Fig. 1. Schematic model outlining the role of Sho1, Cdc42 and Ste20 in the HOG MAP kinase pathway. The membrane-bound protein Sho1 anchors the Hog1 MAP kinase module to the membrane through its interaction with the Pbs2 MAPKK, which in turn interacts with the Ste11 MAPKKK and Hog1 MAP kinase. In response to osmotic stress, Cdc42-bound, membrane-localized Ste20 phosphorylates Ste11, which leads to the activation of the Pbs2 MAPKK and the Hog1 MAP kinase, and eventually to the osmo-adaptive response. Exactly how osmotic stress induces Ste20 activation is unclear. 50BD, Ste50 binding domain; CRIB, Cdc42/Rac interactive binding domain; PPP, proline-rich sequence.