Refined solution structure of the anti-mammal and anti-insect LqqIII scorpion toxin: Comparison with other scorpion toxins

The solution structure of the anti‐mammal and anti‐insect LqqIII toxin from the scorpion Leiurus quinquestriatus quinquestriatuswas refined and compared with other long‐chain scorpion toxins. This structure, determined by 1H‐NMR and molecular modeling, involves an α‐helix (18–29) linked to a three‐stranded β‐sheet (2–6, 33–39, and 43–51) by two disulfide bridges. The average RMSD between the 15 best structures and the mean structure is 0.71 Å for Cα atoms. Comparison between LqqIII, the potent anti‐mammal AaHII, and the weakly active variant‐3 toxins revealed that the LqqIII three‐dimensional structure is closer to that of AaHII than to the variant‐3 structure. Moreover, striking analogies were observed between the electrostatic and hydrophobic potentials of LqqIII and AaHII. Several residues are well conserved in long‐chain scorpion toxin sequences and seem to be important in protein structure stability and function. Some of them are involved in the CSαβ (Cysteine Stabilized α‐helix β‐sheet) motif. A comparison between the sequences of the RII rat brain and the Drosophila extracellular loops forming scorpion toxin binding‐sites of Na+ channels displays differences in the subsites interacting with anti‐mammal or anti‐insect toxins. This suggests that hydrophobic as well as electrostatic interactions are essential for the binding and specificity of long‐chain scorpion toxins. Proteins 28:360–374, 1997 © 1997 Wiley‐Liss, Inc.