Channelrhodopsin-2

Channelrhodopsin-2 is a light-gated ion channel of the channelrhodopsin family. As such, Channelrhodopsin-2 (ChR2) contains not only a light-activated transduction mechanism, but the channel pore itself, which can pass cations like sodium, calcium, and a variety of other small cations such as potassium.

Mechanics
Channelrhodopsin-2 consists of a 7-transmembrane helix protein, as in many other rhodopsoins, but ChR2 has a covalently linked retinal. The peak absorbance of the Channelrhodopsin-2 retinal complex is about 460 nm. When the all-trans retinal complex absorbs light, it induces a conformational change, probably to 13-cis-retinal. This conformational change introduces a further conformational change in the transmembrane protein opening the pore, to at least 6A. The 13-cis-retinal naturally relaxes with time back to the all-trans-retinal which closes the pore, stopping the flow of ions.

The 7-transmembrane nature of Channelrhodopsin-2 is fairly rare to ion channels which usually consist of similar repeating parts.

Applications
This makes depolarization of excitable cells very fast, robust, and useful for bioengineering and neuroscience applications, including photostimulation of neurons for probing of neural circuits. Channelrhodopsin-2 and the yellow light-activated chloride pump halorhodopsin together enable multiple-color optical activation and silencing of neural activity. The C-terminal end of ChR2 extends well into the intracelluar space, whereas the N-terminal end consists of the 7-transmembrane section. As such, the C-terminus can be replaced by the green fluorescent protein (GFP).