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 domai n, structurally homologous to the voltage-gated K+ channels, and a cytoplas mic ligand-binding domain. These two modules are joined by similar to 90 co nserved amino acids, the Clinker, whose precise role in the mechanism of ch annel activation by cyclic nucleotides is poorly understood. We examined cy clic nucleotide-gated channels from bovine photoreceptors and Caenorhabditi s 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 Clinker (the L2 region) a s an important determinant of activation properties. An increase in both th e efficacy of gating and apparent affinity for cGMP and cAMP can be conferr ed onto the photoreceptor channel by the replacement of its L2 region with that of the C. elegans channel. Three residues within this region largely a ccount for this effect. Despite the profound effect of the C-linker region on ligand gating, the identity of the Clinker does not affect the spontaneo us, ligand-independent open probability. Based on a cyclic allosteric model of activation, we propose that the Clinker couples the opening reaction in the transmembrane core region to the enhancement of the affinity of the op en channel for agonist, which underlies ligand gating.