Hog1MAPK

Hog1 MAPK is a serine/threonine protein kinase in the budding yeast Saccharomyces cerevisiae. It is the central effector of the High Osmolarity Glycerol (HOG) pathway and coordinates cellular adaptation to hyperosmotic stress. Upon osmotic shock, two upstream osmosensors transmit signals to Hog1 via the MAP kinase kinase Pbs2. The Sln1-Ypd1-Ssk1 branch and the Sho1 branch converge on Pbs2, which directly phosphorylates Hog1 at a conserved TXY activation motif. Phosphorylated Hog1 translocates to the nucleus and orchestrates transcriptional and metabolic changes necessary for osmotic tolerance.

Transcriptional responses include induction of glycerol biosynthesis and transport genes, notably GPD1 and GPP2, enabling accumulation of glycerol as a compatible solute to restore turgor. Hog1 also modulates the activity of transcription factors such as Hot1 and Sko1, shaping a broader osmoresponsive gene program. In addition to gene expression changes, Hog1 influences cell cycle progression and metabolic reprogramming during osmotic stress. It also participates in cross-talk with other stress pathways, including responses to oxidative and temperature stress.

Regulation and termination: Hog1 activity is reversed by dual-specificity phosphatases Ptp2 and Ptp3, which dephosphorylate Hog1 to restore the kinase to its inactive state once homeostasis is reestablished. Conserved across fungi, Hog1 homologs serve similar roles in osmotic adaptation in other yeasts and filamentous fungi.

---