Aspartic protease (PubMed:21266539). Appears to act as negative regulator of constitutive exocytosis (PubMed:10330187, PubMed:12051757). May act at the level of secretory vesicle docking and fusion as a competitive inhibitor of SNARE assembly (PubMed:12925750). Acts as a linker between the 19S proteasome and polyubiquitinated proteins like the HO endonuclease and UFO1 via UBA domain interactions with ubiquitin for their subsequent degradation (PubMed:17144915, PubMed:16478980). Required for S-phase checkpoint control (PubMed:11238935, PubMed:17144915). Inhibited by the proteinase inhibitors indinavir, lopinavir, nelfinavir, isovaleryl pepstatin, ritonavir, saquinavir and tipranavir. Forms homodimers. Interacts with RAD23. These interactions are mediated by the UBA domain. Is also able to bind ubiquitin and polyubiquitinated proteins. Interacts with the SNAREs SNC1, SNC2, SSO1, TLG1 and TLG2. Binding to SSO1 is promoted by the phosphorylation of 'Ser-49' of SSO1 by TKP1. By DNA damage via PDR3. Present with 6510 molecules/cell in log phase SD medium. Belongs to the DDI1 family.