An aspartate ring at the TolC tunnel entrance determines ion selectivity and presents a target for blocking by large cations

The TolC protein of Escherichia coli comprises an outer membrane beta-barrel channel and a contiguous alpha-helical tunnel spanning the periplasm, providing an exit duct for protein export and multidrug efflux. It forms a single transmembrane pore that is open to the outside of the cell but constricted at the peri-plasmic tunnel entrance. This sole constriction is lined by a ring of six aspartate residues, two in each of the three identical monomers. When these were replaced by alanines, the resulting TolC(DADA) protein reconstituted normally in black lipid membranes but showed altered electrophysiological characteristics. In particular, it had lost the strong pH dependence of the wild type and had switched ion selectivity from cations to anions. The function of wild-type TolC as a membrane pore was severely inhibited by divalent and trivalent cations entering the channel tunnel from the channel ('extracellular') side. Divalent cations bound reversibly to effect complete blocking of the transmembrane ion flux. Trivalent cations were more potent. Hexamminecobalt bound at nanomolar concentrations allowed visualization of single blocking events, whereas the smaller Cr3+ cation bound irreversibly and could also access the cation binding site via the tunnel entrance. The inhibitory cations had no effect on the mutant TolC(DADA) , supporting the view that the aspartate ring is the cation binding site. The electronegative entrance is widely conserved throughout the TolC family, which is essential for efflux and export by Gram-negative bacteria, suggesting that it could present a general target for drugs.