Proteasome polyubiquitin receptor; non-ATPase subunit of the 19S regulatory particle (RP) of the 26S proteasome that links the RP base to the lid; N-terminus plays a role in the assembly of the regulatory particle (RP); ubiquitin (Ub)-interacting motif selectively binds to polyubiquitin chains and contributes to the allosteric Ub sensor activity of Rpn11p that promotes substrate engagement by the 26S proteasome; homolog of the mammalian S5a protein