Residues at the cytoplasmic end of transmembrane helix 2 determine the signal output of the Tar_Ec chemoreceptor

Baseline signal output and communication between the periplasmic and cytoplasmic domains of the Escherichia coli aspartate chemoreceptor Tar_Ec are both strongly influenced by residues at the C-terminus of transmembrane helix 2 (TM2). In particular, the cytoplasmic aromatic anchor, composed of residues Trp-209 and Tyr-210 in wild-type Tar_Ec, is important for determining the CheA kinase-stimulating activity of the receptor and its ability to respond to chemoeffector-induced stimuli. Here, we have studied the effect on Tar_Ec function of the six-residue sequence at positions 207–212. Moving various combinations of aromatic residues among these positions generates substantial changes in receptor activity. Trp has the largest effect on function, both in maintaining normal activity and in altering activity when it is moved. Tyr has a weaker effect, and Phe has the weakest; however, all three aromatic residues can alter signal output when they are placed in novel positions. We also find that Gly-211 plays an important role in receptor function, perhaps because of the flexibility it introduces into the TM2–HAMP domain connector. The conservation of this Gly residue in the high-abundance chemoreceptors of E. coli and Salmonella enterica suggests that it may be important for the nuanced, bidirectional transmembrane signaling that occurs in these proteins.