Toll-like receptor 5 (TLR5) participates in the innate immune response to microbial agents. Toll-like receptors are classified into various types, depending on their primary sequence and types of ligands they recognise. TLR5 mediates detection of bacterial flagellins, both from from both Gram-positive and Gram-negative bacteria [, ]. TLR5 signaling acts via MYD88. In MyD88-dependent signaling, ligand binding to TLR triggers sequential recruitment of MyD88, members of IRAK (interleukin-1 receptora ssociated kinase) family and TRAF6, that results in nuclear factor (NF)-kB activation and induction of cytokine secretion and production of inflammatory cytokines including IL-8 and TNF-alpha. Genetic variation in TLR5 have been associated with systemic lupus erythematosus, an inflammatory and often febrile multisystemic disorder of connective tissue [].
Bacterial flagella are responsible for motility and chemotaxis []. They comprise a basal body, a hook and a filament, the latter accounting for 98% of the mass []. Flagellin is the subunit protein that polymerises to form the flagella [], the subunits being transported through the centre of the filament to the tip, where they then polymerise []. Both the N- and C-termini of the subunit protein, which are α-helical in structure [], are required to mediate polymerisation. Although no export or assembly, consensus sequences have been identified: Ala, Val, Leu, Ile, Gly, Ser, Thr, Asn, Gln and Asp tend to make up around 90% of the sequence, Cys and Trp being absent [].Flagellin plays a role in the activation of innate and adaptive immunity. It is an specific ligand for Toll-like receptor 5 (TLR5) in the host, which has lead to great interest to use it as adjuvant for vaccines [, , ]. The protein is also recognised by the intracellular NAIP5/NLRC4 inflammasome receptor []. This entry represents the N-terminal domain of Flagellin and similar bacterial proteins. This domain comes together with the C-terminal domain () to form the D0 and D1 structural domains []. These domains are responsible for the activation of TLR5 [, ].
