REPRESSION OF YEAST STE12 TRANSCRIPTION FACTOR BY DIRECT BINDING OF UNPHOSPHORYLATED KSS1 MAPK AND ITS REGULATION BY THE STE7 MEK

The mitogen-activated protein kinase (MAPK) Kss1 has a dual role in re gulating filamentous (invasive) growth of the yeast Saccharomyces cere visiae. The stimulatory function of Kss1 requires both its catalytic a ctivity and its activation by the MAPK/ERK kinase (MEK) Ste7; in contr ast, the inhibitory function of Kss1 requires neither. This study exam ines the mechanism by which Kss1 inhibits invasive growth, and how Ste 7 action overcomes this inhibition. We found that unphosphorylated Kss 1 binds directly to the transcription factor Ste12, that this binding is necessary for Kss1-mediated repression of Ste12, and that Ste7-medi ated phosphorylation of Kss1 weakens Kss1-Ste12 interaction and reliev es Kss1-mediated repression. Relative to Kss1, the MAPK Fus3 binds les s strongly to Ste12 and is correspondingly a weaker inhibitor of invas ive growth. Analysis of Kss1 mutants indicated that the activation loo p of Kss1 controls binding to Ste12. Potent repression of a transcript ion factor by its physical interaction with the unactivated isoform of a protein kinase, and relief of this repression by activation of the kinase, is a novel mechanism for signal-dependent regulation of gene e xpression.