Nuclear control of stress signaling in yeast

Response to a changing environment requires the adjustment of several cellular processes ranging from enzymatic activity, gene expression to the cell cycle. These processes are also interdependent. In S.cerevisiae many genes (up to 10% of all genes) change their expression pattern in a common environmental response with the general stress transcription factor Msn2 as a major mediator. Msn2 is an integration point of several types of stress signals. The protein kinase A, TOR signalling pathways inactivate Msn2 in rich nutrient conditions. The question how these different cues are acting on Msn2 has been investigated by several labs in the past. Several regulatory mechanisms impinging on Msn2 are known such as intracellular localization, phosphorylation and stability. However, the actual stress sensing mechanism regulating ScMsn2 activity remains elusive. Work proposed here will aim at the molecular mechanisms of stress sensing and the role of PKA and TOR for nutrient signalling. Two key observations are the basis for the proposed work. Msn2 nuclear accumulation can be uncoupled from chromatin recruitment. Such an intranuclear control of transcription factors is a reoccurring theme both in lower and higher eukaryotes and exists in parallel to the regulation of nucleo-cytoplasmic distribution. In the case of Msn2 a tight connection between regulated recruitment to DNA and activation exists. We will attempt to find nuclear regulatory factors in order to elucidate the underlying mechanism to break new ground for the understanding of environmental signal transduction. The second observation is based on sequence comparisons of Msn2 related proteins in fungi. The comparative analysis unveils a very restricted region of similarity in the N- terminal region of Msn2 which is required for nuclear export. In particular we will concentrate on the mechanism how this domain is contributing to Msn2 regulation by stress and nutrient sensing pathways. The functional conservation of the N-terminal domain which includes homologues from C.glabrata, K. lactis but excludes C.albicans is a guide for Msn2 structure/function analysis to help elucidation of novel stress regulatory mechanisms.

Response to a changing environment requires the adjustment of cellular processes. These range from enzymatic activity to gene expression and to such basic processes as cell division. All organisms are able to adjust their gene expression pattern to adjust them to the environment. We are using baker`s yeast (Saccharomyces cerevisiae) and the related yeast Candida glabrata to investigate how genes are regulated in response to environmental stress. In yeast many genes (up to 10% of all genes) change their expression pattern in a common environmental response. The project focussed on the elucidation of the mechanisms mediating stress signals to the cell nucleus. Stress response is regulated and cells need this control. This principle emerged immediately from our observation that transcription of stress genes is per se stressful. After intense transcription activity, regions in the genome harbouring stress genes demand attention of chromatin remodelling enzymes to re-establish ordered structures. We could also show that the toxicity of arsenic is brought about by triggering intense acute stress response signals. A major result of the work is the observation that the protein phosphatase 2A has a prominent role for transmitting stress signals in the cell nucleus. This suggests a novel function for this highly conserved enzyme. Furthermore, we extended the analysis of yeast stress response to a related human pathogenic yeast called Candida glabrata and could gain insights how this organism communicates with the environment.