Biogenesis of the yeast vacuole (lysosome). Signal sequence deletion of the vacuolar aspartic proteinase yscA does not block maturation of vacuolar proteinases.

Abstract

The activation process of vacuolar (lysosomal) proteinases in the yeast Saccharomyces cerevisiae is initiated by the PRA1 (PEP4) gene product, proteinase yscA. To elucidate the importance of the different domains of proteinase yscA in a first attempt the signal sequence of the enzyme was deleted using site-directed mutagenesis. Proteinase yscA delta S underwent processing to an active enzyme, although the amount of mature enzyme was considerably reduced as compared to wild type. Isolation of vacuoles of the mutant strain showed mature-sized proteinase yscA in this organelle. Pro-proteinase yscB is converted to 5-10% to the mature enzyme, whereas procarboxypeptidase yscY is matured, although with reduced kinetics. Thus, the signal sequence of proteinase yscA is not essential for delivery to the vacuole and the activation of a certain amount of the yeast vacuolar proteolytic capacity.