G protein signaling governing cell fate decisions involves opposing Galpha subunits in Cryptococcus neoformans

Communication between cells and their environments is often mediated by G protein-coupled receptors and cognate G proteins. In fungi, one such signaling cascade is the mating pathway triggered by pheromone/pheromone receptor recognition. Unlike Saccharomyces cerevisiae, which expresses two Galpha subunits, most filamentous ascomycetes and basidiomycetes have three Galpha subunits. Previous studies have defined the Galpha subunit acting upstream of the cAMP-protein kinase A pathway, but it has been unclear which Galpha subunit is coupled to the pheromone receptor and response pathway. Here we report that in the pathogenic basidiomycetous yeast Cryptococcus neoformans, two Galpha subunits (Gpa2, Gpa3) sense pheromone and govern mating. gpa2 gpa3 double mutants, but neither gpa2 nor gpa3 single mutants, are sterile in bilateral crosses. By contrast, deletion of GPA3 (but not GPA2) constitutively activates pheromone response and filamentation. Expression of GPA2 and GPA3 is differentially regulated: GPA3 expression is induced by nutrient-limitation, whereas GPA2 is induced during mating. Based on the phenotype of dominant active alleles, Gpa2 and Gpa3 signal in opposition: Gpa2 promotes mating, whereas Gpa3 inhibits. The incorporation of an additional Galpha into the regulatory circuit enabled increased signaling complexity and facilitated cell fate decisions involving choice between yeast growth and filamentous asexual/sexual development.