title: Helical packing of needles from functionally altered Shigella type III secretion systems.
abstract: Gram-negative bacteria commonly interact with eukaryotic host cells using type III secretion systems (TTSSs or secretons), which comprise cytoplasmic, transmembrane and extracellular domains. The extracellular domain is a hollow needle-like structure protruding 60 nm beyond the bacterial surface. The TTSS is activated to transfer bacterial proteins directly into a host cell only upon physical contact with the target cell. We showed previously that the monomer of the Shigella flexneri needle, MxiH, assembles into a helical structure with parameters similar to those defining the architecture of the extracellular components of bacterial flagella. By analogy with flagella, which are known to exist in different helical states, we proposed that changes in the helical packing of the needle might be used to sense host cell contact. Here, we show that, on the contrary, mutations within MxiH that lock the TTSS into altered secretion states do not detectably alter the helical packing of needles. This implies that either: (1) host cell contact is signalled through the TTSS via helical changes in the needle that are significantly smaller than those linked to structural changes in the flagellar filament and therefore too small to be detected by our analysis methods or (2) that signal transduction in this system occurs via a novel molecular mechanism.
authors.affiliation: Laboratory of Molecular Biophysics, University of Oxford, Oxford OX1 3QU, UK.
authors.forename: Frank S
authors.initials: FS
authors.lastname: Cordes
authors.forename: Sarah
authors.initials: S
authors.lastname: Daniell
authors.forename: Roma
authors.initials: R
authors.lastname: Kenjale
authors.forename: Saroj
authors.initials: S
authors.lastname: Saurya
authors.forename: Wendy L
authors.initials: WL
authors.lastname: Picking
authors.forename: William D
authors.initials: WD
authors.lastname: Picking
authors.forename: Frank
authors.initials: F
authors.lastname: Booy
authors.forename: Susan M
authors.initials: SM
authors.lastname: Lea
authors.forename: Ariel
authors.initials: A
authors.lastname: Blocker
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D005407:Flagella; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D021381:Protein Transport; D012763:Shigella flexneri / Q000737:chemistry* / Q000235:genetics / Q000648:ultrastructure; D015398:Signal Transduction
chemical_list: D001426:Bacterial Proteins; C413572:MxiH protein, Shigella