The capacity for regulation of gene expression enables successful adaptation of organisms to the environment. Modulation of gene expression in response to the environment has at least three components: (1) external signals are recognized and converted into information that can be transmitted to genes in need of regulation (signal transduction); (2) DNA binding proteins with the ability to activate transcription (transcriptional activators) recognize specific DNA binding sites in the regulatory regions of appropriate genes; and (3) transcriptional activators trigger transcription by interacting with the transcriptional machinery. Our research uses molecular, genetic and biochemical techniques in investigating the mechanics of signal detection and transcriptional activation in response to external stimuli using the single-cell model eukaryote, Saccharomyces cerevisiae (yeast).
The response of yeast cells to osmotic stress is mediated by the SLN1 signaling pathway consisting of a histidine/aspartate phosphorelay in which phosphate is transferred from histidine to aspartate residues within and between proteins. The SLN1 signaling pathway is branched. A shared sensor-kinase and phosphorelay protein signal to two downstream receiver proteins. One receiver regulates a p38-like signal transduction cascade, ultimately increasing expression of a set of osmotic response genes. The other receiver protein, Skn7, binds to DNA and activates genes involved in control of cell growth and cell wall integrity. The two branches of the pathway are reciprocally regulated. When the activity of one branch is increased, the activity of the other is decreased. This suggests cells may need to coordinate the osmotic response with aspects of cell growth.
The osmotic stress response is widely believed to be stimulated in response to changes in pressure against the fungal cell wall. However, our recent experiments indicate that this is an oversimplification and that specific cell wall molecules have important roles in activating and repressing the sensor/kinase. We have been dissecting these early events by identifying the molecules involved and their relationships to one another in an attempt to clarify the molecular details of osmotic stress sensing.
The transcription factor, Skn7, is responsive to both osmotic and oxidative stress. The mechanisms by which this factor mounts a stress-specific response are unclear. Activation of Skn7 by Sln1-dependent aspartyl phosphorylation is critical for its response to changes in osmotic pressure, however, the steps involved in oxidative stress activation of Skn7 remain uncharacterized. We have been investigating the involvement of direct redox regulation of Skn7, oxidative stress specific phosphorylation events and oxidative stress dependent protein-protein interactions in the Skn7 dependent oxidative stress response.
We have used a variety of approaches including genetic screens, biochemical assays, and microarray analysis to identify some of the signaling molecules involved in each of the two branches of the SLN1 environmental stress pathways, to learn more about the stimuli and how they are sensed, and to identify the target genes of the SLN1 pathways. We continue to investigate issues including, but not limited to (1) how is information transferred between molecules of the pathway? (2) what additional environmental, physical and chemical signals activate the pathway? and (3) what is the functional relationship between the two branches of the pathway?
Keywords: Signal Transduction, Osmotic Stress, Oxidative Stress, Histidine Kinase, Cell Wall