RECEPTOR CLUSTERING AS A CELLULAR MECHANISM TO CONTROL SENSITIVITY

Chemotactic bacteria such as Escherichia coli can detect and respond t o extremely low concentrations of attractants, concentrations of less than 5 nM in the case of aspartate(1), They also sense gradients of at tractants extending over five orders of magnitude in concentration (up to 1 mM aspartate)(2,3). Here we consider the possibility that this c ombination of sensitivity and range of response depends on the cluster ing of chemotactic receptors on the surface of the bacterium(4). We ex amine what will happen if ligand binding changes the activity of a rec eptor, propagating this change in activity to neighbouring receptors i n a cluster(5,6). Calculations based on these assumptions show that se nsitivity to extracellular ligands increases with the extent of spread of activity through an array of receptors, but that the range of conc entrations over which the array works is severely diminished. However, a combination of low threshold of response and wide dynamic range can be attained if the cell has both clusters and single receptors on its surface, particularly if the extent of activity spread can adapt to e xternal conditions. A mechanism of this kind can account quantitativel y for the sensitivity and response range of E. coli to aspartate.