Latent specificity of molecular recognition in sodium channels engineered to discriminate between two "indistinguishable'' mu-conotoxins

mu -Conotoxins (mu -CTX) are potent oligopeptide blockers of sodium channel s. The best characterized forms of mu -CTX, GIIIA and GIIIB, have similar p rimary and three-dimensional structures and comparable potencies (IC50 simi lar to 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) th at decrease GIIIB blocking affinity similar to 200-fold, but reduce GIIIA a ffinity by only similar to4-fold, compared with wild-type channels. Synthet ic CL-CTX GIIIA mutants reveal that the critical residue for differential r ecognition is at position 14, the site of the only charge difference betwee n the two toxin isoforms. Therefore, engineered Na+ channels, but not wild- type channels, can discriminate between two highly homologous conotoxins. L atent specificity of toxin-channel interactions, such as that revealed here , is a principle worthy of exploitation in the design and construction of i mproved biosensors.