RING-finger E3 ubiquitin ligase which catalyzes the formation of both 'Lys-48'- and 'Lys-63'-linked polyubiquitin chains. Plays an important role in virulence by acting as an anti-apoptotic factor. 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. Localizes to viral factories, the sites of virus replication. The RING-type zinc finger domain is required for E3 ligase activity. Belongs to the poxviridae p28 protein family.