Type VI secretion system

Architecture and secretion mechanism

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-envelop 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

Phage tail-like complex

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.

Baseplate complex

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.

Membrane complex

The T6SS membrane complex is responsible for anchoring the apparatus to the cellular membrane, and provides the channel through which substrates are propelled by the contraction of the phage tail-like tubule. This large (1.7 md) complex is formed from 10 interacting units of a heterotrimer containing TssJ, TssM and TssL. It is believed to span from the inner membrane to the outer membrane of the Gram negative bacterial cell envelope, forming a channel that opens and closes with a unique iris-like mechanism.

Substrate recognition

Unlike substrates of other secretion systems (such as the general secretory pathway or secretion systems III and VI), those of the T6SS do not have any universally identifying features. Instead, they are recognized and selected for secretion through one of two structural components of the apparatus. One class of substrates binds within the pore of an Hcp hexamer. Since substrates are unstable in the absence of this interaction, it is thought that the substrate-Hcp complexes are secreted together, rather than Hcp serving as a passive tubule through which substrates pass. Members of the second class of substrates are targeted for secretion via interaction with the phage tail spike-like protein VgrG. These substrates are often modular proteins that a PAAR domain for interaction with VgrG at one end. There are also instances where a VgrG and a substrate are both part of the same protein.

Anti-Eukaryotic Type VI Secretion Systems

Although the ancestral function of the T6SS appears to be targeting of bacteria, a handful of systems have been identified that have evolved to target eukaryotic cells. In general, these eukaryote-targeting systems are involved in causing disease. For example, the intracellular pathogen Francisella tularensis requires the activity of a T6SS to escape from phagosomes and replicate in the cytoplasm of macrophages. The mechanism by which secreted proteins facilitate F. tularensis virulence is still unknown. The T6SS of Vibrio cholerae has a dual role, being able to target both bacteria and eukaryotic cells. At least one substrate it secretes is specialized for eukaryotic cell-targeting, functioning by crosslinking the cytoskeleton protein actin. Burkholderia pseudomallei and Edwardsiella tarda are two other organisms which possess a T6SS that appears dedicated for eukaryotic targeting.

Antibacterial Type VI Secretion Systems

A wide range of Gram-negative bacteria have been shown to have antibacterial T6SSs, including opportunistic pathogens such as Pseudomonas aeruginosa, obligate commensal species that inhabit the human gut (Bacteroides spp.), and plant-associated bacteria such as Agrobacterium tumefaciens. These systems exert antibacterial activity via the function of their secreted substrates. All characterized bacterial-targeting T6SS proteins act as toxins, either by killing or preventing the growth of target cells. The mechanisms of toxicity toward target cells exhibited by T6SS substrates are diverse, but tend to involve targeting of highly conserved bacterial structures, including degradation of the cell wall through amidase or glycohydrolase activity, disruption of cell membranes through lipase activity or pore formation, cleavage of DNA, and degradation of the essential metabolite NAD+. Bacteria prevent T6S-mediate intoxication of kin cells by producing immunity determinants specific to each secreted toxin. The immunity proteins function by binding to the toxin proteins, often at their active site, and blocking their activity.

GacS/Rsm Two Component System

Some research has gone into regulation of T6SS by two component systems. In P. aeruginosa, it has been observed that the GacS/Rsm two-component system is involved in type VI secretion system regulation. This system regulates the expression of Rsm small regulatory RNA molecules, and has also been implicated in biofilm formation. Upon the GacS/Rsm pathway stimulation, an increase in Rsm molecules leads to inhibition of mRNA-binding protein RsmA. RsmA is a translational inhibitor that binds to sequences near the ribosome-binding site for T6SS gene expression. This level of regulation has also been observed in P. fluorescens and P. syringae

Quorum Sensing

There are various examples in which quorum sensing regulates T6SS. In Vibrio cholerae T6SS studies, it has been observed that serotype O37 has high vas gene expression. Serotypes O139 and O1 on the other hand exhibit the opposite, with markedly low vas gene expression. It has been suggested that the differences in expression are attributable to differences in quorum-sensing levels. In Vibrio cholerae, autoinducer-1 (AI-1) signals are detected by LuxQ, a sensor kinase. LuxQ activates LuxU, which then acts on LuxO, a DNA-binding protein which represses HapR gene expression. HapR is thought to LuxO deletions resulted in strong induction of vas gene expression, and hence T6SS expression, demonstrating that T6SS is regulated in some form by quorum sensing. However, O1 strains with LuxO deletions still had relatively quiescent T6SS compared to the O37 strain, suggesting that additional factors are also involved.