1
Chemoreceptors in signalling complexes: shifted conformation and asymmetric coupling.
Mol Microbiol. 2010 Dec; 78(5):1313-23
All selected articles haveThis article has been added to your saved articles
You can follow/unfollow articles via MyF1000 or via the article or browse pages.
31 Jan 2011
Technical Advance
DOI: 10.3410/f.7891956.8246054
The general presumption has been that the allosteric state of receptors (which make CheA kinase active or inactive) generally drives CheA kinase in a straightforward (that is, tightly coupled) way. However, in a careful, well-conceived, comprehensive and fascinating set of experiments, in which binding...Continue reading
The general presumption has been that the allosteric state of receptors (which make CheA kinase active or inactive) generally drives CheA kinase in a straightforward (that is, tightly coupled) way. However, in a careful, well-conceived, comprehensive and fascinating set of experiments, in which binding of the attractant aspartate to the receptor Tar as a function of adaptational state was varied, rate of methylation (an adaptational reaction) and CheA kinase activity (which mediates the degree of excitation) were measured. The Hazelbauer lab discovered that there is cooperativity in the influence of aspartate on kinase activity but not on receptor conformation or ligand (aspartate) binding.
A number of intriguing observations have emerged from this work. The 'adaptation state' was changed by substituting glutamine residues, which resemble methylglutamate residues, for glutamate residues in a mutant lacking both the methyltransferase and the methylesterase (the adaptation normally occurs by methylation of glutamate residues). Increased receptor adaptation (by substituting glutamines for glutamates) reduced only slightly the affinity of the receptor for aspartate (hyperbolic in vitro binding curve) but increased 100-fold the concentration of aspartate at which half-maximal kinase activity occurred when receptors having no glutamines versus three glutamines were compared (cooperative in vitro activity). The effect of this adaptational modification was stepwise: the greater the number of residues modified, the greater the shift in the concentration of aspartate at which half-maximal kinase activity occurred. Thus, as more receptors become modified through adaptation in vivo, the dynamic range at which chemotactic responses occur would be greatly extended compared to what they would be if kinase activity directly reflected binding of aspartate in a tightly coupled way. Measurement of methylation rate in vitro showed a three-fold shift toward high concentrations for receptors having three glutamines compared with no glutamines, but methylation activity as a function of aspartate concentration was non-cooperative (but, as stated above, binding of aspartate was hardly affected by substituting glutamines for glutamates). Methylation rate reflects receptor conformation. In one conformation, the kinase is active, methylation is inactive, ligand (aspartate) affinity is lower, and demethylation activity is inactive. In the other, the kinase is inactive, methylation is active, ligand affinity is higher, and demethylation activity is active. Thus, there appear to be two factors involved in the very large effect on kinase activity of substituting glutamines for glutamates in the receptors, one on coupling ligand (aspartate) occupancy to receptor conformation (as reflected in rate of methylation) and another on coupling receptor conformation to kinase activity.
A number of intriguing observations have emerged from this work. The 'adaptation state' was changed by substituting glutamine residues, which resemble methylglutamate residues, for glutamate residues in a mutant lacking both the methyltransferase and the methylesterase (the adaptation normally occurs by methylation of glutamate residues). Increased receptor adaptation (by substituting glutamines for glutamates) reduced only slightly the affinity of the receptor for aspartate (hyperbolic in vitro binding curve) but increased 100-fold the concentration of aspartate at which half-maximal kinase activity occurred when receptors having no glutamines versus three glutamines were compared (cooperative in vitro activity). The effect of this adaptational modification was stepwise: the greater the number of residues modified, the greater the shift in the concentration of aspartate at which half-maximal kinase activity occurred. Thus, as more receptors become modified through adaptation in vivo, the dynamic range at which chemotactic responses occur would be greatly extended compared to what they would be if kinase activity directly reflected binding of aspartate in a tightly coupled way. Measurement of methylation rate in vitro showed a three-fold shift toward high concentrations for receptors having three glutamines compared with no glutamines, but methylation activity as a function of aspartate concentration was non-cooperative (but, as stated above, binding of aspartate was hardly affected by substituting glutamines for glutamates). Methylation rate reflects receptor conformation. In one conformation, the kinase is active, methylation is inactive, ligand (aspartate) affinity is lower, and demethylation activity is inactive. In the other, the kinase is inactive, methylation is active, ligand affinity is higher, and demethylation activity is active. Thus, there appear to be two factors involved in the very large effect on kinase activity of substituting glutamines for glutamates in the receptors, one on coupling ligand (aspartate) occupancy to receptor conformation (as reflected in rate of methylation) and another on coupling receptor conformation to kinase activity.
Disclosures
None declared
Add Comment
Ordal G W: F1000Prime Recommendation of [Amin DN and Hazelbauer GL, Mol Microbiol 2010, 78(5):1313-23]. In F1000Prime, 31 Jan 2011; DOI: 10.3410/f.7891956.8246054. F1000Prime.com/7891956#eval8246054
F1000Prime Recommendations, Dissents and Comments for [Amin DN and Hazelbauer GL, Mol Microbiol 2010, 78(5):1313-23]. In F1000Prime, 21 May 2013; F1000Prime.com/7891956
Only registered/signed-in users can make comments.
Sign in or Subscribe to create your user account and join the discussion.
Sign in or Subscribe to create your user account and join the discussion.
No comments yet.
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.
© 2010 Blackwell Publishing Ltd.
DOI: 10.1111/j.1365-2958.2010.07408.x
PMID: 21091513
