A novel broad spectrum venom metalloproteinase autoinhibitor in the rattlesnake <i>Crotalus atrox</i> evolved via a shift in paralog function.

Abstract

The complexity of snake venom composition reflects adaptation to the diversity of prey and may be driven at times by a coevolutionary arms race between snakes and venom-resistant prey. However, many snakes are also resistant to their own venom due to serum-borne inhibitors of venom toxins, which raises the question of how snake autoinhibitors maintain their efficacy as venom proteins evolve. To investigate this potential three-way arms race among venom, prey, and autoinhibitors, we have identified and traced the evolutionary origin of serum inhibitors of snake venom metalloproteinases (SVMPs) in the Western Diamondback rattlesnake <i>Crotalus atrox</i> which possesses the largest known battery of SVMP genes among crotalids examined. We found that <i>C. atrox</i> expresses five members of a Fetuin A-related metalloproteinase inhibitor family but that one family member, FETUA-3, is the major SVMP inhibitor that binds to approximately 20 different <i>C. atrox</i> SVMPs and inhibits activities of all three SVMP classes. We show that the <i>fetua-3</i> gene arose deep within crotalid evolution before the origin of New World species but, surprisingly, <i>fetua-3</i> belongs to a different paralog group than previously identified SVMP inhibitors in Asian and South American crotalids. Conversely, the <i>C. atrox</i> FETUA-2 ortholog of previously characterized crotalid SVMP inhibitors shows limited activity against <i>C. atrox</i> SVMPs. These results reveal that there has been a functional evolutionary shift in the major SVMP inhibitor in the <i>C. atrox</i> lineage as the SVMP family expanded and diversified in the <i>Crotalus</i> lineage. This broad-spectrum inhibitor may be of potential therapeutic interest.