BmTx3, a scorpion toxin with two putative functional faces separately active on A-type K+ and HERG currents

A novel HERG channel blocker was isolated from the venom of the scorpion Buthus martensi Karsch, sequenced and characterized at the pharmacological level after chemical synthesis. According to the determined amino acid sequence, the cDNA and genomic genes were then cloned. The genomic gene consists of two exons interrupted by an intron of 65 bp at position -6 upstream from the mature toxin. The protein sequence of this toxin was completely identical with that of a known A-type K+ current blocker BmTx3, belonging to scorpion alpha-KTx subfamily 15. Thus BmTx3 is the first reported a-KTx peptide also showing HERG-blocking activity, like gamma-KTx peptides. Moreover, different from classical a-KTx peptides, such as charybdotoxin, BmTx3 cannot block Shaker-type K+ channels. Phylogenetic tree analysis reveals that this toxin takes an intermediate position between classical alpha-KTx and gamma-KTX toxins. From a structural point of view, we propose that two separate functional faces might exist on the BmTx3 molecule, responsible for the two different K+-current-blocking functions. Face A, composed of Arg(18) and Lys(19) in the a-helix side, might correspond to HERG blocking activity, whereas Face 13, containing a putative functional dyad (Lys(27) and Tyr(36)) in the P-sheet side, might correspond to A-type blocking activity. A specific deletion mutant with the disrupted Face 13, BmTx3-Y36P37del, loses the A-type current-blocking activity, but keeps a similar HERG-blocking activity, as seen with the wild-type toxin.