Polar clustering of the chemoreceptor complex in Escherichia coli occurs in the absence of complete CheA function

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.

Identification and localization of a methyl-accepting chemotaxis protein in Rhodobacter sphaeroides

Genes coding for a classical membrane spanning chemoreceptor (mcpG) and a response regulator (cheY4) were identified in a region of Rhodobacter sphaeroides DNA unlinked to either of the two previously identified chemosensory operons. Immunogold electron microscopy had shown that the expression of chemoreceptors in R. sphaeroides varies with growth conditions. Using GFP fused to the newly identified McpG, we examined the targeting of this single methyl-accepting chemotaxis protein (MCP) under different growth conditions. The gene encoding the C-terminal McpG-GFP fusion was introduced by homologous recombination into the chromosome, replacing the wild-type gene. The resultant protein localized to the poles of the cell under aerobic, photoheterotrophic and anaerobic dark conditions, demonstrating that this MCP is expressed under all three growth conditions. More protein was always found at one pole than the other. The polar fluorescence increased during the cell cycle, with protein becoming evident at the second pole around the time of septation. At division, each daughter cell had a label at one pole, but the intensity of fluorescence was higher in the daughter cell containing the original labelled pole. McpG localization was not altered in a che Operon 1 deletion strain, lacking CheW1 and CheA1, but a che Operon 2 deletion strain, lacking CheW2, CheW3 and CheA2, showed significantly reduced polar localization. This observation indicates that polar localization of McpG depends on Che proteins encoded by Operon 2, but not homologues encoded by Operon 1.