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EMBO J. 2019 Mar 15. pii: e100886. doi: 10.15252/embj.2018100886. [Epub ahead of print]

In situ and high-resolution cryo-EM structure of a bacterial type VI secretion system membrane complex.

Author information

1
CNRS UMR 5234 Microbiologie Fondamentale et Pathogénicité, Bordeaux, France.
2
Institut Européen de Chimie et Biologie, University of Bordeaux, Pessac, France.
3
Laboratoire d'Ingénierie des Systèmes Macromoléculaires (LISM), Institut de Microbiologie de la Méditerranée (IMM), UMR7255, Aix-Marseille Université - CNRS, Marseille, France.
4
Institute of Molecular Biology & Biophysics, Eidgenössische Technische Hochschule Zürich, Zürich, Switzerland.
5
Institut Pasteur, Structural Bioinformatics Unit, Department of Structural Biology and Chemistry, CNRS UMR 3528, C3BI USR 3756, Paris, France.
6
Institute of Molecular Biology & Biophysics, Eidgenössische Technische Hochschule Zürich, Zürich, Switzerland pilhofer@biol.ethz.ch edurand@imm.cnrs.fr edurand@inserm.fr r.fronzes@iecb.u-bordeaux.fr.
7
Laboratoire d'Ingénierie des Systèmes Macromoléculaires (LISM), Institut de Microbiologie de la Méditerranée (IMM), UMR7255, INSERM, Marseille, France pilhofer@biol.ethz.ch edurand@imm.cnrs.fr edurand@inserm.fr r.fronzes@iecb.u-bordeaux.fr.
8
CNRS UMR 5234 Microbiologie Fondamentale et Pathogénicité, Bordeaux, France pilhofer@biol.ethz.ch edurand@imm.cnrs.fr edurand@inserm.fr r.fronzes@iecb.u-bordeaux.fr.

Abstract

Bacteria have evolved macromolecular machineries that secrete effectors and toxins to survive and thrive in diverse environments. The type VI secretion system (T6SS) is a contractile machine that is related to Myoviridae phages. It is composed of a phage tail-like structure inserted in the bacterial cell envelope by a membrane complex (MC) comprising the TssJ, TssL and TssM proteins. We previously reported the low-resolution negative-stain electron microscopy structure of the enteroaggregative Escherichia coli MC and proposed a rotational 5-fold symmetry with a TssJ:TssL:TssM stoichiometry of 2:2:2. Here, cryo-electron tomography analyses of the T6SS MC confirm the 5-fold symmetry in situ and identify the regions of the structure that insert into the bacterial membranes. A high-resolution model obtained by single-particle cryo-electron microscopy highlights new features: five additional copies of TssJ, yielding a TssJ:TssL:TssM stoichiometry of 3:2:2, an 11-residue loop in TssM, protruding inside the lumen of the MC and constituting a functionally important periplasmic gate, and hinge regions. Based on these data, we propose an updated model on MC structure and dynamics during T6SS assembly and function.

KEYWORDS:

cryo‐electron microscopy; membrane protein complex; type VI secretion systems

PMID:
30877094
DOI:
10.15252/embj.2018100886
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