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 |
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Download this section (compressed HTML)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
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