Extracellular protons accelerate hERG channel deactivation by destabilizing voltage sensor relaxation

Area of Science:

• Cardiovascular Physiology
• Ion Channel Biophysics
• Molecular Cardiology

Background:

• hERG channels mediate the I_Kr current, essential for cardiac action potential repolarization.
• hERG's slow deactivation and mode shift behavior are linked to voltage sensor relaxation, but the mechanism is unclear.

Purpose of the Study:

• To investigate the structural determinants of voltage sensor relaxation in hERG channels.
• To elucidate the mechanism by which extracellular protons influence hERG deactivation kinetics.

Main Methods:

• Utilized extracellular acidification (pH 6.5) to study hERG channel gating.
• Employed gating current recordings and voltage clamp fluorimetry to measure voltage sensor dynamics.
• Investigated the role of specific acidic residues (D509) in S3.

Main Results:

• Extracellular acidification destabilized voltage sensor relaxation, shifting deactivation voltage dependence by ~20 mV.
• The pH dependence of mode shift loss correlated with accelerated deactivation kinetics.
• Neutralizing D509 mimicked proton effects, implicating S3 acidic residues in stabilizing the relaxed state.

Conclusions:

• Voltage sensor relaxation is mechanistically linked to slower pore gate closure in hERG channels.
• Acidic residues in the S3 segment stabilize the relaxed voltage sensor state, contributing to slow deactivation.
• Protonation of these residues accelerates hERG deactivation by destabilizing voltage sensor relaxation.