Plant ferritins have some unique structural and functional features. Most of these features can be related to the plant-specific ''extension peptide" (EP), which exists in the Nterminus of the mature region of a plant ferritin. Recent crystallographic analysis of a plant ferritin revealed the structure of the EP, however, two points remain unclear: (i) whether the structures of well-conserved EP of plant ferritins are common in all plants, and (ii) whether the EP truly contributes to the shell stability of the plant ferritin oligomer. To clarify these matters, we have cloned a green-plant-type ferritin cDNA from a green alga, Ulva pertusa, and investigated its crystal structure. Ulva pertusa ferritin (UpFER) has a plant-ferritin-specific extension peptide composed of 28 amino acid residues. In the crystal structure of UpFER, the EP lay on and interacted with the neighboring threefold symmetry-related subunit. The amino acid residues involved in the interaction were very highly conserved among plant ferritins. The EPs masked the hydrophobic pockets on the ferritin shell surface by lying on them, and this made the ferritin oligomer more hydrophilic. Furthermore, differential scanning calorimetric analysis of the native and its EP-deletion mutant suggested that the EP contributed to the thermal stability of the plant ferritin shell. Thus, the shell stability and surface hydrophobicity of plant ferritin were controlled by the presence or absence of the plant-ferritin-specific EP. This regulation can account for those processes such as shell stability, degradation, and association of plant ferritin, which are significantly related to iron utilization in plants.

Iron is an essential element for virtually all kingdoms of life, and especially for primary producers in ocean ecosystems. To date, the molecular mechanism of iron utilization by macroalgae remains largely unknown. To elucidate the strategy of iron acquisition and storage in macroalgae, we focused on the function of the iron storage protein ferritin in the sea lettuce, Ulva pertusa, which has abundant iron content. Judging from the primary structure, U. pertusa ferritin (UpFer) can be classified as a land-plant-type ferritin, which is usually found in plastids. The gene of UpFer was expressed in the peripheral, central and rhizoid parts. Western blot analysis showed that UpFER was present and functioned in processed 26- and 22-kDa forms. Furthermore, recombinant UpFER had iron incorporation activity comparable to other ferritins. These results suggest that ferritin also functions as an iron storage protein as in unicellular algae and land plants.

Densely packed invertebrate communities have been observed around deep-sea hydrothermal vents and cold seeps, despite the high concentrations of hydrogen sulfide present in these environments. The species that inhabit these environments have been reported to accumulate the sulfur-containing amino acid, thiotaurine. This non-toxic amino acid is synthesized from its precursor, hypotaurine and ambient hydrogen sulfide. In this study, we compared the levels of thiotaurine and hypotaurine in the gills and mantles of two deep-sea mussels (Bathymodiolus septemdierum and B. platifrons) and a shallow-water mussel (Mytilus galloprovincialis) to investigate their degree of dependence on the thiotaurine/hypotaurine system. The levels of thiotaurine and hypotaurine in the gill were higher than those present in the mantle in the two deep-sea species but lower or similar in M. galloprovincialis. The levels of thiotaurine and hypotaurine were the highest in the gill of B. septemdierum, estimated at 6.56 and 13.50 µmol/g wet weights, respectively. Moreover, B. septemdierum exhibited a higher thiotaurine/(hypotaurine+thiotaurine) ratio in the gill, B. septemdierum than that did B. platifrons and M. galloprovincialis. These results suggest that the levels of thiotaurine and hypotaurine in Bathymodiolus may be related to the ambient sulfide levels in their habitats.

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