Asymmetric architecture and adaptation of Treponema flagella

Spirochetes exhibit a distinctive corkscrew-like motility driven by periplasmic flagella that wrap around the cell body in a supercoiled configuration, yet the structural basis of this propulsion remains poorly understood. Here we combine cryo-electron microscopy, cryo-electron tomography, and genetic and biochemical analyses to determine the assembly and adaptation principles of the supercoiled flagellar filament in Treponema denticola, a major periodontal pathogen. Near-atomic structures reveal a glycosylated FlaB flagellin core encased by an asymmetric sheath. The major sheath protein FlaA forms the bulk of the sheath and mechanically couples to the core through defined interfaces required for efficient motility, whereas four minor sheath proteins (FlaA1, FlaA2, FlaAP1, and FlaAP2) assemble along the concave side of the filament to accommodate intrinsic curvature. Disruption of this asymmetric core–sheath organization compromises force transmission and impairs motility, establishing coordinated asymmetric assembly as a fundamental mechanism underlying spirochetal motility. Here, the authors present cryo-EM structures of the intact asymmetric core–sheath supercoiled flagellar filament in Treponema denticola, a major periodontal pathogen, revealing a mechanism underlying distinctive spirochetal morphology and motility.