Kinase-active Signaling Complexes of Bacterial Chemoreceptors Do Not Contain Proposed Receptor−Receptor Contacts Observed in Crystal Structures

The receptor dimers that mediate bacterial chemotaxis form high-order signaling complexes with CheW and the kinase CheA. From the packing arrangement in two crystal structures of different receptor cytoplasmic fragments, two different models have been proposed for receptor signaling arrays: the trimers-of-dimers and hedgerow models. Here we identified an interdimer distance that differs substantially in the two models, labeled the atoms defining this distance through isotopic enrichment, and measured it with (19)F-(13)C REDOR. This was done in two types of receptor samples: isolated bacterial membranes containing overexpressed, intact receptor and soluble receptor fragments reconstituted into kinase-active signaling complexes. In both cases, the distance found was not compatible with the receptor dimer-dimer contacts observed in the trimers-of-dimers or in the hedgerow models. Comparisons of simulated and observed REDOR dephasing were used to deduce a closest approach distance at this interface, which provides a constraint for the possible arrangements of receptor assemblies.