TRANSFER OF A BETA-HAIRPIN FROM THE FUNCTIONAL SITE OF SNAKE CURAREMIMETIC TOXINS TO THE ALPHA BETA SCAFFOLD OF SCORPION TOXINS - 3-DIMENSIONAL SOLUTION STRUCTURE OF THE CHIMERIC PROTEIN/

The alpha/beta scorpion fold is shared by scorpion toxins, insect defe nsins, and plant thionins. This small and functionally versatile templ ate contains an alpha-helix and a triple beta-sheet linked by three di sulfide bridges. With the view to introduce novel functional centers w ithin this fold, we replaced the sequence (the cysteines and glycines excepted) of the original beta-hairpin of a scorpion toxin by the sequ ence of a beta-hairpin that forms part of the site by which snake neur otoxins bind to nicotinic acetylcholine receptors (AcChOR). The result ing chimeric protein, synthesized by chemical means, binds to AcChOR, though with a lower affinity than the snake toxins [Drakopoulou, E., Z inn-Justin, S., Guenneugues, M., Gilquin, B., Menez, A., & Vita, C. (1 996) J. Biol. Chem. 271, 11979-11987]. The work described in this pape r is an attempt to clarify the structural consequences associated with the transfer of the beta-hairpin. We report the determination of the three-dimensional solution structure of the chimeric protein by proton NMR spectroscopy and molecular dynamics calculations. Comparison of t he structure of the chimera with those of the scorpion alpha/beta toxi n and of the snake neurotoxin shows that (i) the new protein folds as an alpha/beta motif and (ii) the beta-hairpins of the chimera and of t he curaremimetic toxin adopt a similar conformation. A closer inspecti on of the differences between the structures of the original and trans ferred beta-hairpins allows rationalization of the biological properti es of the chimera.