Crystal structure of a highly acidic neurotoxin from scorpion Buthus tamulus at 2.2 Å resolution reveals novel structural features

Abstract

The crystal structure of a highly acidic neurotoxin from the scorpion Buthus tamulus has been determined at 2.2 Å resolution. The amino acid sequence determination shows that the polypeptide chain has 64 amino acid residues. The pI measurement gave a value of 4.3 which is one of the lowest pI values reported so far for a scorpion toxin. As observed in other α-toxins, it contains four disulphide bridges, Cys12-Cys63, Cys16-Cys36, Cys22-Cys46, and Cys26-Cys48. The crystal structure reveals the presence of two crystallographically independent molecules in the asymmetric unit. The conformations of two molecules are identical with an r.m.s. value of 0.3 Å for their C^α tracings. The overall fold of the toxin is very similar to other scorpion α-toxins. It is a βαββ protein. The β-sheet involves residues Glu2-Ile6 (strand β1), Asp32-Trp39 (strand β3) and Val45-Val55 (strand β4). The single α-helix formed is by residues Asn19-Asp28 (α2). The structure shows a trans peptide bond between residues 9 and 10 in the five-membered reverse turn Asp8-Cys12. This suggests that this toxin belongs to classical α-toxin subfamily. The surface features of the present toxin are highly characteristic, the first (A-site) has residues, Phe18, Trp38 and Trp39 that protrude outwardly presumably to interact with its receptor. There is another novel face (N-site) of this neurotoxin that contains several negatively charged residues such as, Glu2, Asp3, Asp32, Glu49 and Asp50 which are clustered in a small region of the toxin structure. On yet another face (P-site) in a triangular arrangement, with respect to the above two faces there are several positively charged residues, Arg58, Lys62 and Arg64 that also protrude outwardly for a potentially potent interaction with other molecules. This toxin with three strong features appears to be one of the most toxic molecules reported so far. In this sense, it may be a new subclass of neurotoxins with the largest number of hot spots.