Type VI secretion requires a dynamic contractile phage tail-like structure

Cryo‐ EM reconstruction of Type VI secretion system baseplate and sheath distal end

“…The previous cryo‐ET studies of V. cholerae , M. xanthus, and A. asiaticus showed that T6SS sheath–tube complex is connected to the cell envelope through a cone‐shaped density (Basler et al , 2012; Böck et al , 2017; Chang et al , 2017). In our baseplate reconstruction, the periphery of the wedge likely composed of TssE/F/G is decorated by six copies of a trimeric protein TssK.…”

“…In that case, the membrane complex could induce changes in TssK structure, which would propagate through its interaction with TssE/F/G wedge to trigger sheath contraction. However, live‐cell imaging of T6SS sheath dynamics in V. cholerae provided no evidence for a triggering mechanism based on cell–cell contact (Basler & Mekalanos, 2012; Basler et al , 2012). Conversely, it is possible that rearrangement of baseplate proteins prior to sheath contraction changes TssK conformation, and this results in opening of the connected membrane complex to allow passage of the cargo‐loaded spike and tube.…”

“…The whole T6SS was first observed at low resolution using cryo‐electron tomography (cryo‐ET) in Vibrio cholerae (Basler et al , 2012) and more recently in Myxococcus xanthus (Chang et al , 2017) and Amoebophilus asiaticus (Böck et al , 2017). The system is composed of a long sheath surrounding an inner tube attached to a baseplate and anchored to a membrane complex that spans both membranes (Brunet et al , 2014, 2015; Durand et al , 2015).…”

“…The sheath was shown to assemble across the entire width of a cell, and this allowed to use live‐cell fluorescence microscopy to study its dynamics and subcellular localization (Basler et al , 2012; Brunet et al , 2013; Gerc et al , 2015; Zoued et al , 2016; Vettiger et al , 2017). In V. cholerae , sheath assembly takes approximately 20 s (speed of assembly 30–40 nm/s) and is followed by fast contraction in less than 2 ms (speed of contraction higher than 800 μm/s) (Basler et al , 2012; Vettiger et al , 2017). Rapid sheath contraction propels the tip complex at the end of the inner tube formed from stacks of Hcp rings into the target cell periplasm or cytosol (Vettiger & Basler, 2016).…”

“…Rapid sheath contraction propels the tip complex at the end of the inner tube formed from stacks of Hcp rings into the target cell periplasm or cytosol (Vettiger & Basler, 2016). In contrast to phages and many other contractile nanomachines, which translocate proteins only once by a single sheath contraction (Nakayama et al , 2000; Ge et al , 2015; Hu et al , 2015), the contracted T6SS sheath is disassembled by a cytosolic unfoldase ClpV or ClpB to allow for repeated protein secretion (Bönemann et al , 2009; Pietrosiuk et al , 2011; Basler & Mekalanos, 2012; Basler et al , 2012; Kapitein et al , 2013; Förster et al , 2014; Brodmann et al , 2017). …”