Diverse β-bungarotoxin isoforms manifest different affinities to voltage-gated potassium channels of Kv1.x subfamily

β-Bungarotoxins (β-BuTx), consisting of covalently bound phospholipase A₂ subunit (A-chain), a member of group Ia of secretory phospholipases A₂, and non-enzymatic subunit (B-chain) structurally related to Kunitz-type protease inhibitors, block presynaptic neuromuscular transmission via a not completely defined mechanism of action. In vivo physiological studies revealed that the B-chain is targeting voltage-gated potassium channels of not identified subtypes. In our work, six β-BuTx isoforms were isolated from Bungarus multicinctus krait venom and characterized by mass spectrometry revealing that isoforms differ in the A- and B-chain composition. Their secondary structures determined by a circular dichroism spectroscopy were similar, while phospholipase activities differed by 1.4-2.6 times between isoforms. The β-BuTx isoforms were found to bind to the extracellular pore blocker binding site of human Kv1.1, Kv1.3 and Kv1.6 channels with submicromolar and micromolar affinities depending on the type of constituent chains. Electrophysiology data demonstrated the ability of β-BuTx isoforms to block human Kv1.3 channels with different efficiency. Though the properties of β-BuTx can depend on their subunit composition, the presented data identify human Kv1.1, Kv1.3 and Kv1.6 channels as pharmacological targets for the particular β-BuTx isoforms.