RESUMO
The chemotaxis system of Escherichia coli makes use of an extended two-component sensory response pathway in which CheA, an autophosphorylating protein histidine kinase (PHK) rapidly passes its phosphoryl group to CheY, a phospho-accepting response regulator protein (RR). The CheA-->CheY phospho-transfer reaction is 100-1000 times faster than the His-->Asp phospho-relays that operate in other (non-chemotaxis) two-component regulatory systems, suggesting that CheA and CheY have unique features that enhance His-->Asp phospho-transfer kinetics. One such feature could be the P2 domain of CheA. P2 encompasses a binding site for CheY, but an analogous RR-binding domain is not found in other PHKs. In previous work, we removed P2 from CheA, and this decreased the catalytic efficiency of CheA-->CheY phospho-transfer by a factor of 50-100. Here we examined the kinetics of the binding interactions between CheY and P2. The rapid association reaction (k(assn) approximately 10(8)M(-1)s(-1) at 25 degrees C and micro=0.03 M) exhibited a simple first-order dependence on P2 concentration and appeared to be largely diffusion-limited. Ionic strength (micro) had a moderate effect on k(assn) in a manner predictable based on the calculated electrostatic interaction energy of the protein binding surfaces and the expected Debye-Hückel shielding. The speed of binding reflects, in part, electrostatic interactions, but there is also an important contribution from the inherent plasticity of the complex and the resulting flexibility that this allows during the process of complex formation. Our results support the idea that the P2 domain of CheA contributes to the overall speed of phospho-transfer by promoting rapid association between CheY and CheA. However, this alone does not account for the ability of the chemotaxis system to operate much more rapidly than other two-component systems: k(cat) differences indicate that CheA and CheY also achieve the chemical events of phospho-transfer more rapidly than do PHK-RR pairs of slower systems.
