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 scorpio n toxin that selectively blocks the voltage-gated potassium-channel KV 1.3, has been determined by NMR spectroscopy. The toxin was isotopical ly labeled with C-13 and N-15 and studied using two-dimensional homonu clear and three- and four-dimensional heteronuclear NMR spectroscopy. The final structure was determined using 501 constraints, comprising 4 22 NOE constraints, 60 dihedral angle constraints, 9 disulfide constra ints, and 10 hydrogen bond constraints. Structures were initially dete rmined 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 f rom residues 11 to 20 and includes two proline residues at positions 1 5 and 16. A loop at residues 21-24 leads into a two-strand antiparalle l 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 re lated toxins charybdotoxin and iberiotoxin, insert in a manner that ex tends the beta-sheet by one residue. Otherwise, the global structure i s very similar to that of these two other toxins. The longer sheet may have implications for channel selectivity.