Molecular Determinants of Filament Capping Proteins Required for the Formation of Functional Flagella in Gram-Negative Bacteria

Biomolecules. 2021 Sep 22;11(10):1397. doi: 10.3390/biom11101397.

Abstract

Bacterial flagella are cell surface protein appendages that are critical for motility and pathogenesis. Flagellar filaments are tubular structures constructed from thousands of copies of the protein flagellin, or FliC, arranged in helical fashion. Individual unfolded FliC subunits traverse the filament pore and are folded and sorted into place with the assistance of the flagellar capping protein complex, an oligomer of the FliD protein. The FliD filament cap is a stool-like structure, with its D2 and D3 domains forming a flat head region, and its D1 domain leg-like structures extending perpendicularly from the head towards the inner core of the filament. Here, using an approach combining bacterial genetics, motility assays, electron microscopy and molecular modeling, we define, in numerous Gram-negative bacteria, which regions of FliD are critical for interaction with FliC subunits and result in the formation of functional flagella. Our data indicate that the D1 domain of FliD is its sole functionally important domain, and that its flexible coiled coil region comprised of helices at its extreme N- and C-termini controls compatibility with the FliC filament. FliD sequences from different bacterial species in the head region are well tolerated. Additionally, head domains can be replaced by small peptides and larger head domains from different species and still produce functional flagella.

Keywords: FliD; Pseudomonas aeruginosa; bacterial flagella; filament cap.

MeSH terms

  • Bacterial Proteins / genetics*
  • Bacterial Proteins / ultrastructure
  • Escherichia coli / genetics
  • Escherichia coli / pathogenicity
  • Escherichia coli / ultrastructure
  • Escherichia coli Proteins / genetics*
  • Escherichia coli Proteins / ultrastructure
  • Flagella / chemistry
  • Flagella / genetics
  • Flagella / ultrastructure
  • Flagellin / genetics*
  • Flagellin / ultrastructure
  • Gram-Negative Bacteria / genetics
  • Gram-Negative Bacteria / pathogenicity
  • Intermediate Filaments / genetics
  • Membrane Proteins / genetics*
  • Microscopy, Electron
  • Models, Molecular
  • Protein Domains / genetics
  • Pseudomonas aeruginosa / genetics
  • Pseudomonas aeruginosa / pathogenicity
  • Pseudomonas aeruginosa / ultrastructure

Substances

  • Bacterial Proteins
  • Escherichia coli Proteins
  • FliC protein, E coli
  • Membrane Proteins
  • Flagellin
  • FlaD protein, Bacteria