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Cryo-EM structure of the extended type VI secretion system sheath–tube complex

Abstract

The bacterial type VI secretion system (T6SS) uses contraction of a long sheath to quickly thrust a tube with associated effectors across membranes of eukaryotic and bacterial cells1,2,3,4,5. Only limited structural information is available about the inherently unstable precontraction state of the T6SS. Here, we obtain a 3.7 Å resolution structure of a non-contractile sheath–tube complex using cryo-electron microscopy and show that it resembles the extended T6SS inside Vibrio cholerae cells. We build a pseudo-atomic model of the complete sheath–tube assembly, which provides a mechanistic understanding of coupling sheath contraction with pushing and rotating the inner tube for efficient target membrane penetration. Our data further show that sheath contraction exposes a buried recognition domain to specifically trigger the disassembly and recycling of the T6SS sheath by the cognate ATP-dependent unfoldase ClpV.

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Fig. 1: Cryo-EM structure of the VipA-N3 sheath–tube complex closely resembles the intracellular wild-type extended sheath–tube complex.
Fig. 2: Model of T6SS sheath contraction.
Fig. 3: Domain 3 of the VipA-N3 sheath is inaccessible to ClpV.
Fig. 4: Structure of the Hcp tube and its interaction with the sheath.

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Acknowledgements

The work was supported by Swiss National Science Foundation (SNSF) grant 31003A_159525 and the University of Basel. H.S. acknowledges support from the SNSF NCCR TransCure. Calculations were performed at sciCORE (http://scicore.unibas.ch/) scientific computing core facility at the University of Basel. We acknowledge S. Ursich for the help in sample preparation for cryo-ET.

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Contributions

J.W. collected cryo-electron microscopy data, performed image processing and generated atomic models. M.Br. isolated and purified the sheaths. M.K. performed some initial electron microscopy data collection and data analysis. D.C.-D. provided support and contributed to data analysis. K.N.G. and H.S. provided support and supervised data collection. T.M. contributed to and advised on atomic model building. M.Ba. conceived the project and analysed the data. M.Br., J.W. and M.Ba. wrote the manuscript. All authors read the manuscript.

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Correspondence to Marek Basler.

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Supplementary Figures 1–5, Supplementary Tables 1–6, Supplementary Video legends.

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Supplementary Video 1

Details of tomography reconstruction of the wild-type extended sheath, VipA-N3 sheath structure and Hcp tube.

Supplementary Video 2

Proposed mechanism of T6SS assembly and contraction.

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Wang, J., Brackmann, M., Castaño-Díez, D. et al. Cryo-EM structure of the extended type VI secretion system sheath–tube complex. Nat Microbiol 2, 1507–1512 (2017). https://doi.org/10.1038/s41564-017-0020-7

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