NMR solution structure of butantoxin

The NMR structure of a new toxin, butantoxin (BuTX), which is present in th e venoms of the three Brazilian scorpions Tityus serrulatus, Tityus bahiens is, and Tityus stigmurus, has been investigated. This toxin was shown to re versibly block the Shaker B potassium channels (K-d similar to 660 nM) and inhibit the proliferation of T-cells and the interleukin-2 production of an tigen-stimulated T-helper cells. BuTX is a 40 amino acid basic protein stab ilized by the four disulfide bridges: Cys2-Cys5, Cys10-Cys31, Cys16-Cys36, and Cys20-Cys38, The latter three are conserved among all members of the sh ort-chain scorpion toxin family, while the first is unique to BuTX The thre e-dimensional structure of BuTX was determined using H-1-NMR spectroscopy. NOESY, phase sensitive COSY (PH-COSY), and amide hydrogen exchange data wer e used to generate constraints for molecular modeling calculations. Distanc e geometry and simulated annealing calculations were performed to generate a family of 49 structures free of constraint violations. The secondary stru cture of BuTX consists of a short 21/2 turn alpha-helix (Glu15-Phe23) and a beta-sheet. The beta-sheet is composed of two well-defined antiparallel st rands (Gly29-Met32 and Lys35-Cys38) connected by a type-I' beta-turn (Asn33 -Asn34), Residues Cys5-Ala9 form a quasi-third strand of the beta-sheet, Th e N-terminal C2-C5 disulfide bridge unique to this toxin does not appear to confer stability to the protein. (C) 2000 Academic Press.