Abstract
SummaryFlagellar 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 S higella 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‐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 T hermotoga 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.
Highlights
Non-flagellar type III secretion systems (NF-T3SS) are essential for initiating infection by many pathogenic Gramnegative bacteria (Cornelis, 2006)
We show that alternative translation occurs to generate full-length Spa33 (Spa33-FL) and a shorter C-terminal fragment (Spa33-C) within S. flexneri, with the production of both being required for assembly and function of the NF-T3SS
As S. flexneri Spa33-FL/Spa33-C appear to mimic characteristics of FliM/FliN as the previous (FliN) respectively from the flagellar-Cring, we considered whether the complexes formed by
Summary
Non-flagellar type III secretion systems (NF-T3SS) are essential for initiating infection by many pathogenic Gramnegative bacteria (Cornelis, 2006) These include several enteropathogenic species such as Salmonella, Escherichia, Yersinia and, the primary focus of this study, Shigella, which invades a variety of cells in the intestinal tract and causes over one million deaths annually from bacterial dysentery or shigellosis (Kotloff et al, 1999). This large protein export apparatus (Fig. S1) is composed of ∼ 25 different proteins with both species specific and unified Sct names and functions as a molecular syringe, injecting virulence factors from the bacterial cytoplasm directly into the host-cell (Marlovits and Stebbins, 2010; Abrusci et al, 2014). Interactions between SctQ and the other essential cytoplasmic components SctN, SctL and SctK (Fig. S1) were identified in various species (Jackson and Plano, 2000; Johnson and Blocker, 2008; Johnson et al, 2008; Biemans-Oldehinkel et al, 2011; Lara-Tejero et al, 2011)
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