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Nat Microbiol. 2017 Apr 10;2:17047. doi: 10.1038/nmicrobiol.2017.47.

Structure of the mycobacterial ESX-5 type VII secretion system membrane complex by single-particle analysis.

Author information

1
European Molecular Biology Laboratory, Hamburg Unit, Notkestraße 85, 22607 Hamburg, Germany.
2
Research Institute of Molecular Pathology (IMP), Vienna Biocenter (VBC), Campus-Vienna-Biocenter 1, 1030 Vienna, Austria.
3
Institute of Molecular Biotechnology of the Austrian Academy of Sciences, Vienna Biocenter (VBC), Dr. Bohr-Gasse 3, 1030 Vienna, Austria.
4
University Medical Centre Hamburg-Eppendorf (UKE), Martinistraße 52, 20246 Hamburg, Germany.
5
Vrije Universiteit Amsterdam, De Boelelaan 1108, 1081 HZ Amsterdam, The Netherlands.
6
VU University Medical Center, De Boelelaan 1108, 1081 HZ Amsterdam, The Netherlands.
7
European Molecular Biology Laboratory, Meyerhofstraße 1, 69117 Heidelberg, Germany.
8
Centre for Structural Systems Biology (CSSB), Notkestraße 85, 22607 Hamburg, Germany.
9
Deutsches Elektronen-Synchrotron (DESY), Notkestraße 85, 22607 Hamburg, Germany.

Abstract

Mycobacteria are characterized by their impermeable outer membrane, which is rich in mycolic acids1. To transport substrates across this complex cell envelope, mycobacteria rely on type VII (also known as ESX) secretion systems2. In Mycobacterium tuberculosis, these ESX systems are essential for growth and full virulence and therefore represent an attractive target for anti-tuberculosis drugs3. However, the molecular details underlying type VII secretion are largely unknown, due to a lack of structural information. Here, we report the molecular architecture of the ESX-5 membrane complex from Mycobacterium xenopi determined at 13 Å resolution by electron microscopy. The four core proteins of the ESX-5 complex (EccB5, EccC5, EccD5 and EccE5) assemble with equimolar stoichiometry into an oligomeric assembly that displays six-fold symmetry. This membrane-associated complex seems to be embedded exclusively in the inner membrane, which indicates that additional components are required to translocate substrates across the mycobacterial outer membrane. Furthermore, the extended cytosolic domains of the EccC ATPase, which interact with secretion effectors, are highly flexible, suggesting an as yet unseen mode of substrate interaction. Comparison of our results with known structures of other bacterial secretion systems demonstrates that the architecture of type VII secretion system is fundamentally different, suggesting an alternative secretion mechanism.

PMID:
28394313
DOI:
10.1038/nmicrobiol.2017.47
[Indexed for MEDLINE]

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