title: Characterisation of Shigella Spa33 and Thermotoga FliM/N reveals a new model for C-ring assembly in T3SS.
abstract: Flagellar type III secretion systems (T3SS) contain an essential cytoplasmic-ring (C-ring) largely composed of two proteins FliM and FliN, whereas an analogous substructure for the closely related non-flagellar (NF) T3SS has not been observed in situ. We show that the spa33 gene encoding the putative NF-T3SS C-ring component in Shigella flexneri is alternatively translated to produce both full-length (Spa33-FL) and a short variant (Spa33-C), with both required for secretion. They associate in a 1:2 complex (Spa33-FL/C2) that further oligomerises into elongated arrays in vitro. The structure of Spa33-C2 and identification of an unexpected intramolecular pseudodimer in Spa33-FL reveal a molecular model for their higher order assembly within NF-T3SS. Spa33-FL and Spa33-C are identified as functional counterparts of a FliM-FliN fusion and free FliN respectively. Furthermore, we show that Thermotoga maritima FliM and FliN form a 1:3 complex structurally equivalent to Spa33-FL/C2 , allowing us to propose a unified model for C-ring assembly by NF-T3SS and flagellar-T3SS.
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Melanie A
authors.initials: MA
authors.lastname: McDowell
authors.affiliation: Department of Chemistry, University of Oxford, Oxford, UK.
authors.forename: Julien
authors.initials: J
authors.lastname: Marcoux
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Gareth
authors.initials: G
authors.lastname: McVicker
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Steven
authors.initials: S
authors.lastname: Johnson
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Yu Hang
authors.initials: YH
authors.lastname: Fong
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Stevens
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Lesley A H
authors.initials: LA
authors.lastname: Bowman
authors.affiliation: Department of Chemistry, University of Oxford, Oxford, UK.
authors.forename: Matteo T
authors.initials: MT
authors.lastname: Degiacomi
authors.affiliation: Department of Chemistry, University of Oxford, Oxford, UK.
authors.forename: Jun
authors.initials: J
authors.lastname: Yan
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Adam
authors.initials: A
authors.lastname: Wise
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Miriam E
authors.initials: ME
authors.lastname: Friede
authors.affiliation: Department of Chemistry, University of Oxford, Oxford, UK.
authors.forename: Justin L P
authors.initials: JL
authors.lastname: Benesch
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Janet E
authors.initials: JE
authors.lastname: Deane
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Christoph M
authors.initials: CM
authors.lastname: Tang
authors.affiliation: Department of Chemistry, University of Oxford, Oxford, UK.
authors.forename: Carol V
authors.initials: CV
authors.lastname: Robinson
authors.affiliation: Sir William Dunn School of Pathology, University of Oxford, Oxford, UK.
authors.forename: Susan M
authors.initials: SM
authors.lastname: Lea
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D003460:Crystallization; D018360:Crystallography, X-Ray; D005407:Flagella / Q000502:physiology; D013058:Mass Spectrometry; D008958:Models, Molecular; D011487:Protein Conformation; D055503:Protein Multimerization; D012763:Shigella flexneri / Q000235:genetics* / Q000502:physiology; D020124:Thermotoga maritima / Q000502:physiology*; D000069296:Type III Secretion Systems / Q000502:physiology*
chemical_list: D001426:Bacterial Proteins; C077389:FliN protein, Bacteria; D000069296:Type III Secretion Systems; C066821:FliM protein, Bacteria