Plays a role in the translocation of terminally misfolded proteins from the endoplasmic reticulum lumen to the cytoplasm and their degradation by the proteasome (PubMed:18711132, PubMed:21857022). Plays a role in lipid droplet formation (PubMed:21857022). Induces lipid droplet clustering (PubMed:24039768). Recruits ubiquitin-conjugating enzyme UBE2G2 to lipid droplets which facilitates its interaction with ubiquitin ligases AMFR/gp78 and RNF139/TRC8, leading to sterol-induced ubiquitination of HMGCR and its subsequent proteasomal degradation (PubMed:23223569, PubMed:21127063). Also required for the degradation of INSIG1, SREBF1 and SREBF2 (PubMed:23223569). Plays a role in regulating assembly and secretion of very low density lipoprotein particles and stability of apolipoprotein APOB (PubMed:28183703). (Microbial infection) Following Dengue virus infection, required for induction of lipophagy which facilitates production of virus progeny particles. Identified in a complex that contains SEL1L, OS9, FAF2/UBXD8, UBE2J1/UBC6E and AUP1 (PubMed:18711132). Interacts with the cytoplasmic tail of ITGA2B, ITGA1, ITGA2, ITGA5, ITGAV and ITGAM (PubMed:12042322). Interacts (via C-terminus) with ubiquitin-conjugating enzyme UBE2G2; the interaction recruits UBE2G2 to lipid droplets (PubMed:21127063, PubMed:23223569). Interacts with ubiquitin ligases AMFR/gp78 and RNF139/TRC8; this promotes interaction of UBE2G2 with AMFR and RNF139 (PubMed:23223569). Interacts with apolipoprotein APOB (PubMed:28183703). (Microbial infection) Interacts with Dengue virus NS4A; the interaction occurs in the presence of Dengue virus NS4B and induces lipophagy which facilitates production of virus progeny. (Microbial infection) Upon Dengue virus infection, relocates from lipid droplets to autophagosomes. Detected in blood platelets and leukocytes (at protein level). Ubiquitous. Highly expressed in placenta, liver, kidney, skeletal muscle, heart and brain. (Microbial infection) By Dengue virus infection (at protein level). The CUE domain is required for interaction with the ER quality control machinery and misfolded substrates, ubiquitination, lipid clustering and interaction with AMFR but is not required for localization to lipid droplets. Monoubiquitinated and diubiquitinated. (Microbial infection) Not ubiquitinated following Dengue virus infection. Belongs to the AUP1 family. Extended N-terminus.