Common Molecular Determinants of Tarantula Huwentoxin-IV Inhibition of Na+ Channel Voltage Sensors in Domains II and IV

The voltage sensors of domains II and IV of sodium channels are important determinants of activation and inactivation, respectively. Animal toxins that alter electrophysiological excitability of muscles and neurons often modify sodium channel activation by selectively interacting with domain II and inactivation by selectively interacting with domain IV. This suggests that there may be substantial differences between the toxin-binding sites in these two important domains. Here we explore the ability of the tarantula huwentoxin-IV (HWTX-IV) to inhibit the activity of the domain II and IV voltage sensors. HWTX-IV is specific for domain II, and we identify five residues in the S1-S2 (Glu-753) and S3-S4 (Glu-811, Leu-814, Asp-816, and Glu-818) regions of domain II that are crucial for inhibition of activation by HWTX-IV. These data indicate that a single residue in the S3-S4 linker (Glu-818 in hNav1.7) is crucial for allowing HWTX-IV to interact with the other key residues and trap the voltage sensor in the closed configuration. Mutagenesis analysis indicates that the five corresponding residues in domain IV are all critical for endowing HWTX-IV with the ability to inhibit fast inactivation. Our data suggest that the toxin-binding motif in domain II is conserved in domain IV. Increasing our understanding of the molecular determinants of toxin interactions with voltage-gated sodium channels may permit development of enhanced isoform-specific voltage-gating modifiers.