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 balloon acts as some air is let out. “We think that when the membrane thickens in response to osmotic stress, that changes the orientation of the receptors, making them move closer together,” says Berg.
How changes in relative receptor position stimulate kinase activity is still unknown. The group is now investigating whether chemoattractant stimuli cause receptors to move further apart. They are also looking downstream at the effect of such mechanical perturbations on the flagellar motor's control of direction.