Differences in the polar clustering of the high- and low-abundance chemoreceptors of Escherichia coli

The chemosensory complexes in Escherichia coli are localized predominantly in large aggregates at one or both of the cell poles, however, neither the role of the polar localization nor the role of the clustering is understood. In E. coli, the two classes of chemoreceptors or transducers, high- and low-abundance, differ in their ability to support chemotaxis when expressed as the sole chemoreceptor type in the cell. In this study, we examined both the contribution of individual chemoreceptors to polar clustering and the ability of each chemoreceptor type to cluster in the absence of all others. We found that polar clustering of methyl-accepting chemotaxis proteins (MCPs) is not dependent on any one chemoreceptor type. Remarkably, when expressed individually at similar levels, the chemoreceptors display differential clustering abilities. The high-abundance transducers cluster at the cell pole almost as well as do the MCPs in cells expressing all four species, whereas the low-abundance transducers, although polar, are not particularly clustered. CheA and CheW distributions in strains expressing only one chemoreceptor type coincide with MCP localization, indicating that the low-abundance chemoreceptors are competent for ternary complex formation but are defective in aggregation. These studies reveal that, in contrast to our previous model, polarity of the chemoreceptors is independent of clustering, suggesting that the polar localization of the chemoreceptors is not simply caused by diffusion limitations on large protein aggregates.