The multiple site binding of carboxypeptidase Y inhibitor (IC) to the cognate proteinase. Implications for the biological roles of the phosphatidylethanolamine-binding protein.

The serine carboxypeptidase inhibitor in the cytoplasm of Saccharomyces cerevisiae, IC, specifically inhibits vacuolar carboxypeptidase Y (CPY) and belongs to a functionally unknown family of phosphatidylethanolamine-binding proteins (PEBPs). In the presence of 1 M guanidine hydrochloride, a CPY-IC complex is formed and is almost fully activated. The reactivities of phenylmethylsulfonyl fluoride, p-chloromercuribenzoic acid, and diisopropyl fluorophosphate toward the complex are considerably increased in 1 M guanidine hydrochloride, indicating that IC contains a binding site other than its inhibitory reactive site. IC is able to form the complex with diisopropyl fluorophosphate-modified CPY. Tryptic digestion of the complex indicates that two fragments from IC are involved in complex formation with CPY. These findings demonstrate the multiple site binding of IC with CPY. Considering the fact that mouse PEBP has recently been identified as a novel thrombin inhibitor, the binding that characterizes the CPY-IC complex could be a common feature of PEBPs.