{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,5]],"date-time":"2026-10-05T11:28:47Z","timestamp":1791199727715,"version":"4.2.0"},"reference-count":32,"publisher":"Wiley","issue":"5","license":[{"start":{"date-parts":[[2002,5,23]],"date-time":"2002-05-23T00:00:00Z","timestamp":1022112000000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/onlinelibrary.wiley.com\/termsAndConditions#vor"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Molecular Microbiology"],"published-print":{"date-parts":[[2002,6]]},"abstract":"<jats:title>Summary<\/jats:title><jats:p>The TolC protein of <jats:italic>Escherichia coli<\/jats:italic> comprises an outer membrane \u03b2\u2010barrel\n channel and a contiguous \u03b1\u2010helical tunnel spanning the periplasm, providing\n an exit duct for protein export and multidrug efflux. It forms a single transmembrane\n pore that is open to the outside of the cell but constricted at the peri\u2010plasmic\n tunnel entrance. This sole constriction is lined by a ring of six aspartate residues,\n two in each of the three identical monomers. When these were replaced by alanines,\n the resulting TolC<jats:sup>DADA<\/jats:sup> protein reconstituted normally in black lipid membranes\n but showed altered electrophysiological characteristics. In particular, it had lost\n the strong pH dependence of the wild type and had switched ion selectivity from cations\n to anions. The function of wild\u2010type TolC as a membrane pore was severely inhibited\n by divalent and trivalent cations entering the channel tunnel from the channel (\u2018extracellular\u2019)\n side. Divalent cations bound reversibly to effect complete blocking of the transmembrane\n ion flux. Trivalent cations were more potent. Hexamminecobalt bound at nanomolar\n concentrations allowed visualization of single blocking events, whereas the smaller\n Cr<jats:sup>3<jats:bold>+<\/jats:bold><\/jats:sup> cation bound irreversibly and could also access the cation\n binding site via the tunnel entrance. The inhibitory cations had no effect on the\n mutant TolC<jats:sup>DADA<\/jats:sup>, 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\u2010negative bacteria, suggesting that it could present a general target for drugs.<\/jats:p>","DOI":"10.1046\/j.1365-2958.2002.02898.x","type":"journal-article","created":{"date-parts":[[2003,3,12]],"date-time":"2003-03-12T11:44:24Z","timestamp":1047469464000},"page":"1131-1139","source":"Crossref","is-referenced-by-count":87,"title":["An aspartate ring at the TolC tunnel entrance determines ion selectivity and presents a target for blocking by large 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