CRYSTAL STRUCTURE OF AN ACIDIC NEUROTOXIN FROM SCORPION BUTHUS MARTENSII KARSCH AT 1.85Å RESOLUTION. Da-Cheng Wang, Hong-Min Li, Lei Jin, Zong-Hao Zeng, Instltute of Biophyslcs, Chinese Academy of Sciences, Beijing 100101, P.R. China

The crystal structure of an acidic scorpion neurotoxin BmK M8 has been determined by using molecular replacement method. It is the first acidic [[alpha]]-scorpion neurotoxin reported so far. Compared with the basic and most potent [[alpha]]-toxin, AaH II, the BmK M8 is a relatively inactive toxin (1100 times less active than AaH II) with an unusually low isoelectric point (5.3). The much drop in toxin lethality is accompanying the acidic property of the molecule. The refined structure showed new functionally important area and some features of the toxin-receptor binding. In association with the information from antibody mapping for localization of binding site on sodium channel, a multiposition mode for the toxin-receptor interaction and three "toxic regions" related to the binding process, including Face A, Face B and Site C, are proposed. The Face A, mainly consisted of Tyr5, 35, 47, [[alpha]]-amino group, Arg2 and Asp3, may be more essential for the binding; the Face B, mainly comprised conserved residues Tyrl4, 21, Lys28 and Val55, may contribute to the high efficacy of the binding process and substitutions of acidic residues in this area will strongly weaken the toxic activity; and the Site C formed by Lys58 and Arg62 from C-terminal and Arg41 and Tyr42 from loop 38~44 may be involved in the binding site-specificity. On basis of these, a toxin-sodium channl binding model will be proposed.

BmK M8 was purified from Chinese scorpion Buthus martensii Karsch. The crystals of BmK M8 belong to space group P2₁ and contains one molecule per asymmetric unit. The structure model of BmK M8 has been refined to a R factor of 18.1% with data from 8Å to 1.85Å.