title: S4 movement in a mammalian HCN channel.
abstract: Hyperpolarization-activated, cyclic nucleotide-gated ion channels (HCN) mediate an inward cation current that contributes to spontaneous rhythmic firing activity in the heart and the brain. HCN channels share sequence homology with depolarization-activated Kv channels, including six transmembrane domains and a positively charged S4 segment. S4 has been shown to function as the voltage sensor and to undergo a voltage-dependent movement in the Shaker K+ channel (a Kv channel) and in the spHCN channel (an HCN channel from sea urchin). However, it is still unknown whether S4 undergoes a similar movement in mammalian HCN channels. In this study, we used cysteine accessibility to determine whether there is voltage-dependent S4 movement in a mammalian HCN1 channel. Six cysteine mutations (R247C, T249C, I251C, S253C, L254C, and S261C) were used to assess S4 movement of the heterologously expressed HCN1 channel in Xenopus oocytes. We found a state-dependent accessibility for four S4 residues: T249C and S253C from the extracellular solution, and L254C and S261C from the internal solution. We conclude that S4 moves in a voltage-dependent manner in HCN1 channels, similar to its movement in the spHCN channel. This S4 movement suggests that the role of S4 as a voltage sensor is conserved in HCN channels. In addition, to determine the reason for the different cAMP modulation and the different voltage range of activation in spHCN channels compared with HCN1 channels, we constructed a COOH-terminal-deleted spHCN. This channel appeared to be similar to a COOH-terminal-deleted HCN1 channel, suggesting that the main functional differences between spHCN and HCN1 channels are due to differences in their COOH termini or in the interaction between the COOH terminus and the rest of the channel protein in spHCN channels compared with HCN1 channels.
authors.affiliation: Neurological Sciences Institute, Oregon Health & Science University, Beaverton, OR 97006, USA.
authors.forename: Sriharsha
authors.initials: S
authors.lastname: Vemana
authors.forename: Shilpi
authors.initials: S
authors.lastname: Pandey
authors.forename: H Peter
authors.initials: HP
authors.lastname: Larsson
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000502:physiology* / Q000648:ultrastructure*; D000242:Cyclic AMP / Q000378:metabolism; D054815:Cyclic Nucleotide-Gated Cation Channels; D003545:Cysteine; D004594:Electrophysiology; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D015640:Ion Channel Gating / Q000502:physiology*; D007473:Ion Channels / Q000737:chemistry / Q000502:physiology*; D008322:Mammals; D008564:Membrane Potentials / Q000502:physiology*; D051379:Mice; D016297:Mutagenesis, Site-Directed; D009865:Oocytes; D015221:Potassium Channels / Q000502:physiology; D012617:Sea Urchins; D051637:Shaker Superfamily of Potassium Channels; D014981:Xenopus
chemical_list: D054815:Cyclic Nucleotide-Gated Cation Channels; C578666:Hcn1 protein, mouse; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D015221:Potassium Channels; D051637:Shaker Superfamily of Potassium Channels; D000242:Cyclic AMP; D003545:Cysteine