PROTEIN PHOSPHATASE 2C ACTS INDEPENDENTLY OF STRESS-ACTIVATED KINASE CASCADE TO REGULATE THE STRESS-RESPONSE IN FISSION YEAST

Stress-activated signal transduction pathways, which are largely conse rved among a broad spectrum of eukaryotic species, have a crucial role in the survival of many forms of stress. It is therefore important to discover how these pathways are both positively and negatively regula ted. Recent genetic studies have implicated protein phosphatase 2C (PP 2C) as a novel negative regulator of stress response pathways in both budding and fission yeasts, Moreover, it was hypothesized that PP2C de phosphorylates one or more components of protein kinase cascades that are at the core of stress-activated signal transduction pathways, Here in we present genetic and biochemical studies of the fission yeast Sch izosaccharomyces pombe that disprove this hypothesis and indicate that PP2C instead negatively regulates a downstream element of the pathway . First, high expression of PPSC produces phenotypes that are inconsis tent with negative regulation of the Wik1-Wis1-Spc1 stress-activated k inase cascade. Second, high expression of PPSC leads to sustained acti vating tyrosine phosphorylation of Spc1. Third, Spc1-dependent phospho rylation of Atf1, a transcription factor substrate of Spc1, is unaffec ted by high expression of PP2C, Fourth, high expression of PP2C suppre sses Atf1-dependent transcription of a stress-response gene, These stu dies strongly suggest that PP2C acts down stream of Spc1 kinase in the stress-activated signal transduction pathway.