Phosphorylation of Acid-soluble Nucleolar Proteins of Novikoff Hepatoma Ascites Cells in Vivo

Analysis of the oligomeric states of nucleophosmin using size exclusion chromatography

Nucleophosmin (NPM1) is a multifunctional phosphoprotein which plays important roles in diverse biological processes. NPM1 can form homo- or hetero-oligomers through its N-terminal region, and bind DNA and RNA through its C-terminal region. However, the monomer-oligomer distribution of NPM1, and the extent of NPM1 binding and unbinding to RNA in living cells, are not fully understood. In this work, we analysed molecular complexes of NPM1 using size exclusion chromatography. We found that a substantial fraction of NPM1 behaves as an oligomer in HeLa cells. Furthermore, we identified three distinct oligomeric states of NPM1 using molecular characterization techniques such as subcellular localization and RNA binding. Finally, we found that heterozygous expression of a leukemia-associated NPM1 mutant significantly decreases the RNA binding level. Our data demonstrate that size exclusion chromatography provides a powerful tool for analysing NPM1 oligomers.

Crosstalk between the nucleolus and the DNA damage response

Nucleolar function and the cellular response to DNA damage have long been studied as distinct disciplines. New research and a new appreciation for proteins holding multiple functional roles, however, is beginning to change the way we think about the crosstalk among distinct cellular processes. Here, we focus on the crosstalk between the DNA damage response and the nucleolus, including a comprehensive review of the literature that reveals a role for conventional DNA repair proteins in ribosome biogenesis, and conversely, ribosome biogenesis proteins in DNA repair. Furthermore, with recent advances in nucleolar proteomics and a growing list of proteins that localize to the nucleolus, it is likely that we will continue to identify new DNA repair proteins with a nucleolar-specific role. Given the importance of ribosome biogenesis and DNA repair in essential cellular processes and the role that they play in diverse pathologies, continued elucidation of the overlap between these two disciplines will be essential to the advancement of both fields and to the development of novel therapeutics.

Ribosomal protein S6 kinase from TOP mRNAs to cell size

Ribosomal protein S6 kinase (S6K) has been implicated in the phosphorylation of multiple substrates and is subject to activation by a wide variety of signals that converge at mammalian target of rapamycin (mTOR). In the course of the search for its physiological role, it was proposed that S6K activation and ribosomal protein S6 (rpS6) phosphorylation account for the translational activation of a subgroup of transcripts, the TOP mRNAs. The structural hallmark of these mRNAs is an oligopyrimidine tract at their 5'-terminus, known as the 5'-TOP motif. TOP mRNAs consists of about 90 members that encode multiple components of the translational machinery, such as ribosomal proteins and translation factors. The translation efficiency of TOP mRNAs indeed correlates with S6K activation and rpS6 phosphorylation, yet recent biochemical and genetic studies have established that, although S6K and TOP mRNAs respond to similar signals and are regulated by mTOR, they maintain no cause and effect relationship. Instead, S6K is primarily involved in regulation of cell size, and affects glucose homeostasis, but is dispensable for global protein synthesis, whereas translational efficiency of TOP mRNAs is a determinant of the cellular protein synthesis capacity. Despite extensive studies of their function and mode of regulation, the mechanism underlying the effect of S6K on the cell size, as well as the trans-acting factor that mediates the translational control of TOP mRNAs, still await their identification.

Molecular pathogenesis of acute promyelocytic leukaemia and APL variants

It has been 12 years since the simultaneous discovery of the unique sensitivity of acute promyelocytic leukaemia (APL) to differentiation therapy with all-trans retinoic acid (ATRA) and the discovery that the retinoic acid receptor alpha (RARalpha) gene was rearranged in APL. Nearly 98% of cases of APL are associated with t(15;17) chromosomal translocation and fusion of the PML gene to that encoding RARalpha to yield an abnormal receptor with the capability of de-regulating gene expression in the haematopoietic cell, causing differentiation block and eventually the development of leukaemia. Since this original discovery, four other translocations were described in APL. In each of these the RARalpha gene is fused to different partner genes, all yielding aberrant nuclear receptors. These fusion proteins share in common the ability to repress rather than activate retinoic acid targets, one so strongly that the result is an ATRA-resistant form of the disease. In addition each of the partner proteins is important for normal cell growth and development. In this chapter we explore the biology of the RARalpha, the fusion proteins created in APL and the normal forms of the partner proteins. Through continued study of this disease it is hoped that novel treatments, potentially more applicable to other forms of leukaemia, may arise.

Translocations of the RARalpha gene in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) has been recognized as a distinct clinical entity for over 40 years. Although relatively rare among hematopoietic malignancies (approximately 10% of AML cases), this disease has attracted a particularly good share of attention by becoming the first human cancer in which all-trans-retinoic acid (ATRA), a physiologically active derivative of vitamin A, was able to induce complete remission (CR). ATRA induced remission is not associated with rapid cell death, as in the case of conventional chemotherapy, but with a restoration of the 'normal' granulocytic differentiation pathway. With this remarkable medical success story APL has overnight become a paradigm for the differentiation therapy of cancer. A few years later, excitement with APL was further enhanced by the discovery that a cytogenetic marker for this disease, the t(15:17) reciprocal chromosomal translocation, involves a fusion between the retinoic acid receptor alpha (RARalpha) gene and a previously unknown locus named promyelocytic leukemia (PML). Consequence of this gene rearrangement is expression of the PML-RARalpha chimeric oncoprotein, which is responsible for the cellular transformation as well as ATRA response that is observed in APL. Since this initial discovery, a number of different translocation partner genes of RARalpha have been reported in rarer cases of APL, strongly suggesting that disruption of RARalpha underlies its pathogenesis. This article reviews various rearrangements of the RARalpha gene that have so far been described in literature, functions of the proteins encoded by the different RARalpha partner loci, and implications that these may have for the molecular pathogenesis of APL.

Low levels of NPM gene expression in UV-sensitive human cell lines

Nucleophosmin (NPM) is a major nuclear matrix protein associated with neoplastic growth in various cell types. We recently suggested that expression of the NPM gene is involved in an increased resistance to UV irradiation in human cells against the cell-killing effects of UV (mainly 254nm wavelength far-ultraviolet ray) [Y. Higuchi, K. Kita, H. Nakanishi, X-L. Wang, S. Sugaya, H. Tanzawa, H. Yamamori, K. Sugita, A. Yamaura, N. Suzuki, Biochem. Biophys. Res. Commun. 248 (1998) 597-602]. In the present study, expression levels of the NPM gene were examined in human cell lines with a high sensitivity to UV cell-killing. Cockayne syndrome patient-derived cell lines, CSAI and CSBI, and the Xeroderma pigmentosum patient-derived cell line, XP2OS(SV), XP13KY, XP3KA, XP6BE(SV), XP101OS and XP3BR(SV), have been investigated for their NPM mRNA expression with Northern blotting analysis. All of these UV-sensitive cells demonstrated lower expression levels compared with those of normal fibroblast cells, FF, or an UV-resistant cell line, UH(r)-10; quite a lower level of expression in XP205(SV) cells after UV irradiation in contrast to a distinguishable increase in the expression in UV(r)- cells. These results confirmed an intimate correlation between degree of UV sensitivity and expression levels of the NPM gene in human cells.

Antibodies against a novel nucleolar and cytoplasmic antigen (p105-p42) present in the sera of patients with a subset of rheumatoid arthritis (RA) with signs of scleroderma

We identified three patients (two of them relatives) with RA and signs of scleroderma whose sera contained a high titre of IgG class antibodies against the nucleoli and the nucleoplasm of cells of different mammalian origins. Sera from these patients uniformly immunoprecipitated four polypeptides, from a 35S-methionine-labelled HeLa cell extract, whose mol. wts were 120, 105, 95 and 42 kD. Of these, the 95-kD protein was highly phosphorylated. By immunoblotting, these sera reacted with 105-, 95- and 42-kD proteins and affinity-purified antibodies from these, demonstrating that 105- and 95-kD proteins shared cross-reactive epitopes. Moreover, affinity-purified antibodies from each of these proteins immunoprecipitated the whole complex. Localization studies using immunoelectron microscopy and in vivo actinomycin-D-treated cells demonstrated that the 105-, 95- and 42-kD proteins were present in the granular component of the nucleolus and the nucleoplasm. In addition, the 105- and 95-kD were present in free polyribosomes as well as ribosomes attached to endoplasmic reticulum. Pulse/chase experiments strongly suggested that the complex was accomplished shortly after a 10-min pulse. It was preferentially present in the nucleus after a 2 h chase and in both nucleus and cytoplasm after a 5 h chase. We conclude that a protein complex with a main nucleolar distribution is a new autoantigen (p105-p42) recognized by autoantibodies present in the serum of a subgroup of patients with RA and scleroderma signs. These antibodies could be useful as diagnostic markers and as tools for further studies involving the biology of the nucleolus.

Effect of laminin on the nuclear localization of nucleolin in rat intestinal epithelial IEC-6 cells

Laminin is a major component of extracellular matrix. The mechanism of action of laminin on cell proliferation, differentiation, and migration is not fully understood. In this study, we investigated the role of extracellular matrix, especially laminin, on the cellular localization of the nuclear protein, nucleolin, and on cell proliferation. Immunofluorescent and western blot analysis indicated that nucleolin was translocated most efficiently to the nucleus in the small intestinal rat epithelial cell line (IEC-6) when cultured on laminin-coated plates. Specifically, nucleolin was observed predominantly in cytoplasm in the cells cultured without laminin. In contrast, nuclear localization was observed in the cells cultured on laminin. This effect of laminin on nucleolin translocation was time-dependent. Laminin was also observed to stimulate proliferation of IEC-6 cells in serum free medium. Our results suggest that laminin alters the distribution of nucleolin which may be an early signal for cell proliferation.

Nucleolin: a multifunctional major nucleolar phosphoprotein

Nucleolin is a major protein of exponentially growing eukaryotic cells where it is present in abundance at the heart of the nucleolus. It is highly conserved during evolution. Nucleolin contains a specific bipartite nuclear localization signal sequence and possesses a number of unusual structural features. It has unique tripartite structure and each domain performs a specific function by interacting with DNA or RNA or proteins. Nucleolin exhibits intrinsic self-cleaving, DNA helicase, RNA helicase and DNA-dependent ATPase activities. Nucleolin also acts as a sequence-specific RNA binding protein, an autoantigen, and as the component of a B cell specific transcription factor. Its phosphorylation by cdc2, CK2, and PKC-zeta modulate some of its activities. This multifunctional protein has been implicated to be involved directly or indirectly in many metabolic processes such as ribosome biogenesis (which includes rDNA transcription, pre-rRNA synthesis, rRNA processing, ribosomal assembly and maturation), cytokinesis, nucleogenesis, cell proliferation and growth, cytoplasmic-nucleolar transport of ribosomal components, transcriptional repression, replication, signal transduction, inducing chromatin decondensation and many more (see text). In plants it is developmentally, cell-cycle, and light regulated. The regulation of all these functions of a single protein seems to be a challenging puzzle.

Expression of nucleophosmin/B23 in normal and neoplastic colorectal mucosa

Nucleophosmin/B23 is a 38 kD molecular phosphoprotein involved in ribosome assembly and transport. In view of the fact that nucleophosmin/B23 appears to be more abundant in tumour cells than in normal cells, the mRNA expression and immunohistochemical localization of nucleophosmin/B23 were investigated in 19 samples of non-neoplastic mucosa, six adenomas, and 16 adenocarcinomas of the colorectum. Northern blot analysis revealed that nucleophosmin/B23 mRNA is expressed at a higher level in adenomas and carcinomas than in non-neoplastic mucosa of the colorectum. Immunohistochemical staining of formalin-fixed, paraffin-embedded tissue sections after microwave antigen retrieval, using a nucleophosmin/B23-specific monoclonal antibody, showed almost exclusively diffuse nuclear reactivity of a majority of the epithelial cells in non-neoplastic mucosa: in adenomas, reactivity was almost exclusively nucleolar and in carcinomas, nuclear as well as nucleolar staining was observed. During mitosis, the immunoreactivity of nucleophosmin/B23 appears in the cytoplasm. The results indicate that the expression of nucleophosmin/B23 is higher in neoplastic than in non-neoplastic colorectal mucosa. Furthermore, the pattern of nucleophosmin/B23 expression shifts from nuclear to nucleolar early in the adenoma-carcinoma sequence. The exact function of nucleophosmin/B23 in colorectal carcinogenesis remains to be determined.

Identification of mRNAs that show modulated expression during colon carcinoma cell differentiation

To investigate the working hypotheses that stem cells or their early descendants are prime targets for neoplastic transformation, and that the degree to which a neoplasm retains the immature phenotype is an important determinant of tumor aggressiveness, we have identified several mRNAs that are downregulated during the in vitro differentiation of HT29-D4 colon carcinoma cells. These genes include heat-shock cognate protein Hsc70, adenylosuccinate lyase, B23/nucleophosmin, alpha-tubulin, and a novel gene designated DS-1. The DS-1 mRNA has a length of approximately 0.9 kb and is downregulated 4.7-fold upon differentiation. From the DS-1 cDNA, a protein of 206 amino acids with a molecular mass of 24 kDa and an isoelectric point of 10.9 can be deduced. An antiserum directed against a synthetic peptide detected a minor band of the expected size in Western blots, as well as a major band of lower size that may represent a processed form of the protein.

Protein localization to the nucleolus: a search for targeting domains in nucleolin

Nucleolin, a major nucleolar phosphoprotein, is presumed to function in rDNA transcription, rRNA packaging and ribosome assembly. Its primary sequence was highly conserved during evolution and suggests a multi-domain structure. To identify structural elements required for nuclear uptake and nucleolar accumulation of nucleolin, we used site-directed mutagenesis to introduce point- and deletion-mutations into a chicken nucleolin cDNA. Following transient expression in mammalian cells, the intracellular distribution of the corresponding wild-type and mutant proteins was determined by indirect immunofluorescence microscopy. We found that nucleolin contains a functional nuclear localization signal (KRKKEMANKSAPEAKKKK) that conforms exactly to the consensus proposed recently for a bipartite signal (Robbins, J., Dilworth, S.M., Laskey, R.A. and Dingwall, C. (1991) Cell 64, 615-623). Concerning nucleolar localization, we found that the N-terminal 250 amino acids of nucleolin are dispensible, but deletion of either the centrally located RNA-binding motifs (the RNP domain) or the glycine/arginine-rich C terminus (the GR domain) resulted in an exclusively nucleoplasmic distribution. Although both of these latter domains were required for correct subcellular localization of nucleolin, they were not sufficient to target non-nucleolar proteins to the nucleolus. From these results we conclude that nucleolin does not contain a single, linear nucleolar targeting signal. Instead, we propose that the protein uses a bipartite NLS to enter the nucleus and then accumulates within the nucleolus by virtue of binding to other nucleolar components (probably rRNA) via its RNP and GR domains.

Protein B23 (M.W./pI = 37 kD/5.1) is the only major protein extracted from HeLa nucleoli with 3M urea

HeLa nucleoli were isolated using the NP-40 method and subsequently extracted with 3M urea. The extract was incubated at 60 degrees C for 30 min, and precipitated proteins were removed by centrifugation. The supernatant was analyzed by one- and two-dimensional SDS polyacrylamide gel electrophoresis (PAGE). Protein B23 was the only major protein extracted from HeLa nucleoli by this procedure. Using this procedure, 1 mg of protein B23 was obtained from 2 g of HeLa cells. The purity of the extracted protein B23 was 98%, as measured by one- and two-dimensional gel electrophoresis.

Nucleolar protein B23 translocation after deferoxamine treatment in a human leukemia cell line

Localization of nucleolar protein B23 in HL-60 cells under the treatment by iron chelator deferoxamine (DSF) was studied using indirect immunofluorescence. Bright nucleolar fluorescence was observed in exponentially growing control cells. The addition of DSF in the culture system resulted in time- and dose-dependent induction of protein B23 translocation from nucleoli to nucleoplasm, inhibition of cell growth, DNA and RNA synthesis. The addition of FeCl3 at culture initiation completely reversed the effects of DSF. Furthermore, significant numbers of HL-60 cells could be rescued from the effects of DSF when iron was added back as late as 24 hr after exposure to DSF. Cells resumed their abilities to grow and to synthesize DNA and RNA upon the iron rescue. Protein B23, accordingly, relocated from nucleoplasm to nucleoli. These results indicate the importance of iron for proliferation of leukemic cells and localization of protein B23 in nucleoli. Preribosomal ribonucleoprotein particles (pre-rRNPs) were extracted from isolated nucleoli of HL-60 cells and fractionated on sucrose density gradients. Protein B23 was found to be co-localized with the pre-rRNPs as determined by ELISA assays. No such B23-associated pre-rRNPs or other pre-rRNP fractions were obtained from nucleoli of DSF-treated cells. These results suggest that one of the effects of the anti-proliferative action of DSF is the inhibition of rRNA synthesis in nucleoli. Due to the lack of new synthesis of rRNA in nucleoli, protein B23 loses its binding target and translocates into the nucleoplasm. B23 translocation, as observed by immunofluorescence, may be a simple and rapid method for assessing inhibition of cell growth in response to anti-proliferative drugs such as deferoxamine in cancer chemotherapy.

Enhanced phosphorylation of a nucleolar 110-kDa protein in rat liver by dietary manipulation

RNA polymerase 1 activity and nucleolar volume have been reported to increase in hepatocytes from rats fed a protein-free diet. Phosphorylation in vitro of a 110-kDa protein was enhanced in nuclei and nucleoli from livers of rats fed a protein-free diet. In nuclear extracts the 110-kDa protein in heat-treated nuclei was much more phosphorylated than from control liver. In contrast, casein kinase activity in the nuclear extract from control liver was comparable to that from livers of rats fed a protein-free diet. Nuclear extracts from control rat liver and livers of rats fed a protein-free diet were fractionated by DEAE-cellulose column chromatography. Casein kinase II (NII) eluted at around 0.17 M NaCl scarcely phosphorylates the 110-kDa protein. Chromatography of the nuclear extract from livers of rats fed a protein-free diet, but not from control liver, yielded fractions which eluted at 0.21-0.25 M NaCl and predominantly phosphorylated the 110-kDa protein. The phosphorylation of 110-kDa protein was not appreciably affected by a heparin concentration of 5 micrograms/ml, which completely inhibited casein kinase II. In addition, phosphorylation of the 110-kDa protein in liver nucleoli from rats fed a protein-free diet showed a lower sensitivity to heparin than that in control rat liver nucleoli. These results suggest that enhanced phosphorylation of the nuclear 110-kDa protein in livers from rats fed a protein-free diet is due to the induction of a 110-kDa protein kinase distinct from casein kinase II.

Identification of protein components reactive with anti-PM/Scl autoantibodies

The PM/Scl antigen from mammalian cells has been characterized as a nucleolar and nucleoplasmic molecular complex containing at least 16 polypeptides ranging in molecular weight from 110 to 20 kD. Of these polypeptides, we have found those of 68, 39 and 20 kD to be in a phosphorilated form. Whereas the entire complex was precipitated by all the anti-PM/Scl sera tested, in immunoblots the antibodies specifically recognized determinants on the 110-kD protein. This protein was immunoprecipitated more preferentially from nucleoli extracts than from total cell extracts. Moreover, this protein disappeared from the immunoprecipitates when treated with DNAse. Likewise, the immunoblot reaction of the specific antibodies with the 110-kD protein was abolished by treatment of the extracts with DNAse and trypsin, and was resistant when extracts were treated with RNAse. Affinity-purified antibodies from this protein selectively stained the nucleoli and the nucleoplasm of the mammalian cells. Moreover, when the cultured cells used in immunofluorescence were treated with DNAse, the affinity purified antibodies from the 110-kD protein gave negative fluorescence. However, when whole anti-PM/Scl sera were used, a nucleolar and nucleoplasmic staining was found. We conclude that the 110-kD protein has at least one of the autoimmunogenic epitopes of the PM/Scl antigen, recognized by all anti-PM/Scl sera tested. Other epitopes differing in their DNAse sensitivity may also be present in the PM/Scl antigen.

Structure and developmental expression of chicken nucleolin and NO38: coordinate expression of two abundant non-ribosomal nucleolar proteins

We report the complete primary structures of two major chicken non-ribosomal nucleolar proteins known as nucleolin/C23 and NO38/B23, respectively. By comparison with homologous proteins from other species, this sequence information contributes to the identification of evolutionarily conserved motifs that may be relevant to the function and subcellular distribution of the two proteins. Using cDNA probes and monoclonal antibodies, we have also studied the expression of nucleolin and NO38 in the course of chicken embryogenesis. In all tissues examined, Northern analyses revealed single hybridization signals for nucleolin (at 3.0 kb) and NO38 (at 1.6 kb), and no evidence was obtained for multiple protein products. In total embryos between days 3 and 11 after egg laying, nucleolin and NO38 mRNA and protein levels decreased in parallel (2-5-fold), suggesting transcriptional down-regulation of expression. Coordinate expression of nucleolin and NO38 was observed also when examining individual tissues at various stages of development. Interestingly, however, there was no consistent correlation between relative mRNA and protein levels. In particular, several adult tissues contained exceedingly low levels of either nucleolin or NO38, despite the presence of large amounts of corresponding mRNAs. From these results we conclude, first, that the expression of nucleolin and NO38 is controlled coordinately, and, second, that regulation is likely to involve both transcriptional and posttranscriptional mechanisms.

Specific dephosphorylation by phosphatases 1 and 2A of a nuclear protein structurally and immunologically related to nucleolin. Possible influence on the regulation of rRNA synthesis

A new nuclear substrate (N-60) for phosphatase 1 and 2Ac has been described. In contrast to nucleolin (C23), to which it is structurally and immunologically related, N-60 becomes dephosphorylated to 51% and 41% by phosphatases 1 and 2Ac, respectively, within 10 min. Incubation up to 20 min led to a complete dephosphorylation of N-60. The two other phosphatases tested (2B and 2C) did not dephosphorylate protein N-60 to the same extent as phosphatases 1 and 2Ac. In the case of nucleolin only 18% phosphate was released by all four phosphatases tested. The activity of both phosphatases, 1 and 2A, could be blocked by tumour promoter okadaic acid (100 nM) when N-60 was used as a substrate. These results support the notion that the observed okadaic-acid-induced hyperphosphorylation of N-60 in intact human fibroblasts may be caused by specific inhibition of phosphatases involved in the process of rDNA transcription.

Nucleolin (C23), a physiological substrate for casein kinase II

Nucleolin (C23), a 110 kDa phosphoprotein, which is mainly found in the nucleolus has been shown to be a physiological substrate for casein kinase II (CKII). Nucleolin was identified and characterized by immunodetection using an anti-nucleolin antibody. Phosphopeptide patterns from nucleolin phosphorylated by purified casein kinase II and of phosphorylated nucleolin which had been isolated from tumor cells grown in the presence of [32P]-o-phosphate, were identical. The partial tryptic digest revealed nine phosphopeptides. Nucleolin isolated from Krebs II mouse ascites cells was phosphorylated by purified casein kinase II with about two moles phosphate per one mole of nucleolin.

Effects of actinomycin D analogs on nucleolar phosphoprotein B23 (37,000 daltons/pI 5.1)

Localization of protein B23 in HeLa cells after treatment with actinomycin D and its analogs was studied using indirect immunofluorescence. Bright nucleolar fluorescence was observed in control HeLa cells. After treatment with actinomycin D (250 ng/ml) for 2 hr, a uniform nucleoplasmic fluorescence was observed. Similar results were obtained with the actinomycin analogs, actinomycin Z5 and actinomycin K2T. Only after a much longer incubation (24 hr) with actinomycin 4-4'-gly was nucleoplasmic fluorescence observed. Actinomycin D, actinomycin Z5, and actinomycin K2T inhibited [3H]uridine incorporation into the trichloroacetic acid insoluble fraction of HeLa cells with IC50 values of 9.5 +/- 3.2, 59.1 +/- 19.6 and 1423.3 +/- 212.2 ng/ml respectively. No inhibition of [3H]uridine incorporation was observed using actinomycin 4-4'-gly (2000 ng/ml, 2-hr incubation). The order of potency for the loss of nucleolar fluorescence and the concurrent increase in nucleoplasmic fluorescence was actinomycin D greater than actinomycin Z5 greater than actinomycin K2T greater than actinomycin 4-4'-gly, which correlated with the order of their IC50 values for inhibition of [3H]uridine incorporation. Studies of the effects of actinomycin D and its analogs on RNA synthesis and localization of protein B23 indicated that there is a direct relationship between the B23 "translocation" from nucleolus to nucleoplasm and the inhibition of RNA synthesis. At 45-55% inhibition of RNA synthesis, both nuclear and nucleolar B23 immunofluorescence were observed. At 75-85% inhibition, only a uniform nucleoplasmic fluorescence was observed.

Alterations in immunolocalization of the phosphoprotein B23 in HeLa cells during serum starvation

Bright nucleolar immunofluorescence was observed in HeLa S3 cells by immunostaining with a monoclonal antibody to the nucleolar phosphoprotein B23 (MW 37 kD/pI 5.1). After 48 h of incubation in a serum-free medium, the nucleolar fluorescence was diminished and a general nuclear immunofluorescence was observed. This change in localization of fluorescence indicated that protein B23 had migrated out of the nucleoli. No gross morphological change in nucleoli was observed by light microscopy and the immunolocalization of another nucleolar phosphoprotein, C23, was unaffected by serum deprivation. Relocation of protein B23 in nucleoli was observed after refeeding with serum-containing medium. This re-entry process was not observed after treatment with actinomycin D (50 ng/ml-5 micrograms/ml), but the process was unaffected by cycloheximide (0.2 mM). Quantitation of protein B23 in the nucleoli of the control (fed) or starved HeLa cells was done by ELISA immunoassay. A marked decrease in the amount of protein B23 occurred in the nucleoli of the starved cells (11.8 micrograms B23/mgDNA) as compared with the control nucleoli (20.8 micrograms B23/mgDNA). The amount of protein B23 in the nucleoplasm (excluding nucleoli) was 70% higher in the starved cells. Protein B23 was analysed by one- and two-dimensional PAGE. Three components of protein B23 with slightly different molecular weights and pIs (37 kD/5.1, 35 kD/5.1 and 35 kD/5.3) were observed in nucleoli. The lower molecular weight components were predominantly found in the nucleoplasm.

Characterization of low-molecular-weight acid phosphoprotein phosphatase associated with rat splenic cell nucleoli

Evidence is presented on the existence of an acid phosphoprotein phosphatase (APPase) associated with rat splenic cell nucleoli. The enzyme is purified 1250-fold from 0.3 M NaCl nucleolar extract by means of chromatography on P cellulose and Sephacryl S-200. The nucleolar acid phosphoprotein phosphatase is a very basic protein (pI 8.3) and shows maximal activity at pH 5.8. It dephosphorylates acidic phosphoproteins (casein and phosvitin), ATP, and p-nitrophenyl phosphate, but not basic phosphoproteins (histones and protamine phosphate). The enzyme activity is very dependent on reducing agents, especially on ascorbic acid. Divalent and monovalent cations did not affect phosphatase activity, but heavier divalent metals, Co2+ and Zn2+, strongly inhibit the enzyme activity. The activity was also inhibited by N-ethylmaleimide, indicating a requirement for free sulfhydryl groups. The estimated molecular weight of the purified enzyme is approximately 38,000 by gel filtration and sedimentation in sucrose gradient concentration.

Localization of nucleolar phosphoproteins B23 and C23 during mitosis

Nucleolar phosphoproteins B23 and C23 were simultaneously localized in unsynchronized male rat-kangaroo PtK2 cells during mitosis using a mouse monoclonal antibody against protein B23 and a rabbit antibody against protein C23. The distribution of proteins B23 and C23 during mitosis was compared with the distribution of the silver staining protein. During interphase, proteins B23 and C23 were both localized to the nucleolus. As the nucleolus disappeared in prophase, the distribution of protein B23 became nucleoplasmic, whereas most of protein C23 remained associated with the disappearing nucleolus. Throughout metaphase and anaphase protein B23 was found associated with the chromosomes, whereas protein C23 seemed to disappear. When the nucleolus reformed during telophase, protein C23 appeared first in 'prenucleolar bodies' and then in the nucleolus, whereas protein B23 did not appear in the nucleolus until late telophase or early G1 phase. Silver staining during mitosis closely paralleled the distribution of protein C23, supporting previous conclusions that protein C23 is a silver staining nucleolus organizer region (NOR) protein [19, 20].

Sequence of a phosphorylation site in nucleolar protein B23

The sites of phosphorylation in protein B23, a silver-staining preribosomal ribonucleoprotein particle protein, were analyzed by tryptic peptide mapping. Three 32P peptides were found using in vitro labeling of nucleoli. An additional unlabeled phosphopeptide was identified by amino acid analysis. The sequence of the latter was Asp-Thr(P)-Pro-Ala-Lys. These results suggest that protein B23 contains one class of site labeled rapidly in vitro and another type of site phosphorylated only in vivo.

Identification and characterization of ribosomal proteins phosphorylated in vaccinia-virus-infected HeLa cells

Two-dimensional analysis of 32P-labelled ribosomal proteins revealed three proteins which are phosphorylated in vaccinia-virus-infected HeLa cells. All three proteins belong to the 40-S ribosomal subunits and were identified as S2, S6 and S16. The ribosomal protein S6 is phosphorylated also in uninfected HeLa cells. Phosphoserine was detected in all three proteins, phosphothreonine only in the protein S2. Phosphorylation of these ribosomal proteins in infected cells is dependent on the multiplicity of the viral infection and increases during the first six hours of infection. All three proteins are also phosphorylated in virus-infected cells treated with cycloheximide and in cells infected with ultraviolet-irradiated virus. This suggests that the phosphorylation reaction involves a vaccinia virion-associated protein kinase.

Nucleolar phosphoprotein phosphatase from Novikoff hepatoma and rat liver: characterization and partial purification

Phosphoprotein phosphatase which dephosphorylates 32P-labeled nucleolar protein substrates was found in nucleoli of Novikoff hepatoma ascites cells and normal rat liver. The activity was extracted in high yield from nucleoli with 0.01 M Bis/Tris (pH 7.0). Low ionic strength was also required for activity: the activity was only 50% of maximum in 0.075 M NaCl. Activity was affected differently by various divalent cations: MgCl2 had little effect: CaCl2, MnCl2 and CoCl2 above 4 mM inhibited the activity 30--60%; ZnCl2 above 2 mM completely destroyed the activity. EDTA had no effect, indicating that divalent cations are probably not required. The enzyme activity was enhanced 20% by 5--8 mM dithiothreitol and was inhibited 60% by 7--10 mM N-ethylmaleimide indicating a requirement for free sulfhydryl groups. The Km of the extracted enzyme for 32P-labeled nucleolar protein was 0.6 mg/ml. The phosphatase was capable of dephosporylating the major phosphorylated nucleolar proteins C23-24 and B23-24 and also histone H1. The enzyme was purified more than 200-fold on hydroxyapatite followed by DEAE-Sephadex, which resolved the activity into three major components. The activity of enzyme extracted from Novikoff hepatoma nucleoli was approximately 2.5 times greater than from normal liver nucleoli.

Acidic phosphoproteins of the 60-S ribosomal subunits from HeLa cells

Two-dimensional analysis of the ribosomal proteins from 60-S subunits of HeLa cells revealed a triplet of acidic proteins, L40a, L40b and L40c, of identical molecular weight (13,700), which can be separated only on the basis of their charge differences. Two of the spots, L40b and L40c, become labeled after incubation of the cells with inorganic [32P]phosphate. The electrophoretic behavior and molecular weights of these proteins support the notion that the proteins L40b and L40c, are phosphorylated forms of the protein L40a. The same proteins can be phosphorylated also in vitro by a HeLa protein kinase on 60-S subunits but not on 80-S ribosomes. The inaccessibility of L40 proteins to the phosphorylation in vitro on 80-S ribosomes suggests that they are located in the interface between the 40-S and 60-S subunits.

Cyclic nucleotide metabolism in solid tumor tissues

The examination of the regulation of the system of 3'-5' cyclic nucleotide monophosphates has only begun in cancer tissues. In human cancers, these studies are notably non-existent. However, in animal cancers, especially the Morris hepatomas, enough data has been gathered that, while risky, certain trends seem to begin to appear. Cyclic AMP is constant or lowered, while cyclic GMP is elevated in the fast growing hepatomas. Regulation of adenylate cyclase by protein hormones is reduced, while regulation by epinephrine may be increased. Binding of glucagon is decreased in the fast growing hepatomas. Guanylate cyclase, while being predominantly cytoplasmic in the normal liver, is predominantly membrane bound in the tumors. The liver enzyme is also readily stimulated by several chemical carcinogens. The cyclic GMP phosphodiesterases are decreased in these tumors; while the cAMP phosphodiesterases are increased. Although the cyclic nucleotide dependent protein kinases (histone as substrate) are altered in the hepatomas, observations of unique cyclic nucleotide binding proteins or cAMP independent protein kinases in cancer tissues may be of even greater significance for the development of or the maintenance of the neoplastic state of cells.

A nucleolus-specific phosphoprotein in mouse ascites tumor cells

Analysis of proteins in nucleoli and chromatin of mouse ascites tumor cells labeled with [32P]orthophosphate by SDS-polyacrylamide gel electrophoresis showed that a highly radioactive protein was localized in the nucleoli. This protein was purified and the final preparation appeared as a single component on hydroxylapatite column chromatography with or without SDS. This protein was found to be a nucleolus-specific phosphoprotein with a molecular weight of 120 000. The phosphate moiety in this protein turned over very rapidly whereas the protein itself was stable. When the nucleoli were disrupted by EDTA treatment, this unique protein was found as a major protein constituent of the ultracentrifugal supernatant.

Partial purification and properties of a chromatin-associated phosphoprotein kinase from rat liver nuclei

A phosphoprotein kinase (EC 2.7.1.37) KIVb, from rat liver nuclei, was purified 75-fold by phosphocellulose chromatography and gel filtration on Sephadex G-200. The enzyme, which has an apparent molecular weight of 55 000, phosphorylates casein and chromatin-bound nonhistone proteins more readily than histones or ribosomal proteins. It exhibits an absolute requirement for divalent cation with optimum activity at 15--20 mM Mg2+. Maximal kinase activity is achieved at 100 mM NaCl. The pH vs. activity curve is biphasic with optima at pH 6.5 and pH 8.0. The Km value for casein is 280 mug/ml and the Km for ATP is 6-10(-6) M. Kinase KIVb phosphorylates numerous nonhistone nuclear proteins as shown by electrophoretic analysis. The addition of kinase KIVb to reaction mixtures containing nonhistone proteins results in the phosphorylation of a spectrum of polypeptides similar to those that are phosphorylated by endogenous nuclear kinases. Nonhistone proteins bound to chromatin appear to be better substrates for KIVb than nonhistones dissociated from chromatin. A comparison of nuclear phosphoproteins phosphorylated either in the intact animal or in vitro (by the addition of kinase KIVb) indicates some differences and some similarities in the patterns of phosphorylation.