Ill. Scientists have learned a tiny molecular clutch disengages the flagellum’s tail from the engine that powers its rotation.
The action of the protein they discovered, EpsE, is very similar to that of a car clutch. In cars, the clutch controls whether a car’s engine is connected to the parts that spin its wheels. With the engine and gears disengaged from each other, the car may continue to move, but only because of its prior momentum; the wheels are no longer powered.
EpsE is thought to “sit down,” as Kearns describes it, on the flagellum’s rotor, a donut-shaped structure at the base of the flagellum. EpsE’s interaction with a rotor protein called FliG causes a shape change in the rotor that disengages it from the flagellum’s proton-powered engine.