CAS Researchers Reveal Bacterial Flagellar Motor Structure, Assembly, and Evolution

Breakthrough in Bacterial Motility: CAS Unveils Complex Flagellar Motor Architecture

The Chinese Academy of Sciences (CAS) has led a groundbreaking study revealing the intricate structure, assembly process, and evolutionary history of the bacterial flagellar motor in Campylobacter jejuni, a common foodborne pathogen. Published in Nature Microbiology on January 9, 2026, this research moves beyond the well-studied simpler motors of Escherichia coli and Salmonella enterica, highlighting adaptations in more complex systems prevalent in diverse bacteria.7876 Researchers from the South China Sea Institute of Oceanology (SCSIO), CAS Key Laboratory of Tropical Marine Bio-resources and Ecology in Guangzhou, collaborated with Yale University and Shandong University to construct a near-complete model of this motor using advanced imaging techniques.

This discovery underscores China's growing prominence in structural biology, particularly in microbiology research conducted at CAS-affiliated institutions and the University of Chinese Academy of Sciences (UCAS). The findings provide critical insights into how bacteria achieve high-torque propulsion, essential for survival in viscous environments like the gut.

Understanding the Bacterial Flagellar Motor: From Basics to Complexity

The bacterial flagellar motor (BFM) is a rotary nanomachine embedded in the cell membrane, powering the helical flagellum filament to propel bacteria at speeds up to 100 body lengths per second. Comprising a rotor (MS-ring, C-ring, rod) and multiple stator units (typically MotA/MotB complexes that harness proton motive force), the BFM exemplifies evolutionary engineering. In model organisms like E. coli, 8-11 stators generate modest torque, but species like C. jejuni employ up to 17 for enhanced performance.78

BFMs vary across phyla: proton-driven in Proteobacteria, sodium-driven in Vibrio. Periplasmic scaffolds, absent or minimal in simple motors, stabilize stators in complex ones, preventing futile rotation and optimizing energy use. This CAS-led work dissects these scaffolds in C. jejuni, revealing species-specific innovations.

Key Structural Innovations in the Campylobacter jejuni Flagellar Motor

The study identifies three novel periplasmic scaffolds: an E-ring formed by 17 FlgY homodimers encircling the MS-ring; a spoke-and-rim structure of PflA-PflB linking the E-ring to a peripheral cage; and the cage itself, composed of 34 heterotetrameric units (FcpMNO tetramers alternating with PflD in extended or contracted conformations).78 These form a lattice that anchors 17 stator complexes, excluding peptidoglycan and enabling high torque (~3,288 pN nm estimated, versus ~1,600 pN nm in E. coli).

High-resolution cryo-EM (3.23 Å) of the PflA-PflB complex shows TPR (tetratricopeptide repeat) motifs in PflA (with β-sandwich domain) dimerizing via PflB's α-helices and β-sheets, creating 17 spokes radiating from the E-ring.Full structural model in Nature Microbiology

• E-ring: ARM-like superhelix domains in FlgY stabilize the base.
• Spoke-rim: 1:1 PflA-PflB stoichiometry, interacts with FliL arcs for stator recruitment.
• Cage: Dynamic conformations regulate stator occupancy (~80% in wild-type).

Step-by-Step Assembly Pathway Revealed

Using targeted knockouts (ΔflgY, ΔpflA/B/C/D, ΔfcpMNO, ΔrpoN), researchers mapped assembly. Inner membrane-proximal components (MS/C-rings, E-ring, spokes/rim, partial cage, initial stators) form early, independent of sigma factors RpoN/FliA and before rod export. Rod penetration follows, with medial disk (PflC/PflD) and full cage completing post-stator insertion.78

1. MS-ring and C-ring (38-40 subunits, wider than E. coli's 26) anchor in inner membrane.
2. E-ring and spoke-rim assemble, recruiting ~17 stators.
3. Partial cage (17 units) links to medial disk; stators stabilize remainder.
4. Outer LP-rings and hook-filament extrude via type III secretion system.

Motility assays confirm: mutants lacking scaffolds show reduced stator occupancy (e.g., 10% in ΔpflD) and paralyzed swarming.

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Evolutionary Insights: Ancient Origins and Co-options

Phylogenetic analyses (HMM searches, IQ-TREE on 120 markers) trace the E-ring/spokes to the last bacterial common ancestor, present in 66% of flagellated species but absent in β/γ-Proteobacteria. The cage is unique to Campylobacterota, exapted from type IV pilus (T4P) components (PilM/N/O/P/Q/F homologs in FcpMNO/PflD), co-evolving with F3 chemosensory systems but not T4P itself.78

This challenges E. coli-centric views, showing complex motors as ancestral with simplifications in models. No dynamic stator exchange; fixed arrays suit stable niches.

Advanced Methods Powering the Discovery

Cryo-electron tomography (cryo-ET) on Titan Krios/Glacios achieved ~12 Å resolution via subtomogram averaging (i3 package). Single-particle cryo-EM refined PflA-PflB. AlphaFold3 modeled uncrystallized proteins; co-IP, MST, bacterial two-hybrid validated interactions; RNA-seq/qPCR assessed regulation.78 Funded by NSFC grants 32470031/32370189, this integrates CAS's expertise in marine microbiology.

Implications for Pathogenesis and Antibiotic Development

C. jejuni, causing 1.5 million US cases yearly, relies on polar flagella for gut colonization. Scaffolds enable viscous mucus traversal; disrupting Pfl/FlgY/Fcp could immobilize without resistance, as stators differ from human ion channels. Biotech applications: biomimetic motors for nanorobots, inspired by BFM's efficiency.PubMed abstract

In China, rising campylobacteriosis prompts such research; ties to UCAS training programs in structural biology.

China's Leadership in Flagellar Research

CAS Guangzhou's SCSIO, with UCAS and Shandong University, exemplifies China's investment in cryo-EM facilities. Lead author Xueyin Feng (PhD UCAS) highlights interdisciplinary teams. Prior CAS works (e.g., 2023 stator torque, 2022 switching) build momentum.40 For aspiring researchers, explore research jobs or postdoc positions at Chinese institutions via AcademicJobs.com.

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Future Outlook and Research Opportunities

Dynamic imaging (time-resolved cryo-ET) could visualize assembly live; comparative studies across Campylobacterota test exaptation. Implications for synthetic biology: engineer high-torque motors. In higher education, this spurs career advice for microbiologists. Chinese universities seek talent in microbial tech—check China higher ed jobs, university jobs, and faculty roles. Engage further at Rate My Professor.