Autoactivation of proteinase A initiates activation of yeast vacuolar zymogens

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Abstract
The Saccharomyces cerevisiae PEP4 gene encodes proteinase A, an aspartyl protease. pep4 mutants are defective in the activation of many vacuolar hydrolases, including proteinase B. We have expressed a pep4 mutation which directs the accumulation of pro-proteinase A with a defective active site. Co-expression with PEP4 leads to normal processing, i.e. the mutant zymogen is functional as a substrate for the maturation reaction in trans. We conclude that wild-type pro-proteinase A has the ability to mediate its own activation. Elimination of the co-expressed PEP4 gene did not effectively stop the processing of the mutant zymogen, owing to a strong, proteinase-B-dependent, phenotypic lag. In a proteinase-B-negative strain, processing of pro-proteinase A led to an active form of a higher molecular mass than the normal mature form.