The type VI secretion toolkit

Abstract

Bacterial secretion systems are macromolecular complexes that release virulence factors into the medium or translocate them into the target host cell. These systems are widespread in bacteria allowing them to infect eukaryotic cells and survive or replicate within them. A new secretion system, the type VI secretion system (T6SS), was recently described and characterized in several pathogens. Genomic data suggest that T6SS exist in most bacteria that come into close contact with eukaryotic cells, including plant and animal pathogens. Many research groups are now investigating the underlying mechanisms and the way in which the effector proteins translocated through this machine subvert host defences. This review provides an overview of our current knowledge about type VI secretion, focusing on gene regulation, components of the secretion machine, substrate secretion and the cellular consequences for the host cell.

Figure 1

Gene organization of type VI secretion clusters. Homologues and related genes are indicated with the same colour, whereas genes with no homologues in other type VI secretion systems are coloured white. Characteristic features of the gene products are indicated in supplementary Table 2 online. Genes necessary for type VI secretion determined by the systematic deletion approach on the Edwardsiella tarda Evp cluster are indicated by (+). FHA, forkhead-associated protein; Stk, Ser/Thr kinase; Stp, Ser/Thr phosphatase.

Figure 2

Structural information on type VI secretion subunits. (A) Crystal structure of the Hcp protein. Top and edge-on ribbon views of the Hcp protein assembled as a hexameric ring-shaped structure delimiting a 40 Å cavity. (B) Domain organization of VgrG. The modular structure of the VgrG protein family is shown (a). The carboxy-terminal domain, which probably represents the ‘true effector', carries an activity (see text for details). VgrG proteins assemble as trimers and show structural homology with the hub proteins of the bacteriophage T4 base-plate. The amino-terminal domain has homology with the bacteriophage T4 gp27 protein (lateral and top view of the gp27 trimer shown in panel (b)). The central domain shares homology with the bacteriophage T4 gp5 C-terminal domain. This triangular prism forms a 110 Å-long puncturing needle-like structure (c). Models have been drawn using Pymol (http://pymol.sourceforge.net/) after Mougous et al, 2006 (A) and Kanamaru et al, 2002 (B).

Figure 3

A model for type VI secrection system assembly and function. A putative model that integrates the current data is proposed. An inner membrane channel formed by the IcmF-like and IcmH-like proteins interacts at the cytoplasmic side of the IM with a complex composed of the probable cytosolic type VI secrection (T6S) subunits and the ClpV AAA⁺ ATPase. Recruitment of the ClpV multimer is induced by the regulation of forkhead-associated (FHA) phosphorylation through the activities of PpkA and PppA, and by the presence of the Hcp protein. A multimer of the putative lipoprotein in association with periplasmic subunits is shown at the outer membrane. Putative routes for substrate translocation are depicted through the cell envelope and the host-cell membrane (blue arrow) including a ‘one-step' mechanism through a unique channel, and a ‘two-step' mechanism, in which both steps are catalysed by T6S subunits with transient accumulation in the periplasm (P). This hypothetical model shows a trimeric VgrG inserted into the OM through the amino-terminal domain and puncturing the host cell through the needle-like structure formed by the central domains, releasing the activity domain into the host cytosol (for eukaryotic-like activities) or in the medium (for binding or adhesion activities). J, L, M and H (TssJ, TssL, TssM and TssH respectively) represent the T6S core components, following the nomenclature of Shalom et al, 2007.

Eric Cascales