Non-crystallographic symmetry of crystal of neutral phospholipase A2 from Agkistrodon halys Pallas

Agkistrodotoxin, a neutral phospholipase A2 with high presynaptic neurotoxicity from the venom of Agkistrodon halys Pallas, has been crystallized by hanging drop vapor diffusion method. The crystal belongs to P21 space group with the cell dimensions a = 10.836 nm, b=8.486nm, c = 7.082nm, β=109.87$ showing C2 pseu-do-symmetry. Diffraction data to 0. 26 nm resolution have been collected on a Siemens X-200B area detector. C2 pseu-do-symmetry suggests that there exists a non-crystallographic two-fold axis parallel to crystallographic b axis. Self-rotation function calculation with different integrated radius and resolution ranges using the program POLARRFN yields four stable high peaks corresponding to three more non-crystallographic two-fold axis and one special non-crystal-lographic symmetry. The molecules in the asymmetric unit are suggested to be arranged in a manner of "dimer of dimers" by inference.