DETERMINATION OF THE 3-DIMENSIONAL STRUCTURE OF MARGATOXIN BY H-1, C-13, N-15 TRIPLE-RESONANCE NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

The solution structure of the 39-residue peptide margatoxin, a scorpion toxin that selectively blocks the voltage-gated potassium-channel KV1.3, has been determined by NMR spectroscopy. The toxin was isotopically labeled with C-13 and N-15 and studied using two-dimensional homonuclear and three- and four-dimensional heteronuclear NMR spectroscopy. The final structure was determined using 501 constraints, comprising 422 NOE constraints, 60 dihedral angle constraints, 9 disulfide constraints, and 10 hydrogen bond constraints. Structures were initially determined with the program PEGASUS and subsequently refined with X-PLOR. The average rms deviation from a calculated average structure for the backbone atoms of residues 3-38 is 0.40 Angstrom. A helix is present from residues 11 to 20 and includes two proline residues at positions 15 and 16. A loop at residues 21-24 leads into a two-strand antiparallel sheet from residues 25 to 38 with a turn at residues 30-33. Residues 3-6 run adjacent to the 33-38 strand but do not form a canonical beta-strand. The two additional residues of margatoxin, relative to the related toxins charybdotoxin and iberiotoxin, insert in a manner that extends the beta-sheet by one residue. Otherwise, the global structure is very similar to that of these two other toxins. The longer sheet may have implications for channel selectivity.