High expression of the yeast syntaxin-related Vam3 protein suppresses the protein transport defects of a pep12 null mutant.

The Pep12 protein of Saccharomyces cerevisiae is a member of the syntaxin family thought to function as target membrane receptor (t-SNARE) for vesicular intermediates travelling between the Golgi apparatus and the vacuole. Exploiting the temperature-sensitive growth phenotype of pep12 deletion strains, we identified VAM3 as a multicopy suppressor. Vam3p is another syntaxin-related protein which on high expression restored vacuole acidification of pep12 null mutants and effectively suppressed their sorting and maturation defects of vacuolar hydrolases. We conclude that Vam3p acts either as a bypass suppressor or by functionally replacing Pep12p at an endosomal, prevacuolar compartment.