Bacterial Chemotaxis: a New Player in Response Regulator Dephosphorylation

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Abstract
Escherichia coli chemotaxis is arguably the best understood of all biological behaviors, but even this relatively "simple" system continues to offer up new molecular insights from Evo- lution's bag of signaling tricks. The latest surprise comes from Silversmith et al. (15), who report in this issue how the X-ray structure of a mutant CheY protein led them to the discovery of a CheY residue that plays a special role in this response regulator's phosphorylation activities. Their story not only clar- ifies a mechanistically murky step in chemotactic signal trans- duction but also provides us with a nice example of molecular detective work. receptor population to restore the prestimulus proportions of the two signaling states. The signaling cascade begins with CheA, which donates its phosphoryl groups to two competing response regulators, CheB and CheY, thereby activating them. Phospho-CheY binds to the FliM protein in the flagellar motors to augment clockwise rotation. Phospho-CheB, an MCP-specific methyles- terase, demethylates MCP molecules to shift them to the coun- terclockwise (kinase-off) state. Its counterpart, CheR, an MCP-specific methyltransferase, operates at a constant rate that is not modulated by stimuli. Thus, net changes in MCP methylation state are brought about by changes in the relative rates of the methylation and demethylation reactions through feedback control of CheB activity.