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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.

Original publication

DOI

10.1038/nsmb.2452

Type

Journal article

Journal

Nat Struct Mol Biol

Publication Date

01/2013

Volume

20

Pages

99 - 104

Keywords

Amino Acid Sequence, Bacterial Outer Membrane Proteins, Bacterial Secretion Systems, Biological Transport, Cell Membrane, Models, Molecular, Molecular Sequence Data, Protein Conformation, Protein Structure, Secondary, Shigella flexneri