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Inhibition of Sodium Channel Gating by Trapping the Domain II Voltage Sensor with Protoxin II

Department of Pharmacology, University of Washington, Seattle, Washington (S.S., T.S., W.A.C.) and Merck Research Laboratories, West Point, Pennsylvania (R.L.K.)

ProTx-II, an inhibitory cysteine knot toxin from the tarantula Thrixopelma pruriens, inhibits voltage-gated sodium channels. Using the cut-open oocyte preparation for electrophysiological recording, we show here that ProTx-II impedes movement of the gating charges of the sodium channel voltage sensors and reduces maximum activation of sodium conductance. At a concentration of 1 µM, the toxin inhibits 65.3 ± 4.1% of the sodium conductance and 24.6 ± 6.8% of the gating current of brain Na_v1.2a channels, with a specific effect on rapidly moving gating charge. Strong positive prepulses can reverse the inhibitory effect of ProTx-II, indicating voltage-dependent dissociation of the toxin. Voltage-dependent reversal of the ProTx-II effect is more rapid for cardiac Na_v1.5 channels, suggesting subtype-specific action of this toxin. Voltage-dependent binding and block of gating current are hallmarks of gating modifier toxins, which act by binding to the extracellular end of the S4 voltage sensors of ion channels. The mutation L833C in the S3-S4 linker in domain II reduces affinity for ProTx-II, and mutation of the outermost two gating-charge-carrying arginine residues in the IIS4 voltage sensor to glutamine abolishes voltage-dependent reversal of toxin action and toxin block of gating current. Our results support a voltage-sensor-trapping model for ProTx-II action in which the bound toxin impedes the normal outward gating movement of the IIS4 transmembrane segment, traps the domain II voltage sensor module in its resting state, and thereby inhibits channel activation.

Address correspondence to: William Catterall, University of Washington School of Medicine Department of Pharmacology, Box 357280, Seattle, WA 98195-7280. E-mail: wcatt{at}u.washington.edu