Cyclic Nucleotide-gated Ion Channel - Ligand Selectivity

Ligand Selectivity

By measuring the currents activated in excised inside-out membrane patches upon superfusion with varying ligand concentrations, the ligand sensitivity and selectivity of both endogenously and exogenously expressed CNG channels have been studied. All native CNG channels react to both cAMP and cGMP, but smaller concentrations of cGMP than of cAMP are needed to activate and open the channels. CNG channels are sharply selective between cGMP and cAMP in rods and cones, whereas in OSNs, the channels respond equally well to both ligands. CNG channels found in OSNs are much more sensitive to both cGMP and cAMP than photoreceptor CNG channels. Studies of dose response relations showed that channel activation is greatly dependent on cGMP concentration; several cGMP molecules bind to the channel in a cooperative manner. Since each subunit contains a single cNMP-binding site, and homomeric and heteromeric channels most likely form a tetrameric complex, a maximum of four ligand molecules can bind to the channel. Selectivity can be achieved by differential control of the affinity for binding of the ligand, efficacy of gating, or a combination of both. Binding affinity means how tightly cyclic nucleotides bind to the channel. Efficacy refers to the ability of ligand to activate and open the channel once it is bound. Although these processes are useful in understanding selectivity, they are inextricably coupled to each other that it is very difficult to experimentally separate one from another.

CNG channels do not discriminate between Na+ and K+ ions, and they also allow Ca2+ and Mg2+ to pass through, although at a slower rate. The passage of these divalent ions inhibit the current carried by Na+ and K+. A highly conserved residue of glutamic acid in the selectivity filter of CNG channels has been found to form a high-affinity binding site for Ca2+. Moreover, a bacterial nonselective cation channel called the NaK channel hosts a selectivity filter sequence similar to that of CNG channels. In the crystal structure of the NaK channel, a discrete Ca2+-binding site at the extracellular opening of the pore has been identified.