Molecular biology of scorpion toxins active on potassium channels

Peptidyl scorpion toxins are known to block diverse types of K+ channels wi th high affinity and, thus, can be used as powerful tools to study the phys iological role of the ionic selectivity, and the architecture of the pore-r egion of this class of channels. Yet, diversity among K+ channels is large and there has been a profusion of research for new selective ligands in ord er to elucidate their mechanisms of action and pharmacology significance. S corpion toxins active on K+ channels are short polypeptides of about 30 to 40 amino acid residues, cross-linked by three or four disulfide bridges. Th ey display a high degree of primary sequence homologies. H-1 Nuclear Magnet ic Resonance (NMR) analysis has demonstrated that these toxins are composed of an alpha-helix and a two-stranded antiparallel beta-sheet, linked by tw o disulfide bridges. This structural motif is also found in the insect defe nsins. A 370 bp cDNA encoding the Kaliotoxin 2 (KTX2) precursor (a 37 amino acid residues peptide purified from the North African scorpion Androctonus australis and acting as a high affinity blocker of K+ channels) was obtain ed by PCR amplification and the organization of the KTX2 precursor depicted . This precursor is composed of a signal peptide followed by the mature tox in. The transcriptional unit and the promotor region of the gene encoding K TX2 was then amplified from the genomic DNA of Androctonus australis and it s sequence determined. A single intron of 87 bp, located close to the regio n encoding the C-terminal part of the signal peptide, was found. Its A+T co ntent was particularly high (up to 86%). The transcription unit of the gene was 390 bp long. Regulatory consensus sequences were identified. The genes of scorpion `short' toxins active on K+ channels are organized similarly t o those of the scorpion `long' toxins active on Na+ channels and not like t hose of structurally related insect defensins, which are intronless.