Membrane recruitment of the kinase cascade scaffold protein Ste5 by the Gβγ complex underlies activation of the yeast pheromone response pathway

In the Sacchoromyces cerevisiae pheromone response pathway, the G beta gamma complex activates downstream responses by an unknown mechanism involving a MAP kinase cascade, the PAK-like kinase Ste20, and a Rho family GTPase, Cdc42. Here G beta gamma show that G beta gamma must remain membrane-associated after release from G alpha to activate the downstream pathway. We also show that pheromone stimulates translocation of the kinase cascade scaffold protein Ste5 to the cell surface. This recruitment requires G beta gamma function and the G beta gamma-binding domain of Ste5, but not the kinases downstream of G beta gamma, suggesting that it is mediated by G beta gamma itself. Furthermore, this event has functional significance, as artificial targeting of Ste5 to the plasma membrane, but not intracellular membranes, activates the pathway in the absence of pheromone or G beta gamma. Remarkably, although independent of G beta gamma, activation by membrane-targeted Ste5 requires Ste20, Cdc42, and Cdc24, indicating that their participation in this pathway does not require them to be activated by G beta gamma. Thus, membrane recruitment of Ste5 defines a molecular activity for G beta gamma. Moreover, our results suggest that this event promotes kinase cascade activation by delivering the Ste5-associated kinases to the cell surface kinase Ste20, whose function may depend on Cdc42 and Cdc24.