Function of nucleophosmin/B23, a nucleolar acidic protein, as a histone chaperone

1,2,3,4-Diepoxybutane-induced DNA-protein cross-linking in human fibrosarcoma (HT1080) cells

1,2,3,4-Diepoxybutane (DEB) is the key carcinogenic metabolite of 1,3-butadiene (BD), an important industrial and environmental chemical present in urban air and in cigarette smoke. DEB is a genotoxic bis-electrophile capable of cross-linking cellular biomolecules to form DNA-DNA and DNA-protein cross-links (DPCs). In the present work, mass spectrometry-based proteomics was employed to characterize DEB-mediated DNA-protein cross-linking in human fibrosarcoma (HT1080) cells. Over 150 proteins including histones, high mobility group proteins, transcription factors, splicing factors, and tubulins were found among those covalently cross-linked to chromosomal DNA in the presence of DEB. A large portion of the cross-linked proteins are known factors involved in DNA binding, transcriptional regulation, cell signaling, DNA repair, and DNA damage response. HPLC-ESI(+)-MS/MS analysis of total proteolytic digests revealed the presence of 1-(S-cysteinyl)-4-(guan-7-yl)-2,3-butanediol conjugates, confirming that DEB forms DPCs between cysteine thiols within proteins and the N-7 guanine positions within DNA. However, relatively high concentrations of DEB were required to achieve significant DPC formation, indicating that it is a poor cross-linking agent as compared to antitumor nitrogen mustards and platinum compounds.

Nucleophosmin mutations in acute myeloid leukemia: a tale of protein unfolding and mislocalization

Nucleophosmin (NPM1) is an abundant, ubiquitously expressed protein mainly localized at nucleoli but continuously shuttling between nucleus and cytoplasm. NPM1 plays a role in several cellular functions, including ribosome biogenesis and export, centrosome duplication, chromatin remodeling, DNA repair, and response to stress stimuli. Much of the interest in this protein arises from its relevance in human malignancies. NPM1 is frequently overexpressed in solid tumors and is the target of several chromosomal translocations in hematologic neoplasms. Notably, NPM1 has been characterized as the most frequently mutated gene in acute myeloid leukemia (AML). Mutations alter the C-terminal DNA-binding domain of the protein and result in its aberrant nuclear export and stable cytosolic localization. In this review, we focus on the leukemia-associated NPM1 C-terminal domain and describe its structure, function, and the effect exerted by leukemic mutations. Finally, we discuss the possibility to target NPM1 for the treatment of cancer and, in particular, of AML patients with mutated NPM1 gene.

Functional studies of MP62 during male chromatin decondensation in sea urchins

In amphibians, sperm histone transition post-fertilization during male pronucleus formation is commanded by histone chaperone Nucleoplasmin (NPM). Here, we report the first studies to analyze the participation of a Nucleoplasmin-like protein on male chromatin remodeling in sea urchins. In this report, we present the molecular characterization of a nucleoplasmin-like protein that is present in non fertilized eggs and early zygotes in sea urchin specie Tetrapygus niger. This protein, named MP62 can interact with sperm histones in vitro. By male chromatin decondensation assays and immunodepletion experiments in vitro, we have demonstrated that this protein is responsible for sperm nucleosome disorganization. Furthermore, as amphibian nucleoplasmin MP62 is phosphorylated in vivo immediately post-fertilization and this phosphorylation is dependent on CDK-cyclin activities found after fertilization. As we shown, olomoucine and roscovitine inhibits male nucleosome decondensation, sperm histone replacement in vitro and MP62 phosphorylation in vivo. This is the first report of a nucleoplasmin-like activity in sea urchins participating during male pronucleus formation post-fecundation.

Vertebrate Spt2 is a novel nucleolar histone chaperone that assists in ribosomal DNA transcription

In eukaryotes, transcription occurs in the chromatin context with the assistance of histone-binding proteins, such as chromatin/nucleosome remodeling factors and histone chaperones. However, it is unclear how each remodeling factor or histone chaperone functions in transcription. Here, we identify a novel histone-binding protein, Spt2, in higher eukaryotes. Recombinant human Spt2 binds to histones and DNA, and promotes nucleosome assembly in vitro. Spt2 accumulates in nucleoli and interacts with RNA polymerase I in chicken DT40 cells, suggesting its involvement in ribosomal RNA transcription. Consistently, Spt2-deficient chicken DT40 cells are sensitive to RNA polymerase I inhibitors and exhibit decreased transcription activity, as shown by a transcription run-on assay. Domain analyses of Spt2 revealed that the C-terminal region, containing the region homologous to yeast Spt2, is responsible for histone binding, while the central region is essential for nucleolar localization and DNA binding. Based on these results, we conclude that vertebrate Spt2 is a novel histone chaperone with a separate DNA-binding domain that facilitates ribosomal DNA transcription through chromatin remodeling during transcription.

Chromatin organization, epigenetics and differentiation: an evolutionary perspective

Genome packaging is a universal phenomenon from prokaryotes to higher mammals. Genomic constituents and forces have however, travelled a long evolutionary route. Both DNA and protein elements constitute the genome and also aid in its dynamicity. With the evolution of organisms, these have experienced several structural and functional changes. These evolutionary changes were made to meet the challenging scenario of evolving organisms. This review discusses in detail the evolutionary perspective and functionality gain in the phenomena of genome organization and epigenetics.

The nuclear chaperone nucleophosmin escorts an Epstein-Barr Virus nuclear antigen to establish transcriptional cascades for latent infection in human B cells

Epstein-Barr Virus (EBV) is an oncogenic γ-herpesvirus that capably establishes both latent and lytic modes of infection in host cells and causes malignant diseases in humans. Nuclear antigen 2 (EBNA2)-mediated transcription of both cellular and viral genes is essential for the establishment and maintenance of the EBV latency program in B lymphocytes. Here, we employed a protein affinity pull-down and LC-MS/MS analysis to identify nucleophosmin (NPM1) as one of the cellular proteins bound to EBNA2. Additionally, the specific domains that are responsible for protein-protein interactions were characterized as EBNA2 residues 300 to 360 and the oligomerization domain (OD) of NPM1. As in c-MYC, dramatic NPM1 expression was induced in EBV positively infected B cells after three days of viral infection, and both EBNA2 and EBNALP were implicated in the transactivation of the NPM1 promoter. Depletion of NPM1 with the lentivirus-expressed short-hairpin RNAs (shRNAs) effectively abrogated EBNA2-dependent transcription and transformation outgrowth of lymphoblastoid cells. Notably, the ATP-bound state of NPM1 was required to induce assembly of a protein complex containing EBNA2, RBP-Jκ, and NPM1 by stabilizing the interaction of EBNA2 with RBP-Jκ. In a NPM1-knockdown cell line, we demonstrated that an EBNA2-mediated transcription defect was fully restored by the ectopic expression of NPM1. Our findings highlight the essential role of NPM1 in chaperoning EBNA2 onto the latency-associated membrane protein 1 (LMP1) promoters, which is coordinated with the subsequent activation of transcriptional cascades through RBP-Jκ during EBV infection. These data advance our understanding of EBV pathology and further imply that NPM1 can be exploited as a therapeutic target for EBV-associated diseases.

Histone displacement during nucleotide excision repair

Nucleotide excision repair (NER) is an important DNA repair mechanism required for cellular resistance against UV light and toxic chemicals such as those found in tobacco smoke. In living cells, NER efficiently detects and removes DNA lesions within the large nuclear macromolecular complex called chromatin. The condensed nature of chromatin inhibits many DNA metabolizing activities, including NER. In order to promote efficient repair, detection of a lesion not only has to activate the NER pathway but also chromatin remodeling. In general, such remodeling is thought on the one hand to precede NER, thus allowing repair proteins to efficiently access DNA. On the other hand, after completion of the repair, the chromatin must be returned to its previous undamaged state. Chromatin remodeling can refer to three separate but interconnected processes, histone post-translational modifications, insertion of histone variants and histone displacement (including nucleosome sliding). Here we review current knowledge, and speculate about current unknowns, regarding those chromatin remodeling activities that physically displace histones before, during and after NER.

Vertebrate nucleoplasmin and NASP: egg histone storage proteins with multiple chaperone activities

Recent reviews have focused on the structure and function of histone chaperones involved in different aspects of somatic cell chromatin metabolism. One of the most dramatic chromatin remodeling processes takes place immediately after fertilization and is mediated by egg histone storage chaperones. These include members of the nucleoplasmin (NPM2/NPM3), which are preferentially associated with histones H2A-H2B in the egg and the nuclear autoantigenic sperm protein (NASP) families. Interestingly, in addition to binding and providing storage to H3/H4 in the egg and in somatic cells, NASP has been shown to be a unique genuine chaperone for histone H1. This review revolves around the structural and functional roles of these two families of chaperones whose activity is modulated by their own post-translational modifications (PTMs), particularly phosphorylation. Beyond their important role in the remodeling of paternal chromatin in the early stages of embryogenesis, NPM and NASP members can interact with a plethora of proteins in addition to histones in somatic cells and play a critical role in processes of functional cell alteration, such as in cancer. Despite their common presence in the egg, these two histone chaperones appear to be evolutionarily unrelated. In contrast to members of the NPM family, which share a common monophyletic evolutionary origin, the different types of NASP appear to have evolved recurrently within different taxa.

Epigeneitc silencing of ribosomal RNA genes by Mybbp1a

BACKGROUND

Transcription of the ribosomal RNA gene repeats by Pol I occurs in the nucleolus and is a fundamental step in ribosome biogenesis and protein translation. Due to tight coordination between ribosome biogenesis and cell proliferation, transcription of rRNA and stable maintenance of rDNA clusters are thought to be under intricate control by intercalated mechanisms, particularly at the epigenetic level.

METHODS AND RESULTS

Here we identify the nucleolar protein Myb-binding protein 1a (Mybbp1a) as a novel negative regulator of rRNA expression. Suppression of rDNA transcription by Mybbp1a was linked to promoter regulation as illustrated by its binding to the chromatin around the hypermethylated, inactive rDNA gene promoters. Our data further showed that downregulation of Mybbp1a abrogated the local DNA methylation levels and histone marks associated with gene silencing, and altered the promoter occupancy of various factors such UBF and HDACs, consequently leading to elevated rRNA expression. Mechanistically, we propose that Mybbp1a maintains rDNA repeats in a silenced state while in association with the negative epigenetic modifiers HDAC1/2.

CONCLUSIONS

Results from our present work reveal a previously unrecognized co-repressor role of Mybbp1a in rRNA expression. They are further consistent with the scenario that Mybbp1a is an integral constituent of the rDNA epigenetic regulation that underlies the balanced state of rDNA clusters.

Nucleophosmin (NPM1/B23) interacts with activating transcription factor 5 (ATF5) protein and promotes proteasome- and caspase-dependent ATF5 degradation in hepatocellular carcinoma cells

Nucleophosmin (NPM1/B23) and the activating transcription factor 5 (ATF5) are both known to subject to cell type-dependent regulation. NPM1 is expressed weakly in hepatocytes and highly expressed in hepatocellular carcinomas (HCC) with a clear correlation between enhanced NPM1 expression and increased tumor grading and poor prognosis, whereas in contrast, ATF5 is expressed abundantly in hepatocytes and down-regulated in HCC. Re-expression of ATF5 in HCC inhibits cell proliferation. We report here that using an unbiased approach, tandem affinity purification (TAP) followed with mass spectrometry (MS), we identified NPM1 as a novel ATF5-interacting protein. Unlike many other NPM1-interacting proteins that interact with the N-terminal oligomerization domain of NPM1, ATF5 binds via its basic leucine zipper to the C-terminal region of NPM1 where its nucleolar localization signal is located. NPM1 association with ATF5, whose staining patterns partially overlap in the nucleoli, promotes ATF5 protein degradation through proteasome-dependent and caspase-dependent pathways. NPM1-c, a mutant NPM1 that is defective in nucleolar localization, failed to stimulate ATF5 polyubiquitination and was unable to down-regulate ATF5. NPM1 interaction with ATF5 displaces HSP70, a known ATF5-interacting protein, from ATF5 protein complexes and antagonizes its role in stabilization of ATF5 protein. NPM1-promoted ATF5 down-regulation diminished ATF5-mediated repression of cAMP-responsive element-dependent gene transcription and abrogates ATF5-induced G(2)/M cell cycle blockade and inhibition of cell proliferation in HCC cells. Our study establishes a mechanistic link between elevated NPM1 expression and depressed ATF5 in HCC and suggests that regulation of ATF5 by NPM1 plays an important role in the proliferation and survival of HCC.

Function of homo- and hetero-oligomers of human nucleoplasmin/nucleophosmin family proteins NPM1, NPM2 and NPM3 during sperm chromatin remodeling

Sperm chromatin remodeling after oocyte entry is the essential step that initiates embryogenesis. This reaction involves the removal of sperm-specific basic proteins and chromatin assembly with histones. In mammals, three nucleoplasmin/nucleophosmin (NPM) family proteins-NPM1, NPM2 and NPM3-expressed in oocytes are presumed to cooperatively regulate sperm chromatin remodeling. We characterized the sperm chromatin decondensation and nucleosome assembly activities of three human NPM proteins. NPM1 and NPM2 mediated nucleosome assembly independently of other NPM proteins, whereas the function of NPM3 was largely dependent on formation of a complex with NPM1. Maximal sperm chromatin remodeling activity of NPM2 required the inhibition of its non-specific nucleic acid-binding activity by phosphorylation. Furthermore, the oligomer formation with NPM1 elicited NPM3 nucleosome assembly and sperm chromatin decondensation activity. NPM3 also suppressed the RNA-binding activity of NPM1, which enhanced the nucleoplasm-nucleolus shuttling of NPM1 in somatic cell nuclei. Our results proposed a novel mechanism whereby three NPM proteins cooperatively regulate chromatin disassembly and assembly in the early embryo and in somatic cells.

B23/nucleophosmin is involved in regulation of adenovirus chromatin structure at late infection stages, but not in virus replication and transcription

B23/nucleophosmin has been identified in vitro as a stimulatory factor for replication of adenovirus DNA complexed with viral basic core proteins. In the present study, the in vivo function of B23 in the adenovirus life cycle was studied. It was found that both the expression of a decoy mutant derived from adenovirus core protein V that tightly associates with B23 and small interfering RNA-mediated depletion of B23 impeded the production of progeny virions. However, B23 depletion did not significantly affect the replication and transcription of the virus genome. Chromatin immunoprecipitation analyses revealed that B23 depletion significantly increased the association of viral DNA with viral core proteins and cellular histones. These results suggest that B23 is involved in the regulation of association and/or dissociation of core proteins and cellular histones with the virus genome. In addition, these results suggest that proper viral chromatin assembly, regulated in part by B23, is crucial for the maturation of infectious virus particles.

Fine-tuning nucleophosmin in macrophage differentiation and activation

M-CSF-driven differentiation of peripheral blood monocytes is one of the sources of tissue macrophages. In humans and mice, the differentiation process involves the activation of caspases that cleave a limited number of proteins. One of these proteins is nucleophosmin (NPM1), a multifunctional and ubiquitous protein. Here, we show that caspases activated in monocytes exposed to M-CSF cleave NPM1 at D213 to generate a 30-kDa N-terminal fragment. The protein is further cleaved into a 20-kDa fragment, which involves cathepsin B. NPM1 fragments contribute to the limited motility, migration, and phagocytosis capabilities of resting macrophages. Their activation with lipopolysaccharides inhibits proteolytic processes and restores expression of the full-length protein that negatively regulates the transcription of genes encoding inflammatory cytokines (eg, NPM1 is recruited with NF-κB on the MCP1 gene promoter to decrease its transcription). In mice with heterozygous npm gene deletion, cytokine production in response to lipopolysaccharides, including CXCL1 (KC), MCP1, and MIP2, is dramatically enhanced. These results indicate a dual function of NPM1 in M-CSF-differentiated macrophages. Proteolysis of the protein participates in the establishment of a mature macrophage phenotype. In response to inflammatory stimuli, the full-length protein negatively regulates inflammatory cytokine production.

Crystal structure and function of human nucleoplasmin (npm2): a histone chaperone in oocytes and embryos

Human Npm2 is an ortholog of Xenopus nucleoplasmin (Np), a chaperone that binds histones. We have determined the crystal structure of a truncated Npm2-core at 1.9 Å resolution and show that the N-terminal domains of Npm2 and Np form similar pentamers. This allowed us to model an Npm2 decamer which may be formed by hydrogen bonds between quasi-conserved residues in the interface between two pentamers. Interestingly, the Npm2 pentamer lacks a prototypical A1-acidic tract in each of its subunits. This feature may be responsible for the inability of Npm2-core to bind histones. However, Npm2 with a large acidic tract in its C-terminal tail (Npm2-A2) is able to bind histones and form large complexes. Fluorescence resonance energy transfer experiments and biochemical analysis of loop mutations support the premise that nucleoplasmins form decamers when they bind H2A-H2B dimers and H3-H4 tetramers simultaneously. In the absence of histone tetramers, these chaperones bind H2A-H2B dimers with a single pentamer forming the central hub. When taken together, our data provide insights into the mechanism of histone binding by nucleoplasmins.

HJURP is a CENP-A chromatin assembly factor sufficient to form a functional de novo kinetochore

Centromeres of higher eukaryotes are epigenetically marked by the centromere-specific CENP-A nucleosome. New CENP-A recruitment requires the CENP-A histone chaperone HJURP. In this paper, we show that a LacI (Lac repressor) fusion of HJURP drove the stable recruitment of CENP-A to a LacO (Lac operon) array at a noncentromeric locus. Ectopically targeted CENP-A chromatin at the LacO array was sufficient to direct the assembly of a functional centromere as indicated by the recruitment of the constitutive centromere-associated network proteins, the microtubule-binding protein NDC80, and the formation of stable kinetochore-microtubule attachments. An amino-terminal fragment of HJURP was able to assemble CENP-A nucleosomes in vitro, demonstrating that HJURP is a chromatin assembly factor. Furthermore, HJURP recruitment to endogenous centromeres required the Mis18 complex. Together, these data suggest that the role of the Mis18 complex in CENP-A deposition is to recruit HJURP and that the CENP-A nucleosome assembly activity of HJURP is responsible for centromeric chromatin assembly to maintain the epigenetic mark.

Acute myeloid leukemia with mutated nucleophosmin (NPM1): any hope for a targeted therapy?

Acute myeloid leukemia (AML) carrying nucleophosmin (NPM1) mutations displays distinct molecular and clinical-pathological features that led to its inclusion as provisional entity in 2008 WHO classification of myeloid neoplasms. Since NPM1 mutations behave as a founder genetic lesion in AML, they could be an attractive target for therapeutic intervention. Here, we discuss the potential for developing targeted therapies for NPM1-mutated AML with focus on: (i) interfering with the abnormal traffic of the NPM1 leukemic mutant, i.e., its cytoplasmic dislocation; (ii) disrupting the nucleolar structure/function by interfering with residual wild-type nucleophosmin and other nucleolar components acting as hub proteins; and (iii) evaluating the activity of epigenetic drugs (e.g., 5-azacytidine) or agents acting on differentiation and apoptosis. As quantitative assessment of NPM1 mutated transcript copies now provides the means to measure minimal residual disease, we also discuss the potential for intervening in NPM1-mutated AML before overt hematological relapse occurs (so-called pre-emptive therapy).

An extensive tumor array analysis supports tumor suppressive role for nucleophosmin in breast cancer

Nucleophosmin (NPM) is a multifunctional protein involved in a complex network of interactions. The role of NPM in oncogenesis is controversial. The NPM gene (NPM1) is mutated or rearranged in a number of hematological disorders, but such changes have not been detected in solid cancers. However, experiments with cultured NPM-null cells and with mice carrying a single inactivated NPM allele indicate a tumor suppressor function for NPM. To resolve the role of NPM in solid cancers, we examined its expression and localization in histologically normal breast tissue and a large array of human breast carcinoma samples (n = 1160), and also evaluated its association with clinicopathological variables and patient survival. The intensity and localization (nucleolar, nuclear, cytoplasmic) of NPM varied across clinical samples. No mutations explaining the differences were found, but the present findings indicate that expression levels of NPM affected its localization. Our study also revealed a novel granular staining pattern for NPM, which was an independent prognostic factor of poor prognosis. In addition, reduced levels of NPM protein were associated with poor prognosis. Furthermore, luminal epithelial cells of histologically normal breast displayed high levels of NPM and overexpression of NPM in the invasive MDA-MB-231 cells abrogated their growth in soft agar. These results support a tumor suppressive role for NPM in breast cancer.

The BRCA1 ubiquitin ligase and homologous recombination repair

Impairment of homologous recombination (HR), a vital process employed during repair of DNA double strand breaks and stalled DNA replication, provides a valuable opportunity for the cell to become transformed. Once transformed, the impairment turns to be a target for therapy as exemplified by the synthetic lethal strategy such as poly (ADP-ribose) polymerase (PARP) inhibitor for BRCA1/2-defective breast and ovarian cancer. Hence, improving mechanistic understanding of HR has emerged as an urgent issue to address due to the high clinical demand. Ubiquitin modification plays a central role in HR and more than a few E3 ubiquitin ligases have been implicated in the process. However, the significance of the activity of one such key E3 ligase, BRCA1, has not yet been clarified and remains as a major obstacle in the field. Here, we review recent advances in our understanding of BRCA1 function in HR and discuss possible roles of the activity.

The multifunctional protein nucleophosmin (NPM1) is a human linker histone H1 chaperone

Linker histone H1 plays an essential role in chromatin organization. Proper deposition of linker histone H1 as well as its removal is essential for chromatin dynamics and function. Linker histone chaperones perform this important task during chromatin assembly and other DNA-templated phenomena in the cell. Our in vitro data show that the multifunctional histone chaperone NPM1 interacts with linker histone H1 through its first acidic stretch (residues 120-132). Association of NPM1 with linker histone H1 was also observed in cells in culture. NPM1 exhibited remarkable linker histone H1 chaperone activity, as it was able to efficiently deposit histone H1 onto dinucleosomal templates. Overexpression of NPM1 reduced the histone H1 occupancy on the chromatinized template of HIV-1 LTR in TZM-bl cells, which led to enhanced Tat-mediated transactivation. These data identify NPM1 as an important member of the linker histone chaperone family in humans.

Nucleophosmin and its complex network: a possible therapeutic target in hematological diseases

Nucleophosmin (NPM, also known as B23, numatrin or NO38) is a ubiquitously expressed phosphoprotein belonging to the nucleoplasmin family of chaperones. NPM is mainly localized in the nucleolus where it exerts many of its functions, but a proportion of the protein continuously shuttles between the nucleus and the cytoplasm. A growing number of cellular proteins have been described as physical interactors of NPM, and consequently, NPM is thought to have a relevant role in diverse cellular functions, including ribosome biogenesis, centrosome duplication, DNA repair and response to stress. NPM has been implicated in the pathogenesis of several human malignancies and intriguingly, it has been described both as an activating oncogene and a tumor suppressor, depending on cell type and protein levels. In fact, increased NPM expression is associated with different types of solid tumors whereas an impairment of NPM function is characteristic of a subgroup of hematolologic malignancies. A large body of experimental evidence links the deregulation of specific NPM functions to cellular transformation, yet the molecular mechanisms through which NPM contributes to tumorigenesis remain elusive. In this review, we have summarized current knowledge concerning NPM functions, and attempted to interpret its multifaceted and sometimes apparently contradictory activities in the context of both normal cellular homeostasis and neoplastic transformation.

The Multifunctional Nucleolar Protein Nucleophosmin/NPM/B23 and the Nucleoplasmin Family of Proteins

The nucleophosmin (NPM)/nucleoplasmin family of nuclear chaperones has three members: NPM1, NPM2, and NPM3. Nuclear chaperones serve to ensure proper assembly of nucleosomes and proper formation of higher order structures of chromatin. In fact, this family of proteins has such diverse functions in cellular processes such as chromatin remodeling, ribosome biogenesis, genome stability, centrosome replication, cell cycle, transcriptional regulation, apoptosis, and tumor suppression. Of the members of this family, NPM1 is the most studied and is the main focus of this review. NPM2 and NPM3 are less well characterized, and are also discussed wherever appropriate. The structure–function relationship of NPM proteins has largely been worked out. Other than the many processes in which NPM1 takes part, the major interest comes from its involvement in human cancers, particularly acute myeloid leukemia (AML). Its significance stems from the fact that AML with mutated NPM1 accounts for ∼30% of all AML cases and usually has good prognosis. Its clinical importance also comes from its involvement in virus replication, particularly in the era of outbreaks of infectious diseases.

NPM1/B23: A Multifunctional Chaperone in Ribosome Biogenesis and Chromatin Remodeling

At a first glance, ribosome biogenesis and chromatin remodeling are quite different processes, but they share a common problem involving interactions between charged nucleic acids and small basic proteins that may result in unwanted intracellular aggregations. The multifunctional nuclear acidic chaperone NPM1 (B23/nucleophosmin) is active in several stages of ribosome biogenesis, chromatin remodeling, and mitosis as well as in DNA repair, replication and transcription. In addition, NPM1 plays an important role in the Myc-ARF-p53 pathway as well as in SUMO regulation. However, the relative importance of NPM1 in these processes remains unclear. Provided herein is an update on the expanding list of the diverse activities and interacting partners of NPM1. Mechanisms of NPM1 nuclear export functions of NPM1 in the nucleolus and at the mitotic spindle are discussed in relation to tumor development. It is argued that the suggested function of NPM1 as a histone chaperone could explain several, but not all, of the effects observed in cells following changes in NPM1 expression. A future challenge is to understand how NPM1 is activated, recruited, and controlled to carry out its functions.

Identification of FUSE-binding protein 1 as a regulatory mRNA-binding protein that represses nucleophosmin translation

Nucleophosmin (NPM/B23) is a multifunctional oncoprotein whose protein expression levels dictate cellular growth and proliferation rates. NPM is translationally responsive to hyperactive mammalian target of rapamycin (mTOR) signals, but the mechanism of this regulation is not understood. Using chimeric translational reporters, we found that the 3' untranslated region (UTR) of the NPM messenger (m)RNA is sufficient to mediate its translational modulation by mTOR signalling. We show that far upstream element (FUSE)-binding protein 1 (FBP1) interacts specifically with the 3' UTR of NPM to repress translation. Overexpression of FBP1 resulted in translational repression of NPM mRNAs, whereas depletion of FBP1 caused a dramatic increase in NPM translation and resulted in enhanced overall cell proliferation. Thus, we propose that FBP1 is a key regulator of cell growth and proliferation through its ability to selectively bind the NPM 3' UTR and repress NPM translation.

Regulation of nucleolar chromatin by B23/nucleophosmin jointly depends upon its RNA binding activity and transcription factor UBF

Histone chaperones regulate the density of incorporated histone proteins around DNA transcription sites and therefore constitute an important site-specific regulatory mechanism for the control of gene expression. At present, the targeting mechanism conferring this site specificity is unknown. We previously reported that the histone chaperone B23/nucleophosmin associates with rRNA chromatin (r-chromatin) to stimulate rRNA transcription. Here, we report on the mechanism for site-specific targeting of B23 to the r-chromatin. We observed that, during mitosis, B23 was released from chromatin upon inactivation of its RNA binding activity by cdc2 kinase-mediated phosphorylation. The phosphorylation status of B23 was also shown to strongly affect its chromatin binding activity. We further found that r-chromatin binding of B23 was a necessary condition for B23 histone chaperone activity in vivo. In addition, we found that depletion of upstream binding factor (UBF; an rRNA transcription factor) decreased the chromatin binding affinity of B23, which in turn led to an increase in histone density at the r-chromatin. These two major strands of evidence suggest a novel cell cycle-dependent mechanism for the site-specific regulation of histone density via joint RNA- and transcription factor-mediated recruitment of histone chaperones to specific chromosome loci.

The ratio of SRPK1/SRPK1a regulates erythroid differentiation in K562 leukaemic cells

SRPK1, the prototype of the serine/arginine family of kinases, has been implicated in the regulation of multiple cellular processes such as pre-mRNA splicing, chromatin structure, nuclear import and germ cell development. SRPK1a is a much less studied isoform of SRPK1 that contains an extended N-terminal domain and so far has only been detected in human testis. In the present study we show that SRPK1 is the predominant isoform in K562 cells, with the ratio of the two isoforms being critical in determining cell fate. Stable overexpression of SRPK1a induces erythroid differentiation of K562 cells. The induction of globin synthesis was accompanied by a marked decrease in proliferation and a significantly reduced clonogenic potential. Small interfering RNA-mediated down-regulation of SRPK1 in K562 cells results similarly in a decrease in proliferative capacity and induction of globin synthesis. A decreased SRPK1/SRPK1a ratio is also observed upon hemin/DMSO-induced differentiation of K562 cells as well as in normal human erythroid progenitor cells. Mass spectrometric analysis of SRPK1a-associated proteins identified multiple classes of RNA-binding proteins including RNA helicases, heterogeneous nuclear ribonucleoproteins, ribosomal proteins, and mRNA-associated proteins. Several of the SRPK1a-copurifying proteins have been previously identified in ribosomal and pre-ribosomal complexes, thereby suggesting that SRPK1a may play an important role in linking ribosomal assembly and/or function to erythroid differentiation in human leukaemic cells.

Protein B23/nucleophosmin/numatrin nuclear dynamics in relation to protein kinase CK2 and apoptotic activity in prostate cells

Protein B23/nucleophosmin/numatrin (B23) is a key nucleolar/nuclear matrix-associated protein required for cell growth-related functions, such as rRNA synthesis. Protein kinase CK2 (CK2) (formerly casein kinase 2, a protein Ser/Thr kinase signal that is involved in cell growth and cell death) mediates phosphorylation of B23, thereby influencing its functional activity. Here we have delineated the dynamics of B23 and its link to CK2 status in response to altered growth stimuli and induction of apoptosis in cultured prostate cells and in rat prostate cells in vivo. Our studies employing PC-3 and ALVA-41 prostate cancer cells demonstrated colocalization of CK2 and B23 in the nucleus. Further, CK2 and B23 underwent coordinate modulation in the nucleus related to their nucleocytoplasmic shuttling in response to induction of apoptotic activity in cells caused by downregulation of CK2 or by treatment with other apoptosis-inducing agents. These alterations in nuclear association of B23 occurred in the absence of a significant change in the level of cytoplasmic B23. Similar studies in the in vivo model of rat prostate epithelial cells subjected to androgen deprivation (that resulted in loss of nuclear CK2 and induction of apoptosis) demonstrated dynamic modulation of nuclear matrix-associated B23 without a significant change in its cytoplasmic level. These changes were reversed by androgen-mediated growth response in the prostate. Our results suggest that CK2-mediated phosphorylation of B23 is essential for its retention in the nucleus and that coordinated nuclear localization of B23 and CK2 is dynamically regulated in response to altered growth status in the cell.

Involvement of nucleophosmin/B23 in the cellular response to curcumin

Nucleophosmin (NPM/B23) is a nucleolar phosphoprotein involved in cellular response to many different stimuli. Herein, we studied the molecular mechanism of NPM/B23 induction by curcumin, a natural AP-1 inhibitor with antitumor properties. Exposure to 5-30 μM curcumin significantly and dose-dependently increased the level of NPM/B23 in non-transformed NIH 3T3 cells but not HeLa cells and F9 cells. Besides, the transformed F9 and HeLa cells are more sensitive to curcumin-induced cell death and growth inhibition than NIH 3T3 cells. Overexpression of c-Jun, but not c-Fos, decreased ∼40% of NPM/B23 and enhanced the sensitivity of NIH 3T3 cells to 30 μM curcumin. Furthermore, down-regulation of NPM/B23 by transfection with NPM/B23 antisense plasmid enhanced the sensitivity to curcumin-induced cell death and growth inhibition. These results indicated that NPM/B23 expression regulates cellular sensitivity to curcumin. Besides, NPM/B23 knockdown may facilitate as a novel strategy to promote the sensitivity of cancer cells to curcumin.

Acute myeloid leukemia with mutated nucleophosmin (NPM1): molecular, pathological, and clinical features

The NPM1 gene encodes for nucleophosmin, a nucleolus-located shuttling protein that is involved in multiple cell functions, including regulation of ribosome biogenesis, control of centrosome duplication and preservation of ARF tumor suppressor integrity. The NPM1 gene is specifically mutated in about 30% acute myeloid leukemia (AML) but not in other human neoplasms. Mutations cause crucial changes at the C-terminus of the NPM1 protein that are responsible for the aberrant nuclear export and accumulation of NPM1 mutants in the cytoplasm of leukemic cells. Diagnosis of AML with mutated NPM1 can be done using molecular techniques, immunohistochemistry (looking at cytoplasmic dislocation of nucleophosmin that is predictive of NPM1 mutations) and Western blotting with antibodies specifically directed against NPM1 mutants. Because of its distinctive molecular, pathological, immunophenotypic and prognostic features, AML with mutated NPM1 (synonym: NPMc+ AML) has been included, as a new provisional entity, in the 2008 World Health Organization (WHO) classification of myeloid neoplasms.

Nucleophosmin (NPM1) mutations in adult and childhood acute myeloid leukaemia: towards definition of a new leukaemia entity

Nucleophosmin (NPM) is a ubiquitously expressed chaperone protein that shuttles rapidly between the nucleus and cytoplasm, but predominantly resides in the nucleolus. It plays key roles in ribosome biogenesis, centrosome duplication, genomic stability, cell cycle progression and apoptosis. Somatic mutations in exon 12 of the NPM gene (NPM1) are the most frequent genetic abnormality in adult acute myeloid leukaemia (AML), found in approximately 35% of all cases and up to 60% of patients with normal karyotype (NK) AML. In children, NPM1 mutations are far less frequent, occurring in 8-10% of all AML cases, and in approximately 25% of those with a NK. NPM1 mutations lead to aberrant localization of the NPM protein into the cytoplasm, thus the designation, NPMc+ AML. NPMc+ AML is seen predominantly in patients with a NK and is essentially mutually exclusive of recurrent chromosomal translocations. Patients with NPM1 mutations are twice as likely as those who lack an NPM1 mutation to also have a FMS-like tyrosine kinase (FLT3) internal tandem duplication (ITD) mutation. NPMc+ AML is also characterized by a unique gene expression signature and microRNA signature. NPMc+ AML has important prognostic significance, as NPMc+ AML, in the absence of a coexisting FLT3-ITD mutation, is associated with a favourable outcome. NPM1 mutations have also shown great stability during disease evolution, and therefore represent a possible marker for minimal residual disease detection. Given its distinctive biologic and clinical features and its clear clinical relevance, NPMc+ AML is included as a provisional entity in the 2008 WHO classifications. There is still much to be learned about this genetic alteration, including its exact role in leukaemogenesis, how it interacts with other mutations and why it confers a more favourable prognosis. Further, it represents a potential therapeutic target warranting research aimed at identifying novel small molecules with activity in NPMc+ AML.

Modulatory effect of rRNA synthesis and ppUL83 nucleolar compartmentalization on human cytomegalovirus gene expression in vitro

The nucleolus is a nuclear domain involved in the biogenesis of ribosomes, as well as in many other important cellular regulatory activities, such as cell cycle control and mRNA processing. Many viruses, including herpesviruses, are known to exploit the nucleolar compartment during their replication cycle. In a previous study, we demonstrated the preferential targeting and accumulation of the human cytomegalovirus (HCMV) UL83 phosphoprotein (pp65) to the nucleolar compartment and, in particular, to the nucleolar matrix of lytically infected fibroblasts; such targeting was already evident at very early times after infection. Here we have investigated the possible effects of rRNA synthesis inhibition upon the development of HCMV lytic infection, by using either actinomycin D or cisplatin at low concentrations, that are known to selectively inhibit RNA polymerase I activity, whilst leaving RNA polymerase II function unaffected. Following the inhibition of rRNA synthesis by either of the agents used, we observed a significant redistribution of nucleolar proteins within the nucleoplasm and a simultaneous depletion of viral pp65 from the nucleolus; this effect was highly evident in both unextracted cells and in nuclear matrices in situ. Of particular interest, even a brief suppression of rRNA synthesis resulted in a very strong inhibition of the progression of HCMV infection, as was concluded from the absence of accumulation of HCMV major immediate‐early proteins within the nucleus of infected cells. These data suggest that a functional relationship might exist between rRNA synthesis, pp65 localization to the nucleolar matrix and the normal development of HCMV lytic infection. J. Cell. Biochem. 108: 415–423, 2009. © 2009 Wiley‐Liss, Inc.

Role of nucleophosmin in acute myeloid leukemia

Nucleophosmin (NPM) is a nucleolar phosphoprotein implicated in the regulation of multiple cellular functions, which possesses both oncogenic and tumor-suppressor properties. Mutations of the NPM1 gene leading to the expression of a cytoplasmic mutant protein, NPMc+, are the most frequent genetic abnormalities found in acute myeloid leukemias. Acute myeloid leukemias with mutated NPM1 have distinct characteristics, including a significant association with a normal karyotype, involvement of different hematopoietic lineages, a specific gene-expression profile and clinically, a better response to induction therapy and a favorable prognosis. NPMc+ maintains the capacity of wild-type NPM to interact with a variety of cellular proteins, and impairs their activity by delocalizing them to the cytoplasm. In this review we summarize recent discoveries concerning NPM function, and discuss their possible impact on the pathogenesis of acute myeloid leukemias with mutated NPM1.

A proteomics approach for the identification of nucleophosmin and heterogeneous nuclear ribonucleoprotein C1/C2 as chromatin-binding proteins in response to DNA double-strand breaks

Double-strand breaks (DSBs) of chromosomal DNA trigger the cellular response that activates the pathways for DNA repair and cell-cycle checkpoints, and sometimes the pathways leading to cell death if the damage is too severe to be tolerated. Evidence indicates that, upon generation of DNA DSBs, many nuclear proteins that are involved in DNA repair and checkpoints are recruited to chromatin around the DNA lesions. In the present study we used a proteomics approach to identify DNA-damage-induced chromatin-binding proteins in a systematic way. Two-dimensional gel analysis for protein extracts of chromatin from DNA-damage-induced and control HeLa cells identified four proteins as the candidates for DNA-damage-induced chromatin-binding proteins. MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS analysis identified these proteins to be NPM (nucleophosmin), hnRNP (heterogeneous nuclear ribonucleoprotein) C1, hnRNP C2 and 37-kDa laminin-receptor precursor, and the identity of these proteins was further confirmed by immunoblot analysis with specific antibodies. We then demonstrated with chromatin-binding assays that NPM and hnRNP C1/C2, the abundant nuclear proteins with pleiotropic functions, indeed bind to chromatin in a DNA-damage-dependent manner, implicating these proteins in DNA repair and/or damage response. Immunofluorescence experiments showed that NPM, normally present in the nucleoli, is mobilized into the nucleoplasm after DNA damage, and that neither NPM nor hnRNP C1/C2 is actively recruited to the sites of DNA breaks. These results suggest that NPM and hnRNP C1/C2 may function at the levels of the global context of chromatin, rather than by specifically targeting the broken DNA.

HJURP is a cell-cycle-dependent maintenance and deposition factor of CENP-A at centromeres

The histone H3 variant CenH3, called CENP-A in humans, is central in centromeric chromatin to ensure proper chromosome segregation. In the absence of an underlying DNA sequence, it is still unclear how CENP-A deposition at centromeres is determined. Here, we purified non-nucleosomal CENP-A complexes to identify direct CENP-A partners involved in such a mechanism and identified HJURP. HJURP was not detected in H3.1- or H3.3-containing complexes, indicating its specificity for CENP-A. HJURP centromeric localization is cell cycle regulated, and its transient appearance at the centromere coincides precisely with the proposed time window for new CENP-A deposition. Furthermore, HJURP downregulation leads to a major reduction in CENP-A at centromeres and impairs deposition of newly synthesized CENP-A, causing mitotic defects. We conclude that HJURP is a key factor for CENP-A deposition and maintenance at centromeres.

Nucleophosmin redistribution following heat shock: a role in heat-induced radiosensitization

Cellular survival from radiation-induced DNA damage requires access to sites of damage for the assembly of repair complexes and the subsequent repair, particularly the repair of DNA double strand breaks (DSB). Hyperthermia causes changes in protein-protein/DNA interactions in the nucleus that block access to sites of DNA damage. Studies presented here indicate that the nucleolar protein, nucleophosmin (NPM), redistributes from the nucleolus following hyperthermia, increases its association with DNA, and blocks access to DNA DSBs. Reduction of NPM significantly reduces heat-induced radiosensitization, but reduced NPM level does not alter radiation sensitivity per se. NPM knockdown reduces heat-induced inhibition of DNA DSB repair. Also, these results suggest that NPM associates with nuclear matrix attachment region DNA in heat-shocked cells.

Acetylated NPM1 localizes in the nucleoplasm and regulates transcriptional activation of genes implicated in oral cancer manifestation

Nucleophosmin (NPM1) is a multifunctional protein involved in the regulation of centrosome duplication, ribosome biogenesis, genomic stability, histone chaperone function, and transcription. Overexpression of NPM1 is associated with cancers of diverse histological origins. Here, we have found that p300-mediated acetylation of NPM1 modulates its subcellular localization and augments its oncogenic potential. Acetylated NPM1 is predominantly localized in the nucleoplasm, where it associates with transcriptionally active RNA polymerase II. Deacetylation of NPM1 is brought about by human SIRT1 and reduces its transcriptional activation potential. Remarkably, increased levels of acetylated NPM1 were found in grade II and III oral squamous cell carcinoma (OSCC) patient samples. Small interfering RNA (siRNA)-mediated knockdown of NPM1 in an OSCC cell line, followed by microarray analysis and chromatin immunoprecipitation experiments, revealed that some of the genes involved in oral cancer malignancy are regulated by NPM1 and have acetylated NPM1 localized at their promoters. Either suppression of p300 by siRNA or mutation of acetylatable lysine residues of NPM1 resulted in reduced occupancy of acetylated NPM1 on the target gene promoter concomitant with its decreased transcript levels. These observations suggest that acetylated NPM1 transcriptionally regulates genes involved in cell survival and proliferation during carcinogenesis.

Nucleophosmin protein expression level, but not threonine 198 phosphorylation, is essential in growth and proliferation

Nucleophosmin (NPM), an oligomeric phosphoprotein and nucleolar target of the ARF tumor suppressor, contributes to several critical cellular processes. Previous studies have shown that the human NPM's phosphorylation by cyclin E-cyclin-dependent kinase 2 (cdk2) on threonine (Thr) 199 regulates its translocation from the centrosome during cell cycle progression. Given our previous finding that ARF directly binds NPM, impeding its transit to the cytoplasm and arresting cells before S-phase entry, we hypothesized that ARF might also inhibit NPM phosphorylation. However, ARF induction did not impair phosphorylation of the cdk2 target residue in murine NPM, Thr198. Furthermore, phosphorylation of Thr198 occurred throughout the cell cycle and was concomitant with increases in overall NPM expression. To investigate the cell's presumed requirement for NPM-Thr198 phosphorylation in promoting the processes of growth and proliferation, we examined the effects of a non-phosphorylatable NPM mutant, T198A, in a clean cell system in which endogenous NPM had been removed by RNA interference. Here, we show that the T198A mutant is fully capable of executing NPM's described roles in nucleocytoplasmic shuttling, ribosome export and cell cycle progression. Moreover, the proliferative defects observed with stable NPM knockdown were restored by mutant NPM-T198A expression. Thus, we demonstrate that the reduction in NPM protein expression blocks cellular growth and proliferation, whereas phosphorylation of NPM-Thr198 is not essential for NPM's capacity to drive cell cycle progression and proliferation.

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.

FACT facilitates chromatin transcription by RNA polymerases I and III

Efficient transcription elongation from a chromatin template requires RNA polymerases (Pols) to negotiate nucleosomes. Our biochemical analyses demonstrate that RNA Pol I can transcribe through nucleosome templates and that this requires structural rearrangement of the nucleosomal core particle. The subunits of the histone chaperone FACT (facilitates chromatin transcription), SSRP1 and Spt16, co-purify and co-immunoprecipitate with mammalian Pol I complexes. In cells, SSRP1 is detectable at the rRNA gene repeats. Crucially, siRNA-mediated repression of FACT subunit expression in cells results in a significant reduction in 47S pre-rRNA levels, whereas synthesis of the first 40 nt of the rRNA is not affected, implying that FACT is important for Pol I transcription elongation through chromatin. FACT also associates with RNA Pol III complexes, is present at the chromatin of genes transcribed by Pol III and facilitates their transcription in cells. Our findings indicate that, beyond the established role in Pol II transcription, FACT has physiological functions in chromatin transcription by all three nuclear RNA Pols. Our data also imply that local chromatin dynamics influence transcription of the active rRNA genes by Pol I and of Pol III-transcribed genes.

Nucleophosmin is required for chromosome congression, proper mitotic spindle formation, and kinetochore-microtubule attachment in HeLa cells

Nucleophosmin (NPM) is an abundantly expressed multifunctional nucleolar phosphoprotein. Here we show that depletion of NPM by RNA interference causes defects in cell division, followed by an arrest of DNA synthesis due to activation of a p53-dependent checkpoint response in HeLa cells. Depletion of NPM leads to mitotic arrest due to spindle checkpoint activation. The mitotic cells arrested by NPM depletion have defects in chromosome congression, proper mitotic spindle and centrosome formation, as well as defects in kinetochore-microtubule attachments. Loss of NPM thus causes severe mitotic defects and delayed mitotic progression. These findings indicate that NPM is essential for mitotic progression and cell proliferation.

Chromatin: linking structure and function in the nucleolus

The nucleolus is an informative model structure for studying how chromatin-regulated transcription relates to nuclear organisation. In this review, we describe how chromatin controls nucleolar structure through both the modulation of rDNA activity by convergently-evolved remodelling complexes and by direct effects upon rDNA packaging. This packaging not only regulates transcription but may also be important for suppressing internal recombination between tandem rDNA repeats. The identification of nucleolar histone chaperones and novel chromatin proteins by mass spectrometry suggests that structure-specific chromatin components remain to be characterised and may regulate the nucleolus in novel ways. However, it also suggests that there is considerable overlap between nucleolar and non-nucleolar-chromatin components. We conclude that a fuller understanding of nucleolar chromatin will be essential for understanding how gene organisation is linked with nuclear architecture.

Nucleophosmin serves as a rate-limiting nuclear export chaperone for the Mammalian ribosome

Nucleophosmin (NPM) (B23) is an essential protein in mouse development and cell growth; however, it has been assigned numerous roles in very diverse cellular processes. Here, we present a unified mechanism for NPM's role in cell growth; NPM directs the nuclear export of both 40S and 60S ribosomal subunits. NPM interacts with rRNA and large and small ribosomal subunit proteins and also colocalizes with large and small ribosomal subunit proteins in the nucleolus, nucleus, and cytoplasm. The transduction of NPM shuttling-defective mutants or the loss of Npm1 inhibited the nuclear export of both the 40S and 60S ribosomal subunits, reduced the available pool of cytoplasmic polysomes, and diminished overall protein synthesis without affecting rRNA processing or ribosome assembly. While the inhibition of NPM shuttling can block cellular proliferation, the dramatic effects on ribosome export occur prior to cell cycle inhibition. Modest increases in NPM expression amplified the export of newly synthesized rRNAs, resulting in increased rates of protein synthesis and indicating that NPM is rate limiting in this pathway. These results support the idea that NPM-regulated ribosome export is a fundamental process in cell growth.

Host factors for replication and transcription of the influenza virus genome

For replication and transcription of the influenza virus genome of eight-segmented and negative-stranded RNAs, not only viral factors but also host-derived cellular factors (host factors) are required. This paper focuses on the identification and characterisation of the host factors involved in replication and transcription of the influenza virus genome, reviewing recent progresses in the related molecular mechanisms. Functional assay systems for screening of host factors using cell-free reconstitution systems and an yeast-based influenza virus replicon system are highlighted. We have summarised the property of host factors comprehensively and provided a clue for the perspective in the determination mechanism of host range and virulence and the development of a new strategy to control the influenza virus.

Nucleophosmin and its AML-associated mutant regulate c-Myc turnover through Fbw7 gamma

Mutations leading to aberrant cytoplasmic localization of nucleophosmin (NPM) are the most frequent genetic alteration in acute myelogenous leukemia (AML). NPM binds the Arf tumor suppressor and protects it from degradation. The AML-associated NPM mutant (NPMmut) also binds p19Arf but is unable to protect it from degradation, which suggests that inactivation of p19Arf contributes to leukemogenesis in AMLs. We report here that NPM regulates turnover of the c-Myc oncoprotein by acting on the F-box protein Fbw7γ, a component of the E3 ligase complex involved in the ubiquitination and proteasome degradation of c-Myc. NPM was required for nucleolar localization and stabilization of Fbw7γ. As a consequence, c-Myc was stabilized in cells lacking NPM. Expression of NPMmut also led to c-Myc stabilization because of its ability to interact with Fbw7γ and delocalize it to the cytoplasm, where it is degraded. Because Fbw7 induces degradation of other growth-promoting proteins, the NPM–Fbw7 interaction emerges as a central tumor suppressor mechanism in human cancer.

Structure and function of ribosomal RNA gene chromatin

Transcription of the major ribosomal RNAs by Pol I (RNA polymerase I) is a key determinant of ribosome biogenesis, driving cell growth and proliferation in eukaryotes. Hundreds of copies of rRNA genes are present in each cell, and there is evidence that the cellular control of Pol I transcription involves adjustments to the number of rRNA genes actively engaged in transcription, as well as to the rate of transcription from each active gene. Chromatin structure is inextricably linked to rRNA gene activity, and the present review highlights recent advances in this area.

Characterisation of the Bax-nucleophosmin interaction: the importance of the Bax C-terminus

The molecular chaperone nucleophosmin has been identified as a novel Bax binding protein with this interaction proposed to be a key event in the activation and translocation of Bax in mitochondrial dysfunction and apoptotic cell death. Using a proximity assay, we have quantitatively defined the high affinity and saturable interaction between Bax and nucleophosmin indicative of a competitive and specific mechanism. Binding of full length Bax to nucleophosmin was only observed after conformational change was induced using non-ionic detergents (e.g., NP-40). The Bax-nucleophosmin interaction was inhibited by a Bax C-terminal antibody (IC(50) = 1 nM) but minimally affected by antibodies directed against either the N-terminus or alpha-helices 4 and 5. Bcl-2 and p53 inhibited the interaction between full length activated Bax and nucleophosmin. The proximity assay based on the Bax-nucleophosmin interaction was robust and reproducible (Z' = 0.50) facilitating its use for screening a small chemical library. A low molecular weight non-peptide compound, 2-(5-methyl-2-phenyl-1,3-thiazol-4-yl)ethanohydrazide, partially inhibited the Bax-nucleophosmin interaction (IC(50) = 100 nM) and also attenuated UV-induced cell death of HEK293 cells. The present investigations demonstrate the importance of exposure of the C-terminus of Bax for its interaction with nucleophosmin. These protein-protein interaction assays provide a technical approach both for the study of Bax-interacting proteins and for the discovery of novel anti-apoptotic agents.

Interaction between Agrobacterium tumefaciens oncoprotein 6b and a tobacco nucleolar protein that is homologous to TNP1 encoded by a transposable element of Antirrhinum majus

When gene 6b on the T-DNA of Agrobacterium tumefaciens is transferred to plant cells, its expression causes plant hormone-independent division of cells in in vitro culture and abnormal cell growth, which induces various morphological defects in 6b-expressing transgenic Arabidopsis thaliana and Nicotiana tabacum plants. Protein 6b localizes to the nuclei, a requirement for the abnormal cell growth, and binds to a tobacco nuclear protein called NtSIP1 and histone H3. In addition, 6b has histone chaperone-like activity in vitro and affects the expression of various plant genes, including cell division-related genes and meristem-related class 1 KNOX homeobox genes, in transgenic Arabidopsis. Here, we report that 6b binds to a newly identified protein NtSIP2, whose amino acid sequence is predicted to be 30% identical and 51% similar to that of the TNP1 protein encoded by the transposon Tam1 of Antirrhinum majus. Immunolocalization analysis using anti-T7 antibodies showed nucleolar localization of most of the T7 epitope-tagged NtSIP2 proteins. A similar analysis with the T7-tagged 6b protein also showed subnucleolar as well as nuclear localization of the 6b protein. These results suggest the involvement of 6b along with NtSIP2 in certain molecular processes in the nucleolus as well as the nucleoplasm.

Identification and characterization of nucleoplasmin 3 as a histone-binding protein in embryonic stem cells

Embryonic stem (ES) cells are thought to have unique chromatin structures responsible for their capacity for self-renewal and pluripotency. To examine this possibility, we sought nuclear proteins in mouse ES cells that specifically bind to histones using a pull-down assay with synthetic peptides of histone H3 and H4 tail domain as baits. Nuclear proteins preferentially bound to the latter. We identified 45 proteins associated with the histone H4 tail and grouped them into four categories: 10 chromatin remodeling proteins, five histone chaperones, two histone modification-related proteins, and 28 other proteins. mRNA expression levels of 20 proteins selected from these 45 proteins were compared between undifferentiated and retinoic acid (RA)-induced differentiated ES cells. All of the genes were similarly expressed in both states of ES cells, except nucleoplasmin 3 (NPM3) that was expressed at a higher level in the undifferentiated cells. NPM3 proteins were localized in the nucleoli and nuclei of the cells and expression was decreased during RA-induced differentiation. When transfected with NPM3 gene, ES cells significantly increased their proliferation compared with control cells. The present study strongly suggests that NPM3 is a chromatin remodeling protein responsible for the unique chromatin structure and replicative capacity of ES cells.

Sperm nucleosomes disassembly is a requirement for histones proteolysis during male pronucleus formation

We had previously reported that a cysteine-protease catalyzes the sperm histones (SpH) degradation associated to male chromatin remodeling in sea urchins. We found that this protease selectively degraded the SpH leaving maternal cleavage stage (CS) histone variants unaffected, therefore we named it SpH-protease. It is yet unknown if the SpH-protease catalyzes the SpH degradation while these histones are organized as nucleosomes or if alternatively these histones should be released from DNA before their proteolysis. To investigate this issue we had performed an in vitro assay in which polynucleosomes were exposed to the active purified protease. As shown in this report, we found that sperm histones organized as nucleosomes remains unaffected after their incubation with the protease. In contrast the SpH unbound and free from DNA were readily degraded. Interestingly, we also found that free DNA inhibits SpH proteolysis in a dose-dependent manner, further strengthening the requirement of SpH release from DNA before in order to be degraded by the SpH-protease. In this context, we have also investigated the presence of a sperm-nucleosome disassembly activity (SNDA) after fertilization. We found a SNDA associated to the nuclear extracts from zygotes that were harvested during the time of male chromatin remodeling. This SNDA was undetectable in the nuclear extracts from unfertilized eggs and in zygotes harvested after the fusion of both pronuclei. We postulate that this SNDA is responsible for the SpH release from DNA which is required for their degradation by the cysteine-protease associated to male chromatin remodeling after fertilization.