A problem for inositol signaling is to understand the significance of the kinases that convert inositol hexakisphosphate to diphosphoinositol polyphosphates. This kinase activity is catalyzed by Kcs1p in the yeast Saccharomyces cerevisiae. A kcs1Delta yeast strain that was transformed with a specifically "kinase-dead" kcs1p mutant did not synthesize diphosphoinositol polyphosphates, and the cells contained a fragmented vacuolar compartment. Biogenesis of the yeast vacuole also required another functional domain in Kcs1p, which contains two leucine heptad repeats. The kinase activity of Kcs1p was also found to sustain cell growth and integrity of the cell wall and to promote adaptive responses to salt stress. Thus, the synthesis of diphosphoinositol polyphosphates has wide ranging physiological significance. Furthermore, we showed that these phenotypic responses to Kcs1p deletion also arise when synthesis of precursor material for the diphosphoinositol polyphosphates is blocked in arg82Delta cells. This metabolic block was partially bypassed, and the phenotype was partially rescued, when Kcs1p was overexpressed in the arg82Delta cells. This was due, in part, to the ability of Kcs1p to phosphorylate a wider range of substrates than previously appreciated. Our results show that diphosphoinositol polyphosphate synthase activity is essential for biogenesis of the yeast vacuole and the cell's responses to certain environmental stresses.
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