Agkistrodotoxin is a phospholipase A2-type presynaptic neurotoxin from Agkistrodon halys pallas. The neurotoxin crystallizes in a new crystal form, R3, under basic conditions. The crystal structure has been determined at 0.28 nm resolution by the molecular replacement technique and refined to a crystallographic R factor of 0.23 and reasonable stereochemistry. The crystal structure of agkistrodotoxin in R3 form is compared with those of the other two crystal forms. The extensive intermolecular hydrophobic interactions occurring at the interfacial recognition site of each molecule play an important role in the crystalpacking for all the three crystal forms. The possible biological implications of the hydrophobic interactions are discussed.

Key words: NEUROTOXINS, PHOSPHOLIPASE, X-RAY DIFFRACTION ANALYSIS, CRYSTAL STRUCTURE