Active-site residues of procarboxypeptidase Y are accessible to chemical modification

A high-affinity inhibitor of yeast carboxypeptidase Y is encoded by TFS1 and shows homology to a family of lipid binding proteins

A 25-kDa inhibitor of the vacuolar enzyme carboxypeptidase Y from Saccharomyces cerevisiae has been characterized. The inhibitor, Ic, binds tightly with an apparent Ki of 0.1 nM. Consistent with a cytoplasmic localization, Ic is soluble and contains no sequences which could serve as potential signals for transport into the endoplasmic reticulum. Surprisingly, Ic is encoded by TFS1, which has previously been isolated as a high-copy suppressor of cdc25-1. CDC25 encodes the putative GTP exchange factor for Ras1p/Ras2p in yeast. In an attempt to rationalize this finding, we looked for a physiological relationship by deleting or overexpressing the gene for carboxypeptidase Y in a cdc25-1 strain. However, this did not change the phenotype of this mutant strain. Ic is the first member of a new family of protease inhibitors. The inhibitor is not hydrolyzed on binding to CPY. It has fairly high degree of specificity, showing a 200-fold higher Ki toward a carboxypeptidase from Candida albicans which is highly homologous to carboxypeptidase Y. The TFS1 gene product shows extensive similarity to a class of proteins termed "21-23-kDa lipid binding proteins", members of which are found in several higher eukaryotes, including man. These proteins are highly abundant in some tissues (e.g., brain) and have in general been found to bind lipids. Considering their homology to Ic, it is tempting to speculate that they may also be inhibitors of serine carboxypeptidases.

Inhibition of alpha-lytic protease by pro region C-terminal steric occlusion of the active site

alpha-Lytic protease, a chymotrypsin-like serine protease, is synthesized with an N-terminal 166 amino acid pro region which is absolutely required for folding of the protease. The pro region is also the most potent inhibitor of the protease known with a Ki of approximately 10(-10) M. Compared to its role in the folding reaction, relatively little is known about the mechanism by which the pro region inhibits the mature protease. While proteinaceous protease inhibitors generally function by occluding the active sites of their respective targets [Bode, W., & Huber, R. (1992) Eur. J. Biochem. 204, 433-451], the pro region of alpha-lytic protease with its dual roles in folding and inhibition might be expected to show a novel mechanism of inhibition. However, experiments that probe both the structural and enzymatic consequences of pro region binding indicate that the pro region does not measurably distort the protease active site. Instead, the catalytic site is fully functional in the complex. Pro region inhibition of the protease is due to simple steric obstruction; the pro region C-terminus lies in the substrate binding sites of the protease. The implications of these results are discussed with regard to alpha-lytic protease maturation and folding. In addition, the proposed mechanism of alpha-lytic protease pro region inhibition is discussed with respect to data from other pro region families.