Abstract
Flagella are crucial for bacterial motility and pathogenesis. The flagellar capping protein (FliD) regulates filament assembly by chaperoning and sorting flagellin (FliC) proteins after they traverse the hollow filament and exit the growing flagellum tip. In the absence of FliD, flagella are not formed, resulting in impaired motility and infectivity. Here, we report the 2.2 Å resolution X-ray crystal structure of FliD from Pseudomonas aeruginosa, the first high-resolution structure of any FliD protein from any bacterium. Using this evidence in combination with a multitude of biophysical and functional analyses, we find that Pseudomonas FliD exhibits unexpected structural similarity to other flagellar proteins at the domain level, adopts a unique hexameric oligomeric state, and depends on flexible determinants for oligomerization. Considering that the flagellin filaments on which FliD oligomers are affixed vary in protofilament number between bacteria, our results suggest that FliD oligomer stoichiometries vary across bacteria to complement their filament assemblies.
Highlights
Pathogenic bacteria cause a multitude of deadly human diseases
At the gross structural level, our studies show that the oligomeric states differ between FliD protein assemblies in diverse bacteria
Salmonella has long served as the model for bacterial flagellum structure and function
Summary
Pathogenic bacteria cause a multitude of deadly human diseases. Many of these microbes possess flagella, molecular machines responsible for cell motility, adherence to host cells, and pathogenicity (Duan et al, 2013; Haiko and Westerlund-Wikstrom, 2013). A FliD ( called HAP2) oligomer forms the cap protein complex that is located at the tip of the flagellar filament (Yonekura et al, 2000).
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