Dark venom: the obscure function of xibalbin2 from marine, cave-dwelling remipede crustaceans

Marine venoms are promising bioresources for potential agrochemical and pharmaceutical applications. However, only toxins of few better-studied taxa such as cones, cnidarians, cephalopods and echinoderms are functionally more detailed known. The majority of venomous species, especially marine invertebrates and their toxin bioactivity remain unknown. The only described crustacean venom derives from Xibalbanus tulumensis (Crustacea: Remipedia) that inhabits anchialine underwater caves. Recent proteo-transcriptomic studies revealed, that its venom is dominated by enzymatic proteins but includes several components, which feature a knottin-like cysteine scaffold and are potentially neurotoxic. In this study, the highest expressed knottin-like venom component xibalbin₂ with an unusual ten-cysteine scaffold was recombinantly produced in E. coli adapting a fusion protein strategy. Our comprehensive screening approach revealed no significant antimicrobial, antifungal nor neurotoxic activity. Xibalbin₂ further seems to lack ion channel based neurotoxic activity, which could hint to a highly specific target of this novel, potentially lineage-unique venom component. Its function remains obscure and could be linked to a synergistic mode-of-action with other venom components or currently unknown targets, which both need to be further investigated.