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

McDowell, Melanie A.; Marcoux, Julien; McVicker, Gareth; Johnson, Steven; Fong, Yu Hang; Stevens, Rebecca; Bowman, Lesley A. H.; Degiacomi, Matteo T.; Yan, Jun; Wise, Adam; Friede, Miriam E.; Benesch, Justin L. P.; Deane, Janet E.; Tang, Christoph M.; Robinson, Carol V.; Lea, Susan M.

doi:10.1111/mmi.13267

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

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McDowell, M. A., Marcoux, J., McVicker, G., Johnson, S., Fong, Y. H., Stevens, R., et al. (2016). Characterisation of Shigella Spa33 and Thermotoga FliM/N reveals a new model for C-ring assembly in T3SS. Molecular Microbiology, 99(4), 749-766. doi:10.1111/mmi.13267.

Cite as: https://hdl.handle.net/21.11116/0000-0009-67CC-E

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.