Crystal structure determination of a neutral neurotoxin BmK M4 from Buthus martensii Karsch at 0.20 nm

BmK M4 is a neutral neurotoxin in the BmK toxin series. It is medially toxic and belongs to group III α-toxins. The purified sample was crystallized in rhombic space group P61. Using an X-ray diffraction technique, the crystal structure of BmK M4 was revealed by molecular replacement at 0.20 nm resolution. The model was refined. The final crystallographicR factor was 0.142 and the freeR factor was 0.173. The root mean square deviation is 0.001 5 nm for the bond length and 1.753° for the bond angles. 64 water molecules were added to the asymmetric unit. The refined structure showed an unusual non-prolylcis peptide bond at residue 10. The structure was compared with group II α-toxin BmK M8 (an acidic, weak toxin). The potential structural implications of thecis peptide bond were discussed.