Model of the structure and gating motions of an HCN channel based on high affinity metal bridges. (A) Stereo view of a model of the cAMP-bound closed state of the spHCN channel, based on the crystal structures of KcsA (Protein Data Bank accession no. 1K4C) and the cAMP-bound CNBD of spHCN (Protein Data Bank accession no. 2PTM). Residue D471 of spHCN is in approximately the same position as KcsA-118, based on the equivalence of spHCN-468 to KcsA-115 (Rothberg et al., 2003). The uncoiled strands between the S6 helices and the A′ helices include the spHCN residues ⁴⁷²SSS⁴⁷⁴. (B) Stereo view of the locations of C-linker residues that when mutated to cysteine form metal bridges with 364C to produce lock-open (green spheres) and lock-closed (red spheres) effects. Spheres of ∼6-Å diameter are centered around the β carbon of each C-linker residue involved in bridges with 364C. (C) Model of possible motions occurring during HCN channel gating. A view from the extracellular side of the channel, showing the locations of lock-open (green) and lock-closed (red) effects with 364C, superimposed on the closed-state model. Cylinders represent each S4–S5 linker, and asterisks indicate the positions of 364C residues. The proposed motions of 364C residues (dashed arrows) and the lower ends of the S6 helices (solid arrows) during channel opening are indicated. The color of the numbers for the C-linker residues studied matches the ribbon color for the corresponding subunit.