Abstract
The type VI secretion systems (T6SS) are present in about a quarter of all Gram-negative bacteria. Several key components of T6SS are evolutionarily related to components of contractile nanomachines such as phages and R-type pyocins. The T6SS assembly is initiated by formation of a membrane complex that binds a phage-like baseplate with a sharp spike, and this is followed by polymerization of a long rigid inner tube and an outer contractile sheath. Effectors are preloaded onto the spike or into the tube during the assembly by various mechanisms. Contraction of the sheath releases an unprecedented amount of energy, which is used to thrust the spike and tube with the associated effectors out of the effector cell and across membranes of both bacterial and eukaryotic target cells. Subunits of the contracted sheath are recycled by T6SS-specific unfoldase to allow for a new round of assembly. Live-cell imaging has shown that the assembly is highly dynamic and its subcellular localization is in certain bacteria regulated with a remarkable precision. Through the action of effectors, T6SS has mainly been shown to contribute to pathogenicity and competition between bacteria. This review summarizes the knowledge that has contributed to our current understanding of T6SS mode of action.
Highlights
The type VI secretion systems (T6SS) are present in about a quarter of all Gramnegative bacteria
After characterization of T6SS in V. cholerae, one of the three T6SS clusters of Pseudomonas aeruginosa was shown to secrete haemolysincorregulated protein (Hcp) in vitro and because the Hcp was detected in the sputum of cystic fibrosis patients infected by P. aeruginosa it was suggested that this system could be important for pathogenesis [6]
Energetics of the T6SS is interesting considering the fact that it seems as if only few molecules of spike-associated effectors are secreted with each contraction and since the T6SS seems relatively costly, considering the loss of Hcp and potentially large consumption of ATP during refolding of the contracted sheath
Summary
Gram-negative bacteria use various secretion systems to deliver proteins from the bacterial cytosol to the extracellular space or into target cells, and quite often these systems are important virulence factors [1]. Transport of proteins across a barrier needs a source of energy and the early analyses of T6SS cluster components identified two putative ATPases, TssM (IcmF) and ClpV (TssH). Hcp crystal structure suggested that stacks of Hcp hexamers could form a channel for T6SS substrates [6]. Overall, these initial studies clearly showed that a conserved gene cluster was responsible for protein secretion and virulence by a mechanism distinct from previously described secretion systems [2,6]. I will review the progress that has been made towards understanding the molecular mechanism of protein secretion by T6SS and discuss its unique mode of action
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