A novel toxin form the scorpion Androctonus australis blocks Shaker K+ channels expressed in Xenopus oocytes

The Shaker B potassium channel expressed in Xenopus laevis oocytes is blocked, in a total reversible manner from the outside part, by a new toxin (Aa1) composed of 40 amino acid residues, purified from the venom of the North African scorpion Androctonus australis Garzoni. The experiments were performed with patch-clamp technique in the outside-out configuration. The half blocking concentration is approximately 4.5 microM with a 1:1 stoichiometry. The activation and inactivation kinetics of the current are not modified by the blocking mechanism. The binding affinity is not voltage dependent. These results suggest a simple bimolecular mechanism of blockade by which the toxin occludes the external vestibule of the channel and thereby inhibits the K+ ions conduction.