Background: In the budding yeast Saccharomyces cerevisiae, components of a single mitogen-activated protein (MAP) kinase pathway transduce two distinct signals, each of which activates an independent developmental 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 filamentous growth, while the other, Kss 1, preferentially triggers filamentous growth. Both kinases activate the same transcription factor, Ste 12, which can stimulate gene expression specific to each of the developmental programmes. The precise mechanism by which these MAP kinases activate Ste 12, however, is not clear.
Results: Two newly identified proteins, Rst 1 and Rst 2 (also known as Dig1 and Dig2), were found to associate physically with Fus3 and Ste12. Rst1 and Rst2 were prominent substrates in kinase reactions of Fus3 immune complexes from pheromone-treated cells. Association of Fus3 with Ste12 required Rst1 and Rst2, and activation of Fus3 by pheromone caused release of Ste12 from the Fus3 complex. Although rst1 and rst2 single mutants had no obvious phenotype, 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 upstream kinases. Consistent with Rst1 and Rst2 having a role in Ste12 regulation, both proteins were localized to the nucleus.
Conclusions: Rst1 and Rst2 repress the mating and filamentous growth responses of S. cerevisiae by directly inhibiting Ste12. Activation of Fus3 or Kss1 may cause phosphorylation-dependent release of Ste12 from Rst1/Rst2 and thereby activate Ste12-dependent transcription.