Fig. 1: Schematic diagram of the flagellar protein export apparatus.

The flagellar protein export apparatus is composed of a transmembrane export gate complex made of FlhA, FlhB, FliP, FliQ, and FliR and a cytoplasmic ATPase ring complex consisting of FliH, FliI, and FliJ. The export gate complex is located inside the MS ring and utilizes proton motive force (PMF) across the cytoplasmic membrane (CM) to drive proton (H⁺)-coupled flagellar protein export. FliP, FliQ, and FliR form a polypeptide channel complex. The N-terminal transmembrane domain of FlhB (FlhB_TM) associates with the FliP/FliQ/FliR complex, and its C-terminal cytoplasmic domain (FlhB_C) projects into the central cavity of the C ring. FlhA forms a homo-nonamer through interactions between the C-terminal cytoplasmic domain of FlhA (FlhA_C), and its N-terminal transmembrane domain (FlhA_TM) acts as a transmembrane H⁺ channel. The cytoplasmic ATPase ring complex associates with the C ring through an interaction between FliH and a C ring protein, FliN. ATP hydrolysis by the FliI ATPase activates the export gate complex through an interaction between FliJ and FlhA_C, allowing the gate complex to become an active protein transporter to couple the proton flow through the FlhA proton channel to the translocation of export substrates into the polypeptide channel of the FliP/FliQ/FliR complex. The FliH₂FliI complex is thought to act as a dynamic carrier to bring export substrates from the cytoplasm to the export gate complex.