Tryptophan residues flanking the second transmembrane helix (TM2) set the signaling state of the Tar chemoreceptor.
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The chemoreceptors of Escherichia coli are homodimeric membrane proteins that cluster in patches near the cell poles. They convert environmental stimuli into intracellular signals that control flagellar rotation. The functional domains of a receptor are physically separated by the cell membrane. Chemoeffectors bind to the extracellular (periplasmic) domain, and the cytoplasmic domain mediates signaling and adaptation. These two domains communicate through the second transmembrane helix (TM2) that connects them. In the high-abundance receptors Tar and Tsr, TM2 is flanked by tryptophan residues, which should localize preferentially to the interfacial zone between the polar and hydrophobic layers of the phospholipid bilayer. To investigate the functional significance of the Trp residues that flank TM2 of Tar, we used site-directed mutagenesis to generate the W192A and W209A substitutions. The W192A protein retains full activity in vivo and in vitro, but it increases the K(i) for aspartate in the in vitro assay 3-fold. The W209A replacement eliminates receptor-mediated stimulation of CheA in vitro, and it leads to an increased level of adaptive methylation in vivo. This phenotype in some respects mimics the changes seen upon binding aspartate. Since the W209A substitution may cause the C-terminus of TM2 to protrude farther into the cytoplasm, these results reinforce the hypothesis that aspartate binding causes a similar displacement. Moving Trp to each position from residue 206 to residue 212 generated a wide variety of Tar signaling states that are generally consistent with the predictions of the piston model of transmembrane signaling. None of these receptors was completely locked in one signaling mode, although most showed pronounced signaling biases. Our findings suggest that the Trp residues flanking TM2, especially Trp-209, are important in setting the baseline activity and ligand sensitivity of the Tar receptor. We also conclude that the Tyr-210 residue plays at least an auxiliary role in this control.