VAM3P, A NEW MEMBER OF SYNTAXIN RELATED PROTEIN, IS REQUIRED FOR VACUOLAR ASSEMBLY IN THE YEAST SACCHAROMYCES-CEREVISIAE

Syntaxins are thought to participate in the specific interactions betw een vesicles and acceptor membranes in intracellular protein trafficki ng, VAM3 of Saccharomyces cerevisiae encodes a 33 kDa protein (Vam3p) with a hydrophobic transmembrane segment at its C terminus, Vam3p has structural similarities to syntaxins of yeast, animal and plant cells. Delta vam3 cells accumulated spherical structures of 200-600 nm in di ameter, but lacked normal large vacuolar compartments, Loss of functio n of Vam3p resulted in inefficient processing of vacuolar proteins pro teinase A, proteinase B and carboxypeptidase Y, and defective maturati on of alkaline phosphatase. Subcellular fractionation and immunofluore scence microscopy showed that Vam3p was localized to the vacuolar memb ranes, Vam3p was accumulated in certain regions of the vacuolar membra nes. We conclude from these observations that Vam3p is a novel member of syntaxin in the vacuoles and it provides the t-SNARE function in a late step of the vacuolar assembly.