CRYSTAL-STRUCTURE OF AN ACETYLCHOLINESTERASE-FASCICULIN COMPLEX - INTERACTION OF A 3-FINGERED TOXIN FROM SNAKE-VENOM WITH ITS TARGET

Background: Fasciculin (FAS), a 61-residue polypeptide purified from m amba venom, is a three-fingered toxin which is a powerful reversible i nhibitor of acetylcholinesterase (AChE). Solution of the three-dimensi onal structure of the AChE/FAS complex would provide the first structu re of a three-fingered toxin complexed with its target. Results: The s tructure of a complex between Torpedo californica AChE and fasciculin- II (FAS-II), from the venom of the green mamba (Dendroaspis angusticep s) was solved by molecular replacement techniques, and refined at 3.0 Angstrom resolution to an R-factor of 0.231. The structure reveals a s toichiometric complex with one FAS molecule bound to each AChE subunit . The AChE and FAS conformations in the complex are very similar to th ose in their isolated structures. FAS is bound at the 'peripheral' ani onic site of AChE, sealing the narrow gorge leading to the active site , with the dipole moments of the two molecules roughly aligned. The hi gh affinity of FAS for AChE is due to a remarkable surface complementa rity, involving a large contact area (similar to 2000 Angstrom(2)) and many residues either unique to FAS or rare in other three-fingered to xins. The first loop, or finger, of FAS reaches down the outer surface of the thin aspect of the gorge. The second loop inserts into the gor ge, with an unusual stacking interaction between Met33 in FAS and Trp2 79 in AChE. The third loop points away from the gorge, but the C-termi nal residue makes contact with the enzyme. Conclusions: Two conserved aromatic residues in the AChE peripheral anionic site make important c ontacts with FAS. The absence of these residues from chicken and insec t AChEs and from butyrylcholinesterase explains the very large reducti on in the affinity of these enzymes for FAS. Several basic residues in FAS make important contacts with AChE. The complementarity between FA S and AChE is unusual, inasmuch as it involves a number of charged res idues, but lacks any intermolecular salt linkages.