Molecule of the Month: Flagellar Motor

In this 300th installment of the Molecule of the Month, we’ll explore newly revealed atomic details of one of the wonders of the biomolecular world. Flagellar motors have fascinated biologists for decades. They are huge molecular assemblies composed of about 20 types of proteins, which together form rotary motors. These motors are strong enough to turn long corkscrew-shaped flagella many times the length of the cell. The Salmonella motor shown here turns at a speed of about 18,000 revolutions per minute. Other bacteria have motors that spin even faster.

Remarkably, bacteria steer their swimming direction by reversing the rotation of their flagellar motors. Salmonella and Escherichia coli bacteria have about ten flagella scattered around the surface of the cell. When the motor turns counterclockwise, all of the flagella bundle together and power the cell forward in one direction. If, however, the cell decides that it’s not going the right direction, it switches the motor to the clockwise direction. This causes all the flagella to separate and the cell tumbles randomly in place. Then it switches the motor back to the counterclockwise direction, the flagella bundle up again, and the cell swims in a new, hopefully better, direction.

Flagellar motors have many moving parts that have been studied separately using X-ray crystallography and cryo-electron microscopy. This image shows the clockwise form of the motor, combining several PDB entries (8ucs, 8upl, 7cgo, 2zvy, 1f4v). At the top, the hook connects the motor to the long flagellum. The LP ring surrounds the central axle and reduces friction with the outer membrane and peptidoglycan. Power is generated by about 11 rotary motors, termed stators. They are themselves individual rotary motors driven by flow of hydrogen ions across the inner membrane. These small motors together turn the large C-ring, which turns the central axle and the flagellum. CheY proteins determine the direction of the rotation.

The motor switches rotation with a direct approach that is quite similar to machines in our macroscale world. The C-ring acts like a large gear and the stators act like smaller gears. In the counterclockwise state, shown here at the top from PDB ID 8uox and 8ucs, the stators are positioned outside the C-ring, and when the gears mesh the C-ring is driven in the opposite direction from the stator rotation. When CheY binds, the shape of the C-ring changes slightly and the stator binds on the inside of the ring (PDB ID 8upl, 8ucs, 1f4v). In that position, the C-ring is driven in the same direction as the rotation of the stator.