Abstract
Many pigments, such as melanins, are widely distributed throughout the animal kingdom. Others have arisen as novelties in particular lineages, for example, the Green Fluorescent Protein (GFP) found in cnidarians. While GFPs, widely used as fluorescent tags in biomedical research, are the most famous cnidarian example, other novel proteins have also been identified within this phylum. A blue protein that contains a Kringle (KR) domain inserted within a Frizzled cysteine-rich domain (Fz-CRD) was previously described from the jellyfish Rhizostoma pulmo (named rpulFKz1), however little is known about this pigment’s evolution or distribution among cnidarians. We performed a systematic search for homologs of this protein in published genomes and transcriptomes of 93 cnidarians. Phylogenetic analyses revealed eight predicted proteins that possess both domains in the same arrangement and that fall within the same clade as rpulFKz1. The sequence of one of these proteins contains motifs that match sequenced peptides of Cassio Blue, the blue pigment from Cassiopea xamachana. Another one of these proteins belongs to Stomolophus meleagris, and chemical studies on blue pigments that may occur in this genus have shown similarities to rpulFKz1 and Cassio Blue. Therefore, we hypothesize that the eight rpulFKz1 homologs identified are also pigment precursors. All precursors identified were exclusive to jellyfish in the order Rhizostomeae, so we herein name this new pigment family “rhizostomins.” Not all rhizostomes analyzed are blue, however, so these rhizostomin proteins may also be responsible for other colors, or perform other biochemical and biophysical roles. Previous studies have hypothesized that cnidarian pigments are photoprotective, and this study serves as basis for future investigations not only on the function of rhizostomins, but also on potential biotechnological applications for these proteins.
Highlights
Few features of any organism draw more attention than their color
Biotechnology has been revolutionized by Green Fluorescent Proteins (GFP), a family of protein pigments first identified in the cnidarian Aequorea victoria (Murbach and Shaerer, 1902) (Shimomura et al, 1962), and used as fluorescent tags in various in vivo labeling and detection methods (Matz et al, 2002), which led to a Nobel Prize in Chemistry in 2008
The remaining eight are part of a separate clade that contains the pigment precursor of Rhizostoma pulmo, rpulFKz1. Each of these eight proteins belong to scyphozoan jellyfishes in the order Rhizostomeae. In both phylogenies there are sequences that did not contain both Frizzled cysteine-rich domain (Fz-CRD) and KR domains that fall within this clade, but most still belong to rhizostomes
Summary
Few features of any organism draw more attention than their color. Color can play an important role in an organism’s survival, providing camouflage and photoprotection, and influencing sexual selection and social interactions (Cuthill et al, 2017). Rhizostomins: A Novel Pigment Family that selectively absorb light and can either be naturally produced by the organism or acquired by ingestion (Hendry, 1996). Some common types of pigments in animals are carotenoids, melanins, tetrapyrroles and quinones (Hendry, 1996; Bandaranayake, 2006). These pigments can serve a wide range of biophysical and biochemical functions (for an extensive review see Needham, 1974). Biotechnology has been revolutionized by Green Fluorescent Proteins (GFP), a family of protein pigments first identified in the cnidarian Aequorea victoria (Murbach and Shaerer, 1902) (Shimomura et al, 1962), and used as fluorescent tags in various in vivo labeling and detection methods (Matz et al, 2002), which led to a Nobel Prize in Chemistry in 2008
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