Phosphorylation of the MEKK Ste11p by the PAK-like kinase Ste20p is required for MAP kinase signaling in vivo

Background: Many signals are transduced from the cell surface to the nucleu s through mitogen-activated protein (MAP) kinase cascades. Activation of MA P kinase requires phosphorylation by MEK, which in turn is controlled by Ra f, Mos or a group of structurally related kinases termed MEKKs. It is not u nderstood how MEKKs are regulated by extracellular signals. In yeast, the M EKK Ste11p functions in multiple MAP kinase cascades activated in response to pheromones, high osmolarity and nutrient starvation. Genetic evidence su ggests that the p21-activated protein kinase (PAK) Ste20p functions upstrea m of Ste11p, and Ste20p has been shown to phosphorylate Ste11p in vitro. Results: Ste20p phosphorylated Ste11p on Ser302 and/or Ser306 and Thr307 in yeast, residues that are conserved in MEKKs of other organisms. Mutating t hese sites to non-phosphorylatable residues abolished Ste11p function, wher eas changing them to aspartic acid to mimic the phosphorylated form constit utively activated Ste11p in vivo in a Ste20p-independent manner. The amino- terminal regulatory domain of Ste11p interacted with its catalytic domain, and overexpression of a small amino-terminal fragment of Ste11p was able to inhibit signaling in response to pheromones. Mutational analysis suggested that this interaction was regulated by phosphorylation and dependent on Th r596, which is located in the substrate cleft of the catalytic domain. Conclusions: Our results suggest that, in response to multiple extracellula r signals, phosphorylation of Ste11p by Ste20p removes an amino-terminal in hibitory domain, leading to activation of the Ste11 protein kinase. This me chanism may serve as a paradigm for the activation of mammalian MEKKs.