Cesium chloride sensing and signaling in Saccharomyces cerevisiae: an interplay among the HOG and CWI MAPK pathways and the transcription factor Yaf9.

In yeast, many environmental stimuli are sensed and signaled by the MAP kinases pathways. In a previous work, we showed that cesium chloride activates the HOG pathway and modulates the transcription of several genes, especially those involved in cell wall biosynthesis and organization. The response to cesium was largely overlapping with the response to salt and osmotic stress. However, when low cesium chloride concentrations were used, a specific response was eventually elicited. The cesium-specific response involved the Yaf9 protein and its activity of chromatin remodeling and transcription regulation. In this paper we show that the osmotic activity of cesium salt is detected and signaled by the two branches downstream of the Sln1 and Sho1 sensors of the HOG pathway, that seem to possess different but exchangeables functions in cesium signaling. However, the cesium-specific response mediated by Yaf9, that counteracts the efficiency of the HOG pathway, is not routed by these sensors. In addition, the cesium response also involves the cell wall integrity (CWI) pathway, which is activated by low concentration of cesium chloride. Mutations blocking the CWI pathway show sensitivity to this salt.