Fig1p Facilitates Ca²⁺ Influx and Cell Fusion during Mating of Saccharomyces cerevisiae*

Abstract

During the mating process of yeast cells, two Ca²⁺ influx pathways become activated. The resulting elevation of cytosolic free Ca²⁺ activates downstream signaling factors that promote long term survival of unmated cells, but the roles of Ca²⁺ in conjugation have not been described. The high affinity Ca²⁺ influx system is composed of Cch1p and Mid1p and sensitive to feedback inhibition by calcineurin, a Ca²⁺/calmodulin-dependent protein phosphatase. To identify components and regulators of the low affinity Ca²⁺ influx system (LACS), we screened a collection of pheromone-responsive genes that when deleted lead to defects in LACS activity but not high affinity Ca²⁺ influx system activity. Numerous factors implicated in polarized morphogenesis and cell fusion (Fus1p, Fus2p, Rvs161p, Bni1p, Spa2p, and Pea2p) were found to be necessary for LACS activity. Each of these factors was also required for activation of the cell integrity mitogen-activated protein kinase cascade during the response to α-factor. Interestingly a polytopic plasma membrane protein, Fig1p, was required for LACS activity but not required for activation of Mpk1p mitogen-activated protein kinase. Mpk1p was not required for LACS activity, suggesting Mpk1p and Fig1p define two independent branches in the pheromone response pathways. Fig1p-deficient mutants exhibit defects in the cell-cell fusion step of mating, but unlike other fus1 and fus2 mutants the fusion defect of fig1 mutants can be largely suppressed by high Ca²⁺ conditions, which bypass the requirement for LACS. These findings suggest Fig1p is an important component or regulator of LACS and provide the first evidence for a role of Ca²⁺ signals in the cell fusion step of mating.