Escherichia coli possesses a chemotactic receptor signaling system that allows it to sense and respond to its environment. Under physiological conditions, the kinase CheA associates with the serine-binding transmembrane receptor. CheA is active in this apocomplex; however, serine binding to the receptor inhibits catalytic activity of CheA. The serine-binding transmembrane receptor is reversibly methylated on glutamate residues in proportion to the concentration of serine in its surroundings. However, the role of receptor modification in the adaptive response to serine has not been clearly defined. Li and Weis identified that low external concentrations of serine actually accelerated the formation of active CheA/receptor complexes, at concentrations insufficient to inhibit CheA kinase activity. Inhibition of CheA kinase activity in complexes depended upon the amount of receptor modification, such that compared to unmodified receptors, the fully modified receptors showed greater steady-state phosphorylation of CheA. The difference in kinase activity between unmodified and fully modified receptors is apparently due to reduced binding affinity for serine by fully modified receptors. Thus, intracellular modification of the transmembrane receptor weakens the affinity of serine for the extracellular domain of the receptor.
Li, G., and Weis, R.M. (2000) Covalent modification regulates ligand binding to receptor complexes in the chemosensory system of Escherichia coli. Cell 100: 357-365. [Online Journal]