Cyclic nucleotide-gated (CNG) channels are ion channels which are activated by the binding of cGMP or cAMP ^[1]. Cyclic nucleotide gated channel contains two kind subunits, alpha-subunit (alpha 1, alpha 2, alpha 3, alpha 4) and beta-subunit (beta 1, beta 3).

A CNG channel consists of four subunits around a central pore. Each protein subunit consists 6 transmembrane segments (S1-S6), a P-loop, intracellular amino terminal region, and carboxy terminal region. The P-loop and S6 segments around the pore, which plays a role in ion conduction. There is a cyclic nucleotide binding domain (CNBD) and connection region to the S6 segment in the carboxy terminal. There is a post-CNDB region in the amino terminal ^[2].

The channel contains three CNGA3 and one CNGB3 subunits. Arg403 (yellow sphere) in the pore helix of CNGB3 projects into an asymmetric selectivity filter and forms hydrogen bonds with two pore-lining backbone carbonyl oxygens. Arg442 in S6 of CNGB3 protrudes into and occludes the pore below the hydrophobic cavity gate previously observed in homotetrameric CNGA channels. It is interesting that Arg403Gln is a disease mutation, and Arg442 is replaced by glutamine in some animal species with dichromatic or monochromatic vision ^[3].