title: Charge immobilization of the voltage sensor in domain IV is independent of sodium current inactivation.
abstract: Recovery from fast inactivation in voltage-dependent Na+ channels is associated with a slow component in the time course of gating charge during repolarization (i.e. charge immobilization), which results from the slow movement of the S4 segments in domains III and IV (S4-DIII and S4-DIV). Previous studies have shown that the non-specific removal of fast inactivation by the proteolytic enzyme pronase eliminated charge immobilization, while the specific removal of fast inactivation (by intracellular MTSET modification of a cysteine substituted for the phenylalanine in the IFM motif, ICMMTSET, in the inactivation particle formed by the linker between domains III and IV) only reduced the amount of charge immobilization by nearly one-half. To investigate the molecular origin of the remaining slow component of charge immobilization we studied the human cardiac Na+ channel (hH1a) in which the outermost arginine in the S4-DIV, which contributes approximately 20% to total gating charge (Qmax), was mutated to a cysteine (R1C-DIV). Gating charge could be fully restored in R1C-DIV by exposure to extracellular MTSEA, a positively charged methanethiosulphonate reagent. The RIC-DIV mutation was combined with ICMMTSET to remove fast inactivation, and the gating currents of R1C-DIV-ICM(MTSET) were recorded before and after modification with MTSEAo. Prior to MTSEAo, the time course of the gating charge during repolarization (off-charge) was best described by a single fast time constant. After MTSEA, the off-charge had both fast and slow components, with the slow component accounting for nearly 35% of Qmax. These results demonstrate that the slow movement of the S4-DIV during repolarization is not dependent upon the normal binding of the inactivation particle.
authors.affiliation: The Nora Eccles Harrison Cardiovascular Research & Training Institute and Department of Internal Medicine, University of Utah, Salt Lake City, UT 84112, USA. michael@cvrti.utah.edu
authors.forename: Michael F
authors.initials: MF
authors.lastname: Sheets
authors.forename: Dorothy A
authors.initials: DA
authors.lastname: Hanck
mesh_terms: D019943:Amino Acid Substitution; D002460:Cell Line; D004553:Electric Conductivity; D005020:Ethyl Methanesulfonate / Q000031:analogs & derivatives* / Q000494:pharmacology; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D007668:Kidney / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D009124:Muscle Proteins / Q000502:physiology*; D016297:Mutagenesis, Site-Directed; D062554:NAV1.5 Voltage-Gated Sodium Channel; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000378:metabolism; D015222:Sodium Channels / Q000502:physiology*; D013329:Structure-Activity Relationship
chemical_list: D009124:Muscle Proteins; D062554:NAV1.5 Voltage-Gated Sodium Channel; D011994:Recombinant Proteins; C568320:SCN5A protein, human; D015222:Sodium Channels; C093171:methanethiosulfonate ethylammonium; D005020:Ethyl Methanesulfonate