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

UNLABELLED: 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 spirochete's 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.

IMPORTANCE: Spirochetal motility requires the stable engagement of torque-generating stator units, yet the mechanisms that secure these complexes have remained unknown. Here, we identify FlcE as a previously unrecognized collar protein that functions as a molecular latch by restraining the FliL-stator complex. Loss of FlcE destabilizes FliL-stator assemblies and severely impairs motility, demonstrating that peripheral scaffolding is essential for sustaining high torque output in the Lyme disease spirochete Borrelia burgdorferi . The conservation of tetratricopeptide repeat domains in FlcE and other bacterial flagellar proteins further suggests a broader mechanism in which specialized scaffolds anchor and stabilize stator complexes, enabling high torque generation. Because motility is a central virulence determinant in many pathogenic bacteria-including B. burgdorferi during transmission from the tick to the mammalian host and subsequent infection-defining the mechanistic role of FlcE provides important insight into the molecular basis of motility-driven pathogenesis and suggests new strategies for disrupting bacterial infection.