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 Centruroides margaritatus. 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(V)1.2 with high affinity (K(d) = 65 nM). The conductance–voltage relationship of K(V)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(2+)-activated K(Ca)2.2 and K(Ca)3.1 channels, with K(d) = 502 nM, and K(d) = 58 nM, respectively. However, the peptide does not inhibit hK(V)1.1, hK(V)1.3, hK(V)1.4, hK(V)1.5, hK(V)1.6, hK(V)11.1, mK(Ca)1.1 K(+) channels or the hNa(V)1.5 and hNa(V)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.