Non-essential E3 ubiquitin-protein ligase that specifically catalyzes 'Lys-29'- and 'Lys-48'-linked polyubiquitin chains (By similarity). Accepts ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfers the ubiquitin to targeted substrates (By similarity). Associates with the proteasome and promotes elongation of ubiquitin chains on substrates bound to the proteasome (PubMed:17190603). Elongation of ubiquitin chains on substrates bound to the proteasome promotes proteasomal processivity (PubMed:20008553). Also promotes ubiquitin elongation of 26S proteasome subunit RPN10 (PubMed:17190603). Involved in the stress response required to maintain cell fitness following heat-shock: acts by mediating ubiquitination of cytosolic misfolded proteins, leading to their subsequent degradation (PubMed:21983566). 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 19S proteasomes. Cells are viable. Present with 3120 molecules/cell in log phase SD medium. Belongs to the UBE3C family.