Abstract
Batrachotoxin (BTX), an alkaloid from skin secretions of dendrobatid frogs, causes paralysis and death by facilitating activation and inhibiting deactivation of eukaryotic voltage-gated sodium (Nav) channels, which underlie action potentials in nerve, muscle, and heart. A full understanding of the mechanism by which BTX modifies eukaryotic Nav gating awaits determination of high-resolution structures of functional toxin-channel complexes. Here, we investigate the action of BTX on the homotetrameric prokaryotic Nav channels NaChBac and NavSp1. By combining mutational analysis and whole-cell patch clamp with molecular and kinetic modeling, we show that BTX hinders deactivation and facilitates activation in a use-dependent fashion. Our molecular model shows the horseshoe-shaped BTX molecule bound within the open pore, forming hydrophobic H-bonds and cation-π contacts with the pore-lining helices, leaving space for partially dehydrated sodium ions to permeate through the hydrophilic inner surface of the horseshoe. We infer that bulky BTX, bound at the level of the gating-hinge residues, prevents the S6 rearrangements that are necessary for closure of the activation gate. Our results reveal general similarities to, and differences from, BTX actions on eukaryotic Nav channels, whose major subunit is a single polypeptide formed by four concatenated, homologous, nonidentical domains that form a pseudosymmetric pore. Our determination of the mechanism by which BTX activates homotetrameric voltage-gated channels reveals further similarities between eukaryotic and prokaryotic Nav channels and emphasizes the tractability of bacterial Nav channels as models of voltage-dependent ion channel gating. The results contribute toward a deeper, atomic-level understanding of use-dependent natural and synthetic Nav channel agonists and antagonists, despite their overlapping binding motifs on the channel proteins.
Highlights
More than 30 years after the publication of Boris Khodorov’s review, “Batrachotoxin as a tool to study voltage-sensitive sodium channels of excitable membranes” (Khodorov, 1985), its relevance endures
Batrachotoxin (BTX) is an excitatory component in the skin secretions of dendrobatid frogs, which advertise their lethal armament with their gaudy colors (Daly et al, 1965; Tokuyama et al, 1969; Albuquerque et al, 1971)
All experiments reported below were performed after ≥5-min conditioning with 25-ms depolarizing pulses at 1 Hz from Vh = −120 mV and with 10 μM BTX in the patch pipette to attain a comparable level of channel modification in all cells
Summary
More than 30 years after the publication of Boris Khodorov’s review, “Batrachotoxin as a tool to study voltage-sensitive sodium channels of excitable membranes” (Khodorov, 1985), its relevance endures.
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