Reconstruction of the chemotaxis receptor–kinase assembly

Abstract

In bacterial chemotaxis, an assembly of transmembrane receptors, the CheA histidine kinase and the adaptor protein CheW processes environmental stimuli to regulate motility. The structure of a Thermotoga maritima receptor cytoplasmic domain defines CheA interaction regions and metal ion-coordinating charge centers that undergo chemical modification to tune receptor response. Dimeric CheA-CheW, defined by crystallography and pulsed ESR, positions two CheWs to form a cleft that is lined with residues important for receptor interactions and sized to clamp one receptor dimer. CheW residues involved in kinase activation map to interfaces that orient the CheW clamps. CheA regulatory domains associate in crystals through conserved hydrophobic surfaces. Such CheA self-contacts align the CheW receptor clamps for binding receptor tips. Linking layers of ternary complexes with close-packed receptors generates a lattice with reasonable component ratios, cooperative interactions among receptors and accessible sites for modification enzymes.

CheA Kinase of Bacterial Chemotaxis: Chemical Mapping of Four Essential Docking Sites

“…The models share both similarities and differences with recently proposed alternative models based on crystallographic and EPR data (55). Most of the disagreements among the models arise from different assumptions about the relative spatial positions of CheA domains P3, P4, and P5, emphasizing the sensitivity of current models to interdomain geometries that remain undetermined for the assembled core complex.…”

Bacterial chemoreceptors: high-performance signaling in networked arrays

“…Electron microscopic image analysis of particles assembled from a soluble receptor fragment, CheA and CheW also shows trimers, but with significantly altered geometry in which the three dimers are not arranged in a threefold symmetry [36,37]. The hedgerow arrangement in crystals of an archaeal receptor fragment suggests that archaeal CheA and CheW might associate with separated dimers [34]. Whether the structural diversity implied by these in vitro studies occurs in vivo remains an open question.…”

“…The skewed sets of heptads are well placed to facilitate bending of the four-helix bundle at the hinge. The two available X-ray structures of chemoreceptor cytoplasmic fragments, from two distantly related species, are indeed bent at the position of the glycine hinge [25,34,46], and substitution of larger residues for hinge glycines appears to lock the receptor in the on-or off-state [46]. Finally, in vivo studies of fluorescently-tagged receptors showed that dimers move closer together on repellent stimulation [47] and further apart on attractant binding [48], movements that could depend on bending at the glycine hinge.…”

“…For instance, the kinase control modules of chemoreceptors from E. coli and from the evolutionarily distant species, Thermotoga maritima, are both anti-parallel, four-helix bundles formed by the dimerization of helical hairpins [25,34]. Impressively, recent analysis of >2000 kinase control module sequences from ~150 species revealed an orderly pattern of sequence relatedness [7].…”

“…The one exceptional site (residue 373 in Tsr) is a phenylalanine in the E. coli chemoreceptor family and a polar, usually charged, residue in all others. This difference could conceivably destabilize the trimer interface in other chemoreceptors, and indeed a cytoplasmic fragment of a Thermotoga maritima receptor that carries a glutamic acid at this position crystallized not as trimers-of-dimers but rather as 'hedgerows', lines of parallel receptors interacting along their long axes [34]. It should be possible to elucidate the higher-order organization of receptors in other bacteria through in vivo cross-linking approaches such as those used in E. coli.…”