Structural Basis for α-K Toxin Specificity for K+ Channels Revealed through the Solution 1H NMR Structures of Two Noxiustoxin−Iberiotoxin Chimeras

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Abstract
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 α/β turn and in the length of the α-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 α/β turn in NxTX are replaced with six amino acids from the corresponding α/β 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 α/β scaffold structure typical of the α-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 β-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 α/β turn with NxTX−IbTX I resembling that of IbTX and with NxTX−IbTX II resembling that of NxTX. These differences in the β-sheet and α/β 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.