Interactions Between The C-Linker And The S4-S5 Linker Mediate Gating In Cnga1 Channels

Cyclic Nucleotide Gated (CNG) channels underlie sensory transduction in photoreceptors and olfactory epithelium.They belong to the family of voltage-gated ion channels, but open following binding of cyclic nucleotides (CNs).While electrophysiology combined with mutagenesis have identified the channel pore and the binding domain for CNs, conformational changes associated with gating have remained elusive. In this study we combine electrophysiology, site-directed mutagenesis, homology modelling and single molecule force spectroscopy (SMFS) to determine the molecular mechanisms underlying gating. Gating is mediated by electrostatic interactions between aspartate D413 in the C-linker and arginine R297 in the S4-S5 linker. Indeed when residues in position 413 and 297 are cross-linked, the mutant channels are locked in the open state. These electrostatic interactions induce an upward motion combined with a small rotation of the transmembrane α-helix S5 leading to conformational rearrangements of residues flanking the pore so that the lumen of the pore widens and the channel opens. Therefore gating of CNG channels is not mediated by a motion of the transmembrane α-helix S6 as in usual voltage gated Na+,K+ and Ca2+ channels, but by interactions between the cytoplasmic domain and the S4-S5 linker, possibly shared also by HCN channels (Decher et al., 2004).