Cm28, a scorpion toxin having a unique primary structure, inhibits K_V1.2 and K_V1.3 with high affinity

The Cm28 in the venom of Centruroides margaritatus is a short peptide consisting of 27 amino acid residues with a mol wt of 2,820 D. Cm28 has <40% similarity with other known α-KTx from scorpions and lacks the typical functional dyad (lysine–tyrosine) required to block K_V channels. However, its unique sequence contains the three disulfide-bond traits of the α-KTx scorpion toxin family. We propose that Cm28 is the first example of a new subfamily of α-KTxs, registered with the systematic number α-KTx32.1. Cm28 inhibited voltage-gated K⁺ channels K_V1.2 and K_V1.3 with K_d values of 0.96 and 1.3 nM, respectively. There was no significant shift in the conductance–voltage (G-V) relationship for any of the channels in the presence of toxin. Toxin binding kinetics showed that the association and dissociation rates are consistent with a bimolecular interaction between the peptide and the channel. Based on these, we conclude that Cm28 is not a gating modifier but rather a pore blocker. In a selectivity assay, Cm28 at 150 nM concentration (>100× K_d value for K_V1.3) did not inhibit K_V1.5, K_V11.1, K_Ca1.1, and K_Ca3.1 K⁺ channels; Na_V1.5 and Na_V1.4 Na⁺ channels; or the hH_V1 H⁺ channel but blocked ∼27% of the K_V1.1 current. In a biological functional assay, Cm28 strongly inhibited the expression of the activation markers interleukin-2 receptor and CD40 ligand in anti-CD3–activated human CD4⁺ effector memory T lymphocytes. Cm28, due to its unique structure, may serve as a template for the generation of novel peptides targeting K_V1.3 in autoimmune diseases.