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Query: UNIPROT:P02794 (ferritin)
 17,525 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The translation of ferritin mRNA and degradation of transferrin receptor mRNA are regulated by the interaction of an RNA-binding protein, the iron-responsive element binding protein (IRE-BP), with RNA stem-loop structures known as iron-responsive elements (IREs) contained within these transcripts. IRE-BP produced in iron-replete cells has aconitase (EC 4.2.1.3) activity. The protein shows extensive sequence homology with mitochondrial aconitase, and sequences of peptides prepared from cytosolic aconitase are identical with peptides of IRE-BP. As an active aconitase, IRE-BP is expected to have an Fe-S cluster, in analogy to other aconitases. This Fe-S cluster has been implicated as the region of the protein that senses intracellular iron levels and accordingly modifies the ability of the IRE-BP to interact with IREs. Expression of the IRE-BP in cultured cells has revealed that the IRE-BP functions either as an active aconitase, when the cells are iron-replete, or as an active RNA-binding protein, when the cells are iron-depleted. We compare properties of purified authentic cytosolic aconitase from beef liver with those of IRE-BP from tissue culture cells and establish that characteristics of the physiologically relevant form of the protein from iron-depleted cells resemble those of cytosolic aconitase apoprotein. We demonstrate that loss of the labile fourth iron atom of the Fe-S cluster results in loss of aconitase activity, but that more extensive cluster alteration is required before the IRE-BP acquires the capacity to bind RNA with the affinity seen in vivo. These results are consistent with a model in which the cubane Fe-S cluster is disassembled when intracellular iron is depleted.
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PMID:Cellular regulation of the iron-responsive element binding protein: disassembly of the cubane iron-sulfur cluster results in high-affinity RNA binding. 128 44

In recent reports attention has been drawn to the extensive amino acid homology between pig heart, yeast, and Escherichia coli aconitases (EC 4.2.1.3) and the iron-responsive element binding protein (IRE-BP) of mammalian cells [Rouault, T. A., Stout, C. D., Kaptain, S., Harford, J. B. & Klausner, R. D. (1991) Cell 64, 881-883.; Hentze, M. W. & Argos, P. (1991) Nucleic Acids Res. 19, 1739-1740.; Prodromou, C., Artymiuk, P. J. & Guest, J. R. (1992) Eur. J. Biochem. 204, 599-609]. Iron-responsive elements (IREs) are stem-loop structures located in the untranslated regions of mRNAs. IRE-BP is required in the posttranscriptional regulation of ferritin mRNA translation and stabilization of transferrin receptor mRNA. In spite of substantial homology between the amino acid sequences of mammalian mitochondrial aconitase and IRE-BP, the mitochondrial protein does not bind IREs. However, there is a second aconitase, found only in the cytosol of mammalian tissues, that might serve as an IRE-BP. To test this possibility, we have prepared sufficient quantities of the heretofore poorly characterized beef liver cytosolic aconitase. This enzyme is isolated largely in its active [4Fe-4S] form and has a turnover number similar to that of mitochondrial aconitase. The EPR spectra of the two enzymes are markedly different. The amino acid composition, molecular weight, isoelectric point, and the sequences of six random peptides clearly show that these physicochemical and structural characteristics are identical to those of IRE-BP, and that c-aconitase is distinctly different from m-aconitase. In addition, both cytosolic aconitase and IRE-BP can have aconitase activity or function as IRE-BPs, as shown in the following paper and elsewhere [Zheng, L. Kennedy, M. C., Blondin, G. A., Beinert, H. & Zalkin, H. (1992) Arch. Biochem. Biophys., in press]. This leads us to the conclusion that cytosolic aconitase is IRE-BP.
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PMID:Purification and characterization of cytosolic aconitase from beef liver and its relationship to the iron-responsive element binding protein. 133 46

Post-transcriptional regulation of genes important in iron metabolism, ferritin and the transferrin receptor (TfR), is achieved through regulated binding of a cytosolic protein, the iron-responsive element binding protein (IRE-BP), to RNA stem-loop motifs known as iron-responsive elements (IREs). Binding of the IRE-BP represses ferritin translation and represses degradation of the TfR mRNA. The IRE-BP senses iron levels and accordingly modifies binding to IREs through a novel sensing mechanism. An iron-sulfur cluster of the IRE-BP reversibly binds iron; when cytosolic iron levels are depleted, the cluster becomes depleted of iron and the IRE-BP acquires the capacity to bind IREs. When cytosolic iron levels are replete, the IRE-BP loses RNA binding capacity, but acquires enzymatic activity as a functional aconitase. RNA binding and aconitase activity are mutually exclusive activities of the IRE-BP, and the state of the iron-sulfur cluster determines how the IRE-BP will function.
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PMID:An iron-sulfur cluster plays a novel regulatory role in the iron-responsive element binding protein. 142 65

Maintenance of cellular iron homeostasis demands the coordination of iron uptake, intracellular storage, and utilization. Recent investigations suggest that a single genetic regulatory system orchestrates the expression of proteins with central importance for all three aspects of cellular iron metabolism at the level of mRNA stability and translation. Two components of this regulatory system have been defined: a cis-acting mRNA sequence/structure motif called "iron-responsive element" (IRE) and a specific trans-acting cytoplasmic binding protein, here referred to as "IRE-binding protein" (IRE-BP). As an early event in the regulatory cascade, cellular iron deprivation induces the IRE-binding activity of IRE-BP, whereas binding activity is reduced in iron-replete cells. IRE-BP is highly homologous to the iron-sulphur (Fe-S) protein aconitase which strongly suggests that IRE-BP is an Fe-S protein itself. Control over IRE-BP activity by the cellular iron status is exerted post-translationally and likely involves changes between (4Fe-4S) and (3Fe-4S) states of the postulated IRE-BP Fe-S cluster. In addition, post-translational regulation of IRE-BP activity via heme has been proposed. Subsequent to its activation, IRE-BP binds with high affinity to single IREs contained in the 5' untranslated regions (UTRs) of ferritin and erythroid 5-aminolevulinic acid synthase (eALAS) mRNAs. The binding represses translation of these proteins involved in iron storage and utilization, respectively. In contrast, iron uptake is largely regulated via multiple IREs in the 3' UTR of transferrin receptor (TfR) mRNA. TfR-IREs are required for the iron-sensitive control of TfR mRNA stability. IRE-BP binding stabilizes TfR gene transcripts against as yet undefined ribonucleases. As a result of these regulatory interactions, iron starvation induces the expression of TfR, thereby increasing iron uptake, and represses the synthesis of proteins involved in iron storage and utilization. As cellular iron levels rise, the homeostatic balance is maintained by lowering iron uptake and increasing iron storage in ferritin.
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PMID:Coordination of cellular iron metabolism by post-transcriptional gene regulation. 143 80

The 5' end of porcine mitochondrial aconitase mRNA contains an iron responsive element (IRE)-like secondary structure (T. Dandekar, R. Stripecke, N. K. Gray, B. Goosen, A. Constable, H. E. Johansson, and M. W. Hentze (1991) EMBO J. 10, 1903-1909). A protein from a liver extract binds to a mitochondrial aconitase RNA probe and supports the identification of this sequence as an IRE. Purified cytosolic aconitase but not the mitochondrial enzyme binds to this IRE as well as to a ferritin IRE. All forms of cytosolic aconitase, [4Fe-4S] enzyme, [3Fe-4S] enzyme and apoenzyme bind with similar affinity. A Kd of 0.25 nM was calculated for the apoaconitase-IRE interaction from Scatchard analysis. These results support the conclusion that cytosolic aconitase is an IRE-binding protein which may regulate translation of mitochondrial aconitase mRNA.
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PMID:Binding of cytosolic aconitase to the iron responsive element of porcine mitochondrial aconitase mRNA. 144 77

Several mechanisms of posttranscriptional gene regulation are involved in regulation of the expression of essential proteins of iron metabolism. Coordinate regulation of ferritin and transferrin receptor expression is produced by binding of a cytosolic protein, the iron-responsive element binding protein (IRE-BP) to specific stem-loop structures present in target RNAs. The affinity of this protein for its cognate RNA is regulated by the cell in response to changes in iron availability. The IRE-BP demonstrates a striking level of amino acid sequence identity to the iron-sulfur (Fe-S) protein mitochondrial aconitase. Moreover, the recombinant IRE-BP has aconitase function. The lability of the Fe-S cluster in mitochondrial aconitase has led us to propose that the mechanism by which iron levels are sensed by the IRE-BP involves changes in an Fe-S cluster in the IRE-BP. In this study, we demonstrate that procedures aimed at altering the IRE-BP Fe-S cluster in vitro reciprocally alter the RNA binding and aconitase activity of the IRE-BP. The changes in the RNA binding of the protein produced in vitro appear to match the previously described alterations of the protein in response to iron availability in the cell. Furthermore, iron manipulation of cells correlates with the activation or inactivation of the IRE-BP aconitase activity. The results are consistent with a model for the posttranslational regulation of the IRE-BP in which the Fe-S cluster is altered in response to the availability of intracellular iron and this, in turn, regulates the RNA-binding activity.
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PMID:Reciprocal control of RNA-binding and aconitase activity in the regulation of the iron-responsive element binding protein: role of the iron-sulfur cluster. 150 65

The iron-responsive element-binding protein (IRE-BP) is an RNA-binding protein that regulates the expression of several mRNAs in response to availability of cellular iron. The iron-dependent control of IRE-BP activity has been reconstituted in vitro. Incubation of purified IRE-BP with iron salts in the presence of the reducing agent cysteine decreases IRE-BP binding to the cognate RNA element. The specificity of this effect is established by several parameters: (i) the interaction of the spliceosomal protein U1A with its U1 small nuclear RNA target sequence as an internal control is unaffected by iron perturbations, (ii) non-iron metals fail to mimic the iron effect, and (iii) iron chelator activates the IRE-binding activity of IRE-BP and titrates the effect of iron salts. Modulation of IRE-BP activity by chelatable iron is reversible and thus does not involve permanent alterations of the integrity of the protein. These findings accurately mirror the physiological basis for iron regulation of transferrin receptor mRNA stability as well as ferritin and erythroid 5-aminolevulinate synthase mRNA translation in vivo. We discuss these data vis-a-vis the structural homology of IRE-BP with the iron-sulfur protein aconitase and propose a mechanism by which the same cytoplasmic protein serves a dual function as an RNA-binding factor and an enzyme.
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PMID:Modulation of the RNA-binding activity of a regulatory protein by iron in vitro: switching between enzymatic and genetic function? 158 91

Iron-responsive elements (IREs) are regulatory RNA elements which serve as specific binding sites for the IRE-binding protein (IRE-BP). Interaction between IREs and IRE-BP induces repression of ferritin mRNA translation and transferrin receptor mRNA stabilization. We describe the identification of extensive amino acid sequence homology between IRE-BP and two known isomerases, aconitase and isopropylmalate (IPM) isomerase. We discuss the implications of this observation with regard to structure/function relationships of IRE-BP. The structural conservation between a regulatory RNA-binding protein and two enzymes involved in intermediary metabolism provides a surprising example of the functional flexibility in biological structures.
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PMID:Homology between IRE-BP, a regulatory RNA-binding protein, aconitase, and isopropylmalate isomerase. 190 2

Biosynthesis of nitric oxide (NO) from L-arginine modulates activity of iron-dependent enzymes, including mitochondrial acontiase, an [Fe-S] protein. We examined the effect of NO on the activity of iron regulatory factor (IRF), a cytoplasmic protein which modulates both ferritin mRNA translation and transferrin receptor mRNA stability by binding to specific mRNA sequences called iron responsive elements (IREs). Murine macrophages were activated with interferon-gamma and lipopolysaccharide to induce NO synthase activity and cultured in the presence or absence of NG-substituted analogues of L-arginine which served as selective inhibitors of NO synthesis. Measurement of the nitrite concentration in the culture medium was taken as an index of NO production. Mitochondria-free cytosols were then prepared and aconitase activity as well as IRE binding activity and induction of IRE binding activity were correlated and depended on NO synthesis after IFN-gamma and/or LPS stimulation. Authentic NO gas as well as the NO-generating compound 3-morpholinosydnonimine (SIN-1) also conversely modulated aconitase and IRE binding activities of purified recombinant IRF. These results provide evidence that endogenously produced NO may modulate the post-transcriptional regulation of genes involved in iron homeostasis and support the hypothesis that the [Fe-S] cluster of IRF mediates iron-dependent regulation.
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PMID:Biosynthesis of nitric oxide activates iron regulatory factor in macrophages. 750 26

The control of cellular iron homeostasis involves the coordinate post-transcriptional regulation of ferritin mRNA translation and transferring receptor mRNA stability. These regulatory events are mediated by a soluble cytoplasmic protein, iron regulatory factor (IRF), which binds specifically to mRNA hairpin structures, termed iron-responsive elements (IREs), in the respective mRNAs. IRF is modulated by variations of cellular iron levels and exists as either an apo-protein or a [4Fe-4S]-cluster protein. The two conformations show distinct, mutually exclusive functions. High-affinity IRE binding is observed with the apo-form induced by iron deprivation, but is lost under high iron conditions when IRF is converted to the [4Fe-4S]-cluster form which shows cytoplasmic aconitase activity. Moreover, IRE binding is inactivated by the sulfhydryl-oxidizing agent diamide and fully activated in vitro by 2% 2-mercapto-ethanol, whereas alkylation of IRF inhibits IRE binding. In the present study, we analyzed each of the above features using site-directed mutants of recombinant human IRF. The results support the bifunctional nature of IRF. We conclude that cysteines 437, 503 and 506 anchor the [4Fe-4S]-cluster, and are essential to the aconitase activity. Mutagenesis changing any of the cysteines to serine leads to constitutive RNA binding in 0.02% 2-mercaptoethanol. Cysteine 437 is particularly critical to the RNA-protein interaction. The spontaneous or diamide-induced formation of disulfide bonds between cysteines 437 and 503 or 437 and 506, in apo-IRF, as well as its alkylation by N-ethylmaleimide, inhibit binding to the IRE.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Mutational analysis of the [4Fe-4S]-cluster converting iron regulatory factor from its RNA-binding form to cytoplasmic aconitase. 750 61


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