Figure 5.

Sequential flagellar assembly process revealed in B. burgdorferi.⁵⁰ In the pre-T3S assembly state, most flagellar motor components except for the flagellar rod have been assembled, which includes the MS ring, the C ring, the stators, the export apparatus, and the unique periplasmic structure collar. The secretion channel in the MS ring is closed (first panel). The exportation of rod substrates opens the channel, and proximal rod substrates [FliE, FlgB, FlgC, and FlhO (FlgF homologue)] cooperatively assemble into a stable proximal rod structure (second panel). The distal rod protein FlgG adds onto the proximal rod and polymerizes until it reaches a determined length. With the completion of rod assembly, the hook cap is exported and the P ring is assembled around the rod (third panel). The hook assembly is promoted by a hook cap.⁸³ With the completion of hook assembly, the hook–filament junction and filament cap are exported (fourth panel). The filament assembly is promoted by the filament cap (fifth panel).⁸²