E3 ubiquitin-protein ligase which accepts ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfers the ubiquitin to targeted substrates. Essential in early embryonic development to prevent apoptotic death. 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. Shuttles between the nucleus and the cytoplasm. In the nucleus, delocalizes from the nucleolus to the nucleoplasm in response to DNA damage. Predominantly expressed in brain, liver, kidney, testes and ovary. Up-regulated approximately 4-fold in G2 when compared to S phase. Down-regulated approximately 3-fold by gamma-irradiation. Ubiquitin ligase activity is mediated by two distinct domains, PHD-type zinc fingers 2 and 3. The use of these distinct domains may allow ubiquitination of different targets by each domain. The HECT domain is catalytically inactive and does not contribute to this activity.