A motor protein in the inner ear plays an important role in amplifying sounds, allowing us to hear very soft noises and subtle differences in pitch. Now researchers from Tbingen in Germany have characterized its electromechanical properties in rats.

Once a sound hits the eardrum, it is transmitted to the inner ear, the cochlea, and its sensitive hair cells. Filaments on the tips of the so-called inner hair cells start to vibrate, and the cells convert this movement into electrical signals that allow us to hear sound. The outer hair cells, on the other hand, convert part of the electrical signal back into movement, thereby amplifying the incoming sound. And in response to a change in membrane voltage, they rapidly alter their length by up to 5 percent.

What mediates this behavior is a recently discovered motor molecule called prestin. This membrane protein contains a charged voltage sensor that moves in response to a change in the membrane potential and exerts a mechanical force on structural proteins inside the cell. Many prestin molecules combined lead to a change in cell length.

In todays issue of PNAS, the researchers show that upon electrical stimulation, prestin from rats indeed generates a small mechanical force. But it also works the other way round: when the scientists applied pressure to a cell containing prestin, a stronger electrical signal was necessary to trigger the change in length. What part of prestin forms the voltage sensor, though, remains unknown and will require studies of its three-dimensional shape.