Light-gated ion channel

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.^[1] Photoreceptor proteins, which act in a similar manner to light-gated ion channels 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.^[2] 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
- Volvox channelrhodopsin-1
- Anion-conducting channelrhodopsin

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.^[3]^[4]^[5]

References

↑ "Engineering Light-Gated Ion Channels" — Biochemistry, 45 (51), 15129 -15141, 2006
↑ "A covalently bound photoisomerizable agonist. Comparison with reversibly bound agonists at electrophorus electroplaques" — The Journal of General Physiology, Vol 75, 207-232
↑ "Photosensitive artificial membranes based on azobenzene and spirobenzopyran derivatives" — Tetrahedron Volume 50, Issue 14, 4 April 1994, Pages 4039-4070
↑ "Lipid-Mediated Light Activation of a Mechanosensitive Channel of Large Conductance" — Langmuir, 20 (17), 6985 -6987, 2004
↑ "Engineering of a light-gated potassium channel" - Science, Vol. 348 no. 6235 pp. 707-710, 2015

Membrane transport protein: ion channels (TC 1A)

Ca²⁺: Calcium channel

Ligand-gated	Inositol trisphosphate receptor 1 2 3 Ryanodine receptor 1 2 3

Voltage-gated	L-type/Ca_vα 1.1 1.2 1.3 1.4 N-type/Ca_vα2.2 P-type/Ca_vα 2.1 Q-type/Ca_vα2.1 R-type/Ca_vα2.3 T-type/Ca_vα 3.1 3.2 3.3 α₂δ-subunits 1 2 β-subunits β1 β2 β3 β4 γ-subunits γ1 γ2 γ3 γ4 Cation channels of sperm 1 2 3 4 Two-pore channel 1 2

Na⁺: Sodium channel

Constitutively active	Epithelial sodium channel α β γ δ

Proton-gated	Amiloride-sensitive cation channel 1 2 3 4

Voltage-gated	Na_vα 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 7A Na_vβ 1 2 3 4

K⁺: Potassium channel

Calcium-activated	BK channel α1 β1 β2 β3 β4 SK channel SK1 SK2 SK3 IK channel IK1 K_Ca 1.1 2.1 2.2 2.3 3.1 4.1 4.2 5.1

Inward-rectifier	K_ATP K_ir 1.1 2.1 2.2 2.3 2.4 2.6 GIRK/K_ir 3.1 3.2 3.3 3.4 K_ir 4.1 4.2 5.1 6.1 6.2 7.1

Tandem pore domain	K2P 1 2 3 4 5 6 7 9 10 12 13 15 16 17 18

Voltage-gated	K_vα1-6 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 2.1 2.2 3.1 3.2 3.3 3.4 4.1 4.2 4.3 5.1 6.1 6.2 6.3 6.4 K_vα7-12 7.1 7.2 7.3 7.4 7.5 8.1 8.2 9.1 9.2 9.3 10.1 10.2 11.1/hERG 11.2 11.3 12.1 12.2 12.3 K_vβ 1 2 3 KCNIP 1 2 3 4 minK/ISK minK/ISK-like MiRP 1 2 3 Shaker gene

Miscellaneous

Cl⁻: Chloride channel	Calcium-activated chloride channels Anoctamin ANO1 Bestrophin 1 2 Chloride Channel Accessory 1 2 3 4 CFTR CLCN 1 2 3 4 5 6 7 KA KB CLIC 1 2 3 4 5 6 L1 CLNS 1A 1B

H⁺: Proton channel	HVCN1

M⁺: CNG cation channel	α 1 2 3 4 β 1 2 3 HCN FC 1 2 3 4

M⁺: TRP cation channel	TRPA (1) TRPC 1 2 3 4 4AP 5 6 7 TRPM 1 2 3 4 5 6 7 8 TRPML 1 2 3 TRPN TRPP 1 2 TRPV 1 2 3 4 5 6

H₂O (+ solutes): Porin	Aquaporin 0 1 2 3 4 5 6 7 8 9 Voltage-dependent anion channel 1 2 3 General bacterial porin family

Cytoplasm: Gap junction	Connexin: A GJA1 GJA3 GJA4 GJA5 GJA8 GJA9 GJA10 B GJB1 GJB2 GJB3 GJB4 GJB5 GJB6 GJB7 C GJC1 GJC2 GJC3 D GJD2 GJD3 GJD4) Innexin

By gating mechanism

Ion channel class	Ligand-gated Light-gated Voltage-gated Stretch-activated

see also disorders

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