A HIGH-AFFINITY INHIBITOR OF YEAST CARBOXYPEPTIDASE-Y IS ENCODED BY TFSI AND SHOWS HOMOLOGY TO A FAMILY OF LIPID-BINDING PROTEINS

A 25-kDa inhibitor of the vacuolar enzyme carboxypeptidase Y from Sacc haromyces cerevisiae has been characterized. The inhibitor, I-c, binds tightly with an apparent K-i of 0.1 nM. Consistent with a cytoplasmic localization, I-c is soluble and contains no sequences which could se rve as potential signals for transport into the endoplasmic reticulum. Surprisingly, I-c is encoded by TFS1, which has previously been isola ted as a high-copy suppressor of cdc25-1. CDC25 encodes the putative G TP exchange factor for Ras1p/Ras2p in yeast. In an attempt to rational ize this finding, we looked for a physiological relationship by deleti ng or overexpressing the gene for carboxypeptidase Y in a cdc25-1 stra in. However, this did not change the phenotype of this mutant strain. I-c is the first member of a new family of protease inhibitors. The in hibitor is not hydrolyzed on binding to CPY, It has fairly high degree of specificity, showing a 200-fold higher K-i toward a carboxypeptida se from Candida albicans which is highly homologous to carboxypeptidas e Y. The TFS1 gene product shows extensive similarity to a class of pr oteins termed ''21-23-kDa lipid binding proteins'', members of which a re found in several higher eukaryotes, including man. These proteins a re highly abundant in some tissues (e.g., brain) and have in general b een found to bind lipids. Considering their homology to I-c, it is tem pting to speculate that they may also be inhibitors of serine carboxyp eptidases.