Four polypeptide components of green mamba venom selectively block certain potassium channels in rat brain synaptosomes.

Venom from the green mamba (Dendroaspis angusticeps) blocked 86Rb efflux through voltage-gated K channels in rat brain synaptosomes. Crude venom inhibited both rapidly inactivating, 4-aminopyridine-sensitive K channels, and noninactivating, phencyclidine-sensitive, K channels. Fractionation of the venom by size exclusion chromatography and cation exchange high performance liquid chromatography yielded four 7000-dalton polypeptides (designated alpha-, beta-, gamma-, and delta-DaTX) that blocked synaptosome K channels. Two of these toxins, alpha- and delta-DaTX (10-100 nM), preferentially blocked the inactivating voltage-gated K channels. The other two toxins, beta- and gamma-DaTX, preferentially blocked the noninactivating voltage-gated K channels. The amino acid composition of these four toxins indicated that alpha-DaTX is identical to dendrotoxin [Br. J. Pharmacol. 77:153-161 (1982)] and toxin C13S2C3 [Hoppe-Seyler's Z. Physiol. Chem. 361:661-674 (1980)]; the composition and partial sequence analysis indicate that delta-DaTX is identical to toxin C13S1C3 [Hoppe-Seyler's Z. Physiol. Chem. 361:661-674 (1980)]. Beta- and gamma-DaTX have not previously been identified. Partial amino acid sequences of beta- and gamma-DaTX and the published sequences of alpha- and delta-DaTX reveal that the C-terminal segments of all four toxins are homologous. The C-terminal segments are also homologous with a number of nontoxic proteinase inhibitors. This raises the possibility that the N-terminal rather than the C-terminal regions are more likely responsible for the K channel blocking activity. The N-terminal portions of alpha- and delta-DaTX have some sequence homologies, but they have no obvious homologies with either beta- or gamma-DaTX. The finding of structurally similar peptide toxins with preferential activities toward different K channels may lead to the development of useful probes of K channel structure and may provide the means to distinguish among different K channels biochemically as well as physiologically.