PubMed Health⌕ Search

Biomedical subjects

E C Theil

Publications and source records attributed to E C Theil.

At least 19 recordsLinked to original sources
PubMed1992

A possible role for the conserved trimer interface of ferritin in iron incorporation.

Ferritin is a large protein, highly conserved among higher eukaryotes, which reversibly stores iron as a mineral of hydrated ferric oxide. Twenty-four polypeptides assemble to form a hollow coat with the mineral inside. Multiple steps occur in iron core formation. First, Fe2+ enters the protein. Then, several alternate paths may be followed which include oxidation at site(s) on the protein, oxidation on the core surface, and mineralization. Sequence variations occur among ferritin subunits which are classified as H or L; Fe2+ oxidation at sites on the protein appears to be H-subunit-specific or protein-specific. Other steps of ferritin core formation are likely to involve conserved sites in ferritins. Since incorporation of Fe2+ into the protein must precede any of the other steps in core formation, it may involve sites conserved among the various ferritin proteins. In this study, accessibility of Fe2+ to 1,10-phenanthroline, previously shown to be inaccessible to Fe2+ inside ferritin, was used to measure Fe2+ incorporation in two different ferritins under various conditions. Horse spleen ferritin (L/H = 10-20:1) and sheep spleen ferritin (L/H = 1:1.6) were compared. The results showed that iron incorporation measured as inaccessibility of Fe2+ to 1,10-phenanthroline increased with pH. The effect was the same for both proteins, indicating that a step in iron core formation common among ferritins was being measured. Conserved sites previously proposed for different steps in ferritin core formation are at the interfaces of pairs and trios of subunits. Dinitrophenol cross-links, which modify pairs of subunits and affect iron oxidation, had no effect on Fe2+ incorporation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

The iron regulatory region of ferritin mRNA is also a positive control element for iron-independent translation.

The iron regulatory element (IRE) in the 5'-untranslated region of ferritin mRNA interacts with a specific regulator protein (P-90, IRE-BP, or FRP) to block translation. High cellular iron changes the IRE/P-90 interaction to relax the translational block and allow polyribosome formation. We now show that the IRE and base-paired flanking regions also enhance translation in the absence of P-90, explaining the high translational efficiency of deregulated ferritin mRNA observed previously. The effect of the IRE on translational efficiency was examined by comparing four sets of mRNAs: (1) +/- IRE in animal (frog) ferritin, regulated translationally by iron in vivo; (2) +/- animal IRE fused with plant (soybean) ferritin, regulated transcriptionally by iron in vivo; (3) repositioned IRE in animal ferritin; (4) mutated IRE in animal ferritin with G16A substitution, which decreases P-90 binding (negative control). The IRE region increased translational efficiency of both the animal ferritin and the heterologous IRE/soybean ferritin fusion mRNAs; the effect was observed in cell-free translation systems from either plants (wheat germ) or animals (rabbit reticulocyte). Repositioning the IRE further from the 5' cap eliminated positive control of translation. The single base mutation had no effect, indicating that positive and negative translational control involves different sections of the IRE region. Thus, the IRE region in ferritin mRNA encodes both positive translational control and, when combined with the regulator protein P-90, negative translational control.

Animals↗
PubMed1991

Ferritin gene transcription is regulated by iron in soybean cell cultures.

Iron-regulated ferritin synthesis in animals is dominated by translational control of stored mRNA; iron-induced transcription of ferritin genes, when it occurs, changes the subunit composition of ferritin mRNA and protein and is coupled to translational control. Ferritins in plants and animals have evolved from a common progenitor, based on the similarity of protein sequence; however, sequence divergence occurs in the C termini; structure prediction suggests that plant ferritin has the E-helix, which, in horse ferritin, forms a large channel at the tetrameric interface. In contemporary plants, a transit peptide is encoded by ferritin mRNA to target the protein to plastids. Iron-regulated synthesis of ferritin in plants and animals appears to be very different since the 50- to 60-fold increases of ferritin protein, previously observed to be induced by iron in cultured soybean cells, is accompanied by an equivalent accumulation of hybridizable ferritin mRNA and by increased transcription of ferritin genes. Ferritin mRNA from iron-induced cells and the constitutive ferritin mRNA from soybean hypocotyls are identical. The iron-induced protein is translocated normally to plastids. Differences in animal ferritin structure coincide with the various iron storage functions (reserve for iron proteins and detoxification). In contrast, the constancy of structure of soybean ferritin, iron-induced and constitutive, coupled with the potential for vacuolar storage of excess iron in plants suggest that rapid synthesis of ferritin from a stored ferritin mRNA may not be needed in plants for detoxification of iron.

Amino Acid Sequence↗
PubMed1991

Ferritin mRNA: interactions of iron regulatory element with translational regulator protein P-90 and the effect on base-paired flanking regions.

The ferritin iron regulatory element (IRE), a conserved sequence of 28 nucleotides in a hairpin loop, is a conserved mRNA-specific translational regulatory element; flanking the IRE are regions of varying sequence, which form 9-17 base pairs close to the 5' cap. P-90 is a ferritin mRNA-specific translation regulatory protein purified from animal liver and reticulocytes. To study the P-90-RNA interaction, protein nucleases (RNase S1 and T1) and chemical nucleases FeEDTA and/or 1,10-phenanthroline-Cu were used as probes of an oligonucleotide (n = 55), containing the IRE and flanking regions (FL), and natural ferritin mRNA. Footprints and "toeprints" showed that P-90 binding was confined to the stem and loop of the IRE itself. However, P-90 altered the structure of the flanking region by increasing base stacking or helicity (RNase V1 sensitivity). Comparison of the reactivity of the IRE and flanking regions in natural mRNA and the 55-mer showed that long-range interactions included protecting bulges, single-stranded, and stacked regions from protein nucleases as well as stabilizing the P-90-RNA interaction. Structural integration of the IRE with the base-paired flanking regions was indicated by common features of reactivity (periodic hypersensitivity to FeEDTA) and changes in the FL region caused by P-90. The increased secondary structure of the IRE flanking regions caused by P-90 binding to the IRE provides a likely mechanism for blocking initiation of ferritin mRNA translation, since the combined structure (IRE + FL) is so close (8-17 nucleotides) to the cap.

Animals↗
PubMed1991

Ferritin mRNA probed, near the iron regulatory region, with protein and chemical (1,10-phenanthroline-Cu) nucleases. A possible role for base-paired flanking regions.

Iron stimulates ferritin synthesis in whole cells and animals, by increasing the entry of ferritin mRNA into polyribosomes. Dissection of the regulation at the molecular level has identified a 28-nucleotide, conserved, regulatory sequence (IRE = iron regulatory element) in the 5' non-coding region of ferritin mRNAs, plus trans-acting factor(s), one of which is a 90-kDa protein. The site of iron action is not entirely characterized but may involve heme; sequences in the 3' non-coding region of ferritin mRNA can modulate regulation. Ferritin mRNA is the first eukaryotic mRNA for which a conserved regulatory sequence and regulator protein have been identified. The same RNA-protein motif is used, through iron-dependent degradation of transferrin receptor mRNA, to decrease synthesis of the receptor and cellular iron uptake. The regulatory structure of the transferrin receptor mRNA is composed, in part, of five copies of the IRE in the 3' non-coding region. IRE structure, probed by cleavage with RNases T1, V1, 1,10-phenanthroline-Cu or modification with dimethyl sulfate, is a hairpin loop with conformational variations dependent on magnesium; a base-paired region flanking the IRE is also structurally sensitive to magnesium. Similar results were obtained with a synthetic 55-mer containing the IRE and with a full-length in vitro transcript with a G----A substitution in the loop.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Sequence↗
PubMed1990

Evidence for conservation of ferritin sequences among plants and animals and for a transit peptide in soybean.

Ferritin is a large multisubunit protein that stores iron in plants, animals, and bacteria. In animals, the protein is mainly cytoplasmic and is highly conserved, while in plants ferritin is found in chloroplasts and other plastids. Ferritin is synthesized in plants as a larger precursor of the mature subunit. There is no sequence information for ferritin from plants, except an NH2-terminal peptide of 35 residues which shows little similarity to any known ferritin sequences or transit peptides (Laulhere, J. P., Laboure, A. M., and Briat, J. F. (1989) J. Biol. Chem. 264, 3629-3635). To understand the genetic origin and the location of ferritin synthesis in plant cells, as well as the structure of ferritin from plants, we have sequenced both CNBr peptides from pea seed ferritin and nucleotides of a soybean hypocotyl ferritin cDNA, identified using a frog ferritin cDNA as a probe. Comparison of pea and soybean sequences showed an identity of 89%. Alignment of the plant ferritin sequences with animal ferritins showed 55-65% sequence identity in the common regions. However, a peptide of 28 amino acids extended the NH2 terminus of the plant ferritins. Furthermore, the cDNA encoded additional amino acids which appear to be a transit peptide. None of the sequences in soybean ferritin were found in the tobacco chloroplast genome, suggesting, as does the transit peptide, a nuclear location of ferritin gene(s) in plants. Plant ferritin mRNA is 400-500 nucleotides longer than animal ferritin mRNAs, a difference accounted for in part by the extra peptides encoded. The size of soybean ferritin mRNA was constant in different tissues but expression varied in different tissues (leaf greater than hypocotyl). Thus, higher plants and animal ferritins display sequence homology and differential tissue expression. An ancient, common progenitor apparently gave rise to contemporary eukaryotic ferritins after specific modifications, e.g. transport to plasmids.

Amino Acid Sequence↗
PubMed1990

Structure of the 5' untranslated regulatory region of ferritin mRNA studied in solution.

Ferritin mRNAs are the first eukaryotic mRNAs for which a conserved, translational regulatory sequence has been identified. The sequence of twenty-eight nucleotides, called the IRE (iron regulatory element), is found in the 5'-noncoding region and is required for enhanced translation of ferritin mRNA by excess cellular iron; regulation occurs at initiation. The prediction of secondary structure in the IRE is a hairpin loop. We now report an analysis of the IRE structure in solution studied in natural ferritin mRNAs [H and H'(M) subunits] by primer extension, after modification or cleavage by dimethyl sulfate, RNAases T1 and V1, and the chemical nuclease 1, 10-phenanthroline-copper (OPCu) which cleaves single-stranded and bulged regions of RNA. Overall, the structure in solution of the ferritin mRNA regulatory region is a hairpin loop, with magnesium-sensitive features, in which half the stem is provided by the IRE and half by flanking regions; only secondary structure is conserved in the flanking regions. Predicted bulges or internal loops along the stem were clearly detected by OPCu but were missed by the more bulky probe RNAase T1, indicating the efficacy of OPCu in probing subtle features of RNA structure. Magnesium-dependent deviations from the predicted structure were observed in the stem between the hairpin loop and the bulge at C6. The location of the IRE in relation to the initiator AUG or the cap is variable in different ferritin mRNAs. However, the number of nucleotides in the base-paired flanking regions of known ferritin mRNAs is proportional to the distance of the IRE from the cap and places the secondary/tertiary structure 8-10 nucleotides from the cap where interference with initiation is likely.

Base Sequence↗
PubMed1990

Regulation of ferritin and transferrin receptor mRNAs.

Iron regulates the synthesis of two proteins critical for iron metabolism, ferritin and the transferrin receptor, through novel mRNA/protein interactions. The mRNA regulatory sequence (iron-responsive element (IRE)) occurs in the 5'-untranslated region of all ferritin mRNAs and is repeated as five variations in the 3'-untranslated region of transferrin receptor mRNA. When iron is in excess, ferritin synthesis and iron storage increase. At the same time, transferrin receptor synthesis and iron uptake decrease. Location of the common IRE regulatory sequence in different noncoding regions of the two mRNAs may explain how iron can have opposite metabolic effects; when the IRE is in the 5'-untranslated region of ferritin mRNA, translation is enhanced by excess iron whereas the presence of the IREs in the 3'-untranslated region of the transferrin receptor mRNA leads to iron-dependent degradation. How and where iron actually acts is not yet known. A soluble 90-kDa regulatory protein which has been recently purified to homogeneity from liver and red cells specifically blocks translation of ferritin mRNA and binds IRE sequences but does not appear to be an iron-binding protein. The protein is the first specific eukaryotic mRNA regulator identified and confirms predictions 20 years old. Concerted regulation by iron of ferritin and transferrin receptor mRNAs may also define a more general strategy for using common mRNA sequences to coordinate the synthesis of metabolically related proteins.

Animals↗
PubMed1990

Iron environment in ferritin with large amounts of phosphate, from Azotobacter vinelandii and horse spleen, analyzed using extended X-ray absorption fine structure (EXAFS).

The iron core of proteins in the ferritin family displays structural variations that include phosphate content as well as the number and the degree of ordering of the iron atoms. Earlier studies had shown that ferritin iron cores naturally high in phosphate, e.g., Azotobacter vinelandii (AV) ferritin (Fe:P ratio = 1:1.7), had decreased long-range order. Here, the influence of phosphate on the local structure around iron in ferritin cores is reported, comparing the EXAFS of AV ferritin, reconstituted ferritin [the protein coats of horse spleen ferritin mixed with Fe(II) with and without phosphate at pH 7] (Fe:P ratio = 1:0.25), and native horse spleen ferritin (Fe:P ratio = 1:0.125); reconstituted horse spleen ferritin without phosphate was indistinguishable from native horse spleen ferritin (HSF) in the analysis. In contrast, when the phosphate content was high in AV ferritin and horse spleen ferritin reconstituted with phosphate, the average iron atom had five to six phosphorus neighbors at 3.17 A. Moreover, the number of detectable iron neighbors was lower when phosphate was high or present during reconstitution (2-3 vs 5-6), and the interatomic distance was longer (3.50 vs 3.03 A), indicating that some phosphate bridges neighboring iron atoms. However, the decrease in the number of detectable iron-iron neighbors compared to HSF and the higher number of Fe-P interactions relative to Fe-Fe interactions suggest that some phosphate ligands were chain termini, or blocked crystal growth, and/or introduced defects which contributed both to the long-range disorder and to altered redox properties previously observed in AV ferritin.

Animals↗
PubMed1990

The ferritin family of iron storage proteins.

The ferritins are a family of proteins produced in a variety of amounts and types depending on the state of development of an animal, or the state of differentiation of a particular cell type, or the environment. Iron storage is the main function of the ferritins when iron is needed for intracellular use (housekeeping) for iron proteins such as ribonucleotide reductase, cytochromes, oxidases, nitrogenases, or photosynthetic reaction centers or for extracellular use by other cells (specialized). Under abnormal conditions, such as the breach of transferrin-receptor-controlled incorporation of iron, ferritin can also serve to detoxify excess intracellular iron. The structure of ferritin is very complex, consisting of a protein coat of 24 polypeptide subunits, approximately 20 kDa, which surrounds an inorganic phase of hydrous ferric oxide. The polypeptide subunits, bundles of four alpha helices, display remarkable conservation of sequence among plants and animals, which is probably related to the necessity of forming the hollow sphere pierced by 14 channels through which iron may pass. In spite of the conserved regions of sequence, there are multiple genes for ferritin polypeptide subunits within an organism; at the moment three distinct subunit types, H H'(or M), and L, have been identified which are expressed in a cell-specific fashion. How many different subunit types exist, the influence on function, and the number of genes required to encode them are currently being actively investigated. Not only does the protein coat of ferritin display variations, the inorganic phase of ferritin can vary as well. For instance, differences can occur in the number of Fe atoms (up to 4500), as well as in the phosphorus content and in the degree of hydration and order. Such observations have depended on the use of a variety of physical techniques such as X-ray diffraction, EXAFS, and Mössbauer spectroscopy. The same approaches, as well as EPR spectroscopy, have been used to monitor the path taken by Fe as it passes from mononuclear Fe(II) outside the protein coat to polynuclear Fe(III) inside the protein coat. Both mononuclear Fe(II) and Fe(III) have been observed, as well as dimeric Fe(II)-O-Fe(III), and Fe(III)-oxo bridged clusters attached to the protein. A possible protein site for the Fe(III) cluster is a groove on the inner surface of the dimeric interface, suggested by the structure and from the affect of natural cross-links between subunit pairs.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1990

Rapid reduction of iron in horse spleen ferritin by thioglycolic acid measured by dispersive X-ray absorption spectroscopy.

The release of iron from ferritin is important in the formation of iron proteins and for the management of diseases in both animals and plants associated with abnormal accumulations of ferritin iron. Much more iron can be released experimentally by reduction of the ferric hydrous oxide core than by chelation of Fe3+ which has led to the notion that reduction is also the major aspect of iron release in vivo. Variations in the kinetics of reduction of the mineral core of ferritin have been attributed to the redox potential of the reductant, redox properties of the iron core, the structure of the protein coat, the analytical method used to detect Fe2+ and reactions at the surface of the mineral. Direct measurements of the oxidation state of the iron during reduction has never been used to analyze the kinetics of reduction, although Mössbauer spectroscopy has been used to confirm the extent of reduction after electrochemical reduction using dispersive X-ray absorption spectroscopy (DXAS). We show that the near edge of X-ray absorption spectra (XANES) can be used to quantify the relative amounts of Fe2+ and Fe3+ in mixtures of the hydrated ions. Since the nearest neighbors of iron in the ferritin iron core do not change during reduction, XANES can be used to monitor directly the reduction of the ferritin iron core. Previous studies of iron core reduction which measured by Fe2+.bipyridyl formation, or coulometric reduction with different mediators, suggested that rates depended mainly on the redox potential of the electron donor.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Ferritin mRNA translation, structure, and gene transcription during development of animals and plants.

Changes in transcription/accumulation of ferritin mRNA accompany cell differentiation and development as cells establish the pattern of iron storage and metabolism that matches their specialized features. Signals which induce changes in ferritin gene expression include hormones (thyrotropin in thyroid cells), monokines (tumor necrosis factor alpha in myoblasts, and preadipocytes), phagocytosis and/or inducing agents (erythrophagocytosis in macrophages, phorbol esters in promyelocytes, and dimethyslulfoxide in proerythroblasts), heat shock (in red cells) and light (in leaf mesophyll). Iron, which has a major effect on the translation of ferritin mRNA, also leads to synthesis and/or accumulation of ferritin mRNA in hepatocytes, adult red cells, HeLa cells, and undifferentiated plant cells. Iron-induced changes in ferritin mRNA composition allow cells to synthesize large amounts of different ferritins in response to excess iron. Whether iron will act both on translation and transcription may relate to the uses of stored iron by the cell (iron reserve for other cells? detoxification?) and the magnitude of the iron signal relative to the existing pool of ferritin mRNA. Translational regulation of ferritin mRNA depends on two distinctive features of the RNA: (1) conserved regulatory sequence of 28 nucleotides occurs in the 5' noncoding region (iron 'regulatory' or 'responsive' element, IRE) which interacts with cytoplasmic regulator proteins, present in animal cells, that block initiation; sequences in the 3' untranslated region can modulate translation when the IRE is present. (2) Ferritin mRNA, either deregulated by iron induction in cells or isolated from cells (polyA+), forms initiation complexes efficiently compared to other cellular mRNAs. The structure of the IRE, which is also found in the 3' untranslated region of the iron-destabilized transferrin receptor mRNA, has begun to be analyzed by computer prediction, site-directed mutagenesis, and solution behavior. The results to date show that the IRE is a hairpin with bulges and loops that is stacked upon a base-paired flanking region (FL) with varying sequence. Interactions of the IRE+FL with other parts of ferritin mRNA are indicated by comparing the reactivity of natural ferritin mRNA with an oligomer (n = 55). The structure of the stem nearest the hairpin loop is sequence-dependent and flexible. Variations in the distance of the IRE from the cap in various ferritin mRNAs, compensated by the length of FL, and the structural properties of the IRE+FL suggest that the 5' regulatory structure of ferritin mRNA includes the IRE+FL.

Animals↗
PubMed1989

Crosslinks between intramolecular pairs of ferritin subunits: effects on both H and L subunits and on immunoreactivity of sheep spleen ferritin.

Ferritin is a multisubunit protein, controlling iron storage, with a protein coat composed of 24 subunits (up to three distinct types) in different proportions depending on cell type. Little is known about the subunit interactions in ferritin protein coats composed of heterologous subunits, despite the relevance to ferritin structure and ferritin function (iron uptake and release). Synthetic crosslinking is a convenient way to probe subunit contacts. Crosslinks between subunit pairs in ferritin protein coats are also a natural post-translational modification which coincides with different iron content in ferritin from sheep spleen; ferritin from sheep spleen also contains H and L subunits. Crosslinks synthesized by the reaction of ferritin low in natural crosslinks with difluorodinitrobenzene (F2DNB) reproduced the effects of the natural crosslinks on iron uptake and release. We now extend our observations on the structural effects of natural and synthetic crosslinks to include immunoreactivity of the assembled protein, with monoclonal antibodies as a probe. We also demonstrate, for the first time, ferritin peptides involved in an apparent H- and L-subunit contact: two peptides decreased 4X in cyanogen bromide peptide maps after F2DNB crosslinking were residues L-96-138 and H-66-96; the major DNP-dipeptide was Lys-DNP-Lys. Using the structure of an all L-subunit ferritin as a model, the most likely site for the H-L DNP crosslink is L-Lys 104 (C helix) and H-Lys 67 (B helix). The B helix forms the internal subunit dimer interface, a putative site of iron core nucleation. Alteration by crosslinks of the B helix could, therefore, explain the effect of crosslinks on ferritin iron uptake, release, and iron content.

Amino Acid Sequence↗
PubMed1989

A comparison of an undecairon(III) complex with the ferritin iron core.

The iron core of ferritin is comprised of up to 4,500 Fe(III) atoms as Fe2O3.nH2O, which is maintained in solution by a surrounding, spherical coat of protein. Organisms as diverse as bacteria and man use the ferritin iron-protein complex as a reservoir of stored iron for other essential proteins. To extend studies of the steps in polynuclear iron core formation, a recently characterized undecairon(III) oxo-hydroxo aggregate [Fe11 complex] (Gorun et al., J. Am. Chem. Soc. 109, 3337 [1987]) was examined by x-ray absorption spectroscopy as a model for an intermediate. The results, which are comparable to the previous x-ray diffraction studies, show near neighbors (Fe-O) at 1.90 A that are distinct from those in ferritin and a longer distance of 2.02 A. However, contributions from neighbors (Fe-C) known to exist at ca. 2.7 A were obscured by a highly ordered Fe-Fe interaction and were not detectable in the Fe11 complex in contrast to a previously characterized Fe(III) cluster bound to the protein coat. Of the two Fe-Fe interactions detectable in the Fe11 complex, the shortest, at 3.0 A is particularly interesting, occurring at the same distance as a full shell (CN = 6) in ferritin, but having fewer Fe neighbors (CN = 2-3) characteristic of an intermediate in core formation. The incomplete Fe-Fe shell is much more ordered than in ferritin, suggesting that the disorder in ferritin cores may be associated with the later steps of the core growth. Differences between the Fe11 complex and the full core of ferritin indicate the possibility of intermediates in ferritin iron formation that might be like Fe11.

Ferritins↗
PubMed1988

The importance of the 3'-untranslated region in the translational control of ferritin mRNA.

Ferritin synthesis provides a dramatic example of translational control; stored ferritin mRNA is translated at relatively low rates which can increase 40-50 times when cellular iron levels increase. Although it is not known if agents other than cellular iron levels can release the repression of ferritin mRNA in vivo, the repression appears to be eliminated during the isolation of poly(A+) RNA, judged by translation in wheat germ lysates (WG). Using the bullfrog tadpole as a model, because of the abundance of ferritin-rich embryonic red cells, we now show specific repression of ferritin mRNA in the isolated poly(A+) RNA translated in rabbit reticulocyte lysates (RR) (RR/WG = 25%). Repression of ferritin mRNA was associated with the inability to form polyribosomes in analogy to iron-poor cells in vivo. The addition of various complexes of iron did not relieve the repression, suggesting that in vivo at least part of the effect of iron may be indirect and mediated by factors absent in the cell-free system; all three ferritin subunit mRNAs (H, M, and L) appeared to be regulated coordinately in vitro and in vivo as well. Comparison of transcripts of DNA encoding the M subunit of ferritin, but containing deletions in the 3'-untranslated (UT) region, showed that a region 70 nucleotides long was important for repression. Comparison of secondary structures predicted for the eight known ferritin subunit mRNAs from humans, rats, chickens, and frogs indicates that a region involved in base pairing common to all the mRNAs is eliminated when the 3'-UT region is shortened to 24 nucleotides. Although regions in the 5'-UT of mRNAs, including ferritin, have been shown to be involved in translational regulation, it is clear that complete regulation can involve both the 3'-UT and the 5'-UT regions, mediated, presumably, by secondary and tertiary interactions along the mRNA molecule.

Animals↗
PubMed1987

Stabilization of iron in a ferrous form by ferritin. A study using dispersive and conventional x-ray absorption spectroscopy.

Stabilization of iron in a bioavailable form is the function of ferritin, a protein of 24 subunits forming a coat around a core of less than or equal to 4500 hydrated iron atoms. The core of ferritin isolated from tissues contains Fe3+, but Fe2+ is required for experimental core formation in protein coats; reduction of Fe3+ to Fe2+ facilitates iron removal from protein coats. Using the differences in x-ray absorption spectra (x-ray absorption near edge structure) between Fe2+ and Fe3+ to monitor reconstitution of ferritin from Fe2+ and protein coats, we observed stabilization of Fe2+, apparently inside the coat. Mixtures of Fe2+ and Fe3+ persisted for greater than or equal to 16 h in air indicating that, in vivo, some iron in ferritin could be stored as Fe2+ and with Fe3+ could yield magnetite.

Animals↗
PubMed1987

Differences in the regulation of messenger RNA for housekeeping and specialized-cell ferritin. A comparison of three distinct ferritin complementary DNAs, the corresponding subunits, and identification of the first processed in amphibia.

The ferritin family is a widespread group of proteins that maintain iron in a soluble form and also protect against the toxic effects of excess iron. The structure and sequence of the proteins are highly conserved. However, the cell-specific features of structure which occur within the same organism indicate cell specificity of gene expression and may be related to variations in types of iron storage, i.e. specialized-cell ferritin (stored iron is for other cell types) versus housekeeping ferritin (stored iron is for intracellular purposes related to normal or stress metabolism); the protein structure may also affect rates of iron turnover. Iron induces ferritin synthesis and accumulation by recruiting stored ferritin mRNA that is efficiently translated in cells specialized for iron storage. For the first time we show the occurrence of three different cDNAs from bullfrog tadpoles, corresponding to three subunits of the protein: H, M, and L. Thus, ferritin can be encoded by at least three different mRNAs and probably three different genes, in contrast to the older idea of two, H and L; the subunits maintain the conserved sequences of known ferritins and have similar predicted masses, 20.5, 20.6, and 19.9 kDa, but have distinct mobilities in denaturing gels. Ferritin subunit expression is cell specific; more of the H and L chain mRNAs are expressed in red cells than in liver. Ferritin expression is regulated by transcription (or mRNA stability) in adult red cells; cellular levels of ferritin mRNA were 20% that of embryonic red cells, and L subunit mRNA increased 2.5 times with excess iron. Ferritin expression is also regulated during translation in adult red cells; iron recruits stored ferritin mRNA, but only during certain stages of red cell maturation, in contrast to embryonic red cells. The developmental differences in ferritin expression are discussed in relation to the shift from specialized-cell ferritin to housekeeping ferritin in red cells of the embryonic versus adult lines.

Amino Acid Sequence↗