FIG. 5.

High osmolarity inhibits phosphotransfer in the phosphorelay system and causes activation of the HOG MAPK cascade. Normal osmolarity conditions (left) stimulate phosphotransfer and result in phosphorylation of Ssk1. Phosphorylated Ssk1 cannot activate the MAPK cascade, and consequently there is no response. High osmolarity (right) inhibits Sln1 by an unknown mechanism. Phosphotransfer is blocked, and unphosphorylated Ssk1 is proposed to activate the MAPK cascade, resulting in the stress response. Arrows indicate activation; lines with bars indicate inhibition. The dashed rectangle around the MAPK cascade (left) indicates that it is inactive. The dashed rectangle around Ypd1 (right) indicates that the protein is present but not phosphorylated. See the text for details.