Interactions among the Subunits of the G Protein Involved in Saccharomyces cerevisiae Mating†

The SCG1 (GPA1), STE4, and STE18 genes of Saccharomyces cerevisiae encode mating-pathway components whose amino acid sequences are similar to those of the alpha, beta, and gamma subunits, respectively, of mammalian G proteins. Genetic evidence suggests that the STE4 and STE18 gene products interact. The mating defects of a set of ste4 mutants were partially suppressed by the overexpression of STE18, and, moreover, a combination of partially defective ste4 and ste18 alleles created a totally sterile phenotype, whereas such synthetic sterility was not observed when the ste18 allele was combined with a weakly sterile ste11 allele. Others have provided genetic evidence consistent with an interaction between the SCG1 (GPA1) and STE4 gene products. We have examined the physical interactions of these subunits by using an in vivo protein association assay. The STE4 and STE18 gene products associated with each other, and this association was disrupted by a mutation in the STE4 gene product whose phenotype was partially suppressed by overexpression of STE18. The STE4 and SCG1 (GPA1) gene products also interacted in the assay, whereas we detected no association of the SCG1 (GPA1) and STE18 gene products.