Abstract
Many of the small, cysteine-rich ion-channel modulatory peptides found in Cnidaria are distantly related to vertebrate defensins (of the trans-defensin superfamily). Transcriptomic and proteomic studies of the endemic Australian speckled sea anemone (Oulactis sp.) yielded homologous peptides to known defensin sequences. We extended these data using existing and custom-built hidden Markov models to extract defensin-like families from the transcriptomes of seven endemic Australian cnidarian species. Newly sequenced transcriptomes include three species of Actiniaria (true sea anemones); the speckled anemone (Oulactis sp.), Oulactis muscosa, Dofleinia cf. armata and a species of Corallimorpharia, Rhodactis sp. We analyzed these novel defensin-like sequences along with published homologues to study the evolution of their physico-chemical properties in vertebrate and invertebrate fauna. The cnidarian trans-defensins form a distinct cluster within the chemical space of the superfamily, with a unique set of motifs and biophysical properties. This cluster contains identifiable subgroups, whose distribution in chemical space also correlates with the divergent evolution of their structures. These sequences, currently restricted to cnidarians, form an evolutionarily distinct clade within the trans-defensin superfamily.
Highlights
IntroductionDefensins are short proteins that can be classified into two superfamilies (cis- and trans-) with independent evolutionary origins
Defensins are short proteins that can be classified into two superfamilies with independent evolutionary origins
4.1 Taxonomic distribution and biological source Defensin-like sequences identified in the tentacles of the sea anemone Oulactis sp. had the greatest similarity to the PFAM protein families of Defensin 4 and less so to Toxin 4 (Nv1)
Summary
Defensins are short proteins that can be classified into two superfamilies (cis- and trans-) with independent evolutionary origins. Together, these superfamilies encompass members with a range of biological activities including antimicrobial peptides (AMPs), neurotoxins, and enzyme inhibitors (Shafee, Lay, Phan, Anderson, & Hulett, 2017). Cnidaria are unusual in that they contain sequences from both superfamilies (Shafee, Lay, Hulett, & Anderson, 2016; Shafee et al, 2017):, for example antimicrobial hydramacins from the cis-defensins, and enzyme inhibitors and toxins from the trans-defensins. Most members of the trans-defensin superfamily that have been functionally characterised are antimicrobial, a number of defensin-like proteins (DLPs) have been recruited to alternative biological functions. The fold has been recruited as enzyme inhibitors in ticks (e.g. TCI) and a sea anemone (helianthamide) (Arolas et al, 2005; Tysoe et al, 2016)
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