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 (PubMed:18331836). Catalyzes monoubiquitination of 26S proteasome subunit PSMC2/RPT1 (PubMed:24811749).