The Srk1 protein kinase is a target for the Sty1 stress-activated MAPK in fission yeast

The fission yeast stress-activated Sty1/Spc1 MAPK pathway responds to a similar range of stresses as do the mammalian p38 and SAPK/JNK MAPK pathways. In addition, sty1(-) cells are sterile and exhibit a G(2) cell cycle delay, indicating additional roles of Sty1 in meiosis and cell cycle progression. To identify novel proteins involved in stress responses, a microarray analysis of the Schizosaccharomyces pombe genome was performed to find genes that are up-regulated following exposure to stress in a Sty1-dependent manner. One such gene identified, srk1(+) (Sty1-regulated kinase 1), encodes a putative serine/threonine kinase homologous to mammalian calmodulin kinases. At the C terminus of Srk1 is a putative MAPK binding motif similar to that in the p38 substrates, MAPK-activated protein kinases 2 and 3. Indeed, we find that Srk1 is present in a complex with the Sty1 MAPK and is directly phosphorylated by Sty1. Furthermore, upon stress, Srk1 translocates from the cytoplasm to the nucleus in a process that is dependent on the Sty1 MAPK. Finally, we show that Srk1 has a role in regulating meiosis in fission yeast; following nitrogen limitation, srk1(-) cells enter meiosis significantly faster than wild-type cells and overexpression of srk1(-) inhibits the nitrogen starvation-induced arrest in G(1).