The bacterial type VI secretion system (T6SS) is a nanomachine that punctures the membranes of competing bacteria and, during pathogenesis, those of host cells. In addition to the tail, which is composed of a contractile sheath and a tipped arrow that punctures the membrane of the prey cell, T6SS function relies on the TssJ−TssL−TssM membrane complex, which is an assembly point and anchor for the tail. Now, Durand et al. describe the structure of the TssJ−TssL−TssM complex from Escherichia coli at 11.6 Å resolution. 10 copies of each protein are arranged with fivefold symmetry: in the periplasm, 10 pillars (TssJ and TssM) surround a 15–20 Å channel and are connected to 10 arches (TssM); these arches extend to a base (TssM and TssL) located in the inner membrane and the cytoplasm. The authors propose a mechanism of action in which the inner TssM pillars are pushed outwards to create a larger ring that accomodates the tail during secretion; this would protect the membrane of the attacking cell, which might otherwise be punctured by the contraction of the tail.
References
Durand, E. et al. Biogenesis and structure of a type VI secretion membrane core complex. Nature http://dx.doi.org/10.1038/nature14667 (2015)
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Attar, N. T6SS cases its own tail. Nat Rev Microbiol 13, 527 (2015). https://doi.org/10.1038/nrmicro3543
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DOI: https://doi.org/10.1038/nrmicro3543