Srk1 kinase suppresses mitosis in response to heat stress in fission yeast
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Fission yeast Srk1 kinase is essential for cellular responses to extracellular stimuli. It is activated downstream of the MAPK Spc1 and participates in controlling mitotic entry by directly phosphorylating and inhibiting Cdc25 phosphatase during the normal cell cycle and also in response to osmotic stress. Following phosphorylation, Cdc25 is exported out of the nucleus. Heat stress caused by a temperature shift from 25°C to 36°C, which is within the normal temperature tolerance of fission yeast, temporarily inhibits nuclear division in wildtype cells. The same response is seen in cells deleted for the Pyp1 and Pyp2 phosphatases which normally serve to down regulate the Spc1 stress response. I have shown that the transient block in nuclear division caused by temperature shifts within the physiological range does not occur in srk1- cells but instead there is a stimulation of mitosis and cell division in response to the heat stress. This pattern of mitotic stimulation is phenocopied in cdc25-9a cells where nine putative Srk1 phosphorylation sites on Cdc25 are changed to alanine. Cells lacking srk1, however, display the same cell cycle progression pattern as the wildtype cells in response to osmotic stress. This experiment clearly distinguishes separate pathways for these two stress responses. Also, Cdc25 is found to be phosphorylated after a mild heat stress and seems to be exported out of the nucleus. These data indicate that Srk1 kinase plays the central role in regulating mitotic entry in response to mild heat stress by negatively regulating the Cdc25 tyrosine phosphatase. Thus my work highlights the role of Srk1 kinase in cell cycle regulation and is consistent with the Spc1 MAPK cascade linking the G2/M transition to extracellular stress. This response to mild heat stress is important because temperature sensitive mutants are frequently used in fission yeast research and this work shows that a temperature shift from 25°C to 36°C, which is within the physiological growth range, can trigger a stress response.