Orthorhombic crystals (space group P212121, a = 45.94 A, b = 40.68 A, c = 29.93 A) of the potent scorpion a-toxin II from Androctonus australis Hector were grown using sterile techniques. The structure was solved by a combination of heavy-atom and model phasing. Subsequently, it was refrned at 1.8 A resolution by a fast-Fourier restrained leastsquares procedure. The crystallographic R factor is 0.152 for data with 7.0 A > d > 1.8 A and F > 2.5o(F) and 0.177 when all data are considered. Eighty-nine solvent molecules have been incorporated into the model. The dense core formed by the a-helical and antiparallel ,8-sheet moieties and three of the four disulfide bridges is similar in variant 3, a toxin purified from the North American scorpion Centruroides sculpturatus, and in toxin II. However, the two molecules differ markedly in the orientation of loops protruding from the core. Toxin II seems to contain several highly ordered solvent molecules.Eight of them occupy a cavity consisting of the C-terminal region and a loop found only in scorpion a-toxins. The highly reactive and pharmacologically important Lys-58 is found at one of the extremes of this cavity, where it establishes a series of hydrogen bonds with protein and solvent atoms. The reactivities of the five lysine residues of toxin H are highly correlated with the formation of hydrogen bonds, hydrophobic interactions, and salt links.Scorpion toxins constitute a family of small basic proteins that are responsible for the neurotoxicity of the venoms (1-3). Although they show significant sequence homology (2-4), scorpion toxins display various degrees of toxicity toward different animal classes. For example, venoms from Buthinae scorpions contain toxins that are preferentially directed against mammals, insects, or crustaceans (2,5,6). Mammaldirected toxins have been divided into two groups, a and f (7), depending on their mode of action, cooperative binding with other toxins, and dependence of membrane potential for binding. The a-toxins are typically found in Buthinae venoms and they prolong the Na+ inactivation phase of the action potential (8). 8-Toxins, on the other hand, were first described in Centrurinae venoms and are known to affect the Na+ activation phase (7). As may have been expected, a-toxins and 8-toxins do not bind to the same site on the Na+ channel (9-15).The three-dimensional structure of variant 3 from the scorpion Centruroides sculpturatus Ewing has been determined at 3 A resolution and refined at 1.8 A resolution (16,17). Although variant 3 is only weakly toxic, it is structurally very close to the potent p-toxins. Based on this structure Fontecilla-Camps et al. (18) tTo whom reprint requests should be sent at present address:

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Chemical modification of some trifunctional amino acid residues in toxins I, II, and III of the scorpion Androctonus australis Hector have been performed. The results indicate: (1) Reduction and methylation of one disulfide bridge destroy toxic activity of toxin II. (2) The only tryptophan residue of toxin II (position 38) is not included in the active site of the molecule. (3) Modification of five carboxylates out of the seven contained in toxin II suppresses the toxic activity. (4) Acetylation of the lysine and tyrosine residues in toxin II leads to the loss of both toxic and antigenic activity. Treatment of the acetylated toxin by hydroxylamine restores partially the antigenic activity. In the case of toxin I, total acetylation abolishes only the toxic activity. It is concluded that at least one tyrosine residue must be involved in an antigenic site of toxin II. (5) Citraconylation of toxins II and III leads to complete loss of toxicity; decitraconylation restores full activity. (6) Guanidination of toxin II does not affect its toxicity significantly. (7) Alkylation of toxin II by iodoacetic acid affects both amino groups and histidine residues. The loss of toxicity is mainly due to the modification of the lysine residues. In the case of toxin I, the kinetics of toxicity loss closely parallel the covalent modification of one lysine residue.

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The complete amino acid sequence of toxin XI of the North African scorpion Buthus occitanus tunetanus has been elucidated by automatic sequencing of the reduced and alkylated toxin and of the peptides obtained after tryptic cleavage restricted to arginyl bonds. This toxin is structurally homologous to toxin II of Androctonus australis Hector, the most active among the α‐toxins, but is far less potent, both in vivo and in vitro. This work points out 12 mutations, many of which are conservative. Nevertheless, the most striking difference is the replacement of the lysine residue at position 58, known to be important in the activity of AaH toxin II, by a valine residue. Thus, it seems that the presence of a positive charge at this location facilitates the interactions between the receptor on the sodium channel and the α‐type toxins.

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