Structure and Function of the Voltage Sensor of Sodium Channels Probed by a β-Scorpion Toxin

• Published in: The Journal of biological chemistry 2006-07, Vol.281 (30), p.21332-21344
• Main Authors: Cestèle, Sandrine, Yarov-Yarovoy, Vladimir, Qu, Yusheng, Sampieri, François, Scheuer, Todd, Catterall, William A.
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Cell Line; Humans; Kidney - cytology; Kinetics; Life Sciences; Models, Molecular; Molecular Sequence Data; Mutagenesis, Site-Directed; Mutation; Protein Binding; Scorpion Venoms - chemistry; Scorpiones; Sequence Homology, Amino Acid; Sodium Channels - chemistry; Structure-Activity Relationship
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M603814200

Voltage sensing by voltage-gated sodium channels determines the electrical excitability of cells, but the molecular mechanism is unknown. β-Scorpion toxins bind specifically to neurotoxin receptor site 4 and induce a negative shift in the voltage dependence of activation through a voltage sensor-trapping mechanism. Kinetic analysis showed that β-scorpion toxin binds to the resting state, and subsequently the bound toxin traps the voltage sensor in the activated state in a voltage-dependent but concentration-independent manner. The rate of voltage sensor trapping can be fit by a two-step model, in which the first step is voltage-dependent and correlates with the outward gating movement of the IIS4 segment, whereas the second step is voltage-independent and results in shifted voltage dependence of activation of the channel. Mutations of Glu779 in extracellular loop IIS1-S2 and both Glu837 and Leu840 in extracellular loop IIS3-S4 reduce the binding affinity of β-scorpion toxin. Mutations of positively charged and hydrophobic amino acid residues in the IIS4 segment do not affect β-scorpion toxin binding but alter voltage dependence of activation and enhance β-scorpion toxin action. Structural modeling with the Rosetta algorithm yielded a three-dimensional model of the toxin-receptor complex with the IIS4 voltage sensor at the extracellular surface. Our results provide mechanistic and structural insight into the voltage sensor-trapping mode of scorpion toxin action, define the position of the voltage sensor in the resting state of the sodium channel, and favor voltage-sensing models in which the S4 segment spans the membrane in both resting and activated states.