Site-directed mutations altering the CAAX box of Ste18, the yeast pheromone-response pathway G gamma subunit

The STE18 gene encodes the gamma subunit of the G protein which functions in the Saccharomyces cerevisiae pheromone-response pathway. The STE18 gene product undergoes a post-translational processing at the carboxyl terminus directed by the CCAAX box motif CCTLM110. A variety of site-directed mutations of this sequence have been constructed to test the role of this motif on Ste18 function. Mutations which change or eliminate the cysteine at position 107 abolish Ste18-dependent mating, and thus the cysteine (C107) is essential for Ste18 function. However, inactivation of the prenyltransferase by disruption of DPR1 has only a minor effect on Ste18-dependent mating. Mutation of cysteine 106 to serine significantly reduces but does not eliminate Ste18 function. Deletion of the C-terminal TLM sequence or modification of the ultimate methionine to lysine, arginine or leucine, all changes which do not affect the CAAX box cysteines, have only minor effects on Ste18-dependent mating. Intriguingly, these latter mutations dramatically compromise Ste18 function in cells which are deleted for Gpa1, the alpha subunit of the G protein. In addition, overexpression of these mutant versions of STE18 causes a dominant negative phenotype and inhibits the constitutive mating response generated by GPA1 deletion in cells which contain a functional STE18 gene. These results suggest that the C terminus of Ste18 and the Gpa1 protein have overlapping roles in some aspect of yeast G protein function such as membrane targeting.