Serine/threonine protein kinases of the Ste20/PAK family have been implicated in the signalling from heterotrimeric G proteins to mitogen-activated protein (MAP) kinase cascades1,2. In the yeast Saccharomyces cerevisiae, Ste20 is involved in transmitting the mating-pheromone signal from the βγ-subunits (encoded by the STE4 and STE18 genes, respectively) of a heterotrimeric G protein to a downstream MAP kinase cascade1. We have identified a binding site for the G-protein β-subunit (Gβ) in the non-catalytic carboxy-terminal regions of Ste20 and its mammalian homologues, the p21-activated protein kinases (PAKs). Association of Gβ with this site in Ste20 was regulated by binding of pheromone to the receptor. Mutations in Gβ and Ste20 that prevented this association blocked activation of the MAP kinase cascade. Considering the high degree of structural and functional conservation of Ste20/PAK family members and G-protein subunits, our results provide a possible model for a role of these kinases in Gβγ-mediated signal transduction in organisms ranging from yeast to mammals.
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We thank A. Nantel for helpful discussion, D. Harcus, D. Dignard and L. Johnson for excellent technical assistance, R. Tsien for supplying the GFP plasmid and S. Sprang for providing the Gβ1 structure coordinates. The NRC publication number for this work is 39976.
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