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Review
. 2014 Feb;17(100):24-31.
doi: 10.1016/j.mib.2013.11.001. Epub 2013 Dec 5.

Structural organisation of the type IV secretion systems

Affiliations
Review

Structural organisation of the type IV secretion systems

Gabriel Waksman et al. Curr Opin Microbiol. 2014 Feb.

Abstract

Type IV secretion (T4S) systems are large dynamic nanomachines that transport DNAs and/or proteins through the membranes of bacteria. Because of their complexity and multi-protein organisation, T4S systems have been extremely challenging to study structurally. However in the past five years significant milestones have been achieved by X-ray crystallography and cryo-electron microscopy. This review describes some of the more recent advances: the structures of some of the protein components of the T4S systems and the complete core complex structure that was determined using electron microscopy.

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Figures

Figure 1
Figure 1
Overall organisation of the T4S system. VirD4 (in pink), VirB11 (in blue), VirB4 (in gold) ATPases, polytopic VirB6 (in purple), bitopic VirB8 (in light green) and VirB3 (in orange) form the cytoplasmic IM part of the complex. VirB7 (in brown), VirB9 (in green), and VirB10 (in blue) compose the periplasmic part of the secretion system. VirB2 and VirB5 constitute the outer part of the secretion system. Red dot indicates the path of the substrate through the machinery as established by Cascales and Christie [22]. The stoichiometry of the various components in a native, fully assembled, T4S system is unknown.
Figure 2
Figure 2
Structures of T4S system components or domains. (a) VirD4: structure of the soluble domain of TrwB. One subunit of the hexamer is shown (coils are shown in cyan, helices in blue, and strands in red). (b) VirB4: C-terminal domain of Thermoanaerobacter pseudethanolicus VirB4; (c) VirB11: crystal structures of B. suis VirB11 and H. pylori HP5025, CTD-C terminal domain, NTD-N terminal domain (shown in red dashed oval on B. suis VirB11). Red arrow indicates the shift of the NTD in H. pylori Vir11 compared to B. suis VirB11. (d) VirB8: crystal structure of the periplasmic domain (C-terminal domain) of VirB8 from B. suis (left panel), A. tumefaciens (middle panel), TraM214–322 protein (right panel); the central and C-terminal domains of the TcpC99–359 structure are shown in the bottom panel. (e) VirB5: crystal structure of TraC encoded by the E. coli conjugative plasmid pKM101. (f) Crystal structure of the CC's O-layer composed of VirB7 (in dark red), VirB9CT (in green) and VirB10CT (blue). All structures are shown in the ribbon representation.
Figure 3
Figure 3
Electron microscopy of the T4S system. (a) Structure of the CC complex at 15 Å; (b) structure of the CC complex at 12 Å; (c) schematic illustration of the regions of TraN/VirB7, TraO/VirB9, and TraF/VirB10 present in the CCelastase complex. Domains corresponding to the VirB9 binding domain (B9BD), the signal peptide (SP), the N-terminal trans-membrane (TM) helix and the C-terminal domains (CTD) are shown in darker colours; (d) Cryo-EM structure of the CCelastase complex. (e) Cutaway view of the superposition of the difference map (in green) between the full length CC and CCelastase and the cryo-EM structure of the CCelastase complex (in gold).
Figure 4
Figure 4
Schematic model for the full length organisation of the core complex. (a) Four TraF/VirB10CT subunits of the 14-mer present in the O-layer atomic structure are shown. One subunit is highlighted in blue. The density of one subunit column in the difference map (in light green) is shown with the tentative docking of the three TraF/VirB10NT α-helical regions. This subunit is located immediately below TraF/VirB10CT shown in dark blue. The connections are shown in dashed lines. (b) Central slice of the full length CC with fitted O-layer atomic structure (in cyan) and atomic models obtained for TraO/VirB9NT (in dark red) and the α-helices predicted in TraF/VirB10NT (shown as in (a)).

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References

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