Ubiquitin-protein ligase that may function as an E3 ligase in conjunction with specific E1 and E2 ligases. May also function as an E4 ligase mediating the assembly of polyubiquitin chain assembly on substrates monoubiquitinated by another E3 ubiquitin 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. Protein modification; protein ubiquitination. The U-box domain is required for the ubiquitin protein ligase activity. Belongs to the ubiquitin conjugation factor E4 family. Sequencing errors.