Chemical synthesis and characterization of Pi1, a scorpion toxin from Pandinus imperator active on K+ channels

Pi1 is a 35-residue toxin cross-linked by four disulfide bridges that has b een isolated from the venom of the chactidae scorpion Pandinus imperator. D ue to its very low abundance in the venom, we have chemically synthesized t his toxin in order to study its biological activity. Enzyme-based proteolyt ic cleavage of the synthetic Pi1 (sPi1) demonstrates half-cystine pairings between Cys4-Cys25, Cys10-Cys30, Cys14-Cys32 and Cys20-Cys35, which is in a greement with the disulfide bridge organization initially reported on the n atural toxin. In vivo, intracerebroventricular injection of sPi1 in mice pr oduces lethal effects with an LD50 of 0.2 mu g per mouse. In vitro, the app lication of sPi1 induces drastic inhibition of Shaker B (IC50 of 23 nM) and rat Kv1.2 channels (IC50 of 0.44 nM) heterologously expressed in Xenopus l aevis oocytes. No effect was observed on rat Kv1.1 and Kv1.3 currents upon synthetic peptide application. Also, sPi1 is able to compete with I-125-lab eled apamin for binding onto rat brain synaptosomes with an IC50 of 55 pM. Overall, these results demonstrate that sPi1 displays a large spectrum of a ctivities by blocking both SK- and Kv1-types of K+ channels; a selectivity reminiscent of that of maurotoxin, another structurally related four disulf ide-bridged scorpion toxin that exhibits a different half-cystine pairing p attern.