In situ and high-resolution Cryo-EM structure of the Type VI secretion membrane complex

Bacteria have evolved macromolecular machineries that secrete effectors and toxins to survive and thrive in diverse environments. The type VI secretion system (T6SS) is a contractile machine that is composed of a baseplate that contains a spike onto which an inner tube is built, surrounded by a contractile sheath. The T6SS is an intracellular machine inserted in the bacterial membranes by a membrane complex. This membrane complex (MC) comprises three proteins: TssJ, TssL and TssM. We previously reported the low-resolution negative stain electron microscopy structure of the enteroaggregative Escherichia coli MC and proposed a rotational 5-fold symmetry with a TssJ:TssL:TssM stoichiometry of 2:2:2. Here, cryo-electron tomography and single particle analysis CryoEM of the T6SS MC confirmed the 5-fold symmetry in situ and identified the regions of the structure that insert into the bacterial membranes. A high resolution model obtained by reveals its global architecture and highlights new features. Based on these data, we revisit the model on the mechanism of action of the MC during T6SS assembly and function.