Abstract
The Type VI secretion system (T6SS) is a widespread weapon dedicated to the delivery of toxin proteins into eukaryotic and prokaryotic cells. The 13 T6SS subunits assemble a cytoplasmic contractile structure anchored to the cell envelope by a membrane-spanning complex. This structure is evolutionarily, structurally and functionally related to the tail of contractile bacteriophages. In bacteriophages, the tail assembles onto a protein complex, referred to as the baseplate, that not only serves as a platform during assembly of the tube and sheath, but also triggers the contraction of the sheath. Although progress has been made in understanding T6SS assembly and function, the composition of the T6SS baseplate remains mostly unknown. Here, we report that six T6SS proteins–TssA, TssE, TssF, TssG, TssK and VgrG–are required for proper assembly of the T6SS tail tube, and a complex between VgrG, TssE,-F and-G could be isolated. In addition, we demonstrate that TssF and TssG share limited sequence homologies with known phage components, and we report the interaction network between these subunits and other baseplate and tail components. In agreement with the baseplate being the assembly platform for the tail, fluorescence microscopy analyses of functional GFP-TssF and TssK-GFP fusion proteins show that these proteins assemble stable and static clusters on which the sheath polymerizes. Finally, we show that recruitment of the baseplate to the apparatus requires initial positioning of the membrane complex and contacts between TssG and the inner membrane TssM protein.
Highlights
In the environment, bacteria endure an intense warfare
The Type VI secretion system (T6SS) is a contractile machine functionally comparable to a crossbow: an inner tube is wrapped by a contractile structure
In addition to TssE and VgrG that have been shown to share homologies with the bacteriophage gp25 and gp27-gp5 proteins, the TssF and TssG proteins have homologies with bacteriophage components
Summary
Bacteria endure an intense warfare. Bacteria collaborate or compete to acquire nutrients or to efficiently colonize a niche. Several T6SSs have been shown to be required for full virulence towards different eukaryotic cells, most T6SSs shape bacterial communities through inter-bacteria interactions [1]. In both cases, T6SSs inject toxic effectors into target/recipient cells. A number of anti-bacterial toxins have been recently identified and carry a versatile repertoire of cytotoxic activities such as peptidoglycan hydrolases, phospholipases or DNases [1,5,6] Delivery of these toxins into the periplasm or cytoplasm of the target cell leads to a rapid lysis that usually occurs within minutes [7,8,9]
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