Follow the gold ball. Attaching a bead to ATP synthase allowed researchers to track how its motor ratchets.

Cellular Motor Caught In the Act

A team of researchers has snapped some of the most detailed moving pictures ever of a key molecular motor. The work should yield insights into how one of nature's most "splendid machines" builds the energy molecule ATP.

ATP, or adenosine triphosphate, is the main source of energy for cellular processes in bacteria, plants, and animals. In muscles, for example, ATP supplies the energy necessary for contraction. ATP is made by the enzyme ATP synthase, which uses a rotory motor to construct the molecule out of adenosine diphosphate (ADP) plus inorganic phosphate.

The motor is made up of a central rodlike shaft surrounded by a cylinder. When making ATP, the motor rotates in one direction; when breaking it down to make energy, the motor rotates in reverse. In earlier research by Ryohei Yasuda and colleagues at Keio University in Yokohama, Japan, the motor appeared to crank around in 120-degree jerks. But the filament they attached to the shaft in order to track the rotation put drag on the system; that obscured the relationship between the chemical reaction and the workings of the motor's subunits.

To overcome this limitation, Yasuda's team substituted a 40-nanometer-diameter gold bead for the clumsy filament. The bead is attached to the tip of the central shaft obliquely, so that during rotation it traces a circular path. Laser imaging allowed them to capture 8000 images per second of the rotating bead. And this clearly showed that each 120-degree step is actually made up of two movements during the "burning" of ATP. The shaft first turns 90 degrees and rotates the remaining 30 degrees, with each step taking only a fraction of a millisecond, the team reports in the 19 April issue of Nature. Furthermore, by carefully controlling the supply of ATP, the researchers demonstrated that the 90-degree turn happens as ATP binds to the cylinder. The second motion is probably driven by the release of the resulting ADP and phosphate products.

The finding "gives new insight as to how the actual products [of the reaction] interact with the molecule," says Paul Boyer, professor emeritus of biochemistry at the University of California, Los Angeles, who won the 1997 Nobel Prize in Chemistry for his work on ATP. "It's a nice step forward," he adds.