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Mol Cells. 2015;38(2):180-6. doi: 10.14348/molcells.2015.2277. Epub 2015 Jan 15.

Structure of the tripartite multidrug efflux pump AcrAB-TolC suggests an alternative assembly mode.

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Department of Manufacturing Pharmacy, Pusan National University, Busan 609-735, Korea.
Division of Electron Microscopic Research, Korea Basic Science Institute, Dajeon 169-148, Korea.
Department of Food and Animal Biotechnology, Department of Agricultural Biotechnology, Center for Food and Bioconvergence, Research Institute for Agricultural and Life Sciences, Seoul National University, Seoul 151-742, Korea.
Department of Life Science, Chung-Ang University, Seoul 156-756, Korea.
Division of Magnetic Resonance, Korea Basic Science Institute, Chungbuk 363-883, Korea.


Escherichia coli AcrAB-TolC is a multidrug efflux pump that expels a wide range of toxic substrates. The dynamic nature of the binding or low affinity between the components has impeded elucidation of how the three components assemble in the functional state. Here, we created fusion proteins composed of AcrB, a transmembrane linker, and two copies of AcrA. The fusion protein exhibited acridine pumping activity, suggesting that the protein reflects the functional structure in vivo. To discern the assembling mode with TolC, the AcrBA fusion protein was incubated with TolC or a chimeric protein containing the TolC aperture tip region. Three-dimensional structures of the complex proteins were determined through transmission electron microscopy. The overall structure exemplifies the adaptor bridging model, wherein the funnel-like AcrA hexamer forms an intermeshing cogwheel interaction with the α-barrel tip region of TolC, and a direct interaction between AcrB and TolC is not allowed. These observations provide a structural blueprint for understanding multidrug resistance in pathogenic Gram-negative bacteria.


complex structure; electron microscopy; membrane protein; multidrug efflux pump

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