Control of chemotactic signal gain via modulation of a pre-formed receptor array

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.