A Selective Tether Recruits Activated Response Regulator CheB to Its Chemoreceptor Substrate.

ABSTRACT

Methylesterase/deamidase CheB is a key component of bacterial chemotaxis systems. It is also a prominent example of a two-component response regulator in which the effector domain is an enzyme. Like other response regulators, CheB is activated by phosphorylation of an aspartyl residue in its regulatory domain, creating an open conformation between its two domains. Studies of CheB in Escherichia coli and related organisms have shown that its enzymatic action is also enhanced by a pentapeptide-binding site for the enzyme at the chemoreceptor carboxyl terminus. Related carboxyl-terminal pentapeptides are found on >25,000 chemoreceptor sequences distributed across 11 bacterial phyla and many bacterial species, in which they presumably play similar roles. Yet, little is known about the interrelationship of CheB phosphorylation, pentapeptide binding, and interactions with its substrate methylesters and amides on the body of the chemoreceptor. We investigated by characterizing the binding kinetics of CheB to Nanodisc-inserted chemoreceptor dimers. The resulting kinetic and thermodynamic constants revealed a synergy between CheB phosphorylation and pentapeptide binding in which a phosphorylation mimic enhanced pentapeptide binding, and the pentapeptide served not only as a high-affinity tether for CheB but also selected the activated conformation of the enzyme. The basis of this selection was revealed by molecular modeling that predicted a pentapeptide-binding site on CheB which existed only in the open, activated enzyme. Recruitment of activated enzyme by selective tethering represents a previously unappreciated strategy for regulating response regulator action, one that may well occur in other two-component systems. IMPORTANCE Two-component signal transduction systems are a primary means by which bacteria sense and respond to their environment. Response regulators are key components of these systems. Phosphorylation of response regulators by cognate histidine kinases generate active conformations which act on specific targets, DNA sequences or proteins. The targets have been considered passive in this process. Our characterization of interaction between response regulator CheB and its target chemoreceptor revealed active participation of the target in response regulator action. We found that a pentapeptide sequence at the carboxyl terminus of Escherichia coli chemoreceptors is a selective tether that binds only phosphorylated CheB, thus selecting the form of this two-component enzyme active for covalent modification of the selecting chemoreceptor. Analogous pentapeptides are found on chemoreceptors in many bacterial species and are presumably also selective tethers. There may well be other, uncharacterized examples of active participation of target molecules in response to regulator action.