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

EMBO J. 2019 May 15;38(10):e100886. doi: 10.15252/embj.2018100886. Epub 2019 Mar 15.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Secretion Systems / chemistry
  • Bacterial Secretion Systems / metabolism
  • Cell Membrane / chemistry
  • Cell Membrane / metabolism
  • Cryoelectron Microscopy
  • Escherichia coli / metabolism
  • Escherichia coli Proteins / chemistry
  • Escherichia coli Proteins / metabolism
  • Membrane Proteins / chemistry
  • Membrane Proteins / metabolism
  • Models, Molecular
  • Protein Binding
  • Protein Structure, Quaternary
  • Type VI Secretion Systems / chemistry*
  • Type VI Secretion Systems / metabolism*

Substances

  • Bacterial Secretion Systems
  • Escherichia coli Proteins
  • Membrane Proteins
  • Type VI Secretion Systems

Associated data

  • PDB/6HS7