Molecular biology of scorpion toxins active on potassium channels

Abstract

Peptidyl scorpion toxins are known to block diverse types of K+ channels with high affinity and, thus, can be used as powerful tools to study the physiological role of the ionic selectivity, and the architecture of the pore-region 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 order to elucidate their mechanisms of action and pharmacology significance. Scorpion toxins active on K+ channels are short polypeptides of about 30 to 40 amino acid residues, cross-linked by three or four disulfide bridges. They display a high degree of primary sequence homologies. 1H Nuclear Magnetic Resonance (NMR) analysis has demonstrated that these toxins are composed of an α-helix and a two-stranded antiparallel β-sheet, linked by two disulfide bridges. This structural motif is also found in the insect defensins. 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 obtained by PCR amplification and the organization of the KTX2 precursor depicted. This precursor is composed of a signal peptide followed by the mature toxin. The transcriptional unit and the promotor region of the gene encoding KTX2 was then amplified from the genomic DNA of Androctonus australis and its sequence determined. A single intron of 87 bp, located close to the region encoding the C-terminal part of the signal peptide, was found. Its A+T content 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 to those of the scorpion ‘long’ toxins active on Na+ channels and not like those of structurally related insect defensins, which are intronless.