The NMR structure of a new toxin, butantoxin (BuTX), which is present in the venoms of the three Brazilian scorpions
Tityus serrulatus, Tityus bahiensis, an
d Tityus stigmurus, has been investigate
d. This toxin was shown to reversibly block the
Shaker B potassium channels (
Kd ![]()
der=0 SRC=/images/
glyphs/BQ1.GIF> 660 nM) an
d inhibit the proliferation of T-cells an
d the interleukin-2 pro
duction of antigen-stimulate
d T-helper cells. BuTX is a 40 amino aci
d basic protein stabilize
d by the four
disulfi
de bri
dges: C
ys2-Cys5, Cys10-Cys31, Cys16-Cys36, an
d C
ys20-Cys38. The latter three are conserve
d among all members of the short-chain scorpion toxin family, while the first is unique to BuTX. The three-
dimensional structure of BuTX was
determine
d using
1H-NMR spectroscopy. NOESY, phase sensitive COSY (PH-COSY), an
d ami
de hy
drogen exchange
data were use
d to generate constraints for molecular mo
deling calculations. Distance geometry an
d simulate
d annealing calculations were performe
d to generate a family of 49 structures free of constraint violations. The secon
dary structure of BuTX consists of a short 2 turn α-helix (Glu15-
Phe23) an
d a β-sheet. The β-sheet is compose
d of two well-
define
d antiparallel stran
ds (Gly29-Met32 an
d Lys35-Cys38) connecte
d by a type-I′ β-turn (Asn33-Asn34). Resi
dues Cys5-Ala9 form a quasi-thir
d stran
d of the β-sheet. The N-terminal C2-C5
disulfi
de bri
dge unique to this toxin
does not appear to confer stability to the protein.