Tetramerization Dynamics of C-terminal Domain Underlies Isoform-specific cAMP Gating in Hyperpolarization-activated Cyclic Nucleotide-gated Channels

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are
dually activated by hyperpolarization and binding of cAMP to their
cyclic nucleotide binding domain (CNBD). HCN isoforms respond
differently to cAMP; binding of cAMP shifts activation of HCN2 and HCN4
by 17 mV but shifts that of HCN1 by only 2-4 mV. To explain the
peculiarity of HCN1, we solved the crystal structures and performed a
biochemical-biophysical characterization of the C-terminal domain
(C-linker plus CNBD) of the three isoforms. Our main finding is that
tetramerization of the C-terminal domain of HCN1 occurs at basal cAMP
concentrations, whereas those of HCN2 and HCN4 require cAMP saturating
levels. Therefore, HCN1 responds less markedly than HCN2 and HCN4 to
cAMP increase because its CNBD is already partly tetrameric. This is
confirmed by voltage clamp experiments showing that the right-shifted
position of V-1/2 in HCN1 is correlated with its propensity to
tetramerize in vitro. These data underscore that ligand-induced CNBD
tetramerization removes tonic inhibition from the pore of HCN channels.