4 POLYPEPTIDE COMPONENTS OF GREEN MAMBA VENOM SELECTIVELY BLOCK CERTAIN POTASSIUM CHANNELS IN RAT-BRAIN SYNAPTOSOMES

Abstract

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 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 to 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.