Structural insights into bacterial flagellar hooks similarities and specificities

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• The flagellar hook protein (FlgE) exhibits significant variability in amino acid composition and length across bacterial species.
• Understanding the structural basis of this variability is crucial for comprehending bacterial motility.

Purpose of the Study:

• To elucidate the structural organization of the bacterial flagellar hook protein (FlgE).
• To investigate the conserved and variable regions within FlgE and their contribution to flagellar hook structure and function.

Main Methods:

• X-ray crystallography was employed to determine the structures of FlgE fragments from *Campylobacter jejuni* and *Caulobacter crescentus*.
• A high-resolution model of the *Caulobacter* hook was generated.
• Functional analysis involved motility assays in a genetically modified *Salmonella enterica* strain.

Main Results:

• Bacterial flagellar hooks are structurally divisible into two main parts: a conserved domain common to all flagellated bacteria and a hyper-variable domain specific to each bacterium.
• Despite variations in size and structure, the flagellar hook consistently comprises 11 protofilaments.
• A *Salmonella enterica* strain engineered to express *C. jejuni* FlgE with extra domains remained fully motile, indicating functional adaptability.

Conclusions:

• The bacterial flagellar hook possesses a conserved structural core and a variable region that accommodates species-specific adaptations.
• FlgE structural plasticity does not impede overall flagellar hook assembly or function, ensuring bacterial motility.
• This study provides critical insights into the structure-function relationship of the bacterial flagellar hook.