Binds and presumably selects ubiquitin-conjugates for destruction. Prefers multiubiquitin chains rather than single ubiquitins, with a binding affinity for 'Lys-48'-linked ubiquitin chains. Acts as a ubiquitin receptor that associates with the 26S proteasomal docking subunit RPN10 for the indirect recognition of ubiquitinated substrates of ubiquitin/26S proteasome-mediated proteolysis (UPP). Interacts with 'Lys-48'-linked polyubiquitin chains via its UBA domain. Interacts with RPN10 via its ubiquitin-like domain. Interacts with PEX2 and PEX12. Ubiquitous.