Bacterial flagellar capping proteins adopt diverse oligomeric states

Flagella are crucial for bacterial motility and pathogenesis. The flagellar capping protein (FIiD) regulates filament assembly by chaperoning and sorting flagellin (FIiC) proteins after they traverse the hollow filament and exit the growing flagellum tip. In the absence of FIiD, flagella are not formed, resulting in impaired motility and infectivity. Here, we report the 2.2 angstrom resolution X-ray crystal structure of FIiD from Pseudomonas aeruginosa, the first high-resolution structure of any FIiD protein from any bacterium. Using this evidence in combination with a multitude of biophysical and functional analyses, we find that Pseudomonas FIiD 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 FIiD oligomers are affixed vary in protofilament number between bacteria, our results suggest that FIiD oligomer stoichiometries vary across bacteria to complement their filament assemblies.