Weak Cation Selectivity in HCN Channels Results From K⁺-Mediated Release of Na⁺ From Selectivity Filter Binding Sites

Area of Science:

• Biophysics
• Molecular Biology
• Cardiovascular Physiology

Background:

• Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels generate the pacemaker current essential for biological timing, including heartbeats.
• Understanding ion permeation through HCN channels is key to explaining their unique functional properties.

Purpose of the Study:

• To investigate the potential of mean force for potassium and sodium ion conduction through the open HCN4 channel pore.
• To elucidate the energetic basis for low unitary conductance and weak selectivity in HCN channels.

Main Methods:

• Umbrella sampling simulations were employed to explore the free energy landscape of ion permeation.
• Analysis focused on the energetic barriers within the selectivity filter (SF) and ion binding sites.

Main Results:

• High energetic barriers (>3-5 kJ/mol) within the HCN4 SF impede ion diffusion, explaining low unitary conductance.
• Sodium ions bind more strongly to the SF than potassium ions.
• Potassium ions facilitate the release of bound sodium ions, a mechanism distinct from potassium-selective channels.

Conclusions:

• The weak selectivity of HCN channels arises from having only two ion binding sites in the SF.
• Ion transport and selectivity in HCN channels are governed by sodium acting as a weak blocker, released by potassium.
• This mechanism differs from that of potassium-selective channels, highlighting unique HCN channel gating properties.