Reverse Engineering of Osmo-Adaptaion Machinery in Yeast

Source: Mettetal et al. (2008). The frequency dependence of osmo-adaptation in Saccharomyces cerevisiae. Science 319: 482-484.

This is a very fun paper for someone with my interests in this field. An MIT-Harvard coalition of forces is employed to reverse engineer a well-studied pathway in yeast and to define the dynamics of the cellular system. Below, the MAP kinase pathway is shown which leads to the activation of Hog1 (a transcription factor) upon osmotic stress and in turn its localization to the nucleus. Once inside the nucleus, Hog1 changes the expression level of many genes with roles in stress response. In this study, the authors employ a microscopy based technique to measure Hog1-YFP nucleus localization in response to osmotic shock (the response in normalized using Nrd1-RFP, a strictly nuclear protein). This set-up is employed in a chemostat with stepwise addition and depletion of medium containing excess NaCl. The figure below (from the original paper) shows this experimental set-up.

The authors record the responses in wild-type and pbs2 mutants. Reduced expression of Pbs2 decreases the level of Hog1 and enables them to measure the Hog1 independent feedback loops. They interpret a successful predictive model from the data and based on the fitted parameters they comment on the dynamics of the pathway. First of all, the cell recovers after 5 min which is much lower than the time needed for gene-expression based changes (>15min). Then, why is Hog1 needed to activate and deactivate so many genes? The authors address this by inhibiting protein synthesis using cycloheximide. Their results shows that the hog1 induced changes in protein levels render the cells more competent in recovering from the shock in the future challenges. In other words, Hog1 activation carves a short memory of osmotic shock in the cell.