biostudies-literatureUnknownMartin FSNIAID NIH HHSNIGMS NIH HHSSpirochete pathogens are among the most invasive bacteria known, causing syphilis, Lyme disease, and leptospirosis. Their tissue penetration depends on periplasmic flagellar filaments that, unlike other bacterial flagella, are encased in a spirochete-specific multi-protein sheath and deform the cell body into motile waves. How these filaments achieve the mechanical properties needed for invasive motility has remained unclear. Here we determine complete atomic structures of the <i>Leptospira</i> endoflagellar filament, revealing an elaborate sheath of 9 to 12 distinct asymmetrically arranged proteins. We show that the flagellin variant forming the filament core determines sheath composition, producing curvatures ranging from ∼3.5 µm ^-1 to ∼5 µm ^-1 . The lower-curvaturebioRxiv : the preprint server for biology2026.02.02.703089https://www.ebi.ac.uk/biostudies/studies/S-EPMC12889604biostudies-literatureCore-sheath coupling controls flagellar curvature and motility in &lt;i&gt;Leptospira&lt;/i&gt;.PMC12889604T32 GM008283R01 AI052473P01 AI168148R21 AI163663R01 AI182354Mondino SRey MSindelar CVFule LNouchikian LDuran RChamot-Rooke JTrajtenberg FMartin FSBrady MRRodriguez APicardeau MBuschiazzo AKo AILarrieux NWunder EAfalseCore-sheath coupling controls flagellar curvature and motility in &lt;i&gt;Leptospira&lt;/i&gt;.Spirochete pathogens are among the most invasive bacteria known, causing syphilis, Lyme disease, and leptospirosis. Their tissue penetration depends on periplasmic flagellar filaments that, unlike other bacterial flagella, are encased in a spirochete-specific multi-protein sheath and deform the cell body into motile waves. How these filaments achieve the mechanical properties needed for invasive motility has remained unclear. Here we determine complete atomic structures of the <i>Leptospira</i> endoflagellar filament, revealing an elaborate sheath of 9 to 12 distinct asymmetrically arranged proteins. We show that the flagellin variant forming the filament core determines sheath composition, producing curvatures ranging from ∼3.5 µm ^-1 to ∼5 µm ^-1 . The lower-curvature2026-01-01T00:00:00Z2026 Feb2026-07-12T03:14:27.728Z2026-07-12T03:09:13.113ZS-EPMC128896044167652210.64898/2026.02.02.703089