The a and alpha mating types in yeast secrete and respond to the peptide pheromones, a-factor and alpha-factor. The response is initiated by binding of pheromone to a specific receptor present on cells of the opposite mating type. The structures of the a- and alpha-factor receptors are very similar to the structures of receptors in vertebrate signal transduction pathways mediated by G (guanine nucleotide-binding) proteins, including the phototransduction and the beta-adrenergic response pathways. The yeast SST2 and SCG1 genes were isolated by complementation of a pheromone response mutation (sst2). These genes encode proteins that are likely to play important roles in the pheromone response pathway common to both mating types. SST2 is proposed to play a role in desensitization to pheromone similar to the role of arrestin in desensitization of the phototransduction pathway. SCG1 shows a high degree of homology to the G proteins involved in other signal transduction systems. The purpose of the proposed research is to further characterize the roles of the SST2 and SCG1 products in the pheromone response pathway, to test specific models for SST2 and SCG1 function, to identify additional components of this pathway, and to further investigate similarities to vertebrate signal transduction systems. Further analysis of SST2 and SCG1 function will involve examination of the effect of overproduction on pheromone response and mating, screens for change-of- function mutations, and localization of the protein products both in the absence and presence of pheromone. SCG1 function will also be analyzed by assays for guanine nucleotide-binding and GTPase activities associated with G proteins and by construction of site-directed mutations to examine the effects of amino acid substitutions that have been shown to effect the function of other guanine nucleotide-binding proteins. Additional clones that complement sst2 may encode other components of the pheromone response pathway and therefore will be analyzed. Suppressors of sst2 or scg1 loss-of-function and change-of-function mutations will be isolated; mutations in components of the response pathway should be identified among the suppressors. Finally, hybridization analysis has indicated that there are additional yeast sequences homologous to SCG1; these sequences will be analyzed to identify additional G proteins involved in pheromone response and other interesting processes.
