The Type VI Secretion System: A Multipurpose Delivery System with a Phage-Like Machinery

Abstract

Whether they live in the soil, drift in the ocean, survive in the lungs of human hosts or reside on the surfaces of leaves, all bacteria must cope with an array of environmental stressors. Bacteria have evolved an impressive suite of protein secretion systems that enable their survival in hostile environments and facilitate colonization of eukaryotic hosts. Collectively, gram-negative bacteria produce six distinct secretion systems that deliver proteins to the extracellular milieu or directly into the cytosol of host cells. The type VI secretion system (T6SS) was discovered recently and is encoded in at least one fourth of all sequenced gram-negative bacterial genomes. T6SS proteins are evolutionarily and structurally related to phage proteins, and it is likely that the T6SS apparatus is reminiscent of phage injection machinery. Most studies of T6SS function have been conducted in the context of host-pathogen interactions. However, the totality of data suggests that the T6SS is a versatile tool with roles in virulence, symbiosis, interbacterial interactions, and antipathogenesis. This review gives a brief history of T6SS discovery and an overview of the pathway's predicted structure and function. Special attention is paid to research addressing the T6SS of plant-associated bacteria, including pathogens, symbionts and plant growth-promoting rhizobacteria.

Evaluation of the roles played by Hcp and VgrG type 6 secretion system effectors in Aeromonas hydrophila SSU pathogenesis

“…The T6SS is derived from phage injection machinery, and, through horizontal gene transfer, has been acquired and identified in 25 % of sequenced Gram-negative genomes (Records, 2011). The identification of T6SS in human pathogens can be traced back to seminal discoveries made in both Pseudomonas aeruginosa and Vibrio cholerae in which a cluster of chromosomally encoded genes have been found to be involved in bacterial virulence (Mougous et al, 2006;Pukatzki et al, 2006).…”

EliA is required for inducing the stearyl alcohol-mediated expression of secretory proteins and production of polyester in Ralstonia sp. NT80

“…NT80 was isolated from soils that contained fallen plant leaves, it is therefore possible that soil bacteria, such as NT80, may recognize fatty alcohols present in the dead leaves and other dead tissues of plants and consequently may secrete large amounts of lipase to hydrolyse the waxy plant polymer cutin, the most abundant structural component of the plant cuticle, which is composed of covalently cross-linked C16 or C18 oxygenated fatty acids and glycerol (Pollard et al, 2008;Beisson et al, 2012), and obtain a source of carbon. Hcp, another secretory protein identified in this study, is known to form a hexameric ring and is postulated to be an extracellular translocon of the T6SS (Mougous et al, 2006;Records, 2011;Decoin et al, 2014). It was reported earlier that a T6SS-defective mutant of R. solanacearum showed significantly attenuated virulence in tomato (Zhang et al, 2014).…”

“…NT80 could potentially cause disease in plants, its putative virulence proteins are probably induced by the fatty alcohols naturally found in the plant waxes. In this regard, it is noteworthy that the expression of the orthologues of ndk, hcp and lipA is induced by quorum sensing (Goo et al, 2010;Records, 2011). Therefore, it is possible that these genes, in addition to being induced by stearyl alcohol, were also induced by quorum sensing, as addition of stearyl alcohol to the growth medium partially maintained cell viability.…”

“…Out of the 10 identified secreted proteins (listed in Table S2), only four proteins (EliA, LipA, peptidase S8/S53 subtilisin kexin sedolisin and tfp pilus assembly protein tip-associated adhesin PilY1-like protein) contained a putative N-terminal signal peptide sequence, suggesting that they could be secreted by the type II secretion system (T2SS); the following six proteins, although predicted to be extracellular, did not have any known N-terminal signal peptide sequences for secretion (Table S2): Hcp (haemolysin-coregulated protein), Ndk (nucleotide diphosphate kinase), lipoprotein, flagellin domain protein, FlgK and FlgE. Among these six proteins, Hcp forms the extracellular part of the type VI secretion machinery and is released into the culture medium by T6SS (Mougous et al, 2006;Records, 2011;Decoin et al, 2014). Ndk (nucleoside diphosphate kinase) and the flagellum proteins FlgK and FlgE, by contrast, are secreted by the T3SS (Macnab, 2004;Neeld et al, 2014).…”

“…The Hcp identified in this study shared 95 and 71 % identity with the haemolysin-coregulated protein of Ralstonia solanacearum PSI07 and Burkholderia pseudomallei 1026b, respectively (Remenant et al, 2010;Hayden et al, 2012), whereas the Ndk identified in this study shared 97 and 86 % identity with the nucleoside diphosphate kinase of R. solanacearum PSI07 and B. pseudomallei 1026b, respectively (Remenant et al, 2010;Hayden et al, 2012). Of these two proteins, Hcp has been shown to be an extracellular part of the T6SS (Mougous et al, 2006;Records, 2011;Decoin et al, 2014), which has been reported to be involved in secreting virulence-related proteins either out of or directly into the cells of a eukaryotic host (Schell et al, 2007;Burtnick et al, 2011). Secretory Ndk, by contrast, has been shown to be exported via the T3SS and is known to be involved in the virulence of pathogenic bacteria such as Pseudomonas aeruginosa and Mycobacterium tuberculosis (Chakrabarty, 1998;Neeld et al, 2014).…”