Discovery and Characterization of a Distinct Cyclic Nucleotide Binding Pocket in HCN Channels

Abstract

Direct binding of cAMP to the cardiac pacemaker channels mediates largely the autonomic regulation of the heart rate. Binding of cAMP to the canonical binding site in the CNBD of HCN channels, promotes quaternary changes in the C termini that, in turn, affects the transmembrane domain of the channel facilitating pore opening. In the crystal structure of the C-linker + CNBD portion of HCN4, we found a second and yet unknown binding pocket that can be occupied by cyclic nucleotides. Occupancy of this pocket prevents cAMP modulation of the current in HCN4 and in the native current If, leading to heart rate reduction in mouse SAN myocytes. Compound 11, a structurally unrelated molecule selected by docking, reproduces this effect with higher affinity and pinpoints this pocket as a promising drug target. We are currently working at the mechanism by which the distinct binding site disrupts the allosteric pathway of cAMP from the CNBD to the pore.