Lipid bilayers determine the allostery but not intrinsic affinity of cAMP binding to pacemaker channels

Cyclic adenosine monophosphate (cAMP), a second messenger, binds to hyperpolarization and cyclic nucleotide-gated (HCN) ion channels and regulates the automaticity of pacemaking activity. While cellular studies suggest that cAMP binding to HCN channels exhibits unusual cooperativity, recent findings using purified detergent-solubilized channels indicate independent binding to each subunit. This discrepancy raises the question of whether the lipid environment or endogenous cellular cofactors influence cAMP-dependent gating. To address this, we reconstituted purified human HCN channels in nanodiscs and resolved cAMP binding energetics at single-molecule resolution using nanophotonic waveguides. Our measurements reveal that, in contrast to detergent-solubilized channels, cAMP binds cooperatively to HCN channels reconstituted in a variety of lipid nanodiscs. Remarkably, the presence of lipid bilayer promotes ligand-binding allostery but not intrinsic binding affinity. To explore the molecular basis of bilayer-induced allostery, we determine the cryo-EM structure of HCN1 in soy polar lipid nanodiscs at a nominal resolution of 3.77 angstrom resolution. Although the overall architecture is conserved, the average interfacial distance between the transmembrane domain and C-terminal domain of neighboring subunits are shorter in lipid nanodiscs. These findings indicate that the lipid bilayer regulates the function of pacemaker ion channels by enhancing inter-subunit interactions and underscore the fundamental role of membranes in amplifying the gating sensitivity of ion channels by promoting long-range cooperative interactions.