Journal
MOLECULAR MICROBIOLOGY
Volume 99, Issue 4, Pages 749-766Publisher
WILEY
DOI: 10.1111/mmi.13267
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Funding
- Wellcome Trust [083599, 100298]
- Medical Research Council (UK) [G0900888, M011984]
- BBSRC [BB/K004247/1] Funding Source: UKRI
- MRC [MR/M011984/1] Funding Source: UKRI
- Biotechnology and Biological Sciences Research Council [BB/K004247/1] Funding Source: researchfish
- Medical Research Council [G0900888] Funding Source: researchfish
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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 maritimaFliM 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.
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