Role of α-Helical Coiled-coil Interactions in Receptor Dimerization, Signaling, and Adaptation during Bacterial Chemotaxis*

The aspartate receptor, Tar, is a member of a large family of signal transducing membrane receptors that interact with CheA and CheW proteins to mediate the chemotactic responses of bacteria. A highly conserved cytoplasmic region, the signaling domain, is flanked by two sequences, methylated helices 1 and 2 (MH1 and MH2), that are predicted to form alpha-helical coiled-coils. MH1 and MH2 contain glutamine and glutamate residues that are subject to deamidation, methylation, and demethylation. We show that the signaling domain is an independently folding unit that binds CheW. When expressed in vivo the signaling domain inhibits CheA kinase activity, but if MH1 or an unrelated leucine zipper coiled-coil sequence is attached to the signaling domain, CheA is activated. A construct that contains a leucine zipper fused to MH1-signaling domain-MH2 also activates the kinase, both in vivo and in vitro, and this activation is regulated by the level of glutamate modification. These findings support a model for receptor signaling where aspartate binding controls the relative orientation of receptor monomers to favor the formation of coiled-coils between MH1 and/or MH2 between subunits. Glutamate modification may stabilize these coiled-coils by reducing electrostatic repulsion between helices.