E3 ubiquitin-protein ligase which probably functions outside the N-end rule pathway, since it lacks the residues essential for the degradation of N-end rule substrates. Mediates RPN4 ubiquitination and subsequent degradation. 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. Interacts with MUB1, RPN4 and UBC2. The RING-H2 zinc finger is an atypical RING finger with a His ligand in place of the fourth Cys of the classical motif. Present with 49 molecules/cell in log phase SD medium. Deletion produces no detectable phenotype. Belongs to the UBR1 family.