Structure of fasciculin 2 from green mamba snake venom: evidence for unusual loop flexibility

The crystal structure of the snake toxin fasciculin 2, a potent acetylcholinesterase inhibitor from the venom of the green mamba (Dendroaspis angusticeps), has been determined by the molecular-replacement method, using the fasciculin 1 model and refined to 2.0 A resolution. The introduction of an overall anisotropic temperature factor improved significantly the quality of the electron-density map. It suggests, as it was also indicated by the packing, that the thermal motion along the unique axis direction is less pronounced than on the (ab) plane. The final crystallographic R factor is 0.188 for a model having r.m.s. deviations from ideality of 0.016 A for bond lengths and 2.01 degrees for bond angles. As fasciculin 1, fasciculin 2 belongs to the three-finger class of Elapidae toxins, a structural group that also contains the alpha-neurotoxins and the cardiotoxins. Although the two fasciculins have, overall, closely related structures, the conformation of loop I differs appreciably in the two molecules. The presence of detergent in crystallization medium in the case of fasciculin 2 appears to be responsible for the displacement of the loop containing Thr9. This conformational change also results in the formation of a crystallographic dimer that displays extensive intermolecular interactions.