Stress-responsive changes in the mRNA levels of MAPK-related protein kinase (TpMAPK) and NIMA-related protein kinase (TpNrk) in Tetrahymena cells

Abstract

To elucidate the stress signaling pathway in Tetrahymena cells, we have examined the mRNA levels of TpMAPK which shows significant sequence similarity to those of mitogen-activated protein kinases, especially p38 and stress-activated protein kinase (SAPK) involved in various stress responses, and TpNrk that has high sequence homology to other NIMA-related protein kinases (Neks) involved in the control of the cell cycle in response to cold and osmotic stresses. The transcript levels of TpMAPK and TpNrk were sensitive to the decreasing temperature of the culture media; TpMAPK mRNA level was increased to reach the maximal level at 6 hour and TpNrk mRNA peaked at 2 hour after the growth temperature down shift to 6 °C. The time-dependent activations of TpMAPK and TpNrk were also observed by exposure to osmotic stress. TpMAPK mRNA was close to the maximal level at 2 hour after cells were exposed to 50 mM NaCl and at 1 hour after exposure to 100 mM sorbitol, whereas TpNrk mRNA was increased to the highest level at 30 min after exposure to 50 mM NaCl or 100 mM sorbitol. These results suggest that TpMAPK and TpNrk are stress-responsive genes and that the activation of TpNrk appears to precede the TpMAPK activation in response to the cold and osmotic stresses in Tetrahymena cells. Based on these observations, we speculate that TpMAPK and TpNrk genes would be involved in the stress signal transduction pathway in protozoan Tetrahymena cells.