Novel transient cytoplasmic rings stabilize assembling bacterial flagellar motors

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Bacterial flagellar motility is crucial for various cellular functions.
• Previous studies on flagellar assembly primarily used Escherichia coli and Salmonella enterica.
• The in situ assembly process in diverse bacteria remains incompletely understood.

Purpose of the Study:

• To investigate the in situ assembly of bacterial flagellar motors in diverse Proteobacteria.
• To elucidate the structural roles of accessory proteins in flagellar motor construction.
• To identify novel components and mechanisms involved in flagellar assembly.

Main Methods:

• Electron cryo-tomography (cryo-ET) was employed to visualize flagellar motor assembly in situ.
• Studies were conducted on multiple bacterial species including Hylemonella gracilis, Helicobacter pylori, Campylobacter jejuni, Pseudomonas aeruginosa, Pseudomonas fluorescens, and Shewanella oneidensis.
• Mutant strains of Helicobacter pylori were analyzed to determine protein functions.

Main Results:

• In situ structures of flagellar intermediates, including the flagellar type III secretion system core complex (fT3SScc) and MS-ring, were revealed.
• Novel cytoplasmic rings interacting with the FliG torque ring were discovered in high-torque motors.
• These novel rings are essential for stator ring assembly and depend on fT3SScc proteins FliO and FliQ.

Conclusions:

• Flagellar motor assembly is a dynamic process, not a simple accretion of parts.
• Accessory components transiently interact to facilitate the construction of the flagellar nanomachine.
• Novel cytoplasmic rings play a critical role in orchestrating flagellar motor assembly across diverse Proteobacteria.