2 SIMILAR PEPTIDES FROM THE VENOM OF THE SCORPION PANDINUS IMPERATOR,ONE HIGHLY EFFECTIVE BLOCKER AND THE OTHER INACTIVE ON K+ CHANNELS

Two novel peptides, named Pi4 and Pi7, were purified from the venom of the scorpion Pandinus imperator, and their primary structures were de termined. These peptides have 38 amino acids residues, compacted by fo ur disulfide bridges, instead of the normal three found in most K+-cha nnel specific toxins. Both peptides contain 25 identical amino acid re sidues in equivalent positions (about 66% identity), including all eig ht half-cystines. Despite the fact that their C-terminal sequence comp rising amino acid residues 27 to 37 are highly conserved (10 out of 11 amino acids are identical), Pi4 blocks completely and reversibly Shak er B K+-channels (a Kv1.1 sub-family type of channel) at 100 nM concen tration, whereas Pi7 is absolutely inactive at this concentration. Sim ilar effects were observed in binding and displacement experiments to rat brain synaptosomal membranes using I-125-Noxiustoxin, a well known K+-channel specific toxin, In this preparation Pi4 displaces the bind ing of radiolabeled Noxiustoxin with IC50 in the order of 10 nM, where as Pi7 is ineffective at same concentration, Comparative analysis of P i4 and Pi7 sequences with those obtained by site directed mutagenesis of Charybdotoxin, another very well studied K+-channel blocking toxin, shows that the substitution of lysine (in Pi4) for arginine (in Pi7) at position 26, might be one of the important 'point mutations' respon sible for such impressive variation in blocking properties of both tox ins, here described. (C) 1998 Elsevier Science Ltd, All rights reserve d.