Charybdotoxin, a protein inhibitor of single Ca2+-activated K+ channels from mammalian skeletal muscle

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Abstract
The recent development of techniques for recording currents through single ionic channels1 has led to the identification of a K+-specific channel that is activated by cytoplasmic Ca2+ (refs 2–12). The channel has complex properties, being activated by depolarizing voltages and having a voltage-sensitivity that is modulated by cytoplasmic Ca2+ levels. The conduction behaviour of the channel is also unusual, its high ionic selectivity being displayed simultaneously with a very high unitary conductance2,4,12. Very little is known about the biochemistry of this channel, largely due to the lack of a suitable ligand for use as a biochemical probe for the channel. We describe here a protein inhibitor of single Ca2+-activated K+ channels of mammalian skeletal muscle. This inhibitor, a minor component of the venom of the Israeli scorpion, Leiurus quinquestriatus, reversibly blocks the large Ca2+-activated K+ channel in a simple bimolecular reaction. We have partially purified the active component, a basic protein of relative molecular mass (Mr) ∼ 7,000.