In fission yeast the onset of mitosis is brought about by Cdc2/Cdc13 kinase, which is inhibited by the Wee1/Mik1 tyrosine kinases and activated by Cdc25 tyrosine phosphatase. 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 Spc1 promotes the onset of mitosis. Spc1 is a MAP kinase homologue that is activated by Wis1 kinase in response to osmotic stress and nutrient limitation. Spc1 is inactivated by Pyp1, a phosphatase previously identified as a mitotic inhibitor. Pyp1 dephosphorylates only tyrosine-173 of Spc1, unlike the dual-specificity phosphatases that have been shown to regulate other MAP kinases.

Studies of Schizosaccharomyces pombe have played an instrumental role in uncovering how eukaryotic cells regulate the initiation of mitosis. Cdc2, the cyclin-dependent kinase that is the focus of this control, is maintained in an inhibited state during S and G2 attributable to phosphorylation of tyrosine-15 carried out by Weel and Mikl kinases (Dunphy 1994). Cdc2 is activated by Cdc25 tyrosine phosphatase to bring about the onset of mitosis. A major aim of recent studies has been to discover how the mitotic control is linked to checkpoint mechanisms that monitor genome replication and integrity, but recently it has become apparent that fission yeast also links the mitotic control with changes in the extracellular environment that affect cell physiology. A key element of this signal transduction pathway is Spcl, a mitogen-activated protein (MAP) kinase homolog that is also known as Styl (Millar et al. 1995;Shiozaki and Russell 1995). At optimal growth conditions Spcl has a ICorresponding author. basal activity that is important for determining the timing of mitosis, as shown by the fact that spcl-cells are -20% elongated at division compared with wild type. A spcl-mutation causes G2 arrest in cells that have a partial loss of Cdc25 activity, confirming that Spcl activity affects the mitotic control (Millar et al. 1995;Shiozaki and Russell 1995).Spcl becomes highly activated in response to various forms of cytotoxic stress, including high osmolarity, oxidative stress, and high temperature (Millar et al. 1995;Shiozaki and Russell 1995;Degols et al. 1996). Failure to activate Spcl during stress leads to G2 arrest and cell death. Spcl is most closely related to a conserved family of stress-activated protein kinases that includes budding yeast Hoglp (Brewster et al. 1993), Xenopus Mpk2 (Rouse et al. 1994), routine p38 (Han et al. 1994, and human CSBP1 . These kinases have a Thr-Gly-Tyr sequence in the region of activating phosphorylations, whereas other members of the MAP kinase family have Thr-Glu-Tyr or Thr-Pro-Tyr at corresponding positions. The MAPK kinase homolog Wisl (Warbrick and Fantes 1991) activates Spcl by phosphorylating

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Spc1, an osmotic-stress-stimulated mitogen-activated protein kinase (MAPK) homolog in the fission yeast Schizosaccharomyces pombe, is required for the induction of mitosis and survival in high-osmolarity conditions. Spc1, also known as Sty1, is activated by Wis1 MAPK kinase and inhibited by Pyp1 tyrosine phosphatase. Spc1 is most closely related to Saccharomyces cerevisiae Hog1 and mammalian p38 kinases. Whereas Hog1 is specifically responsive to osmotic stress, we report here that Spc1 is activated by multiple forms of stress, including high temperature and oxidative stress. In this regard Spc1 is more similar to mammalian p38. Activation of Spc1 is crucial for survival of various forms of stress. Spc1 regulates expression of genes encoding stress-related proteins such as glycerol-3-phosphate dehydrogenase (gpd1 ؉ ) and trehalose-6-phosphate synthase (tps1 ؉ ). Spc1 also promotes expression of pyp2 ؉ , which encodes a tyrosine phosphatase postulated as a negative regulator of Spc1. This proposal is supported by the finding that Spc1 associates with Pyp2 in vivo and that the amount of Spc1 tyrosine phosphorylation is lower in a Pyp2-overproducing strain than in the wild type. Moreover, the level of stress-stimulated gpd1 ؉ expression is higher in ⌬pyp2 mutants than in the wild type. These findings demonstrate that Spc1 promotes expression of genes involved in stress survival and that Spc1-regulated transcription of one of these genes, pyp2 ؉ , serves to attenuate Spc1 activity. This mechanism of regulation may be commonly employed to modulate MAPK signal transduction pathways in eukaryotic species.Eukaryotic cells have highly effective mechanisms of adapting to environmental changes which cause physiological stress. In several cases it is clear that exposure to environmental insults, such as high temperature, high osmolarity, genotoxic agents, or hydroxyl radicals, leads to stimulation of kinases that phosphorylate transcription factors and thereby modulate gene expression (21). Spc1, a mitogen-activated protein kinase (MAPK) homolog in the fission yeast Schizosaccharomyces pombe, is an example of a stress-stimulated kinase (23, 36). Spc1, also known as Sty1 (23), is activated following exposure to highosmolarity media or growth in suboptimal nutrient conditions. Like all MAPK homologs, Spc1 is activated by a MAPK kinase homolog, in this case Wis1 (40), which phosphorylates threonine 171 and tyrosine 173 of Spc1 (36). The activating tyrosine phosphorylation is convenient for analytical studies because it is easily monitored by immunoblotting with antiphosphotyrosine antibodies. The level of Spc1 tyrosine phosphorylation increases in response to osmotic stress, and this phosphorylation is dependent on Wis1 in vivo (23, 36).The spc1 ϩ gene was first discovered in a genetic screen that identified mutations of five loci that rescued lethal phenotypes caused by loss of protein phosphatase 2C activity (37). Our interest in the stress response arose from the discovery that spc1 Ϫ and wis1 Ϫ (spc2 Ϫ ) mutants grow to a ...

Pom1 kinase recruited to cell ends by the Tea1-Tea4/Wsh3 complex is essential for proper localization of a GAP for Cdc42, Rga4, which ensures bipolar localization of GTP-bound, active Cdc42. Because of the established role of Cdc42 in F-actin formation, these observations provide a new insight into how the microtubule system achieves localized formation of F-actin to generate cell polarity.