Hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels are regulated by the binding of 3′,5′-cyclic adenosine monophosphate (cAMP). Upon binding, structural differences induced in the cyclic nucleotide-binding domain (CNBD) are propagated to the rest of the protein, increasing the extent of channel activation by shifting it to less hyperpolarized voltages. However, the exact structural mechanism underlying this regulation remains unknown. Here, we use long timescale molecular dynamics (MD) simulations of the CNBD to study the structural reorganization induced by cAMP binding. Our simulations are consistent with structural evidence from double electron–electron resonance spectroscopy and transition metal ion FRET experiments. We find that cAMP binding renders CNBDs more rigid and induces folding of the distal end of the C-helix in the binding site, which keeps the site closed. In particular, we were able to capture the changes in the degree of opening of the binding site of three isoforms of the HCN family, that correlate well with the effect of cAMP on their function. These results provide an understanding at the molecular level of how cAMP regulates the opening of HCN channels. Furthermore, we show that a partial agonist modulates the CNBD similarly to cAMP, whereas an antagonist keeps the binding site open.

Linking the ligand modulation of the binding site to the function of HCN ion channels

2019 Fall National Meeting

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