The HCN channel voltage sensor undergoes a large downward motion during hyperpolarization

Nat Struct Mol Biol. 2019 Aug;26(8):686-694. doi: 10.1038/s41594-019-0259-1. Epub 2019 Jul 8.


Voltage-gated ion channels (VGICs) contain positively charged residues within the S4 helix of the voltage-sensing domain (VSD) that are displaced in response to changes in transmembrane voltage, promoting conformational changes that open the pore. Pacemaker hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are unique among VGICs because their open probability is increased by membrane hyperpolarization rather than depolarization. Here we measured the precise movement of the S4 helix of a sea urchin HCN channel using transition metal ion fluorescence resonance energy transfer (tmFRET). We show that the S4 undergoes a substantial (~10 Å) downward movement in response to membrane hyperpolarization. Furthermore, by applying distance constraints determined from tmFRET experiments to Rosetta modeling, we reveal that the carboxy-terminal part of the S4 helix exhibits an unexpected tilting motion during hyperpolarization activation. These data provide a long-sought glimpse of the hyperpolarized state of a functioning VSD and also a framework for understanding the dynamics of reverse gating in HCN channels.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Video-Audio Media

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Cyclic AMP / metabolism
  • Fluorescence Resonance Energy Transfer
  • Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels / chemistry*
  • Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels / genetics
  • Ion Channel Gating / physiology
  • Membrane Potentials
  • Models, Molecular
  • Motion
  • Patch-Clamp Techniques
  • Point Mutation
  • Potassium / metabolism
  • Protein Conformation
  • Protein Domains
  • Recombinant Proteins / chemistry
  • Strongylocentrotus purpuratus / chemistry


  • Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels
  • Recombinant Proteins
  • Cyclic AMP
  • Potassium