INHIBITION OF G-PROTEIN SIGNALING BY DOMINANT GAIN-OF-FUNCTION MUTATIONS IN SST2P, A PHEROMONE DESENSITIZATION FACTOR IN SACCHAROMYCES-CEREVISIAE

Genetic analysis of cell-cell signaling in Saccharomyces cerevisiae ha s led to the identification of a novel factor, known as Sst2p, that pr omotes recovery after pheromone-induced growth arrest (R.K. Chan and C . A. Otte, Mol. Cell. Biol. 2:11-20, 1982). Loss-of-function mutations lead to increased pheromone sensitivity, but this phenotype is partia lly suppressed by overexpression of the G protein alpha subunit gene ( GPA1). Suppression is allele specific, however, suggesting that there is direct interaction between the two gene products. To test this mode l directly, we isolated and characterized several dominant gain-of-fun ction mutants of SST2. These mutations block the normal pheromone resp onse, including a loss of pheromone-stimulated gene transcription, cel l cycle growth arrest, and G protein myristoylation. Although the SST2 mutations confer a pheromone-resistant phenotype, they do not prevent downstream activation by overexpression of G beta (STE4), a constitut ively active G beta mutation (STE4(Hpl)), or a disruption of GPA1. Non e of the SST2 alleles affects the expression or stability of G alpha. These results point to the G protein alpha subunit as being the direct target of Sst2p action and underscore the importance of this novel de sensitization factor in G-protein-mediated signaling.