Structural Basis of TLR5-Flagellin Recognition and Signaling

Abstract

Toll-like receptor 5 (TLR5) binding to bacterial flagellin activates NF-κB signaling and triggers an innate immune response to the invading pathogen. To elucidate the structural basis and mechanistic implications of TLR5-flagellin recognition, we determined the crystal structure of zebrafish TLR5, as a VLR-hybrid protein, in complex with the D1/D2 fragment of Salmonella flagellin, FliC, at 2.47 Å resolution. TLR5 interacts primarily with the three helices of the FliC D1 domain using its lateral side. Two TLR5-FliC 1:1 heterodimers assemble into a 2:2 tail-to-tail signaling complex that is stabilized by quaternary contacts of the FliC D1 domain with the convex surface of the opposing TLR5. The proposed signaling mechanism is supported by structure-guided mutagenesis and deletion analysis on CBLB502, a therapeutic protein derived from FliC.

A speculation on the tandem fasciclin 1 repeat of FLA4 proteins in angiosperms

“…While some LRR ligands such as flagellin bind to the ascending lateral (convex) surface of the LRR-domain, 20 others such as the Spätzle morphogen 21 or the follicle-stimulating hormone 22 interact with the concave LRR surfaces of their respective receptors. In our model the convex surface of the FEI1 ectodomain binds to the surface of the Fas1-2 domain (Figure 3) that is formed by the outward bent β-sheet harbouring the Ser348 residue (marked red in Figure 2).…”

Toll-like receptor 3 recognizes incomplete stem structures in single-stranded viral RNA

“…Structural analyses of TLR3, TLR4, TLR5, TLR2/1 and TLR2/6 ECDs, and their complexes with typical ligands, demonstrated that each TLR possessed ligand-binding sites or pockets and a dimerizing interface, which allowed for ligand-induced receptor dimerization 46,47 . Although there are multiple ligands for each TLR, it remains unknown whether they share common structural features.…”

Regulation of immunity and disease resistance by commensal microbes and chromatin modifications during zebrafish development

“…Similarly, an almost complete set of TLRs has been described in the pufferfish Fugu rubripes and in the zebrafish Danio rerio (12), and, with the exception of TLR4 (13,14), these TLRs seem to be functional orthologs of mammalian TLRs. Thus, they are able to sense the same ligands (15)(16)(17), use similar adaptor molecules for signaling (15), and activate the transcription factor NF-κB (13,15,18). In addition, it has been shown that in zebrafish MyD88 modulates innate immune responses to microbes (19)(20)(21)(22).…”