Roles of molecular interactions in the control of cellular hydration by bacterial osmosensing transporter ProP

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Physical properties of cellular environments vary over time and space. They include the temperature, the hydrostatic pressure and the osmotic pressure. Changing environmental osmotic pressure causes water to flow into or out of cells, thereby changing their hydration and impairing their functions. Water follows solutes, so water fluxes can be mitigated by moving solutes across the membrane that encloses each cell. Those solutes include salts, amino acid-like compounds and sugars. Osmotic downshocks cause cell swelling and trigger solute release via tension-sensitive membrane channels to avoid cell rupture. Osmotic upshocks cause cell shrinkage and trigger solute uptake via osmosensing transporters to forestall dehydration. We aim to understand how osmotically-induced cell dehydration triggers and modulates solute uptake via osmosensing transporter ProP in Escherichia coli and other bacteria. If fact, our work rendered ProP the paradigmatic osmosensing transporter. The ProP protein is embedded in the membrane that surrounds the cell and exposed on both membrane surfaces. ProP binds solutes available outside the cell and releases them inside the cell. The rate at which ProP pumps solutes into cells (ProP activity) depends on the composition of the membrane in which it is embedded and the salinity of the cellular interior. The membrane composition varies over the cell surface, and ProP concentrates at the ends (the poles) of rod-shaped E. coli cells. We will test the hypothesis that both ProP targeting to the cell poles and ProP activity are controlled by salt-dependent self-association and membrane association of a ProP domain that extends into the cellular interior. Key features of that domain will be identified by changing its structure (through gene editing), then examining the function and cellular location of the resulting ProP variants. Advanced chemical techniques (for example, nuclear magnetic resonance and infrared spectroscopies) and supercomputer-based simulations will be used to analyze how self- and membrane-association influence the structures of ProP and its variants. This will allow us to visualize how the structure of ProP depends on its subcellular location and the osmotic pressure. ProP serves as a paradigm for the study of two fundamental phenomena: osmosensing and protein localization within cells. Many biological processes result from structure-specific (lock and key) interactions between solutes and protein or nucleic acid receptors. We aim to understand different principles that allow cells to detect physical stimuli such as osmotic stress. By deducing how E. coli cells sense and control their own physical properties, we elucidate mechanisms that promote the health and survival of microbes, animals and plants. For example, such mechanisms are central to the survival of bacteria in food and water supplies, kidney cells and crop plants in salty soils.