A new type of prosome-like particle, composed of small cytoplasmic RNA and multimers of a 21-kDa protein, inhibits protein synthesis in vitro

Suppression of cell growth by heavy chain ferritin

While producing recombinant rat liver H and L chain ferritin homopolymers using the baculovirus expression system, we noticed that rat liver H chain ferritin, but not L chain ferritin, had a suppressive effect on the growth of Spodoptera frugiperda (Sf-21) cells. Suppression was observed immediately after infection with recombinant H chain ferritin baculovirus prepared from lysed infected cells. Immediate suppression was observed when purified with either recombinant H chain apoferritin or various holoferritins (loaded with 1,970 +/- 50 or 2,520 +/- 90 atoms of iron/ferritin) indicating that suppression was not due to sequestration of iron required for cell growth. Suppression by H chain ferritin was also observed upon attempting to express the protein in Escherichia coli. Strategies for expression of recombinant rat liver H and L ferritin homopolymers in both prokaryotic and eukaryotic expression systems were developed.

Purification of a RNA-binding protein from rat liver. Identification as ferritin L chain and determination of the RNA/protein binding characteristics

In cultured rat hepatocytes the degradation of phosphoenolpyruvate carboxykinase mRNA might be regulated by protein(s), which by binding to the mRNA alter its stability. The 3'-untranslated region of phosphoenolpyruvate carboxykinase mRNA as a potential target was used to select RNA-binding protein(s) from rat liver by the use of gel retardation assays. A cytosolic protein was isolated, which bound to the phosphoenolpyruvate carboxykinase mRNA 3'-untranslated region and other in vitro synthesized RNAs. The protein was purified to homogeneity; it had an apparent molecular mass of 400 kDa and consisted of identical subunits with an apparent size of 24.5 kDa. Sequence analysis of a tryptic peptide from the 24.5-kDa protein revealed its identity with rat ferritin light chain. Binding of ferritin to RNA was abolished after phosphorylation with cAMP-dependent protein kinase and was augmented after dephosphorylation with alkaline phosphatase. Weak binding was observed in extracts from okadaic acid-treated cultured hepatocytes compared with untreated cells. Preincubation of ferritin with an anti-phosphoserine or an anti-phosphothreonine antibody attenuated binding to RNA, while an anti-phosphotyrosine antibody generated a supershift indicating that phosphoserine and phosphothreonine but not phosphotyrosine residues were in close proximity to the RNA-binding region. Ferritin is the iron storage protein in the liver. Binding of ferritin to RNA was diminished in the presence of increasing iron concentrations, whereas the iron chelator desferal was without effect. It is concluded that ferritin might function as RNA-binding protein and that it may have important functions in the general regulation of cellular RNA metabolism.

Prosomes (proteasomes) changes during differentiation are related to the type of inducer

The core of the 26S proteasome, the 20S prosome, is a highly organized multi-protein complex found in large amount in malignant cells. Differentiation of several cell lines, including the monoblastic U937 and the lymphoblastoid CCRF-CEM, is accompanied by a general decrease in the prosome concentration when phorbol-myrirtic-acetate (PMA) and retinoic acid plus dihydroxyvitamine D3 (RA+VD) are used. Incubation of U937 cells for three days with PMA or RA+VD causes differentiation, but the resulting patterns of prosome labeling in the cell and on the plasma membrane are not the same. In contrast, the same kind of prosome changes occur in U937 and CCRF-CEM cells when PMA is used as inducer. The intracellular distribution of prosomes is also linked to malignancy and differentiation. Prosomes are found in the nucleus and the cytoplasm of cancer cells; and treatment with RA+VD decreases the prosomes in the nucleus whereas PMA causes various prosome proteins changes. These results indicate that prosomes are important in cell regulation and in the expression of malignancy.

Changes in the amount and distribution of prosomal subunits during the differentiation of U937 myeloid cells: high expression of p23K

Prosomes (Proteasomes/Multicatalytic proteinase (MCP)-complexes) are protein particles built of 28 subunits in variable composition, having proteinase activity. We have studied the changes in prosomal subunits p29K, p31K and the highly expressed p23K during the differentiation of U937 cells. Control cells had little prosomal subunit p31K in the cytoplasm, while p29K antigen was detected in both the nucleus and cytoplasm; more p23K antigen was found in the cytoplasm than in the nucleus. Flow cytometry demonstrated a biphasic intracellular decrease in prosomes during differentiation induced by phorbol-myristic-acetate (PMA) and retinoic acid plus 1,25-dihydroxycholecalciferol (RA + VD). p23K and p29K decreased both in the cytoplasm and the nucleus of differentiated cells, though the p23K antigen was concentrated near vesicles and the plasma membrane in PMA-induced cells. The p31K antigens disappeared from RA + VD-induced cells, while in PMA-induced cells, cytoplasmic labelling was unchanged and nuclear labelling was increased. Small amounts of prosomal proteins p23K and p29K were found on the outer membrane of un-induced cells. While there was no labelling on the outer membrane of RA + VD-induced cells, p23K protein increased on the plasma membrane of PMA-induced cells. The prosome-like particle protein p21K was not present to any significant extent in the intracellular compartment of control or induced cells; however, p21K was detected on the outer surface of control cells and was increased only in PMA-induced cells. The culture medium of control and induced cells contained no p21K, p23K, p29K or p31K. RA + VD seemed to induce a general decrease of prosomal subunits within the cells and at the outer surface, whereas PMA caused a migration toward the plasma membrane and an increase at the outer surface. These changes in the distribution and type of prosomes in RA + VD- and PMA-induced cells indicate that prosomes may play a part in differentiation, especially p23K which is the most highly expressed protein among those studied and presents the more important changes.

The ferritins: molecular properties, iron storage function and cellular regulation

The iron storage protein, ferritin, plays a key role in iron metabolism. Its ability to sequester the element gives ferritin the dual functions of iron detoxification and iron reserve. The importance of these functions is emphasised by ferritin's ubiquitous distribution among living species. Ferritin's three-dimensional structure is highly conserved. All ferritins have 24 protein subunits arranged in 432 symmetry to give a hollow shell with an 80 A diameter cavity capable of storing up to 4500 Fe(III) atoms as an inorganic complex. Subunits are folded as 4-helix bundles each having a fifth short helix at roughly 60 degrees to the bundle axis. Structural features of ferritins from humans, horse, bullfrog and bacteria are described: all have essentially the same architecture in spite of large variations in primary structure (amino acid sequence identities can be as low as 14%) and the presence in some bacterial ferritins of haem groups. Ferritin molecules isolated from vertebrates are composed of two types of subunit (H and L), whereas those from plants and bacteria contain only H-type chains, where 'H-type' is associated with the presence of centres catalysing the oxidation of two Fe(II) atoms. The similarity between the dinuclear iron centres of ferritin H-chains and those of ribonucleotide reductase and other proteins suggests a possible wider evolutionary linkage. A great deal of research effort is now concentrated on two aspects of ferritin: its functional mechanisms and its regulation. These form the major part of the review. Steps in iron storage within ferritin molecules consist of Fe(II) oxidation, Fe(III) migration and the nucleation and growth of the iron core mineral. H-chains are important for Fe(II) oxidation and L-chains assist in core formation. Iron mobilisation, relevant to ferritin's role as iron reserve, is also discussed. Translational regulation of mammalian ferritin synthesis in response to iron and the apparent links between iron and citrate metabolism through a single molecule with dual function are described. The molecule, when binding a [4Fe-4S] cluster, is a functioning (cytoplasmic) aconitase. When cellular iron is low, loss of the [4Fe-4S] cluster allows the molecule to bind to the 5'-untranslated region (5'-UTR) of the ferritin m-RNA and thus to repress translation. In this form it is known as the iron regulatory protein (IRP) and the stem-loop RNA structure to which it binds is the iron regulatory element (IRE). IREs are found in the 3'-UTR of the transferrin receptor and in the 5'-UTR of erythroid aminolaevulinic acid synthase, enabling tight co-ordination between cellular iron uptake and the synthesis of ferritin and haem. Degradation of ferritin could potentially lead to an increase in toxicity due to uncontrolled release of iron. Degradation within membrane-encapsulated "secondary lysosomes' may avoid this problem and this seems to be the origin of another form of storage iron known as haemosiderin. However, in certain pathological states, massive deposits of "haemosiderin' are found which do not arise directly from ferritin breakdown. Understanding the numerous inter-relationships between the various intracellular iron complexes presents a major challenge.

Distribution of prosome proteins and their relationship with the cytoskeleton in oogenesis of Xenopus laevis

The presence of prosome proteins (p25K and p27K) was shown and their distribution was studied in oogenesis of Xenopus laevis using immunoblotting and immunofluorescence. These proteins form numerous granular clusters of variable size all over the cell. At previtellogenic stages, the prosome antibodies homogeneously stain the oocyte nucleus and the evenly distributed relatively large clusters in the cytoplasm. As the oocyte grows, the pattern of distribution of the prosome proteins undergoes changes: animal-vegetal and cortical gradients appear in the cytoplasm. In the course of oocyte maturation the size of clusters diminishes. Artificial activation of the egg leads to a dorso-ventral gradient in distribution of the prosome proteins. In this way, specific localization of prosome proteins is first visualized during formation of the dorso-ventral polarity. Co-localization of prosome proteins and actin and myosin was found in the oocyte by double staining. Small clusters of prosomes dispersed in the cytoplasm acquire capability of movement (after artificial activation) due, in all likelihood, to persisting connection with the acto-myosin complex of the egg.

The protein of M(r) 21,000 constituting the prosome-like particle of duck erythroblasts is homologous to apoferritin

In duck erythroblasts, two major populations of untranslated messenger (m) RNP can be separated by sucrose gradient centrifugation in low ionic strength. One of these contains globin mRNA associated to protein factors, among them the prosomes. The other, sedimenting in the 35S zone, contains non-globin mRNA. From this '35S' mRNP, a new RNP particle called the prosome-like particle was isolated and characterized [Akhayat, O., Infante, A. A., Infante, D., Martins de Sa, C., Grossi de Sa, M.-F. & Scherrer, K. (1987) Eur. J. Biochem. 170, 23-33]. The PLP is a multimer of a protein of M(r) 21,000, and contains small RNA species. The particle is tightly associated with repressed mRNA and inhibits in vitro protein synthesis. We show here that the protein of M(r) 21,000, constituting the prosome-like particle, is apoferritin. Different approaches confirm the RNP character of this particle and provide evidence that some of its RNA species are tRNA. The hypothesis is discussed as to whether (apo-)ferritin might serve other functions in addition to iron storage.

Distribution and activity of alternatively spliced Alzheimer amyloid peptide precursor and scrapie PrP mRNAs on rat brain polysomes

Mammalian brains contain low levels of the Alzheimer amyloid precursor variants (AAPPs) and the normal form of the scrapie agent protease-resistant protein (PrPc); however, their mRNAs are readily detectable. To understand these discrepancies we have investigated some aspects of the translational regulation of these mRNAs. An accurate blot-hybridization procedure was developed to measure absolute amounts of mRNA. Rat brain contains the following mRNA levels (ng/g tissue) AAPP(695), 170; AAPP(751/770), 63; PrPc, 144; actin, 615; glyceraldehyde-3-phosphate dehydrogenase (G3PDH), 359; ferritin, 148. The method was also used to determine the distribution of mRNAs between translationally active polysomes and translationally inactive ribonucleoprotein protein particles (mRNPs). More than 90% of G3PDH and actin mRNAs were associated with polysomal RNA; whereas, ferritin light chain mRNA was predominantly (90%) in mRNP RNA. The degree of cross-contamination of mRNPs with polysomes was less than 10%. Probes specific for the scrapie PrP protein and the AAPP(695) splice junction revealed that 70% of these mRNAs were associated with polysomes. One-half of AAPP(751/770) mRNAs (which comprise 20-30% of all AAPP mRNA in brain) were found in polysomes. We conclude therefore that both scrapie and AAPP mRNAs are subject to translational regulation in rat brain. Evidence from in vitro translational experiments confirm the message distribution determined by blot hybridization and corroborate the hypothesis that AAPP is subject to partial post-transcriptional regulation. Nevertheless, the low tissue levels of AAPP and PrPc must result primarily from their relatively rapid turnover.

An A + U-rich element RNA-binding factor regulates c-myc mRNA stability in vitro

Transient expression of some proto-oncogenes, cytokines, and transcription factors occurs as a cellular response to growth factors, 12-O-tetradecanoylphorbol-13-acetate, antigen stimulation, or inflammation. Expression of these genes is mediated in part by the rapid turnover of their mRNAs. A + U-rich elements in the 3' untranslated regions of these mRNAs serve as one recognition signal targeting the mRNAs for rapid degradation. I report the identification of a cytosolic factor that both binds to the proto-oncogene c-myc A + U-rich element and specifically destabilizes c-myc mRNA in a cell-free mRNA decay system which reconstitutes mRNA decay processes found in cells. Proteinase K treatment of the factor abolishes its c-myc mRNA degradation activity without affecting its RNA-binding capacity. Thus, RNA substrate binding and degradation appear to be separable functions. These findings should aid in understanding how the cell selectively targets mRNAs for rapid turnover.

An A + U-rich element RNA-binding factor regulates c-myc mRNA stability in vitro

Transient expression of some proto-oncogenes, cytokines, and transcription factors occurs as a cellular response to growth factors, 12-O-tetradecanoylphorbol-13-acetate, antigen stimulation, or inflammation. Expression of these genes is mediated in part by the rapid turnover of their mRNAs. A + U-rich elements in the 3' untranslated regions of these mRNAs serve as one recognition signal targeting the mRNAs for rapid degradation. I report the identification of a cytosolic factor that both binds to the proto-oncogene c-myc A + U-rich element and specifically destabilizes c-myc mRNA in a cell-free mRNA decay system which reconstitutes mRNA decay processes found in cells. Proteinase K treatment of the factor abolishes its c-myc mRNA degradation activity without affecting its RNA-binding capacity. Thus, RNA substrate binding and degradation appear to be separable functions. These findings should aid in understanding how the cell selectively targets mRNAs for rapid turnover.

Prosomes, subcomplexes of untranslated mRNP

PROSOMES are a novel class of small RNP particles of uniform morphology, but of variable RNA (pRNA) and protein composition (about 650,000 MW; 12 nm diameter in the EM). They were discovered as subcomplexes of free mRNP, tightly attached to inactive mRNA in the cytoplasm. The pRNAs hybridize stably to mRNA. Prosomes associate in vitro to mRNA and inhibit cell free protein synthesis inducing an mRNA structure unable to interact with ribosomes. Many types of prosomes were observed. The individual particle is made up by a variable combination of about 20 characteristic proteins and one or several pRNa. Some prosomal proteins are glycosylated, phosphorylated and, possibly, ADP-ribosylated and are highly conserved in evolution whilst others vary with the species and the mRNA population they are associated to. A protease activity was found associated to prosomes. The function(s) of the prosomes is(are) still unknown. The differential inhibition of in vitro protein synthesis points to a capacity to recognize mRNA and to keep it in an inactive state. The observation with the aid of monoclonal antibodies (pMABs) that prosomes and thus mRNP are attached to the intermediate filaments (IF) raises the question if one of the functions of the IF might be in the topological distribution of mRNA within the cell. Similar to the cytokeratin fibers, the prosome networks bridge neighboring cells at specific positions. The nucleus also contains some prosomal antigens, located on chromosomes and on the nuclear matrix. Their presence and distribution in the cell compartments varies with the cell type and the prosomal antigen probed. Oocytes contain large amounts of prosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Prosomes and heat shock complexes in Drosophila melanogaster cells

Prosomes and heat shock protein (HSP) complexes isolated from the cytoplasm of Drosophila cells in culture were biochemically and immunologically characterized. The two complexes were found to separate on sucrose gradients, allowing the analysis of their protein constituents by two-dimensional polyacrylamide gel electrophoresis and by reaction with anti-HSP sera and prosome-specific monoclonal antibodies. All of the prosomal proteins were found to be clearly distinct from the HSP; none of the prosomal proteins was synthesized de novo in heat shock. However, an antiprosome (anti-p27K) monoclonal antibody (mouse anti-duck) recognizing the Drosophila p29K prosomal protein allowed immunoprecipitation from a heat-shocked postmitochondrial supernatant of the crude HSP complex, including the low- and the high-molecular-weight components, in particular the 70 x 10(3)-molecular weight HSP. The highly purified small 16S HSP complex still contained this preexistent p29K prosomal protein, which thus also seems to be a metabolically stable constituent of the HSP complex. The significance of this structural and possibly functional relationship between prosomes and HSP, involving the highly ubiquitous and evolutionarily conserved prosomal protein p27/29K, remains to be elucidated.

Prosomes and heat shock complexes in Drosophila melanogaster cells

Prosomes and heat shock protein (HSP) complexes isolated from the cytoplasm of Drosophila cells in culture were biochemically and immunologically characterized. The two complexes were found to separate on sucrose gradients, allowing the analysis of their protein constituents by two-dimensional polyacrylamide gel electrophoresis and by reaction with anti-HSP sera and prosome-specific monoclonal antibodies. All of the prosomal proteins were found to be clearly distinct from the HSP; none of the prosomal proteins was synthesized de novo in heat shock. However, an antiprosome (anti-p27K) monoclonal antibody (mouse anti-duck) recognizing the Drosophila p29K prosomal protein allowed immunoprecipitation from a heat-shocked postmitochondrial supernatant of the crude HSP complex, including the low- and the high-molecular-weight components, in particular the 70 x 10(3)-molecular weight HSP. The highly purified small 16S HSP complex still contained this preexistent p29K prosomal protein, which thus also seems to be a metabolically stable constituent of the HSP complex. The significance of this structural and possibly functional relationship between prosomes and HSP, involving the highly ubiquitous and evolutionarily conserved prosomal protein p27/29K, remains to be elucidated.

The association of prosomes with some of the intermediate filament networks of the animal cell

The small RNP complexes of defined morphology and biochemical composition termed prosomes, first isolated from the cytoplasm associated with repressed mRNA (Martins de Sa, C., M.-F. Grossi de Sa, O. Akhayat, F. Broders, and K. Scherrer. J. Mol. Biol. 1986. 187:47-493), were found also in the nucleus (Grossi de Sa, M.-F., C. Martins de Sa, F. Harper, O. Coux, O. Akhayat, P. Gounon, J. K. Pal, Y. Florentin, and K. Scherrer. 1988. J. Cell Sci. 89:151-165). Immunofluorescence, immunoelectron microscopy, and immunochemical studies using mAbs directed against some of the prosomal proteins of duck erythroblasts indicate that in the cytoplasm of HeLa and PtK cells, prosome antigens are associated with the intermediate filament network of the cytokeratin type.