Crystal structure determination of an acidic neurotoxin (BmK M8) from scorpion Buthus martensii Karsch at 0.25 nm resolution.

The crystal structure of an acidic neurotoxin, BmK M8, from Chinese scorpion Buthus martensii Karsch was determined at 0.25 nm resolution. The X-ray diffraction data of BmK M8 crystals at 0.25 nm resolution were collected on a Siemens area detector. Using molecular replacement method with a basic scorpion toxin AaH II in a search model, the cross-rotation function, PC-refinement and translation function were calculated by X-PLOR program package. The correct orientation and position of BmK M8 molecule in crystal were determined in a resolution range of 1.5-0.35 nm. The crystallographic refinement was further performed by stereo-chemical restrict least-square technique, followed by simulated annealing, 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 angle from ideality are 0.0017 nm and 2.24 degrees, respectively. The final model of BmK M8 structure is composed of a dense core of secondary structure elements by a stretch of alpha-helix with two and a half turns (residues 19-28) and a three-stranded antiparallel beta-sheet (residues 2-4, 32-37, 45-51). In addition, three loops protruded from the structural core. The general folding properties of BmK M8 molecule were described; a common structure motif which may appear in all scorpion neurotoxins was identified. The conserved aromatic residues and charged residues were found to be distributed on two roughly opposite surfaces of the molecule. The relationship between these two faces and receptor-binding sites are also discussed.