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 (By similarity). Localizes to viral factories, the sites of virus replication. The vaccinia strains LC16m0 and LC16m8 encode a full-length E3 ubiquitin ligase p28 unlike the majority of the vaccinia strains. Belongs to the poxviridae p28 protein family.