E3 ubiquitin-protein ligase that specifically catalyzes 'Lys-29'- and 'Lys-48'-linked polyubiquitin chains (PubMed:11278995, PubMed:12692129, PubMed:16341092, PubMed:16601690, PubMed:24811749, PubMed:24158444, PubMed:25752573, PubMed:25752577, PubMed:34239127, PubMed:33637724, PubMed:32039437). Accepts ubiquitin from the E2 ubiquitin-conjugating enzyme UBE2D1 in the form of a thioester and then directly transfers the ubiquitin to targeted substrates (PubMed:9575161, PubMed:32039437). Associates with the proteasome and promotes elongation of ubiquitin chains on substrates bound to the 26S proteasome (PubMed:24158444, PubMed:28396413, PubMed:31375563). Also catalyzes 'Lys-29'- and 'Lys-48'-linked ubiquitination of 26S proteasome subunit ADRM1/RPN13 in response to proteotoxic stress, impairing the ability of the proteasome to bind and degrade ubiquitin-conjugated proteins (PubMed:24811749, PubMed:31375563). Acts as a negative regulator of autophagy by mediating 'Lys-29'- and 'Lys-48'-linked ubiquitination of PIK3C3/VPS34, promoting its degradation (PubMed:33637724). Can assemble unanchored poly-ubiquitin chains in either 'Lys-29'- or 'Lys-48'-linked polyubiquitin chains; with some preference for 'Lys-48' linkages (PubMed:11278995, PubMed:16601690, PubMed:25752577). Acts as a negative regulator of type I interferon by mediating 'Lys-48'-linked ubiquitination of IRF3 and IRF7, leading to their degradation by the proteasome (PubMed:21167755). Catalyzes ubiquitination and degradation of CAND2 (PubMed:12692129). 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. Interacts with 26S proteasomes (PubMed:12692129, PubMed:24158444). Interacts (via the HECT domain) with UBE2D1 and, less efficiently, with UBE2L3 (PubMed:11278995, PubMed:9575161). Highly expressed in skeletal muscle. Detected at much lower levels in kidney and pancreas. Autoubiquitinated; promoting its own degradation. Belongs to the UBE3C family. Extended N-terminus.