Structural characterization of Pseudomonas aeruginosa flagellar hook FlgE reveals a novel β-hairpin element involved...

The flagellar hook subunit FlgE of Pseudomonas aeruginosa (Pa-FlgE) is a core component of the bacterial surface flagellum. It is also identified as an important virulence factor capable of modulating host inflammatory response. Herein, we report the high-resolution crystal structures of Pa-FlgE domain I (D-I) and domain II (D-II), both of which adopt predominantly β-structure. Structural comparison among Pa-FlgE and its orthologs shows that D-I and the core-barrel of D-II are highly conserved. Two peripheral insertions of variable length and structure, however, are identified in FlgE D-II. In Pa-FlgE, these two insertions fold as a loop element and a β-hairpin element, respectively. Notably, these two elements are solvent-exposed and extend towards one another, and deletion of the two elements either simultaneously or individually is shown to abolish the immunomodulation activity of Pa-FlgE. While the loop element is present in other FlgE orthologs, the β-hairpin is unique to Pa-FlgE, suggesting that P. aeruginosa has evolved this distinctive β-hairpin in flagellar hook to modulate the inflammatory response during infection.