22 March 2006
Robert Endres
Wingreen Lab
Department of Molecular Biology
Princeton University
Adaptation is ubiquitous in biological sensory systems. Specifically, precise adaptation allows bacteria such as Escherichia coli to efficiently chemotax, i.e. to swim up gradients of attractants such as amino acids or sugars. The adaptation mechanism relies on methylation and demethylation (or deamidation) of specific modification sites of the membrane-bound chemoreceptors by the enzymes CheR and CheB, respectively. These enzymes can assist modifying 5-7 nearby receptors when tethered to a particular receptor. Using a free-energy based model for signalling by clusters of chemoreceptors, we show that these "assistance neighborhoods" are necessary for precise adaptation. In agreement with experiment, clusters of receptors of different type exhibit high sensitivity and precise adaptation over a wide range of concentrations, and the response of adapted clusters to addition/removal of attractant scales with the free-energy change of the receptors. We predict two limits of precise adaptation at large attractant concentrations: either receptors become fully methylated and turn off, or receptors become saturated and cease to respond to attractant, but retain their adapted activity.
J E Keymer, R G Endres, M Skoge, Y Meir & N S Wingreen, "Chemosensing in Escherichia coli: two regimes of two-state receptors", PNAS 103:1786-91 2006. PubMed