MAP KINASE-RELATED FUS3 FROM SACCHAROMYCES-CEREVISIAE IS ACTIVATED BY STE7 INVITRO

PHEROMONE-STIMULATED haploid yeast cells undergo a differentiation process that allows them to mate1. Transmission of the intracellular signal involves threonine and tyrosine phosphorylation of the redundant FUS3 and KSS1 kinases, which are members. of the MAP kinase family2-4. FUS3/KSS1 phosphorylation depends on two additional kinases, STE11 and STE7 (refs 2, 5, 6). Genetic analyses predict an ordered pathway where STE11 acts before STE7 and FUS3/KSS1 (refs 2, 7). Here we report that STE7 is a dual-specificity kinase that modifies FUS3 at the appropriate sites and stimulates its catalytic activity in vitro. From these data and previous genetic results, we argue that STE7 is the physiological activator of FUS3. Recent indications that MA kinase activators are related to STE7 suggest that signal transduction pathways in many, if not all, eukaryotic organisms use homologous kinase cascades8-10.