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

Abstract

Two nonhomologous polypeptide toxins, tityustoxin Kα (TsTX-Kα) and tityustoxin Kβ (TsTX-Kβ), 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, α-dendrotoxin (α-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- α-DTX binding and 86Rb efflux experiments. Both TsTX-Kα and ChTX completely displaced specifically bound 125I-α-DTX from synaptic membranes, but TsTX-Kβ had no effect on bound α-DTX. TsTX-Kα and TsTX-Kβ blocked the same noninactivating component of 100 mM K+-stimulated 86Rb efflux in synaptosomes. Both α-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-Kβ and either 600 nM α-DTX or 200 nM ChTX were present. The effects of TsTX-Kα and ChTX on 86Rb efflux were also additive. When TsTX-Kα was added in the presence of α-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-Kα. We conclude that TsTX-Kα unblocks the voltage-gated inactivating K+ channels in synaptosomes when they are blocked by α-DTX, but not when they are blocked by ChTX. TsTX-Kα binds to a site on the inactivating K+ channel that does not occlude the pore; its binding apparently prevents α-DTX (7054 Da), but not ChTX (4300 Da), from blocking the pore. The effects of TsTX-Kα on 125I-α-DTX binding and 86Rb efflux are mimicked by noxiustoxin, which is homologous to TsTX-Kα and ChTX.