GENETIC-RELATIONSHIPS BETWEEN THE G-PROTEIN BETA-GAMMA COMPLEX, STE5P, STE20P AND CDC42P - INVESTIGATION OF EFFECTOR ROLES IN THE YEAST PHEROMONE RESPONSE PATHWAY

The Saccharomyces cerevisiae G protein beta gamma dimer, Ste4p/Ste18p, acts downstream of the alpha subunit, Gpalp, to activate the pheromon e response pathway and therefore must interact with a downstream effec tor. Synthetic sterile mutants that exacerbate the phenotype of ste4-t s mutations were isolated to identify proteins that functionally inter act with Ste4p. The identification of a ste18 mutant indicated that th is screen could identify proteins that interact directly with Ste4p. T he other mutations were in STE5 and the STE20 kinase gene, which act n ear Ste4p in the pathway, and a new gene called STE21. ste20 null muta nts showed residual mating, suggesting that another kinase may provide some function. Overexpression of Ste5p under galactose control activa ted the pheromone response pathway. This activation was dependent on S te4p and Ste18p and partially dependent on Ste20p. These results canno t be explained by the linear pathway of Ste4p --> Ste20p --> Ste5p. Ov erexpression of Cdc42p resulted in a slight increase in pheromone indu ction of a reporter gene, and overexpression of activated forms of Cdc 42p resulted in a further twofold increase. Mutations in pheromone res ponse pathway components did not suppress the lethality associated wit h the activated CDC42 mutations, suggesting that this effect is indepe ndent of the pheromone response pathway.