Characterisation of Shigella Spa33 and Thermotoga FliM/N reveals a new model for C-ring assembly in T3SS

Melanie Mcdowell , Julien Marcoux (1) , Gareth Mcvicker , Steven Johnson (2) , Yu Hang Fong , Rebecca Stevens (3) , Lesley Bowman , Matteo Degiacomi , Jun Yan (4) , Adam Wise , Miriam Friede , Justin Benesch , Janet Deane , Christoph Tang (5) , Carol Robinson (6) , Susan Lea

1 University of Oxford
2 Bureau of Environmental and Coastal Quality
3 GAFL - Génétique et Amélioration des Fruits et Légumes
4 Massachusetts General Hospital [Boston]
5 Imperial College London
6 Loughborough University

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/C 2) that further oligomerises into elongated arrays in vitro. The structure of Spa33-C 2 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/C 2, allowing us to propose a unified model for C-ring assembly by NF-T3SS and flagellar-T3SS.

Domaines: Biologie structurale [q-bio.BM]

Melanie Mcdowell, Julien Marcoux, Gareth Mcvicker, Steven Johnson, Yu Hang Fong, et al.. Characterisation of Shigella Spa33 and Thermotoga FliM/N reveals a new model for C-ring assembly in T3SS. Molecular Microbiology, 2016, 99 (4), pp.749-766. ⟨10.1111/mmi.13267⟩. ⟨hal-02335543⟩