Receptor of ubiquitinated protein targeted to ubiquitin/proteasome-mediated proteolysis (UPP). Relatively weak affinity for both 'Lys-48'- and 'Lys-63'-linked ubiquitin chains with a slight preference for 'Lys-48-'linked chains of three or more ubiquitin units. Homodimer. Belongs to the DDI1 family.