Crystal structure of the bacterial membrane protein ToOC central to multidrug efflux and protein export

Diverse molecules, from small antibacterial drugs to large protein toxins, are exported directly across both cell membranes of Gram-negative bacteria. This export is brought about by the reversible interaction of substrate-sp ecific inner-membrane proteins with an outer-membrane protein of the TolC f amily, thus bypassing the intervening periplasm. Here we report the 2.1-Ang strom crystal structure of TolC from Escherichia coli, revealing a distinct ive and previously unknown fold. Three TolC protomers assemble to form a co ntinuous, solvent-accessible conduit-a 'channel-tunnel' over 140 Angstrom l ong that spans both the outer membrane and periplasmic space. The periplasm ic or proximal end of the tunnel is sealed by sets of coiled helices. We su ggest these could be untwisted by an allosteric mechanism, mediated by prot ein-protein interactions, to open the tunnel. The structure provides an exp lanation of how the cell cytosol is connected to the external environment d uring export, and suggests a general mechanism for the action of bacterial efflux pumps.