Purification and molecular characterization of two inhibitors of yeast proteinase B

A rapid purification procedure for large scale preparations of yeast proteinase B inhibitors 1 and 2 (IB1 and IB2) is described. By disc gel electrophoresis, amino acid analysis, and end-group determinations, each of the inhibitors is homogeneous. Both inhibitors are polypeptides with molecular weights of 8,500, containing 74 residues. No components other than amino acids could be detected. There is no significant difference in the amino acid compositions of the two inhibitors as analyzed after acid hydrolysis. Both polypeptides are characterized by the total absence of arginine, tryptophan, and sulfur-containing amino acid residues. The proteinase B inhibitors of yeast, therefore, differ fundamentally from proteinase inhibitors of many other organisms, which generally contain a large number of disulfide bridges. Both proteinase B inhibitors have threonine as the NH2-terminal residue and -Val-His-Thr-Asn-COO- as the COOH-terminal sequence. Comparison of peptide maps after tryptic digestion reveals that the two inhibitors differ definitely in only a few tryptic peptides. The inhibitors are rapidly inactivated by digestion with carboxypeptidase A from bovine pancreas at pH 8.5. Inactivation occurs stoichiometrically with the release of threonine, the penultimate residue at the COOH-terminal end of both inhibitors.