DIFFERENTIAL-EFFECTS OF COMPARTMENT DEACIDIFICATION ON THE TARGETING OF MEMBRANE AND SOLUBLE-PROTEINS TO THE VACUOLE IN YEAST

Lysosomal/vacuolar protein targeting is dependent on compartment acidi fication. In yeast, sorting of soluble vacuolar proteins such as carbo xypeptidase Y is sensitive to acute changes in vacuolar pH. In contras t, the vacuolar membrane protein alkaline phosphatase is missorted onl y under conditions of chronic deacidification. We have undertaken a te mporal analysis to define further the relationship between compartment acidification and sorting of soluble and membrane vacuolar proteins. Depletion of either the Vma3p or Vma4p subunits of the yeast vacuolar ATPase over time resulted in loss of vacuolar ATPase activity and vacu olar acidification. A kinetic delay in processing of carboxypeptidase Y occurred concomitant with these physiological changes while transpor t of alkaline phosphatase remained unaffected. Carboxypeptidase S, ano ther vacuolar hydrolase that transits through the secretory pathway as an integral membrane protein, displayed a pH sensitivity similar to t hat of soluble vacuolar proteins. These results indicate that compartm ent acidification is tightly coupled to efficient targeting of protein s to the vacuole and that there may be multiple distinct mechanisms fo r targeting of vacuolar membrane proteins.