Substrate-recruiting cofactor of the Cdc48p-Npl4p-Ufd1p segregase; K48-specific Ub chain binding receptor that assists Cdc48p in the dislocation of misfolded, poly(Ub) ERAD substrates prior to proteasomal degradation; role in regulated destruction of ER membrane proteins (Hmg1/2p), cytosolic proteins (Fbp1p) and mislocalized proteins (Cse4p); role in the RADAR pathway; mobilizes membrane-bound transcription factors by regulated Ub/proteasome-dependent processing; contains a UBX domain