Coupling between residues on S4 and S1 defines the voltage-sensor resting conformation in NaChBac.
The voltage sensor is a four-transmembrane helix bundle (S1-S4) that couples changes in membrane potential to conformational alterations in voltage-gated ion channels leading to pore opening and ion conductance. Although the structure of the voltage sensor in activated potassium channels is available, the conformation of the voltage sensor at rest is still obscure, limiting our understanding of the voltage-sensing mechanism. By employing a heterologously expressed Bacillus halodurans sodium channel (NaChBac), we defined constraints that affect the positioning and depolarization-induced outward motion of the S4 segment. We compared macroscopic currents mediated by NaChBac and mutants in which E43 on the S1 segment and the two outermost arginines (R1 and R2) on S4 were substituted. Neutralization of the negatively charged E43 (E43C) had a significant effect on channel gating. A double-mutant cycle analysis of E43 and R1 or R2 suggested changes in pairing during channel activation, implying that the interaction of E43 with R1 stabilizes the voltage sensor in its closed/available state, whereas interaction of E43 with R2 stabilizes the channel open/unavailable state. These constraints on S4 dynamics that define its stepwise movement upon channel activation and positioning at rest are novel, to the best of our knowledge, and compatible with the helical-screw and electrostatic models of S4 motion.
Paldi
Tzur
T
Department of Plant Sciences, George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel. tzur.paldi@gmail.com
Gurevitz
Michael
M
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution / Q000235:genetics; D000596:Amino Acids / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D015640:Ion Channel Gating; D008969:Molecular Sequence Data; D050505:Mutant Proteins / Q000737:chemistry / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D017433:Protein Structure, Secondary; D016415:Sequence Alignment; D015222:Sodium Channels / Q000737:chemistry* / Q000378:metabolism*; D013329:Structure-Activity Relationship
D000596:Amino Acids; D001426:Bacterial Proteins; D050505:Mutant Proteins; C443666:NaChBac protein, bacteria; D015222:Sodium Channels