Bacterial flagellar capping proteins adopt diverse oligomeric states

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Bacterial flagella are critical for motility and pathogenesis.
• Flagellar capping protein (FliD) regulates flagellin (FliC) assembly at the flagellar tip.
• Absence of FliD prevents flagella formation, impairing bacterial motility and infectivity.

Purpose of the Study:

• To determine the high-resolution X-ray crystal structure of FliD from *Pseudomonas aeruginosa*.
• To elucidate the structural and oligomeric properties of FliD.
• To understand the functional implications of FliD structure for flagellar assembly across different bacteria.

Main Methods:

• X-ray crystallography (2.2 Å resolution).
• Biophysical analyses.
• Functional assays.

Main Results:

• First high-resolution structure of any bacterial FliD protein reported.
• *Pseudomonas aeruginosa* FliD exhibits domain-level similarity to other flagellar proteins.
• FliD forms a unique hexameric oligomeric state, dependent on flexible determinants.
• FliD oligomer stoichiometry may vary across bacterial species to match filament assembly.

Conclusions:

• The structure of *Pseudomonas aeruginosa* FliD provides insights into flagellar assembly mechanisms.
• FliD's oligomeric state is adaptable, suggesting a conserved yet flexible role in flagellar biogenesis.
• Variations in FliD oligomerization likely accommodate diverse flagellar filament structures in bacteria.