An essential role of the yeast pheromone-induced Ca2+ signal is to activate calcineurin.

Abstract

Previous studies showed that, in wild-type (MATa) cells, a-factor causes an essential rise in cytosolic Ca2 . We show that calcineurin, the Ca2 /calmodulin-dependent protein phosphatase, is one target of this Ca2+ signal. Calcineurin mutants lose viability when incubated with mating pheromone, and overproduction of constitutively active (Ca2+-independent) calcineurin improves the viability of wild-type cells exposed to pheromone in Ca2+-deficient medium. Thus, one essential consequence of the pheromone-induced rise in cytosolic Ca2+ is activation of calcineurin. Although calcineurin inhibits intracellular Ca sequestration in yeast cells, neither increased extracellular Ca2+ nor defects in vacuolar Ca2 transport bypasses the requirement for calcineurin during the pheromone response. These observations suggest that the essential 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, farl) show decreased dependence on calcineurin during treatment with pheromone. Thus, calcineurin is essential in yeast cells during prolonged exposure to pheromone and especially under conditions of pheromone-induced growth arrest. Ultrastructural examination of pheromone-treated cells indicates that vacuolar morphology is abnormal in calcineurin-deficient cells, suggesting that calcineurin may be required for maintenance of proper vacuolar structure or function during the pheromone response. INTRODUCTIONThe signaling role of Ca21 is highly conserved among eukaryotic cells. Several distinct Ca2+-dependent signaling pathways exist in higher eukaryotes, and some components of these pathways have also been identified in the lower eukaryote Saccharomyces cerevisiae. Protein kinase C, for example, is required for maintenance of yeast cell integrity (for review, Levin and Errede, 1995), and phospholipase C, encoded by the PLC1 gene, is required for a diverse set of metabolic processes (Flick and Thorner, 1993;Payne and Fitzgerald-Hayes, 1993;Yoko-o et al., 1993). Yeast cells also possess calmodulin, and this protein is required for vegetative growth (Davis et al., 1986 (Cyert et al., 1991;Kuno et al., 1991;Liu et al., 1991;Cyert and Thorner, 1992). Calcineurin mutants fail to ,grow in the presence of high concentrations of Na+, Li+, Mn2+, or OH-, and the intracellular concentrations of several ions are altered in these cells (Nakamura et al., 1993;Breuder et al., 1994;Mendoza et al., 1994;Farcasanu et al., 1995;Pozos et al., 1996). Calcineurin may regulate the activity and/or the level of ion transporters in yeast cells. For example, the transcriptional induction of several P-type ATPases (ENA1, PMC1, PMR1) is reduced in calcineurin mutants (Mendoza et al., 1994;Cunningham and Fink, 1996). In addition, the expression of FKS2, a gene whose product is required for synthesis of 13-1,3-glucan in the cell wall, is regulated by calcineurin (Mazur et al., 1995). Several studies have indicated that Ca2+-dependent signal...

Ca+2 Signal is Generated Only Once in the Mating Pheromone Response Pathway in Saccharomyces cerevisiae.

“…Other lines of research also suggest that Ca 2+ plays a regulatory role in the late stage of the pheromone response pathway. Mutants lacking a subunit of the Ca 2+ /calmodulin-dependent phosphoprotein phosphates calcineurin are defective in recovery (Cyert et al, 1991;Cyert and Thorner, 1992) and die in response to the pheromone, like the mid mutants (Moser et al, 1996;Withee et al, 1997). Mutants defective in either calmodulin itself or calmodulin-dependent protein kinase also lose viability after differentiating into shmoos (Moser et al, 1996).…”

Evidence for Antagonistic Regulation of Cell Growth by the Calcineurin and High Osmolarity Glycerol Pathways in Saccharomyces cerevisiae

“…As previously suggested, the osmo-activated Sln1-Ypd1-Ssk1 phosphorelay system may be under a negative feedback regulation by accepting a phosphate group from glycerol 3-phosphate or related metabolic intermediate donors, which may be overproduced by glycerol-3-phosphate dehydrogenase, a target of the HOG pathway (5). The loss of calcineurin activity leads to elevation of the cytosolic Ca 2ϩ concentration (32,39,40). If such a feedback regulatory mechanism that is mediated by a phosphorylated metabolite does exist for the HOG pathway, the elevation of intracellular Ca 2ϩ may cause the formation of an insoluble complex with the phosphate compound, interfering with re-phosphorylation of the two-component system.…”

An Osmotically Induced Cytosolic Ca2+ Transient Activates Calcineurin Signaling to Mediate Ion Homeostasis and Salt Tolerance of Saccharomyces cerevisiae

“…Ca 2ϩ -dependent yeast mating pheromone signaling has been characterized in some detail (1,(22)(23)(24)(25). Mating pheromone causes G 1 arrest and resumption of cell cycle progression by facilitating Ca ext 2ϩ influx, which results in calmodulin activation of calcineurin (1,(22)(23)(24)(25).…”

“…Mating pheromone causes G 1 arrest and resumption of cell cycle progression by facilitating Ca ext 2ϩ influx, which results in calmodulin activation of calcineurin (1,(22)(23)(24)(25). Pheromone-induced Ca ext 2ϩ influx is mediated by Cch1p and Mid1p, which are plasma membrane proteins that by genetic criteria function as a single Ca 2ϩ uptake system (26 -29).…”