C-Linker of Cyclic Nucleotide–gated Channels Controls Coupling of Ligand Binding to Channel Gating

Cyclic nucleotide–gated channels are composed of a core transmembrane domain, structurally homologous to the voltage-gated K⁺ channels, and a cytoplasmic ligand-binding domain. These two modules are joined by ∼90 conserved amino acids, the C-linker, whose precise role in the mechanism of channel activation by cyclic nucleotides is poorly understood. We examined cyclic nucleotide–gated channels from bovine photoreceptors and Caenorhabditis elegans sensory neurons that show marked differences in cyclic nucleotide efficacy and sensitivity. By constructing chimeras from these two channels, we identified a region of 30 amino acids in the C-linker (the L2 region) as an important determinant of activation properties. An increase in both the efficacy of gating and apparent affinity for cGMP and cAMP can be conferred onto the photoreceptor channel by the replacement of its L2 region with that of the C. elegans channel. Three residues within this region largely account for this effect. Despite the profound effect of the C-linker region on ligand gating, the identity of the C-linker does not affect the spontaneous, ligand-independent open probability. Based on a cyclic allosteric model of activation, we propose that the C-linker couples the opening reaction in the transmembrane core region to the enhancement of the affinity of the open channel for agonist, which underlies ligand gating.