Functional characterization of the interaction of Ste50p with Ste11p MAPKKK in Saccharomyces cerevisiae

The Saccharomyces cerevisiae Ste11p protein kinase is a homologue of mammal ian MAPK/ extracellular signal-regulated protein kinase kinase kinases (MAP KKKs or MEKKs) as well as the Schizosaccharomyces pombe Byr2p kinase. Ste11 p functions in several signaling pathways, including those for mating phero mone response and osmotic stress response. The Ste11p kinase has an N-termi nal domain that interacts with other signaling molecules to regulate Ste11p function and direct its activity in these pathways. One of the Ste11p regu lators is Ste50p, and Ste11p and Ste50p associate through their respective N-terminal domains. This interaction relieves a negative activity of the St e11p N terminus, and removal of this negative function is required for Ste1 1p function in the high-osmolarity glycerol (HOG) pathway. The Ste50p/Ste11 p interaction is also important (but not essential) for Ste11p function in the mating pathway; in this pathway binding of the Ste11p N terminus with b oth Ste50p and Ste5p is required, with the Ste5p association playing the ma jor role in Ste11p function. In vitro, Ste50p disrupts an association betwe en the catalytic C terminus and the regulatory N terminus of Ste11p. In add ition, Ste50p appears to modulate Ste11p autophosphorylation and is itself a substrate of the Ste11p kinase. Therefore, both in vivo and in vitro data support a role for Ste50p in the regulation of Ste11p activity.