Self-assembly of the Filament Capping Protein, FliD, of Bacterial Flagella into an Annular Structure

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A bacterial flagellum has a cap structure at the tip of the external filament. The cap is composed of the FliD protein ( M r, 49 × 10 3), and plays an essential role in the polymerization of the filament protein, flagellin, which is believed to be transported through a central channel in the flagellum. A fliD-deficient mutant becomes non-motile because it lacks flagellar filaments and leaks flagellin monomer out into the medium. We have constructed a FliD-overproducing plasmid and purified the protein. The purified FliD at high concentration formed a large complex ( M r, ca. 600 × 10 3) under physiological conditions. The complex was found by electron microscopy to be ring shaped. Image analysis revealed that the complex consisted of five substructures arranged in a pentagonal shape. Its outer diameter, approximately 10 nm, was about the same as that of the cap at the tip of the wild-type flagella. When the annular structure was added to the culture medium of a Salmonella fliDmutant, almost all of the cells became able to swim. Overall, about ten molecules of FliD self-assemble into an annular structure in vitro, forming the functional capping structure by incorporating flagellin at the tip of the flagellar filament in vivo.