Evidence that ferritin is associated with light production in the mucus of the marine worm Chaetopterus

Crystallographic characterization of a marine invertebrate ferritin from the sea cucumber Apostichopus japonicus

Ferritin is considered to be an ubiquitous and conserved iron-binding protein that plays a crucial role in iron storage, detoxification and immune response. Although ferritin is of critical importance for almost all kingdoms of life, there is a lack of knowledge about its role in the marine invertebrate sea cucumber (Apostichopus japonicus). In this study, we characterized the first crystal structure of Apostichopus japonicas ferritin (AjFER) at 2.75 Å resolution. The structure of AjFER shows a 4-3-2 symmetry cage-like hollow shell composed of 24 subunits, mostly similar to the structural characteristics of other known ferritin species, including the conserved ferroxidase center and 3-fold channel. The 3-fold channel consisting of three 3-fold negative amino acid rings suggests a potential pathway in which metal ions can be first captured by Asp120 from the outside environment, attracted by His116 and Cys128 when entering the channel, and then transferred by Glu138 from the 3-fold channel to the ferroxidase site. Overall, the presented crystal structure of AjFER may provide insights into the potential mechanism of the metal transport pathway for related marine invertebrate ferritins.

Conservative and Atypical Ferritins of Sponges

Ferritins comprise a conservative family of proteins found in all species and play an essential role in resistance to redox stress, immune response, and cell differentiation. Sponges (Porifera) are the oldest Metazoa that show unique plasticity and regenerative potential. Here, we characterize the ferritins of two cold-water sponges using proteomics, spectral microscopy, and bioinformatic analysis. The recently duplicated conservative HdF1a/b and atypical HdF2 genes were found in the Halisarca dujardini genome. Multiple related transcripts of HpF1 were identified in the Halichondria panicea transcriptome. Expression of HdF1a/b was much higher than that of HdF2 in all annual seasons and regulated differently during the sponge dissociation/reaggregation. The presence of the MRE and HRE motifs in the HdF1 and HdF2 promotor regions and the IRE motif in mRNAs of HdF1 and HpF indicates that sponge ferritins expression depends on the cellular iron and oxygen levels. The gel electrophoresis combined with specific staining and mass spectrometry confirmed the presence of ferric ions and ferritins in multi-subunit complexes. The 3D modeling predicts the iron-binding capacity of HdF1 and HpF1 at the ferroxidase center and the absence of iron-binding in atypical HdF2. Interestingly, atypical ferritins lacking iron-binding capacity were found in genomes of many invertebrate species. Their function deserves further research.

Iron redox pathway revealed in ferritin via electron transfer analysis

Ferritin protein is involved in biological tissues in the storage and management of iron - an essential micro-nutrient in the majority of living systems. While there are extensive studies on iron-loaded ferritin, its functionality in iron delivery is not completely clear. Here, for the first time, differential pulse voltammetry (DPV) has been successfully adapted to address the challenge of resolving a cascade of fast and co-occurring redox steps in enzymatic systems such as ferritin. Using DPV, comparative analysis of ferritins from two evolutionary-distant organisms has allowed us to propose a stepwise resolution for the complex mix of concurrent redox steps that is inherent to ferritins and to fine-tune the structure-function relationship of each redox step. Indeed, the cyclic conversion between Fe3+ and Fe2+ as well as the different oxidative steps of the various ferroxidase centers already known in ferritins were successfully discriminated, bringing new evidence that both the 3-fold and 4-fold channels can be functional in ferritin.

Chaetopterus variopedatus Bioluminescence: A Review of Light Emission within a Species Complex

Chaetopterus variopedatus has been studied for over a century in terms of its physiology, ecology and life history. One focus of research is on its intrinsic bioluminescent emissions, which can be observed as a blue light emitted from the extremities of individual body segments, or as a secreted mucus. Even though research shows that C. variopedatus is a species complex miscategorized as a single species, all of the variants of this polychaete produce light, which has been investigated in terms of both physiology and biochemistry. Despite decades of study, there are still many questions about the luminescence reaction, and, as of yet, no clear function for light emission exists. This review summarizes the current knowledge on C. variopedatus luminescence in addition to briefly describing its morphology, life cycle and ecology. Possible functions for luminescence were discussed using observations of specimens found in Brazil, along with a comparison of previous studies of other luminescent organisms. Further study will provide a better understanding of how and why C. variopedatus produces luminescence, and purifying the protein and luciferin involved could lead to new bioanalytical applications, as this reaction is unique among all known luminescent systems.

Bioluminescence properties of Thelepus japonicus (Annelida: Terebelliformia)

Terebelliformia is a benthic group of marine annelid worms. The bioluminescence of several species has been reported in taxonomical and histological literature, but very little information is known about the biochemical aspects of this phenomenon. In this study, we examined the basic properties of the luminescence system using an extract of the Japanese terebelliform worm, Thelepus japonicus. The bioluminescence extract was soluble in water, and emitted blue-green light at λ_max 508 nm following the addition of divalent cations. This triggering action was highly specific to Fe²⁺ and addition of ATP, H₂ O₂ or coelenterazine did not enhance activity. The bioluminescence was inactivated by heat treatment and organic solvents, indicating the involvement of a protein component. These results suggested that Thelepus worm produces light using a novel system that differs from that in other known luminescent annelids.

Novel gene encoding a unique luciferase from the fireworm Odontsyllis undecimdonta

Luciferases identified or engineered so far emit violet, blue, blue-green, green, yellow, red or near infra-red light. The unique and beautiful bluish-green bioluminescence of fireworms Odontosyllis spp. has attracted particular interest, however, their molecular basis is totally unknown partly due to the difficulty of animal collection. Here we report a novel type of luciferase gene from the Japanese fireworm O. undecimdonta. The major SDS-PAGE band of the luminous mucus showed luciferase activity. A highly sensitive mass spectrometry analysis in combination with RNA sequencing technique revealed that this band was product of a single gene with no homology to any other sequences in public databases. The recombinant protein of this putative luciferase gene expressed in mammalian cells produced the same unique bluish-green emission peak as the fireworm crude extract, indicating that this novel gene is the genuine fireworm luciferase with an evolutionary different origin from other luciferases previously described. Our findings extend the repertoire of luciferin/luciferase system to previously unavailable wavelength range.

First biochemical and crystallographic characterization of a fast-performing ferritin from a marine invertebrate

Ferritin, a multimeric cage-like enzyme, is integral to iron metabolism across all phyla through the sequestration and storage of iron through efficient ferroxidase activity. While ferritin sequences from ∼900 species have been identified, crystal structures from only 50 species have been reported, the majority from bacterial origin. We recently isolated a secreted ferritin from the marine invertebrate Chaetopterus sp. (parchment tube worm), which resides in muddy coastal seafloors. Here, we present the first ferritin from a marine invertebrate to be crystallized and its biochemical characterization. The initial ferroxidase reaction rate of recombinant Chaetopterus ferritin (ChF) is 8-fold faster than that of recombinant human heavy-chain ferritin (HuHF). To our knowledge, this protein exhibits the fastest catalytic performance ever described for a ferritin variant. In addition to the high-velocity ferroxidase activity, ChF is unique in that it is secreted by Chaetopterus in a bioluminescent mucus. Previous work has linked the availability of Fe²⁺ to this long-lived bioluminescence, suggesting a potential function for the secreted ferritin. Comparative biochemical analyses indicated that both ChF and HuHF showed similar behavior toward changes in pH, temperature, and salt concentration. Comparison of their crystal structures shows no significant differences in the catalytic sites. Notable differences were found in the residues that line both 3-fold and 4-fold pores, potentially leading to increased flexibility, reduced steric hindrance, or a more efficient pathway for Fe²⁺ transportation to the ferroxidase site. These suggested residues could contribute to the understanding of iron translocation through the ferritin shell to the ferroxidase site.