The activation mechanism of the Bacillus subtilis stressosome signalling hub

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The survival of all organisms is dependent on their ability to adapt to changes in their environment and the stress response of Bacillus subtilis, and a number of its close Gram-positive relatives, provides these organisms with a protective mechanism against a wide range of physical and chemical stresses. This research proposal concerns the mechanism by which B. subtilis is able to respond to environmental stimuli. At the top of the stress-response pathway is a structure resembling a small virus that acts as a signalling hub, and has been named the stressosome. This stressosome is composed of a number of proteins that sense stress-inducing stimuli that are related to a protein called RsbR and another small protein, RsbS, that together act to sequester a protein kinase, RsbT, required to kick-start a cascade of interactions that ultimately leads to the activation of over two hundred genes whose products provide the cell with resistance to stress. The structure of the stressosome that we have recently determined, displays an unusual arrangement of protein components providing tantalising clues as to the mechanism of action in response to stress. We aim to use a blue-light responsive signalling protein to generate a stressosome that we can use in structural studies to determine any gross-structural changes in the complex that lead to the release of the protein kinase and thus the mechanism by which the stress response is initiated. Furthermore, we will investigate the role of the RsbS protein, the absence of which causes bacteria to develop as small and sickly cells, in stress-response by creating stressosomes that consist only of an RsbR-like protein. It is also apparent that the proteins related to RsbR may function as signalling modules. A full understanding of how these proteins work is dependent on the determination of their structures by X-ray crystallography, which provides a molecular model of the protein that can be used to assess potential activating signals that are recognised by their specific shapes. Finally, we will investigate the role of a number of other proteins that are known to mediate the length of the stress signal, as the requirement to turn off the stress response in a timely fashion is almost as important as the ability to respond to stress in the first place.