Plays an important role in the regulation of different protein degradation mechanisms and pathways including ubiquitin-proteasome system (UPS), autophagy and endoplasmic reticulum-associated protein degradation (ERAD) pathway. Mediates the proteasomal targeting of misfolded or accumulated proteins for degradation by binding (via UBA domain) to their polyubiquitin chains and by interacting (via ubiquitin-like domain) with the subunits of the proteasome. Plays a role in the ERAD pathway via its interaction with ER-localized proteins UBXN4, VCP and HERPUD1 and may form a link between the polyubiquitinated ERAD substrates and the proteasome. Plays a role in unfolded protein response (UPR) by attenuating the induction of UPR-inducible genes, DDTI3/CHOP, HSPA5 and PDIA2 during ER stress. Involved in the regulation of macroautophagy and autophagosome formation; required for maturation of autophagy-related protein LC3 from the cytosolic form LC3-I to the membrane-bound form LC3-II and may assist in the maturation of autophagosomes to autolysosomes by mediating autophagosome-lysosome fusion. Negatively regulates the TICAM1/TRIF-dependent toll-like receptor signaling pathway by decreasing the abundance of TICAM1 via the autophagic pathway. Promotes the ubiquitination and lysosomal degradation of ORAI1, consequently down-regulating the ORAI1-mediated Ca2+ mobilization (By similarity). Suppresses the maturation and proteasomal degradation of amyloid beta A4 protein (A4) by stimulating the lysine 63 (K63)-linked polyubiquitination. Delays the maturation of A4 by sequestering it in the Golgi apparatus and preventing its transport to the cell surface for subsequent processing (PubMed:22847417). Promotes the surface expression of GABA-A receptors (PubMed:11528422). Ubiquitinates BCL2L10 and thereby stabilizes protein abundance (By similarity). Monomer and homodimer. Heterodimer with UBQLN2. Binds CD47 (By similarity). Binds NBL1 (PubMed:9303440). Binds GABRA1, GABRA2, GABRA3, GABRA6, GABRB1, GABRB2 and GABRB3 (PubMed:11528422). Binds UBE3A, BTRC, P4HB and MTOR. Interacts with the proteasome 19S subunit. Interacts (via ubiquitin-like domain) with TREX1; the interaction is direct and may control TREX1 subcellular location. Forms a complex with UBXN4 and VCP. Interacts (via UBA domain) with UBQLN4 (via ubiquitin-like domain). Found in a complex with UBQLN2 and MAP1LC3A/B/C. The monomeric form interacts with PSEN1 and PSEN2. Interacts with ORAI1. Interacts (via UBA domain) with TICAM1. Interacts with EPS15. Interacts (via UBA domain) with UBA52 and (via ubiquitin-like domain) with PSMD3 and PSMD4. Interacts with HERPUD1. Interacts with MAP1LC3A/B/C in the presence of UBQLN4. Interacts (via ubiquitin-like domain) with EPS15 (via UIM domains) and both the ubiquitinated and non-ubiquitinated forms can interact with EPS15. Interacts (via ubiquitin-like domain) with EPS15L1, HGS (via UIM domain) and STAM2 (via UIM domain) (By similarity). Interacts with BCL2L10/BCL-B; in the cytoplasm (By similarity). Recruited to the ER during ER-associated protein degradation (ERAD). Colocalizes with PSEN1 in the cell membrane and in cytoplasmic juxtanuclear structures called aggresomes. Colocalizes with ORAI1 and TICAM1 in the autophagosome. Colocalizes with EPS15 and HGS in ubiquitin-rich cytoplasmic aggregates that are not endocytic compartments and with EPS15 also in aggresomes (By similarity). Detected in neuronal processes and synapses (PubMed:11528422). The UBA domain mediates binding to PSEN1 and PSEN2. It also binds ubiquitin with micromolar affinity, independently of polyubiquitin linkage type. Essential for its association with microtubule-associated protein 1 light chain 3 (MAP1LC3). The ubiquitin-like domain mediates its association with the subunits of the proteasome. Dimerization is dependent upon the central region of the protein containing the STI1 domains and is independent of its ubiquitin-like and UBA domains. Degraded during both macroautophagy and during chaperone-mediated autophagy (CMA). Phosphorylated. Ubiquitinated.