title: Role of Dynamics in the Autoinhibition and Activation of the Hyperpolarization-activated Cyclic Nucleotide-modulated (HCN) Ion Channels.
abstract: The hyperpolarization-activated cyclic nucleotide-modulated (HCN) ion channels control rhythmicity in neurons and cardiomyocytes. Cyclic AMP allosterically modulates HCN through the cAMP-dependent formation of a tetrameric gating ring spanning the intracellular region (IR) of HCN, to which cAMP binds. Although the apo versus holo conformational changes of the cAMP-binding domain (CBD) have been previously mapped, only limited information is currently available on the HCN IR dynamics, which have been hypothesized to play a critical role in the cAMP-dependent gating of HCN. Here, using molecular dynamics simulations validated and complemented by experimental NMR and CD data, we comparatively analyze HCN IR dynamics in the four states of the thermodynamic cycle arising from the coupling between cAMP binding and tetramerization equilibria. This extensive set of molecular dynamics trajectories captures the active-to-inactive transition that had remained elusive for other CBDs, and it provides unprecedented insight on the role of IR dynamics in HCN autoinhibition and its release by cAMP. Specifically, the IR tetramerization domain becomes more flexible in the monomeric states, removing steric clashes that the apo-CDB structure would otherwise impose. Furthermore, the simulations reveal that the active/inactive structural transition for the apo-monomeric CBD occurs through a manifold of pathways that are more divergent than previously anticipated. Upon cAMP binding, these pathways become disallowed, pre-confining the CBD conformational ensemble to a tetramer-compatible state. This conformational confinement primes the IR for tetramerization and thus provides a model of how cAMP controls HCN channel gating.
authors.affiliation: Departments of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
authors.forename: Bryan
authors.initials: B
authors.lastname: VanSchouwen
authors.affiliation: Departments of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
authors.forename: Madoka
authors.initials: M
authors.lastname: Akimoto
authors.affiliation: Departments of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
authors.forename: Maryam
authors.initials: M
authors.lastname: Sayadi
authors.affiliation: Department of Biomedical Science and Technology, University of Udine, Piazzale Kolbe 4, 33100 Udine, Italy.
authors.forename: Federico
authors.initials: F
authors.lastname: Fogolari
authors.affiliation: Departments of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario L8S 4M1, Canada; Departments of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada. Electronic address: melacin@mcmaster.ca.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Melacini
mesh_terms: D000242:Cyclic AMP / Q000378:metabolism*; D006801:Humans; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels / Q000737:chemistry* / Q000378:metabolism*; D056004:Molecular Dynamics Simulation; D009124:Muscle Proteins / Q000737:chemistry* / Q000378:metabolism*; D015221:Potassium Channels / Q000737:chemistry* / Q000378:metabolism*; D011487:Protein Conformation; D055503:Protein Multimerization; D013816:Thermodynamics
chemical_list: C119587:HCN4 protein, human; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D009124:Muscle Proteins; D015221:Potassium Channels; D000242:Cyclic AMP
keywords: HCN; allosteric regulation; cAMP; cell signaling; gating; molecular dynamics; nuclear magnetic resonance (NMR)