Proteo-transcriptomic analysis identifies potential novel toxins secreted by the predatory, prey-piercing ribbon worm Amphiporus lactifloreus

Nemerteans (ribbon worms) employ toxins to subdue their prey, but research thus far has focused on the small-molecule components of mucus secretions and few protein toxins have been characterized. We carried out a preliminary proteotranscriptomic analysis of putative toxins produced by the hoplonemertean Amphiporus lactifloreus (Hoplonemertea, Amphiporidae). We did not find any variants of known nemertean-specific toxin proteins (neurotoxins, cytotoxins, parbolysins or nemertides) but we identified several toxin-like transcripts expressed strongly in the proboscis, including putative metalloproteinases and sequences resembling sea anemone actitoxins, crown-of-thorn sea star plancitoxins, and multiple classes of inhibitor cystine knot/knottin family proteins. Some of these products were also directly identified in the mucus proteome, supporting their preliminary identification as secreted toxin components. We identified two new nemertean-typical toxin candidates and named them U-nemertotoxin-1 and U-nemertotoxin-2. Our findings provide insight into the largely overlooked venom system of nemerteans and support a hypothesis in which the nemertean proboscis evolved in several steps, from a flesh-melting organ in scavenging nemerteans to a flesh-melting and toxin-secreting venom apparatus in hunting hoplonemerteans.