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. Inhibition of CNG channels Studies have shown the differential inhibition of CNG channels by diacylglycerol (DAG) in Xenopus oocytes. DAG can be used as a closed state inhibitor. Using DAG to inhibit homomultimeric rod channels, or those that contain two or more identical peptide chains, was similar to the inhibition of native rod channels. Using DAG to inhibit the homomultimeric olfactory channels was not as effective, even with a high concentration of DAG. More than one molecule of DAG is needed to inhibit a channel effectively. DAG molecules position themselves to stabilize the close state of a CNG channel, by binding to the channel or by changing the interaction between the lipid bilayer of the cell membrane and the channel. Studies with chimeras rod and olfactory channels suggest that differences in DAG inhibition are due to differences in the attached loop of the transmembrane segment. Physiological significance Photoreceptors In the absence of light, cGMP binds to CNG channels in photoreceptors. This binding causes the channels to open, which allows sodium (Na+) and calcium (Ca2+) ions to flow into the cell causing the outer segment of the photoreceptor to depolarize. This depolarizing flow of ions is known as the dark current. When the retina of the eye detects light, a reaction