May have a ubiquitin-protein ligase activity acting as an E3 ubiquitin-protein ligase or as a ubiquitin-ubiquitin ligase promoting elongation of ubiquitin chains on 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. Interacts with UBE2L3. Interacts with VCP. Enriched in nuclear bodies. Expressed in liver, heart, brain, kidney and testis. The U-box domain mediates interaction with E2 ubiquitin ligases and is required for the ubiquitin-protein ligase activity.