PTH1 VAM3P IS THE SYNTAXIN HOMOLOG AT THE VACUOLAR MEMBRANE OF SACCHAROMYCES-CEREVISIAE REQUIRED FOR THE DELIVERY OF VACUOLAR HYDROLASES/

The PEP12 homolog-Pthlp (Pep twelve homolog 1) is predicted to be simi lar in size to Pep12p, the endosomal syntaxin homolog that mediates do cking of Golgi-derived transport vesicles and, like other members of t he syntaxin family is predicted to be a cytoplasmically oriented, inte gral membrane protein with a C-terminal transmembrane domain. Kinetic analyses indicate thar Delta pth1/vam3 mutants fail to process the sol uble vacuolar hydrolase precursors and that PrA, PrB and most of CpY a ccumulate within the cell in their Golgi-modified P2 precursor forms. This is in contrast to a pep12 mutant in which P2CpY is secreted from the cell. Furthermore, pep12 is epistatic to pth1/vam3 with respect to the CpY secretion phenotype. Alkaline phosphatase, a vacuolar membran e hydrolase, accumulates in its precursor form in the Delta pth1/vam3 mutant. Maturation of pro-aminopeptidase I, a hydrolase precursor deli vered directly to the vacuole from the cytoplasm, is also blocked in t he Delta pth1/vam3 mutant. Subcellular fractionation localizes Pth1/Va m3p to vacuolar membranes. Based on these data, we propose that Pth1/V am3p is the vacuolar syntaxin/t-SNARE homolog that participates in doc king of transport vesicles at the vacuolar membrane and that the funct ion of Pth1/Vam3p impinges on at least three routes of protein deliver y to tile yeast vacuole.