Differential involvement of disulfide bridges on the folding of a scorpion toxin.

Leiurotoxin I is a neurotoxin, blocker of Ca²⁺-activated apamin-sensitive K⁺ channel, purified from the venom of the scorpion Leiurus quinquestriatus hebraeus. It is a 31-residue polypeptide reticulated by three disulfide bridges, i.e. Cys³-Cys²¹, Cys⁸-Cys²⁶ and Cys¹²-Cys²⁸. To investigate the role of these disulfide bridges in the folding of this toxin, analogs lacking one disulfide bridge were synthesized. The structures of two analogs in which two half-cystines were replaced by α-aminobutyrate residues to suppress one disulfide bridge, were analyzed by ¹H NMR. The NMR studies reveal a three-dimensional structure identical with the native toxin for the analog lacking disulfide bridge Cys³-Cys²¹ and a loss of organized structure for another analog lacking disulfide bridge Cys¹²-Cys²⁸. These analogs are, respectively, fully active and only weakly active (2% of the residual activity) when tested in vitro for their ability to interact with their receptor channel and in vivo for their neurotoxic activity in mice. This suggests that disulfide bridge Cys¹²-Cys²⁸ is essential for the folding process. In contrast, the lack of disulfide bridge Cys³-Cys²¹ does not affect the folding and the maintenance of bioactive conformation of Leiurotoxin I. [ABSTRACT FROM AUTHOR]