Signal transduction in Saccharomyces cerevisiae requires tyrosine and threonine phosphorylation of FUS3 and KSS1.

The FUS3 and KSS1 kinases are components of the pheromone-dependent signal transduction pathway in yeast. We show that FUS3 and KSS1 become rapidly phosphorylated after pheromone treatment. Similar to mammalian MAP kinases, this modification occurs at two amino acids of FUS3, threonine-180 and tyrosine-182. A mutation introduced at either position results in complete loss of function in vivo. Amino acid substitutions that destroy catalytic activity of the kinase do not prevent phosphorylation of the mutant products, a result that excludes an autocatalytic activation pathway. The modification of FUS3 is dependent on kinases encoded by the STE11 and STE7 genes. Furthermore, a hyperactive allele of STE11 causes increased phosphorylation of FUS3 in the absence of pheromone stimulation. Thus, either STE7 or STE11 could be the kinase responsible for the phosphorylation of FUS3.