The intracellular proteinases and their inhibitors in yeast. A mutant with altered regulation of proteinase A inhibitor activity.

A mutation of yeast pai1 has been isolated which leads to altered regulation of proteinase A inhibitor activity. Under conditions of derepression, this mutant as compared with wild type has a 70% reduction in proteinase A inhibitor activity, but no change in the activities of proteinase A or B. The growth of strains carrying the pai1 mutation is sensitive to temperature and pH. The altered physiological and biochemical phenotypes of the mutant appear to be the consequences of a single mutation which segregates 2:2 at meiosis. Results obtained with this mutant indicate that proteinase A and its inhibitor are two independently synthesized polypeptide chains rather than two proteins resulting from proteolytic cleavage of a single precursor. Furthermore, proteinase A and its inhibitor appear to be regulated independently of each other. Studies on the mutant also indicate that the regulation of the proteinase A inhibitor is different from that of the proteinase B inhibitor. In wild type cells there is an excess of proteinase A inhibitor over proteinase A, whereas in the mutant cells there is a 2.5-fold excess of proteinase A over proteinase A inhibitor. This excess proteinase A does not lead to detectable alterations in overall protein degradation. However, tryptophan synthase and proteinase B inhibitor proteins which are sensitive to proteinase A attack in vitro show an enhanced loss of activity in the mutant.