Vacuolar ATPase mutants accumulate precursor proteins in a pre-vacuolar compartment

The vacuole of the yeast Saccharomyces cerevisiae contains a proton-translocating ATPase that acidifies the vacuolar lumen and generates an electrochemical potential across the vacuole membrane. Strains with chromosomal disruptions of the genes encoding the A, B, and c subunits of the vacuolar ATPase accumulate precursor forms of the vacuolar membrane protein alkaline phosphatase, and the soluble vacuolar hydrolases carboxypeptidase Y and proteinase A. We have found that the intracellular precursors in delta vat strains accumulate within the secretory pathway at some point before delivery to the vacuole but after transit to the Golgi complex. Purified vacuoles from delta vat cells do not contain the precursor forms of carboxypeptidase Y or alkaline phosphatase. In addition, vacuolar hydrolase-invertase hybrid proteins are inefficiently delivered to the vacuole in delta vat strains as demonstrated by vacuole isolation. Further subcellular fractionation to separate organelles indicate that significant amounts of the carboxypeptidase Y-invertase and alkaline phosphatase-invertase hybrid proteins are located in the late Golgi complex and/or post Golgi compartments.