Type VI secretion system

The type VI secretion system (T6SS) is molecular machine used by a wide range of Gram-negative bacterial species to transport proteins from the interior (cytoplasm or cytosol) of a bacterial cell across the cellular envelope into an adjacent target cell. While often reported that the T6SS was discovered in 2006 by researchers studying the causative agent of cholera, Vibrio cholerae, the first study demonstrating that T6SS genes encode a protein export apparatus was actually published in 2004, in a study of protein secretion by the fish pathogen Edwardsiella tarda.

Since then, Type VI secretion systems have been found in a quarter of all Proteobacterial genomes, including pathogens of animals, plants, and humans, as well as soil, environmental or marine bacteria. While most of the early studies of Type VI secretion focused on its role in the pathogenesis of higher organisms, it is now known to function primarily in interbacterial antagonism.

The T6SS is thought to resemble an inverted phage extending outward from the bacterial cell surface. It consists of 14 proteins that assemble into three sub-complexes: a phage tail-like tubule, a phage baseplate-like structure, and cell-envelope spanning membrane complex. These three subcomplexes work together to transport proteins across the bacterial cell envelope and into a target cell through a contractile mechanism

The phage tail-like component of the T6SS is a dynamic tubular structure that undergoes cycles of assembly and disassembly. It can be up to 600 nm long, and has been visualized extending across the bacterial cytoplasm in electron micrographs. The tubules consist of repeating units of the proteins TssA and TssB (VipB/A) arranged as a sheath around a tube built from stacked hexameric rings of the protein Hcp. At the tip of the Hcp tube sits a trimer of the phage tail spike-like protein VgrG, which is in turn capped by a pointed PAAR domain-containing protein. Contraction of the sheath is thought to propel the Hcp tube, VgrG and associated substrates outside of the bacterial cell, where the VgrG/PAAR spike facilitates penetration of the membrane of a neighboring cell. The tubule structure is dismantled through the action of the ATP-degrading protein ClpV, which sits at the tubule base.

The phage tail-like tubule of the T6SS assembles on a structure analogous to bacteriophage baseplates. It consists of the proteins TssE, TssF, TssG, and TssK. The baseplate and phage tail-like complex interact in the bacterial cytoplasm, and then are recruited to the cell envelope by the membrane complex.

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