Insights into the molecular mechanism for hyperpolarization-dependent activation of HCN channels

Significance Unlike most other voltage-activated ion channels, hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels open in response to hyperpolarizing membrane voltages. The molecular mechanism that is responsible for this unique voltage dependence in HCN channels is unknown. Here, we show that the covalent linkage between the voltage-sensing domain and the pore domain, the S4-S5 linker, is not required for hyperpolarization-dependent activation or ligand-dependent gating, as previously thought. Instead the voltage-sensing domain is inhibitory on the pore domain, and hyperpolarizing voltages relieve this autoinhibition, allowing the pore to open. This model explains the unique hyperpolarization-dependent activation of HCN channels.