In situ structures of periplasmic flagella reveal a distinct cytoplasmic ATPase complex in Borrelia burgdorferi

Abstract

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 combine cryo-electron tomography and mutagenesis approaches to characterize the fT3SS machine in the Lyme disease spirochete Borrelia burgdorferi. We define the fT3SS machine by systematically characterizing mutants lacking key component genes. We discover that a distinct cytosolic ATPase complex is attached to the flagellar C-ring through multiple spoke-like linkers. The ATPase complex not only strengthens structural rigidity of the C-ring, but also undergoes conformational changes in concert with flagellar rotation. Our studies provide structural framework to uncover the unique mechanisms underlying assembly and rotation of the periplasmic flagella and may provide the bases for the development of novel therapeutic strategies against several pathogenic spirochetes.