Fig. 1: Flagellar assembly pathway.

The Salmonella flagellum is composed of the basal body, the hook, the hook–filament junction, the filament and the filament cap. Upon completion of basal body assembly, newly exported FlgE molecules polymerize into the hook structure with the help of the hook cap made of FlgD. When the hook reaches its mature length of about 55 nm, the hook cap is replaced by FlgK. FlgK and FlgL self-assemble at the hook tip in this order to form the junction structure. Then, FliD forms the filament cap at the tip of the junction and promotes the assembly of FliC into the filament. A type III protein export apparatus (fT3SS) is located at the flagellar base and transports flagellar building blocks from the cytoplasm to the distal end of the growing flagellar structure. The fT3SS sometimes secretes the FliK ruler to measure the hook length during hook assembly. When the hook reaches its mature length of about 55 nm, the fT3SS switches its substrate specificity, thereby terminating the export of hook-type proteins (FlgD, FlgE, and FliK) and initiating the export of filament-type proteins (FlgK, FlgL, FliD, and FliC). FlgN, FliT, and FliS act as flagellar type III export chaperones specific for FlgK and FlgL, FliD and FliC, respectively. OM outer membrane, PG peptidoglycan layer, CM cytoplasmic membrane.