AUP1

AUP1 is a type-III membrane protein that resides on both the endoplasmic reticulum and lipid droplets, functioning as a central scaffold in ER-associated degradation (ERAD) and lipid droplet homeostasis. Its C-terminal G2BR domain binds the E2 ubiquitin-conjugating enzyme UBE2G2 with nanomolar affinity, allosterically activating UBE2G2 and recruiting it to the ER membrane where AUP1 engages the HRD1–SEL1L E3 ligase complex via its CUE domain to drive polyubiquitination and proteasomal degradation of misfolded substrates including apoB100 and NKCC2 (PMID:21127063, PMID:21857022, PMID:34879065, PMID:28183703, PMID:38474353). The AUP1–UBE2G2 complex also retains STING in the ER to suppress basal type I interferon signaling, with AUP1 deficiency causing spontaneous STING activation (PMID:40237449). AUP1 possesses acyltransferase activity on lipid droplets that is exploited by flavivirus NS4A to trigger lipophagy, a process negatively regulated by mono-ubiquitylation of AUP1 itself (PMID:29902443).

1. 2010 High

   Establishing AUP1 as a lipid droplet-resident scaffold for UBE2G2 answered how this E2 enzyme is recruited to a specific membrane compartment, revealing that AUP1's monotopic membrane insertion and its C-terminal G2BR domain are functionally separable.

   Evidence Fluorescence microscopy, domain deletion/mutation, co-IP, and in vitro binding in mammalian cells

   PMID:21127063

   Open questions at the time

   • Whether AUP1-UBE2G2 recruitment to LDs has a distinct function from ER recruitment was unclear
   • No structural detail of the G2BR–UBE2G2 interface
   • The E3 ligase partner was not identified
2. 2011 High

   Demonstrating that AUP1 physically associates with the HRD1–SEL1L ERAD complex and is required for misfolded ER protein degradation established AUP1 as a functional ERAD component bridging E2 recruitment to E3 ligase activity, while also linking it to lipid droplet abundance regulation.

   Evidence Reciprocal co-IP, siRNA knockdown with ERAD substrate degradation assays, CUE domain mutant analysis, LD quantification

   PMID:21857022

   Open questions at the time

   • Identity of specific physiological ERAD substrates channeled through AUP1 was unknown
   • Whether the CUE domain binds ubiquitin on substrates or on AUP1 itself was unresolved
   • Mechanism linking AUP1 to LD abundance was not defined
3. 2017 High

   Identifying apoB100 as a direct AUP1-dependent ERAD substrate demonstrated a physiological role for AUP1 in hepatic lipoprotein quality control and VLDL secretion, extending the ERAD scaffold function to a clinically relevant substrate.

   Evidence siRNA knockdown in HepG2 cells, co-IP, metabolic labeling, ubiquitination and VLDL secretion assays

   PMID:28183703

   Open questions at the time

   • Whether AUP1 acts on apoB100 via HRD1 or another E3 was not shown
   • In vivo relevance in animal models was not tested
4. 2018 High

   Revealing that dengue virus NS4A hijacks AUP1's acyltransferase activity to trigger lipophagy — and that mono-ubiquitylation of AUP1 blocks this — uncovered a dual enzymatic function for AUP1 beyond ERAD scaffolding and identified a host–pathogen regulatory switch.

   Evidence CRISPR knockout, functional proteomics/ubiquitylation screen, acyltransferase domain mutants, live imaging, virus production assay

   PMID:29902443

   Open questions at the time

   • The E3 ligase responsible for AUP1 mono-ubiquitylation was not identified
   • Whether AUP1 acyltransferase activity functions independently of viral exploitation was not determined
   • Lipid substrate specificity of the acyltransferase domain was not characterized
5. 2021 High

   The crystal structure of the G2BR–UBE2G2 complex revealed nanomolar-affinity binding via a salt bridge/hydrophobic interface on UBE2G2's backside, explaining how AUP1 allosterically activates ubiquitination and stabilizes UBE2G2 at the ER membrane.

   Evidence X-ray crystallography, isothermal titration calorimetry, in vitro ubiquitination reconstitution, mutagenesis, cell-based ERAD assays

   PMID:34879065

   Open questions at the time

   • Full-length AUP1 structure including the transmembrane and CUE domains is lacking
   • How AUP1's allosteric activation of UBE2G2 is coordinated with E3 engagement was not resolved
6. 2024 Medium

   Demonstrating that AUP1 interacts with the ERAD lectin OS9 and promotes polyubiquitination-dependent degradation of the renal cotransporter NKCC2 expanded the roster of physiological ERAD substrates handled by AUP1 to include an ion transporter.

   Evidence Co-IP, siRNA knockdown, dominant-negative overexpression, proteasome inhibitor treatment

   PMID:38474353

   Open questions at the time

   • Single-lab study; independent confirmation needed
   • Whether AUP1–OS9 interaction is direct or mediated through other ERAD factors was not resolved
   • In vivo renal phenotype of AUP1 deficiency was not tested
7. 2025 Medium

   Showing that the AUP1–UBE2G2 complex retains STING in the ER and prevents its spontaneous activation revealed an unexpected role for the ERAD scaffold in restraining basal innate immune signaling.

   Evidence Co-IP, AUP1 knockout in vitro and in vivo, STING translocation and interferon reporter assays

   PMID:40237449

   Open questions at the time

   • Single-lab study; independent confirmation needed
   • Whether AUP1 promotes STING degradation or purely retains it via physical interaction was not fully distinguished
   • Relevance to autoinflammatory disease in humans is untested

• Key unresolved questions include the full-length structure of AUP1, the identity of the E3 ligase(s) that mono-ubiquitylate AUP1 to regulate its acyltransferase activity, the lipid substrate specificity of the acyltransferase domain, and the in vivo physiological consequences of AUP1 loss in mammalian models.
• No full-length AUP1 structure available
• E3 ligase for AUP1 mono-ubiquitylation unidentified
• Acyltransferase substrate specificity uncharacterized
• No mammalian genetic model phenotypically characterized