Ion channels that open and close in response to light

US chemists and molecular biologists have developed an ion channel that can be opened and closed by exposing it to light at different wavelengths. The work will allow scientists to study these pore-forming proteins in greater detail, and offers several possible applications.

The researchers, led by Dirk Trauner from the University of California, Berkeley, studied a type of ion channel known as an ionotropic receptor. These open in the presence of specific neurotransmitters, in this case glutamate, and are found at the junctions between nerve cells. 

They comprise an extracellular binding domain, shaped like a clam shell, attached to a transmembrane domain. A glutamate molecule binding with the clam-shell-shaped domain causes it to close shut, which in turn causes a conformational change in the transmembrane domain. This opens a channel through the cell membrane that ions, such as calcium, sodium and potassium, can pass through.

Trauner and his team linked a variant of glutamate to a long azobenzene molecule that changes shape on exposure to light of specific wavelength. They attached this compound, termed MAG, to the outside of the extracellular binding domain on an ionotropic receptor that responds to glutamate. 

Exposing MAG to light at a wavelength of 380nm caused the azobenzene molecule to bend round, allowing the glutamate variant to bind with the clam-shell-shaped domain, opening the ion channel. Exposing it to light at a wavelength of 500nm caused the asobenzene molecule to straighten up, removing the glutamate molecule and causing the channel to close.

Trauner is extending this technique to other receptors, such as other ion channels and G-protein coupled receptors, and expanding it into other areas. ’We are working on the development of light-sensitive retinal ganglion cells as a potential remedy for certain kinds of blindness,’ he told Chemistry World. ’In addition, I am very interested in manipulating nature’s molecular machines in such as fashion that they are not only controlled, but also powered, by light.’ Jon Evans