Inositol pyrophosphates modulate S phase progression after pheromone-induced arrest in Saccharomyces cerevisiae.

Several studies have demonstrated the activation of phosphoinositide-specific phospholipase C (Plc) in nuclei of mammalian cells during synchronous progression through the cell cycle, but the downstream targets of Plc-generated inositol 1,4,5-trisphosphate are poorly described. Phospholipid signaling in the budding yeast Saccharomyces cerevisiae shares similarities with endonuclear phospholipid signaling in mammals, and many recent studies point to a role for inositol phosphates, including InsP(5), InsP(6), and inositol pyrophosphates, in mediating the action of Plc. In this study, we investigated the changes in inositol phosphate levels in α-factor-treated S. cerevisiae, which allows cells to progress synchronously through the cell cycle after release from a G(1) block. We found an increase in the activity of Plc1 early after release from the block with a concomitant increase in the levels of InsP(7) and InsP(8). Treatment of cells with the Plc inhibitor U73122 prevented increases in inositol phosphate levels and blocked progression of cells through S phase after pheromone arrest. The enzymatic activity of Kcs1 in vitro and HPLC analysis of [(3)H]inositol-labeled kcs1Δ cells confirmed that Kcs1 is the principal kinase responsible for generation of pyrophosphates in synchronously progressing cells. Analysis of plc1Δ, kcs1Δ, and ddp1Δ yeast mutants further confirmed the role that a Plc1- and Kcs1-mediated increase in pyrophosphates may have in progression through S phase. Our data provide genetic, metabolic, and biochemical evidence that synthesis of inositol pyrophosphates through activation of Plc1 and Kcs1 plays an important role in the signaling response required for cell cycle progression after mating pheromone arrest.