Cryo-electron tomography of periplasmic flagella in Borrelia burgdorferi reveals a distinct cytoplasmic ATPase complex

Periplasmic flagella are essential for the distinct morphology and motility of spirochetes. A flagella-specific type III secretion system (fT3SS) composed of a membrane-bound export apparatus and a cytosolic ATPase complex is responsible for the assembly of the periplasmic flagella. Here, we deployed cryo-electron tomography (cryo-ET) to visualize the fT3SS machine in the Lyme disease spirochete Borrelia burgdorferi. We show, for the first time, that the cytosolic ATPase complex is attached to the flagellar C-ring through multiple spokes to form the “spoke and hub” structure in B. burgdorferi. This structure not only strengthens structural rigidity of the round-shaped C-ring but also appears to rotate with the C-ring. Our studies provide structural insights into the unique mechanisms underlying assembly and rotation of the periplasmic flagella and may provide the basis for the development of novel therapeutic strategies against several pathogenic spirochetes.

周质鞭毛（periplasmic flagella）对于螺旋体独特的形态与运动能力至关重要。由膜结合输出装置与胞质ATP酶复合物构成的鞭毛特异性III型分泌系统（flagella-specific type III secretion system，fT3SS）负责周质鞭毛的组装。本研究利用冷冻电子断层扫描（cryo-electron tomography，cryo-ET）技术，对莱姆病致病菌伯氏疏螺旋体（Borrelia burgdorferi）中的fT3SS机器进行可视化成像。本研究首次证实，在伯氏疏螺旋体中，胞质ATP酶复合物通过多条辐条连接至鞭毛C环，共同形成“辐毂”结构。该结构不仅增强了圆形C环的结构刚性，还可随C环一同旋转。本研究为周质鞭毛组装与旋转的独特机制提供了结构层面的解析，同时可为针对多种致病性螺旋体的新型治疗策略开发奠定基础。