Self-assembly of the filament capping protein, FliD, of bacterial flagella into an annular structure

J Mol Biol. 1996 Jun 21;259(4):679-86. doi: 10.1006/jmbi.1996.0349.


A bacterial flagellum has a cap structure at the tip of the external filament. The cap is composed of the FliD protein (Mr, 49 x 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 (Mr, ca. 600 x 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 fliD mutant, 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.

MeSH terms

  • Bacterial Proteins / chemistry
  • Bacterial Proteins / isolation & purification
  • Bacterial Proteins / metabolism*
  • Bacterial Proteins / ultrastructure
  • Chromatography, Gel
  • Cloning, Molecular
  • Electrophoresis, Polyacrylamide Gel
  • Escherichia coli / genetics
  • Flagella / metabolism*
  • Gene Expression / genetics
  • Microscopy, Electron
  • Models, Molecular
  • Molecular Weight
  • Plasmids / genetics
  • Protein Conformation
  • Salmonella typhimurium / genetics
  • Salmonella typhimurium / metabolism*
  • Salmonella typhimurium / physiology


  • Bacterial Proteins
  • FlaD protein, Bacteria