3 NEW TOXINS FROM THE SCORPION PANDINUS IMPERATOR SELECTIVELY BLOCK CERTAIN VOLTAGE-GATED K+ CHANNELS

Three 35-amino acid peptide K+ channel toxins (pandinotoxins) were pur ified from the venom of the scorpion Pandinus imperator: the toxins ar e designated pandinotoxin (PiTX)-K alpha, PiTX-K beta, and PiTX-K gamm a. In an Rb-86 tracer flux assay on rat brain synaptosomes, all three toxins selectively blocked the component of the K+-stimulated Rb-86 ef flux that corresponds to a voltage-gated, rapidly inactivating (A-type ) K+ current (IC50 = 6, 42, and 100 nM, respectively). These toxins bl ocked neither the noninactivating component of the K+-stimulated Rb-86 efflux (corresponding to a delayed rectifier) nor the Ca2+-dependent component of the Rb-86 efflux (i.e., a Ca2+-activated K+ current) in t hese terminals. PiTX-K alpha, which was expressed by recombinant metho ds, also blocked the Kv1.2 channel expressed in fibroblasts (IC50 = 32 pM). PiTX-K alpha and PiTX-K beta have identical amino acid sequences except for the seventh amino acid: a proline in PiTX-K alpha, and a g lutamic acid in PiTX-K beta. They have substantial sequence homology, especially at the carboxyl termini, with another scorpion toxin, chary bdotoxin (ChTX), which blocks both the Ca2+-activated and the rapidly inactivating, K+-stimulated Rb-86 efflux components in synaptosomes an d the Kv1.2 channel. PiTX-K gamma, however, has much less sequence hom ology. Conserved in all four toxins are three identically positioned d isulfide bridges; an asparagine at position 30; and positive charges a l positions 27, 31, and 34 (based on ChTX numbering). PiTX-K gamma is novel in that it has a fourth pair of cysteines. The PiTX structures w ere computer simulated, using ChTX as a model. We speculate that the t hree-dimensional structures of all three PiTXs resemble that of ChTX: a beta-sheet al the carboxyl terminus, containing three cysteines, is linked to the central alpha-helix by two disulfide bridges (C17-35 and C13-33) and to an extended amino-terminal fragment by the third disul fide bridge (C7-C28). Further analysis of the three-dimensional struct ures reveals differences that may help to explain the selectivity and affinity differences of these toxins.