Crystal structure of FlgL and its implications for flagellar assembly

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Bacteria utilize flagellar rotation for motility, driven by complex protein assemblies.
• The bacterial flagellum comprises over 30 proteins, with FlgL acting as a key junction protein between the hook and filament.
• Limited structural and functional data exists for FlgL, hindering understanding of flagellar assembly.

Purpose of the Study:

• To elucidate the structure of the FlgL protein from different bacterial species.
• To provide structural insights into the interaction between FlgL and flagellin.
• To propose a mechanism for flagellar filament-Hook junction assembly.

Main Methods:

• Crystal structure determination of FlgL from Bacillus cereus (bcFlgL) and Xanthomonas campestris (xcFlgL).
• Comparative structural analysis of bcFlgL and xcFlgL domains.
• Analysis of structural similarities between FlgL domains and other flagellar proteins.

Main Results:

• The crystal structure of bcFlgL revealed a single domain (D1), while xcFlgL exhibited a two-domain structure (D1 and D2).
• The D1 domain of FlgL possesses a conserved rod structure formed by four longitudinal segments.
• The D2 domain of xcFlgL shares structural similarities with the FlgK protein, another junction protein.

Conclusions:

• A novel junction-filament assembly mechanism is proposed, based on quasi-homotypic interactions involving the conserved D1 domain of FlgL.
• The structural similarity between xcFlgL's D2 domain and FlgK suggests a related function and evolutionary link between these junction proteins.
• These findings advance our understanding of bacterial flagellar assembly and protein-protein interactions at the hook-filament junction.