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The type II secretion system: biogenesis, molecular architecture and mechanism

Nature Reviews Microbiology volume 10, pages 336–351 (2012)Cite this article

Subjects

Key Points

  • The type II secretion system (T2SS) is a double-membrane-spanning protein secretion system composed of 12–15 different general secretory pathway (Gsp) proteins that are present in various copy numbers. It is found in a large number of pathogenic and non-pathogenic Gram-negative bacteria.

  • The T2SSs of different species secrete a wide variety of folded exoproteins of different functions, shapes, sizes and quaternary structures. The T2SS secretion signal is still unknown, but it has been suggested that β-complementation is a feature of this signal.

  • The T2SS contains several subassemblies: the outer-membrane secretin, the periplasmic pseudopilus and a cytoplasmic ATPase all interact with components of the inner-membrane platform.

  • Crystal structures have been determined for the majority of T2SS domains, together with binary complexes showing the interaction of the inner-membrane platform with the ATPase and with the outer-membrane secretin, and a ternary complex of three pseudopilins forming the tip of the pseudopilus.

  • Electron microscopy studies indicate that the outer-membrane complex is dodecameric, and indirect evidence suggests that the ATPase is hexameric. The stoichiometry of the inner-membrane platform is still largely a mystery.

  • The current hypothesis regarding the mode of T2SS action is that an exoprotein is captured in the periplasmic vestibule of the outer-membrane secretin, possibly assisted by the inner-membrane protein GspC, and that this induces ATP hydrolysis by the ATPase, leading to conformational changes in the ATPase and the inner-membrane platform. This, in turn, results in elongation of the pseudopilus, which then functions as a piston, opening the periplasmic gate in the outer-membrane secretin to form a channel and then expelling the exoprotein.

Abstract

Many Gram-negative bacteria use the sophisticated type II secretion system (T2SS) to translocate a wide range of proteins from the periplasm across the outer membrane. The inner-membrane platform of the T2SS is the nexus of the system and orchestrates the secretion process through its interactions with the periplasmic filamentous pseudopilus, the dodecameric outer-membrane complex and a cytoplasmic secretion ATPase. Here, recent structural and biochemical information is reviewed to describe our current knowledge of the biogenesis and architecture of the T2SS and its mechanism of action.

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Figure 1: T2SS subassemblies and biogenesis.
Figure 2: Structures of the T2SS pseudopilins.
Figure 3: Structures of the T2SS secretin and pilotin.
Figure 4: Structures and topologies of the secretion ATPase and the inner-membrane components of the T2SS.
Figure 5: A possible mode of action of the T2SS.

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Acknowledgements

The authors thank the many members of their groups who have made important contributions to the studies reported, and special thanks go to J. Abendroth, M. Robien, S. Turley, T. Johnson, M. Patrick and M. Gray. They also thank their many collaborators on the T2SS project, including J. Steyaert and E. Pardon for the preparation of valuable nanobodies, and T. Gonen and his group for electron microscopy studies. This work was supported by awards RO1AI049294 (to M.S.) and RO1AI34501 (to W.G.J.H.) from the US National Institutes of Health, National Institute of Allergy and Infectious Diseases, and earlier support from the Howard Hughes Medical Institute to W.G.J.H. is deeply appreciated.

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  1. Konstantin V. Korotkov

    Present address: Present address: Department of Molecular & Cellular Biochemistry, University of Kentucky, Lexington, Kentucky 40536, USA.,

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  1. Department of Biochemistry and Biomolecular Structure Center, University of Washington, Seattle, 98195, Washington, USA

    Konstantin V. Korotkov & Wim G. J. Hol

  2. Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, 48109, Michigan, USA

    Maria Sandkvist

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Structural data for the type II secretion proteins (PDF 293 kb)

Glossary

General secretory pathway

A traditional name for the type II secretion system (T2SS), in which substrates are transported through the inner membrane via the Sec or Tat pathways. Historically, the T2SS was found to rely on the Sec pathway, hence the use of 'general' in the name. However, many other systems also use the Sec pathway, so the term general secretory pathway is considered to be inaccurate by some scientists.

Sec translocon

A universal pathway for transport of proteins through the cytoplasmic membrane in bacteria and archaea and the endoplasmic reticulum membrane in eukaryotes. The bacterial and archaeal Sec translocon is composed of SecYEG (the integral membrane channel), SecA (the peripheral membrane ATPase), and SecD and SecF (the auxiliary release proteins). Targeting to the Sec translocon relies on the signal recognition particle for co-translational transport or on the cytoplasmic chaperone SecB for post-translational transport.

Signal recognition particle

(SRP). A universally conserved protein–RNA complex that is involved in targeting secreted proteins to the Sec translocon for co-translational transport. The bacterial and archaeal SRP is formed by the protein Ffh and 4.5S RNA.

β-barrel assembly machinery

A machinery for the correct folding and insertion of outer-membrane proteins which have a β-barrel structure.

Lol pathway

A machinery for the transport of outer-membrane lipoproteins from the inner to the outer membrane. The Lol pathway consists of LolCDE (the ABC (ATP-binding cassette) transporter), LolA (the periplasmic chaperone) and LolB (the outer-membrane receptor and release assistant).

Nanobodies

The smallest antigen-binding fragments of the heavy-chain-only antibodies from camelids. Nanobodies have a single immunoglobulin fold domain and three antigen-binding loops.

Crystallization chaperones

Proteins that are used in co-crystallization because they bind a particular target. Examples of crystallization chaperones include antibody fragments and designed scaffold proteins. These chaperones may reduce the conformational heterogeneity of the target protein and/or form favourable crystal contacts.

Walker A and B motifs

Two protein motifs. The Walker A motif, also known as the P-loop (phosphate-binding loop), is a GXXXGK(T/S) motif that is found in many nucleotide-binding proteins and interacts with phosphate groups of the bound nucleotide. The Walker B motif, XXXXD, coordinates Mg2+ and is essential for ATP hydrolysis.

Circular permutation

A change in the protein sequence that leads to a similar three-dimensional structure to that of the original sequence but with a different connectivity.

Ferredoxin fold

A common protein fold with a βαββαβ secondary structure.

PDZ domain

A ubiquitous protein domain of approximately 90 amino acids that is typically involved in protein–protein interactions or signalling. It is commonly found in eukaryotic proteins, but is relatively rare in bacterial proteins. The acronym is derived from the first proteins found to share this domain: postsynaptic density protein 95 (PSD95; also known as DLG4), Disks large 1 (DLG1) and zona occludens 1 (ZO1).

Tat complex

(Twin-Arg translocation complex). A system for the transport of folded proteins via the cytoplasmic membrane of bacteria and archaea or the thylakoid membrane of the plant chloroplast. A conserved twin-Arg motif is present in the amino-terminal signal sequence of substrate proteins.

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Korotkov, K., Sandkvist, M. & Hol, W. The type II secretion system: biogenesis, molecular architecture and mechanism. Nat Rev Microbiol 10, 336–351 (2012). https://doi.org/10.1038/nrmicro2762

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