Isolation of yeast mutants defective in protein targeting to the vacuole

We have constructed a PRC1-SUC2 gene fusion that directs the synthesis in Saccharomyces cerevisiae of a hybrid polypeptide consisting of a 433-residue amino-terminal domain derived from the yeast vacuolar protease carboxypeptidase Y (CPY; EC 3.4.16.1) and a 511-residue carboxyl-terminal domain derived from the secreted yeast enzyme invertase (EC 3.2.1.26). Fractionation data indicated that this amount of CPY primary sequence is sufficient to quantitatively divert invertase to the yeast vacuole. The phenotypic consequence of localizing active invertase to the vacuole has enabled us to select for mutants that "mislocalize" the hybrid protein to the cell surface. The corresponding mutations that lead to this effect are all trans-acting and recessive, and they define at least eight complementation groups. These vacuolar protein targeting (vpt) mutants also exhibit hybrid protein independent defects in wild-type CPY delivery to the yeast vacuole. Precursor forms of CPY accumulate in the mutants and are secreted into the yeast periplasm and extracellular medium. The vpt mutants should provide useful information pertaining to the mechanisms by which yeast cells regulate vacuolar protein traffic.