Scorpion toxins are invaluable pharmacological tools for studying
ion channels and potential drugs for
channelopathies. The long-chain toxins from
scorpion venom with four
disulfide bridges exhibit their unusual bioactivity or biotoxicity by acting on the
sodium channels. However, the functional properties of most toxins are still unclear due to their tiny amounts in crude
venom and their challenging production by chemical and gene engineering techniques. Here, we expressed one of the long-chain α-toxins, BmKM9, found in the
venom of the scorpion Buthus martensii Karsch and characterized its pharmacological properties on
sodium channels. Unlike previous toxin production, the recombinant BmKM9 (rBmKM9) possessed no additional
amino acid residues such as the His-tag and
thrombin cleavage site. The refolded toxin could inhibit the inactivation of rNav1.4, hNav1.5 and hNav1.7
sodium channels. Dose-response experiments were further conducted on these channels. The calculated EC50 values were 131.7±6.6nM for rNav1.4, 454.2±50.1nM for hNav1.5 and 30.9±10.3μM for hNav1.7. The channel activation experiments indicated that the rBmKM9 toxin could shift the activation curves of rNav1.4 and hNav1.5 channels toward a more negative direction and present the typical features of a β-toxin. However, instead of the same activation property on
sodium channels, the rBmKM9 toxin could result in different inactivation shift capabilities on rNav1.4 and hNav1.5 channels. The V1/2 values of the steady-state inactivation were altered to be more positive for rNav1.4 and more negative for hNav1.5. Moreover, the recovery of the
hNav1.5 channel from inactivation was more significantly delayed than that of the rNav1.4 channel by exposure to rBmKM9. Together, these findings highlighted that the rBmKM9 toxin presents the pharmacological properties of both α- and β-toxins, which would increase the challenge to the classical classification of
scorpion toxins. Furthermore, the expression method and functional information on
sodium channels would promote the potential application of toxins and contribute to further channel structural and functional studies.