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.
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We thank N.C. Strynadka for access to InvA coordinates ahead of publication, P. Sansonetti (Institute Pasteur, Paris) for providing the polyclonal antibodies to Ipa proteins, I. Lasa (Universidad Pública de Navarra, Pamplona) for the pKO3blue plasmid, L. De Colibus, C. King and members of the Lea group for general assistance, the staff of the protein crystallography beamlines at the European Synchrotron Radiation Source, Grenoble (France) and DIAMOND facility, Didcot (UK) for assistance in data collection, and S.K. Mazmanian for use of the microaerobic chamber. P.A. is funded by grant 083599/Z/07/Z and J.E.D. by grant WT083599MA, both from the Wellcome Trust to S.M.L.; S.J. is funded by grant G0900888 from the UK Medical Research Council to S.M.L. and C.M.T.; P.R. is funded by the Oxford Martin School Vaccine Design Institute of which S.M.L. is codirector. M.V.-I. is funded by FP7 Marie Curie EIMID-IAPP-217768 grant. M.B. and G.J.J. were supported by the Howard Hughes Medical Institute.
The authors declare no competing financial interests.
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Abrusci, P., Vergara-Irigaray, M., Johnson, S. et al. Architecture of the major component of the type III secretion system export apparatus. Nat Struct Mol Biol 20, 99–104 (2013). https://doi.org/10.1038/nsmb.2452
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