Abstract
Type VI secretion systems (T6SSs) translocate effectors into target cells and are made of a contractile sheath and a tube docked onto a multi-protein transmembrane complex via a baseplate. Although some information is available about the mechanisms of tail contraction leading to effector delivery, the detailed architecture and function of the baseplate remain unknown. Here, we report the 3.7 Å resolution cryo-electron microscopy reconstruction of an enteroaggregative Escherichia coli baseplate subcomplex assembled from TssK, TssF and TssG. The structure reveals two TssK trimers interact with a locally pseudo-3-fold symmetrical complex comprising two copies of TssF and one copy of TssG. TssF and TssG are structurally related to each other and to components of the phage T4 baseplate and of the type IV secretion system, strengthening the evolutionary relationships among these macromolecular machines. These results, together with bacterial two-hybrid assays, provide a structural framework to understand the T6SS baseplate architecture.
Highlights
Type VI secretion systems (T6SSs) translocate effectors into target cells and are made of a contractile sheath and a tube docked onto a multi-protein transmembrane complex via a baseplate
The T6SS needle oligomerizes in the bacterial cytoplasm and comprises an inner tube, made of Hcp hexameric rings arranged with helical symmetry, surrounded by TssB–TssC rings forming the sheath
Crystal structures of the Hcp hexamer and of the VgrG trimeric spike revealed part of the building blocks of tailed phages, R-type pyocins, and T6SSs are structurally similar and led to the hypothesis that these nanomachines are evolutionary and functionally related[11,12,13,14,15,16,17]. These findings were reinforced upon determination of cryo-electron microscopy and cryoelectron tomography structures of the TssB–TssC sheath in the contracted and extended states, which provided insights into the mechanism associated with effector delivery to recipient cells and subsequent recycling of the T6SS needle[18,19,20,21]
Summary
Type VI secretion systems (T6SSs) translocate effectors into target cells and are made of a contractile sheath and a tube docked onto a multi-protein transmembrane complex via a baseplate. Crystal structures of the Hcp hexamer and of the VgrG trimeric spike revealed part of the building blocks of tailed phages, R-type pyocins, and T6SSs are structurally similar and led to the hypothesis that these nanomachines are evolutionary and functionally related[11,12,13,14,15,16,17] These findings were reinforced upon determination of cryo-electron microscopy (cryoEM) and cryoelectron tomography structures of the TssB–TssC sheath in the contracted and extended states, which provided insights into the mechanism associated with effector delivery to recipient cells and subsequent recycling of the T6SS needle[18,19,20,21]. This latter complex is evolutionary related to components of the type IVB secretion system (T4BSS) and demonstrates the mosaic origin of the T6SS apparatus[6]
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