Body shaping under water stress: osmosensing and osmoregulation of solute transport in bacteria

Fluctuation of external osmolarity is one of the most common types of environmental stress factors for all kind of cells, both of prokaryotic and of eukaryotic origin. Cells try to keep their volume and/or turgor pressure constant; consequently, both a decrease (hypoosmotic stress) and an increase (hyperosmotic stress) of the solute concentration (correctly: increase or decrease in water activity) in the surrounding area, respectively, are challenges for cellular metabolism and survival. A common example from the prokaryotic world is the fate of a soil bacterium that, after a sunny day has dried out the soil (hyperosmotic stress), is suddenly exposed to a drop of distilled water from a rain cloud (hypoosmotic stress). The immediate and inevitable passive response to the sudden osmotic shift in the surroundings is fast water efflux out of the cell in the former situation and water influx in the latter. In the worst case, these responses may lead to either loss of cell turgor and plasmolysis or to cell burst. In order to overcome such drastic consequences cells have developed effective mechanisms, namely osmoadaptation, to cope with the two different types of osmotic stress. For a graded reaction to osmotic shifts, cells must be able (1) to sense stimuli related to osmotic stress, (2) to transduce corresponding signals to those systems that properly respond (3) by activating transport or enzymatic functions or (4) by changing gene expression profiles. In this review, membrane proteins involved in the cell's active response to osmotic stress are described. Molecular details of structure, function, and regulation of mechanosensitive efflux channels from various organisms, as well as of osmoregulated uptake systems are discussed.