Voltage Sensor Movements during Hyperpolarization in the HCN Channel.

Area of Science:

• Biophysics
• Molecular Biology
• Neuroscience

Background:

• Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are vital for cardiac and neuronal pacemaker activity.
• These channels display reversed voltage dependence, opening upon hyperpolarization and closing upon depolarization.
• Unlike other voltage-gated ion channels, HCN channels lack domain-swapped voltage sensors.

Purpose of the Study:

• To elucidate the structural mechanisms underlying HCN channel gating.
• To investigate the unique voltage-sensing mechanism of HCN channels.
• To compare HCN channel structure with other voltage-gated ion channel families.

Main Methods:

• Utilized cryoelectron microscopy (cryo-EM) to determine channel structure.
• Introduced a reversible, metal-mediated cross-bridge to stabilize a hyperpolarized conformation.
• Created a chemical mimic of the hyperpolarized state for structural analysis.

Main Results:

• Determined the cryo-EM structure of the HCN channel in a hyperpolarized state.
• Observed significant displacement of the S4 helix towards the cytoplasm by two helical turns.
• Identified a novel S4 helix fragmentation near the cytoplasm, forming two distinct helical segments.

Conclusions:

• The findings suggest a unique mechanism for voltage sensing and allosteric communication in HCN channels.
• The structural data provides insights into how voltage sensors interact with the channel gate.
• This study implies that voltage-gated channel mechanisms are more diverse than previously understood, with distinct movements in HCN channels.