E4 ubiquitin ligase that extends E3-dependent Ub chains; forms branched chains on substrates prior to degradation, catalyzing K48-linked multi-monoubiquitination of K29-linked Ub chains assembled by Ufd4p; binds and converts CxUb, a C-terminally extended Ub, into an unanchored chain to mark client proteins for stress-induced degradation; role in Ub-fusion degradation, OLE membrane fluidity pathway, and the ERAD pathway for misfolded proteins; U-box family member; homologous to human Ufd2a, Ufd2b