Dimerization of Ste5, a mitogen-activated protein kinase cascade scaffold protein, is required for signal transduction

The mitogen-activated protein kinase cascade of the Saccharomyces cerevisiae pheromone response pathway is organized on the Ste5 protein, which binds each of the kinases of the cascade prior to signaling. In this study, a structure-function analysis of Ste5 deletion mutants uncovered new functional domains of the Ste5 protein and revealed that Ste5 dimerizes during the course of normal signal transduction. Dimerization, mediated by two regions in the N-terminal half of Ste5, was first suggested by intragenic complementation between pairs of nonfunctional Ste5 mutants and was confirmed by using the two-hybrid system. Coimmunoprecipitation of differently tagged forms of Ste5 from cells in which the pathway has been activated by Ste5 overexpression further confirmed dimerization. A precise correlation between the biological activity of various Ste5 fragments and dimerization suggests that dimerization is essential for Ste5 function.