NMR SOLUTION STRUCTURE OF A 2-DISULFIDE DERIVATIVE OF CHARYBDOTOXIN -STRUCTURAL EVIDENCE FOR CONSERVATION OF SCORPION TOXIN ALPHA BETA MOTIF AND ITS HYDROPHOBIC SIDE-CHAIN PACKING/

The alpha/beta scorpion fold consisting of a short alpha-helix and bet a-sheet is a structural motif common to scorpion toxins, insect defens ins, and plant gamma-thionins that invariably contains three disulfide s. CHABII is a two-disulfide derivative of the scorpion toxin charybdo toxin (ChTX), chemically synthesized by inserting two L-alpha-aminobut yric acids in place of the two half-cystine residues involved in the d isulfide 13-33. This disulfide is one of the two disulfides which conn ect the alpha-helix to the beta-sheet. The solution structure of CHABI I was determined at pH 6.3 and 5 degrees C using 2D NMR and simulated annealing from 513 distance and 46 dihedral angle constraints. The NMR structure of CHABII is well-defined as judged from the low value of t he averaged backbone rms deviation between the 30 lowest energy struct ures and the energy-minimized mean structure ([rmsd] = 0.65 Angstrom f or the entire sequence and 0.48 Angstrom for the segment 3-36). Analys is and comparison of the solution structures of CHABII and ChTX lead t o the following conclusions: (i) the fold of CHABII is similar to that of ChTX as indicated by the low value of the averaged backbone atomic rms deviation between the 10 lowest energy solution structures of the two proteins (1.44 Angstrom); (ii) the packing of the hydrophobic cor e is well-preserved, underlying the critical structural role of the hy drophobic interactions even for such a small and cysteine-rich protein as ChTX.