Yeast proteinase A is synthesized as a zymogen which transits through the endoplasmic reticulum, the Golgi complex and the endosome to the vacuole. On arrival in the vacuole, activation takes place. It has previously been found that proteinase A can activate autocatalytically ; however, the propeptide of proteinase A shows essentially no similarity to other known aspartic proteinase propeptides. To understand why proteinase A activation occurs rapidly in the vacuole but not at all in earlier compartments, we have purified the zymogen and investigated the conditions that trigger autoactivation and the mechanism of autoactivation. Autoactivation was triggered by acidic pH and its rate increased with increasing ionic strength. Kinetic evidence
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