The extension peptide of plant ferritin from sea lettuce contributes to shell stability and surface hydrophobicity

Abstract

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.

Ferritin Iron Mineralization and Storage: From Structure to Function

“…The EP interacts with a hydrophobic patch at the shell surface and thus increases the Ftn solubility. In addition, the extra contacts between Ftn subunits allow for the increase of the Ftn stability ( Figure 6.1b) [7,8]. This stability is further used during plant germination in which autodegradation of the EP leads to lower stability of the Ftn core and faster iron release.…”

The iron content and ferritin contribution in fresh, dried, and toasted nori, Pyropia yezoensis

“…Generally, oligomeric ferritin can be detected by non-reducing PAGE without heat denaturation, because ferritin oligomer has high stability against the treatment of heat and denaturant. 26,27) Protein samples were prepared by extraction from 0.1 g of raw, dried, and toasted nori using 5 ml of PBS. So, each loaded sample contained soluble protein extracted from 200 μg of each form of nori.…”

“…The cDNA sequence has been submitted to the NCBI database (GenBank ID: AB918149). The amino acid sequence of PyFer shows 38% and 35% sequence identity with ferritin, respectively, from U. pertusa 24,26) (green macroalga) and soybean ferritin subunit No. 4.…”

“…The putative secondary structure deduced from the three-dimensional structure of algal 26) and higher-plant ferritin 28) is shown in Fig. 2(A).…”

“…26,28) The EP region also presents in PyFer sequence in the downstream of putative TP region. Since the core region forming the 4-helix bundle is highly conserved among PyFer, UpFer, and other plant type ferritins, PyFer is also supposed to forms 4-helix bundle subunit, which assembles to spherical 24-mer.…”