CONTROL OF ADAPTATION TO MATING PHEROMONE BY G-PROTEIN BETA-SUBUNITS OF SACCHAROMYCES-CEREVISIAE

The STE4 gene of the yeast Saccharomyces cerevisiae encodes the beta subunit of a heterotrimeric G protein that mediates response to mating pheromones and influences recovery from pheromone-induced growth arrest. To explore how G(beta) subunits regulate response and recovery (adaptation), we isolated and characterized signaling-defective STE4 alleles (STE4(sd)). STE4(sd) mutations resulted in amino acid substitutions in the N-terminal region of Ste4p, proximal to the first of seven repeat units conserved in G protein beta subunits. Genetic tests indicated that STE4(sd) mutations disrupted functions of Ste4p required for inducing pheromone responses. Wild-type cells that overexpressed STE4(sd) alleles displayed apparently normal initial responses to pheromone as judged by quantitative mating, G, arrest and transcriptional assays. However, after undergoing initial G(1) arrest, wild-type cells overexpressing STE4(sd) alleles recovered more quickly from division arrest, suggestive of a hyperadaptive phenotype. Because hyperadaptation occurred when STE4(sd) alleles were overexpressed in cells lacking Sst1p (Bar1p), Sst2p or the C-terminal domain of the alpha-factor receptor, this phenotype did not involve three principal modes of adaptation in yeast. However, hyperadaptation was abolished when STE4(sd) mutations were combined in cis with a deletion that removes a segment of Ste4p (residues 310-346) previously implicated in adaptation to pheromone. These results indicate that G(beta) subunits possess two independent activities, one required for triggering pheromone response and another that promotes adaptation. Potential models for G(beta) subunit-mediated adaptation are discussed.