An osmosensor is a protein that detects changes in water activity, or resulting changes in cell structure or composition, and directs osmoregulatory responses. Operationally, osmosensors are identified as proteins that respond with the same activity or output signal to isotonic aqueous solutions that differ in solute composition (e.g., those containing an electrolyte such as NaCl or a nonelectrolyte such as sucrose). Most detailed analyses of osmo-sensing and osmosignaling have focused on membrane-based osmosensors from moderately osmotolerant Eubacteria. The identified osmoregulatory systems include osmosensory transporters, the histidine kinase components of two-component transcriptional regulatory systems, and mechanosensi-tive channels (Table 5.2). In addition, osmotic pressure may control bacterial transcription without benefit of these sensory or signal transduction mechanisms.
Prototypical osmosensory transporters ProP, BetP, and OpuA can detect osmotic pressure changes and respond by mediating osmoprotectant uptake without the assistance of other proteins. Membrane-embedded histidine kinases KdpD and EnvZ detect changes in osmotic pressure and other signals and then respond by directing cognate response regulators to modulate transcription. KdpD directs KdpE to stimulate transcription ofthe genes encoding K+-motive ATPase KdpFABC. EnvZ directs OmpR to modulate transcription of many genes, among them those encoding the outer membrane porins OmpC and OmpF. Mechanosensitive channels MscL and MscS open in response to in-plane membrane strain, releasing solutes and thereby triggering water efflux. Study of these bacterial systems, each successfully purified and reconstituted in artificial phospholipid vesicles (proteoliposomes), is revealing fundamental principles of osmosensing and osmosignaling.
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