From the identification of gene organization of α conotoxins to the cloning of novel toxins

In the venoms of cone snails, α conotoxins are competitive antagonists of nicotinic acetylcholine receptors. Eleven novel cDNA and eight partial gene sequences (including two pseudogenes) of α conotoxins were identified from five species of cone snail. As expected, every cDNA encodes a precursor of prepropeptide. In all the partial genes of α conotoxins identified, there is a long intron inserted at a fixed position in the pro-region, dividing the encoding region into two exons. The mutation rate in exon I (encoding the signal peptide and a part of pro-region) is much lower than that in exon II (encoding the other part of pro-region, the mature peptide and 3′ untranslational region). Interestingly, the sequences at the 5′ and 3′ end of introns are highly conserved. In addition, in the identified introns exist long dinucleotide (e.g. “GT”, “CA”) or trinucleotide (“CAT”) repeats. In the special case of Pu1.1, there are five almost identical repeats of a 150 bp sequence in the long intron. Taking advantage of the conserved 3′ end sequence of intron, 16 α conotoxins, as well as a pseudogene and three κA conotoxins, were identified from their genomic DNAs. Based on the comparison of these cDNA and gene sequences, a hypothesis of the α conotoxin evolution was proposed.