Structure and function of flagellar cap complex in filament elongation

The bacterial flagellum is a molecular motor that allows motility through the rotation of a long filament protruding from the cell. In enteric pathogens, like Campylobacter jejuni, it facilitates adhesion to surfaces, infection and evasion of the immune system. FliD is an oligomeric capping protein at the tip of the filament and has been shown to play an essential part in filament assembly and adherence. However, the structure of the intact FliD cap complex, and the molecular basis for its interaction and assembly of the filament, remains elusive.

While the first cryo-EM structure of FliD from Salmonella enterica showed a pentameric complex at low resolution, crystal structures of truncated FliD in several other species have revealed a range of crystallographic symmetries. This and a unique oligomerization of C. jejuni flagellar filament into 7 protofilaments suggested that the mechanism of filament assembly across bacterial species is not uniform. The aim was to determine the structure and oligomerization of FliD and flagellar filament across different species to conclude if the varied oligomeric states are functional and analyse the interaction of the two complexes to propose a mechanism of elongation.

This thesis reports the first near-atomic cryo-EM structure of the most complete capping complex to date, revealing a pentameric FliD with previously uncharacterised terminal regions essential to its function. FliD in Serratia marscecens and Pseudomonas aeruginosa forms pentamers of similar dimensions, contrary to the crystal structures. Through construction of FliD mutant strains in C. jejuni and motility assays, the function of the capping protein in motility and filament elongation was analysed. There also was an investigation of potential of C. jejuni FliD in therapeutic development due to it being an antibody target. Finally, this work demonstrates that the native C. jejuni flagellum filament is 11-stranded and proposes a molecular model for the filament-cap interaction.