Molecular Motor: What's Shakin' in Ears?

Researchers have isolated a protein that makes cells stretch and contract when zapped with electricity. Dubbed prestin, the protein may be the long-sought molecular motor that powers the amplifiers in the ears of mammals.

Jimi Hendrix took the phenomenon to an ear-splitting extreme, but feedback is essential for hearing in mammals. Within the ear, a long, narrow membrane, called the basilar membrane, vibrates in response to sounds. Low tones elicit ripples closer to one end, and high tones raise a ruckus nearer the other. In mammals, the membrane is carpeted with cylindrical cells called hair cells. Some of these, known as outer hair cells, push the membrane up and down in sync with the vibration, amplifying the sounds 1000-fold and sharpening the ear's ability to distinguish pitch.

No one knows exactly how the outer hair cells push and pull the membrane, but researchers discovered 15 years ago that the cells lengthen and contract when jolted with electricity, creating more jolts in the process. So according to one theory, the cells act like little pistons to drive the membrane up and down. Neurobiologists Peter Dallos, Jing Zheng, and their team at Northwestern University in Evanston, Illinois, searched for a substance that might power these pistons.

The researchers looked for genes turned on in outer hair cells, but turned off in inner hair cells, which always stay the same length. Once they found a prime suspect, the researchers added the gene to human kidney cells grown in the lab. The modified cells started behaving like outer hair cells: Small voltages caused current to flow in and out of them in the same characteristic way. And when the researchers squished the cells to make them cylindrical, electricity caused the cells to lengthen and contract just like outer hair cells. Prestin, the protein made by the gene, was the molecular motor they sought, the team concludes in the 11 May issue of Nature.

The discovery opens a new phase in the study of mammalian hearing, says Bill Brownell, a biophysicist at Baylor College of Medicine in Houston who discovered the electrical response of outer hair cells. "We still don't know how the protein works," he says. "That's the next big step." But neurobiologist David Corey of Harvard Medical School in Boston thinks that's getting ahead of the game. Another theory holds that the feedback is driven by small fibers that wiggle on the tops of the outer hair cells, he says, and the debate "is definitely not settled."