Characterization and Chemical Synthesis of Cm39 (α-KTx 4.8): A Scorpion Toxin That Inhibits Voltage-Gated K + Channel K V 1.2 and Small- and Intermediate-Conductance Ca 2+ -Activated K + Channels K Ca 2.2 and K Ca 3.1

A novel peptide, Cm39, was identified in the venom of the scorpion . Its primary structure was determined. It consists of 37 amino acid residues with a MW of 3980.2 Da. The full chemical synthesis and proper folding of Cm39 was obtained. Based on amino acid sequence alignment with different K channel inhibitor scorpion toxin (KTx) families and phylogenetic analysis, Cm39 belongs to the α-KTx 4 family and was registered with the systematic number of α-KTx 4.8. Synthetic Cm39 inhibits the voltage-gated K channel hK 1.2 with high affinity (K = 65 nM). The conductance-voltage relationship of K 1.2 was not altered in the presence of Cm39, and the analysis of the toxin binding kinetics was consistent with a bimolecular interaction between the peptide and the channel; therefore, the pore blocking mechanism is proposed for the toxin-channel interaction. Cm39 also inhibits the Ca -activated K 2.2 and K 3.1 channels, with K = 502 nM, and K = 58 nM, respectively. However, the peptide does not inhibit hK 1.1, hK 1.3, hK 1.4, hK 1.5, hK 1.6, hK 11.1, mK 1.1 K channels or the hNa 1.5 and hNa 1.4 Na channels at 1 μM concentrations. Understanding the unusual selectivity profile of Cm39 motivates further experiments to reveal novel interactions with the vestibule of toxin-sensitive channels.