Pheromone response in the yeast Saccharomyces cerevisiae provides a microbial model for studying general features of hormone action and cell division control. When alpha-factor pheromone binds to specific receptors on the surface of yeast --a-- cells, it causes the cells to arrest division in the G1 phase of the cycle. The occupied receptors are continually internalized and resynthesized. After prolonged exposure to alpha-factor, the cells "adapt" to the pheromone and reenter the mitotic cycle. Genetic studies suggest that the receptor belongs to the same structural class as rhodopsin and the beta- adrenergic receptor and that signal transduction is mediated by a heterotrimeric GTP-binding regulatory protein (G protein). Detailed understanding of this very basic process in yeast should provide information for a more general understanding of receptor action and cell division control. A combined genetic and biochemical approach will be applied to this problem. Structural features of the receptor which control its ligand binding, signal transduction, and ligand- mediated internalization activities will be identified by isolating and characterizing receptor mutants with specific defects in each of these properties. Factors which interact with the receptor to mediate signal transduction and receptor internalization will be defined genetically by isolating suppressors which compensate for each of the specific receptor defects. The fate of the receptor and G protein after alpha- factor stimulation will be determined. A purified in vitro assay for signal transduction will be developed in order to examine specific interactions among the receptor and the three G protein subunits and to determine the role that these interactions play in the adaptation process.