T H EJ O U R N A LO FC E L LB I O L O G YRESEARCH ROUNDUP •THE JOURNAL OF CELL BIOLOGY327Text by Alla Katsnelson [email protected]E. colisqueezed into actionEscherichia colichemoreceptors double as osmotic sensors by me-chanically compressing in response to increased osmolarity, say Ady Vaknin and Howard Berg (Harvard University, Cambridge, MA).E. coliis 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 recep-tors’ position in living cells, Vaknin and Berg found that increased osmo-larity 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 dynam-ics. 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
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