Bacterial cellular membranes protrude to form flagella, which are composed of up to about 30 distinct proteins and are essential for motility. The bacterial flagellar type III export apparatuses utilize energy derived from ATP hydrolysis and proton motive force (PMF) across the cell membrane to translocate flagellar proteins across the membrane. Previous studies have demonstrated that the flagellar transmembrane export gates play crucial roles in energy transduction to unfold and transport flagellar proteins for construction of the flagella in a PMF-powered manner. However, the energy transduction mechanisms have not been completely understood.