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 Wh Show more
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. Show less
The Start/G1 phase in the cell cycle is an important period during which cells determine their developmental fate, onset of mitotic progression, or the switch to developmental stages in response to bo Show more
The Start/G1 phase in the cell cycle is an important period during which cells determine their developmental fate, onset of mitotic progression, or the switch to developmental stages in response to both external and internal signals. In the budding yeast Saccharomyces cerevisiae, Whi3, a negative regulator of the G1 cyclins, has been identified as a positive regulator of cell size control and is involved in the regulation of Start. However, the regulatory pathway of Whi3 governing the response to multiple signals remains largely unknown. Here, we show that Whi3 is phosphorylated by the Ras/cAMP-dependent protein kinase (PKA) and that phosphorylation of Ser-568 in Whi3 by PKA plays an inhibitory role in Whi3 function. Phosphorylation of Whi3 by PKA led to its decreased interaction with CLN3 G1 cyclin mRNA and was required for the promotion of G1/S progression. Furthermore, we demonstrate that the phosphomimetic S568D mutation of Whi3 prevented the developmental fate switch to sporulation or invasive growth. Thus, PKA modulated the function of Whi3 by phosphorylation, thus implicating PKA-mediated modulation of Whi3 in multiple cellular events. Show less