E3 ubiquitin-protein ligase which accepts ubiquitin from specific E2 ubiquitin-conjugating enzymes, and transfers it to substrates, generally promoting their degradation by the proteasome (PubMed:15749827). Independently of its E3 ubiquitin-protein ligase activity, acts as an inhibitor of CPSF3 endonuclease activity by blocking CPSF3 active site (PubMed:39032490). . This is the function of UBE3D (ubiquitin protein ligase E3D, ENSG00000118420).