Ca²⁺-Calmodulin Promotes Survival of Pheromone-Induced Growth Arrest by Activation of Calcineurin and Ca²⁺-Calmodulin-Dependent Protein Kinase

Abstract

The cmd1-6 allele contains three mutations that block Ca²⁺ binding to calmodulin from Saccharomyces cerevisiae. We find that strains containing cmd1-6 lose viability during cell cycle arrest induced by the mating pheromone α-factor. The 50% lethal dose (LD₅₀)of α-factor for the calmodulin mutant is almost fivefold below the LD₅₀ for a wild-type strain. The calmodulin mutants are not more sensitive to α-factor, as measured by activation of a pheromone-responsive reporter gene. Two observations indicate that activation of the Ca²⁺-calmodulin-dependent protein phosphatase calcineurin contributes to survival of pheromone-induced arrest. First, deletion of the gene encoding the calcineurin regulatory B subunit, CNB1, from a wild-type strain decreases the LD₅₀ of α-factor but has no further effect on a cmd1-6 strain. Second, a dominant constitutive calcineurin mutant partially restores the ability of the cmd1-6 strain to survive exposure to α-factor. Activation of the Ca²⁺-calmodulin-dependent protein kinase (CaMK) also contributes to survival, thus revealing a new function for this enzyme. Deletion of the CMK1 and CMK2 genes, which encode CaMK, decreases the LD₅₀ of pheromone compared with that for a wild-type strain but again has no effect in a cmd1-6 strain. Furthermore, the LD₅₀ of α-factor for a mutant in which the calcineurin and CaMK genes have been deleted is the same as that for the calmodulin mutant. Finally, the CaMK and calcineurin pathways appear to be independent since the ability of constitutive calcineurin to rescue a cmd1-6 strain is not blocked by deletion of the CaMK genes.