Membrane Curvature and the Tol-Pal Complex Determine Polar Localization of the Chemoreceptor Tar in Escherichia coli.

Chemoreceptors are localized at the cell poles of Escherichia coli and other rod-shaped bacteria. Over the years, different mechanisms have been put forward to explain this polar localization, including stochastic clustering, membrane curvature-driven localization, interactions with the Tol-Pal complex, and nucleoid exclusion. To evaluate these mechanisms, we monitored the cellular localization of the aspartate chemoreceptor Tar in different deletion mutants. We did not find any indication for either stochastic cluster formation or nucleoid exclusion. However, the presence of a functional Tol-Pal complex appeared to be essential to retain Tar at the cell poles. Interestingly, Tar still accumulated at midcell in tol and in pal deletion mutants. In these mutants, the protein appears to gather at the base of division septa, a region characterized by strong membrane curvature. Chemoreceptors, like Tar, form trimers of dimers that bend the cell membrane due to a rigid tripod structure. The curvature approaches the curvature of the cell membrane generated during cell division, and localization of chemoreceptor tripods at curved membrane areas is therefore energetically favorable, as it lowers membrane tension. Indeed, when we introduced mutations in Tar that abolish the rigid tripod structure, the protein was no longer able to accumulate at midcell or the cell poles. These findings favor a model where chemoreceptor localization in E. coli is driven by strong membrane curvature and association with the Tol-Pal complex.IMPORTANCE Bacteria have exquisite mechanisms to sense and adapt to the environment they live in. One such mechanism involves the chemotaxis signal transduction pathway, in which chemoreceptors specifically bind certain attracting or repelling molecules and transduce the signals to the cell. In different rod-shaped bacteria, these chemoreceptors localize specifically to cell poles. Here, we examined the polar localization of the aspartate chemoreceptor Tar in E. coli and found that membrane curvature at cell division sites and the Tol-Pal protein complex localize Tar at cell division sites, the future cell poles. This study shows how membrane curvature can guide localization of proteins in a cell.