AUP1 (Ancient Ubiquitous Protein 1) is a conserved, ubiquitously expressed monotopic membrane protein that resides on the cytosolic face of the endoplasmic reticulum membrane and on the lipid droplet surface monolayer, inserting via an N-terminal hydrophobic hairpin so that both termini face the cytosol. Its principal molecular role is to act as an adaptor that recruits the soluble E2 ubiquitin-conjugating enzyme UBE2G2 (via a C-terminal G2 binding region) to ER-associated degradation (ERAD) ubiquitin ligase machinery, including the HRD1-SEL1L complex and the lipid-droplet/ER E3 ligases gp78/AMFR and TRC8/RNF139. Through this activity AUP1 couples substrate ubiquitination to dislocation and proteasomal degradation of misfolded ER proteins, and drives sterol-regulated ubiquitination and ERAD of HMGCR, INSIG1, SREBF1 and SREBF2. AUP1 also has lipid-droplet-regulatory functions: it binds ubiquitin through an internal CUE domain, is itself monoubiquitinated in a CUE-dependent manner to promote lipid droplet clustering, and its level controls lipid droplet abundance. Additional reported roles include hepatic VLDL/apolipoprotein B assembly and secretion. AUP1 is exploited by flaviviruses, whose NS4A protein relocalizes it to autophagosomes to trigger lipophagy.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005789 endoplasmic reticulum membrane | IBA GO_REF:0000033 | ACCEPT | Summary: ER membrane is the principal compartment where AUP1 acts as an ERAD E2 adaptor; well supported by orthology and direct experiments. Reason: AUP1 is a monotopic ER membrane protein with both termini in the cytosol, where it recruits UBE2G2 to the HRD1-SEL1L/gp78 ERAD machinery. This is a core localization. Supporting Evidence: PMID:23197321 AUP1 is inserted into the membrane of the ER in a monotopic |
| GO:0036503 ERAD pathway | IBA GO_REF:0000033 | ACCEPT | Summary: ERAD participation is the core biological process of AUP1, supported by direct loss-of-function evidence and orthology. Reason: AUP1 associates with the HRD1-SEL1L complex and recruits UBE2G2; depletion impairs degradation of misfolded ER proteins. Supporting Evidence: PMID:21857022 AUP1) physically associates with |
| GO:0005776 autophagosome | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Autophagosome localization is only seen upon Dengue/flavivirus infection, when NS4A relocalizes AUP1 from lipid droplets to autophagosomes; it is not a constitutive localization of the normal protein. Reason: This is a context-dependent (virus-induced) localization rather than the core ER/lipid-droplet residence of AUP1. Supporting Evidence: file:human/AUP1/AUP1-uniprot.txt virus infection, relocates from lipid droplets to autophagosomes. |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: ER membrane is the principal site of AUP1 ERAD activity; well supported. Reason: AUP1 is a monotopic ER membrane protein where it recruits UBE2G2 to the ERAD machinery. Supporting Evidence: PMID:23197321 AUP1 is inserted into the membrane of the ER in a monotopic |
| GO:0005811 lipid droplet | IEA GO_REF:0000044 | ACCEPT | Summary: Lipid droplet surface localization is a core, directly demonstrated localization for AUP1. Reason: AUP1 integrates into the LD surface monolayer in a monotopic fashion with both termini in the cytosol. Supporting Evidence: PMID:21127063 integrates into the LD surface in a monotopic fashion with both termini facing |
| GO:0036503 ERAD pathway | IEA GO_REF:0000002 | ACCEPT | Summary: ERAD participation is the core biological process of AUP1. Reason: AUP1 associates with the HRD1-SEL1L complex and recruits UBE2G2; depletion impairs degradation of misfolded ER proteins. Supporting Evidence: PMID:21857022 AUP1) physically associates with |
| GO:0043130 ubiquitin binding | IEA GO_REF:0000002 | ACCEPT | Summary: AUP1 binds ubiquitin through its internal CUE domain, a known ubiquitin-binding domain; this is a core molecular function underlying its ERAD and LD-clustering roles. Reason: The CUE domain is directly demonstrated as a ubiquitin-binding domain required for ubiquitination and LD clustering. Supporting Evidence: PMID:24039768 its internal CUE domain, which is a known ubiquitin-binding |
| GO:0005515 protein binding | IPI PMID:22119785 Defining human ERAD networks through an integrative mapping ... | MARK AS OVER ANNOTATED | Summary: Generic protein-binding annotation from a proteome-scale ERAD interaction map; uninformative as a molecular function term. Reason: Bare protein binding does not convey AUP1's specific adaptor function; more informative terms (ubiquitin conjugating enzyme binding) are captured elsewhere. |
| GO:0005515 protein binding | IPI PMID:23840749 MOR is not enough: identification of novel mu-opioid recepto... | MARK AS OVER ANNOTATED | Summary: Generic protein binding from a mu-opioid receptor membrane yeast two-hybrid interactome; uninformative. Reason: Bare protein binding from a high-throughput screen does not identify a physiologically interpretable AUP1 function. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: Generic protein binding from a binary interactome reference map; uninformative. Reason: Bare protein binding is too general to represent AUP1 function. |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:12042322 Ancient ubiquitous protein 1 binds to the conserved membrane... | ACCEPT | Summary: Early experimental support for AUP1 ER/membrane association; consistent with the well-established core ER membrane localization. Reason: ER membrane is a core localization for AUP1 ERAD activity. Supporting Evidence: file:human/AUP1/AUP1-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:18711132 SEL1L nucleates a protein complex required for dislocation o... | ACCEPT | Summary: AUP1 is part of the SEL1L-nucleated ER membrane ERAD complex; supports core ER membrane localization. Reason: AUP1 was identified as a functionally important component of the SEL1L-nucleated ER dislocation complex. Supporting Evidence: PMID:18711132 We identified AUP1, UBXD8, UBC6e, and OS9 as functionally important components of this degradation complex in mammalian cells |
| GO:0005789 endoplasmic reticulum membrane | EXP PMID:21857022 Dual role of ancient ubiquitous protein 1 (AUP1) in lipid dr... | ACCEPT | Summary: Direct evidence places AUP1 at the ER membrane within the HRD1-SEL1L complex. Reason: ER membrane is the core compartment for AUP1 ERAD function. Supporting Evidence: PMID:21857022 AUP1) physically associates with |
| GO:0005811 lipid droplet | EXP PMID:28183703 AUP1 (Ancient Ubiquitous Protein 1) Is a Key Determinant of ... | ACCEPT | Summary: AUP1 is demonstrated at the lipid droplet surface in hepatic VLDL studies; core localization. Reason: LD surface localization is a directly demonstrated core localization of AUP1. Supporting Evidence: PMID:28183703 also localizes to the surface of lipid |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-9921605 | ACCEPT | Summary: Reactome traceable assertion of ER membrane localization is consistent with the established core localization. Reason: ER membrane localization is well supported by direct experimental evidence. Supporting Evidence: file:human/AUP1/AUP1-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005515 protein binding | IPI PMID:29902443 Flaviviruses Exploit the Lipid Droplet Protein AUP1 to Trigg... | MARK AS OVER ANNOTATED | Summary: The underlying interaction is the specific AUP1-Dengue NS4A association, but the GO term used is generic protein binding. Reason: Bare protein binding is uninformative; the meaningful (virus-specific) interaction is with NS4A and is not a core physiological function. |
| GO:0005776 autophagosome | IDA PMID:29902443 Flaviviruses Exploit the Lipid Droplet Protein AUP1 to Trigg... | KEEP AS NON CORE | Summary: Autophagosome localization is observed only after Dengue virus infection, when NS4A relocalizes AUP1 from LDs. Reason: A virus-induced relocalization, not the constitutive site of AUP1 action. Supporting Evidence: PMID:29902443 relocalized from lipid droplets to autophagosomes upon infection |
| GO:0005811 lipid droplet | IDA PMID:29902443 Flaviviruses Exploit the Lipid Droplet Protein AUP1 to Trigg... | ACCEPT | Summary: AUP1 is directly observed as a lipid droplet-localized membrane protein; core localization. Reason: LD surface residence is the well-established core localization of AUP1. Supporting Evidence: PMID:29902443 AUP1, a lipid droplet-localized type-III membrane protein |
| GO:0009615 response to virus | IMP PMID:29902443 Flaviviruses Exploit the Lipid Droplet Protein AUP1 to Trigg... | KEEP AS NON CORE | Summary: AUP1 is exploited by flaviviruses to drive virus production; this is a host factor role hijacked by the virus rather than an antiviral/defense function of AUP1. Reason: The phenotype is virus production dependent on AUP1, a context-specific host-pathogen role, not a core conserved physiological function. Supporting Evidence: PMID:29902443 Virus production was abolished in cells deleted for AUP1 |
| GO:0061724 lipophagy | IMP PMID:29902443 Flaviviruses Exploit the Lipid Droplet Protein AUP1 to Trigg... | KEEP AS NON CORE | Summary: AUP1 is required for Dengue-induced lipophagy; demonstrated only in the virus-infection context. Reason: Lipophagy induction is a virus-triggered process exploiting AUP1's acyltransferase activity, not a core constitutive function. Supporting Evidence: PMID:29902443 exploits the acyltransferase activity of AUP1 to trigger lipophagy |
| GO:0005789 endoplasmic reticulum membrane | IDA PMID:23223569 Ancient ubiquitous protein-1 mediates sterol-induced ubiquit... | ACCEPT | Summary: AUP1 acts at lipid droplet-associated ER membranes; core localization. Reason: AUP1 functions in sterol-regulated HMGCR ubiquitination at LD-associated ER membranes. Supporting Evidence: PMID:23223569 lipid droplet-associated ER membranes |
| GO:0005811 lipid droplet | IDA PMID:23223569 Ancient ubiquitous protein-1 mediates sterol-induced ubiquit... | ACCEPT | Summary: AUP1 is a lipid droplet-associated protein recruiting Ubc7/UBE2G2 there; core localization. Reason: LD localization is directly demonstrated and required for AUP1's sterol-regulated ERAD role. Supporting Evidence: PMID:23223569 Aup1 recruits the ubiquitin-conjugating enzyme Ubc7 to |
| GO:0031624 ubiquitin conjugating enzyme binding | IPI PMID:23223569 Ancient ubiquitous protein-1 mediates sterol-induced ubiquit... | ACCEPT | Summary: AUP1 binds and recruits the E2 ubiquitin-conjugating enzyme Ubc7/UBE2G2; this is a core molecular function. Reason: AUP1 recruits Ubc7 (UBE2G2) to lipid droplets and facilitates its binding to gp78 and Trc8, central to its adaptor role. Supporting Evidence: PMID:23223569 Aup1 recruits the ubiquitin-conjugating enzyme Ubc7 to |
| GO:0031625 ubiquitin protein ligase binding | IPI PMID:23223569 Ancient ubiquitous protein-1 mediates sterol-induced ubiquit... | ACCEPT | Summary: AUP1 binds the E3 ubiquitin ligases gp78/AMFR and Trc8/RNF139, bridging them to the E2; core molecular function. Reason: AUP1 facilitates binding of Ubc7 to both gp78 and Trc8, directly demonstrating ligase association. Supporting Evidence: PMID:23223569 facilitates its binding to both gp78 and Trc8 |
| GO:0036503 ERAD pathway | IMP PMID:23223569 Ancient ubiquitous protein-1 mediates sterol-induced ubiquit... | ACCEPT | Summary: AUP1 is required for sterol-regulated ERAD of HMGCR, INSIG1, and SREBF1/2; core process. Reason: AUP1 knockdown blunts sterol-accelerated ubiquitination and ERAD of HMGCR and other sterol-pathway substrates. Supporting Evidence: PMID:23223569 RNAi-mediated knockdown of Aup1 blunts sterol-accelerated ubiquitination |
| GO:1990044 protein localization to lipid droplet | IMP PMID:23223569 Ancient ubiquitous protein-1 mediates sterol-induced ubiquit... | ACCEPT | Summary: AUP1 recruits/localizes the E2 Ubc7 to lipid droplets, supporting a role in protein localization to the LD. Reason: AUP1 recruits Ubc7 to lipid droplets, a directly demonstrated protein-targeting function at the LD. Supporting Evidence: PMID:23223569 Aup1 recruits the ubiquitin-conjugating enzyme Ubc7 to |
| GO:0005783 endoplasmic reticulum | IDA PMID:21857022 Dual role of ancient ubiquitous protein 1 (AUP1) in lipid dr... | ACCEPT | Summary: ER localization (parent of ER membrane) consistent with core ER membrane residence. Reason: AUP1 is an ER protein within the HRD1-SEL1L complex; the more specific ER membrane term is preferred but this is correct. Supporting Evidence: PMID:21857022 AUP1) physically associates with |
| GO:0005789 endoplasmic reticulum membrane | IDA PMID:21127063 Ancient ubiquitous protein 1 (AUP1) localizes to lipid dropl... | ACCEPT | Summary: Direct evidence of AUP1 at the ER membrane; core localization. Reason: ER membrane residence is a core localization for AUP1. Supporting Evidence: file:human/AUP1/AUP1-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005789 endoplasmic reticulum membrane | IDA PMID:23197321 Monotopic topology is required for lipid droplet targeting o... | ACCEPT | Summary: AUP1 inserts into the ER membrane in monotopic hairpin fashion; core localization. Reason: Directly demonstrated monotopic ER membrane insertion. Supporting Evidence: PMID:23197321 AUP1 is inserted into the membrane of the ER in a monotopic |
| GO:0005811 lipid droplet | IDA PMID:21857022 Dual role of ancient ubiquitous protein 1 (AUP1) in lipid dr... | ACCEPT | Summary: AUP1 localizes to lipid droplets and its level controls LD abundance; core localization. Reason: LD localization is directly demonstrated and central to AUP1's LD-regulatory function. Supporting Evidence: PMID:21127063 ancient ubiquitous protein 1 (AUP1) localizes to LDs |
| GO:0005811 lipid droplet | IDA PMID:23197321 Monotopic topology is required for lipid droplet targeting o... | ACCEPT | Summary: AUP1 is transported to the LD hemi-membrane in monotopic fashion; core localization. Reason: Monotopic topology is directly shown to be required for LD targeting. Supporting Evidence: PMID:23197321 subsequently transported to the hemi-membrane of LDs |
| GO:0031624 ubiquitin conjugating enzyme binding | IPI PMID:21857022 Dual role of ancient ubiquitous protein 1 (AUP1) in lipid dr... | ACCEPT | Summary: AUP1 recruits the soluble E2 UBE2G2 in ER quality control; core molecular function. Reason: One of AUP1's functions in ER quality control is to recruit the soluble E2 ubiquitin-conjugating enzyme UBE2G2. Supporting Evidence: PMID:21127063 binds to Ube2g2 |
| GO:0036503 ERAD pathway | IMP PMID:21857022 Dual role of ancient ubiquitous protein 1 (AUP1) in lipid dr... | ACCEPT | Summary: AUP1 depletion impairs degradation of misfolded ER proteins; core process. Reason: Loss-of-function evidence directly supports AUP1 in ERAD. Supporting Evidence: PMID:21857022 AUP1) physically associates with |
| GO:0140042 lipid droplet formation | IMP PMID:21857022 Dual role of ancient ubiquitous protein 1 (AUP1) in lipid dr... | KEEP AS NON CORE | Summary: AUP1 expression level affects the abundance of cellular lipid droplets; a core LD-regulatory function, though regulation of abundance/clustering is the better-supported framing than de novo formation. Reason: The data show AUP1 controls LD abundance/clustering rather than directly catalyzing LD biogenesis; retained as a supported but secondary LD-regulatory role. Supporting Evidence: PMID:21857022 AUP1) in lipid droplet accumulation |
| GO:0005811 lipid droplet | IDA PMID:21127063 Ancient ubiquitous protein 1 (AUP1) localizes to lipid dropl... | ACCEPT | Summary: AUP1 integrates into the LD surface monolayer; core localization. Reason: Directly demonstrated monotopic LD surface localization. Supporting Evidence: PMID:21127063 integrates into the LD surface in a monotopic fashion with both termini facing |
| GO:0031624 ubiquitin conjugating enzyme binding | IPI PMID:21127063 Ancient ubiquitous protein 1 (AUP1) localizes to lipid dropl... | ACCEPT | Summary: AUP1 binds the E2 UBE2G2 via its C-terminal G2BR; core molecular function. Reason: AUP1, by means of its G2BR domain, binds Ube2g2, providing a direct molecular link to the ubiquitination machinery. Supporting Evidence: PMID:21127063 by means of its G2BR |
| GO:0034389 lipid droplet organization | IDA PMID:24039768 Monoubiquitination of ancient ubiquitous protein 1 promotes ... | ACCEPT | Summary: Monoubiquitinated AUP1 on the LD surface induces lipid droplet clustering; a core LD-regulatory function. Reason: AUP1 induces LD cluster formation in a CUE/monoubiquitination-dependent manner. Supporting Evidence: PMID:24039768 the lipid droplet protein AUP1 induces cluster formation |
| GO:1990044 protein localization to lipid droplet | IDA PMID:21127063 Ancient ubiquitous protein 1 (AUP1) localizes to lipid dropl... | ACCEPT | Summary: AUP1 brings the AUP1-Ube2g2 complex to lipid droplets, localizing the E2 to the LD; supported. Reason: AUP1 provides the molecular link recruiting Ube2g2 to lipid droplets. Supporting Evidence: PMID:21127063 presence of the AUP1-Ube2g2 complex at LDs |
| GO:0030970 retrograde protein transport, ER to cytosol | IMP PMID:25660456 Identification of ERAD components essential for dislocation ... | ACCEPT | Summary: AUP1 is required for dislocation (retrotranslocation) of the ERAD substrate NHK from the ER to the cytosol; core process tied to its ERAD adaptor role. Reason: AUP1 was identified as an ERAD component essential for dislocation of NHK, i.e. ER-to-cytosol retrograde transport. Supporting Evidence: PMID:25660456 dislocation, also known as retrotranslocation |
| GO:0070062 extracellular exosome | HDA PMID:18570454 Proteomic analysis of exosomes from human neural stem cells ... | MARK AS OVER ANNOTATED | Summary: High-throughput exosome proteomics localization; inconsistent with AUP1's monotopic ER/LD membrane topology and not a functionally meaningful localization. Reason: This is a mass-spectrometry co-purification hit, not evidence for a functional extracellular/exosomal role for an ER/LD membrane protein. |
| GO:0016020 membrane | HDA PMID:19946888 Defining the membrane proteome of NK cells. | MARK AS OVER ANNOTATED | Summary: Generic membrane from a high-throughput membrane proteome; too unspecific to be informative. Reason: The specific ER membrane and lipid droplet localizations are captured by other annotations; the bare membrane term adds no information. |
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Download this section (compressed HTML)Q: Does AUP1 have an intrinsic enzymatic (acyltransferase) activity, and if so what is its physiological substrate? The flavivirus study refers to an AUP1 acyltransferase domain whose activity is required for lipophagy, but a defined enzymatic function and substrate are not established outside the infection context.
Experiment: Define the substrate repertoire of AUP1-dependent ERAD genome-wide (e.g. proximity labeling or quantitative degradomics in AUP1-knockout vs wild-type cells) to distinguish core ERAD clients from sterol-pathway-specific and lipid-droplet-specific roles.
Experiment: Reconstitute AUP1-UBE2G2-gp78/Trc8 ubiquitination in vitro with defined components to test whether AUP1 is a passive scaffold or actively stimulates E2/E3 activity.
Experiment: Test whether AUP1-dependent ERAD of endogenous polytopic membrane clients such as NKCC2/SLC12A1 and NCC/SLC12A3 requires the G2BR-UBE2G2 interaction and the CUE domain, to determine whether physiological substrate handling uses the same module that maintains UBE2G2 levels and recruits it to the ER membrane.
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