Yeast Cdc42 GTPase and Ste20 PAK-like kinase regulate Sho1-dependent activation of the Hog1 MAPK pathway.

The adaptive response to hyperosmotic stress in yeast, termed the high osmolarity glycerol (HOG) response, is mediated by two independent upstream pathways that converge on the Pbs2 MAP kinase kinase (MAPKK), leading to the activation of the Hog1 MAP kinase. One branch is dependent on the Sho1 transmembrane protein, whose primary role was found to be the binding and translocation of the Pbs2 MAPKK to the plasma membrane, and specifically to sites of polarized growth. The yeast PAK homolog Ste20 is essential for the Sho1-dependent activation of the Hog1 MAP kinase in response to severe osmotic stress. This function of Ste20 in the HOG pathway requires binding of the small GTPase Cdc42. Overexpression of Cdc42 partially complements the osmosensitivity of ste20Delta mutants, perhaps by activating another PAK-like kinase, while a dominant-negative Cdc42 mutant inhibited signaling through the SHO1 branch of the HOG pathway. Since activated Cdc42 translocates Ste20 to sites of polarized growth, the upstream and downstream elements of the HOG pathway are brought together through the membrane targeting function of Sho1 and Cdc42.