In situ structures of periplasmic flagella reveal a distinct cytoplasmic ATPase complex inBorrelia 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.

FlhF regulates the number and configuration of periplasmic flagella in Borrelia burgdorferi

“…burgdorferi cells average 15 μm in length (Charon et al, 2009), which imposes a technical barrier to visualize flagellar assembly and arrangement in whole cells by using cryo-ET. The above and previous cryo-ET studies only focused on one part of B. burgdorferi cells, for example, cell tips (Charon et al, 2009;Qin et al, 2018;Xu, Raddi, Liu, Charon, & Li, 2011;Zhao et al, 2013). Due to this limitation, the configuration of PF and its interaction with the cell cylinder in the periplasmic space have yet to be clearly visualized in context of a whole cell.…”