Selective allosteric coupling in core chemotaxis signaling complexes

Abstract

Bacterial chemotaxis is mediated by signaling complexes that sense chemical gradients and direct bacteria to favorable environments by controlling a histidine kinase as a function of chemoreceptor ligand occupancy. Core signaling complexes contain two trimers of transmembrane chemoreceptor dimers, each trimer binding a coupling protein CheW and a protomer of the kinase dimer. Core complexes assemble into hexagons, and these form hexagonal arrays. The notable cooperativity and amplification in bacterial chemotaxis is thought to reflect allosteric interactions in cores, hexagons, and arrays, but little is known about this presumed allostery. We investigated allostery in core complexes assembled with two chemoreceptor species, each recognizing a different ligand. Chemoreceptors were inserted in Nanodiscs, which rendered them water soluble and allowed isolation of individual complexes. Neighboring dimers in receptor trimers influenced one another's operational ligand affinity, indicating allosteric coupling. However, this coupling did not include the key function of kinase inhibition. Our data indicated that only one receptor dimer could inhibit kinase as a function of ligand occupancy. This selective allosteric coupling corresponded with previously identified structural asymmetry: only one dimer in a trimer contacts kinase and only one CheW. We suggest one of these dimers couples ligand occupancy to kinase inhibition. Additionally, we found that kinase protomers are allosterically coupled, conveying inhibition across the dimer interface. Because kinase dimers connect core complex hexagons, allosteric communication across dimer interfaces provides a pathway for receptor-generated kinase inhibition in one hexagon to spread to another, providing a crucial step for the extensive amplification characteristic of chemotactic signaling.transmembrane receptors | bacterial chemotaxis | allosteric coupling | histidine kinases | Nanodiscs

Networked Chemoreceptors Benefit Bacterial Chemotaxis Performance

“…In cells lacking the sensory adaptation enzymes, interface 2 lesions did not alter the level of kinase activity or the ligand-dependent regulation of the kinase by the receptors, but they essentially abolished the cooperativity of the kinase activity responses. These results imply greatly reduced coupling between receptor signaling units and mirror previous in vitro measurements of receptor signaling complexes embedded in nanodiscs (24). …”

Signal Transduction in Histidine Kinases: Insights from New Structures

“…Symmetric to asymmetric transitions within the homodimer, appear to be ubiquitous in many HK domains and also in their enzymological function. A similar connection between half-of-sites reactivity and asymmetry is seen in homodimeric receptor tyrosine kinases like EGFR (Leahy, 2010) and in chemoreceptors (Li and Hazelbauer, 2014), suggesting that this might be a general strategy for signal transduction. In HKs, asymmetric transitions are often encoded into structures via the fundamental properties of helical bundles such as their hydrophobic registry and side-chain packing propensities.…”

The source of high signal cooperativity in bacterial chemosensory arrays

“…The signaling behaviors of interface 2 mutants closely resemble those of core signaling units in membrane nanodiscs (3), implying that the mutant cells may contain mainly isolated core signaling complexes. Those mutant receptor complexes were distributed uniformly around the cell membrane, with little tendency to cluster at the poles as wild type arrays do.…”