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). Interacts with 'Lys-48'-linked polyubiquitin chains. Interacts with RPN10 via its ubiquitin-like domain. Interacts with UBQ1, UBQ2, UBQ5, UBQ7, UBQ10, UBQ11 and IAA16. Widely expressed in the whole plant. Slow growth with abnormal phyllotaxy, shorter primary root with fewer lateral roots, shorter inflorescences, smaller siliques and reduced seed set, with unfertilized ovules interspersed among seeds of normal appearance. Mutant displays resistance to mitomycin C (MMC). Belongs to the RAD23 family.