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J Biol Chem. 2003 May 9;278(19):17103-7. Epub 2003 Feb 5.

Helical structure of the needle of the type III secretion system of Shigella flexneri.

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Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, United Kingdom.


Gram-negative bacteria commonly interact with animal and plant hosts using type III secretion systems (TTSSs) for translocation of proteins into eukaryotic cells during infection. 10 of the 25 TTSS-encoding genes are homologous to components of the bacterial flagellar basal body, which the TTSS needle complex morphologically resembles. This indicates a common ancestry, although no TTSS sequence homologues for the genes encoding the flagellum are found. We here present an approximately 16-A structure of the central component, the needle, of the TTSS. Although the needle subunit is significantly smaller and shares no sequence homology with the flagellar hook and filament, it shares a common helical architecture ( approximately 5.6 subunits/turn, 24-A helical pitch). This common architecture implies that there will be further mechanistic analogies in the functioning of these two bacterial systems.

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