Evolution of the Cytolytic Pore-Forming Proteins (Actinoporins) in Sea Anemones

Abstract

Sea anemones (Cnidaria, Anthozoa, and Actiniaria) use toxic peptides to incapacitate and immobilize prey and to deter potential predators. Their toxin arsenal is complex, targeting a variety of functionally important protein complexes and macromolecules involved in cellular homeostasis. Among these, actinoporins are one of the better characterized toxins; these venom proteins form a pore in cellular membranes containing sphingomyelin. We used a combined bioinformatic and phylogenetic approach to investigate how actinoporins have evolved across three superfamilies of sea anemones (Actinioidea, Metridioidea, and Actinostoloidea). Our analysis identified 90 candidate actinoporins across 20 species. We also found clusters of six actinoporin-like genes in five species of sea anemone (Nematostella vectensis, Stomphia coccinea, Epiactis japonica, Heteractis crispa, and Diadumene leucolena); these actinoporin-like sequences resembled actinoporins but have a higher sequence similarity with toxins from fungi, cone snails, and Hydra. Comparative analysis of the candidate actinoporins highlighted variable and conserved regions within actinoporins that may pertain to functional variation. Although multiple residues are involved in initiating sphingomyelin recognition and membrane binding, there is a high rate of replacement for a specific tryptophan with leucine (W112L) and other hydrophobic residues. Residues thought to be involved with oligomerization were variable, while those forming the phosphocholine (POC) binding site and the N-terminal region involved with cell membrane penetration were highly conserved.

A process of convergent amplification and tissue‐specific expression dominates the evolution of toxin and toxin‐like genes in sea anemones

“…This suggests that potentially phylogenetic inertia is an important mechanism in the evolution and distribution of TTL genes in sea anemones. Studies investigating the sequence variation of TTL gene families in cnidarians consistently report a similar pattern (Jouiaei et al, ; Macrander et al, , ; Macrander & Daly, ). This is in contrast to what has been observed in other venomous lineages, such as snakes, which show significant differences in their sequence variation and in TTL gene complement within and across lineages (Amazonas et al, ; Chippaux, Williams, & White, ; Dowell et al, ,; Wooldridge et al, ).…”

The chemical armament of reef-building corals: inter- and intra-specific variation and the identification of an unusual actinoporin in Stylophora pistilata

“…Interestingly, Trp112 in Eqt-II, which was shown to be critical for sphingonmyelin recognition and hemolytic activity 69 , is replaced in Δ-Pocilopotoxin-Spi1 by phenylalanine, which is also hydrophobic. This important residue is replaced in other actinoporins, for example by leucine in HALT-1 from Hydra magnipappilata 70 – 72 . A notable difference between the anemone and coral actinoporins is that Δ-Pocilopotoxin-Spi1 has a single Cysteine residue (marked with a blue triangle in Fig.…”

“…Actinoporins are common in sea anemones and Hydra, yet are missing from the transcriptome of at least one scyphozoan jellyfish 16 , 72 . Similar proteins are found also in other non-cnidarian organisms, with the role of most of these non-cnidarian actinoporins currently not known 72 , 74 , 75 . Recently, a database containing the genomes and/or transcriptomes of 20 coral species has been assembled 76 , allowing the presence of genes encoding this family of hemolysins to be mapped across coral diversity.…”

“…In some cnidarians, most notably sea anemones and Hydra, actinoporins are observed as gene families that underwent gene duplication and diversification (Fig. 6 60 , 72 ). Unlike many toxins from cone snails and scorpions, which are under strong positive/diversifying selection, most of the residues in actinoporins are under strong negative/purifying selection, although some might be affected by episodic diversifying selection 39 , 60 , 80 .…”

Characterising Functional Venom Profiles of Anthozoans and Medusozoans within Their Ecological Context

“…Comparative analysis supports the minimal impact of environmental factors on the toxin gene complement, revealing that closely related cnidarian species have more similar toxin gene complements than those that share an ecological niche [16]. Moreover, phylogenetic investigations of sequence variation in cnidarian toxin genes consistently report that toxin gene distribution correlates with species relatedness [49][50][51][52]. These results suggest that speciation is an important driver of toxin gene complement and sequence variation.…”