Fig. 1

Schematic drawing of the flow of phosphoryl groups through the Sln1 pathway of S. cerevisiae in the wild-type version and with a LOV-histidine kinase fusion protein replacing the Sln1 kinase. The left-hand panel shows the flow of phosphoryl groups under wall-stress conditions (maximal activity of native Sln1) and in the dark (maximal activity of the C1, fusion protein). The right-hand panel shows the absence of flow of the phosphoryl groups in the presence of osmotic stress (for the native Sln1) and upon illumination (for the C1 fusion protein). The C1 fusion protein is most active in the dark. This leads to phosphorylation of the Ssk1 and Skn7 response regulators via transfer of the phosphoryl group through the Ypd1 phospho-transfer protein. Phosphorylation of Ssk1 renders it inactive in its role in the HOG1 pathway. Phosphorylation of Skn7 leads to activation of Skn7-dependent genes, such as the mannosyl-transferase, OCH1. Osmotic stress and illumination lead to reduction in the kinase activity of Sln1 and the C1 fusion protein, respectively, and therefore accumulation of the kinase domain of Sln1 in the dephosphorylated form. This causes de-phosphorylation of the Ssk1 response regulator, which can interact with and activate Ssk2 and the Ssk22 kinases of the HOG pathway. Phosphorylation of HOG1 leads to its translocation into nucleus and activation of the genes responsible for osmotic-stress regulation (Fassler and West 2010; Gao and Stock 2009)