ROLE OF ALPHA-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 t ransducing membrane receptors that interact with CheA and CheW protein s 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 glutama te residues that are subject to deamidation, methylation, and demethyl ation. We show that the signaling domain is an independently folding u nit that binds CheW, When expressed in vivo the signaling domain inhib its CheA kinase activity, but if MH1 or an unrelated leucine zipper co iled-coil sequence is attached to the signaling domain, CheA is activa ted, 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, T hese findings support a model for receptor signaling where aspartate b inding 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.