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Assembly, structure, function and regulation of type III secretion systems

A Corrigendum to this article was published on 12 May 2017

This article has been updated

Key Points

  • Type III secretion systems (T3SSs) are protein transport nanomachines that are used by numerous important Gram-negative bacterial pathogens and symbionts to establish trans-kingdom interactions with different hosts. They are essential virulence factors for many notorious bacterial pathogens, including the agents of plague and typhoid fever.

  • T3SSs evolved from the flagellum, which is a key organelle for bacterial motility, and the core components that are involved in the assembly of these complex nanomachines are highly conserved. Although the flagellar systems are largely inherited vertically, the non-flagellar T3SSs can be transmitted through horizontal gene transfer.

  • T3SSs are syringe-shaped, multi-megadalton complexes that are composed of more than 20 proteins and a series of ring structures embedded in the bacterial inner and outer membranes, as well as translocation pores in the host cell membrane. Encircled by these rings is a hollowed conduit that enables the delivery of partially unfolded virulence effector proteins into the host cell.

  • Integrative imaging technologies that combine cryo-electron microscopy (cryo-EM), X-ray crystallography, NMR and computer modelling have enabled the high-resolution visualization of key substructures of the T3SSs and the nanomachine in action. Assembly of the T3SS apparatus and substrate secretion occur in a defined temporal order and hierarchy, and genetic analyses and molecular biology have enabled the identification and functional characterization of the key regulators that control these processes.

  • Effector proteins that are secreted through T3SSs carry out various functions within the host cell, including the manipulation of host immune responses and actin cytoskeletal dynamics, subverting gene expression and post-translational modifications, hijacking signal transduction pathways, and interrupting vesicle transport and endocytic trafficking, all of which can promote bacterial colonization, survival and replication.

  • T3SSs are attractive targets for vaccines and therapeutics owing to their essential roles in bacterial virulence and pathogenicity. By targeting bacterial virulence mechanisms instead of growth, inhibitors of T3SSs may exert less selective pressure on pathogens to develop drug resistance. Structural and functional characterization of T3SSs should facilitate mechanism-based drug design.

Abstract

Type III secretion systems (T3SSs) are protein transport nanomachines that are found in Gram-negative bacterial pathogens and symbionts. Resembling molecular syringes, T3SSs form channels that cross the bacterial envelope and the host cell membrane, which enable bacteria to inject numerous effector proteins into the host cell cytoplasm and establish trans-kingdom interactions with diverse hosts. Recent advances in cryo-electron microscopy and integrative imaging have provided unprecedented views of the architecture and structure of T3SSs. Furthermore, genetic and molecular analyses have elucidated the functions of many effectors and key regulators of T3SS assembly and secretion hierarchy, which is the sequential order by which the protein substrates are secreted. As essential virulence factors, T3SSs are attractive targets for vaccines and therapeutics. This Review summarizes our current knowledge of the structure and function of this important protein secretion machinery. A greater understanding of T3SSs should aid mechanism-based drug design and facilitate their manipulation for biotechnological applications.

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Figure 1: Overview of key structural elements and features of the T3SS basal body.
Figure 2: Schematic diagrams of the 'inside-out' and 'outside-in' models of T3SS assembly.
Figure 3: Key structural elements of the T3SS cytoplasmic ring and sorting platform.
Figure 4: Regulation of type III secretion hierarchy and substrate specificity.
Figure 5: Sensing of host cell contact and effector delivery by T3SSs.

Change history

  • 12 May 2017

    In the above article, a mistake was introduced in table 1; FliJ should have read 'FlgJ' in the row of 'Lytic transglycosylase'. This has now been corrected in the PDF and online. The authors apologize to readers for any confusion caused.

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Acknowledgements

The authors apologize to those colleagues whose studies were not highlighted here owing to space constraints. This work was funded by operating grants from the Canadian Institutes of Health Research (to B.B.F. and N.C.J.S.) and the Howard Hughes International Senior Scholar program (to N.C.J.S.). N.C.J.S. is a Tier I Canada Research Chair in Antibiotic Discovery. B.B.F. is the UBC Peter Wall Distinguished Professor.

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Glossary

Cryo-electron microscopy

(Cryo-EM). A transmission electron microscopy (TEM) technique that is used in structural biology for imaging unstained and unfixed frozen-hydrated specimens at cryogenic (generally liquid nitrogen) temperatures, thus enabling the preservation of their native state.

Basal body

A component of the type III secretion apparatus that is composed of highly oligomerized concentric rings that are embedded in the bacterial inner and outer membranes, excluding any extracellular appendages such as the needle filament.

Needle

The hollow filamentous structure that is formed by helical polymerization of a single protein that lines the inside of the type III secretion system basal body and protrudes to the bacterial surface.

Translocation pore

Hetero-oligomeric complexes of two hydrophobic membrane proteins that contain a central pore and form in the host cell membrane, and that enable the injection of effector proteins.

Secretin

A family of proteins that form large and extremely stable multimeric complexes and channels in the outer membrane of Gram-negative bacteria. Secretins are essential for protein transport across membranes in type II and type III secretion systems, the type IV pilus system, and filamentous bacteriophage assembly and extrusion.

Sorting platform

A substructure that includes the cytoplasmic ring and the ATPase complex. It is located on the bacterial cytoplasmic side of the type III secretion apparatus, where dynamic substrate uploading and secretion takes place.

Secretion hierarchy

The temporal order by which the different categories (early, middle and late) of substrate proteins are secreted by type III secretion systems.

Chaperones

Small proteins in type III secretion systems that maintain substrates in a partially unfolded secretion-competent state and/or prevent undesired premature protein–protein interactions or aggregation. They may also be involved in substrate targeting and secretion hierarchy.

Injectisome

A syringe-like protein secretion apparatus that forms a channel across both the inner membrane and the outer membrane of Gram-negative bacteria, as well as the host cell membrane, and is used by bacteria to deliver bacterial proteins into eukaryotic host cells through type III secretion.

General secretory pathway

(Sec pathway). An essential protein export machinery that transports proteins across the plasma membrane and is evolutionarily conserved in all domains of life. It often recognizes its substrates through an amino-terminal signal peptide, which is cleaved during secretion. The majority of secreted proteins in bacteria are exported through this pathway.

β-Barrel

A closed structure that is formed by twisted and coiled β-sheets that are composed of β-strands. It is a common assembly by certain outer membrane proteins, such as porins and secretins of protein secretion systems, in Gram-negative bacteria.

F/V-type ATPases

Phosphorylation factor-type (F-type) F0F1 ATPases and vacuolar-type (V-type) ATPases are multisubunit protein complexes that couple transmembrane movement and the pumping of ions (protons or Na+) with ATP hydrolysis or synthesis. They are involved in providing the energy for various cellular activities, including protein trafficking and the movement of metabolites.

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Deng, W., Marshall, N., Rowland, J. et al. Assembly, structure, function and regulation of type III secretion systems. Nat Rev Microbiol 15, 323–337 (2017). https://doi.org/10.1038/nrmicro.2017.20

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