Disruption of receptor-G protein coupling in yeast promotes the function of an SST2-dependent adaptation pathway

In the yeast Saccharomyces cerevisiae, a G protein-linked signal transduction pathway mediates response to the oligopeptide mating pheromones a-factor and alpha-factor. Because cellular responses, including G1 arrest, occur transiently, cells can adapt or desensitize and resume growth. To address whether the balance between response and adaptation is influenced by the efficiency of receptor-G protein interaction, we introduced random point mutations in sequences that encode the third cytoplasmic loop of the alpha-factor receptor (STE2 gene product). Three mutations were identified that confer alpha-factor-resistant phenotypes, yet preserve normal cell-surface expression, ligand-binding affinity, and endocytosis of the receptor. However, these mutations confer partial signaling defects, as determined by cell cycle arrest and transcriptional induction assays, as well as in vitro assays of receptor-G protein interaction. Physiological tests suggested that receptors bearing third loop substitutions promote recovery from pheromone-induced growth arrest. Genetic evidence indicated that the third loop and the C-terminal domain of the receptor control independent recovery or adaptation processes. In contrast, receptor third loop substitutions caused rapid adaptation only if cells express a functional SST2 gene. Thus, disruption of pheromone receptor-G protein interaction concomitantly blunts signaling and specifically promotes the function of an SST2-dependent adaptation pathway. Possible functions for the Sst2 protein are discussed.