Structural basis for alpha-K toxin specificity for K+ channels revealed through the solution 1H NMR structures of two noxiustoxin-iberiotoxin chimeras.

Noxiustoxin (NxTX) and iberiotoxin (IbTX) exhibit extraordinary differences in their ability to inhibit current through the large-conductance calcium-activated potassium (maxi-K) and voltage-gated potassium (Kv1.3) channels. The three-dimensional structures of NxTX and IbTX display differences in their alpha/beta turn and in the length of the alpha-carbon backbone. To understand the role of these differences in defining specificity, we constructed two NxTX mutants, NxTX-IbTX I and NxTX-IbTX II, and solved their solution structures by 1H NMR spectroscopy. For NxTX-IbTX I, seven amino acids comprising the alpha/beta turn in NxTX are replaced with six amino acids from the corresponding alpha/beta turn in IbTX (NxTX-YGSSAGA21-27FGVDRF21-26). In addition, NxTX-IbTX II contained the S14W mutation and deletion of the N- and C-terminal residues. Both NxTX-IbTX I and NxTX-IbTX II exhibit an alpha/beta scaffold structure typical of the alpha-K channel toxins. A helix is present from residues 10 to 19 in NxTX-IbTX I and from residues 13 to 19 in NxTX-IbTX II. The beta-sheet, defined by three antiparallel strands, is one residue longer in NxTX-IbTX I relative to NxTX-IbTX II. The two toxins also differ in the structure of the alpha/beta turn with NxTX-IbTX I resembling that of IbTX and with NxTX-IbTX II resembling that of NxTX. These differences in the beta-sheet and alpha/beta turn alter the dimensions of the toxin-channel interaction surface and provide insight into how these NxTX mutations alter K+ channel specificity for the maxi-K and Kv1.3 channels.