Light-gated ion channel

Overview
Light-gated ion channels are a group of transmembrane proteins that form ion channels; pores which open or close in response to light. Most light-gated ion channels have been synthesized in the laboratory for study, though one naturally occurring example, Channelrhodopsin, is currently known. There are, however, many known photoreceptor proteins, which act in a similar manner to light-gated ion channels but are generally G protein coupled receptors and not actually gated ion channels.

Synthetic Purpose
Other types of gated ion channels, ligand-gated and voltage-gated, have been synthesized with a light-gated component in an attempt to better understand their nature and properties. By the addition of a light-gated section, the kinetics and mechanisms of operation can be studied in depth. For example, the addition of a light-gated component allows for the introduction of many highly similar ligands to be introduced to the binding site of a ligand-gated ion channel to assist in the determination of the mechanism. In 1980, the first ion channel to be adapted for study with a light-gated mechanism was the nicotinic acetylcholine receptor. This receptor was well-known at the time, and so was aptly suited to adaptation, and allowed for a study of the kinetics as not allowed before.

Mechanism
Light-gated ion channels function in a similar manner to other gated ion channels. When exposed to a certain stimulus, a conformational change occurs in the transmembrane proteins, which constitute the pore. This conformational change then opens or closes the ion channel, allowing for the flow of ions according to their electrochemical gradient. In the specific case of light-gated ion channels, the transmembrane proteins are usually coupled with a molecule that acts as a photoswitch. Retinal is a good example of a molecular photoswitch and is found in the naturally-occurring Channelrhodopsins. The photoswitch absorbs a specific photon and changes its conformation, which in-turn, changes the conformation of the transmembrane proteins opening or closing the pore through which ions flow.

Examples
Examples of light-gated ion channels occur in both natural and synthetic environments. These include:

Naturally Occurring

 * Channelrhodopsins were the first discovered family of light-gated ion channels.
 * Channelrhodopsin-1
 * Channelrhodopsin-2

Synthetically Adapted

 * nicotinic acetylcholine receptor was the first ion channel to be synthetically adapted with a light-gated mechanism.
 * Many other, fully-synthetic, light-gated channels have produced as well.