Nucleolar localization of nucleophosmin/B23 requires GTP

Functions of the native NPM1 protein and its leukemic mutant

The nucleophosmin (NPM1) gene encodes for the most abundant nucleolar protein. Thanks to its property to act as histone chaperone and to shuttle between the nucleus and cytoplasm, the NPM1 protein is involved in multiple cellular function that are here extensively reviewed and include the formation of the nucleolus through liquid-liquid phase separation, regulation of ribosome biogenesis and transport, control of DNA repair and centrosome duplication as well as response to nucleolar stress. NPM1 is mutated in about 30-35% of adult acute myeloid leukemia (AML). Due to its unique biological and clinical features, NPM1-mutated AML is regarded as a distinct leukemia entity in the WHO 5th edition and ICC classifications of myeloid malignancies. The NPM1 mutant undergoes changes at the C-terminus of the protein that leads to its delocalization in the cytoplasm of the leukemic cells. Here, we focus also on its biological functions discussing the murine models of NPM1 mutations and the various mechanisms that occur at cytoplasmic and nuclear levels to promote and maintain NPM1-mutated AML.

Influence of Chemical and Genetic Manipulations on Cellular Organelles Quantified by Label-Free Optical Diffraction Tomography

Label-free optical diffraction tomography provides three-dimensional imaging of cells and organelles, along with their refractive index (RI) and volume. These physical parameters are valuable for quantitative and accurate analysis of the subcellular microenvironment and its connections to intracellular biological properties. In biological and biochemical cell analysis, various invasive cell manipulations are used, such as temperature change, chemical fixation, live cell staining with fluorescent dye, and gene overexpression of exogenous proteins. However, it is not fully understood how these various manipulations affect the physicochemical properties of different organelles. In this study, we investigated the impact of these manipulations on the cellular properties of single HeLa cells. We found that after cell fixation and an increase in temperature, the RI value of organelles, such as the nucleus and cytoplasm, significantly decreased overall. Interestingly, unlike the cell nuclei, cytoplasmic RI values were hardly detected after membrane permeation, indicating that only intracytoplasmic components were largely lost. Additionally, our findings revealed that the expression of GFP and GFP-tagged proteins significantly increased the RI values of organelles in living cells compared to the less effective RI changes observed with chemical fluorescence staining for cell organelles. The result demonstrates that distinct types of invasive manipulations can alter the microenvironment of organelles in different ways. Our study sheds new light on how chemical and genetic manipulations affect organelles.

Biophysical characterization of organelle-based RNA/protein liquid phases using microfluidics

Living cells contain numerous membrane-less RNA/protein (RNP) bodies that assemble by intracellular liquid-liquid phase separation. The properties of these condensed phase droplets are increasingly recognized as important in their physiological function within living cells, and also through the link to protein aggregation pathologies. However, techniques such as droplet coalescence analysis or standard microrheology do not always enable robust property measurements of model RNA/protein droplets in vitro. Here, we introduce a microfluidic platform that drives protein droplets into a single large phase, which facilitates viscosity measurements using passive microrheology and/or active two-phase flow analysis. We use this technique to study various phase separating proteins from structures including P granules, nucleoli, and Whi3 droplets. In each case, droplets exhibit simple liquid behavior, with shear rate-independent viscosities, over observed timescales. Interestingly, we find that a reported order of magnitude difference between the timescale of Whi3 and LAF-1 droplet coalescence is driven by large differences in surface tension rather than viscosity, with implications for droplet assembly and function. The ability to simultaneously perform active and passive microrheological measurements enables studying the impact of ATP-dependent biological activity on RNP droplets, which is a key area for future research.

Discovery of widespread GTP-binding motifs in genomic DNA and RNA

Biological RNAs that bind small molecules have been implicated in a variety of regulatory and catalytic processes. Inspired by these examples, we used in vitro selection to search a pool of genome-encoded RNA fragments for naturally occurring GTP aptamers. Several aptamer classes were identified, including one (the "G motif") with a G-quadruplex structure. Further analysis revealed that most RNA and DNA G-quadruplexes bind GTP. The G motif is abundant in eukaryotes, and the human genome contains ~75,000 examples with dissociation constants comparable to the GTP concentration of a eukaryotic cell (~300 μM). G-quadruplexes play roles in diverse cellular processes, and our findings raise the possibility that GTP may play a role in the function of these elements. Consistent with this possibility, the sequence requirements of several classes of regulatory G-quadruplexes parallel those of GTP binding.

Nucleolar changes and fibrillarin redistribution following apatone treatment of human bladder carcinoma cells

Ascorbate and menadione (Apatone) in a ratio of 100:1 kills tumor cells by autoschizis. In this study, vitamin-induced changes in nucleolar structure were evaluated as markers of autoschizis. Human bladder carcinoma (T24) cells were overlain with vitamins or with culture medium. Supernatants were removed at 1-hr intervals from 1 to 4 hr, and the cells were washed with PBS and prepared for assay. Apatone produced marked alterations in nucleolar structure including redistribution of nucleolar components, formation of ring-shaped nucleoli, condensation and increase of the proportion of perinucleolar chromatin, and the enlargement of nucleolar fibrillar centers. Immunogold labeling of the nucleolar rRNA revealed a granular localization in treated and sham-treated cells, and immunogold labeling of the rDNA revealed a shift from the fibrillar centers to the condensed perinucleolar chromatin. Fibrillarin staining shifted from the fibrillar centers and adjacent regions to a more homogeneous staining of the entire nucleolus and was consistent with the percentage of autoschizic cells detected by flow cytometry. Because autoschizis entails sequential reactivation of DNase I and DNase II, and because the fibrillarin redistribution following DNase I and Apatone treatment is identical, it appears that the nucleolar and fibrillarin changes are markers of autoschizis.

Immunodetection of nucleolar proteins and ultrastructure of nucleoli of soybean root meristematic cells treated with chilling stress and after recovery

The nucleolar proteins, fibrillarin and nucleophosmin, have been identified immunofluorescently in the root meristematic cells of soybean seedlings under varying experimental conditions: at 25 degrees C (control), chilling at 10 degrees C for 3 h and 4 days and recovery from the chilling stress at 25 degrees C. In each experimental variant, the immunofluorescence signals were present solely at the nucleolar territories. Fluorescent staining for both proteins was mainly in the shape of circular domains that are assumed to correspond to the dense fibrillar component of the nucleoli. The fewest fluorescent domains were observed in the nucleoli of chilled plants, and the highest number was observed in the plants recovered after chilling. This difference in the number of circular domains in the nucleoli of each variant may indicate various levels of these proteins in each variant. Both the number of circular domains and the level of these nucleolar proteins changed with changes in the transcriptional activity of the nucleoli, with the more metabolically active cell having higher numbers of active areas in the nucleolus and higher levels of nucleolar proteins, and conversely. Electron microscopic studies revealed differences in the ultrastructure of the nucleoli in all experimental variants and confirmed that the number of fibrillar centres surrounded by dense fibrillar component was the lowest in the nucleoli of chilled plants, and the highest in the nucleoli of recovered seedlings.

Lysine 263 residue of NPM/B23 is essential for regulating ATP binding and B23 stability

Here, we show that Nucleophsomin/B23 provides lysine 263 as a critical binding site for ATP. Mutagenesis of lysine 263 to asparagine (K263N) disrupts B23 from ATP binding. While B23 WT exclusively localizes to the nucleolus, the B23-K263N is redistributed from the nucleolus to the nucleoplam. Notably, the K263N mutant is unstable, and displayed rapid degradation. Alteration of K263 induced B23 instability through increased ubiquitination and proteaosomal degradation. Moreover, mutation of K263 impedes the mitogenic effect of B23 in PC12 cells. Thus, K263 is a critical site for ATP binding and required for B23 stability, confining B23 in the nucleolus.

Compartmentation of the nucleolar processing proteins in the granular component is a CK2-driven process

To analyze the compartmentation of nucleolar protein complexes, the mechanisms controlling targeting of nucleolar processing proteins onto rRNA transcription sites has been investigated. We studied the reversible disconnection of transcripts and processing proteins using digitonin-permeabilized cells in assays capable of promoting nucleolar reorganization. The assays show that the dynamics of nucleolar reformation is ATP/GTP-dependent, sensitive to temperature, and CK2-driven. We further demonstrate the role of CK2 on the rRNA-processing protein B23. Mutation of the major CK2 site on B23 induces reorganization of nucleolar components that separate from each other. This was confirmed in assays using extracts containing B23 mutated in the CK2-binding sites. We propose that phosphorylation controls the compartmentation of the rRNA-processing proteins and that CK2 is involved in this process.

Nucleolus: from structure to dynamics

The nucleolus, a large nuclear domain, is the ribosome factory of the cells. Ribosomal RNAs are synthesized, processed and assembled with ribosomal proteins in the nucleolus, and the ribosome subunits are then transported to the cytoplasm. In this review, the structural organization of the nucleolus and the dynamics of the nucleolar proteins are discussed in an attempt to link both information. By electron microscopy, three main nucleolar components corresponding to different steps of ribosome biogenesis are identified and the nucleolar organization reflects its activity. Time-lapse videomicroscopy and fluorescent recovery after photobleaching (FRAP) demonstrate that mobility of GFP-tagged nucleolar proteins is slower in the nucleolus than in the nucleoplasm. Fluorescent recovery rates change with inhibition of transcription, decreased temperature and depletion of ATP, indicating that recovery is correlated with cell activity. At the exit of mitosis, the nucleolar processing machinery is first concentrated in prenucleolar bodies (PNBs). The dynamics of the PNBs suggests a steady state favoring residence of processing factors that are then released in a control- and time-dependent manner. Time-lapse analysis of fluorescence resonance energy transfer demonstrates that processing complexes are formed in PNBs. Finally, the nucleolus appears at the center of several trafficking pathways in the nucleus.

A multistep, GTP-driven mechanism controlling the dynamic cycling of nucleostemin

Nucleostemin (NS) was identified as a stem cell– and cancer cell–enriched nucleolar protein that controls the proliferation of these cells. Here, we report the mechanism that regulates its dynamic shuttling between the nucleolus and nucleoplasm. The nucleolar residence of nucleostemin involves a transient and a long-term binding by the basic and GTP-binding domains, and a dissociation mechanism mediated by the COOH-terminal region. This cycle is propelled by the GTP binding state of nucleostemin. We propose that a rapid nucleostemin cycle is designed to translate extra- and intra-cellular signals into the amount of nucleostemin in the nucleolus in a bidirectional and fast manner.

Release of nucleophosmin from the nucleus: Involvement in aloe-emodin-induced human lung non small carcinoma cell apoptosis

Aloe-emodin (1,8-dihydroxy-3-(hydroxymethyl)-anthraquinone) is one of the active constituents from the root and rhizome of Rheum palmatum. Our previous study has demonstrated that aloe-emodin induced a significant change in the expression of lung cancer cell apoptosis-related proteins compared to those of control cells. However, the molecular mechanisms underlying the biological effects of aloe-emodin still remain unknown. Based on these reasons, we were interested in the change of aloe-emodin-induced total protein expression by the proteomics technique during aloe-emodin-induced lung cancer cell apoptosis. Our study applied 2D electrophoresis to analyze the proteins involved in aloe-emodin-induced apoptosis in H460 cells. We found that the release of nucleophosmin from the nucleus to the cytosol and the degradation of nucleophosmin were associated with aloe-emodin-induced H460 cell apoptosis. Our study also demonstrated that the gene expression of nucleophosmin remained unchanged after treatment with aloe-emodin. The aloe-emodin-caused increase in the amount of proform and fragment of nucleophosmin in cytoplasm may be one of the important events for aloe-emodin-induced H460 cell apoptosis.

A molecular view of stem cell and cancer cell self-renewal

With the recent advances in cell biology and molecular genetics, scientists were able to isolate and culture tissue-specific stem cells from various sources and define their properties. The challenge has now shifted to understanding the genetic programs controlling the stem cell state, i.e. self-renewal and multipotential. Cracking the molecular codes that govern the stem cell state turns out to be a difficult task. This is in part because a single gene may exhibit distinct activities when expressed in different cell types. Comprehending the cell-context dependent readout of any given gene requires an integrated knowledge of the complex cellular machinery, a platform which can be provided by the research on stem cells. This review is an attempt to formulate a model for the self-renewal machinery operating in stem cells and cancer cells. Insight into this issue at the molecular and cellular levels will no doubt facilitate the realization of the stem cell potential in both regenerative medicine and anticancer therapy.

Proteasome-dependent processing of nuclear proteins is correlated with their subnuclear localization

Although proteasomes are abundant in the nucleoplasm little is known of proteasome-dependent proteolysis within the nucleus. Thus, we monitored the subcellular distribution of nuclear proteins in correlation with proteasomes. The proteasomal pathway clears away endogenous proteins, regulates numerous cellular processes, and delivers immunocompetent peptides to the antigen presenting machinery. Confocal laser scanning microscopy revealed that histones, splicing factor SC35, spliceosomal components, such as U1-70k or SmB/B('), and PML partially colocalize with 20S proteasomes in nucleoplasmic substructures, whereas the centromeric and nucleolar proteins topoisomerase I, fibrillarin, and UBF did not overlap with proteasomes. The specific inhibition of proteasomal processing with lactacystin induced accumulation of histone protein H2A, SC35, spliceosomal components, and PML, suggesting that these proteins are normally degraded by proteasomes. In contrast, concentrations of centromeric proteins CENP-B and -C and nucleolar proteins remained constant during inhibition of proteasomes. Quantification of fluorescence intensities corroborated that nuclear proteins which colocalize with proteasomes are degraded by proteasome-dependent proteolysis within the nucleoplasm. These data provide evidence that the proteasome proteolytic pathway is involved in processing of nuclear components, and thus may play an important role in the regulation of nuclear structure and function.

Subcellular recruitment of fibrillarin to nucleoplasmic proteasomes: implications for processing of a nucleolar autoantigen

A prerequisite for proteins to interact in a cell is that they are present in the same intracellular compartment. Although it is generally accepted that proteasomes occur in both, the cytoplasm and the nucleus, research has been focusing on cytoplasmic protein breakdown and antigen processing, respectively. Thus, little is known on the functional organization of the proteasome in the nucleus. Here we report that within the nucleus 20S and 26S proteasomes occur throughout the nucleoplasm and partially colocalize with splicing factor-containing speckles. Because proteasomes are absent from the nucleolus, a recruitment system was used to analyze the molecular fate of nucleolar protein fibrillarin: Subtoxic concentrations of mercuric chloride (HgCl(2)) induce subcellular redistribution of fibrillarin and substantial colocalization (33%) with nucleoplasmic proteasomes in different cell lines and in primary cells isolated from mercury-treated mice. Accumulation of fibrillarin and fibrillarin-ubiquitin conjugates in lactacystin-treated cells suggests that proteasome-dependent processing of this autoantigen occurs upon mercury induction. The latter observation might constitute the cell biological basis of autoimmune responses that specifically target fibrillarin in mercury-mouse models and scleroderma.

The mechanism of inhibition of Ran-dependent nuclear transport by cellular ATP depletion

Rran-dependent nuclear transport requires a nuclear pool of RanGTP both for the assembly of export complexes and the disassembly of import complexes. Accordingly, in order for these processes to proceed, Ran-dependent nuclear import and export assays in vitro require the addition of GTP to produce RanGTP. Notably, no ATP requirement can be detected for these transport processes in vitro. But in vivo, when cells are depleted of ATP by the addition of sodium azide and 2-deoxyglucose to block ATP production by oxidative phosphorylation and glycolysis, respectively, Ran-dependent nuclear import and export are rapidly inhibited. This raised the question of whether there is an ATP requirement for these nuclear transport pathways in an intact cell that has remained undetected in vitro. Here we report that the free (but not total) GTP concentration rapidly drops to an undetectable level upon ATP depletion as does the availability of RanGTP. Our conclusion is that the inhibition of Ran-dependent nuclear transport observed upon ATP depletion in vivo results from a shortage of RanGTP rather than the inhibition of some ATP-dependent process.

A novel nucleolar G-protein conserved in eukaryotes

We describe here a novel, evolutionarily conserved set of predicted G-proteins. The founding member of this family, TbNOG1, was identified in a two-hybrid screen as a protein that interacts with NOPP44/46, a nucleolar phosphoprotein of Trypanosoma brucei. The biological relevance of the interaction was verified by co-localization and co-immunoprecipitation. TbNOG1 localized to the trypanosome nucleolus and interacted with domains of NOPP44/46 that are found in several other nucleolar proteins. Genes encoding proteins highly related to TbNOG1 are present in yeast and metazoa, and related G domains are found in bacteria. We show that NOG1 proteins in humans and Saccharomyces cerevisae are also nucleolar. The S. cerevisae NOG1 gene is essential for cell viability, and mutations in the predicted G motifs abrogate function. Together these data suggest that NOG1 may play an important role in nucleolar functions. The GTP-binding region of TbNOG1 is similar to those of Obg and DRG proteins, which, together with NOG, form a newly recognized family of G-proteins, herein named ODN. The ODN family differs significantly from other G-protein families, and shows several diagnostic sequence characteristics. All organisms appear to possess an ODN gene, pointing to the biological significance of this family of G-proteins.

Specific inhibition of rRNA transcription and dynamic relocation of fibrillarin induced by mercury

Current evidence suggests that the nucleolus is composed of different substructures that are dynamic and form in response to the requirement for new ribosome synthesis. Thus, agents that disrupt nucleolar organization may deregulate basic cellular events and eventually contribute to human disease. Here we report that environmentally relevant concentrations (5 microM) of inorganic mercury induce a redistribution of nucleolar protein fibrillarin from the nucleolus to the nucleoplasm in epithelial cell lines. Since treatment with transcription inhibitors led to a similar relocation of fibrillarin, the effects of mercury on transcription were studied by run-on transcription assays: mercuric ions specifically blocked synthesis of ribosomal RNA, whereas activity of RNA polymerase II remained unchanged and occurred throughout the nucleoplasm. Moreover, we show by double-labeling that inhibition of nucleolar transcription and redistribution of fibrillarin occur simultaneously, underlining that fibrillarin relocation is a consequence of the blockade of ribosomal RNA synthesis by mercury. We also detected redistribution of fibrillarin in vivo, e.g., in splenic cells of mice chronically exposed to HgCl(2). Thus, implications of this alteration of nuclear structure and function for mercury-induced autoimmunity are discussed.

The N-terminal half of NPM dissociates from nucleoli of HeLa cells after anticancer drug treatments

NPM (nucleophosmin/B23) is a nucleolar phosphoprotein abundant in tumor cells. It dissociates from nucleoli of cells after treatments with various anticancer drugs. To determine the domain of NPM responsible for nucleolar binding, the N- and C-terminal halves of NPM were fused to GFP (green fluorescent protein) and introduced into HeLa cells. The N-terminal half (aa 1-150) of NPM (GFP-NPM(N)) was found localized in the nucleoli. A stable transformant of GFP-NPM(N) in HeLa cells was prepared and tested for association to nucleoli after anticancer drug treatments. GFP-NPM(N) dissociates from nucleoli after treatments with daunomycin, actinomycin D, camptothecin, and toyocamycin. The dissociation is time- and dose-dependent, and correlates with the cytotoxicity induced by the drugs. These results indicate that a stable transformant of GFP-NPM(N) in HeLa cells may be useful for the screening of anticancer drugs.

A study of correlation between NPM-translocation and apoptosis in cells induced by daunomycin

Human leukemia K562 and HeLa cells were treated with daunomycin (DA) for 1-4 hr. With the indirect immunofluorescence technique, we observed that the nucleolar protein nucleophosmin/B23 (NPM) shifted its location from the nucleolus to the nucleoplasm (NPM-translocation). The degree of NPM-translocation was determined by the relative immunofluorescent intensity in the nucleoli vs the nucleoplasm (defined as localization index, LI). We found that NPM-translocation, as determined by the decrease of LI, correlates with cytotoxicity. The degrees of NPM-translocation, chromatin condensation, and DNA fragmentation in HeLa cells were determined after treatment with 0.1, 0.5 and 1 microg/mL DA for 1 hr. We found that NPM-translocation (LI < 2.5) was observed in cells during the treatment with 0.5 and 1 but not with 0.1 microg/mL DA. Also, cells treated with 1 microg/mL remained in an NPM-translocated state for a longer time (5-6 hr) than those cells treated with 0.5 microg/mL (1-2 hr). Cells treated with 0.5 and 1 microg/mL DA showed increased levels of chromatin condensation beginning at 5 hr after the drug treatment. The number of cells with condensed chromatin increased with both time and drug concentration. No cells with condensed chromatin were observed in samples treated with 0.1 microg/mL DA, which also showed no significant NPM-translocation. Similar results were observed for induction of DNA fragmentation. We found that the drug concentration required for induction of DNA fragmentation and chromatin condensation coincided with the drug concentration required for NPM-translocation. Taken together, these results indicate that NPM-translocation correlates with apoptosis induced by daunomycin.

Modulation of the in situ activity of tissue transglutaminase by calcium and GTP

Tissue transglutaminase (tTG) is a calcium-dependent enzyme that catalyzes the posttranslational modification of proteins by transamidation of specific polypeptide-bound glutamine residues. Previous in vitro studies have demonstrated that the transamidating activity of tTG requires calcium and is inhibited by GTP. To investigate the endogenous regulation of tTG, a quantitative in situ transglutaminase (TG) activity assay was developed. Treatment of human neuroblastoma SH-SY5Y cells with retinoic acid (RA) resulted in a significant increase in tTG levels and in vitro TG activity. In contrast, basal in situ TG activity did not increase concurrently with RA-induced increased tTG levels. However, stimulation of cells with the calcium-mobilizing drug maitotoxin (MTX) resulted in increases in in situ TG activity that correlated (r2 = 0.76) with increased tTG levels. To examine the effects of GTP on in situ TG activity, tiazofurin, a drug that selectively decreases GTP levels, was used. Depletion of GTP resulted in a significant increase in in situ TG activity; however, treatment of SH-SY5Y cells with a combination of MTX and tiazofurin resulted in significantly less in situ TG activity compared with treatment with MTX alone. This raised the possibility of calcium-dependent proteolysis due to the effects of tiazofurin, because in vitro GTP protects tTG against proteolysis by trypsin. Studies with a selective membrane permeable calpain inhibitor indicated that tTG is likely to be an endogenous substrate of calpain, and that depletion of GTP increases tTG degradation after elevation of intracellular calcium levels. TG activity was also increased in response to activation of muscarinic cholinergic receptors, which increases intracellular calcium through inositol 1,4,5-trisphosphate generation. The results of these experiments demonstrate that selective changes in calcium and GTP regulate the activity and levels of tTG in situ.

Quantitation of the nucleophosmin/B23-translocation using imaging analysis

We have previously detected by immunofluorescent assay that the cellular localization of nucleophosmin/B23 (NPM) shifts from the nucleolus to the nucleoplasm (NPM-translocation) after exposure of cells to multiple agents. In order to improve the quantification of the NPM-translocation, we have developed a digital imaging technique. Human Lo leukemia cells, MCF-7 breast carcinoma cells, and fresh human leukemia cells were exposed to anthracyclines or actinomycin D for 4 h. The degree of NPM-translocation was determined and presented as the localization index (LI). Control cells had a LI of about 10, which indicates that the majority of NPM was localized in nucleoli. The LI for drug-treated cells decreased in a dosage- and time-dependent manner. The effect of two classes of anthracycline (daunomycin and aclacinomycin A) and different types of intercalators (daunomycin and actinomycin D) had additive effects on induction of NPM-translocation. The imaging procedure was easily applied to fresh leukemia cells, thus providing useful information regarding drug effects on cancer cells.

Intracellular trafficking of the human immunodeficiency virus type 1 Rev protein: involvement of continued rRNA synthesis in nuclear retention

We have explored the mechanism directing the intracellular trafficking and nucleolar accumulation of the human immunodeficiency virus type 1 (HIV-1) Rev protein. Treatment of Rev-expressing cells with mycophenolic acid, an inhibitor of inosine monophosphate dehydrogenase, resulted in a redistribution of Rev from the nucleoli to the nucleoplasm and cytoplasm. In contrast, a Rev effector domain mutant was retained in the nucleus, indicating the involvement of this domain in the protein's nuclear retention/nucleocytoplasmic transport. Identical results were obtained by inhibiting transcription using actinomycin D or 5,6-dichlorobenzimidazole riboside. All three drugs were found to inhibit biosynthetic labeling of ribosomal RNA and to disrupt nucleolar morphology, suggesting a correlation between nucleolar/nuclear retention of Rev, continued ribosomal RNA synthesis, and intact nucleolar architecture. Results of binding/immunofluorescence assays using isolated, permeabilized nuclei and extracts of cells expressing Rev demonstrated that the protein is able to bind to nucleoli in vitro, in the absence of active cellular processes or eukaryotic posttranslational modifications. Rev derived from actinomycin D-treated cells showed equivalent binding, indicating that the inhibitor did not directly interfere with the ability of the protein to interact with nucleolar structures. Rev's interaction with nucleoli was directed by the protein's arginine-rich RNA-binding/nucleolar localization domain, and was abrogated by pretreatment of the nuclei with RNaseA, indicating a requirement for RNA, probably ribosomal RNA.

A novel intracellular antigen in HL-60 cells that changes in molecular weight after granulocytic and monocytic differentiation

The present study reports the identification and partial characterization of a novel antigen with M(r) 100,000 by a monoclonal antibody (D29A8) that was obtained by immunizing BALB/c mice with nuclei of HL-60 cells. D29A8 detected mainly a nucleolar macromolecule with M(r) 100,000 (p100). On the other hand, when HL-60 cells were induced to differentiate either into a granulocytic or monocytic pathway, the antibody detected mainly a cytoplasmic macromolecule with M(r) 95,000 (p95). Since two subtypes of the antigen (p100 and p95) appear to be present in the same cells that differ in the stage of cell differentiation, the antigen may play an important role in cellular differentiation.

GTP gamma S restores nucleophosmin (NPM) localization to nucleoli of GTP-depleted HeLa cells

Previous studies showed that localization of nucleophosmin/B23 (NPM) to nucleoli requires adequate cellular GTP levels (Finch et al., J Biol Chem 268, 5823-5827, 1993). In order to study whether hydrolysis of GTP plays a role in NPM localization, we introduced a nonhydrolyzable GTP analog into HeLa cells. Cells were first depleted of GTP with the IMP dehydrogenase inhibitor, mycophenolic acid (MA), to induce translocation of NPM from the nucleoli to the nucleoplasm. Non-hydrolyzable GTP analogs were then introduced into cells by electroporation. We found that introduction of the non-hydrolyzable analog, GTP gamma S, was effective in restoring NPM localization to nucleoli. Cells incubated in medium containing G-nucleotides without electroporation showed no effect. To reduce the possibility that cells use guanine from degraded nucleotide to supplement GTP pools via salvage pathways, experiments were also performed in the presence of (6-mercaptopurine) 6MP, a competitive inhibitor of the salvage enzyme, HGPRT (hypoxanthine guanine phosphoribosyl transferase), in addition to MA. Under these conditions, introduction of GTP gamma S still effectively restored the localization of NPM into nucleoli. This study demonstrates that electroporation can be used effectively to introduce nucleotides into cultured cells without excessive loss of viability. Our results also indicate that the GTP dependent localization of NPM to the nucleoli may not require GTP hydrolysis.

Nucleophosmin/B23 (NPM) oligomer is a major and stable entity in HeLa cells

HeLa cell extract was separated by 7% polyacrylamide gel electrophoresis without SDS and subsequently stained with anti-nucleophosmin/B23 (NPM) antibody in a Western blot analysis. Two immunobands were obtained. The major band with a slower electromobility (RF = 0.23) is the NPM oligomer, while the fast-moving minor band is the monomer (RF = 0.56). The oligomer constitutes about 95% of total NPM. The oligomer sedimented faster (10 s) than the monomer in sucrose density gradient centrifugation. Three oligomer bands were identified. NPM oligomer is not affected by treatments with DNase. RNase, 10 mM EDTA, 1 M NaCl, and lyophilization. However, 3 M urea causes reversible dissociation of NPM oligomer into monomer. The level of NPM oligomer remains unchanged in HeLa cells after treatment with the cytotoxic agents, actinomycin D, toyocamycin and camptothecin. These results indicate that NPM oligomer is the major and stable NPM entity in HeLa cells.

Reduced numatrin/B23/nucleophosmin labeling in apoptotic Jurkat T-lymphoblasts

Jurkat T-lymphoblasts were induced to undergo apoptosis by treatment with either EGTA (5 mM/24 h) or a high concentration of lovastatin (100 microM/48 h) to identify proteins that exhibited coordinate regulation between the two treatments and thus provide candidate proteins in the common apoptotic induction pathway. A pure population of apoptotic cells, as determined by morphology, "DNA laddering," and flow cytometry, was obtained by Percoll density gradient centrifugation. Cells of increased buoyant density were clearly apoptotic by all criteria. Following this gradient centrifugation, the cells were labeled with [35S]methionine/cysteine, and lysates were separated by two-dimensional polyacrylamide gel electrophoresis. Surprisingly, the two-dimensional polyacrylamide gel electrophoresis patterns generated from the apoptotic cells did not differ dramatically from that of control cells. Thus, apoptotic Jurkat cells are able to synthesize new proteins and do not exhibit extensive proteolysis. Subsequent quantitative analysis revealed that only five proteins exhibited decreases in turnover that were common to the two treatments. No increases in protein turnover were able to be confirmed across the replicate experiments. One of the proteins that showed decreased labeling by both apoptotic inductions was an abundant nuclear protein with a pI of 5.1 and M(r) 40,000. This protein was identified as numatrin/B23/nucleophosmin (NPM) based on internal amino acid sequence, and this identity was confirmed by immunoblotting and mass spectrometry. NPM is implicated in a range of diverse cellular functions, but its role in apoptosis is unclear.