FlcE latches onto the FliL-stator complex to turbocharge flagellar motility in Borrelia burgdorferi

Periplasmic flagella are essential for the distinctive morphology and motility of the Lyme disease spirochete Borrelia burgdorferi, and motility plays a critical role in its pathogenic lifestyle. These flagella are powered by specialized motors that contain a large spirochete-specific multiprotein collar complex, yet the molecular architecture and mechanisms underlying high-torque motility remain poorly understood. Here, we identify the tetratricopeptide repeat (TPR)-containing protein BB0298 as a previously unrecognized flagellar collar component and rename it FlcE. Loss of flcE results in altered morphology and nearly abolishes the bacterial motility. Using cryo-electron tomography and biochemical analyses, we show that FlcE occupies a unique position within the collar where it surrounds the FliL-stator complex by binding to FliL and the collar protein FlcA. These findings support a model in which FlcE functions as a molecular latch that secures stator assemblies to sustain efficient torque generation. Notably, flcE is conserved across most motile members of the Spirochaetales and is located within the division and cell-wall (dcw) gene cluster. More broadly, the conservation of TPR-containing structural proteins across diverse bacterial flagellar systems suggests a general architectural principle whereby dedicated scaffolds reinforce stator complexes to maximize motor performance under high mechanical loads