Crystallographic characterization of ferritin from Sinonovacula constricta

Structural Insights Into the Effects of Interactions With Iron and Copper Ions on Ferritin From the Blood Clam Tegillarca granosa

In addition to its role as an iron storage protein, ferritin can function as a major detoxification component in the innate immune defense, and Cu²⁺ ions can also play crucial antibacterial roles in the blood clam, Tegillarca granosa. However, the mechanism of interaction between iron and copper in recombinant Tegillarca granosa ferritin (TgFer) remains to be investigated. In this study, we investigated the crystal structure of TgFer and examined the effects of Fe²⁺ and Cu²⁺ ions on the TgFer structure and catalytic activity. The crystal structure revealed that TgFer presented a typically 4–3–2 symmetry in a cage-like, spherical shell composed of 24 identical subunits, featuring highly conserved organization in both the ferroxidase center and the 3-fold channel. Structural and biochemical analyses indicated that the 4-fold channel of TgFer could be serviced as potential binding sites of metal ions. Cu²⁺ ions appear to bind preferentially with the 3-fold channel as well as ferroxidase site over Fe²⁺ ions, possibly inhibiting the ferroxidase activity of TgFer. Our results present a structural and functional characterization of TgFer, providing mechanistic insight into the interactions between TgFer and both Fe²⁺ and Cu²⁺ ions.

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.

Structural comparison of two ferritins from the marine invertebrate Phascolosoma esculenta

For marine invertebrates with no adaptive immune system, ferritin is a major intracellular iron-storage protein with a critical role in innate immunity. Here, we present the crystal structures of two novel ferritins [Fer147 and Phascolosoma esculenta ferritin (PeFer)] from the marine invertebrate P. esculenta, which resides in muddy-bottom coastal regions. Fer147 and PeFer exhibit the 4-3-2 symmetry of cage-like hollow shells containing 24 subunits, similar to other known ferritins. Fer147 and PeFer contain both the conserved ferroxidase center and threefold channels. Subtle structural differences in the putative nucleation sites suggest possible routes of metal ion movement in the protein shells. However, the marked variation in the electrostatic potential of the threefold channels in Fer147 and the fourfold channels in PeFer suggests significant diversity between Fer147 and PeFer in terms of metal ion aggregation and cation exclusion. In summary, the presented crystal structures may serve as references for studies of the iron-storage mechanism of additional ferritins from marine invertebrates.