Mutational analysis of Vam4/Ypt7p function in the vacuolar biogenesis and morphogenesis in the yeast, Saccharomyces cerevisiae

The vacuole is one of the most prominent compartments in yeast cells. The wild-type yeast cells have a large vacuolar compartment which occupies approximately a quarter of the cell volume, while the vam4 mutant cells exhibit highly fragmented vacuolar morphology. We isolated the VAM4 gene and found that the VAM4 is identical to the YPT7 which encodes a member of small GTP-binding protein superfamily. We introduced mutations to the VAM4/YPT7 which alter nucleotide binding characteristics of the gene product specifically, and their activities for the vacuolar morphogenesis were examined by transforming the mutant genes into yeast cells. The Thr22Asn mutation, which was expected to fix the protein in the GDP-bound state, resulted in loss of function in the vacuolar morphogenesis. Subcellular fractionation analysis indicated that the mutant molecule did nor associate with intracellular membranes efficiently. In contrast, Vam4/Ypt7p with the Gln68Leu mutation, which was expected to be the GTP-bound form, complemented the fragmented vacuolar morphology of Delta vam4 mutant cells. Vam4/Ypt7p with the Gln68Leu mutation also complemented the defects in the biogenesis of vacuolar alkaline phosphatase whose maturation requires the proper function of Vam4/Ypt7p. Overexpression of the mutant proteins in wild-type cells did not develop dominant-negative effects on the vacuolar assembly. These results indicated that the GTP-bound form of Vam4/Ypt7p promotes the biogenesis and morphogenesis of the yeast vacuolar compartment.