DIFFERENTIAL INVOLVEMENT OF DISULFIDE BRIDGES ON THE FOLDING OF A SCORPION TOXIN

Leiurotoxin I is a neurotoxin, blocker of Ca2+-activated apamin-sensit ive K+ channel, purified from the venom of the scorpion Leiurus quinqu estriatus hebraeus. It is a 31-residue polypeptide reticulated by thre e disulfide bridges, i.e. Cys(3)-Cys(21), Cys(8)-Cys(26) and Cys(12)-C ys(28). To investigate the role of these disulfide bridges in the fold ing of this toxin, analogs lacking one disulfide bridge were synthesiz ed. The structures of two analogs in which two half-cystines were repl aced by a-aminobutyrate residues to suppress one disulfide bridge, wer e analyzed by H-1 NMR. The NMR studies reveal a three-dimensional stru cture identical with the native toxin for the analog lacking disulfide bridge Cys(3)-Cys(21) and a loss of organized structure for another a nalog lacking disulfide bridge Cys(12)-Cys(28). These analogs are, res pectively, fully active and only weakly active (2% of the residual act ivity) when tested ill vitro for their ability to interact with their receptor channel and in vivo for their neurotoxic activity in mice. Th is suggests that disulfide bridge Cys(3)-Cys(21) is essential for the folding process. In contrast, the lack of disulfide bridge Cys(3)-Cys( 21) does not affect the folding and the maintenance of bioactive confo rmation of Leiurotoxin I. (C) Munksgaard 1997.