Identification of Factors Affecting Polar Localization of Chemoreceptors in Escherichia coli

In prokaryotic cells, several proteins localize at specific sites, such as the cell poles and the septum, and this spatial organization is fundamental for many cellular processes. Clusters of chemoreceptors, a family of transmembrane receptors that mediate chemotaxis, are the canonical example of a spatially localized protein in bacteria. For example, Escherichia coli presents 5 different chemoreceptors (Tsr, Tar, Trg, Tap and Aer) that form trimers-of-dimer clusters embedded into the inner membrane and are mostly found at the cell poles. However, the molecular mechanisms that regulate the polar positioning of chemoreceptors are still undefined. In this study, we demonstrate that the Tol-Pal complex, a multiprotein complex bridging the three layers of the cell envelope, and the major outer membrane lipoprotein, Lpp, might play an important role in polar localization of chemoreceptors in E. coli. We expressed a functional fluorescent-labeled fusion protein, YFP-CheR, in E. coli K-12 strain MG1655 and various E. coli mutants, and analyzed the localization profile of chemoreceptors by fluorescence microscopy. CheR is a methyltransferase that targets a specific C-terminal pentapeptide of the chemoreceptors and, therefore, its localization should reflect that of the chemoreceptor. The overexpression of YFP-CheR in tolA, tolB, tolQ, tolR, and pal deletion mutants produced diffuse fluorescence throughout the cell. Expression in the lpp deletion mutant produced defined CheR foci that were mislocalized compared to expression in the wild-type strain. Whether this represents a specific phenomenon for chemoreceptors or a general conserved mechanism for polar localization of proteins in E. coli is still under investigation.