Cell-cycle control linked to extracellular environment by MAP kinase pathway in fission yeast

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Abstract
IN fission yeast the onset of mitosis is brought about by Cdc2/ Cdcl3 kinase, which is inhibited by the Weel/Mikl tyrosine kin-ases and activated by Cdc25 tyrosine phosphatase1. This control network integrates many signals, including those that monitor DNA replication, DNA damage and cell size. We report here that a fission yeast MAP kinase pathway links the cell-cycle G2/M control with changes in the extracellular environment that affect cell physiology. Fission yeast spc1 mutants have a G2 delay that is greatly exacerbated by growth in high osmolarity media and nutrient limitation. A lethal interaction of spc1 and cdc25 mutations shows that Spcl promotes the onset of mitosis. Spcl is a MAP kinase homologue that is activated by Wisl kinase in response to osmotic stress and nutrient limitation. Spcl is inactivated by Pypl, a phosphatase previously identified as a mitotic inhibitor2,3. Pypl dephosphorylates only tyrosine-173 of Spcl, unlike the dual-specificity phosphatases that have been shown to regulate other MAP kinases4.