REGULATION OF THE MATING PHEROMONE AND INVASIVE GROWTH-RESPONSES IN YEAST BY 2 MAP KINASE SUBSTRATES

Background: In the budding yeast Saccharomyces cerevisiae, components of a single mitogen-activated protein (MAP) kinase pathway transduce t wo distinct signals, each of which activates an independent developmen tal programme: peptide mating pheromones initiate the mating response. whereas nutrient limitation initiates filamentous growth. One of the MAP kinases in this pathway, Fus3, triggers mating but antagonizes fil amentous growth, while the other, Kss1, preferentially triggers filame ntous growth. Both kinases activate the same transcription factor, Ste 12, which can stimulate gene expression specific to each of the develo pmental programmes. The precise mechanism by which these MAP kinases a ctivate Ste12, however, is not clear. Results: Two newly identified pr oteins, Rst1 and Rst2 (also known as Dig1 and Dig2), were found to ass ociate physically with Fus3 and Ste12. Rst1 and Rst2 were prominent su bstrates in kinase reactions of Puss immune complexes from pheromone-t reated cells. Association of Fus3 with Ste12 required Rst1 and Rst2, a nd activation of Fuss by pheromone caused release of Ste12 from the Fu s3 complex. Although rst1 and rst2 single mutants had no obvious pheno type, both filamentous growth and mating-specific gene expression were constitutive in rst1 rst2 double mutants. The phenotype of rst1 rst2 cells required Ste12 function, but did not require the function of ups tream kinases. Consistent with Rst1 and Rst2 having a role in Ste12 re gulation, both proteins were localized to the nucleus. Conclusions: Rs t1 and Rst2 repress the mating and filamentous growth responses of S. cerevisiae by directly inhibiting Ste12. Activation of Fus3 or Kss1 ma y cause phosphorylation-dependent release of Ste12 from Rst1/Rst2 and thereby activate Ste12-dependent transcription. (C) Current Biology Lt d.