Structural basis for alpha-K toxin specificity for K+ channels revealed through the solution H-1 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 th eir alpha/beta turn and in the length of the (alpha -carbon backbone. To un derstand the role of these differences in defining specificity, we construc ted two NxTX mutants, NxTX-IbTX I and NxTX-IbTX II, and solved their soluti on structures by H-1 NMR spectroscopy. For NxTX-IbTX I, seven amino acids c omprising the alpha/beta turn in NxTX are replaced with six amino acids fro m the corresponding (alpha/beta turn in IbTX (NxTX-YGSSAGA(21-27)FGVDRF(21- 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 a n alpha/beta scaffold structure typical of the alpha -K channel toxins. A h elix 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 stran ds, 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 11 resembling that of NxTX. The se 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 Kvl.3 chann els.