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Commun Biol. 2019 Oct 4;2:358. doi: 10.1038/s42003-019-0604-2. eCollection 2019.

Cryo-EM structure of the bacterial Ton motor subcomplex ExbB-ExbD provides information on structure and stoichiometry.

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1Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892 USA.
2Laboratory of Membrane Proteins and Structural Biology, Biochemistry and Biophysics Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD 20892 USA.
3Center for Molecular Microscopy, Center for Cancer Research, National Cancer institute, National Institutes of Health, Bethesda, MD 20892 USA.
4Cancer Research Technology Program, Frederick National Laboratory for Cancer Research, Leidos Biomedical Research Inc, Frederick, MD 21701 USA.
5Laboratoire d'Ingénierie des Systèmes Macromoléculaires, UMR7255 CNRS/Aix-Marseille Université, Institut de Microbiologie de la Méditerranée, 13402 Marseille Cedex 20, France.


The TonB-ExbB-ExbD molecular motor harnesses the proton motive force across the bacterial inner membrane to couple energy to transporters at the outer membrane, facilitating uptake of essential nutrients such as iron and cobalamine. TonB physically interacts with the nutrient-loaded transporter to exert a force that opens an import pathway across the outer membrane. Until recently, no high-resolution structural information was available for this unique molecular motor. We published the first crystal structure of ExbB-ExbD in 2016 and showed that five copies of ExbB are arranged as a pentamer around a single copy of ExbD. However, our spectroscopic experiments clearly indicated that two copies of ExbD are present in the complex. To resolve this ambiguity, we used single-particle cryo-electron microscopy to show that the ExbB pentamer encloses a dimer of ExbD in its transmembrane pore, and not a monomer as previously reported. The revised stoichiometry has implications for motor function.


Bacterial structural biology; Cryoelectron microscopy

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