Bacterial Flagellar Motor

7CGO

Tan et al., 2021 (PDB 7CGO) — cryo-EM structure of the Salmonella flagellar motor–hook complex.

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Reaction cycle

1Run · Stator units pump ions across the membrane, driving the rotor — CCW rotation sends the cell on a smooth run

Moving parts

Rotor — MS-ring, rod & hook (one rigid drive-shaft)

How it works

The bacterial flagellar motor is a rotary nanomachine that spins a corkscrew filament to swim the cell — the only true wheel-and-axle in biology. Unlike ATP synthase it burns no ATP: ion flux (H⁺ or Na⁺) through ring-shaped stator units in the membrane applies torque to the rotor, driving it at up to ~1000 revolutions per second and reversing it to switch the cell between smooth runs and tumbles. This cryo-EM structure resolves the motor–hook complex: the FliF MS-ring embedded in the inner membrane, the rod (FlgB/C/F/G) that acts as the drive shaft, and the flexible FlgE hook — a universal joint that transmits the spin to the filament. The whole rod–ring–hook column turns as one rigid body; here it rotates continuously through the fixed membrane (the C-ring and stators that power it are not resolved in this map). At 335,000 atoms, the motor is deposited only as mmCIF — far too large for the legacy PDB format.