Characterization of the CheA S /CheZ complex: a specific interaction resulting in enhanced dephosphorylating activity on CheY‐phosphate

The cheA gene encodes two overlapping polypeptides with a common carboxyl terminus: CheA L and CheA S . CheA L plays a central role in the Escherichia coli chemotaxis signalling pathway by autophosphorylation and transferring the phosphate to both CheY and CheB. On the other hand, the physiological functions of CheA S remain unknown. We have observed that overproduction of CheA S in wild‐type cells increased counterclockwise‐biased flagellar rotation, and this effect is dependent on the presence of CheZ. CheZ specifically facilitates CheY‐phosphate (CheY‐P) dephosphorylation and generates a smooth swimming signal. A physical interaction was detected between CheZ and CheA S in wild‐type cell lysates by immunoprecipitation. The CheA S /CheZ interaction does not require other chemotaxis components, as we could form the complex using purified CheA S and CheZ proteins. The ability of CheA S to bind to CheZ depends on its being in the reduced state. We found that under non‐reducing conditions, CheA S appears to form intermolecular disulphide bonds and loses the ability to bind to CheZ. Finally, the CheA S /CheZ complex formed in vitro shows a greater dephosphorylating activity on CheY‐P than does free CheZ.