INHIBITORY AND ACTIVATING FUNCTIONS FOR MAPK KSS1 IN THE SACCHAROMYCES-CEREVISIAE FILAMENTOUS-GROWTH SIGNALING PATHWAY

Mitogen-activated protein kinase (MAPK) cascades are conserved signall ing modules that regulate responses to diverse extracellular stimuli, developmental cues and environmental stresses (reviewed in refs 1-3). A MAPK is phosphorylated and activated by a MAPK kinase (MAPKK), which is activated by an upstream protein kinase, such as Raf, Mos or a MAP KK kinase. Ste7, a MAPKK in the yeast Saccharomyces cerevisiae, is req uired for two developmental pathways: mating(4) and invasive (filament ous) growth(5,6). Kss1 and Fus3, the MAPK targets of Ste7, are require d for mating(7,8), but their role in invasive growth has been unclear.