Solution structure of a beta-neurotoxin from the New World scorpion Centruroides sculpturatus Ewing

We report the detailed solution structure of the 7.2 kDa protein CsE-I, a b eta-neurotoxin from the New World scorpion Centruroides sculpturatus Ewing. This toxin binds to sodium channels, but unlike the alpha-neurotoxins, shi fts the voltage of activation toward more negative potentials causing the m embrane to fire spontaneously. Sequence-specific proton NMR assignments wer e made using 600 MHz 2D-NMR data. Distance geometry and dynamical simulated annealing refinements were performed using experimental distance and torsi on angle constraints from NOESY and pH-COSY data. A family of 40 structures without constraint violations was generated, and an energy-minimized avera ge structure was computed. The backbone conformation of the CsE-I toxin sho ws similar secondary structural features as the prototypical alpha-neurotox in, CsE-v3, and is characterized by a short 2 1/2-turn alpha-helix and a 3- strand antiparallel beta-sheet, both held together by disulfide bridges. Th e RMSD for the backbone atoms between CsE-I and CsE-v3 is 1.48 Angstrom. De spite this similarity in the overall backbone folding, the these two protei ns show some important differences in the primary structure (sequence) and electrostatic potential surfaces. Our studies provide a basis for unravelli ng the role of these differences in relation to the known differences in th e receptor sites on the voltage sensitive sodium channel for the alpha- and beta-neurotoxins. (C) 1999 Academic Press.