title: Structure and function of the voltage sensor of sodium channels probed by a beta-scorpion toxin.
abstract: Voltage sensing by voltage-gated sodium channels determines the electrical excitability of cells, but the molecular mechanism is unknown. beta-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 beta-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 Glu(779) in extracellular loop IIS1-S2 and both Glu(837) and Leu(840) in extracellular loop IIS3-S4 reduce the binding affinity of beta-scorpion toxin. Mutations of positively charged and hydrophobic amino acid residues in the IIS4 segment do not affect beta-scorpion toxin binding but alter voltage dependence of activation and enhance beta-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.
authors: Department of Pharmacology, University of Washington, Seattle, Washington 98195-7280; Facultéde Medecine Nord, Université de la Mediterranée, Bd. Pierre Dramard, 13916 Marseille Cedex 20, France.
authors: Sandrine
authors: S
authors: Cestèle
authors: Department of Pharmacology, University of Washington, Seattle, Washington 98195-7280.
authors: Vladimir
authors: V
authors: Yarov-Yarovoy
authors: Department of Pharmacology, University of Washington, Seattle, Washington 98195-7280.
authors: Yusheng
authors: Y
authors: Qu
authors: Facultéde Medecine Nord, Université de la Mediterranée, Bd. Pierre Dramard, 13916 Marseille Cedex 20, France.
authors: François
authors: F
authors: Sampieri
authors: Department of Pharmacology, University of Washington, Seattle, Washington 98195-7280.
authors: Todd
authors: T
authors: Scheuer
authors: Department of Pharmacology, University of Washington, Seattle, Washington 98195-7280. Electronic address: wcatt@u.washington.edu.
authors: William A
authors: WA
authors: Catterall