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A cyclic nucleotide-gated ion channel is any ion channel that opens in the presence of cyclic nucleotides.

Contents 1 Mechanism 2 Functions 3 Examples 3.1 Alpha 3.2 Hyperpolarization-activated 4 References 5 External links

The channels are gated by a chemical ligand (the cyclic nucleotide) but they are more similar in structure to the family of voltage-gated ion channels than to the ligand gated ones. In fact, the cyclic nucleotide-gated channels often have positively or negatively charged areas that may respond to changes in membrane potential. The purpose and function of these charged areas are not yet fully understood.

Cyclic nucleotide-gated channels are particularly important in several systems:

• The mammalian olfactory system.^[1]

• In the visual system, a cGMP (cyclic guanosine monophosphate) gated channel is found in the outer membrane of retinal photoreceptor cells. In response to high levels of cGMP, the channels are open and allow positively charged ions to flow into the cell, causing depolarization. This is the state of the cell in the dark (called the dark current), but a photon striking a photoreceptor in the cell causes a chain reaction that results in lower levels of cGMP and therefore hyperpolarization. Thus, these cells are actually more active in dark than in light.

• Funny currents have been associated with the HCN channels.^[2]

"Cyclic nucleotide gated channel alpha" subunits include CNGA1, CNGA2, CNGA3, and CNGA4. "Beta" subunits include CNGB1 and CNGB3.

Other examples include the following "hyperpolarization activated cyclic nucleotide-gated potassium channels" (HCN): HCN1, HCN2, HCN3, HCN4

1. ^ Jenkins PM, Hurd TW, Zhang L, et al (2006). "Ciliary targeting of olfactory CNG channels requires the CNGB1b subunit and the kinesin-2 motor protein, KIF17". Curr. Biol. 16 (12): 1211-6. doi:10.1016/j.cub.2006.04.034. PMID 16782012. 
2. ^ DiFrancesco D (2006). "Serious workings of the funny current". Prog. Biophys. Mol. Biol. 90 (1-3): 13-25. doi:10.1016/j.pbiomolbio.2005.05.001. PMID 15975637. 

• MeSH cyclic-nucleotide+gated+ion+channels
• Overview at wustl.edu

v • d • e Membrane transport protein: ion channels
Ca2+: Calcium channel	Voltage-dependent calcium channel (L-type/Cavα(1.1, 1.2, 1.3, 1.4), N-type, P-type/Cavα(2.1), Q-type, R-type, T-type, β-subunits (β1, β2, β4), γ-subunits (γ2) • Inositol triphosphate receptor • Ryanodine receptor • Cation channels of sperm • Two-pore channel
Na+: Sodium channel	Navα (1.1, 1.2, 1.4, 1.5, 1.7, 1.9) • Navβ (1, 3, 4) • Epithelial sodium channel
K+: Potassium channel	Voltage-gated (Kv(1.1, 1.2, 1.3, 1.4, 1.5, 2.1, 4.2, 4.3, 7.1, 7.2, 7.3, 7.4, 10.1, 11.1/hERG), Shaker gene, KCNE1 • Calcium-activated (BK channel, SK channel, SK3) • Inward-rectifier Kir (1.1, 2.1, 2.2, 2.3, 3.1, 3.2, 3.4, 4.1, 4.2, 6.1, 6.2)) • Tandem pore domain K2P (1, 2, 3, 4, 6, 9)
Cl-: Chloride channel	Cystic fibrosis transmembrane conductance regulator
Porin	Aquaporin (1, 2, 3, 4) • Voltage-dependent anion channel
Cations: TRP	TRPA (1) • TRPC (1, 2, 3, 4, 4AP, 5, 6, 7) • TRPM (1, 2, 3, 4, 5, 6, 7, 8) • TRPML (Mucolipin-1) • TRPP (1, 2)• TRPV (1, 2, 3, 4, 5, 6)
Other/general	Voltage-gated ion channel • Ligand-gated ion channel • Cyclic nucleotide-gated ion channel (α1, α3, β3, H1, H2, H4) • Stretch-activated ion channel