Modular architecture and assembly blueprint of bacterial flagella. (A) A model of flagellar assembly in B. burgdorferi. In the pre-T3S assembly state, many flagellar components assemble, including the MS ring, the C ring, the stators, the export apparatus, and the collar. The secretion channel in the MS ring is closed (first panel). In the presence of FliE and FlgB, rod substrates can be secreted but are unable to form a stable structure (second panel) until all of the proximal rod substrates (FliE, FlgB, FlgC, and FlhO) are present (third panel). The distal rod protein FlgG adds onto the proximal rod and polymerizes until it reaches a determined length (fourth panel). A hook cap composed of FlgD forms at the distal end of the rod (fourth panel) and promotes hook assembly (fifth panel). Assembly of the filament (FlaA and FlaB) is promoted by the filament cap (FliD) (sixth panel). (B) A cartoon model of a PF. Five rod proteins assemble sequentially on top of the channel domain of the MS ring and are enclosed by the socket domain of the MS ring and the P ring. The FlgG distal rod in the PF is shorter than that in the external flagellum (C). (D) A cartoon model of type III injectisome shows that the rod is anchored on a structure similar to the channel domain of the flagellar motor. The rod is a straight tube formed by one protein, PrgJ.