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Insights into α-Κ Toxin Specificity for K+ Channels Revealed through Mutations in Noxiustoxin†

Noxiustoxin (NxTX) displays an extraordinary ability to discriminate between large conductance, calcium-activated potassium (maxi-K) channels and voltage-gated potassium (Kv1.3) channels. To identify features that contribute to this specificity, we constructed several NxTX mutants and examined their effects on whole cell current through Kv1.3 channels and on current through single maxi-K channels. Recombinant NxTX and the site-specific mutants (P10S, S14W, A25R, A25Δ) all inhibited Kv1.3 channels with Kd values of 6, 30, 0.6, 112, and 166 nM, respectively. In contrast, these same NxTX mutants had no effect on maxi-K channel activity with estimated Kd values exceeding 1 mM. To examine the role of the α-carbon backbone in binding specificity, we constructed four NxTX chimeras, which altered the backbone length and the α/β turn. For each of these chimeras, six amino acids comprising the α/β turn in iberiotoxin (IbTX) replaced the corresponding seven amino acids in NxTX (NxTX-YGSSAGA21-27-FGVDRG21-26). The ch...