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

Journal article

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: McDowell, M; Marcoux, J; McVicker, G; Johnson, S; Fong, Y

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: John Wiley & Sons Ltd
• Journal: Molecular Microbiology
• Volume: 99
• Issue: 4
• Pages: 749–766
• Publication date: 2015-12-23
• Acceptance date: 2015-11-02
• DOI: 10.1111/mmi.13267
• ISSN: 0950-382X and 1365-2958
• Pmid: 26538516
• Language: English
• Keywords: SBTMR