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

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 cells (1-5) . 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.The structure of the extended sheath-tube complex of the type VI secretion system from Vibrio cholerae elucidates the molecular mechanisms by which conformational changes in the sheath enable the inner tube to penetrate target cells.The structure of the extended sheath-tube complex of the type VI secretion system from Vibrio cholerae elucidates the molecular mechanisms by which conformational changes in the sheath enable the inner tube to penetrate target cells.

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