FliS/flagellin/FliW heterotrimer couples type III secretion and flagellin homeostasis

Abstract

Flagellin is amongst the most abundant proteins in flagellated bacterial species and constitutes the major building block of the flagellar filament. The proteins FliW and FliS serve in the post-transcriptional control of flagellin and guide the protein to the flagellar type III secretion system (fT3SS), respectively. Here, we present the high-resolution structure of FliS/flagellin heterodimer and show that FliS and FliW bind to opposing interfaces located at the N- and C-termini of flagellin. The FliS/flagellin/FliW heterotrimer is able to interact with FlhA-C suggesting that FliW and FliS are released during flagellin export. After release, FliW and FliS are recycled to execute a new round of post-transcriptional regulation and targeting. Taken together, our study provides a mechanism explaining how FliW and FliS synchronize the production of flagellin with the capacity of the fT3SS to secrete flagellin.

Crystal Structure of Flagellar Export Chaperone FliS in Complex With Flagellin and HP1076 of Helicobacter pylori

“…Helices α1 and α2 were oriented in a half-opened hairpin structure and interacted with helices α3 and α4 of FliS. Helix 3 was embedded in the flagellar binding pocket, which was consistent with other FliS-FliC structural models (Evdokimov et al, 2003;Altegoer et al, 2018;Xing et al, 2018). Helix 4 was located near the faces of helices α1 and α2 of FliS.…”

“…The H. pylori flagellins FlaA and FlaB each contain four domains, consistent with other flagellin homologs (Beatson et al, 2006). Although the FliS-FliC structures have been resolved in A. aeolicus, B. subtilis and S. Typhimurium (Evdokimov et al, 2003;Altegoer et al, 2018;Xing et al, 2018), recent studies have demonstrated that the C-terminal D0 domain of flagellin is critical to the ability of H. pylori to evade TLR-5-mediated host recognition (Forstnerič et al, 2017). Therefore, the atomic details of FlaA/FlaB and the mechanism of binding to FliS are important.…”

“…From the structural model of S. Typhimurium flagellin (Samatey et al, 2001;Yonekura et al, 2003), this segment is likely to be disordered in the absence of the N-terminal D1 helix and the hairpin structure. In general, the structure of FliS-FlaBc-HP1076 ternary complex reveals that FliS interacts with FlaB using a conserved binding mode demonstrated previously in A. aeolicus, B. subtilis, and S. Typhimurium (Evdokimov et al, 2003;Altegoer et al, 2018;Xing et al, 2018). However, the helical arrangement of the D0 domain of FlaB in the ternary complex is different.…”

“…The C-terminal D0 domain of FlaB contains four helices with the first two helices folded in a half-opened hairpin structure and interact with helices α2-α3 of FliS, whereas the last two helices wrap around the opening end and one side of the helix bundle of FliS as in other flagellin homologs. Taken together the structural information from the protofilament and FliS-FliC models (Evdokimov et al, 2003;Altegoer et al, 2018;Xing et al, 2018), it appears that the conformation of the C-terminal D0 segment is dependent on its interacting protein. This segment is in three helices (or four helices in H. pylori) when interacting with FliS.…”

“…FliS also regulates flagellar gene expression by binding to and suppressing the secretion of the anti-sigma factor FlgM (Yokoseki et al, 1996;Xu et al, 2014). Previous studies have revealed the structural basis of the FliS-FliC interaction in Aquifex aeolicus and Bacillus subtilis, as well as the FlhA-FliS-FliC interactions in S. Typhimurium (Evdokimov et al, 2003;Altegoer et al, 2018;Xing et al, 2018). The 40 C-terminal residues of FliC form three helices that wrap around the core of FliS, which has a four-helix bundle structure, in an extended, horseshoe-like conformation.…”