Extension and refinement of the recognition motif for Toll-like receptor 5 activation by flagellin

Abstract

TLRs sense conserved and essential molecular components of microbes that invade multicellular organisms. The wide range of TLR agonists, differing in size and shape, is recognized either through a single or a pair of binding sites on the ectodomains of TLRs. TLR5 recognizes bacterial flagellin through two distinct binding sites on the ectodomain, the first facilitating primary binding of flagellin and the second guiding receptor dimerization necessary for signaling. The regions of flagellin recognized by TLR5 encompass key functional regions within the D1 domain of flagellin, which is also required for the assembly of functional flagella. In addition to previously identified binding sites at the N-terminal and central segment of the TLR5 ectodomain, we extended the TLR5'-D1 interaction interface on TLR5 and showed a species-specific recognition relevance of this extended region. In addition, we showed that the loop and following β-hairpin region of flagellin, previously proposed to participate in the TLR5-flagellin dimerization interface, is not accountable for these species-specific differences. We further identified residues that contribute to the interaction between two TLR5 ectodomains in an active signaling complex. Our work demonstrates that flagellin is recognized by TLR5 through a more extensive interaction surface than previously characterized.

Escape of TLR5 Recognition by Leptospira spp.: A Rationale for Atypical Endoflagella

“…The consensus sites for TLR5 recognition in the flagellin FliC are localized at stacking sites between the flagellin monomers and therefore are not accessible when the filament is formed. Hence, when polymerized, the interaction domain of FliC with TLR5 is masked, therefore whole flagella do not signal through TLR5, which occurs only when FliC is monomeric ( 41 , 50 , 51 ). Intact purified periplasmic flagella from Treponema denticola were not able to activate TLR5 as well ( 52 ).…”

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

“…Bacterial flagellar filaments also play a critical role in innate immunity in βand γ-proteobacteria (Hayashi et al, 2001;Andersen-Nissen et al, 2005). In addition to the D1 domain, the D0 domain is required for full TLR5 activation (Yoon et al, 2012;Song et al, 2017;Ivičak-Kocjan et al, 2018). However, βand ε-proteobacteria are able to evade the TLR5 activation (Andersen-Nissen et al, 2005).…”

“…In gram-negative βand γ-proteobacteria, FliC also serves as a pathogen-associated molecular pattern that is recognized by Toll-like receptors (TLRs) on host innate immune cells (Hayashi et al, 2001;Andersen-Nissen et al, 2005;Song et al, 2017). The interaction between the D1 domain of FliC and the ectodomain of TLR5 was determined (Yoon et al, 2012;Song et al, 2017;Ivičak-Kocjan et al, 2018). Recent mutagenesis studies reported that the D0 domain also contributes to TRL5 activation (Forstnerič et al, 2017;Song et al, 2017).…”

The Role of Flagellin B in Vibrio anguillarum-Induced Intestinal Immunity and Functional Domain Identification

“…Importantly, flagellins are usually species-specific ( 27 ), and there are significant differences in the immune properties of flagellins from different bacterial sources ( 28 ). Even from the same bacteria, the immunogenicities of different flagellins can differ markedly ( 29 ).…”

“…However, due to variation in the sequences and domains of flagellins, flagellins from diverse bacterial species use different TLR5 recognition mechanisms ( 34 ). For activation, the receptor must undergo a ligand-induced conformational change ( 27 ). In the present study, we aimed to identify the specific ligand-binding sites on the surface of ligand-receptor complexes.…”