Structures of a venom peptide from distantly related arthropods have revealed the origin of such peptide toxins. The protein from a spider, Tegenaria agrestis, and that from a centipede, Scolopendra mutilans show very similar 3D folds despite having only around 22% sequence identity. The two structures are shown superimposed in the accompanying image.

A recent study indicated that this fold is similar to that of a hormone from a third arthropod, the Kuruma prawn and suggests that both predators, the spider and the centipede, have re-purposed an ancestral hormone into a toxin. This study is a great example of where structural studies can elucidate relationships between proteins that is hidden at the sequence level.

The two structures are available from PDB; that of the centipede PDB entry 2mun and the spider peptide 2ksl. The prawn hormone is PDB entry 1j0t. The paper, by Undheim et al., is published in Structure, DOI:10.1016/j.str.2015.05.003