μ-Conotoxins (μ-CTX) are potent oligopeptide blockers of sodium channels. The best characterized forms of μ-CTX, GIIIA and GIIIB, have similar primary and three-dimensional structures and comparable potencies (IC50 ∼30 nM) for block of wild-type skeletal muscle Na+ channels. The two toxins are thus considered to be indistinguishable by their target channels. We have found mutations in the domain II pore region (D762K and E765K) that decrease GIIIB blocking affinity ∼200-fold, but reduce GIIIA affinity by only ∼4-fold, compared with wild-type channels. Synthetic μ-CTX GIIIA mutants reveal that the critical residue for differential recognition is at position 14, the site of the only charge difference between the two toxin isoforms. Therefore, engineered Na+ channels, but not wild-type channels, can discriminate between two highly homologous conotoxins. Latent specificity of toxin - Channel interactions, such as that revealed here, is a principle worthy of exploitation in the design and construction of improved biosensors.
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