Variable Signaling Team Sizes of Escherichia coli Chemoreceptors: A New Level of Adaptation
Transmembrane receptors of the chemotaxis network in Escherichia coli allow bacteria to sense chemicals in the environment, allowing cells to swim towards nutrients (attractant chemicals) and away from repellents (toxic chemicals). The chemotaxis network possesses remarkable signaling properties including high sensitivity to small changes in chemical concentration over a wide range of ambient concentrations. These signaling properties rely on adaptation and on receptor clustering, which occurs at multiple length scales. At a small scale, the chemotaxis receptors form homodimers which then assemble into larger signaling teams in which receptors of different chemical specificities are intermixed (with trimers of dimers believed to be the smallest signaling unit). At a larger scale, ~10,000 receptors form large polar and lateral receptor clusters. However, very little is known quantitatively about the sizes of receptor signaling teams. From in vivo fluorescence resonance energy transfer (FRET) signaling data, we have developed a biophysical theory for signaling by teams of chemotaxis receptors of mixed types. We examined both adapting receptors in the presence of the modification enzymes CheR and CheB, as well as genetically engineered receptors in particular modification states in non-adapting cells. As a result of this quantitative data analysis, we discovered a new level of adaptation in the E. coli chemotaxis network. Specifically, we found that signaling team size increases by up to three-fold with receptor modification, such as occurs during adaptation to an attractant. Furthermore, we present a theory that the observed variation in signaling team size is a novel adaptive mechanism to optimally measure noisy ligand concentrations.