Recognizes and binds polyubiquitin chains (PubMed:11584278, PubMed:16138082). Acts as a linker between the 19S proteasome and polyubiquitinated proteins via UBA domain interactions with ubiquitin for their subsequent degradation. Aspartic protease. Appears to act as negative regulator of constitutive exocytosis. May act at the level of secretory vesicle docking and fusion as a competitive inhibitor of SNARE assembly. Required for S-phase checkpoint control (By similarity). Homodimer (PubMed:16138082). Interacts (via UBA domain) with polyubiquitin (polyUb) chains (via Lys-48-linked polyUbs) (PubMed:11584278, PubMed:16138082). Has weak binding affinity for monoubiquitin (PubMed:16138082). According to another report, has no affinity for monoubiquitin (PubMed:11584278). The UBA domain specifically recognizes Lys-48-linked diubiquitin units (Ub2), which is mediated by the two binding sites within a single UBA domain. Belongs to the DDI1 family.