The Type VI Secretion System is a well-characterized complex that bacteria use to either defend themselves or to attack other cells, either prokaryotic or eukaryotic. It is composed of a membrane-anchoring complex and a baseplate that allow for the assembly of a tube capped by a spike and surrounded by a helical sheath. When needed, the sheath contracts releasing the tube and the spike with its cargo towards the target cell, leading to its death. The baseplate is a key component of such system, as it not only anchors the tube and sheath to the membrane complex, but also contributes to their assembly.
Here we present the first high-resolution cryo-EM structure of a wedge of the baseplate complex and the membrane complex, and use that structure to model the full secretion system assembly. We also compare the T6SS it to its closest structural homologue of which a high resolution is available, the T4 phage baseplate.
The structure unveils not only potential sites essential for the interaction between components of the system that could be disrupted by targeted antimicrobials, it also provides unprecedented insight into the mechanism of assembly and function of the baseplate and the larger complex of the Type VI Secretion System.