Free energy landscape remodeling of the cardiac pacemaker channel explains the molecular basis of familial sinus bradycardia

J Biol Chem. 2017 Apr 14;292(15):6414-6428. doi: 10.1074/jbc.M116.773697. Epub 2017 Feb 7.

Abstract

The hyperpolarization-activated and cyclic nucleotide-modulated ion channel (HCN) drives the pacemaker activity in the heart, and its malfunction can result in heart disorders. One such disorder, familial sinus bradycardia, is caused by the S672R mutation in HCN, whose electrophysiological phenotypes include a negative shift in the channel activation voltage and an accelerated HCN deactivation. The outcomes of these changes are abnormally low resting heart rates. However, the molecular mechanism underlying these electrophysiological changes is currently not fully understood. Crystallographic investigations indicate that the S672R mutation causes limited changes in the structure of the HCN intracellular gating tetramer, but its effects on protein dynamics are unknown. Here, we utilize comparative S672R versus WT NMR analyses to show that the S672R mutation results in extensive perturbations of the dynamics in both apo- and holo-forms of the HCN4 isoform, reflecting how S672R remodels the free energy landscape for the modulation of HCN4 by cAMP, i.e. the primary cyclic nucleotide modulator of HCN channels. We show that the S672R mutation results in a constitutive shift of the dynamic auto-inhibitory equilibrium toward inactive states of HCN4 and broadens the free-energy well of the apo-form, enhancing the millisecond to microsecond dynamics of the holo-form at sites critical for gating cAMP binding. These S672R-induced variations in dynamics provide a molecular basis for the electrophysiological phenotypes of this mutation and demonstrate that the pathogenic effects of the S672R mutation can be rationalized primarily in terms of modulations of protein dynamics.

Keywords: HCN; allosteric regulation; cell signaling; cyclic AMP (cAMP); free energy landscape; nuclear magnetic resonance (NMR); protein dynamic.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Substitution
  • Cyclic AMP / chemistry*
  • Cyclic AMP / genetics
  • Cyclic AMP / metabolism*
  • Female
  • Humans
  • Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels / chemistry*
  • Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels / genetics
  • Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels / metabolism*
  • Male
  • Muscle Proteins / chemistry*
  • Muscle Proteins / genetics
  • Muscle Proteins / metabolism*
  • Mutation, Missense*
  • Potassium Channels / chemistry*
  • Potassium Channels / genetics
  • Potassium Channels / metabolism*
  • Protein Domains
  • Sick Sinus Syndrome / congenital*
  • Sick Sinus Syndrome / genetics
  • Sick Sinus Syndrome / metabolism
  • Structure-Activity Relationship

Substances

  • HCN4 protein, human
  • Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels
  • Muscle Proteins
  • Potassium Channels
  • Cyclic AMP

Supplementary concepts

  • Sick Sinus Syndrome 1, Autosomal Recessive

Associated data

  • PDB/2MNG
  • PDB/3OTF
  • PDB/4HBN