May be involved in nucleotide excision repair (By similarity). 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) (By similarity). Interacts with 'Lys-48'-linked polyubiquitin chains. Interacts with RPN10 via its ubiquitin-like domain. Widely expressed in the whole plant. No visible phenotype. Belongs to the RAD23 family.