Abstract
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are essential for rhythmic activity in the heart and brain, and mutations in HCN channels are linked to heart arrhythmia and epilepsy. HCN channels belong to the family of voltage-gated K+ (Kv) channels. However, why HCN channels are activated by hyperpolarization whereas Kv channels are activated by depolarization is not clear. Here we reverse the voltage dependence of HCN channels by mutating only two residues located at the interface between the voltage sensor and the pore domain such that the channels now open upon depolarization instead of hyperpolarization. Our data indicate that what determines whether HCN channels open by hyperpolarizations or depolarizations are small differences in the energies of the closed and open states, due to different interactions between the voltage sensor and the pore in the different channels.
Highlights
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are essential for rhythmic activity in the heart and brain, and mutations in HCN channels are linked to heart arrhythmia and epilepsy[1,2]
To distinguish whether the ΔQWE channel is voltage independent because the movement of the S4 is uncoupled from the gate or because S4 does not move, we used voltage clamp fluorometry (VCF) to simultaneously measure S4 movement by the fluorescence from fluorophores attached to S49
We show here that hyperpolarization-activated HCN channels can be turned into depolarization-activated channels by just two mutations, suggesting that there is a delicate balance of forces that makes HCN channels activated by hyperpolarization
Summary
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are essential for rhythmic activity in the heart and brain, and mutations in HCN channels are linked to heart arrhythmia and epilepsy. Hyperpolarization-activated HCN channels and depolarization-activated Kv10.1 (EAG) channels have very similar tetrameric structures with six transmembrane segments (S1–S6) per subunit: S1–S4 form the voltage-sensing domain (VSD) and S5–S6 form the pore domain (PD)[7,8]. In both Kv and HCN channels, S4 is the positively charged voltage sensor[3,9,10,11] and the C-terminal part of S6 forms the gate[7,12,13,14]. Our data indicate that what determines whether HCN channels open by hyperpolarizations or depolarizations are small differences in the energies of the closed and open states, owing to different interactions between the voltage sensor and the pore in the different channels
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