Catalyzes the covalent attachment of ubiquitin to other proteins. Also acts as an E3 ubiquitin-protein ligase. 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. 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. Protein modification; protein ubiquitination. The RING-type zinc finger domain mediates binding to an E2 ubiquitin-conjugating enzyme. In the C-terminal section; belongs to the ubiquitin-conjugating enzyme family.