Scyllatoxin (also leiurotoxin I) is a toxin, from the scorpion Leiurus quinquestriatus hebraeus, which blocks small-conductance Ca2+-activated K+ channels. Leiurotoxin I is a 31-residue peptide, with a helix and a short antiparallel β-sheet. This toxin is stabilized by disulfide bonds: Cys8-Cys26 and Cys12-Cys28 is bound to the β-sheet, Cys3-Cys21 is bound to an N-terminal segment preceding the helix. Leiurotoxin adopts the ά/β motif. Especially the positively charged residues (Arg6 and Arg13, which are located in the ά helix) are important for the expression of toxin biological activities and for its receptor affinity. Scyllatoxin is a blocker of small-conductance Ca2+– activated K+ channels at 10-13–10-11 M concentrations in various cell types. This toxin shows similarity in its physiological activity and binding specificity to apamin, but both toxins show no structural similarity.
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