The type VI secretion system (T6SS) is a supra‐molecular bacterial complex that resembles phage tails. It is a killing machine which fires toxins into target cells upon contraction of its TssBC sheath. Here, we show that TssA1 is a T6SS component forming dodecameric ring structures whose dimensions match those of the TssBC sheath and which can accommodate the inner Hcp tube. The TssA1 ring complex binds the T6SS sheath and impacts its behaviour in vivo. In the phage, the first disc of the gp18 sheath sits on a baseplate wherein gp6 is a dodecameric ring. We found remarkable sequence and structural similarities between TssA1 and gp6 C‐termini, and propose that TssA1 could be a baseplate component of the T6SS. Furthermore, we identified similarities between TssK1 and gp8, the former interacting with TssA1 while the latter is found in the outer radius of the gp6 ring. These observations, combined with similarities between TssF and gp6N‐terminus or TssG and gp53, lead us to propose a comparative model between the phage baseplate and the T6SS.

Insertion of folded proteins into the outer membrane of Gramnegative bacteria is mediated by the essential b-barrel assembly machine (Bam). Here, we report the native structure and mechanism of a core component of this complex, BamE, and show that it is exclusively monomeric in its native environment of the periplasm, but is able to adopt a distinct dimeric conformation in the cytoplasm. BamE is shown to bind specifically to phosphatidylglycerol, and comprehensive mutagenesis and interaction studies have mapped key determinants for complex binding, outer membrane integrity and cell viability, as well as revealing the role of BamE within the Bam complex.

Highlights d The structure of the VgrG2b C-terminal domain presents a metallopeptidase fold d VgrG2b exerts antibacterial activity in the periplasmic space d Toxicity of VgrG2b is counteracted by a cognate periplasmic immunity protein d VgrG2b C-ter -intoxicated prey cells bleb at the midcell and lyse Authors

Background: The type VI secretion system (T6SS) is a bacterial device similar to bacteriophages.Results: Pseudomonas aeruginosa HsiB and HsiC proteins form a T6SS subcomplex, which polymerizes into tubules via the C terminus of HsiC.Conclusion: HsiB and HsiC form a structure with similarity to the bacteriophage tail sheath.Significance: TssB and TssC members of the T6SS form a complex resembling the bacteriophage tail sheath and mimic the gp18 protein.