Escherichia coli chemoreceptors double as osmotic sensors by mechanically compressing in response to increased osmolarity, say Ady Vaknin and Howard Berg (Harvard University, Cambridge, MA).
E. coli is always on the look-out for a better environment. As it swims, chemoattractant receptors talk to the flagellar motors, thus orienting the bacterium's travels. Using fluorescence polarization to image the receptors' position in living cells, Vaknin and Berg found that increased osmolarity caused receptors, joined in triplets like the legs of a tripod, to move closer together by about 10%. The squeeze stimulates kinase activity and the subsequent signaling pathway, prompting the bacterium to swim away from the potentially damaging environment.
This compression can be explained by simple cell membrane dynamics. As osmotic stress increases, water leaves the cell. Reduced pressure from within causes a slackness in the membrane and an increase in its thickness—much as a rubber...
