRAMD1A localizes to ER-PM contact sites where it functions in cholesterol
transport. The
GRAM domain senses accessible PM cholesterol and recruits the protein to ER-PM
contacts.
action: ACCEPT
reason: >-
ER-PM contact site localization is central to GRAMD1A function. The protein
is recruited
to these sites upon increases in accessible PM cholesterol, where it mediates
sterol
transfer. This is a core cellular component annotation.
supported_by:
- reference_id: PMID:29469807
supporting_text: >-
GRAMD1a marks distinct ER-PM contacts in a PI(4,5)P2-independent manner
- term:
id: GO:0005776
label: autophagosome
evidence_type: IEA
original_reference_id: GO_REF:0000044
review:
summary: >-
This annotation derives from UniProt subcellular location vocabulary mapping
based on
PMID:31222192 which reports GRAMD1A accumulates at sites of autophagosome
initiation.
However, GRAMD1A is not an autophagosome-resident protein; rather its cholesterol
transfer
function affects autophagosome biogenesis. This is an over-annotation that
conflates
functional effect with true localization.
action: REMOVE
reason: >-
The annotation to autophagosome as a cellular component is misleading. GRAMD1A
is an
ER-anchored protein that transiently localizes to ER-PM and ER-Golgi contact
sites.
PMID:31222192 shows GRAMD1A affects autophagosome biogenesis through its cholesterol
transfer function, but this does not mean GRAMD1A is a constituent of autophagosomes.
The core function of GRAMD1A is nonvesicular cholesterol transport at membrane
contact
sites, and any autophagy effects are downstream consequences of cholesterol
redistribution.
- term:
id: GO:0005789
label: endoplasmic reticulum membrane
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: >-
Duplicate annotation for ER membrane localization via automated combined IEA
methods.
This is consistent with the IBA annotation and experimental evidence.
action: ACCEPT
reason: >-
Correct localization. Duplicate of the IBA annotation but acceptable as it
represents
independent computational evidence supporting ER membrane localization.
- term:
id: GO:0005886
label: plasma membrane
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: >-
Duplicate annotation for PM association via automated combined IEA methods.
GRAMD1A
contacts the PM at ER-PM membrane contact sites through its GRAM domain.
action: ACCEPT
reason: >-
While GRAMD1A is not a PM-resident protein, it does functionally associate
with the PM
at membrane contact sites. This annotation is acceptable as it captures the
PM-binding
function via the GRAM domain.
- term:
id: GO:0006869
label: lipid transport
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: >-
Broad parent term annotation derived from UniProt keyword mapping. GRAMD1A
does function
in lipid transport, specifically sterol/cholesterol transport.
action: ACCEPT
reason: >-
This is a correct but overly broad annotation. GRAMD1A specifically transports
sterols/
cholesterol. The more specific child terms (intracellular sterol transport,
intracellular
cholesterol transport) are preferable, but this parent term annotation is
not incorrect.
- term:
id: GO:0006914
label: autophagy
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: >-
This annotation derives from UniProt keyword mapping based on PMID:31222192.
While that
paper reports GRAMD1A affects autophagosome biogenesis, this is a secondary
consequence
of GRAMD1A's cholesterol transport function, not its primary role. Autophagy
is not the
core function of GRAMD1A.
action: MARK_AS_OVER_ANNOTATED
reason: >-
GRAMD1A's primary function is nonvesicular cholesterol transport at ER-PM
and ER-Golgi
membrane contact sites. The autophagy connection reported in PMID:31222192
appears to
be a downstream consequence of cholesterol redistribution affecting autophagosome
biogenesis, rather than a direct role in autophagy machinery. This annotation
risks
misrepresenting the gene's function. The core Aster family papers [PMID:30220461,
PMID:31724953, PMID:37735529] make no mention of autophagy - they focus entirely
on
cholesterol transport and SREBP regulation. This annotation should be reconsidered
or at minimum marked as non-core.
- term:
id: GO:0008289
label: lipid binding
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: >-
Broad parent term annotation for lipid binding derived from UniProt keyword
mapping.
GRAMD1A binds cholesterol via its VASt domain and phosphatidylserine/anionic
lipids
via its GRAM domain.
action: ACCEPT
reason: >-
Correct but broad. GRAMD1A does bind lipids - specifically cholesterol (VASt
domain)
and phosphatidylserine (GRAM domain). More specific terms like cholesterol
binding
(GO:0015485) are preferable, but this parent term is accurate.
- term:
id: GO:0031410
label: cytoplasmic vesicle
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: >-
This annotation derives from UniProt keyword mapping, likely from the autophagosome
connection. However, GRAMD1A is not a vesicular protein - it mediates nonvesicular
cholesterol transport at membrane contact sites.
action: REMOVE
reason: >-
This annotation is misleading. GRAMD1A is an ER-anchored membrane contact
site protein
that specifically functions in NONvesicular cholesterol transport. It is not
associated
with cytoplasmic vesicles in its normal function. This annotation likely derives
from
the autophagosome keyword, which itself is an over-annotation.
- term:
id: GO:0044232
label: organelle membrane contact site
evidence_type: IEA
original_reference_id: GO_REF:0000117
review:
summary: >-
GRAMD1A localizes to organelle membrane contact sites, specifically ER-PM
and ER-Golgi
contact sites, where it functions in cholesterol transport.
action: ACCEPT
reason: >-
Accurate annotation. GRAMD1A functions at membrane contact sites. This parent
term
encompasses both ER-PM contact sites (primary) and ER-Golgi contact sites
(recently
established role). The more specific term GO:0140268 is also annotated.
supported_by:
- reference_id: PMID:37735529
supporting_text: >-
GRAMD1s transport excess cholesterol from the Golgi to the ER, thereby
preventing its build-up
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:25416956
review:
summary: >-
High-throughput interactome study detecting binary protein-protein interactions.
The term
'protein binding' is too generic to be informative about molecular function.
action: REMOVE
reason: >-
Per curation guidelines, 'protein binding' does not convey meaningful functional
information. GRAMD1A does interact with proteins (forms complexes with GRAMD1B/C,
interacts
with MAL per IntAct), but the generic protein binding term should be avoided
in favor of
more specific interaction terms where possible. This high-throughput data
does not
illuminate the specific functional context of interactions.
supported_by:
- reference_id: PMID:25416956
supporting_text: A proteome-scale map of the human interactome
network.
- term:
id: GO:0005515
label: protein binding
evidence_type: IPI
original_reference_id: PMID:32296183
review:
summary: >-
Duplicate protein binding annotation from a second high-throughput interactome
study (HuRI).
action: REMOVE
reason: >-
Same reasoning as above - 'protein binding' is too vague to be informative.
GRAMD1A does
form homo- and heteromeric complexes with GRAMD1B/C via transmembrane and
luminal regions,
but this generic annotation does not capture that specificity.
supported_by:
- reference_id: PMID:32296183
supporting_text: Apr 8. A reference map of the human binary protein
interactome.
- term:
id: GO:0015485
label: cholesterol binding
evidence_type: IEA
original_reference_id: GO_REF:0000107
review:
summary: >-
Automated transfer annotation for cholesterol binding via Ensembl Compara.
Consistent
with other annotations and the known function of the VASt/ASTER domain.
action: ACCEPT
reason: >-
Correct annotation. Duplicate evidence for cholesterol binding, which is a
core
molecular function of GRAMD1A mediated by its VASt domain.
- term:
id: GO:0032367
label: intracellular cholesterol transport
evidence_type: IEA
original_reference_id: GO_REF:0000107
review:
summary: >-
More specific than GO:0032366 (intracellular sterol transport). GRAMD1A specifically
transports cholesterol from PM to ER and from Golgi to ER.
action: ACCEPT
reason: >-
Excellent annotation - more specific than the parent term and accurately captures
the
core biological process of GRAMD1A. This is the primary function of the protein.
- term:
id: GO:0120020
label: cholesterol transfer activity
evidence_type: IEA
original_reference_id: GO_REF:0000107
review:
summary: >-
Automated transfer annotation for cholesterol transfer activity. Consistent
with
IBA annotation and biochemical evidence.
action: ACCEPT
reason: >-
Correct core molecular function annotation. Duplicate evidence supporting
the
cholesterol transfer activity of the VASt domain.
- term:
id: GO:0140268
label: endoplasmic reticulum-plasma membrane contact site
evidence_type: IEA
original_reference_id: GO_REF:0000107
review:
summary: >-
Automated transfer annotation for ER-PM contact site localization. Consistent
with
IBA annotation and experimental evidence.
action: ACCEPT
reason: >-
Correct core cellular component annotation. ER-PM contact site localization
is
central to GRAMD1A function.
- term:
id: GO:0032367
label: intracellular cholesterol transport
evidence_type: ISS
original_reference_id: GO_REF:0000024
review:
summary: >-
Manual transfer annotation based on sequence similarity. This is the core
biological
process of GRAMD1A.
action: ACCEPT
reason: >-
Correct and specific biological process annotation. Intracellular cholesterol
transport (PM to ER, Golgi to ER) is the primary function of GRAMD1A.
- term:
id: GO:0005886
label: plasma membrane
evidence_type: IDA
original_reference_id: GO_REF:0000052
review:
summary: >-
IDA annotation based on curation of immunofluorescence data showing PM localization.
action: ACCEPT
reason: >-
Experimental evidence supporting PM association. GRAMD1A contacts the PM at
ER-PM
membrane contact sites via its GRAM domain.
- term:
id: GO:0005886
label: plasma membrane
evidence_type: IDA
original_reference_id: PMID:29469807
review:
summary: >-
Direct experimental evidence from Besprozvannaya et al. 2018 showing GRAMD1a
localizes
to ER-PM contact sites and contacts the plasma membrane.
action: ACCEPT
reason: >-
High-quality experimental evidence. PMID:29469807 demonstrates that GRAMD1a-eGFP
co-localizes with PM markers at ER-PM contact sites.
supported_by:
- reference_id: PMID:29469807
supporting_text: >-
GRAMD1a and GRAMD2a localize to ER-PM contact sites
- term:
id: GO:0015485
label: cholesterol binding
evidence_type: ISS
original_reference_id: GO_REF:0000024
review:
summary: >-
Manual sequence similarity transfer annotation for cholesterol binding. The
VASt
domain is well-characterized as a sterol-binding domain.
action: ACCEPT
reason: >-
Correct molecular function annotation with strong evolutionary support. The
VASt
domain is conserved and structurally characterized for sterol binding.
- term:
id: GO:0120020
label: cholesterol transfer activity
evidence_type: ISS
original_reference_id: GO_REF:0000024
review:
summary: >-
Manual sequence similarity transfer annotation for cholesterol transfer activity.
action: ACCEPT
reason: >-
Correct core molecular function annotation. The Aster family is defined by
its
cholesterol transfer activity mediated by the VASt domain.
- term:
id: GO:0140268
label: endoplasmic reticulum-plasma membrane contact site
evidence_type: ISS
original_reference_id: GO_REF:0000024
review:
summary: >-
Manual sequence similarity transfer annotation for ER-PM contact site localization.
action: ACCEPT
reason: >-
Correct core cellular component annotation. ER-PM MCS localization is conserved
across the Aster/GRAMD1 family.
- term:
id: GO:0005789
label: endoplasmic reticulum membrane
evidence_type: IDA
original_reference_id: PMID:29469807
review:
summary: >-
Direct experimental evidence from Besprozvannaya et al. 2018 showing GRAMD1a
co-localizes with ER marker Sec61beta.
action: ACCEPT
reason: >-
High-quality experimental evidence demonstrating ER membrane localization.
supported_by:
- reference_id: PMID:29469807
supporting_text: >-
GRAMD1a-eGFP with mCherry-Sec61beta and lyn-mCherry Z-stack, sample images
are displayed in Figure 1C
- term:
id: GO:0044232
label: organelle membrane contact site
evidence_type: IDA
original_reference_id: PMID:29469807
review:
summary: >-
Direct experimental evidence showing GRAMD1a localizes to membrane contact
sites
(specifically ER-PM MCS) in human cells.
action: ACCEPT
reason: >-
High-quality experimental evidence. PMID:29469807 is a key paper establishing
GRAMD1a as an ER-PM membrane contact site protein.
supported_by:
- reference_id: PMID:29469807
supporting_text: >-
Our analysis of the human family members, GRAMD1a and GRAMD2a, demonstrates
that they are ER-PM MCS proteins, which mark separate regions of the plasma
membrane (PM) and perform distinct functions in vivo
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:0000052
title: Gene Ontology annotation based on curation of immunofluorescence data
findings: []
- id: GO_REF:0000107
title: Automatic transfer of experimentally verified manual GO annotation
data to orthologs using Ensembl Compara
findings: []
- id: GO_REF:0000117
title: Electronic Gene Ontology annotations created by ARBA machine learning
models
findings: []
- id: GO_REF:0000120
title: Combined Automated Annotation using Multiple IEA Methods
findings: []
- id: PMID:25416956
title: A proteome-scale map of the human interactome network.
findings:
- statement: High-throughput binary protein-protein interaction screen
- id: PMID:29469807
title: GRAM domain proteins specialize functionally distinct ER-PM contact
sites in human cells.
findings:
- statement: GRAMD1a and GRAMD2a localize to distinct ER-PM contact sites
supporting_text: >-
Our analysis of the human family members, GRAMD1a and GRAMD2a, demonstrates
that they are ER-PM MCS proteins, which mark separate regions of the plasma
membrane (PM) and perform distinct functions in vivo
- statement: GRAMD1a localizes to ER-PM contacts in a
PI(4,5)P2-independent manner
supporting_text: >-
GRAMD1a marks distinct ER-PM contacts in a PI(4,5)P2-independent manner
- statement: GRAM domain mediates PM association
supporting_text: >-
GRAM domain-dependent targeting of GRAMD1a and GRAMD2a to ER-PM MCSs
- statement: GRAMD1a co-localizes with ER marker Sec61beta
supporting_text: >-
GRAMD1a-eGFP with mCherry-Sec61beta and lyn-mCherry Z-stack, sample images
are displayed in Figure 1C
- id: PMID:30220461
title: Aster proteins facilitate nonvesicular plasma membrane to ER
cholesterol transport in mammalian cells.
findings:
- statement: Established Aster nomenclature (GRAMD1A = Aster-A)
supporting_text: >-
three ER-resident proteins (Aster-A, -B, -C) that bind cholesterol and facilitate
its removal from the plasma membrane
- statement: VASt domain binds cholesterol
supporting_text: >-
three ER-resident proteins (Aster-A, -B, -C) that bind cholesterol
- statement: Asters mediate PM to ER cholesterol transport
supporting_text: >-
Aster Proteins Facilitate Nonvesicular Plasma Membrane to ER Cholesterol
Transport in Mammalian Cells
- statement: GRAM domain senses accessible cholesterol
supporting_text: >-
Aster N-terminal GRAM domain binds phosphatidylserine and mediates Aster
recruitment to plasma membrane-ER contact sites in response to cholesterol
accumulation in the plasma membrane
- id: PMID:31222192
title: The cholesterol transfer protein GRAMD1A regulates autophagosome
biogenesis.
findings:
- statement: Reports GRAMD1A affects autophagosome biogenesis
- statement: Effect is mediated through cholesterol transfer activity
- statement: Represents secondary function, not core role
- id: PMID:31724953
title: Movement of accessible plasma membrane cholesterol by the GRAMD1
lipid transfer protein complex.
findings:
- statement: GRAMD1 proteins move accessible PM cholesterol to ER
supporting_text: >-
endoplasmic reticulum (ER)-anchored lipid transfer proteins, the GRAMD1s,
sense and transport accessible PM cholesterol to the ER
- statement: Transfer suppresses SREBP-2 activation
supporting_text: >-
Cells that lack all three GRAMD1s exhibit striking expansion of the accessible
pool of PM cholesterol as a result of less efficient PM to ER transport
of accessible cholesterol
- statement: GRAM domain is coincidence detector for cholesterol and PS
supporting_text: >-
GRAMD1s bind to one another and populate ER-PM contacts by sensing a transient
expansion of the accessible pool of PM cholesterol via their GRAM domains
- statement: GRAMD1A forms complexes with GRAMD1B/C
supporting_text: >-
GRAMD1s bind to one another and populate ER-PM contacts
- id: PMID:32296183
title: A reference map of the human binary protein interactome.
findings:
- statement: HuRI interactome study; high-throughput PPI screen
- id: PMID:37735529
title: Regulation of cellular cholesterol distribution via non-vesicular
lipid transport at ER-Golgi contact sites.
findings:
- statement: GRAMD1s also function at ER-Golgi contacts
supporting_text: >-
GRAMD1s transport excess cholesterol from the Golgi to the ER, thereby preventing
its build-up
- statement: Export excess Golgi cholesterol to ER
supporting_text: >-
GRAMD1s transport excess cholesterol from the Golgi to the ER
- statement: Prevents chronic SREBP-2 activation
supporting_text: >-
This is accompanied by chronic activation of the SREBP-2 signalling pathway
- id: file:human/GRAMD1A/GRAMD1A-deep-research-falcon.md
title: Deep research report on GRAMD1A
findings: []
core_functions:
- description: >-
Nonvesicular cholesterol transfer from plasma membrane to endoplasmic reticulum
at ER-PM membrane contact sites
molecular_function:
id: GO:0120020
label: cholesterol transfer activity
directly_involved_in:
- id: GO:0032367
label: intracellular cholesterol transport
locations:
- id: GO:0140268
label: endoplasmic reticulum-plasma membrane contact site
- description: >-
Cholesterol binding via VASt/StART-like domain enabling sterol sensing and transfer
molecular_function:
id: GO:0015485
label: cholesterol binding
locations:
- id: GO:0005789
label: endoplasmic reticulum membrane
proposed_new_terms: []
suggested_questions:
- question: What is the relative contribution of GRAMD1A vs GRAMD1B/C to
cholesterol transport in different tissues?
- question: Does GRAMD1A have tissue-specific functions beyond cholesterol
transport?
- question: What regulates GRAMD1A expression and activity?
suggested_experiments:
- description: Tissue-specific knockout studies to determine physiological
roles in different organs
hypothesis: GRAMD1A may have tissue-specific roles distinct from GRAMD1B/C
- description: Structural studies of full-length GRAMD1A to understand
conformational changes during cholesterol transfer
hypothesis: Cholesterol binding may induce conformational changes that
facilitate transfer
- description: Investigation of post-translational modifications regulating
GRAMD1A activity
hypothesis: Phosphorylation or other PTMs may regulate GRAMD1A recruitment
to membrane contact sites