Solution structure of BmKTX, a K+ blocker toxin from the Chinese scorpion Buthus Martensi

BmKTX is a toxin recently purified from the venom of Buthus Martensi, which belongs to the kaliotoxin family. We have determined its solution structure by use of conventional two-dimensional NMR techniques followed by distance-geometry and energy minimization. The calculated structure is composed of a short alpha-helix (residues 14 to 20) connected by a tight turn to a two-stranded antiparallel beta-sheet (sequences 25-27 and 32-34). The beta-turn connecting these strands belongs to type I. The N-terminal segment (sequence 1 to 8) runs parallel to the beta-sheet although it cannot be considered as a third strand. Comparison of the conformation of BmKTX and toxins of the kaliotoxin family clearly demonstrates that they are highly related. Therefore, analysis of the residues belonging to the interacting surface of those toxins allows us to propose a functional map of BmKTX slightly different from the one of KTX and AgTX2, which may explain the variations in affinities of these toxins towards the Kv1.3 channels.