Subunit stoichiometry of CNG channels in photoreceptors and olfactory neurons.

The dark current of photoreceptors. CNG channels that are kept open in the dark by the relatively high concentration of cGMP allowing an inward current, carried by Na⁺, and to a lesser extent Ca²⁺. The steady inward current is balanced by a steady outward current through K⁺ channels in the inner segment, completing the circuit. Ion concentration gradients are maintained by the Na⁺/Ca²⁺‐K⁺ exchanger in the outer segment and the Na⁺/K⁺ ATPase active transporter in the inner segment.

Membrane topology of CNG channels. For simplicity, this figure shows two of the four subunits comprising CNG channels. Similarly to voltage‐gated K⁺ channels, CNG channels are tetramers where each subunit contains six transmembrane domains, intracellular N‐ and C‐termini, positive charges in the fourth transmembrane domain (S4), and a P‐loop lining the pore of the channel. In addition, CNG channels contain a cyclic nucleotide‐binding domain in the C‐terminus of each subunit, connected to the transmembrane domain by a region called the C‐linker. The P‐loop, lining the pore, is shown in red, and the CNBD is shown in blue with cGMP bound.

Structures of domains of related ion channels. (a) Structure of the NaK selectivity filter (PDB: 2AHY). Two Na⁺‐binding sites are shown in purple and one Ca²⁺‐binding site is shown in green. D66 is important for Ca²⁺ binding in NaK, as the homologous aspartate in CNG channels. However, D66 does not directly coordinate the Ca²⁺ ion, as had been previously predicted. Rather, the backbone carbonyl groups from G67 stabilize the Ca²⁺ ion and D66 indirectly stabilizes the positioning of G67. (b) Structure of the cytoplasmic domain from a single subunit of HCN2 channels (PDB: 1Q5O). The C‐linker, shown at the top, contains six α‐helices separated by short loops. The cyclic nucleotide‐binding domain contains four α‐helices with a β‐roll separating the first two α‐helices. cAMP is shown bound between the β‐roll and the final α‐helix, called the C helix. Although only one subunit is shown, this domain folds as a stable tetramer with intersubunit contacts between the C‐linkers. (c) Structure of the P‐loop and S6 of MlotiK1 (PDB: 3BEH). Residues spacefilled in blue represent residues blocking the permeation pathway in the structure. Residues spacefilled in red represent residues in the selectivity filter accessible to cysteine modification in closed CNG channels.

Model of CNG channel gating. Cartoon illustrating conformational changes in response to ligand binding and channel opening. Although CNG channels are tetrameric proteins with 4‐fold symmetry, only 2 subunits are illustrated for simplicity. In the closed, unliganded state, salt bridges hold the C‐linkers of multiple subunits together. The C helices (blue cylinders) from multiple subunits are in close proximity in the closed state. Cyclic nucleotides initially bind by interacting with residues in the β‐roll of the cyclic nucleotide‐binding domain. This triggers a conformational change leading to channel opening. This conformational change involves a relative movement of the C helices away from each other and towards the β‐rolls, and destabilization of the tetramerization of the C‐linkers. The details of this model remain to be determined.