AUP1

AUP1 is a monotopic membrane protein of lipid droplets and the ER that acts as an adaptor coupling lipid droplet biology to the ubiquitin-proteasome system and ER-associated degradation (ERAD) (PMID:21127063, PMID:21857022). Its C-terminal G2BR domain binds the E2 ubiquitin-conjugating enzyme UBE2G2 with low-nanomolar affinity at the backside of the enzyme, recruiting it to the ER membrane, protecting it from rapid degradation, and allosterically activating ubiquitination in conjunction with ERAD E3 ligases (PMID:21127063, PMID:34879065). Through its CUE domain AUP1 physically associates with the HRD1-SEL1L dislocation complex, and its depletion impairs degradation of misfolded ER proteins while altering cellular lipid droplet abundance (PMID:21857022). AUP1-driven ubiquitination governs the turnover of specific ERAD substrates, including the ER-retained form of the cotransporter NKCC2 (acting with the ER lectin OS9) and apolipoprotein B-100, where AUP1 loss reduces substrate ubiquitination, increases lipid droplet size, and enhances apoB100/VLDL secretion (PMID:28183703, PMID:38474353). Beyond degradation, AUP1's acyltransferase domain drives lipophagy that is co-opted by dengue virus through its NS4A protein, with AUP1 mono-ubiquitylation disrupting the NS4A interaction and inhibiting acyltransferase activity (PMID:29902443). Together with UBE2G2, AUP1 also retains STING in the ER to restrain basal type I interferon signaling, and its loss promotes spontaneous STING activation and altered antiviral resistance (PMID:40237449).

1. 2009 Medium

   Established a conserved autophagy-related role for the gene by showing the yeast ortholog is required for stationary-phase mitophagy and retrograde signaling, framing AUP1 within organelle quality-control biology.

   Evidence Genetic deletion (aup1Δ, rtg3Δ) with mitophagy assays, Rtg3 phosphorylation gel-shift, and RT-qPCR of RTG target genes in yeast

   PMID:19840933

   Open questions at the time

   • Whether the mammalian protein retains the mitophagy/RTG-signaling role is not addressed
   • Mechanism linking Aup1 to Rtg3 phosphorylation not defined
2. 2010 High

   Defined AUP1 as a monotopic lipid droplet surface protein that binds the E2 enzyme UBE2G2 through a discrete C-terminal G2BR domain, separating its membrane targeting from its enzyme-recruiting function.

   Evidence Confocal microscopy/fractionation for LD topology plus Co-IP and domain deletion/point mutation for G2BR–UBE2G2 binding

   PMID:21127063

   Open questions at the time

   • Catalytic consequence of UBE2G2 recruitment not yet established
   • E3 ligase partners unidentified at this stage
3. 2011 High

   Placed AUP1 mechanistically in ERAD by showing it associates with the HRD1-SEL1L complex, uses its CUE domain to regulate polyubiquitylation, and links lipid droplet abundance to ER quality control.

   Evidence Co-IP with HRD1-SEL1L, siRNA knockdown ERAD substrate degradation assays, CUE domain mutagenesis, and LD quantification

   PMID:21857022

   Open questions at the time

   • Specific endogenous ERAD substrates not identified here
   • Structural basis of UBE2G2 activation unresolved
4. 2017 Medium

   Identified apoB100 as a physiologically relevant AUP1-controlled substrate, connecting AUP1-mediated ubiquitination to hepatic lipoprotein secretion and lipid droplet size.

   Evidence siRNA knockdown, Co-IP of AUP1–apoB100, metabolic labeling/secretion assays, ubiquitination assays, and LD imaging in HepG2 cells

   PMID:28183703

   Open questions at the time

   • Direct vs. indirect nature of AUP1–apoB100 interaction not fully resolved
   • In vivo VLDL regulation not tested
5. 2018 High

   Revealed a degradation-independent function: AUP1's acyltransferase domain drives lipophagy that dengue virus exploits via NS4A, with mono-ubiquitylation acting as a switch on this activity.

   Evidence Ubiquitylation proteomics screen, Co-IP of AUP1–NS4A, acyltransferase point-mutant rescue, CRISPR/siRNA KO with virus titer, and imaging of LD-to-autophagosome relocalization

   PMID:29902443

   Open questions at the time

   • Endogenous (non-viral) substrates of the acyltransferase activity unknown
   • Enzyme(s) catalyzing AUP1 mono-ubiquitylation not identified
6. 2021 High

   Provided atomic-resolution mechanism for E2 activation by solving the G2BR–UBE2G2 structure and showing G2BR binding allosterically stimulates ubiquitination and stabilizes UBE2G2 at the ER.

   Evidence X-ray crystallography of the G2BR–UBE2G2 complex, biophysical affinity measurement, in vitro ubiquitination reconstitution, and cell-based ERAD assays with G2BR mutants

   PMID:34879065

   Open questions at the time

   • How allosteric activation integrates with specific E3 ligases at the membrane not fully mapped
7. 2024 Medium

   Extended AUP1's substrate repertoire to membrane transporters by demonstrating it promotes OS9-coupled ER retention and proteasomal degradation of NKCC2 and the related NCC.

   Evidence Reciprocal Co-IPs (AUP1–NKCC2, AUP1–OS9, AUP1–NCC), overexpression/knockdown with MG132 rescue, and dominant-negative ubiquitination assays

   PMID:38474353

   Open questions at the time

   • Physiological impact on renal ion transport not established in vivo
   • Which E3 ligase mediates NKCC2 ubiquitination not defined
8. 2025 Medium

   Uncovered an innate-immune role: AUP1 with UBE2G2 retains STING in the ER to suppress basal type I IFN, linking AUP1's ER residency to antiviral signaling control.

   Evidence Co-IP (AUP1–STING, UBE2G2–STING), AUP1/UBE2G2 KO STING ER-to-Golgi translocation assays, IFN reporter assays, and in vitro/in vivo virus infection

   PMID:40237449

   Open questions at the time

   • Whether STING retention requires ubiquitination or only physical sequestration unclear
   • Reciprocal validation across cell types limited to single lab

• How AUP1's two biochemical activities — UBE2G2-dependent ubiquitination and lipid-droplet acyltransferase activity — are coordinated within a single protein, and what determines substrate and pathway selection, remains unresolved.
• No unified model coupling acyltransferase and ubiquitin-adaptor functions
• Regulatory inputs (e.g. AUP1 ubiquitylation) governing pathway choice not characterized
• Disease relevance in mammals not established by direct genetic evidence in the corpus