The type III secretion injectisome

Key Points

  • The type III secretion injectisome is a nanomachine that delivers bacterial proteins into the cytosol of eukaryotic target cells. It is evolutionarily related to the flagellum, with which it shares structural and functional similarities.

  • It consists of a basal structure made of several rings spanning the inner and the outer membranes, connected by a central tube. A dodecameric ATPase forms a ring structure at the cytoplasmic side of this basal structure.

  • On top of the basal body is a short, stiff needle or a needle and a filament (animal pathogens) or a pilus (plant pathogens). This distal structure allows bacteria to reach the plasma membrane of the target cell.

  • The needle terminates with a specific tip structure. This structure functions as a scaffold for the formation of the translocation pore.

  • The length of the needle is controlled and adapted to match the length of various macromolecules at the surface of the bacterium and the host cell.

  • The export apparatus, localized in the basal structure, exports the protein subunits that form the external elements of the injectisome. When assembly is complete, the export apparatus changes its substrate specificity and is ready to export the pore formers and the effector proteins. Export of the proteins will only occur on contact with a target cell.

  • Contact to a eukaryotic cell membrane triggers export, by a complex mechanism that is not understood. In some cases, the presence of cholesterol in the target membrane is required. Delivery of proteins by the T3SS is a fast process.

  • The assembly and operation requires the presence of specific cytosolic chaperones dedicated either to effector proteins (class I), to the pore formers (class II) or to substructures subunits (class III). The common main function could be to hide polymerization or aggregation-prone domains in the bacterial cytosol.

Abstract

The type III secretion injectisome is a complex nanomachine that allows bacteria to deliver protein effectors across eukaryotic cellular membranes. In recent years, significant progress has been made in our understanding of its structure, assembly and mode of operation. The principal structural components of the injectisome, from the base located in the bacterial cytosol to the tip of the needle protruding from the cell surface, have been investigated in detail. The structures of several constituent proteins were solved at the atomic level and important insights into the assembly process have been gained. However, despite the ongoing concerted efforts of molecular and structural biologists, the role of many of the constituent components of this nanomachine remain unknown.

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Figure 1: Structure of the flagellum and the injectisomes.
Figure 2: Structure of the injectisome.
Figure 3: Class I and class III chaperone structures.
Figure 4: LcrV forms a structure at the tip of the needle.
Figure 5: Hypothetical model of the function of the LcrV tip complex.
Figure 6: Control of the length of the needle and the hook.
Figure 7: The role of the needle structure.

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Acknowledgements

This review does not claim to be comprehensive. For the sake of coherence, the Yersinia Ysc archetype was taken as a unifying thread, which means that there is a bias in favour of work on this organism. I apologize to colleagues whose work could not be cited for these two reasons. I sincerely thank P. Broz and M. Letzelter for help in the conception of the illustrations and for challenging discussions. I am grateful to P. Broz, M. Letzelter and I. Sorg for discussions, information and critical assessment of the manuscript. I also thank J. Galan for exchange of information. Work in my laboratory is supported by the Swiss National Science Foundation.

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DATABASES

Entrez Genome Project

Aeromonas salmonicida

Escherichia coli

Pseudomonas aeruginosa

Pseudomonas syringae

S. typhimurium

Shigella flexneri

Yersinia enterocolitica

Yersinia pseudotuberculosis

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Glossary

Injectisome

The injectisome is a nanomachine that evolved for the delivery of bacterial proteins, by type III secretion, across eukaryotic cell membranes. In the present stage of knowledge, it consists of a basal structure, which resembles the basal structure of the flagellum, surmounted by either a needle, a needle and a filament or a long pilus.

Flagellum

The flagellum is a motility organelle consisting of a rotating long filament connected to a rotary motor by a short curved structure called the hook. The motor is powered by the flow of ions down an electrochemical gradient across the cytoplasmic membrane into the cell. The ions are typically H+ (protons) in Escherichia coli and enterobacteria and Na+ in alkalophiles and marine Vibrio species.

Needle complex

The needle complex is the part of the injectisome that was characterized in great detail by cryo-EM28,30,85. This structure contains neither the ATPase nor the putative C ring.

Basal structure

Here, the basal structure is defined as the injectisome without its needle, filament or pilus.

Type II secretion apparatus

The type II secretion apparatus is a complex nanomachine that translocates proteins across the outer membrane. This machine involves a secretin in the outer membrane and a dynamic short pilus that functions as a piston.

Type IV pili

Type IV pili are retractable pili involved in adherence and motility and found on diverse bacteria. They are related to the piston of the type II secretion apparatus.

Lipoprotein

A Lipoprotein (LP) is a protein that is synthesized with a signal peptide followed by a cysteine onto which a diacylglycerol is covalently attached by a thioether bond during export. LPs insert either in the plasma membrane or in the outer membrane.

AAA+

(ATPases associated with various cellular activities) The AAA+ family is a large and functionally diverse group of enzymes that can induce conformational changes in a wide range of substrate proteins. The defining feature of the family is a structurally conserved ATPase domain that assembles into oligomeric rings and undergoes conformational changes during cycles of nucleotide binding and hydrolysis. AAA+ are associated with several ATP-dependent bacterial proteases, including ClpXP and ClpAP. They unfold proteins and translocate the unfolded polypeptide into the proteolytic chamber for degradation. See Ref. 152 for a review.

General secretory pathway

(Sec pathway) The General Secretory pathway is the most essential bacterial export pathway. It is involved in the assembly of inner membrane proteins and it translocates many proteins across the plasma membrane. The Sec machine recognizes its substrates by an amino-terminal signal peptide that is cleaved off during translocation.

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Cornelis, G. The type III secretion injectisome. Nat Rev Microbiol 4, 811–825 (2006). https://doi.org/10.1038/nrmicro1526

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