Atomic Structure of T6SS Reveals Interlaced Array Essential to Function

Type VI secretion systems (T6SSs) are newly identified contractile nanomachines that translocate effector proteins across bacterial membranes. The Francisella pathogenicity island, required for bacterial phagosome escape, intracellular replication, and virulence, was presumed to encode a T6SS-like apparatus. Here, we experimentally confirm the identity of this T6SS and, by cryo electron microscopy (cryoEM), show the structure of its post-contraction sheath at 3.7 Å resolution. We demonstrate the assembly of this T6SS by IglA/IglB and secretion of its putative effector proteins in response to environmental stimuli. The sheath has a quaternary structure with handedness opposite that of contracted sheath of T4 phage tail and is organized in an interlaced two-dimensional array by means of β sheet augmentation. By structure-based mutagenesis, we show that this interlacing is essential to secretion, phagosomal escape, and intracellular replication. Our atomic model of the T6SS will facilitate design of drugs targeting this highly prevalent secretion apparatus. [Display omitted] •Specific environmental stimuli trigger Francisella T6SS assembly and secretion•Atomic structure of T6SS sheath provides targets for mutagenesis and drug design•Two-dimensional interlacing of T6SS sheath proteins is essential to its function•Lack of ClpV and different sheath structure support an alternative functional state An atomic structure of the Francisella novicida type VI secretion system sheath, determined by cryoelectron microscopy, reveals a mesh-like architecture that is required for functional translocation of secreted proteins.