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Na channel; insect selectivity; Xenopus oocytes; scorpion toxin; gating; Drosophila Para
Abstract:
The ability of the excitatory anti-insect-selective scorpion toxin AahIT (Androctonus australis hector) to exclusively bind to and modify the insect voltage-gated sodium channel (NaCh) makes it a unique tool to unravel the structural differences between mammalian and insect channels, a prerequisite in the design of selective pesticides. To localize the insect NaCh domain that binds AahIT, we constructed a chimeric channel composed of rat brain NaCh alpha-subunit (rBIIA) in which domain-2 (D2) was replaced by that of Drosophila Para (paralytic temperature-sensitive). The choice of D2 was dictated by the similarity between AahIT and scorpion beta- toxins pertaining to both their binding and action and the essential role of D2 in the beta-toxins binding site on mammalian channels. Expression of the chimera rBIIA-ParaD2 in Xenopus oocytes gave rise to voltage-gated and TTX-sensitive NaChs that, like rBIIA, were sensitive to scorpion alpha-toxins and regulated by the auxiliary subunit beta(1) but not by the insect TipE. Notably, like Drosophila Para/TipE, but unlike rBIIA/beta(1), the chimera gained sensitivity to AahIT, indicating that the phyletic selectivity of AahIT is conferred by the insect NaCh D2. Furthermore, the chimera acquired additional insect channel properties; its activation was shifted to more positive potentials, and the effect of alpha- toxins was potentiated. Our results highlight the key role of D2 in the selective recognition of anti-insect excitatory toxins and in the modulation of NaCh gating. We also provide a methodological approach to the study of ion channels that are difficult to express in model expression systems.
