AN ESSENTIAL ROLE OF THE YEAST PHEROMONE-INDUCED CA2+ SIGNAL IS TO ACTIVATE CALCINEURIN

Previous studies showed that, in wild-type (MATa) cells, alpha-factor causes an essential rise in cytosolic Ca2+ We show that calcineurin, t he Ca2+/calmodulin-dependent protein phosphatase, is one target of thi s Ca2+ signal. Calcineurin mutants lose viability when incubated with mating pheromone, and overproduction of constitutively active (Ca2+-in dependent) calcineurin improves the viability of wild-type cells expos ed to pheromone in Ca2+-deficient medium. Thus, one essential conseque nce of the pheromone-induced rise in cytosolic Ca2+ is activation of c alcineurin. Although calcineurin inhibits intracellular Ca2+ sequestra tion in yeast cells, neither increased extracellular Ca2+ nor defects in vacuolar Ca2+ transport bypasses the requirement for calcineurin du ring the pheromone response. These observations suggest that the essen tial function of calcineurin in the pheromone response may be distinct from its modulation of intracellular Ca2+ levels. Mutants that do not undergo pheromone-induced cell cycle arrest (fus3, far1) show decreas ed dependence on calcineurin during treatment with pheromone. Thus, ca lcineurin is essential in yeast cells during prolonged exposure to phe romone and especially under conditions of pheromone-induced growth arr est. Ultrastructural examination of pheromone-treated cells indicates that vacuolar morphology is abnormal in calcineurin-deficient cells, s uggesting that calcineurin may be required for maintenance of proper v acuolar structure or function during the pheromone response.