Control of cell cycle progression by the last MAPK Hog1

Abstract

Exposure of yeast to increases in extracellular osmolarity activates the stress-activated Hog1 MAP kinase, which is essential for cell survival upon osmotic stress. Activation of the Hog1 MAPK results in cell growth arrest, suggesting a possible role of the MAP kinase in the control of the cell cycle. Our results have shown that Hog1 activation resulted in accumulation of cells in the G1/S and G2/M transitions. At G1, Hog1 regulates the cell cycle progression by a dual mechanism that involves downregulation of G1 cyclin expression and direct targeting of the CDK-inhibitor protein Sic1. The MAPK interacts with Sic1, and phosphorylates a single residue of Sic1, which, in combination with the downregulation of cyclin expression, results in Sic1 stabilization and inhibition of cell cycle progression. Consistently, sic1_ cells, or cells containing a SIC1 allele mutated in the Hog1 phosphorylation site, are unable to arrest at G1 phase after Hog1 activation, and become sensitive to osmostress. Together, our data indicate that Sic1 is the molecular target for Hog1 that is required to modulate cell cycle progression in response to stress at G1. On the other hand, activation of the Hog1 MAPK also results in an increase of cells in the G2 phase. Arrested cells displayed down regulation of the Clb2-Cdc28 kinase activity and consequently enlarged buds, defects in spindle formation and orientation. These effects were prevented by deletion of the SWE1 gene. Thus, swe1Ä cells failed to arrest at G2, which resulted in a premature entry into mitosis and mislocalization of nuclei. Consistently, swe1Ä cells were osmosensitive. Swe1 degradation was reduced in response to activation of Hog1. Swe1 accumulation is mediated by the activity of the complex Hsl1-Hsl7. Hog1 phosphorylates a single residue at the regulatory domain of Hsl1, which leads to the mislocalization of Hsl7 from the bud neck, and consequent Swe1 accumulation. In addition, Hog1 downregulates G2 cyclin expression, reinforcing the inhibition of cell cycle progression at G2/M. These results indicate that Hog1 imposes a delay in critical phases of cell cycle progression necessary for proper cellular adaptation to new extracellular conditions.