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Structural insights into substrate recognition by the type VII secretion system.

Shuhui Wang1,2,3, Kaixuan Zhou4, Xiaolin Yang1,2,3

  • 1Shanghai Institute for Advanced Immunochemical Studies and School of Life Science and Technology, ShanghaiTech University, Shanghai, 201210, China.

Protein & Cell
|November 24, 2019
PubMed
Summary

Structural insights reveal how Mycobacterium tuberculosis (Mtb) type VII secretion systems (T7SSs) recognize and transport virulence factors. This research elucidates the molecular mechanisms underlying substrate selection and secretion in Mtb T7SSs.

Keywords:
ATPaseMycobacterium tuberculosissubstrate recognitiontype VII secretion systemvirulence factor

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Area of Science:

  • Structural biology
  • Microbiology
  • Molecular mechanisms of bacterial secretion

Background:

  • Type VII secretion systems (T7SSs) are crucial for virulence in pathogenic bacteria like Mycobacterium tuberculosis (Mtb).
  • The ESX-1 system (early secreted antigen 6 kilodaltons system 1) is a key Mtb T7SS involved in delivering effector proteins into host macrophages.
  • The precise molecular mechanisms of substrate recognition and translocation by T7SSs remain largely unknown.

Purpose of the Study:

  • To determine the high-resolution crystal structures of the C-terminal ATPase3 domains of EccC subunits from four Mtb T7SS subtypes.
  • To elucidate the molecular basis of substrate recognition and the role of signaling sequences in T7SS-mediated secretion.
  • To propose a model for the substrate translocation channel within the T7SS complex.

Main Methods:

  • High-resolution X-ray crystallography was used to determine the structures of EccC ATPase3 domains.
  • Co-crystallization of EccCb1 with a substrate peptide (EsxB C-terminal) identified the substrate recognition site.
  • Comparative structural analysis and molecular modeling were employed to understand substrate binding and translocation.

Main Results:

  • The crystal structures revealed a conserved RecA-like fold and Mg-ATP binding site in the EccC ATPase3 domains.
  • The structure of EccCb1-EsxB complex pinpointed the substrate recognition site, highlighting the 'LxxxMxF' motif's role in ESX-1 substrate binding.
  • Significant variations in signal recognition pockets across T7SS subtypes suggest specific substrate recognition mechanisms for each system.
  • A hexameric model of EccC ATPase3 proposed a translocation channel (~25-Å diameter) capable of accommodating helix-bundle proteins.

Conclusions:

  • This study provides novel molecular insights into how different Mtb T7SS subtypes specifically recognize their substrates.
  • The findings suggest a potential mechanism for the transport of virulence factors through a central channel in the T7SS complex.
  • Understanding these secretion mechanisms is critical for developing new therapeutic strategies against Mtb infections.