SOLUTION STRUCTURE OF LSIII, A LONG NEUROTOXIN FROM THE VENOM OF LATICAUDA-SEMIFASCIATA

We report the sequence-specific proton assignments and solution struct ure of the long neurotoxin LSIII from the venom of Laticauda semifasci ata determined by two- and three-dimensional H-1 NMR. Input for struct ure calculations consisted of 497 NOE-derived distance restraints and 45 dihedral angle restraints obtained from J couplings. A two-partide- per-residue representation of protein structure was used to generate 2 00 initial structures which were then subjected to all-atom refinement by simulated annealing. Twenty-three final structures consistent with the experimental restraints were obtained; the average atomic RMS dif ference between the individual structures and the mean structure was 0 .82 Angstrom for the backbone heavy atoms and 1.3 Angstrom for all hea vy atoms (residues 1-26, 37-60). The main elements of regular secondar y structure are a three-stranded antiparallel beta-sheet and three fin ger-like loops protruding from a globular core, consistent with previo usly reported structures of long neurotoxins. The end of the prominent loop II, which is involved in binding to acetylcholine receptor, is d isordered relative to the rest of the molecule. A novel finding of thi s study is that the loop has a well defined local structure; this and other observations suggest this region moves as a rigid body. We propo se that this motion is a heretofore unrecognized general feature of lo ng neurotoxins, with specific consequences for binding to the acetylch oline receptor.