One of our on-going efforts has been identification of physiological substrates of the Cln-Cdc28 kinases. We isolated two novel proteins, called Rst1 and Rst2 (for regulator of sterile twelve), that specifically interact with Cln1 and Cln2. We discovered that Rst1/2 also physically associate with the MAP kinases of the pheromone pathway, Fus3 and Kss1. Genetic analysis showed that Rst1/2 are redundant inhibitors of pheromone-dependent gene
expression and filamentous growth. Thus, rst1 rst2 double mutants but not either single mutant constitutively express pheromone-inducible and filamentous growth-specific genes. Our working model is that Rst1/2 bind directly to the Ste12 transcription factor and repress transcriptional activation. Upon stimulation of the MAP kinases, Rst1/2 are phosphorylated and released from Ste12, thereby liberating Ste12 to activate transcription
(Figure). Thus MAP kinases may regulate gene expression not only by phosphorylation dependent stimulation of transcriptional activators, but also by phosphorylation-dependent inactivation of transcriptional repressors.