Studying the selectivity of scorpion αKTx15 toxins towards potassium channels

Scorpion toxins affecting K⁺ channels are pore blockers that prolong action potentials when interacting with voltage-gated K⁺ channels (Kv). Here, we report the isolation and characterization of the first selective blocker of the human Kv4.1 K⁺ channel from the venom of the scorpion Buthacus leptochelys. This 37 amino-acid long toxin, Bl31381-2, was exogenously expressed, and its activity was tested on human K⁺ channels expressed in Xenopus oocytes. Bl31381-2 blocks Kv4.1 (EC₅₀=24±3nM) selectively while not affecting the currents of other 21 human potassium channels, including Kv4.2 and Kv4.3. Based on a 3D channel-toxin model, we found that replacing one turret residue (Asn355 in Kv4.1) with a lysine residue was sufficient to eliminate toxin binding. Accordingly, the mutation of toxin residue Ala25, which is predicted to reside close to channel residue Asn355, was equally capable of preventing toxin binding.

A paralog toxin, Bl31381-1, differing from Bl31381-2 in five amino acids, has non-selective activity and blocks several other voltage-gated potassium channels. By mutating Bl31381-2 residues to those found in Bl31381-1, we could pinpoint the residues essential to its selectivity. Thus, combining structural considerations and site-directed mutagenesis, we learned about Bl31381-2 toxin`s structural basis for activity and selectivity.

We suggest that Bl31381-2 could assist in determining the importance of Kv4.1 channels in both neurons and the heart. Furthermore, as overexpression of Kv4.1 channels was associated with cancer cell proliferation and poor prognosis in some cancer types, we propose Bl31381-2 as a lead compound to reduce the malignancy of specific cancer types.