Functions as a proteasomal ubiquitin receptor. Binds and presumably selects ubiquitin-conjugates for destruction. Prefers multiubiquitin chains rather than single ubiquitins, with a binding affinity for 'Lys-63'-linked ubiquitin chains. Interacts with 'Lys-63'-linked polyubiquitin chains. Interacts with DSK2a. The PH domain mediates interactions with ubiquitin and DSK2A. No visible phenotype. Belongs to the RPN13 family.