Cyclic Nucleotide-Gated (CNG) channels belong to the superfamily of voltage-gated ion channels. Although permeable to various ions such as the monovalent Na+ and K+ ions, and the divalent Ca2+ ion, they are gated by the intracellular binding of the cyclic nucleotides cAMP and cGMP and not by voltage per se (CNGs bind preferably to cGMP).nSix channels form this subfamily: The A subunit (CNGA1-4) and the B subunit (CNGB1 and CNGB3). Functional entities are formed by the assembly of four subunits. Each subunit consists of six membrane spanning domains, where the fourth transmembrane domain is highly positively charged, a pore domain between transmembrane domains five and six, and intracellular N- and C-termini. The cyclic nucleotide binding domain is located in the C-terminal region and is responsible for the channel gating upon cyclic nucleotide binding
