Figure 1

Export switching mechanism of the flagellar type III protein export apparatus. The flagellar type III protein export apparatus, which is composed of a transmembrane export gate complex (indicated as Export gate) and a cytoplasmic ATPase ring complex consisting of FliH, FliI and FliJ, transport the hook protein FlgE during hook assembly (left panel). FliK is also exported during hook assembly and acts as a ruler to measure the hook length. When the hook length reaches to about 55 nm, the C-terminal domain of FliK binds to the C-terminal domain of a transmembrane export gate protein FlhB (FlhB_C), switching export specificity of the export apparatus (middle panel). As a result, the export apparatus terminates the export of FlgE and FliK and initiates the export of filament-type export substrates such as FlgK, FlgL, FliC and FliD (right panel). The C-terminal cytoplasmic domain of FlhA (FlhA_C) provides binding sites for the filament-type substrates in complex with their cognate export chaperones.