[Scorpion toxins].

Abstract

Scorpion are known to use a cocktail of toxins to immobilize their prey. So, their venoms constitute a complex mixture of polypeptides exhibiting different pharmacological activities. These polypeptides are small (between 30 and 70 amino acids long), basic and highly reticulated (3 or 4 disulfide bridges). They bind with very high affinities to specific targets, which are different ionic channels of excitable cells. Thus, they constitute usefull tools for the neurobiologist. The scorpion toxins can be divided into: 1) "long chain toxins" (60-70 amino acids residues cross-linked by 4 disulfide bridges) that affect exclusively voltage-dependent Na+ channels of excitable cells; 2) "short chain toxins" (30-40 amino acids residues cross-linked by 3 disulfide bridges) that block several types of K+ channels in different cells (not only potassium channels of excitable cells are affected by these toxins, but also those of erythrocytes or lymphocytes). Specificities or affinities of "long" and "short" chain toxins have been studied using structure-function relationships. Conformational analysis gave the three dimensional structures of an alpha, beta and anti-insect selective toxins. The molecules show a dense core of secondary structure, 2 1/2 turns of alpha-helice, and a short segment of anti-parallel beta-sheet, which exists in all known structures of scorpion toxins, irrespective of their size, sequence and function. From cDNA libraries, full-length cDNAs of about 370 nucleotides encoding precursors of these toxins have been isolated. Sequence analysis of these cDNAs show that the precursors contain signal peptides of about 20 amino acid residues. In addition, precursors of the toxins on mammalian Na+ have extensions at their COOH-terminal ends and have to be processed by specific exopeptidases to give the mature toxins. The cloning and sequencing of their genes revealed that they contain two exons and one intron near the end of the signal peptide sequence of the toxins precursors.