MULTIPLE-MODES OF ACTIVATION OF THE STRESS-RESPONSIVE MAP KINASE PATHWAY IN FISSION YEAST

The Schizosaccharomyces pombe wis1(+) gene is essential for cell survi val under stress conditions. The MAPKK homologue Wis1 is required for activation of the MAPK homologue Spc1, and integrity of the Wis1-Spc1 pathway is required for survival in extreme conditions of heat, osmola rity, oxidation or limited nutrition, We show here that Wis4, a protei n kinase of a new MAPKKK class, phosphorylates Wis1 in vitro and activ ates it in vivo, Win1 is also required for full activation of Wis1, an d Win1 rather than Wis4 mediates the osmotic stress signal. Surprising ly, the pathway can still be activated by heat or oxidative stress ind ependently of the phosphorylation of two conserved Wis1 residues, Evid ence is presented that the Pyp1 protein tyrosine phosphatase, which de phosphorylates Spc1, is central to this alternative activation mechani sm.