cAMP binding to closed pacemaker ion channels is cooperative

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 121; no. 9; p. e2315132121
• Main Authors: Kuschke, Stefan, Thon, Susanne, Sattler, Christian, Schwabe, Tina, Benndorf, Klaus, Schmauder, Ralf
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 27.02.2024
• Subjects: Binding sites; Biological Sciences; Biophysical Phenomena; Cardiomyocytes; Cell membranes; Cooperativity; Cyclic AMP; Cyclic AMP - metabolism; Cyclic Nucleotide-Gated Cation Channels - metabolism; Electric potential; Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels - metabolism; Ion Channel Gating - physiology; Ion channels; Ion channels (cyclic nucleotide-gated); Ligands; Pacemakers; Physical Sciences; Receptor mechanisms; Receptors; Voltage; Waveguides; single molecule; cooperativity; cAMP; TIRF; HCN-channels
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.2315132121

The cooperative action of the subunits in oligomeric receptors enables fine-tuning of receptor activation, as demonstrated for the regulation of voltage-activated HCN pacemaker ion channels by relating cAMP binding to channel activation in ensemble signals. HCN channels generate electric rhythmicity in specialized brain neurons and cardiomyocytes. There is conflicting evidence on whether binding cooperativity does exist independent of channel activation or not, as recently reported for detergent-solubilized receptors positioned in zero-mode waveguides. Here, we show positive cooperativity in ligand binding to closed HCN2 channels in native cell membranes by following the binding of individual fluorescence-labeled cAMP molecules. Kinetic modeling reveals that the affinity of the still empty binding sites rises with increased degree of occupation and that the transition of the channel to a flip state is promoted accordingly. We conclude that ligand binding to the subunits in closed HCN2 channels not pre-activated by voltage is already cooperative. Hence, cooperativity is not causally linked to channel activation by voltage. Our analysis also shows that single-molecule binding measurements at equilibrium can quantify cooperativity in ligand binding to receptors in native membranes.