␣-Dendrotoxin, a 59-amino acid basic peptide from the venom of Dendroaspis angusticeps (green mamba snake), potently blocks some but not all voltage-dependent potassium channels. Here we have investigated the relative contribution of the individual ␣-subunits constituting functional Kv1.1 potassium channels to ␣-dendrotoxin binding. Three residues critical for ␣-dendrotoxin binding and located in the loop between domains S5 and S6 were mutated (A352P, E353S, and Y379H), and multimeric cDNAs were constructed encoding homoand heterotetrameric channels composed of all possible ratios of wild-type and mutant ␣-subunits. Complete mutant channels were about 200-fold less sensitive for the ␣-dendrotoxin block than complete wild-type channels, which is attributable to a smaller association rate. Analysis of the bimolecular reaction between ␣-dendrotoxin and the different homo-and heteromeric channel constructs revealed that the association rate depends on the number of wild-type ␣-subunits in the functional channel. Furthermore, we observed a linear relationship between the number of wild-type ␣-subunits in functional channels and the free energy for ␣-dendrotoxin binding, providing evidence that all four ␣-subunits must interact with ␣-dendrotoxin to produce a high affinity binding site.

A disulfide-rich, low-molecular-mass toxin-like peptide has been isolated from Parabuthus schlechteri venom using gel filtration, ion exchange, and reversed phase chromatography. Partial characterization of this peptide reveals a relationship with four-disulfide bridge proteins belonging to the family of short' insectotoxins (44% residue identity). In recognition hereof, the peptide was named PBITx1 (sITx10). Our work also reports on the deduced sequences of two other`short' insectotoxins from Buthus eupeus, I Q and I R , and it provides a consensus sequence and nomenclature for all known`short' insectotoxins. Finally, sequence similarities with K + channel blockers (charybdotoxin, U U-conotoxin), and a Cl 3 channel blocker (chlorotoxin) are highlighted.z 1998 Federation of European Biochemical Societies.

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