Regulation of MAPK function by direct interaction with the mating-specific Galpha in yeast.

The mating response of the budding yeast Saccharomyces cerevisiae is mediated by a prototypical heterotrimeric GTP-binding protein (G protein) and mitogen-activated protein kinase (MAPK) cascade. Although signal transmission by such pathways has been modeled in detail, postreceptor down-regulation is less well understood. The pheromone-responsive G protein alpha subunit (Galpha) of yeast down-regulates the mating signal, but its targets are unknown. We have found that Galpha binds directly to the mating-specific MAPK in yeast cells responding to pheromone. This interaction contributes both to modulation of the mating signal and to the chemotropic response, and it demonstrates direct communication between the top and bottom of a Galpha-MAPK pathway.