Tityustoxin K alpha blocks voltage-gated noninactivating K+ channels and unblocks inactivating K+ channels blocked by alpha-dendrotoxin in synaptosomes

Two nonhomologous polypeptide toxins, tityustoxin K alpha (TsTX-K alpha) and tityustoxin K beta (TsTX-K beta), purified from the venom of the Brazilian scorpion Tityus serrulatus, selectively block voltage-gated noninactivating K+ channels in synaptosomes (IC50 values of 8 nM and 30 nM, respectively). In contrast, alpha-dendrotoxin (alpha-DTX) and charybdotoxin (ChTX) block voltage-gated inactivating K+ channels in synaptosomes (IC50 values of 90 nM and 40 nM, respectively). We studied interactions among these toxins in 125I-alpha-DTX binding and 86Rb efflux experiments. Both TsTX-K alpha and ChTX completely displaced specifically bound 125I-alpha-DTX from synaptic membranes, but TsTX-K beta had no effect on bound alpha-DTX. TsTX-K alpha and TsTX-K beta blocked the same noninactivating component of 100 mM K(+)-stimulated 86Rb efflux in synaptosomes. Both alpha-DTX and ChTX blocked the same inactivating component of the K(+)-stimulated 86Rb efflux in synaptosomes. Both the inactivating and the noninactivating components of the 100 mM K(+)-stimulated 86Rb efflux were completely blocked when 200 nM TsTX-K beta and either 600 nM alpha-DTX or 200 nM ChTX were present. The effects of TsTX-K alpha and ChTX on 86Rb efflux were also additive. When TsTX-K alpha was added in the presence of alpha-DTX, however, only the noninactivating component of the K(+)-stimulated efflux was blocked. The inactivating component could then be blocked by ChTX, which is structurally homologous to TsTX-K alpha. We conclude that TsTX-K alpha unblocks the voltage-gated inactivating K+ channels in synaptosomes when they are blocked by alpha-DTX, but not when they are blocked by ChTX. TsTX-K alpha binds to a site on the inactivating K+ channel that does not occlude the pore; its binding apparently prevents alpha-DTX (7054 Da), but not ChTX (4300 Da), from blocking the pore. The effects of TsTX-K alpha on 125I-alpha-DTX binding and 86Rb efflux are mimicked by noxiustoxin, which is homologous to TsTX-K alpha and ChTX.