Structure and composition of the Shigella flexneri‘needle complex’, a part of its type III secreton

Authors

  • Ariel Blocker,

    Corresponding author
    1. Unité de Pathogénie Microbienne Moléculaire, INSERM U389, Institut Pasteur, 25–28 rue du Dr Roux, 75724 Paris Cedex 15, France.
    • *For correspondence. E-mail ablocker@pasteur.fr; Tel. (+33) 1 40 61 37 71; Fax (+33) 1 45 68 53 98. †Present address: Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE, UK. ‡The first three authors contributed equally to this work.

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  • Noureddine Jouihri,

    1. Université Libre de Bruxelles, Faculté de Médecine, Laboratoire de Bactériologie Moléculaire, Bâtiment G/E, CP 614, B-1070 Brussels, Belgium.
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  • Eric Larquet,

    1. Station Centrale de Microscopie Electronique et
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  • Pierre Gounon,

    1. Station Centrale de Microscopie Electronique et
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  • Frank Ebel,

    1. Unité de Génétique Moléculaire, CNRS 1773, Institut Pasteur, 25–28 rue du Dr Roux, 75724 Paris Cedex 15, France.
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  • Claude Parsot,

    1. Unité de Pathogénie Microbienne Moléculaire, INSERM U389, Institut Pasteur, 25–28 rue du Dr Roux, 75724 Paris Cedex 15, France.
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  • Philippe Sansonetti,

    1. Unité de Pathogénie Microbienne Moléculaire, INSERM U389, Institut Pasteur, 25–28 rue du Dr Roux, 75724 Paris Cedex 15, France.
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  • Abdelmounaaïm Allaoui

    1. Université Libre de Bruxelles, Faculté de Médecine, Laboratoire de Bactériologie Moléculaire, Bâtiment G/E, CP 614, B-1070 Brussels, Belgium.
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Abstract

Type III secretion systems (TTSSs or secretons), essential virulence determinants of many Gram-negative bacteria, serve to translocate proteins directly from the bacteria into the host cytoplasm. Electron microscopy (EM) indicates that the TTSSs of Shigella flexneri are composed of: (1) an external needle; (2) a transmembrane domain; and (3) a cytoplasmic bulb. EM analysis of purified and negatively stained parts 1, 2 and a portion of 3 of the TTSS, together termed the ‘needle complex’ (NC), produced an average image at 17 Å resolution in which a base, an outer ring and a needle, inserted through the ring into the base, could be discerned. This analysis and cryoEM images of NCs indicated that the needle and base contain a central 2–3 nm canal. Five major NC components, MxiD, MxiG, MxiJ, MxiH and MxiI, were identified by N-terminal sequencing. MxiG and MxiJ are predicted to be inner membrane proteins and presumably form the base. MxiD is predicted to be an outer membrane protein and to form the outer ring. MxiH and MxiI are small hydrophilic proteins. Mutants lacking either of these proteins formed needleless secretons and were unable to secrete Ipa proteins. As MxiH was present in NCs in large molar excess, we propose that it is the major needle component. MxiI may cap at the external needle tip.

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