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

Abstract

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 spirochetes 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. ImportanceSpirochetal 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.