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

• Chiara Rapisarda ^[1] ; Yassine Cherrak ^[2] ; Romain Kooger ^[3] ; Victoria Schmidt ^[2] ; Riccardo Pellarin ^[4] ; Laureen Logger ^[2] ; Eric Cascales ^[2] ; Martin Pilhofer ^[3] ; Eric Durand ^[2] ; Rémi Fronzes ^[1]
  1. [1] University of Bordeaux

     University of Bordeaux

     Arrondissement de Bordeaux, Francia

     
  2. [2] Institut de Microbiologie de la Méditerranée

     Institut de Microbiologie de la Méditerranée

     Arrondissement de Marseille, Francia

     
  3. [3] 4 Institute of Molecular Biology & Biophysics Eidgenössische Technische Hochschule Zürich Zürich Switzerland
  4. [4] 5 Institut Pasteur Structural Bioinformatics Unit Department of Structural Biology and Chemistry CNRS UMR 3528, C3BI USR 3756 Paris France

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• Localización: EMBO journal: European Molecular Biology Organization, ISSN 0261-4189, Vol. 38, Nº. 10, 2019
• Idioma: inglés
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  • 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.