Abstract
The remarkably wide dynamic range of the chemotactic pathway of Escherichia coli, a model signal transduction system, is achieved by methylation/amidation of the transmembrane chemoreceptors that regulate the histidine kinase CheA in response to extracellular stimuli. The chemoreceptors cluster at a cell pole together with CheA and the adaptor CheW. Several lines of evidence have led to models that assume high cooperativity and sensitivity via collaboration of receptor dimers within a cluster. Here, using in vivo disulfide cross-linking assays, we have demonstrated a well defined arrangement of the aspartate chemoreceptor (Tar). The differential effects of amidation on cross-linking at different positions indicate that amidation alters the relative orientation of Tar dimers to each other (presumably inducing rotational displacements) without much affecting the conformation of the periplasmic domains. Interestingly, the effect of aspartate on cross-linking at any position tested was roughly opposite to that of receptor amidation. Furthermore, amidation attenuated the effects of aspartate by several orders of magnitude. These results suggest that receptor covalent modification controls signal gain by altering the arrangement or packing of receptor dimers in a pre-formed cluster.
Highlights
The resulting ternary complexes form a cluster at a cell pole (8 –10)
Receptor clustering has been implicated in signal gain control by methylation of specific glutamate residues that is responsible for adaptation to persisting stimuli
Receptor Dimers Are Organized into a Well Defined Array—To probe the relative orientation of receptor dimers in a polar cluster, we systematically introduced Cys residues within or near loop 2–3 of the periplasmic domain of the aspartate chemoreceptor Tar (Fig. 1)
Summary
The resulting ternary complexes form a cluster at a cell pole (8 –10). Attractant binding to the Tar dimer, which is formed regardless of its ligand occupancy state [11], induces a small but critical inward displacement of a membrane-spanning ␣-helix of one subunit [12,13,14,15,16,17]. These results suggest that receptor amidation (methylation) controls signal gain by altering the arrangement or packing of receptor dimers in a pre-formed cluster.
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