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 channels, have been divided into two groups, the alpha and beta toxins, according to their activities. The beta-toxin class includes the groups of excitatory and depressant toxins, which differ in their mode of action and are highly specific against insects. The three-dimensional structures of several alpha and beta toxins have been determined at high resolution, but no detailed 3D structure of an excitatory toxin has been presented so far. RESULTS: The crystal structure of an anti-insect excitatory toxin from the scorpion Buthotus judaicus, Bj-xtrIT, has been determined at 2.1 A resolution and refined to an R factor of 0.209. The first 59 residues form a closely packed module, structurally similar to the conserved alpha and beta toxins ('long toxins') affecting sodium channels. The last 17 residues form a C-terminal extension not previously seen in scorpion toxins. It comprises a short alpha helix anchored to the N-terminal module by a disulfide bridge and is followed 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 interaction with the channel in several long toxins. CONCLUSIONS: Replacement of the last seven residues by a single glycine abolishes the activity of Bj-xtrIT, strongly suggesting that these residues are intimately involved in the interaction with the channel. Taken together with the partial shielding of the conserved hydrophobic surface and the proximity of the C terminus to an adjacent surface rich in charged residues, it seems likely that the bioactive surface of Bj-xtrIT is formed by residues surrounding the C terminus. The 3D structure and a recently developed expression system for Bj-xtrIT pave the way for identifying the structural determinants involved in the bioactivity and anti-insect specificity of excitatory toxins.