E3 ubiquitin-protein ligase which accepts ubiquitin specifically from endoplasmic reticulum-associated E2 ligases, and transfers it to substrates promoting their degradation. Mediates the degradation of endoplasmic reticulum proteins (ERQC), also called ER-associated degradation (ERAD). Component of the hrd1 ubiquitin ligase complex, which is part of the ERAD-L and ERAD-M pathways responsible for the rapid degradation of soluble lumenal and membrane proteins with misfolded lumenal domains (ERAD-L), or ER-membrane proteins with misfolded transmembrane domains (ERAD-M) (By similarity). Together with ubc7, required for the degradation of the transcription factor sre1 precursor in the absence of its binding partner scp1. 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. Protein modification; protein ubiquitination. Belongs to the HRD1 family.