Three-dimensional structure of a protein from scorpion venom: a new structural class of neurotoxins.

Abstract

The 3-dimensional crystal structure of variant-3, toxin from the scorpion Centruroides sculpturatus was determined at 3 .ANG. resolution. Phases were obtained by use of K2PtCL4 and K2IrCl6 derivatives. The most prominent secondary structural features were 2 1/2 turns of .alpha.-helix and a 3-strand stretch of antiparallel .beta.-sheet, which ran parallel to the .alpha.-helix. The helix was connected to the middle strand of the .beta.-sheet by 2 disulfide bridges; a 3rd disulfide bridge was located nearby. Several loops extended out of this dense core of secondary structure. The largest loop was joined to the COOH terminus of the molecule by the 4th disulfide bridge. The overall shape of the molecules resembled a right-hand fist: the .alpha.-helix ran along the knuckles of the fist; the .beta.-sheet along the 2nd and 3rd joints of the fingers; the thumb defined by 2 short loops composed of residues 16-21 and residues 41-46; the wrist corresponded to the COOH terminal stretch of residues 52-65 and a loop composed of residues 5-14; the 2nd joint of the little finger was near the NH2 terminus of the molecule. The .alpha.-carbon backbone displayed a large flat surface along the 2nd joints of the fingers and the heel of the hand in the fist model. Severl conserved residues in the scorpion neurotoxins were clustered on this surface, which may play a role in interactions of scorpion toxins with Na channels of excitable membranes.