Mitogen-activated protein kinase Hog1 mediates adaptation to G1 checkpoint arrest during arsenite and hyperosmotic stress

Cells slow down cell cycle progression in order to adapt to unfavorable stress conditions. Yeast (Saccharomyces cerevisiae) responds to osmotic stress by triggering G₁ and G₂ checkpoint delays that are dependent on the mitogen-activated protein kinase (MAPK) Hog1. The high-osmolarity glycerol (HOG) pathway is also activated by arsenite, and the hog1Δ mutant is highly sensitive to arsenite, partly due to increased arsenite influx into hog1Δ cells. Yeast cell cycle regulation in response to arsenite and the role of Hog1 in this process have not yet been analyzed. Here, we found that long-term exposure to arsenite led to transient G₁ and G₂ delays in wild-type cells, whereas cells that lack the HOG1 gene or are defective in Hog1 kinase activity displayed persistent G₁ cell cycle arrest. Elevated levels of intracellular arsenite and “cross talk” between the HOG and pheromone response pathways, observed in arsenite-treated hog1Δ cells, prolonged the G₁ delay but did not cause a persistent G₁ arrest. In contrast, deletion of the SIC1 gene encoding a cyclin-dependent kinase inhibitor fully suppressed the observed block of G₁ exit in hog1Δ cells. Moreover, the Sic1 protein was stabilized in arsenite-treated hog1Δ cells. Interestingly, Sic1-dependent persistent G₁ arrest was also observed in hog1Δ cells during hyperosmotic stress. Taken together, our data point to an important role of the Hog1 kinase in adaptation to stress-induced G₁ cell cycle arrest.