Three new toxins from the scorpion Pandinus imperator selectively block certain voltage-gated K+ channels

RS Rogowski, JH Collins, TJ O'Neill, TA Gustafson, TR Werkman, MA Rogawski, TC Tenenholz, DJ Weber and MP Blaustein

Department of Physiology, School of Medicine, University of Maryland, Baltimore 21201, USA.

Three 35-amino acid peptide K+ channel toxins (pandinotoxins) were purified from the venom of the scorpion Pandinus imperaton the toxins are designated pandinotoxin (PiTX)-K alpha, PiTX-K beta, and PiTX-K gamma. In an 86Rb tracer flux assay on rat brain synaptosomes, all three toxins selectively blocked the component of the K(+)-stimulated 86Rb efflux that corresponds to a voltage-gated, rapidly inactivating (A-type) K+ current (IC50 = 6, 42, and 100 nM, respectively). These toxins blocked neither the noninactivating component of the K(+)- stimulated 86Rb efflux (corresponding to a delayed rectifier) nor the Ca(2+)-dependent component of the 86Rb efflux (i.e., a Ca(2+)-activated K+ current) in these terminals. PiTX-K alpha, which was expressed by recombinant methods, 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 glutamic acid in PiTX-K beta. They have substantial sequence homology, especially at the carboxyl termini, with another scorpion toxin, charybdotoxin (ChTX), which blocks both the Ca(2+)- activated and the rapidly inactivating. K(+)-stimulated 86Rb efflux components in synaptosomes and the Kv 1.2 channel PiTX-K gamma, however, has much less sequence homology. Conserved in all four toxins are three identically positioned disulfide bridges; an asparagine at position 30; and positive charges at 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 were computer simulated, using ChTX as a model. We speculate that the three-dimensional structures of all three PiTXs resemble that of ChTX: a beta-sheet at the carboxyl terminus, containing three cysteines, is linked to the central alpha-helix by two disulfide bridges (C17-C35 and C13-C33) and to an extended amino- terminal fragment by the third disulfide bridge (C7-C28). Further analysis of the three-dimensional structures reveals differences that may help to explain the selectivity and affinity differences of these toxins.

Volume 50, Issue 5, pp. 1167-1177, 11/01/1996
Copyright © 1996 by American Society for Pharmacology and Experimental Therapeutics

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