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Elife. 2014 Sep 19;3. doi: 10.7554/eLife.03145.

Coupling of remote alternating-access transport mechanisms for protons and substrates in the multidrug efflux pump AcrB.

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Institute of Biochemistry, Goethe University, Frankfurt am Main, Germany.
Institute of Physiology, University of Zurich, Zurich, Switzerland.
Theoretical Molecular Biophysics Section, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, United States.
Department of Biology, University of Konstanz, Konstanz, Germany.


Membrane transporters of the RND superfamily confer multidrug resistance to pathogenic bacteria, and are essential for cholesterol metabolism and embryonic development in humans. We use high-resolution X-ray crystallography and computational methods to delineate the mechanism of the homotrimeric RND-type proton/drug antiporter AcrB, the active component of the major efflux system AcrAB-TolC in Escherichia coli, and one most complex and intriguing membrane transporters known to date. Analysis of wildtype AcrB and four functionally-inactive variants reveals an unprecedented mechanism that involves two remote alternating-access conformational cycles within each protomer, namely one for protons in the transmembrane region and another for drugs in the periplasmic domain, 50 Å apart. Each of these cycles entails two distinct types of collective motions of two structural repeats, coupled by flanking α-helices that project from the membrane. Moreover, we rationalize how the cross-talk among protomers across the trimerization interface might lead to a more kinetically efficient efflux system.


E. coli; H+ transport; antibiotic resistance; biochemistry; biophysics; drug resistance; drug transport; efflux pump; mechanistic coupling; structural biology

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