Architecture of the major component of the type III secretion system export apparatus

Nat Struct Mol Biol. 2013 Jan;20(1):99-104. doi: 10.1038/nsmb.2452. Epub 2012 Dec 9.


Type III secretion systems (T3SSs) are bacterial membrane-embedded nanomachines designed to export specifically targeted proteins from the bacterial cytoplasm. Secretion through T3SS is governed by a subset of inner membrane proteins termed the 'export apparatus'. We show that a key member of the Shigella flexneri export apparatus, MxiA, assembles into a ring essential for secretion in vivo. The ring-forming interfaces are well-conserved in both nonflagellar and flagellar homologs, implying that the ring is an evolutionarily conserved feature in these systems. Electron cryo-tomography revealed a T3SS-associated cytoplasmic torus of size and shape corresponding to those of the MxiA ring aligned to the secretion channel located between the secretion pore and the ATPase complex. This defines the molecular architecture of the dominant component of the export apparatus and allows us to propose a model for the molecular mechanisms controlling secretion.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Bacterial Outer Membrane Proteins / chemistry*
  • Bacterial Outer Membrane Proteins / metabolism*
  • Bacterial Secretion Systems*
  • Biological Transport
  • Cell Membrane / metabolism
  • Models, Molecular
  • Molecular Sequence Data
  • Protein Conformation
  • Protein Structure, Secondary
  • Shigella flexneri / enzymology
  • Shigella flexneri / metabolism*


  • Bacterial Outer Membrane Proteins
  • Bacterial Secretion Systems
  • mxiA protein, Shigella flexneri

Associated data

  • PDB/4A5P