CRYSTAL-STRUCTURE DETERMINATION OF AN ACIDIC NEUROTOXIN (BMK M8) FROMSCORPION BUTHUS-MARTENSII KARSCH AT 0.25 NM RESOLUTION

The crystal structure of an acidic neurotoxin, BmK M8, from Chinese sc orpion Buthus martensii Karsch was determined at 0.25nm resolution. Th e X-ray diffraction data of BmK M8 crystals at 0.25nm resolution were collected on a Siemens area detector. Using molecular replacement meth od with a basic scorpion toxin AaH II in a search model, the cross-rot ation function, PC-refinement and translation function were calculated by X-PLOR program package. The correct orientation and position of Bm K M8 molecule in crystal were determined in a resolution range of 1.5- 0.35nm. The crystallographic refinement was further performed by stere ochemical restrict least-sq;are technique, followed by simulated annea ling, slow-cooling protocols. The final crystallographic R-factor at 0 .8-0.25 nm is 0.171. The standard deviations of bond length and bond a ngle from ideality are 0.001 7 nm and 2.24 degrees, respectively. The final model of BmK M8 structure is composed of a dense core of seconda ry structure elements by a stretch of alpha-helix with two and a half turns (residues 19-28) and a three-stranded antiparallel beta-sheet (r esidues 2-4, 32-37, 45-51). In addition, three loops protruded from th e structural core. The general folding properties of BmK M8 molecule w ere described; a common structure motif which may appear in all scorpi on neurotoxins was identified. The conserved aromatic residues and cha rged residues were found to be:distributed on two roughly opposite sur faces of the molecule. The relationship between these two faces and re ceptor-binding sites are also discussed.