The Origin and Evolution of Metalloproteinases in the Venom of Snakes

Background: Snake venom metalloproteinases (SVMPs) are a pathologically-important, often major, toxin component of snake venoms, particularly in the venoms of viperid snakes. The SVMPs are members of the large multi-locus adamalysin gene family alongside ADAM (a disintegrin and metalloproteinase) and ADAMTS (ADAM with thrombospondin motifs) proteins. Here we discuss the evolution of SVMPs from: (i) their single ancestral recruitment into the venom of advanced snakes, to (ii) the diverse structural and functional isoforms observed in venom today. Methods: Phylogenetic analyses were used to reconstruct the evolutionary history of the adamlysins, with a focus on the SVMPs. Subsequently, the mode and tempo of SVMP evolution was analysed using ancestral sequence reconstructions, positive selection tests and macromolecular structure modeling. Results and Discussion: The ancestral recruitment of SVMPs into venom resulted from the duplication of an ADAM28-like gene. Consequently, basal SVMPs are most closely related to reptilian ADAM28 and mammalian ADAM28, ADAM7 and ADAM decysin-1 proteins. Notably, ancestral SVMPs exhibit complete conservation of cysteine residues with their non-venom ADAM homologs, demonstrating that the gain and loss of cysteine residues thought to be important for facilitating structural changes/post-translational modifications are the direct result of mutations following the recruitment of SVMPs into venom. Following this recruitment event, novel SVMP domain scaffolds have been generated in viperid snakes (P-II and P-I classes) through the duplication of SVMP genes coupled with the action of positive selection. P-III SVMPs first evolved into the P-II structure through the single evolutionary loss of the cysteine-rich domain, whilst multiple independent losses of the P-II disintegrin domain have resulted in convergent evolution of P-I SVMPs. In both instances of domain loss, adaptive evolution is a major driving force – positive selection was found to predominately act on amino acid residues predicted to be surface-exposed on the molecular surface of new SVMP scaffolds. Conclusions: These results highlight how changes to the genetic structure of venom toxins can catalyze the accelerated evolution of novel proteins and facilitate major structural and functional alterations. The generation of different molecular scaffolds (P-I, P-II and P-III SVMPs) encoded by the same multi-locus gene family appears to facilitate protein neofunctionalization, whilst also presenting an evolutionary advantage through the retention of multiple genes capable of encoding functionally distinct proteins.