May function as E3 ubiquitin-protein ligase that catalyzes the transfer of ubiquitin onto target proteins. May play a role in DNA repair via its role in the synthesis of 'Lys-63'-linked polyubiquitin chains that recruit proteins involved in repair to sites of DNA damage by alkylating agents. 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.