Cyclic nucleotide-gated channels: shedding light on the opening of a channel pore

The term 'gating' refers to the allosteric transition that opens and closes the pore of an ion channel. Channel gating has been the focus of intense investigation, but its structural basis remains elusive. The crystal structure of KcsA, a bacterial potassium channel, has provided a framework for new studies of gating in many channel proteins, including cyclic nucleotide-gated (CNG) channels, the focus of this review. The sequence of CNG channels is similar to that of KcsA in the region around the pore domain, having a pore helix, a selectivity filter and an inner helix. Site-directed cysteine substitutions at the presumptive pore helix of CNG1 have provided evidence for the rotation of this helix during gating. Similarly, kinetic analysis and studies with channel blockers have provided indirect evidence for movement of the selectivity filter during gating. In the case of the inner helix, a conformational change in this region also seems to occur during channel gating, as illustrated by the spontaneous formation of disulphide bridges between the inner helices of different CNG subunits when the channel is closed, but not when it is open. The linker between the inner helix and the intracellular cyclic nucleotide-binding domain is crucial for the allosteric coupling between ligand binding and channel opening. It has been found that histidine residues that are present in part of the linker region are capable of coordinating Ni2+ ions between subunits, indicating their spatial proximity. Histidine-substitution experiments show that this region of the linker rotates during gating. On the basis of these and other observations, a new structural model for CNG channel gating is emerging. Opening of the channel involves a clockwise rotation of the distal portion of the linker segment. This rigid body movement unwinds the helical bundle at the bottom of the inner helix, leading to a significant increase in the diameter of the pore. The movement of the inner helix then initiates rearrangements in a gate that is presumably located in the selectivity filter.