Cell Biology

Seeing Signaling

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Science  18 Apr 2014:
Vol. 344, Issue 6181, pp. 236
DOI: 10.1126/science.344.6181.236-c

A classic model for the study of cell signaling is the bacterial chemotaxis system. Fukuoka et al. describe the latest step in “seeing” just how this simple yet elegant mechanism really works. The basics of the system are known. Receptors on the cell surface bind attractant or repellent molecules, and this leads to alterations in the phosphorylation of a regulatory protein known as CheY. CheY in turn interacts with the molecular motor of the flagellum, altering its direction of rotation. Bacteria were engineered to express CheY protein that was fluorescently tagged. This enabled the use of total internal reflection fluorescence microscopy to monitor the interaction of CheY with the flagellar motor in single bacterial cells, while at the same time observing the direction of rotation of the flagellar motor. The rotor turned clockwise when phosphorylated CheY (CheY-P) was bound and counterclockwise when CheY-P dissociated. Although the motor has been estimated to contain approximately 34 subunits that can bind CheY-P, calculations showed that clockwise-turning motors bound 13 ± 7 CheY-P molecules, so not all the subunits needed to be bound to alter the rotor's direction. In addition, the affinity for binding of CheY-P to the motor in its clockwise-moving state was about fivefold greater than that of a counterclockwise-moving motor. Dissociation of CheY-P from the motor occurred with a ∼70-ms time frame—much too fast to be controlled by dephosphorylation of CheY-P.

Sci. Signal. 7, ra32 (2014).

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