Type IV secretion systems are secretion nanomachines spanning the two membranes of Gram-negative bacteria. Three proteins, VirB7, VirB9 and VirB10, assemble into a 1.05 megadalton (MDa) core spanning the inner and outer membranes. This core consists of 14 copies of each of the proteins and forms two layers, the I and O layers, inserting in the inner and outer membrane, respectively. Here we present the crystal structure of a ∼0.6 MDa outer-membrane complex containing the entire O layer. This structure is the largest determined for an outer-membrane channel and is unprecedented in being composed of three proteins. Unexpectedly, this structure identifies VirB10 as the outer-membrane channel with a unique hydrophobic double-helical transmembrane region. This structure establishes VirB10 as the only known protein crossing both membranes of Gram-negative bacteria. Comparison of the cryo-electron microscopy (cryo-EM) and crystallographic structures points to conformational changes regulating channel opening and closing.
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This work was funded by Wellcome Trust grant 082227 to G.W. We thank A. Thompson and the staff of beamline PROXIMA 1 at Soleil, the staff of beamline ID14.4 at the European Synchrotron Radiation Facility, and H. Saibil, E. Orlova and P. Christie for comments on the manuscript. We thank A. Kumar for help in implementing the immunofluorescence experiments.
Author Contributions V.C. produced the complex, optimized crystals, and built, refined and analysed the structure. R.F. designed the purification protocol, produced the complex, grew the first crystals, optimized crystals and analysed the structure. S.D. and J.N. solved the structure by molecular replacement and provided the electron density map. N.C. collected crystallographic data. G.W. supervised the work, analysed the structure and wrote the paper.
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Chandran, V., Fronzes, R., Duquerroy, S. et al. Structure of the outer membrane complex of a type IV secretion system . Nature 462, 1011–1015 (2009). https://doi.org/10.1038/nature08588
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