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 alpha-KTx from scorpions and lacks the typical functional dyad (lysine-tyrosine) required to block KV channels. However, its unique sequence contains the three disulfide-bond traits of the alpha-KTx scorpion toxin family. We propose that Cm28 is the first example of a new subfamily of alpha-KTxs, registered with the systematic number alpha-KTx32.1. Cm28 inhibited voltage-gated K+ channels KV1.2 and KV1.3 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 (>100x Kd value for KV1.3) did not inhibit K(V)1.5, K(V)11.1, K(Ca)1.1, and K(Ca)3.1 K+ channels; Na(V)1.5 and Na(V)1.4 Na+ channels; or the hH(V)1 H+ channel but blocked similar to 27% of the K(V)1.1 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(V)1.3 in autoimmune diseases.