Chemoreceptors in signalling complexes: shifted conformation and asymmetric coupling

Bacterial chemotaxis is mediated by signalling complexes of chemoreceptors, histidine kinase CheA and coupling protein CheW. Interactions in complexes profoundly affect the kinase. We investigated effects of these interactions on chemoreceptors by comparing receptors alone and in complexes. Assays of initial rates of methylation indicated that signalling complexes shifted receptor conformation towards the methylation-on, higher-ligand-affinity, kinase-off state, tuning receptors for greater sensitivity. In contrast, transmembrane and conformational signalling within chemoreceptors was essentially unaltered, consistent with other evidence identifying receptor dimers as the fundamental units of such signalling. In signalling complexes, coupling of ligand binding to kinase activity is cooperative and the dynamic range of kinase control expanded > 100-fold by receptor adaptational modification. We observed no cooperativity in influence of ligand on receptor conformation, only on kinase activity. However, receptor modification generated increased dynamic range in a stepwise fashion, partly in coupling ligand to receptor conformation and partly in coupling receptor conformation to kinase activity. Thus, receptors and kinase were not equivalently affected by interactions in signalling complexes or by ligand binding and adaptational modification, indicating asymmetrical coupling between them. This has implications for mechanisms of precise adaptation. Coupling might vary, providing a previously unappreciated locus for sensory control.     

Differences in signalling by directly and indirectly binding ligands in bacterial chemotaxis

In chemotaxis of Escherichia coli and other bacteria, extracellular stimuli are perceived by transmembrane receptors that bind their ligands either directly, or indirectly through periplasmic‐binding proteins (BPs). As BPs are also involved in ligand uptake, they provide a link between chemotaxis and nutrient utilization by cells. However, signalling by indirectly binding ligands remains much less understood than signalling by directly binding ligands. Here, we compared intracellular responses mediated by both types of ligands and developed a new mathematical model for signalling by indirectly binding ligands. We show that indirect binding allows cells to better control sensitivity to specific ligands in response to their nutrient environment and to coordinate chemotaxis with ligand transport, but at the cost of the dynamic range being much narrower than for directly binding ligands. We further demonstrate that signal integration by the chemosensory complexes does not depend on the type of ligand. Overall, our data suggest that the distinction between signalling by directly and indirectly binding ligands is more physiologically important than the traditional distinction between high‐ and low‐abundance receptors.     

Toll-like receptors as molecular switches

Members of the Toll family of single-pass transmembrane receptors are key mediators of innate immunity in both vertebrates and invertebrates. They respond to various pathogen-associated stimuli and transduce the complex signalling responses that are required for inflammation and for the subsequent development of adaptive immunity. Here, we propose a molecular mechanism for signalling by the Toll and Toll-like receptors that involves a series of protein conformational changes initiated by dimerization of their extracellular domains. The initial dimerization event, which is triggered by the interaction of the receptor with its ligand, might disrupt a pre-formed but non-functional dimer. Formation of a stable receptor-ligand complex then relieves constitutive autoinhibition, enabling receptor-receptor association of the extracellular juxtamembrane regions and cytoplasmic signalling domains. This activation process constitutes a tightly regulated, unidirectional molecular switch.