A recent study investigates the structure and mechanism of bacterial flagellum assembly, particularly focusing on flagellin incorporation in significant human pathogens like Salmonella enterica and Campylobacter jejuni.

Utilizing advanced techniques such as cryogenic electron tomography and single-particle cryo-electron microscopy, researchers visualized the structures and dynamics of flagellum components at high resolutions.

The flagellum is composed of three main components: the basal body, hook, and filament, with the hook serving as a joint that connects the filament to the basal body, thereby facilitating bacterial motility.

The study also characterized the intact hook-filament junction (HFJ), made up of FlgK and FlgL proteins, highlighting its role in stabilizing the connection between the hook and filament.

Structural analysis reveals that the flagellar filament consists of a superhelical assembly of flagellin subunits, with the filament cap complex, composed of the protein FliD, playing a crucial role in flagellin incorporation.

Researchers determined the structure of the filament cap complex in its native environment and at various stages of flagellin incorporation, identifying key molecular steps in the process.

Results indicate that specific mutations in FliD and HFJ proteins significantly impair filament assembly and bacterial motility, underscoring the importance of these proteins in flagellum function.

Additionally, mutagenesis and functional assays revealed that the terminal domains of FliD are essential for the proper folding and incorporation of flagellin into the filament.

These findings provide an in-depth understanding of the molecular mechanisms underlying flagellin insertion and flagellar structure, which could inform therapeutic strategies against bacterial infections.