THE AB INITIO STRUCTURE AND REFINEMENT OF A SCORPION PROTEIN TOXIN. G.D. Smith^1,2, R.H. Blessing¹, S.E. Ealick³, J.C. Fontecilla-Camps⁴, H.A. Hauptman¹, D. Housset⁴, D.A. Langs¹, R. Miller^1,5. ¹Hauptman-Woodward Med. Res. Inst., 73 High St., Buffalo, NY 14203 USA; ²Roswell Park Cancer Institute, Elm & Carlton St., Buffalo, NY 14263 USA; ³Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, NY 14853 USA; ⁴Laboratoire de Cristallographie et Cristallisation des Proteines, Institut de Biologie Structurale Jean Pierre Ebel-CEA, CNRS, 41, Avenue des Martyrs, 38027 Grenoble Cedex 1 France; ⁵Dept. of Computer Science, State Univ. of New York at Buffalo, Buffalo, NY, 14260 USA.

The structure of a 64 residue scorpion toxin, which crystallizes in space group P2₁2₁2₁, has been determined ab initio at 0.96Å using the SnB program. A total of 50,000 triples were generated from 5,000 phases and the value of R_min clearly revealed a single solution amongst the 1619 trials, using 255 Shake-and-Bake cycles per randomly generated trial structure. The number of residues in the protein and the amino acid sequence were deliberately withheld from the Buffalo group; the only known information was that the protein was composed of approximately 500 atoms and contained 4 disulfide bonds. A very conservative initial starting model consisted of five fragments varying in length from 5 to 18 residues for a total of 241 atoms. The entire structure was revealed following multiple cycles of Xplor refinement and Fourier maps, and only four residues were incorrectly identified; three of these residues were subsequently found to be seriously disordered while the fourth suffered from high thermal motion. The structure has been refined to a residual of 0.160 for 512 protein atoms, 112 water molecules, and 418 protein hydrogen atoms. A mean phase error of 19.1deg. was calculated for the difference between the SnB phases and the final refined phases. It is estimated that given the sequence, 88% of the backbone atoms and 30% of the side chain atoms could have been extracted from the initial SnB phase set. Research supported by NIH grant GM-46733 and NSF grant IRI9412415.