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Microbes are directly exposed to the elements, and one of the most acute insults they can experience is a rapid change in osmotic environment. The forces that are generated by even small changes in the osmolarity are massive. In response to increases in the osmolarity of the medium, also called osmotic upshock, the cell transports and synthesizes cytoplasmic osmoprotectants, which thus help to maintain its turgor. A subsequent severe osmotic downshock is more than an inconvenience, it is life threatening. The cell swells, its cell-wall becomes compromised, and without immediate action the organism would lyse. Largeconductance mechanosensitive channels within the cytoplasmic membrane prevent this needless death by serving as biological emergency release valves. Two such bacterial mechanosensitive channel families have been extensively studied: MscL and MscS. For each, a crystal structure of a family member has been obtained, and detailed models for structural changes that occur upon gating have been postulated. As we learn more of the molecular mechanisms by which these channels sense and respond to membrane tension, we discover similarities not only between these two relatively distant families, but also potentially with the more complex mechanosensory systems of eukaryotic organisms. [ABSTRACT FROM AUTHOR]

What are the stimuli that are sensed by mechanoreceptors? Researchers have now begun to address this question and determine the molecular mechanisms underlying channels that are gated by mechanical forces. Two quite different models now exist. The first is that the channels are ‘tethered' to cytoskeleton and or extracellular components, which thus exert forces on the channel that lead to gating. The second model predicts that the channel protein directly senses biophysical changes that occur within the membrane when it is under tension. Several lines of evidence indicate that many putative mechanosensitive channels are indeed tethered by other proteins, however in many instances the exact role this tethering plays in mechanosensing has yet to be fully clarified. On the other hand, the cloning and study of bacterial mechanosensitive channels demonstrated that channels can directly sense tension within the membrane. Evidence obtained from several of the more complex eukaryotic mechanosensory systems suggests that a number of eukaryotic channels from divergent families similarly sense tension within the membrane. [ABSTRACT FROM AUTHOR]