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

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 similar to16-Angstrom 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 (similar to5.6 subunits/turn, 24-Angstrom helical pitch). This common architecture implies that there will be further mechanistic analogies in the functioning of these two bacterial systems.