Ubiquitin-protein ligase that probably functions as an E3 ligase in conjunction with specific E1 and E2 ligases. May also function as an E4 ligase mediating the assembly of polyubiquitin chains on substrates ubiquitinated by another E3 ubiquitin ligase. Mediates 'Lys-48'-linked polyubiquitination of substrates. 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. The disease is caused by variants affecting the gene represented in this entry. Belongs to the ubiquitin conjugation factor E4 family. Contaminating sequence. Potential poly-A sequence. Contaminating sequence. Potential poly-A sequence. Extended N-terminus.