Localization of Receptor Site on Insect Sodium Channel for Depressant β-toxin BmK IT2

BackgroundBmK IT2 is regarded as a receptor site-4 modulator of sodium channels with depressant insect toxicity. It also displays anti-nociceptive and anti-convulsant activities in rat models. In this study, the potency and efficacy of BmK IT2 were for the first time assessed and compared among four sodium channel isoforms expressed in Xenopus oocytes. Combined with molecular approach, the receptor site of BmK IT2 was further localized.Principal Findings2 µM BmK IT2 strongly shifted the activation of DmNav1, the sodium channel from Drosophila, to more hyperpolarized potentials; whereas it hardly affected the gating properties of rNav1.2, rNav1.3 and mNav1.6, three mammalian central neuronal sodium channel subtypes. (1) Mutations of Glu896, Leu899, Gly904 in extracellular loop Domain II S3–S4 of DmNav1 abolished the functional action of BmK IT2. (2) BmK IT2-preference for DmNav1 could be conferred by Domain III. Analysis of subsequent DmNav1 mutants highlighted the residues in Domain III pore loop, esp. Ile1529 was critical for recognition and binding of BmK IT2.Conclusions/SignificanceIn this study, BmK IT2 displayed total insect-selectivity. Two binding regions, comprising domains II and III of DmNav1, play separated but indispensable roles in the interaction with BmK IT2. The insensitivity of Nav1.2, Nav1.3 and Nav1.6 to BmK IT2 suggests other isoforms or mechanism might be involved in the suppressive activity of BmK IT2 in rat pathological models.