Plays a role in protein homeostasis by regulating both the translocation and the ubiquitin-mediated proteasomal degradation of nascent proteins at the endoplasmic reticulum. It is involved in the regulation of signal-mediated translocation of proteins into the endoplasmic reticulum. It also plays a role in the ubiquitin-mediated proteasomal degradation of proteins for which signal-mediated translocation to the endoplasmic reticulum has failed. May therefore function in the endoplasmic reticulum stress-induced pre-emptive quality control, a mechanism that selectively attenuates the translocation of newly synthesized proteins into the endoplasmic reticulum and reroutes them to the cytosol for proteasomal degradation (By similarity). By controlling the steady-state expression of the IGF1R receptor, indirectly regulates the insulin-like growth factor receptor signaling pathway (PubMed:26692333). Binds 'Lys-48'-linked polyubiquitin chains of ubiquitinated proteins (PubMed:24160817). Associates with the proteasome complex; upon exposure to arsenite (By similarity). Interacts (via VIM motif) with VCP; the interaction is direct (PubMed:21896481, PubMed:24160817, PubMed:26337389). Interacts with BAG6 (PubMed:24160817, PubMed:26337389). Interacts with IGF1R (nascent precursor form) (PubMed:26692333). Interacts with DERL1, FAF2, NPLOC4 and UFD1; probably through VCP (PubMed:24160817). Down-regulated by the miRNA miR-125a-3p in myelo-proliferative neoplasms. The UIM domains specifically bind 'Lys-48'-linked ubiquitin polymers. The UIM domains mediate interaction with polyubiquitinated proteins. Phosphorylated by MAPK14. Phosphorylation has no effect on association with the proteasome complex.