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Identification of minor inner-membrane components of the Shigella type III secretion system ‘needle complex’

Sebastian F. Zenk^1,

David Stabat^1,

Steven Johnson^1,2,

¹ Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE, UK
² Laboratory of Molecular Biophysics, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK

Correspondence
Ariel J. Blocker
ariel.blocker{at}path.ox.ac.uk

Type III secretion systems (T3SSs or secretons) are central virulence factors of many Gram-negative bacteria, used to inject protein effectors of virulence into eukaryotic host cells. Their overall morphology, consisting of a cytoplasmic region, an inner- and outer-membrane section and an extracellular needle, is conserved in various species. A portion of the secreton, containing the transmembrane regions and needle, has been isolated biochemically and termed the ‘needle complex’ (NC). However, there are still unsolved questions concerning the nature and relative arrangement of the proteins assembling the NC. Until these are resolved, the mode of function of the NC cannot be clarified. This paper describes an affinity purification method that enables highly efficient purification of Shigella NCs under near-physiological conditions. Using this method, three new minor components of the NC were identified by mass spectrometry: IpaD, a known component of the needle tip complex, and two predicted components of its central inner-membrane export apparatus, Spa40 and Spa24. A further minor component of the NC, MxiM, is only detected by immunoblotting. MxiM is a ‘pilotin’-type protein for the outer-membrane ‘secretin’ ring formed of MxiD. As expected, it localized to the outer rim of the upper ring of NCs, validating the other findings.

Present address: Institut für Klinische Mikrobiologie, Immunologie und Hygiene, FAU Erlangen-Nürnberg, Wasserturmstraße 3–5, D-91054 Erlangen, Germany.

Present address: Université Pierre et Marie Curie, Paris VI, 4 place Jussieu, 75252, Paris cedex 03, France.

Present address: Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE, UK.

Supplementary material is available with the online version of this paper.

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