Characterisation of Shigella Spa33 and Thermotoga FliM/N reveals a new model for C‐ring assembly in T3SS |
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| Authors: | Melanie A McDowell Julien Marcoux Gareth McVicker Steven Johnson Yu Hang Fong Rebecca Stevens Lesley A H Bowman Matteo T Degiacomi Jun Yan Adam Wise Miriam E Friede Justin L P Benesch Janet E Deane Christoph M Tang Carol V Robinson Susan M Lea |
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| Institution: | 1. Sir William Dunn School of Pathology, University of Oxford, Oxford, UK;2. Department of Chemistry, University of Oxford, Oxford, UK |
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| 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. |
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