Mapping the interaction site for the tarantula toxin hainantoxin-IV (β-TRTX-Hn2a) in the voltage sensor module of domain II of voltage-gated sodium channels.

Peptide toxins often have pharmacological applications and are powerful tools for investigating the structure-function relationships of voltage-gated sodium channels (VGSCs). Although a group of potential VGSC inhibitors have been reported from tarantula venoms, little is known about the mechanism of their interaction with VGSCs. In this study, we showed that hainantoxin-IV (β-TRTX-Hn2a, HNTX-IV in brief), a 35-residue peptide from Ornithoctonus hainana venom, preferentially inhibited rNav1.2, rNav1.3 and hNav1.7 compared with rNav1.4 and hNav1.5. hNav1.7 was the most sensitive to HNTX-IV (IC50∼21nM). In contrast to many other tarantula toxins that affect VGSCs, HNTX-IV at subsaturating concentrations did not alter activation and inactivation kinetics in the physiological range of voltages, while very large depolarization above +70mV could partially activate toxin-bound hNav1.7 channel, indicating that HNTX-IV acts as a gating modifier rather than a pore blocker. Site-directed mutagenesis indicated that the toxin bound to site 4, which was located on the extracellular S3-S4 linker of hNav1.7 domain II. Mutants E753Q, D816N and E818Q of hNav1.7 decreased toxin affinity for hNav1.7 by 2.0-, 3.3- and 130-fold, respectively. In silico docking indicated that a three-toed claw substructure formed by residues with close contacts in the interface between HNTX-IV and hNav1.7 domain II stabilized the toxin-channel complex, impeding movement of the domain II voltage sensor and inhibiting hNav1.7 activation. Our data provide structural details for structure-based drug design and a useful template for the design of highly selective inhibitors of a specific subtype of VGSCs.