GENETIC INTERACTIONS BETWEEN A PEP7 MUTATION AND THE PEP12 AND VPS45 GENES - EVIDENCE FOR A NOVEL SNARE COMPONENT IN TRANSPORT BETWEEN THE SACCHAROMYCES-CEREVISIAE GOLGI-COMPLEX AND ENDOSOME

The PEP7 gene from Saccharomyces cerevisiae encodes a 59-kD hydrophili c polypeptide that is required for transport of soluble vacuolar hydro lase precursors from the TGN to the endosome. This study presents the results of a high-copy suppression analysis of pep7-20 mutant phenotyp es. This analysis demonstrated that both VPS45 and PEP12 are allele-sp ecific high-copy suppressors of pep7-20 mutant phenotypes. Overexpress ion of VPS45 was able to completely suppress the Zn2+ sensitivity and partially suppress the carboxypeptidase Y deficiency. Overexpression o f PEP12 was able to do the same, but to a lesser extent Vps45p and Pep 12p are Sec1p and syntaxin (t-SNARE) homologues, respectively, and are also thought to function in transport between the TGN and endosome. T wo additional vacuole pathway SNARE complex homologues, Vps33p (Sec1p) and Pth1p (syntaxin), when overexpressed, were unable to suppress pep 7-20 or any other pep7 allele, further supporting the specificity of t he interactions of pep7-20 with PEP12 and VPS45. Because several other vesicle docking/fusion reactions take place in the cell without disce rnible participation of Pep7p homologues, we suggest that Pep7p is a s tep-specific regulator of docking and/or fusion of TGN-derived transpo rt vesicles onto the endosome.