Evidence of Antagonistic Regulation of Restart from G1 Delay in Response to Osmotic Stress by the Hog1 and Whi3 in Budding Yeast

Hog1 of Saccharomyces cerevisiae is activated by hyperosmotic stress, and this leads to cell-cycle delay in G₁, but the mechanism by which cells restart from G₁ delay remains elusive. We found that Whi3, a negative regulator of G₁ cyclin, counteracted Hog1 in the restart from G₁ delay caused by osmotic stress. We have found that phosphorylation of Ser-568 in Whi3 by RAS/cAMP-dependent protein kinase (PKA) plays an inhibitory role in Whi3 function. In this study we found that the phosphomimetic Whi3 S568D mutant, like the Δwhi3 strain, slightly suppressed G₁ delay of Δhog1 cells under osmotic stress conditions, whereas the non-phosphorylatable S568A mutation of Whi3 caused prolonged G₁ arrest of Δhog1 cells. These results indicate that Hog1 activity is required for restart from G₁ arrest under osmotic stress conditions, whereas Whi3 acts as a negative regulator for this restart mechanism.