Integrative Structural Biology of Spirochetal Endoflagella: compositional complexity and architectural asymmetry determine filament curvature and functional integrity

Spirochete pathogens cause syphilis, Lyme disease and leptospirosis, ranking among the most invasive bacteria known. Their tissue-penetrating motility relies on periplasmic flagellar filaments that, unlike the extracellular flagella of other bacteria, are encased in a multi-protein sheath and deform the cell body into motile waves. How these filaments achieve the mechanical properties required for invasive motility has remained unclear. Integrating cryo-electron microscopy, X-ray crystallography and mass spectrometry, we determined complete atomic structures of the Leptospira endoflagellar filament from wild-type and mutant strains of L. biflexa and L. interrogans. The structures reveal an elaborate sheath of 9 to 12 distinct, asymmetrically arranged proteins surrounding a flagellin (FlaB) core. The core flagellin variant determines sheath composition, producing curvatures ranging from ~3.5 to ~5 μm^-1. The lower-curvature architecture of pathogenic L. interrogans proves essential for motility in viscous environments and during infection. We also report the crystal structure of FlaA1, the first structure of a FlaA protein, a family universal to spirochetes. FlaA1 adopts a Carbohydrate Binding Module fold and coordinates a calcium ion structuring a β-hairpin loop likely involved in carbohydrate binding. These results establish that Leptospira achieves environment-specific motility through modular core–sheath coupling, linking atomic-scale structural plasticity to swimming behavior. Conservation of key sheath components suggests this mechanism may extend across the Spirochete phylum.