Molecular Rearrangements in the Ligand-Binding Domain of Cyclic Nucleotide–Gated Channels

Cyclic nucleotide–gated (CNG) channels are activated in response to the direct binding of cyclic nucleotides to an intracellular domain. This domain is thought to contain a β roll and two α helices, designated the B and C helices. To probe the conformational changes occurring in the ligand-binding domain during channel activation, we used the substituted cysteine accessibility method (SCAM). We found that a residue in the β roll, C505, is more accessible in unliganded channels than in liganded channels, whereas a residue in the C helix, G597C, is more accessible in closed channels than in open channels. These results support a molecular mechanism for channel activation in which the ligand initially binds to the β roll, followed by an opening allosteric transition involving the relative movement of the C helix toward the β roll.