FIG. 1.

The osmosensing signal transduction pathway in yeast. Hyperosmotic stress inhibits the phosphorelay system initiated from the Sln1p membrane-associated histidine kinase, which functions as a homodimer, to cause reduced levels of phosphorylation of Ypd1p and the response regulator Ssk1p. Unphosphorylated Ssk1p activates the Ssk2p MAPKKK, which in turn activates the downstream HOG MAPK cascade. Skn7p is another response regulator phosphorylated by the phosphorelay system, which is a transcriptional factor. Another sensor system that is dependent on the membrane anchor Sho1p also functions, in which Pbs2p is activated by Ste11p (36, 47, 51, 52). Msb2p is also implicated in osmosensing (not shown) (43). MAPKK, MAPK kinase.