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A second S4 movement opens hyperpolarization-activated HCN channels

Xiaoan Wu1, Rosamary Ramentol1, Marta E Perez1

  • 1Department of Physiology and Biophysics, University of Miami, Miami, FL 33136.

Proceedings of the National Academy of Sciences of the United States of America
|September 10, 2021
PubMed

Insights

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels open via a two-step S4 movement. A specific mutation reveals a second S4 movement linked to channel gating, uncovering the mechanism of HCN channel opening.

Area of Science:

  • Biophysics
  • Molecular Biology
  • Cardiology

Background:

  • Pacemaker cell rhythmic activity relies on hyperpolarization-activated cyclic nucleotide-gated (HCN) channels.
  • The S4 transmembrane segment acts as a voltage sensor in HCN channels, similar to K+ channels.

Purpose of the Study:

  • To elucidate the mechanism by which S4 movement in HCN channels couples to channel opening at hyperpolarized voltages.
  • To investigate the distinct voltage-dependent steps of S4 movement and their relation to channel gating.

Main Methods:

  • Voltage clamp fluorometry was employed to monitor S4 movement in HCN channels.
  • Site-directed mutagenesis was used to create and study the E356A mutation in sea urchin HCN channels.
  • Cysteine accessibility studies and molecular modeling provided structural insights.

Main Results:

  • S4 in HCN channels exhibits a two-step movement in response to hyperpolarization, with the second step correlating with channel opening.
  • The E356A mutation in S4 uncoupled the voltage dependence of the main S4 movement from channel opening.
  • This mutation revealed a distinct S4 movement at negative voltages associated with gate opening and suggested an intracellular crevice formation.

Conclusions:

  • The study identified a two-step S4 movement mechanism underlying HCN channel gating.
  • The E356A mutation provides a tool to dissect the coupling between voltage sensing and pore opening.
  • Structural models suggest that S4 movement creates an intracellular crevice, facilitating pore opening by S5 and S6 segments.

Keywords:
S4 movementS4-to-gate couplingmolecular modelsspHCNvoltage clamp fluorometry

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