E3 ubiquitin-protein ligase which accepts ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and transfers it to its substrates (PubMed:10373495, PubMed:16772533, PubMed:19204938, PubMed:19233847, PubMed:19325566, PubMed:19591933, PubMed:22645313, PubMed:24273172, PubMed:24728990). Several substrates have been identified including the ARNTL/BMAL1, ARC, RAD23A and RAD23B, MCM7 (which is involved in DNA replication), annexin A1, the PML tumor suppressor, and the cell cycle regulator CDKN1B (PubMed:10373495, PubMed:19204938, PubMed:19325566, PubMed:19591933, PubMed:22645313, PubMed:24728990). Additionally, may function as a cellular quality control ubiquitin ligase by helping the degradation of the cytoplasmic misfolded proteins (PubMed:19233847). Finally, UBE3A also promotes its own degradation in vivo. Plays an important role in the regulation of the circadian clock: involved in the ubiquitination of the core clock component ARNTL/BMAL1, leading to its proteasomal degradation (PubMed:24728990). Acts as transcriptional coactivator of progesterone receptor PGR upon progesterone hormone activation (PubMed:16772533). Acts as a regulator of synaptic development by mediating ubiquitination and degradation of ARC (By similarity). Synergizes with WBP2 in enhancing PGR activity (PubMed:16772533). (Microbial infection) Catalyzes the high-risk human papilloma virus E6-mediated ubiquitination of p53/TP53, contributing to the neoplastic progression of cells infected by these viruses. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. The active form is probably a homotrimer. Binds UBQLN1 and UBQLN2. Interacts with the 26S proteasome. Interacts with BPY2. Interacts with HIF1AN, MAPK6 AND NEURL4; interaction with MAPK6 may be mediated by NEURL4. Interacts with the proteasomal subunit PSMD4. Interacts with ESR1 and WBP2 (PubMed:16772533, PubMed:21642474). Interacts with ARNTL/BMAL1 (PubMed:24728990). Interacts with ARC (By similarity). (Microbial infection) Interacts with HCV core protein and targets it to degradation. (Microbial infection) Interacts with the E6 protein of the cancer-associated human papillomavirus types 16 and 18. The E6/E6-AP complex binds to and targets the p53/TP53 tumor-suppressor protein for ubiquitin-mediated proteolysis. Phosphorylation at Tyr-659 by ABL1 impairs E3 ligase activity and protects p53/TP53 from degradation in (HPV)-infected cells. The disease is caused by variants affecting the gene represented in this entry. A cysteine residue is required for ubiquitin-thioester formation.