AN EXCITATORY SCORPION TOXIN WITH A DISTINCTIVE FEATURE - AN ADDITIONAL ALPHA-HELIX AT THE C-TERMINUS AND ITS IMPLICATIONS FOR INTERACTION WITH INSECT SODIUM-CHANNELS

Background: Scorpion neurotoxins, which bind and modulate sodium chann els, have been divided into two groups, the alpha and beta toxins, acc ording to their activities. The beta-toxin class includes the groups o f excitatory and depressant toxins, which differ in their mode of acti on and are highly specific against insects. The three-dimensional stru ctures of several alpha and beta toxins have been determined at high r esolution, but no detailed 3D structure of an excitatory toxin has bee n presented so far. Results: The crystal structure of an anti-insect e xcitatory toxin from the scorpion Buthotus judaicus, Bj-xtrlT, has bee n determined at 2.1 Angstrom resolution and refined to an R factor of 0.209. The first 59 residues form a closely packed module, structurall y similar to the conserved alpha and beta toxins ('long toxins') affec ting sodium channels. The last 17 residues form a C-terminal extension not previously seen in scorpion toxins. It comprises a short a helix anchored to the N-terminal module by a disulfide bridge and is followe d by a highly mobile stretch of seven residues, of which only four are seen in the electron-density map. This mobile peptide covers part of a conserved hydrophobic surface that is thought to be essential for in teraction with the channel in several long toxins. Conclusions: Replac ement of the last seven residues by a single glycine abolishes the act ivity of Bj-xtrlT, strongly suggesting that these residues are intimat ely involved in the interaction with the channel. Taken together with the partial shielding of the conserved hydrophobic surface and the pro ximity of the C terminus to an adjacent surface rich in charged residu es, it seems likely that the bioactive surface of Bj-xtrlT is formed b y residues surrounding the C terminus. The 3D structure and a recently developed expression system for Bj-xtrlT pave the way for identifying the structural determinants involved in the bioactivity and anti-inse ct specificity of excitatory toxins.