Catalyzes the covalent attachment of ubiquitin to other proteins. Acts as an essential factor of the anaphase promoting complex/cyclosome (APC/C), a cell cycle-regulated ubiquitin ligase that is essential for the transition from metaphase to anaphase in mitosis. Involved in both degradation of proteins responsible for maintaining sister chromatid cohesion at the onset of anaphase and of mitotic cyclins A and B at the exit of mitosis. Acts by initiating polyubiquitin chains on APC/C substrates, leading to the degradation of APC/C substrates by the proteasome and promoting mitotic exit. S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine + [E2 ubiquitin-conjugating enzyme]-L-cysteine = [E1 ubiquitin-activating enzyme]-L-cysteine + S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine. S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E1 ubiquitin-activating enzyme]-L-cysteine + N(6)-monoubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Component of the APC/C complex. Autoubiquitinated by the APC/C complex, leading to its degradation by the proteasome. Belongs to the ubiquitin-conjugating enzyme family.