Cloning, expression and characterization of horse L-ferritin in Escherichia coli

The synergistic mechanisms of apo-ferritin structural transitions and Au(iii) ion transportation: molecular dynamics simulations with the Markov state model

Due to its unique structure, recent years have witnessed the use of apo-ferritin to accumulate various non-natural metal ions as a scaffold for nanomaterial synthesis. However, the transport mechanism of metal ions into the cavity of apo-ferritin is still unclear, limiting the rational design and controllable preparation of nanomaterials. Here, we conducted all-atom classical molecular dynamics (MD) simulations combined with Markov state models (MSMs) to explore the transportation behavior of Au(iii) ions. We exhibited the complete transportation paths of Au(iii) from solution into the apo-ferritin cage at the atomic level. We also revealed that the transportation of Au(iii) ions is accompanied by coupled protein structural changes. It is shown that the 3-fold axis channel serves as the only entrance with the longest residence time of Au(iii) ions. Besides, there are eight binding clusters and five 3-fold structural metastable states, which are important during Au(iii) transportation. The conformational changes of His118, Asp127, and Glu130, acting as doors, were observed to highly correlate with the Au(iii) ion's position. The MSM analysis and Potential Mean Force (PMF) calculation suggest a remarkable energy barrier near Glu130, making it the rate-limiting step of the whole process. The dominant transportation pathway is from cluster 3 in the 3-fold channel to the inner cavity to cluster 5 on the inner surface, and then to cluster 6. These findings provide inspiration and theoretical guidance for the further rational design and preparation of new nanomaterials using apo-ferritin.

Synthesis of rare earth doped yttrium-vanadate nanoparticles encapsulated within apoferritin

Luminescent europium (Eu) and dysprosium (Dy) doped yttrium-vanadate (Y-V) nanoparticles (NPs) were synthesized in the cavity of the protein, apoferritin. Y-V NPs were synthesized by incubating a solution of apoferritin with Y(3+) and VO3(-) ions in the presence of ethylene diamine-N-N'-diacetic acid (EDDA). EDDA plays an important role in preventing Y-vanadate precipitation in bulk solution by chelating the Y(3+) ions. Using high resolution electron microscopy, the obtained NPs in the apoferritin cavities were confirmed to be amorphous, and to consist of Y and V. Eu-doped Y-V (Y-V:Eu) NPs were synthesized by the same procedure as Y-V NPs, except that Eu(NO3)3 was added. Y-V:Eu NPs exhibited a strong absorption peak due to the O-V charge transfer transition and remarkable luminescence at 618 nm due to the (5)D0 → (7)F2 transition. The luminescence lifetime of Y:Eu and Y-V:Eu NPs measured in H2O and D2O solution showed reduction of non-radiative transition to the O-H vibration in Y-V:Eu NPs. Accordingly, Y-V NPs showed strong luminescence compared to Y:Eu NPs. Dy-doped Y-V NPs were also synthesized in apoferritin cavities and showed luminescence peaks at 482 nm and 572 nm, corresponding to (4)F9/2 → (6)H15/2 and (4)F9/2 → (6)H13/2 transitions. These NPs stably dispersed in water solution since their aggregation was prevented by the protein shell. NPs encapsulated in the protein are likely to be biocompatible and would have significant potential for biological imaging applications.

Structural analysis of haemin demetallation by L-chain apoferritins

There are extensive structural similarities between eukaryotic and prokaryotic ferritins. However, there is one essential difference between these two types of ferritins: bacterioferritins contain haem whereas eukaryotic ferritins are considered to be non-haem proteins. In vitro experiments had shown that horse spleen apoferritin or recombinant horse L chain apoferritins, when co-crystallised with haemin, undergoes demetallation of the porphyrin. In the present study a cofactor has been isolated directly from horse spleen apoferritin and from crystals of the mutant horse L chain apoferritin (E53Q, E56Q, E57Q, E60Q and R59M) which had been co-crystallised with haemin. In both cases the HPLC/ESI-MS results confirm that the cofactor is a N-ethylprotoporphyrin IX. Crystal structures of wild type L chain horse apoferritin and its three mutants co-crystallised with haemin have been determined to high resolution and in all cases a metal-free molecule derived from haemin was found in the hydrophobic pocket, close to the two-fold axis. The X-ray structure of the E53Q, E56Q, E57Q, E60Q+R59M recombinant horse L-chain apoferritin has been obtained at a higher resolution (1.16Å) than previously reported for any mammalian apoferritins. Similar evidence for a metal-free molecule derived from haemin was found in the electron density map of horse spleen apoferritin (at a resolution of 1.5Å). The out-of-plane distortion of the observed porphyrin is clearly compatible with an N-alkyl porphyrin. We conclude that L-chain ferritins are capable of binding and demetallating haemin, generating in the process N-ethylprotoporphyrin IX both in vivo and in vitro.

Sequence analysis of canine and equine ferritin H and L subunit cDNAs

Canine and equine ferritin H and L subunit cDNA clones were obtained using reverse transcriptase-polymerase chain reaction (RT-PCR) and TA cloning from various tissues. Canine liver and spleen ferritin H subunit cDNA clones contained an open reading frame for the same 182-amino acid protein as that reported in canine brain ferritin H subunit cDNA although there were substitutions in the 3'-noncoding regions. Ferritin L subunit cDNA clones from canine liver, spleen, and kidney showed identical coding sequences encoding the 174-amino acid protein except for a single nucleotide substitution in kidney (C474G). The H subunit nucleotide sequences of equine leukocyte and spleen were identical to the fragment encoding the 181-amino acid protein in equine peripheral blood mononuclear cells, with the exception of one substitution seen in both leukocyte and spleen sequences (C234T). The nucleotide sequence of equine leukocyte ferritin L subunit showed 7 substitutions compared with the published equine liver L subunit sequence with two substitutions at positions 281 and 282 resulting in an amino acid substitution of P94L. The amino acid residues involved in the ferroxidase center and in iron nucleation were perfectly conserved in H and L subunits of canine and equine ferritins, respectively.

Hexagonal close-packed array formed by selective adsorption onto hexagonal patterns

A patterned two-dimensional hexagonally ordered array of ferritin molecules, the outer surfaces of which had been genetically modified by titanium (Ti) specific binding peptides (minT1-LF), was realized in a self-assembling manner on a hexagonal Ti thin film island made on a silicon substrate. The optimum degree of order was realized at the pH with the maximum selectivity of minT1-LF adsorption on the Ti surface with respect to the silicon dioxide (SiO2) surface. Quartz crystal microbalance (QCM) measurement revealed that minT1-LF adsorbed onto the Ti surface strongly and irreversibly, but adsorbed onto the silicon dioxide surface weakly and reversibly. It was suggested that the concentration of minT1-LF on the Ti pattern promotes hexagonal close-packed ordering and axis aligning.

Size-controlled one-pot synthesis of fluorescent cadmium sulfide semiconductor nanoparticles in an apoferritin cavity

A simple size-controlled synthesis of cadmium sulfide (CdS) nanoparticle (NP) cores in the cavity of apoferritin from horse spleen (HsAFr) was performed by a slow chemical reaction synthesis and a two-step synthesis protocol. We found that the CdS NP core synthesis was slow and that premature CdS NP cores were formed in the apoferritin cavity when the concentration of ammonia water was low. It was proven that the control of the ammonia water concentration can govern the CdS NP core synthesis and successfully produce size-controlled CdS NP cores with diameters from 4.7 to 7.1 nm with narrow size dispersion. X-ray powder diffraction (XRD), energy dispersive spectroscopy (EDS) analysis and high-resolution transmission electron microscopy (HR-TEM) observation characterized the CdS NP cores obtained as cubic polycrystalline NPs, which showed photoluminescence with red shifts depending on their diameters. From the research of CdS NP core synthesis in the recombinant apoferritins, the zeta potential of apoferritin is important for the biomineralization of CdS NP cores in the apoferritin cavity. These synthesized CdS NPs with different photoluminescence properties will be applicable in a wide variety of nano-applications.

Sequence analysis of dolphin ferritin H and L subunits and possible iron-dependent translational control of dolphin ferritin gene

Background

Iron-storage protein, ferritin plays a central role in iron metabolism. Ferritin has dual function to store iron and segregate iron for protection of iron-catalyzed reactive oxygen species. Tissue ferritin is composed of two kinds of subunits (H: heavy chain or heart-type subunit; L: light chain or liver-type subunit). Ferritin gene expression is controlled at translational level in iron-dependent manner or at transcriptional level in iron-independent manner. However, sequencing analysis of marine mammalian ferritin subunits has not yet been performed fully. The purpose of this study is to reveal cDNA-derived amino acid sequences of cetacean ferritin H and L subunits, and demonstrate the possibility of expression of these subunits, especially H subunit, by iron.

Methods

Sequence analyses of cetacean ferritin H and L subunits were performed by direct sequencing of polymerase chain reaction (PCR) fragments from cDNAs generated via reverse transcription-PCR of leukocyte total RNA prepared from blood samples of six different dolphin species (Pseudorca crassidens, Lagenorhynchus obliquidens, Grampus griseus, Globicephala macrorhynchus, Tursiops truncatus, and Delphinapterus leucas). The putative iron-responsive element sequence in the 5'-untranslated region of the six different dolphin species was revealed by direct sequencing of PCR fragments obtained using leukocyte genomic DNA.

Results

Dolphin H and L subunits consist of 182 and 174 amino acids, respectively, and amino acid sequence identities of ferritin subunits among these dolphins are highly conserved (H: 99–100%, (99→98) ; L: 98–100%). The conserved 28 bp IRE sequence was located -144 bp upstream from the initiation codon in the six different dolphin species.

Conclusion

These results indicate that six different dolphin species have conserved ferritin sequences, and suggest that these genes are iron-dependently expressed.

In Vitro Synthesis of Calcium Nanoparticles Using the Protein Cage of Apoferritin

Recently the creation of calcium compounds with a highly controlled ultrastructure is noted as next generation materials for biomedical applications. Here we propose the novel method for synthsizing calcium nanoparticles using iron strange protein, apoferritin. Apoferritin was incubated in saturated Ca(HCO3)2 solution at 18 °C. Temperature of the reaction solution was then increased to 37 °C and left for 2 hours to make CaCO3 sedimentated. After removing the sediments in the bulk solution by centrifugation, the supernatant was concentrated. Saturated Ca(HCO3)2 was added to it and the mixed solution was incubated at 37 °C for 30 min. This process was repeated four times. With a Transmission Electron Microscope (TEM), nearly spherical particles with a diameter of about 6 nm were observed to form in the cavity of apoferritin. The nanoparticles were observed to have a lattice structure of spacing about 0.22 nm with high resolution TEM. With Energy Dispersive X-ray spectroscopy (EDS) analysis, the peak of Ca (Kα; 3.7 keV) was detected from a synthesized nanoparticle. According to the solvent condition, nanoparticles formed in the apoferritin cavity would be CaCO3.

Mass spectrometry studies of demetallation of haemin by recombinant horse L chain apoferritin and its mutant (E 53,56,57,60 Q)

An essential difference between eukaryotic ferritins and bacterioferritins is that the latter contain naturally, in vivo haem as Fe-protoporphyrin IX. This haem is located in a hydrophobic pocket along the 2-fold symmetry axes and is liganded by two Met 52. However, in in vivo studies, a cofactor has been isolated in horse spleen apoferritin similar to protoporphyrin IX; in in vitro experiments, it has been shown that horse spleen apoferritin is able to interact with haem. Studies of haemin (Fe(III)-PPIX) incorporation into horse spleen apoferritin have been carried out, which show that the metal free porphyrin is found in a corresponding pocket to haem in bacterioferritins [Précigoux, G., Yariv, J., Gallois, B., Dautant, A., Courseille, C. and Langlois, d'Estaintot B. (1994) A crystallographic study of haem binding to ferritin. Acta Cryst. D 50, 739-743]. A mechanism of demetallation of haemin by L-chain apoferritin was proposed [Crichton, R.R., Soruco, J.A., Roland, F., Michaux, M.A., Gallois, B., Précigoux, G., Mahy, J.P. and Mansuy. (1997) Remarkable ability of horse spleen apoferritin to demetallate hemin and to metallate protoporphyrin IX as a function of pH. J. P. Biochem. 36, 49, 15049-15054]: this involved four Glu residues (53,56,57,60) situated at the entrance of the hydrophobic pocket and appeared to be favoured by acidic conditions. To verify this mechanism, we have mutated these four Glu to Gln and examined demetallation in both acidic and basic conditions. In this paper, we report the mass spectrometry studies of L-chain apoferritin and its mutant incubated with haemin and analysed after different times of incubation: 15 days, 2 months, 6 months, 9 months and 12 months. These studies show that the recombinant L-chain apoferritin and its mutant are able to demetallate haemin to give a hydroxyethyl protoporphyrin IX derivative in a dimeric form [Macieira, S., Martins, B. M. and Huber, R. (2003) Oxygen-dependent coproporphyrinogen IX oxidase from Escherichia coli: one-step purification and biochemical characterization. FEMS. Microbiology Letters 226, 31-37].

Optical and EPR spectroscopic studies of demetallation of hemin by L-chain apoferritins

Earlier crystallographic and spectroscopic studies had shown that horse spleen apoferritin was capable of removing the metal ion from hemin (Fe(III)-protoporphyrin IX) [G. Précigoux, J. Yariv, B. Gallois, A. Dautant, C. Courseille, B. Langlois d'Estaintot, Acta Cryst. D50 (1994) 739-743; R.R. Crichton, J.A. Soruco, F. Roland, M.A. Michaux, B. Gallois, G. Précigoux, J.-P. Mahy, D. Mansuy, Biochemistry 36 (1997) 15049-15054]. We have carried out a detailed re-analysis of this phenomenon using both horse spleen and recombinant horse L-chain apoferritins, by electron paramagnetic resonance spectroscopy (EPR) to unequivocally distinguish between heme and non-heme iron. On the basis of site-directed mutagenesis and chemical modification of carboxyl residues, our results show that the UV-visible difference spectroscopic method that was used to establish the mechanism of demetallation is not representative of hemin demetallation. EPR spectroscopy does establish, as in the initial crystallographic investigation, that hemin demetallation occurs, but it is much slower. The signal at g=4.3 corresponding to high spin non-heme-iron (III) increases while the signal at g=6 corresponding to heme-iron decreases. Demetallation by the mutant protein, while slower than the wild-type, still occurs, suggesting that the mechanism of demetallation does not only involve the cluster of four glutamate residues (Glu 53, 56, 57, 60), proposed in earlier studies. However, the mutant protein had lost its capacity to incorporate iron, as had the native protein in which the four Glu residues had been chemically modified. Interestingly, a signal at g=1.94 is also observed. This signal most likely corresponds to a mixed-valence Fe(II)-Fe(III) cluster suggesting that a redox reaction may also be involved in the mechanism of demetallation.

EPR Studies of Recombinant Horse L-Chain Apoferritin and its Mutant (E 53,56,57,60 Q) with Haemin

Structural similarities between ferritins and bacterioferritins have been extensively demonstrated. However, there is an essential difference between these two types of ferritins: whereas bacterioferritins bind haem, in-vivo, as Fe(II)-protoporphyrin IX (this haem is located in a hydrophobic pocket along the 2-fold symmetry axes and is liganded by two axial Met 52 residues), eukaryotic ferritins are non-haem iron proteins. However, in in-vivo studies, a cofactor has been isolated from horse spleen apoferritin similar to protoporphyrin IX; in in-vitro experiments, it has been shown that horse spleen apoferritin is able to interact with haemin (Fe(III)-protoporphyrin IX). Studies of haemin incorporation into horse spleen apoferritin have been carried out, which show that the metal free porphyrin is found in a pocket similar to that which binds haem in bacterioferritins (Précigoux et al. 1994 Acta Cryst D50, 739-743). A mechanism of demetallation of haemin by L-chain apoferritins was subsequently proposed (Crichton et al. 1997 Biochem 36, 15049-15054) which involved four Glu residues (E 53,56,57,60) situated at the entrance of the hydrophobic pocket and appeared to be favoured by acidic conditions. To verify this mechanism, these four Glu have been mutated to Gln in recombinant horse L-chain apoferritin. We report here the EPR spectra of recombinant horse L-chain apoferritin and its mutant with haemin in basic and acidic conditions. These studies confirm the ability of recombinant L-chain apoferritin and its mutant to incorporate and demetallate the haemin in acidic and basic conditions.

Fabrication of ZnSe Nanoparticles in the Apoferritin Cavity by Designing a Slow Chemical Reaction System

Zinc selenide nanoparticles (ZnSe NPs) were synthesized in the cavity of the cage-shaped protein apoferritin by designing a slow chemical reaction system, which employs tetraaminezinc ion and selenourea. The chemical synthesis of ZnSe NPs was realized in a spatially selective manner from an aqueous solution, and ZnSe cores were formed in almost all apoferritin cavities with little bulk precipitation. Three factors are found to be important for ZnSe NP synthesis in the apoferritin cavity: (1) the threefold channel, which selectively introduces zinc ion into the apoferritin cavity, (2) the apoferritin internal potential, which favors zinc ion accumulation in the cavity, and (3) the nucleation site, which nucleates ZnSe inside the cavity. The characterization of the synthesized ZnSe NPs by X-ray powder diffraction and energy-dispersive spectrometry revealed that the synthesized NPs are a collection of cubic ZnSe polycrystals. It was shown that the 500 degrees C heat treatment for 1 h under nitrogen gas transformed the polycrystalline ZnSe core into a single crystal, and single-crystal ZnSe NPs free of protein were obtained.

Fabrication of nickel and chromium nanoparticles using the protein cage of apoferritin

The iron storage protein, apoferritin, has a cavity in which iron is oxidized and stored as a hydrated oxide core. The size of the core is about 7 nm in diameter and is regulated by the cavity size. The cavity can be utilized as a nanoreactor to grow inorganic crystals. We incubated apoferritin in nickel or chromium salt solutions to fabricate hydroxide nanoparticles in the cavity. By using a solution containing dissolved carbon dioxide and by precisely controlling the pH, we succeeded in fabricating nickel and chromium cores. During the hydroxylation process of nickel ions a large portion of the apoferritin precipitated through bulk precipitation of nickel hydroxide. Bulk precipitation was suppressed by adding ammonium ions. However, even in the presence of ammonium ions the core did not form using a degassed solution. We concluded that carbonate ions were indispensable for core formation and that the ammonium ions prevented precipitation in the bulk solution. The optimized condition for nickel core formation was 0.3 mg/mL horse spleen apoferritin and 5 mM ammonium nickel sulfate in water containing dissolved carbon dioxide. The pH was maintained at 8.65 using two buffer solutions: 150 mM HEPES (pH 7.5) and 195 mM CAPSO (pH 9.5) with 20 mM ammonium at 23 degrees C. The pH had not changed after 48 h. After 24 h of incubation, all apoferritins remained in the supernatant and all of them had cores. Recombinant L-ferritin showed less precipitation even above a pH of 8.65. A chromium core was formed under the following conditions: 0.1 mg/mL apoferritin, 1 mM ammonium chromium sulfate, 100 mM HEPES (pH 7.5) with a solution containing dissolved carbon dioxide. About 80% of the supernatant apoferritin (0.07 mg/mL) formed a core. In nickel and chromium core formation, carbonate ions would play an important role in accelerating the hydroxylation in the apoferritin cavity compared to the bulk solution outside.

Evolution of the acute phase response: iron release by echinoderm (Asterias forbesi) coelomocytes, and cloning of an echinoderm ferritin molecule

That the plasma concentration of certain divalent cations change during an inflammatory insult provides a major host defense response in vertebrate animals. This study was designed to investigate the involvement of iron sequestration in invertebrate immune responses. A ferritin molecule was cloned from an echinoderm coelomocyte cDNA library. The amino acid sequence showed sequence homology with vertebrate ferritin. The cDNA contained a conserved iron responsive element sequence. Studies showed that stimulated coelomocytes released iron into in vitro culture supernatants. The amount of iron in the supernatants decreased over time when the amebocytes were stimulated with LPS or PMA. Coelomocytes increased expression of ferritin mRNA after stimulation. In vertebrates, cytokines can cause changes in iron levels in macrophages. Similarly, echinoderm macrokines produced decreases in iron levels in coelomocyte supernatant fluids. These results suggest that echinoderm ferritin is an acute phase protein and suggest that sequestration of iron is an ancient host defense response in animals.

Cloning and sequence analysis of cDNA for heavy-chain ferritin from the Canis familiaris

Ferritin serves as a storage protein for iron in animals. Complementary DNA encoding a heavy chain ferritin was cloned from the brain of Canis familiaris. The dog ferritin cDNA encodes a 182 amino acid that shows high levels of amino acid identity with vertebrate ferritins (90-98%). Near the cap region of the 5'-untranslated region, the dog H-ferritin mRNA displays a 28-nucleotide sequence that is exactly conserved in the corresponding region of the human and pig H-ferritin mRNA, thus making this sequence a prime candidate for involvement in the known translational regulation of H-ferritin by iron.

Sequencing of cDNA clones that encode bovine ferritin H and L chains

The molecular weight of the liver-type subunit (L) of bovine ferritin is much larger than that of the heart-type subunit (H) as determined by SDS-PAGE (L, 20.5 kDa; H, 18.4 kDa). The migration of these two subunits on SDS-PAGE gels, relative to each other, is opposite to that reported for ferritin L and H subunits in other mammalian species (L, 19 kDa; H, 21 kDa). To determine the cause of this anomaly, full-length cDNA clones of the bovine L and H chains were isolated from a bovine spleen gamma gt11 cDNA library and sequenced. The amino acid sequences of the L and H chains of bovine ferritin, deduced from their cDNA sequences, contained open reading frames coding for 174 and 180 amino acid residues with calculated molecular weights of 19,856 and 20,920 Da, respectively. The deduced amino acid sequence of the L chain shows 86%, 84%, 87%, 83% and 83% homology with the amino acid sequences of horse, human, rabbit, rat and mouse L chains, respectively. The H chain displays a higher homology with the human, rat and mouse H chains (91%, 92% and 93%, respectively). In addition, the bovine L chain did not contain the extra octapeptide present in rodent L chains, and bovine, L and H chains did not react with concanavalin A. The bovine L and H chains expressed using a baculovirus expression system showed almost the same mobilities as those of bovine spleen ferritin, respectively, by SDS-PAGE. These results suggest that the much slower mobility of the bovine L chain compared with other mammalian L chains on SDS-PAGE cannot be attributed to insertion(s) of amino acid(s) or peptide(s) into the L chain, to the deletion(s) of them of it or to the addition of carbohydrate chains(s) but may result from significant differences in the binding affinity of SDS for bovine ferritin L chains.

Comparison of the three-dimensional structures of recombinant human H and horse L ferritins at high resolution

Mammalian ferritins are 24-mers assembled from two types of polypeptide chain which provide the molecule with different functions. H(eavy) chains catalyse the first step in iron storage, the oxidation of iron(II). L(ight) chains promote the nucleation of the mineral ferrihydrite enabling storage of iron(III) inside the protein shell. We report here the comparison of the three-dimensional structures of recombinant human H chain (HuHF) and horse L chain (HoLF) ferritin homopolymers, which have been refined at 1.9 A resolution. There is 53% sequence identity between these molecules, and the two structures are very similar, the H and L subunit alpha-carbons superposing to within 0.5 A rms deviation with 41 water molecules in common. Nevertheless, there are significant important differences which can be related to differences in function. In particular, the centres of the four-helix bundles contain distinctive groups of hydrophilic residues which have been associated with ferroxidase activity in H chains and enhanced stability in L chains. L chains contain a group of glutamates associated with mineralisation within the iron storage cavity of the protein.

Protein arrays: concepts and subjects

To adapt proteins, the materials in life, for use as materials in science and technology, we focused not only on the biological aspects (functional aspects) but also on the material aspects as matter (structural and physical aspects). Engineering with protein arrays will develop under such consideration and advance toward stable devices made of protein molecules. The protein arrays with 2D crystalline order provide a primary model of macroscopic protein-based devices. The combination of protein engineering, the leading edge of life science, and array engineering, the leading edge of materials science, will provide clues to the controlled integration of protein molecules to a form of functional supramolecules on proper surfaces.

Two-dimensional crystals of apoferritin

A simple 2D crystallization method using unfolded protein film as a supporting film of crystals was described, which allows modification of protein surfaces by injecting chemical reagents into the subphase after the crystal formation. As an example, glutaraldehyde was used to cross-link adjacent proteins and then stabilize protein crystals. The second layer of other proteins can also be formed on the apoferritin array using cross-linkers. The array of apoferritin is not only beneficial for electron crystallography but also for practical applications. For example, apoferritin produces a mineral core with a size which can be adjusted by the size to the cavity (i.e. 6 nm). Fabrication of such a small size of well defined fine particles is currently not easy using physical or chemical procedures. Using apoferritin, however, it is easy to produce uniform fine particles. If the core is designed to add interesting properties such as magnetism it is possible to make the highest class of magnetic film with ferritin 2D crystals. Basic researches toward practical applications of 2D protein crystal is now under way in various fields. The well defined size and function of protein molecules will benefit to many applications. The function and crystalline order can be designed by site-directed mutagenesis with the development of protein engineering.

Single molecular functional assay of ferritin arrays

In situ functional assay of each ferritin molecule in single-layer 2D arrays for horse spleen apoferritin and recombinant horse L- and human H-apoferritins was conducted by observing the iron-cores formed in the arrays by TEM. The study of the time-course, pH-dependence, and temperature-dependence of the function confirmed the iron-core formation to be due to the native function of apoferritins in array. Dark-field TEM imaging revealed that there was crystallinity in the cores in the array of recombinant human H-apoferritin. This iron-core formation was perfectly preserved in the array even after 3 months of storage at room temperature and low humidity. Moreover, about 50% of the function was found to remain in the array after it was exposed to 150 degrees C in vacuum for 1 hr.

Ferritin gene organization: differences between plants and animals suggest possible kingdom-specific selective constraints

Ferritin, a protein widespread in nature, concentrates iron approximately 10(11)-10(12)-fold above the solubility within a spherical shell of 24 subunits; it derives in plants and animals from a common ancestor (based on sequence) but displays a cytoplasmic location in animals compared to the plastid in contemporary plants. Ferritin gene regulation in plants and animals is altered by development, hormones, and excess iron; iron signals target DNA in plants but mRNA in animals. Evolution has thus conserved the two end points of ferritin gene expression, the physiological signals and the protein structure, while allowing some divergence of the genetic mechanisms. Comparison of ferritin gene organization in plants and animals, made possible by the cloning of a dicot (soybean) ferritin gene presented here and the recent cloning of two monocot (maize) ferritin genes, shows evolutionary divergence in ferritin gene organization between plants and animals but conservation among plants or among animals; divergence in the genetic mechanism for iron regulation is reflected by the absence in all three plant genes of the IRE, a highly conserved, noncoding sequence in vertebrate animal ferritin mRNA. In plant ferritin genes, the number of introns (n = 7) is higher than in animals (n = 3). Second, no intron positions are conserved when ferritin genes of plants and animals are compared, although all ferritin gene introns are in the coding region; within kingdoms, the intron positions in ferritin genes are conserved. Finally, secondary protein structure has no apparent relationship to intron/exon boundaries in plant ferritin genes, whereas in animal ferritin genes the correspondence is high. The structural differences in introns/exons among phylogenetically related ferritin coding sequences and the high conservation of the gene structure within plant or animal kingdoms of the gene structure within plant or animal kingdoms suggest that kingdom-specific functional constraints may exist to maintain a particular intron/exon pattern within ferritin genes. In the case of plants, where ferritin gene intron placement is unrelated to triplet codons or protein structure, and where ferritin is targeted to the plastid, the selection pressure on gene organization may relate to RNA function and plastid/nuclear signaling.

Control of crystal forms of apoferritin by site-directed mutagenesis

Surface charges of protein molecules are not only important to biological functions but also crucial to the molecular assembly responsible for crystallization. Appropriate alteration in the surface charge distribution of a protein molecule induces new molecular alignment in the proper direction in the crystal and, hence, controls the crystal form. Apoferritin molecules are known to crystallize in two- and three-dimensional forms in the presence of cadmium ions, which bridge neighboring protein molecules. Here we report a controlled transformation of the apoferritin 2-D crystal by site-directed mutagenesis. In mutant apoferritin, two amino acid residues binding a cadmium-ion through their negative charge, were replaced by one type of nonionic amino acid residues. The amino acid residues, Asp-84 and Gln-86 in the sequence of recombinant (i.e., wild-type) horse L-apoferritin, were replaced by Ser. The wild-type apoferritin yielded a hexagonal lattice 2-D crystal in the presence of cadmium ions. In contrast, the mutant apoferritin yielded two types of oblique crystals independent of the presence of cadmium ions. Image reconstruction of electron micrographs of the mutant crystals made clear that the mutant apoferritin molecules oriented themselves with the 2-fold symmetry axis perpendicular to the crystal plane in both crystals, while the wild-type apoferritin molecules oriented themselves with the 3-fold symmetry axis perpendicular to the crystal plane. The changes of crystal forms and molecular orientation in the 2-D crystals were well explained by a change of the electrostatic interactions induced by the mutagenesis.

Purification and characterization of recombinant pea-seed ferritins expressed in Escherichia coli: influence of N-terminus deletions on protein solubility and core formation in vitro

Plant ferritin subunits are synthesized as precursor molecules; the transit peptide (TP) in their NH2 extremity, responsible for plastid targeting, is cleaved during translocation to this compartment. In addition, the N-terminus of the mature subunit contains a plant-specific sequence named extension peptide (EP) [Ragland, Briat, Gagnon, Laulhère, Massenet, and Theil, E.C. (1990) J. Biol. Chem. 265, 18339-18344], the function of which is unknown. A novel pea-seed ferritin cDNA, with a consensus ferroxidase centre conserved within H-type animal ferritins has been characterized. This pea-seed ferritin cDNA has been engineered using oligonucleotide-directed mutagenesis to produce DNA fragments (1) corresponding to the wild-type (WT) ferritin precursor, (2) with the TP deleted, (3) with both the TP and the plant specific EP sequences deleted and (4) containing the TP but with the EP deleted. These four DNA fragments have been cloned in an Escherichia coli expression vector to produce the corresponding recombinant pea-seed ferritins. Expression at 37 degrees C led to the accumulation of recombinant pea-seed ferritins in inclusion bodies, whatever the construct introduced in E. coli. Expression at 25 degrees C in the presence of sorbitol and betaine allowed soluble proteins to accumulate when constructs with the TP deleted were used; under this condition, E. coli cells transformed with constructs containing the TP were unable to accumulate recombinant protein. Recombinant ferritins purified from inclusion bodies were found to be assembled only when the TP was deleted; however assembled ferritin under this condition had a ferroxidase activity undetectable at acid pH. On the other hand, soluble recombinant ferritins with the TP deleted and expressed at 25 degrees C were purified as 24-mers containing an average of 40-50 iron atoms per molecule. Despite the conservation in the plant ferritin subunit of a consensus ferroxidase centre, the iron uptake activity in vitro at pH 6.8 was found to be lower than that of the recombinant human H-ferritin, though it was much more active than the recombinant human L-ferritin. The recombinant ferritin with both the TP and the EP deleted (r delta TP/EP) assembled correctly as a 24-mer; it has slightly higher ferroxidase activity and decreased solubility compared with the wild-type protein with the TP deleted (r delta TP). In addition, on denaturation by urea followed by renaturation by dialysis the r delta TP/EP protein showed a 25% increase in core-formation in vitro compared with the r delta TP protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Heterogeneities in ferritin dimers as characterized by gel filtration, nuclear magnetic resonance, electrophoresis, transmission electron microscopy, and gene engineering techniques

To understand the mechanism underlying the preferential dimerization of ferritin shells, we studied monomers and dimers from both horse spleen and recombinant horse L-apoferritin by using gel filtration, nuclear magnetic resonance, electrophoresis, transmission electron microscopy, and gene engineering techniques. Our study of the kinetics of dimer-monomer dissociation that is produced by heating revealed the presence of at least two types of dimers, namely, weakly and strongly linked dimers with activation energies of 124 +/- 14 and 157 +/- 16 kJ/mol, respectively. Our study using thiol reagents indicated that the dimerization in horse spleen ferritin is partially mediated by disulfide bridges being formed between H-chains. Our analysis of the components that resulted from the dimer-monomer dissociation further clarified that these dimers form interdigitation structures. In summary, five types of dimers were identified in horse spleen apoferritin: reversible dimers with very weak interaction, non-sulfide dimers with weak interaction, non-sulfide dimers with strong interaction, disulfide dimers linked only by disulfide bridges, and disulfide dimers linked by disulfide bridges and having other interactions.