Nuclear dot antigens may specify transcriptional domains in the nucleus

The Fate of Speckled Protein 100 (Sp100) During Herpesviruses Infection

The constitutive expression of Speckled-100 (Sp100) is known to restrict the replication of many clinically important DNA viruses. This pre-existing (intrinsic) immune defense to virus infection can be further upregulated upon interferon (IFN) stimulation as a component of the innate immune response. In humans, Sp100 is encoded by a single gene locus, which can produce alternatively spliced isoforms. The widely studied Sp100A, Sp100B, Sp100C and Sp100HMG have functions associated with the transcriptional regulation of viral and cellular chromatin, either directly through their characteristic DNA-binding domains, or indirectly through post-translational modification (PTM) and associated protein interaction networks. Sp100 isoforms are resident component proteins of promyelocytic leukemia-nuclear bodies (PML-NBs), dynamic nuclear sub-structures which regulate host immune defenses against many pathogens. In the case of human herpesviruses, multiple protein antagonists are expressed to relieve viral DNA genome transcriptional silencing imposed by PML-NB and Sp100-derived proteinaceous structures, thereby stimulating viral propagation, pathogenesis, and transmission to new hosts. This review details how different Sp100 isoforms are manipulated during herpesviruses HSV1, VZV, HCMV, EBV, and KSHV infection, identifying gaps in our current knowledge, and highlighting future areas of research.

Meta-Analysis of Antinuclear Antibodies in the Diagnosis of Antimitochondrial Antibody-Negative Primary Biliary Cholangitis

Objective

The diagnostic value of antinuclear antibodies (ANAs) including anti-gp210 and anti-sp100 for primary biliary cholangitis/cirrhosis (PBC) has been widely reported. However, their diagnostic performances for antimitochondrial antibody- (AMA-) negative PBC were less well elucidated. Therefore, the aim of the current meta-analysis was to evaluate the diagnostic accuracy of ANAs in patients with AMA-negative PBC.

Materials and Methods

Literature on the diagnostic value of biomarkers for AMA-negative PBC was systematically searched in PubMed, MEDLINE, EMBASE, and the Cochrane Library. The qualities of the retrieved studies were assessed by the Quality Assessment of Diagnostic Accuracy Studies-version 2 (QUADAS-2) scale. Pooled sensitivity and specificity of the biomarkers were calculated with random-effects models. The areas under the summary receiver operating characteristic (AUSROC) curves were used to evaluate the overall diagnostic performance of ANAs.

Results

A total of 11 studies (400 AMA-negative PBC patients and 6217 controls) were finally included in the meta-analysis. ANAs had an overall sensitivity of 27% (95% CI: 20%, 35%) and specificity of 98% (95% CI: 97%, 99%). The pooled sensitivities for anti-gp210 and anti-sp100 were 23% (95% CI: 13%, 37%) and 25% (95% CI: 13%, 43%), respectively, and their specificities were 99% (95% CI: 97%, 100%) and 97% (95% CI: 93%, 98%), respectively.

Conclusions

ANAs exhibited high specificity but low sensitivity and therefore could be used as reliable biomarkers to reduce the necessity of liver histology.

Src nuclear localization and its prognostic relevance in human osteosarcoma

Osteosarcoma is the most common malignant bone tumor in children and young adults. The identification of proteins which exhibit different subcellular localization in low- versus high-risk osteosarcoma can be instrumental to obtain prognostic information and to develop innovative therapeutic strategies. Beside the well-characterized membrane and cytoplasmic localization of Src protein, this study evaluated the prognostic relevance of its so-far unknown nuclear compartmentalization. We analyzed the subcellular distribution of total and activated (pY418) Src in a tissue microarray including 60 osteosarcoma samples. Immunohistochemical analyses revealed a variable pattern of Src expression and localization, ranging from negative to high-stained nuclei combined with a substantial cytoplasmic staining for total and activated forms. The analysis of Kaplan-Meier survival curves in relationship to the diverse permutations of cytoplasmic and nuclear staining suggested a correlation between Src subcellular localization and the overall survival (OS) of osteosarcoma patients. In order to explain this different subcellular localization, normal osteoblasts and three osteosarcoma cell lines were used to investigate the molecular mechanism. Once confirmed a variable Src localization also in these cell lines, we demonstrated a correlation between the N-myristoyltransferase enzymes expression and activity and the Src nuclear content. In conclusion, these results described a so-far unknown Src nuclear localization in osteosarcoma cells, suggesting that the combined detection of nuclear and cytoplasmic Src levels can be used as a prognostic marker for osteosarcoma patient survival. A correlation between the N-myristoyltransferase enzymes and the Src subcellular localization was described as well.

The replication initiator protein of a geminivirus interacts with host monoubiquitination machinery and stimulates transcription of the viral genome

Geminiviruses constitute a group of plant viruses, with a ssDNA genome, whose replication in the nucleus of an infected cell requires the function of geminivirus-encoded replication initiator protein (Rep). Our results suggest that monoubiquitinated histone 2B (H2B-ub) promotes tri-methylation of histone 3 at lysine 4 (H3-K4me3) on the promoter of Chilli leaf curl virus (ChiLCV). We isolated homologues of two major components of the monoubiquitination machinery: UBIQUITIN-CONJUGATING ENZYME2 (NbUBC2) and HISTONE MONOUBIQUITINATION1 (NbHUB1) from N. benthamiana. ChiLCV failed to cause disease in NbUBC2-, and NbHUB1-silenced plants, at the same time, H2B-ub and H3-K4me3 modifications were decreased, and the occupancy of RNA polymerase II on the viral promoter was reduced as well. In further investigations, Rep protein of ChiLCV was found to re-localize NbUBC2 from the cytoplasm to the nucleoplasm, like NbHUB1, the cognate partner of NbUBC2. Rep was observed to interact and co-localize with NbHUB1 and NbUBC2 in the nuclei of the infected cells. In summary, the current study reveals that the ChiLCV Rep protein binds the viral genome and interacts with NbUBC2 and NbHUB1 for the monoubiquitination of histone 2B that subsequently promotes trimethylation of histone 3 at lysine 4 on ChiLCV mini-chromosomes and enhances transcription of the viral genes.

Sp100A promotes chromatin decondensation at a cytomegalovirus-promoter-regulated transcription site

Promyelocytic leukemia nuclear bodies (PML-NBs)/nuclear domain 10s (ND10s) are nuclear structures that contain many transcriptional and chromatin regulatory factors. One of these, Sp100, is expressed from a single-copy gene and spliced into four isoforms (A, B, C, and HMG), which differentially regulate transcription. Here we evaluate Sp100 function in single cells using an inducible cytomegalovirus-promoter-regulated transgene, visualized as a chromatinized transcription site. Sp100A is the isoform most strongly recruited to the transgene array, and it significantly increases chromatin decondensation. However, Sp100A cannot overcome Daxx- and α-thalassemia mental retardation, X-linked (ATRX)-mediated transcriptional repression, which indicates that PML-NB/ND10 factors function within a regulatory hierarchy. Sp100A increases and Sp100B, which contains a SAND domain, decreases acetyl-lysine regulatory factor levels at activated sites, suggesting that Sp100 isoforms differentially regulate transcription by modulating lysine acetylation. In contrast to Daxx, ATRX, and PML, Sp100 is recruited to activated arrays in cells expressing the herpes simplex virus type 1 E3 ubiquitin ligase, ICP0, which degrades all Sp100 isoforms except unsumoylated Sp100A. The recruitment Sp100A(K297R), which cannot be sumoylated, further suggests that sumoylation plays an important role in regulating Sp100 isoform levels at transcription sites. This study provides insight into the ways in which viruses may modulate Sp100 to promote their replication cycles.

NANOG regulates glioma stem cells and is essential in vivo acting in a cross-functional network with GLI1 and p53

A cohort of genes associated with embryonic stem (ES) cell behaviour, including NANOG, are expressed in a number of human cancers. They form an ES-like signature we first described in glioblastoma multiforme (GBM), a highly invasive and incurable brain tumour. We have also shown that HEDGEHOG-GLI (HH-GLI) signalling is required for GBM growth, stem cell expansion and the expression of this (ES)-like stemness signature. Here, we address the function of NANOG in human GBMs and its relationship with HH-GLI activity. We find that NANOG modulates gliomasphere clonogenicity, CD133(+) stem cell cell behavior and proliferation, and is regulated by HH-GLI signalling. However, GLI1 also requires NANOG activity forming a positive loop, which is negatively controlled by p53 and vice versa. NANOG is essential for GBM tumourigenicity in orthotopic xenografts and it is epistatic to HH-GLI activity. Our data establish NANOG as a novel HH-GLI mediator essential for GBMs. We propose that this function is conserved and that tumour growth and stem cell behaviour rely on the status of a functional GLI1-NANOG-p53 network.

SCFCdc4-mediated degradation of the Hac1p transcription factor regulates the unfolded protein response in Saccharomyces cerevisiae

The Saccharomyces cerevisiae basic leucine zipper transcription factor Hac1p is synthesized in response to the accumulation of unfolded polypeptides in the lumen of the endoplasmic reticulum (ER), and it is responsible for up-regulation of approximately 5% of all yeast genes, including ER-resident chaperones and protein-folding catalysts. Hac1p is one of the most short-lived yeast proteins, having a half-life of approximately 1.5 min. Here, we have shown that Hac1p harbors a functional PEST degron and that degradation of Hac1p by the proteasome involves the E2 ubiquitin-conjugating enzyme Ubc3/Cdc34p and the SCF(Cdc4) E3 complex. Consistent with the known nuclear localization of Cdc4p, rapid degradation of Hac1p requires the presence of a functional nuclear localization sequence, which we demonstrated to involve basic residues in the sequence (29)RKRAKTK(35). Two-hybrid analysis demonstrated that the PEST-dependent interaction of Hac1p with Cdc4p requires Ser146 and Ser149. Turnover of Hac1p may be dependent on transcription because it is inhibited in cell mutants lacking Srb10 kinase, a component of the SRB/mediator module of the RNA polymerase II holoenzyme. Stabilization of Hac1p by point mutation or deletion, or as the consequence of defects in components of the degradation pathway, results in increased unfolded protein response element-dependent transcription and improved cell viability under ER stress conditions.

Mediation of Epstein-Barr virus EBNA-LP transcriptional coactivation by Sp100

The Epstein-Barr virus (EBV) EBNA-LP protein is important for EBV-mediated B-cell immortalization and is a potent gene-specific coactivator of the viral transcriptional activator, EBNA2. The mechanism(s) by which EBNA-LP functions as a coactivator remains an important question in the biology of EBV-induced B-cell immortalization. In this study, we found that EBNA-LP interacts with the promyelocytic leukemia nuclear body (PML NB)-associated protein Sp100 and displaces Sp100 and heterochromatin protein 1alpha (HP1alpha) from PML NBs. Interaction between EBNA-LP and Sp100 was mediated through conserved region 3 in EBNA-LP and the PML NB targeting domain in Sp100. Overexpression of Sp100 lacking the N-terminal PML NB targeting domain, but not a mutant form of Sp100 lacking the HP1alpha interaction domain, was sufficient to coactivate EBNA2 in a gene-specific manner independent of EBNA-LP. These findings suggest that Sp100 is a major mediator of EBNA-LP coactivation. These studies indicate that modulation of PML NB-associated proteins may be important for establishment of latent viral infections, and also identify a convenient model system to investigate the functions of Sp100.

Characterization of nuclear localization and export signals of the major tegument protein VP8 of bovine herpesvirus-1

Bovine herpesvirus-1 (BHV-1) VP8 is found in the nucleus immediately after infection. Transient expression of VP8 fused to yellow fluorescent protein (YFP) in COS-7 cells confirmed the nuclear localization of VP8 in the absence of other viral proteins. VP8 has four putative nuclear localization signals (NLS). Deletion of pat4 ((51)RRPR(54)) or pat7 ((48)PRVRRPR(54)) NLS2 abrogated nuclear accumulation, whereas deletion of (48)PRV(50) did not, so pat4 NLS2 is critical for nuclear localization of VP8. Furthermore, NLS1 ((11)RRPRR(15)), pat4 NLS2, and pat7 NLS2 were all capable of transporting the majority of YFP to the nucleus. Finally, a 12-amino-acid peptide with the sequence RRPRRPRVRRPR directed all of YFP into the nucleus, suggesting that reiteration of the RRPR motif makes the nuclear localization more efficient. Heterokaryon assays demonstrated that VP8 is also capable of shuttling between the nucleus and cytoplasm of the cell. Deletion mutant analysis revealed that this property is attributed to a leucine-rich nuclear export sequence (NES) consisting of amino acids (485)LSAYLTLFVAL(495). This leucine-rich NES caused transport of YFP to the cytoplasm. These results demonstrate that VP8 shuttles between the nucleus and cytoplasm.

Nuclear DNA helicase II is recruited to IFN-alpha-activated transcription sites at PML nuclear bodies

It is known that nuclear DNA helicase II (NDH II) links CREB-binding protein directly to RNA polymerase II holoenzyme, and that this interaction is essential for gene activation by CREB. Here, we report for the first time that some NDH II/RNA helicase A is a component of promyelocytic leukemia nuclear bodies (PML NBs). An autoimmune serum specific for PML NBs was identified and used in immunoprecipitation experiments. NDH II was present in the immunoprecipitates as shown by mass spectrometry and by immunoblotting. Immunofluorescence and ultrastructural studies showed that NDH II colocalizes with a small subset of PML NBs in control cells, however, colocalizes with practically all bodies in interferon-alpha-stimulated cells. After interferon stimulation, more PML NBs were found to contain newly synthesized RNA, as indicated by bromouridine incorporation. PML NBs also contain RNA polymerase II. The association of NDH II with PML NBs was transcriptionally dependent, and NDH II was present in all bodies with nascent RNA. Blocking of mRNA synthesis caused NDH II relocalization from nucleoplasm to nucleoli. Based on the data, we suggest that NDH II recruitment to PML NBs is connected with transcriptional regulation of interferon-alpha-inducible genes attached to PML NBs.

Mammalian transcription factors in yeast: strangers in a familiar land

Many transcription factors in human cells have functional orthologues in yeast, and a common experimental theme has been to define the function of the yeast protein and then test whether the mammalian version behaves similarly. Although, at first glance, this approach does not seem feasible for factors that do not have yeast counterparts, mammalian transcriptional activators or repressors can be expressed directly in yeast. Often, the mammalian factor retains function in yeast, and this allows investigators to exploit the experimental tractability of yeast to ask a diverse set of questions.

Interferon-induced antiviral Mx1 GTPase is associated with components of the SUMO-1 system and promyelocytic leukemia protein nuclear bodies

Mx proteins are interferon-induced large GTPases, some of which have antiviral activity against a variety of viruses. The murine Mx1 protein accumulates in the nucleus of interferon-treated cells and is active against members of the Orthomyxoviridae family, such as the influenza viruses and Thogoto virus. The mechanism by which Mx1 exerts its antiviral action is still unclear, but an involvement of undefined nuclear factors has been postulated. Using the yeast two-hybrid system, we identified cellular proteins that interact with Mx1 protein. The Mx1 interactors were mainly nuclear proteins. They included Sp100, Daxx, and Bloom's syndrome protein (BLM), all of which are known to localize to specific subnuclear domains called promyelocytic leukemia protein nuclear bodies (PML NBs). In addition, components of the SUMO-1 protein modification system were identified as Mx1-interacting proteins, namely the small ubiquitin-like modifier SUMO-1 and SAE2, which represents subunit 2 of the SUMO-1 activating enzyme. Analysis of the subcellular localization of Mx1 and some of these interacting proteins by confocal microscopy revealed a close spatial association of Mx1 with PML NBs. This suggests a role of PML NBs and SUMO-1 in the antiviral action of Mx1 and may allow us to discover novel functions of this large GTPase.

Transcriptional regulation in acute promyelocytic leukemia

It has been 10 years since the seminal discovery that a mutant form of a retinoid acid receptor (RARalpha) is associated with acute promyelocytic leukemia (APL). This finding, coupled with the remarkable success of retinoic acid (RA), the natural ligand of RARalpha, in the treatment of APL, has made APL a unique model system in the study of oncogenic conversion of transcription factors in hematological malignancies. Indeed, subsequent basic and clinical studies showed that chromosomal translocation involving the RARalpha gene is the cytogenetic hallmark of APL and that these mutant forms of RARs are the oncogenes in APL that interfere with the proliferation and differentiation pathways controlled by both RAR and their fusion partners. However, it was not until recently that the role of aberrant transcriptional regulation in the pathogenesis of APL was revealed. In this review, we summarize the biochemical and biological mechanisms of transcriptional regulation by mutant RARs and their corresponding wild-type fusion partner PML and PLZF. These studies have been instrumental in our understanding of the process of leukemogenesis in general and have laid the scientific foundation for the novel concept of transcription therapy in the treatment of human cancer.

The transcription coactivator CBP is a dynamic component of the promyelocytic leukemia nuclear body

The transcription coactivator and histone acetyltransferase CAMP response element-binding protein (CBP) has been demonstrated to accumulate in promyelocytic leukemia (PML) bodies. We show that this accumulation is cell type specific. In cells where CBP does not normally accumulate in PML bodies, it can be induced to accumulate in PML bodies through overexpression of either CBP or Pml, but not Sp100. Using fluorescence recovery after photobleaching, we demonstrate that CBP moves rapidly into and out of PML bodies. In contrast, Pml and Sp100 are relatively immobile in the nucleoplasm and within PML nuclear bodies. They possess the characteristics expected of proteins that would play a structural role in the integrity of these subnuclear domains. Our results are consistent with CBP being a dynamic component of PML bodies and that the steady-state level in these structures can be modulated by Pml.

Regulation of matrix attachment region-dependent, lymphocyte-restricted transcription through differential localization within promyelocytic leukemia nuclear bodies

Bright (B cell regulator of IgH transcription) transactivates the immunoglobulin heavy chain (IgH) intronic enhancer, Emicro, by binding to matrix attachment regions (MARs), sites necessary for DNA attachment to the nuclear matrix. Here we report that Bright interacts with the ubiquitous autoantigen Sp100, a component of promyelocytic leukemia nuclear bodies (PML NBs), and with LYSp100B/Sp140, the lymphoid-restricted homolog of Sp100. Both in intact cells and in nuclear matrix preparations, the majority of Bright and Sp100 colocalize within PML NBs. In contrast, Bright colocalizes with only a small fraction of LYSp100B while inducing a redistribution of the majority of LYSp100B from its associated nuclear domains (LANDs) into nucleoplasm and cytoplasm. Sp100 represses the MAR-binding and transactivation activity of Bright. LYSp100B interacts more weakly with Bright but requires significantly higher levels than Sp100 to inhibit MAR binding. However, it strongly stimulates Bright transactivation through E mu. We suggest that Sp100 and LYSp100B interactions with Bright have different consequences for IgH transcription, potentially through differential association of E mu MARs with nuclear matrix- associated PML NBs and LANDs.

Promyelocytic leukemia (PML) nuclear bodies are protein structures that do not accumulate RNA

The promyelocytic leukemia (PML) nuclear body (also referred to as ND10, POD, and Kr body) is involved in oncogenesis and viral infection. This subnuclear domain has been reported to be rich in RNA and a site of nascent RNA synthesis, implicating its direct involvement in the regulation of gene expression. We used an analytical transmission electron microscopic method to determine the structure and composition of PML nuclear bodies and the surrounding nucleoplasm. Electron spectroscopic imaging (ESI) demonstrates that the core of the PML nuclear body is a dense, protein-based structure, 250 nm in diameter, which does not contain detectable nucleic acid. Although PML nuclear bodies contain neither chromatin nor nascent RNA, newly synthesized RNA is associated with the periphery of the PML nuclear body, and is found within the chromatin-depleted region of the nucleoplasm immediately surrounding the core of the PML nuclear body. We further show that the RNA does not accumulate in the protein core of the structure. Our results dismiss the hypothesis that the PML nuclear body is a site of transcription, but support the model in which the PML nuclear body may contribute to the formation of a favorable nuclear environment for the expression of specific genes.

Colocalization within the nucleolus of two highly related IFN-induced human nuclear phosphoproteins with nucleolin

We have previously reported the identification of two interferon (IFN)-induced cDNAs which code for two proteins, named 41 and 75, which have homology to a number of proteins involved in regulating gene expression. Here we establish that these cDNAs correspond to in vivo synthesized mRNAs. Expression of the 41 and 75 cDNAs, both in vitro and in vivo, generated proteins of 30 and 68 kDa, respectively. In a variety of mammalian cells, 41 and 75 were found to be located in the nucleus, with 41 being localized to the nucleolus, whereas 75, although it is mainly concentrated at the periphery of the nucleolus, is also found throughout the nucleoplasm. Treatment with interferon results in a translocation of 41 to the periphery of the nucleolus and it is in this region that the two proteins colocalize. 41 and 75 were found to colocalize with nucleolin but not with B23 or fibrillarin, three nucleolar proteins involved in ribosome synthesis. This colocalization was not affected by low concentrations of actinomycin D. In view of this and since 41 and 75 have homology to proteins involved in regulating gene expression, we suggest that, in association with nucleolin, they play a role in ribosome biogenesis.

The nuclear dot protein sp100, characterization of domains necessary for dimerization, subcellular localization, and modification by small ubiquitin-like modifiers

The Sp100 and promyelocytic leukemia proteins (PML) are constituents of nuclear domains, known as nuclear dots (NDs) or PML bodies, and are both covalently modified by the small ubiquitin-related protein SUMO-1. NDs play a role in autoimmunity, virus infections, and in the etiology of acute promyelocytic leukemia. To date, little is known about the function of the Sp100 protein. Here we analyzed Sp100 domains that determine its subcellular localization, dimerization, and SUMOylation. A functional nuclear localization signal and an ND-targeting region that coincides with an Sp100 homodimerization domain were mapped. Sequences similar to the Sp100 homodimerization/ND-targeting region occur in several other proteins and constitute a novel protein motif, termed HSR domain. The lysine residue of the Sp100 protein, to which SUMO-1 is covalently linked, was mapped within and may therefore modulate the previously described HP1 protein-binding site. A consensus sequence for SUMOylation of proteins in general is suggested. SUMOylation strictly depended on a functional nuclear localization signal but was not necessary for nuclear import or ND targeting. A three-dimensional structure of Sp100, which supports the mapping data and provides additional information on Sp100 structure/function relationships, was generated by computer modeling. Taken together, our studies indicate the existence of well defined Sp100 domains with functions in ND targeting, nuclear import, nuclear SUMOylation, and protein-protein interaction.

Splice variants of the nuclear dot-associated Sp100 protein contain homologies to HMG-1 and a human nuclear phosphoprotein-box motif

Sp100 and PML are interferon-inducible proteins associated with a new class of nuclear domains (known as nuclear dots or PML bodies) which play a role in tumorigenesis, virus infections, and autoimmunity. While PML is extensively alternatively spliced, only two splice variants are known for Sp100. Here we describe the identification and characterization of several Sp100 splice variant proteins and support their existence by elucidation of the 3'-end of the Sp100 gene. Some of the splice variants contain a domain of significant sequence similarity with two previously described highly related interferon-inducible nuclear phosphoproteins as well as to suppressin and DEAF-1, which altogether define a novel protein motif, termed HNPP-box. One class of splice variants contains an almost complete and highly conserved copy of the DNA-binding high mobility group 1 protein sequence and thus represent novel HMG-box proteins. When expressed transiently, both major classes of Sp100 splice variant proteins localize in part to nuclear dots/PML bodies and in addition to different nuclear domains. Furthermore, PML was occasionally redistributed. These data indicate that alternatively spliced Sp100 proteins are expressed, differ in part in localization from Sp100, and might bind to chromatin via the HMG domain.

The product of the mammalian orthologue of the Saccharomyces cerevisiae HBS1 gene is phylogenetically related to eukaryotic release factor 3 (eRF3) but does not carry eRF3-like activity

We describe here the cloning and sequencing of human and mouse cDNAs encoding a putative GTP binding protein. Sequence comparison shows that these cDNAs (named eRFS) are likely to represent the orthologues of the yeast Saccharomyces cerevisiae HBS1 gene and that the C-terminal domains of the encoded proteins share structural features with eukaryotic elongation factor eEF-1A and release factor 3 (eRF3) families. The phylogenetic analysis suggests that eRFS proteins and Hbs1p form a cluster of orthologous sequences branching with the eRF3 family. Nevertheless, in yeast, the human eRFS protein and Hbs1p do not complement eRF3/Sup35p thermosensitive mutation and do not interact with eRF1.

Nuclear domain 10, the site of DNA virus transcription and replication

Within the highly organized nuclear structure, specific nuclear domains (ND10) are defined by accumulations of proteins that can be interferon-upregulated, implicating ND10 as sites of a nuclear defense mechanism. Compatible with such a mechanism is the deposition of herpesvirus, adenovirus, and papovavirus genomes at the periphery of ND10. However, these DNA viruses begin their transcription at ND10 and consequently initiate replication at these sites, suggesting that viruses have evolved ways to circumvent this potential cellular defense and exploit it. Other ND10-associated proteins belong to ubiquitin-related pathways. These findings, together with the accumulation of various overexpressed cellular and viral proteins, suggest that ND10 function as nuclear dumps or as nuclear depots. Consistent with the recruitment or deposition of various proteins and viral genomes adjacent to ND10, ND10 themselves may only be protein accumulations at specific but as yet undefined nuclear deposition sites. The concept of specific nuclear deposition sites may explain the juxtaposition of various nuclear bodies and allows testable predictions about a potential supramolecular regulatory mechanism whereby proteins are selectively segregated or released by global changes induced in nuclear functions such as viral infections, stress, or hormonal induction.

Ret finger protein is a normal component of PML nuclear bodies and interacts directly with PML

The ret finger protein (rfp) is a member of the B-box zinc finger gene family many of which may function in growth regulation and in the appropriate context become oncogenic. Members of this family are nuclear proteins that possess a characteristic tripartite motif consisting of the RING and B-box zinc binding domains and a coiled-coil domain. The promyelocytic leukemia gene (PML), another B-box family member, produces a protein product that is detected within punctate nuclear structures called PML nuclear bodies (NBs) or PML oncogenic domains (PODs). These NBs are complex structures that consist of a number of different proteins many of which have yet to be identified. In the disease acute promyelocytic leukemia (APL) a fusion protein, PML-RARA, is produced through the t(15:17) translocation. In APL the morphology of the NBs is altered. We report that rfp co-localizes with PML in a subset of the PML NBs and that it interacts directly with PML. This interaction is mediated through the rfp B-box and the distal two coils. In contrast, homomultimerization of rfp preferentially involves the B-box and the proximal coil. The association of rfp with the PML NBs is altered by mutations that affect rfp/PML interaction and in NB4 cells that are derived from APL patients. When treated with retinoic acid, rfp reassociates with the NBs in a pattern similar to non APL cells. Additionally, we found that rfp colocalizes with PML-RARA protein produced in APL patients. These results suggest that rfp, along with the other known/unknown components of PML NBs, have an important role in regulating cellular growth and differentiation.

The Spa2-related protein, Sph1p, is important for polarized growth in yeast

The Saccharomyces cerevisiae protein Sph1p is both structurally and functionally related to the polarity protein, Spa2p. Sph1p and Spa2p are predicted to share three 100-amino acid domains each exceeding 30% sequence identity, and the amino-terminal domain of each protein contains a direct repeat common to Homo sapiens and Caenorhabditis elegans protein sequences. sph1- and spa2-deleted cells possess defects in mating projection morphology and pseudohyphal growth. sph1(Delta) spa2(Delta) double mutants also exhibit a strong haploid invasive growth defect and an exacerbated mating projection defect relative to either sph1(Delta) or spa2(Delta) single mutants. Consistent with a role in polarized growth, Sph1p localizes to growth sites in a cell cycle-dependent manner: Sph1p concentrates as a cortical patch at the presumptive bud site in unbudded cells, at the tip of small, medium and large buds, and at the bud neck prior to cytokinesis. In pheromone-treated cells, Sph1p localizes to the tip of the mating projection. Proper localization of Sph1p to sites of active growth during budding and mating requires Spa2p. Sph1p interacts in the two-hybrid system with three mitogen-activated protein (MAP) kinase kinases (MAPKKs): Mkk1p and Mkk2p, which function in the cell wall integrity/cell polarization MAP kinase pathway, and Ste7p, which operates in the pheromone and pseudohyphal signaling response pathways. Sph1p also interacts weakly with STE11, the MAPKKK known to activate STE7. Moreover, two-hybrid interactions between SPH1 and STE7 and STE11 occur independently of STE5, a proposed scaffolding protein which interacts with several members of this MAP kinase module. We speculate that Spa2p and Sph1p may function during pseudohyphal and haploid invasive growth to help tether this MAP kinase module to sites of polarized growth. Our results indicate that Spa2p and Sph1p comprise two related proteins important for the control of cell morphogenesis in yeast.

Pheromone-regulated genes required for yeast mating differentiation

Yeast cells mate by an inducible pathway that involves agglutination, mating projection formation, cell fusion, and nuclear fusion. To obtain insight into the mating differentiation of Saccharomyces cerevisiae, we carried out a large-scale transposon tagging screen to identify genes whose expression is regulated by mating pheromone. 91,200 transformants containing random lacZ insertions were screened for beta-galactosidase (beta-gal) expression in the presence and absence of alpha factor, and 189 strains containing pheromone-regulated lacZ insertions were identified. Transposon insertion alleles corresponding to 20 genes that are novel or had not previously been known to be pheromone regulated were examined for effects on the mating process. Mutations in four novel genes, FIG1, FIG2, KAR5/ FIG3, and FIG4 were found to cause mating defects. Three of the proteins encoded by these genes, Fig1p, Fig2p, and Fig4p, are dispensible for cell polarization in uniform concentrations of mating pheromone, but are required for normal cell polarization in mating mixtures, conditions that involve cell-cell communication. Fig1p and Fig2p are also important for cell fusion and conjugation bridge shape, respectively. The fourth protein, Kar5p/Fig3p, is required for nuclear fusion. Fig1p and Fig2p are likely to act at the cell surface as Fig1:: beta-gal and Fig2::beta-gal fusion proteins localize to the periphery of mating cells. Fig4p is a member of a family of eukaryotic proteins that contain a domain homologous to the yeast Sac1p. Our results indicate that a variety of novel genes are expressed specifically during mating differentiation to mediate proper cell morphogenesis, cell fusion, and other steps of the mating process.

The ubiquitin-homology gene PIC1: characterization of mouse (Pic1) and human (UBL1) genes and pseudogenes

The human ubiquitin-homology domain protein PIC1 interacts with the acute promyelocytic leukemia protein PML, and both proteins form part of the large, nuclear, multiprotein complexes known as PML nuclear bodies. The normal punctate immunohistochemical staining pattern of these complexes is disrupted by viral infection or interferon treatment and in blast cells from patients with acute promyelocytic leukemia. We have characterized the murine homologue of PIC1 and have found that the predicted amino acid sequences of the mouse and human proteins are identical. High levels of Pic1 mRNA were detected in a range of mouse tissues. Pic1 genomic clones were isolated, and the organization of the gene was determined. Two processed Pic1 pseudogenes were also isolated and characterized. Through FISH, the chromosomal localizations of the mouse Pic1 gene and the two pseudogenes were determined. Human PIC1 (HGMW-approved symbol UBL1)-related sequences were isolated from human genomic DNA and were shown to represent processed pseudogenes. The role of PIC1 in a variety of cellular processes is discussed.

Evidence for covalent modification of the nuclear dot-associated proteins PML and Sp100 by PIC1/SUMO-1

PML and Sp100 proteins are associated with nuclear domains, known as nuclear dots (NDs). They were discovered in the context of leukemic transformation and as an autoantigen in primary biliary cirrhosis, respectively. Both proteins are expressed in the form of many COOH-terminally spliced variants, and their expression is enhanced by interferons (IFN). The recent finding that PIC1/SUMO-1, a small ubiquitin-like protein, is covalently linked to the RanGAP1 protein of the nuclear pore complex and also binds PML in yeast cells led us to determine whether PML is covalently modified by PIC1/SUMO-1 and whether the same is true for Sp100. We found an immune reaction of PML and Sp100 proteins with a PIC1/SUMO-1-specific monoclonal antibody by immunoblotting when using cell extracts prepared from stably transfected cells inducibly expressing one isoform of each protein as well as from nontransfected cells. In contrast, both proteins did not react when synthesized in vitro. Immunofluorescence staining showed that PIC1/SUMO-1 colocalized with Sp100 and PML in NDs except in mitotic cells, in which PML and Sp100 are dissociated. Cell fractionation and immunoblotting demonstrated that PIC1/SUMO-1 immunoreactive Sp100 in IFN-treated and untreated cells was exclusively nuclear, whereas nonmodified Sp100 was also found in the cytoplasm. Taken together, these data strongly suggest covalent modification of specific nuclear isoforms of Sp100 and PML by PIC1/SUMO-1. This modification may play a regulatory role in ND structure, composition, and function.

Nuclear dots: actors on many stages

Nuclear dots (NDs), alternatively designated nuclear bodies (NBs), PML oncogenic domains (PODs), nuclear domain 10 (ND10) or Kr-bodies, became a major topic for researchers in many fields only recently. Originally described as an autoantigenic target in patients with primary biliary cirrhosis, they are now also known to play a role in development of acute promyelocytic leukemia (APL) and possibly other forms of neoplasia. Size, number and composition of NDs are regulated throughout the cell cycle. Infection with herpes simplex virus, adenovirus, cytomegalovirus, Epstein-Barr-virus, influenza virus and human T cell lymphotropic virus type I (HTLV I) strongly modifies ND structure through viral regulatory proteins. Due to this finding and because at least three of the cellular ND proteins are highly interferon-inducible, a function of NDs in early viral infection or in antiviral response has been postulated. Functional data are currently available only for two of the ND-associated proteins. The Sp100 protein seems to have transcriptional transactivating property, whereas the promyelocytic leukemia protein (PML) was reported to suppress growth and transformation. Here, we give a brief overview of the data currently available on NDs. Thus, we hope to link seemingly unrelated findings in the literature on oncology, virology, cell biology and immunology.

Prevalence, kinetics, and therapeutic modulation of autoantibodies against Sp100 and promyelocytic leukemia protein in a large cohort of patients with primary biliary cirrhosis

Antinuclear antibodies (ANA) staining nuclear dot structures predominantly occur in primary biliary cirrhosis (PBC) patients and recognize the Sp100 and promyelocytic leukemia protein (PML). From retrospective analysis of sera from a clinically well-defined Canadian series of 170 PBC patients included into a 24-month therapeutic trial of ursodeoxycholic acid (UDCA), we report the prevalence of these ANA and their dynamics in the course of the disease. Using an enzyme-linked immunosorbent assay (ELISA), anti-Sp100 autoantibodies were shown in 35 (21%) patients. Thirty-three patients (19%) had autoantibodies against PML as determined by indirect immunostaining of cells overexpressing PML. Altogether, anti-nuclear dot autoantibodies were present in 25% of the 170 PBC patients. Their occurrence correlated with an unfavorable disease course, because these patients progressed significantly more frequently from early stages (I/II) to late stages (III/IV) within the 24-month observation period (P < .05). During the course of the disease, the autoantibody levels against the Sp100 full-length protein remained nearly constant in all 35 positive patients. However, 9 patients showed remarkable changes in Sp100 epitope recognition as revealed by ELISA and immunoblotting. When the occurrence of these changes and the treatment of the patients were compared retrospectively, it became evident that 8 of the 9 patients had received UDCA (42% of all Sp100-positive patients treated with UDCA). These findings indicate subtle changes of the Sp100 epitope recognition pattern during the natural course of the disease and its induction or acceleration by UDCA treatment. This implies that UDCA can modulate immunoglobulin (Ig) expression not only quantitatively, but also qualitatively.

Gene rearrangements in the molecular pathogenesis of acute promyelocytic leukemia

Acute Promyelocytic Leukemia (APL) is a distinct subtype of myeloid leukemia that in the USA alone affects more than 3,000 individuals every year. APL is characterized by three distinct and unique features: i) the accumulation in the bone marrow of tumor cells with promyelocytic features; ii) the invariable association with specific translocations which always involve chromosome 17 and the Retinoic Acid Receptor alpha (RAR alpha) locus; iii) the exquisite sensitivity of APL blasts to the differentiating action of Retinoic Acid (RA). These features have led APL to become the paradigm for therapeutic approaches utilizing differentiating agents. The last 5 years have provided crucial insights into the molecular basis of APL. RAR alpha translocates in 99% of cases to a gene located on chromosome 15 that we initially named myl and subsequently has been called PML. In a few cases, RAR alpha variably translocates to chromosome 11 where it fuses to the PLZF gene or to a newly described partner, NuMA. In addition, RAR alpha is also found translocated to chromosome 5 where it fuses to the NPM gene. The cloning of variant translocations in APL and the comparative analysis of their associated products is crucial for the understanding of the molecular etiopathogenesis of the disease. The generation of animal models, i.e., transgenic mice expressing the fusion genes, will be instrumental in determining the precise contribution of these fusion genes to leukemogenesis. In fact, mice harboring a PML/RAR alpha transgene whose expression is specifically targeted to the myeloid-promyelocytic lineage develop acute myeloid leukemia with promyelocytic features. Moreover, the functional analysis of the various fusion proteins, as well as RAR alpha partners, is revealing striking common features beneath a misleading structural heterogeneity which unravels a possible unifying molecular mechanism towards APL leukemogenesis.

Structure and expression of the murine Sp100 nuclear dot gene

The human SP100 gene encodes an autoantigen that colocalizes with two other proteins, PML and NDP52, in distinct nuclear domains, called "nuclear dots" (NDs). NDs do not overlap with other known subnuclear structures, and their function is still unknown. Patients suffering from the autoimmune disease primary biliary cirrhosis often produce antibodies against the SP100 protein. The present study describes the structure and expression of the murine Sp100 gene. In the species Mus caroli, Sp100 consists of 17 exons that are distributed over a range of 52 kb. The human and murine Sp100 promoters are very similar, and both harbor an interferon-stimulated response element. Like its human counterpart, the murine Sp100 gene is responsive to interferon treatment. The house mouse, Mus musculus, harbors the Sp100 gene and a second gene with homology to Sp100, the multicopy Sp100-rs gene. However, in contrast to the genuine mouse homolog, Sp100-rs shares only segmental homology with the human Sp100 gene. Replacement of the murine Sp100 gene by a defective copy is now feasible and should shed light on its function in an animal model.

Cellular localization, expression, and structure of the nuclear dot protein 52

Nuclear dots containing PML and Sp100 proteins (NDs) play a role in the development of acute promyelocytic leukemia, are modified after infection with various viruses, and are autoimmunogenic in patients with primary biliary cirrhosis (PBC). PML and Sp100 gene expression is strongly enhanced by interferons (IFN). Based on immunostaining with a monoclonal antibody (mAb C8A2), a third protein, nuclear dot protein 52 (NDP52), was recently localized in NDs. Here we analyzed the cellular localization, expression, and structure of NDP52 in more detail. Our NDP52-specific sera revealed mainly cytoplasmic staining but no ND pattern, neither in untreated nor in IFN-treated cells. Cells transfected with NDP52 expression vectors showed exclusively cytoplasmic staining. In subcellular fractionation experiments, NDP52 was found in cytoplasmic and nuclear fractions. Unlike as described for Sp100 and PML, NDP52 mRNA and protein levels were only marginally enhanced by IFN gamma and not enhanced at all by IFN beta. NDP52 homodimerization but no heterodimerization with Sp100 or PML could be demonstrated. None of the 93 PBC sera tested contained autoantibodies against NDP52. Finally, mAb C8A2 reacted not only with NDP52 but also with a conformation-dependent epitope on the Sp100 protein. These data imply that NDP52 forms homodimers but no heterodimers with Sp100 and PML, lacks autoantigenicity in PBC, localizes mainly in the cytoplasm, and is associated with the nucleus, but not with NDs. Finally, unlike Sp100 and PML, NDP52 expression is neither markedly enhanced nor localization detectably altered by type I and II IFNs.

Heterogeneous nuclear expression of the promyelocytic leukemia (PML) protein in normal and neoplastic human tissues

The RING-finger promyelocytic leukemia (PML) protein is the product of the PML gene that fuses with the retinoic acid receptor-alpha gene in the t(15; 17) translocation of acute promyelocytic leukemia. Wild-type PML localizes in the nucleus with a typical speckled pattern that is a consequence of the concentration of the protein within discrete subnuclear domains known as nuclear bodies. Delocalization of PML from nuclear bodies has been documented in acute promyelocytic leukemia cells and suggested to contribute to leukemogenesis. In an attempt to get new insights into the function of the wild-type PML protein and to investigate whether it displays an altered expression pattern in neoplasms other than acute promyelocytic leukemia, we stained a large number of normal and neoplastic human tissues with a new murine monoclonal antibody (PG-M3) directed against the amino-terminal region of PML. As the PG-M3 epitope is partially resistant to fixatives, only cells that overexpress PML are detected by the antibody in microwave-heated paraffin sections. Among normal tissues, PML was characteristically up-regulated in activated epithelioid histiocytes and fibroblasts in a variety of pathological conditions, columnar epithelium in small active thyroid follicles, well differentiated foamy cells in the center of sebaceous glands, and hypersecretory endometria (Arias-Stella). Interferons, the PML of which is a primary target gene, and estrogens are likely to represent some of the cytokines and/or hormones that may be involved in the up-regulation of PML under these circumstances. In keeping with this concept, we found that PML is frequently overexpressed in Hodgkin and Reed-Sternberg cells of Hodgkin's disease, a tumor of cytokine-producing cells. Among solid tumors, overexpression of PML was frequently found in carcinomas of larynx and thyroid (papillary), epithelial thymomas, and Kaposi's sarcoma, whereas carcinomas of the lung, thyroid (follicular), breast, and colon were frequently negative or weakly PML+. We did not observe any changes in the levels of PML expression as the lesion progressed from benign dysplasia to carcinoma. Our immunohistological data are consistent with the hypothesized growth suppressor function of PML and strongly suggest that PML expression levels are likely to be modulated by a variety of stimuli, including cytokines and hormones.

Identification and characterization of a leukocyte-specific component of the nuclear body

The nuclear body (NB) is a cellular organelle that is involved in the pathogenesis of acute promyelocytic leukemia and viral infection. The NB is also a target of antibodies in the serum of patients with the autoimmune disease primary biliary cirrhosis. In this study, serum from a patient with primary biliary cirrhosis was used to identify a cDNA encoding a novel component of the NB, a 140-kDa protein designated Sp140. The predicted amino acid sequence of the amino-terminal portion of Sp140 was similar to Sp100, a previously identified NB protein. The carboxyl portion of Sp140 contained a zinc-finger domain and a bromodomain, motifs that are present in proteins regulating gene transcription. High levels of Sp140 mRNA were detected in human spleen and peripheral blood leukocytes, but not other human tissues. The level of SP140 mRNA in myeloid precursor cell lines HL60 and NB4 markedly increased in response to chemically induced cellular differentiation. Immunohistochemical techniques were used to demonstrate that SP140 localized to the NB in differentiated HL60 and NB4 cells. The location of Sp140 in the NB, and expression of this gene in cells involved in host defense, suggest that Sp140 may be involved in the pathogenesis of acute promyelocytic leukemia and viral infection.

The interferon (IFN)-stimulated gene Sp100 promoter contains an IFN-gamma activation site and an imperfect IFN-stimulated response element which mediate type I IFN inducibility

Expression of the nuclear domain-associated proteins Sp100, PML, and NDP52, is enhanced by interferons (IFNs) on the mRNA and protein level. Increase both of Sp100 and PML mRNA is due to enhanced transcription of the corresponding genes which occurs independently of cellular protein synthesis immediately upon IFN-beta addition. Here, we describe the molecular cloning and functional analysis of the Sp100 promoter. DNA sequence analysis revealed potential binding sites for several constitutive and IFN-inducible transcription factors. Consistent with the absence of a TATA box and an initiator element, several transcription initiation sites were found. Transient expression studies identified an imperfect IFN-stimulated response element within the first 100 nucleotides upstream of the major transcription start site. This element rendered a heterologous promoter IFN-beta-inducible and bound IFN-stimulated gene factor 2 strongly but IFN-stimulated gene factor 3 only weakly. An IFN-gamma activation site approximately 500 base pairs upstream of the IFN-stimulated response element was found to bind three IFN-alpha/beta activation factors upon IFN-beta induction and conferred both type I and type II IFN inducibility upon a heterologous promoter. These data demonstrate a novel arrangement of a nonoverlapping IFN-gamma activation site and an IFN-stimulated response element mediating type I IFN inducibility, previously not reported for other IFN-stimulable promoters.

The biologic function of PML and its role in acute promyelocytic leukemia

Patients with acute promyelocytic leukemia (APL) are characterized by the presence of a t(15;17) chromosomal translocation. The fusion protein PML-RAR alpha encoded from the breakpoint can form a heterodimer and acts as a dominant negative inhibitor against the normal function of PML. Recently we demonstrated that PML is a growth suppressor and transcription suppressor expressed in all cell lines tested. We also found that PML suppresses the clonogenicity and tumorigenicity of APL-derived NB4 cells, as well as the transformation of rat embryo fibroblasts by cooperative oncogenes and NIH/3T3 by neu. Overexpression of PML in human tumor cell lines induces a remarkable reduction in growth rate in vitro and in vivo. More recently, we have shown that PML is a phosphoprotein associated with the nuclear matrix and that its expression is cell cycle related. PML expression is altered during human oncogenesis, implying that PML may be an anti-oncogene involved not only in APL but also in other oncogenic events. Mutation analysis of the functional domains of PML demonstrated that its ability to form PML nuclear bodies or PODs (PML oncogenic domains) is essential for suppressing growth and transformation. In light of the above studies it appears that disruption of the normal function of PML plays a critical role in the pathogenesis of APL.

Interferon-modulated expression of genes encoding the nuclear-dot-associated proteins Sp100 and promyelocytic leukemia protein (PML)

Promyelocytic leukemia protein (PML) and Sp100 are transcription-regulatory proteins which colocalize in discrete nuclear dots and play a role in autoimmunity, oncogenesis and virus-host interaction. Interferons (IFNs) were shown previously to increase strongly the levels of Sp100 mRNA and protein. Here, we examined which mechanisms lead to upregulation of Sp100 gene expression and whether IFNs also increase expression of the promyelocytic leukemia (PML) gene. We found that both mRNA and protein levels of PML are also strongly upregulated by IFNs. In addition, new Sp100 and PML proteins were detected immunologically after IFN treatment of cells. Nuclear run-on analysis revealed protein-synthesis-independent, rapid IFN-enhanced transcription rates as well as synergistic activation of the Sp100 and PML genes by type-I and type-II IFNs. These data demonstrate that PML and Sp100 belong to the growing family of IFN-stimulated genes (ISGs) upregulated most likely by the transcription factor ISGF3, and indicate that IFNs also qualitatively alter the expression of these two genes.

Characterization of the rat A2a adenosine receptor gene

To understand the molecular basis for the regulation of rat A2a adenosine receptor (A2a-R) expression, we have characterized the rat A2a-R gene and defined its promoter regions. Through a combination of restriction mapping and sequence analysis, we have demonstrated that the rat A2a-R gene is composed of two exons interrupted by a 7.2-kb intron. Primer extension and RNase protection on RNA isolated from PC12 cells suggested that the A2a-R gene encoded two clusters of alternative transcripts. The most upstream transcription start site was designated as +1. The sequence of the proximal 1.5 kb of 5'-flanking region demonstrated no potential TATA box, CCAAT box, or initiator element in the appropriate location. Varying lengths of 5'-flanking regions were inserted into a transient expression vector (pGL2-basic), which contained bacterial luciferase as the reporter gene, to determine its promoter region(s) in PC12 cells, CHOP cells, and C6 cells. Consistent with two clusters of transcription start sites, two independent functional promoter regions (designated P1, -67/-1; and P2, +272/+304) for the rat A2a-R gene were identified. Although both promoters are in use in PC12 cells, only P2 is active in CHOP cells, indicating possible cell line-specific usage of these two promoters.

Target gene identification: target specific transcriptional activation by three murine homeodomain/VP16 hybrid proteins in Saccharomyces cerevisiae

The mammalian homeodomain proteins encoded by Hox genes play an important role in embryonic development by providing positional queues which define developmental identities along the anteroposterior axis of developing organisms. These proteins bind DNA specifically through their homeodomain to sequences containing ATTA cores, and thereby are thought to exert their effect regulating downstream genes. Little is known about the specificity of binding of homeodomain proteins to their sequences and the identity of their target genes. We have developed a transcriptional activation assay in yeast which employs a homeobox/VP16 fusion gene as a transcriptional activator and a target construct in which test fragments of DNA are inserted upstream to a reporter gene. Using this assay, we compared transcriptional activation by three chimeric proteins containing the homeodomains of the mouse homeobox genes, Hoxa-5, Hoxb-6, and Hoxc-8. When tested on previously defined target sequences, strong differential specificities of activation were observed. In an effort to identify enhancers that normally respond to homeodomain transcriptional activators, random fragments of mouse genomic DNA were cloned upstream of the reporter gene. Genomic DNA fragments with distinct activation profiles were obtained and were found to share matches beyond the ATTA core with previously described enhancers. These results demonstrate that the transcriptional activation system in yeast can be used as a convenient system to detect DNA motifs which bind homeodomain proteins, and subsequently, to identify authentic target genes responsive to Hox gene proteins.

GATA transcription factors associate with a novel class of nuclear bodies in erythroblasts and megakaryocytes

The nuclear distribution of GATA transcription factors in murine haemopoietic cells was examined by indirect immunofluorescence. Specific bright foci of GATA-1 fluorescence were observed in erythroleukaemia cells and primary murine erythroblasts and megakaryocytes, in addition to diffuse nucleoplasmic localization. These foci, which were preferentially found adjacent to nucleoli or at the nuclear periphery, did not represent sites of active transcription or binding of GATA-1 to consensus sites in the beta-globin loci. Immunoelectron microscopy demonstrated the presence of intensely labelled structures likely to represent the GATA-1 foci seen by immunofluorescence. The GATA-1 nuclear bodies differed from previously described nuclear structures and there was no co-localization with nuclear antigens involved in RNA processing or other ubiquitous (Spl, c-Jun and TBP) or haemopoietic (NF-E2) transcription factors. Interestingly, GATA-2 and GATA-3 proteins also localized to the same nuclear bodies in cell lines co-expressing GATA-1 and -2 or GATA-1 and -3 gene products. This pattern of distribution is, thus far, unique to the GATA transcription factors and suggests a protein-protein interaction with other components of the nuclear bodies via the GATA zinc finger domain.

Nuclear domains and the nuclear matrix

This overview describes the spatial distribution of several enzymatic machineries and functions in the interphase nucleus. Three general observations can be made. First, many components of the different nuclear machineries are distributed in the nucleus in a characteristic way for each component. They are often found concentrated in specific domains. Second, nuclear machineries for the synthesis and processing of RNA and DNA are associated with an insoluble nuclear structure, called nuclear matrix. Evidently, handling of DNA and RNA is done by immobilized enzyme systems. Finally, the nucleus seems to be divided in two major compartments. One is occupied by compact chromosomes, the other compartment is the space between the chromosomes. In the latter, transcription takes place at the surface of chromosomal domains and it houses the splicing machinery. The relevance of nuclear organization for efficient gene expression is discussed.

Nuclear domain 10 (ND10) associated proteins are also present in nuclear bodies and redistribute to hundreds of nuclear sites after stress

The promyelocytic leukemia protein fused to the retinoic acid receptor alpha in t(15;17) acute promyelocytic leukemia, the primary biliary cirrhosis autoantigen, Sp100, as well as the incompletely characterized protein NDP55, are co-localized in specific immunohistochemically defined nuclear domains (ND10), which are potential equivalents of ultrastructurally defined nuclear bodies. We investigated whether the distribution of these proteins depends on environmental conditions and whether ND10 correlate with nuclear bodies. Certain nuclear bodies and ND10 react in a similar way and share antigens. Interferon exposure doubled the number of ND10 and increased the frequency of nuclear bodies, whereas herpes simplex virus infection or heat shock modify both. Redistribution of ND10-associated proteins to hundreds of small sites throughout the chromatin was inducible by stress in the form of heat shock and exposure to Cd++ ions. The change of distribution was rapid and independent of protein synthesis, and thus not part of the classical heat shock response. The very rapid redistribution of these proteins after heat shock, together with the development of ND10 upon interferon activation, raises the possibility that ND10 represent storage sites of certain matrix proteins readily accessible throughout the chromatin in response to stress or other effectors that induce global nuclear changes.

Two nuclear dot-associated proteins, PML and Sp100, are often co-autoimmunogenic in patients with primary biliary cirrhosis

The nucleoproteins Sp100 and PML, the first an autoantigen predominant in patients with primary biliary cirrhosis (PBC) and the second a transformation and cell growth suppressing protein aberrantly expressed in promyelocytic leukaemia cells, were recently shown to colocalize in dot-like nuclear domains. Here we analysed whether PML, like Sp100, is also an autoantigen in patients with PBC and other autoimmune diseases, and wether both proteins interact directly. Testing sera from autoimmune patients using an immunoprecipitation assay with radiolabelled PML and an immunofluorescence assay based on a cell line overexpressing PML, autoantibodies (Aabs) against PML were found in the majority o anti-Sp100 Aab positive patients. Only very few patients with PBC or other autoimmune diseases contained anti-PML or anti-Sp100 Aabs exclusively. In contrast to Sp100, immunoreactivity of recombinant PML in immunoblots was only weak and was directed to one region. This suggests that anti-PML Aabs recognize fewer and preferentially conformation-dependent epitopes. In an immunoprecipitation assay using in vitro synthesized Sp100 and PML proteins and Abs to recombinant proteins, no direct interaction was observed. Taken together, these data indicate that Aabs against PML are as highly prevalent and specific for patients with PBC as those against Sp100. The colocalization of these autoantigens and the frequent co-occurrence of the corresponding Aabs might reflect an association of both proteins mediated by one or several other proteins.

Evaluation of the expression and intracellular localization of a 44-kDa calmodulin binding protein during exponential growth and quiescence (G0)

We have previously demonstrated that changes in calmodulin (CaM) levels are associated with G1/S transition of the cell cycle and entry into and release from quiescence (G0). CaM mediates its regulation through the specific interaction with different intracellular proteins called calmodulin binding proteins (CaMBPs). This study was designed to evaluate the expression of the CaMBPs during the cell cycle. Mouse C127 cells were synchronized in quiescence (G0) by serum deprivation. Analysis of the CaMBPs by the 125I-labeled CaM ([125I]CaM) overlay procedure on one- and two-dimensional gels revealed many proteins that bind to CaM at any given time during the cell cycle. However, specific expression of a 44-kiloDalton CaMBP (44CaMBP) was observed. As cells entered quiescence (G0) phase, there was a decrease in the CaM binding to the 44CaMBP. During release into the cell cycle from G0 phase, the binding to CaM was maintained at the low level, but reappeared as the cells entered S phase. CaM binding to the 44CaMBP was intense during S phase and decreased as the cells progressed into G2/M. Antibody directed against the 44CaMBP was produced in rabbit. Quantitation of the 44CaMBP by Western blot analysis revealed a similar pattern to that observed by the [125I]CaM overlay procedure during the course of G0 entry and release. The anti-44CaMBP antibody was used to evaluate the intracellular localization of the 44CaMBP by indirect immunofluorescence. A distinctive punctate nuclear staining, Mwas observed. This punctate nuclear staining, observed in all cells during exponential growth, disappeared as the cells entered G0. The nuclear staining remained absent in cells released from G0 until the cells approached and entered the S phase, at which time the punctate nuclear staining reappeared. This staining pattern was then maintained through G2/M progression. Following M phase and entry into G1 phase, the punctate nuclear staining was observed in all G1 cells. Similar analysis for cells synchronized at the G1/S boundary by the double thymidine block procedure revealed that the punctate nuclear staining was present in all cells throughout the entire course of the cell cycle. The immunofluorescence staining pattern for the 44CaMBP was sensitive to the anti-CaM drug W13 at a dose that is known to reversibly block cells at G1/S. No effect was observed by the inactive analog W12. The punctate nuclear staining of the 44CaMBP would appear to be present during all phases of the cell cycle when cells are committed to be in the cell cycle.(ABSTRACT TRUNCATED AT 400 WORDS)

Generation and characterization of novel antibodies highly selective for phosphorylated linker histone H1 in Tetrahymena and HeLa cells

Phosphorylated forms of Tetrahymena macronuclear histone H1 were separated from each other and from dephosphorylated H1 by cation-exchange HPLC. A homogeneous fraction of hyperphosphorylated macronuclear H1 was then used to generate novel polyclonal antibodies highly selective for phosphorylated H1 in Tetrahymena and in human cells. These antibodies fail to recognize dephosphorylated forms of H1 in both organisms and are not reactive with most other nuclear or cytoplasmic phosphoproteins including those induced during mitosis. The selectivity of these antibodies for phosphorylated forms of H1 in Tetrahymena and in HeLa argues strongly that these antibodies recognize highly conserved phosphorylated epitopes found in most H1s and from this standpoint Tetrahymena H1 is not atypical. Using these antibodies in indirect immunofluorescence analyses, we find that a significant fraction of interphase mammalian cells display a strikingly punctate pattern of nuclear fluorescence. As cells enter S-phase, nuclear staining becomes more diffuse, increases significantly, and continues to increase as cells enter mitosis. As cells exit from mitosis, staining with the anti-phosphorylated H1 antibodies is rapidly lost presumably owing to the dephosphorylation of H1. These immunofluorescent data document precisely the cell cycle changes in the level of H1 phosphorylation determined by earlier biochemical studies and suggest that these antibodies represent a powerful new tool to probe the function(s) of H1 phosphorylation in a wide variety of eukaryotic systems.

A novel macromolecular structure is a target of the promyelocyte-retinoic acid receptor oncoprotein

Acute promyelocytic leukemia (APL) is associated with a t(15;17) translocation that creates the promyelocyte-retinoic acid receptor alpha (PML-RAR alpha) fusion gene. Immunohistochemistry demonstrates that PML is a part of a novel macromolecular organelle (including at least three other nuclear proteins) referred to as PML oncogenic domains (PODs). In APL cells, the POD is disrupted into a microparticulate pattern as a consequence of the expression of the PML-RAR oncoprotein. RA treatment of APL cells triggers a reorganization of PML to generate normal-appearing PODs. We propose that PML-RAR is a dominant negative oncoprotein that exerts its putative leukomogenic effect by inhibiting assembly of the POD. According to this proposal, not only is the POD a novel structure, but it can be ascribed an imputed function such that its disruption leads to altered myeloid maturation; this may represent a novel oncogenic target.