E3 ubiquitin-protein ligase that mediates ubiquitination of p53/TP53, leading to its degradation by the proteasome. 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. Expression increases from oocyte stage I/II to reach its maximum in oocyte stage v/vi in unfertilized eggs, and then progressively decreases to become undetectable at the gastrula stage. Belongs to the MDM2/MDM4 family.