COUPLING OF CELL IDENTITY TO SIGNAL RESPONSE IN YEAST - INTERACTION BETWEEN THE ALPHA-1 AND STE12 PROTEINS

In Saccharomyces cerevisiae, the STE12 protein mediates transcriptional induction of cell type-specific genes in response to pheromones. STE12 binds in vitro to the pheromone response elements (PREs) present in the control region of alpha-specific genes. STE12 is also required for transcription of alpha-specific genes, but there is no evidence that it binds directly to these genes. Instead, the MATalpha-encoded protein alpha1 and the MCM1 product bind to the DNA element that is responsible for alpha-specific and alpha-factor-inducible expression. To explore the role of STE12 in the pheromone induction of alpha-specific genes, we cloned STE12 and MATalpha1 homologs from the related yeast Kluyveromyces lactis. The K. lactis STE12 protein did not cooperate with the S. cerevisiae alpha1 protein to promote the overall mating process or the induction of transcription of an alpha-specific gene. However, introduction of both K. lactis STE12 along with K. lactis alpha1 did restore mating, suggesting that an interaction between STE12 and alpha1 is important for alpha-specific gene activation. We also show that bacterially expressed STE12 and alpha1 are able to form a complex in vitro. Thus, we demonstrate a coupling in alpha cells between a protein functioning in cell identity, alpha1, with a protein responsive to the pheromone-induced signal STE12.