Nuclear dot antigens may specify transcriptional domains in the nucleus

PML Body Component Sp100A Restricts Wild-Type Herpes Simplex Virus 1 Infection

Sp100 (speckled protein 100 kDa) is a constituent component of nuclear structure PML (promyelocytic leukemia) bodies, playing important roles in mediating intrinsic and innate immunity. The Sp100 gene encodes four isoforms with distinct roles in the transcriptional regulation of both cellular and viral genes. Since Sp100 is a primary intranuclear target of infected-cell protein 0 (ICP0), an immediate early E3 ligase encoded by herpes simplex virus 1 (HSV-1), previous investigations attempting to analyze the functions of individual Sp100 variants during HSV-1 infection mostly avoided using a wild-type virus. Therefore, the role of Sp100 under natural infection by HSV-1 remains to be clarified. Here, we reappraised the antiviral capacity of four Sp100 isoforms during infection by a nonmutated HSV-1, examined the molecular behavior of the Sp100 protein in detail, and revealed the following intriguing observations. First, Sp100 isoform A (Sp100A) inhibited wild-type HSV-1 propagation in HEp-2, Sp100-/-, and PML-/- cells. Second, endogenous Sp100 is located in both the nucleus and the cytoplasm. During HSV-1 infection, the nuclear Sp100 level decreased drastically upon the detection of ICP0 in the same subcellular compartment, but cytosolic Sp100 remained stable. Third, transfected Sp100A showed subcellular localizations similar to those of endogenous Sp100 and matched the protein size of endogenous cytosolic Sp100. Fourth, HSV-1 infection induced increased secretion of endogenous Sp100 and ectopically expressed Sp100A, which copurified with extracellular vesicles (EVs) but not infectious virions. Fifth, the Sp100A level in secreting cells positively correlated with its level in EVs, and EV-associated Sp100A restricted HSV-1 in recipient cells. IMPORTANCE Previous studies show that the PML body component Sp100 protein is immediately targeted by ICP0 of HSV-1 in the nucleus during productive infection. Therefore, extensive studies investigating the interplay of Sp100 isoforms with HSV-1 were conducted using a mutant virus lacking ICP0 or in the absence of infection. The role of Sp100 variants during natural HSV-1 infection remains blurry. Here, we report that Sp100A potently and independently inhibited wild-type HSV-1 and that during HSV-1 infection, cytosolic Sp100 remained stable and was increasingly secreted into the extracellular space, in association with EVs. Furthermore, the Sp100A level in secreting cells positively correlated with its level in EVs and the anti-HSV-1 potency of these EVs in recipient cells. In summary, this study implies an active antiviral role of Sp100A during wild-type HSV-1 infection and reveals a novel mechanism of Sp100A to restrict HSV-1 through extracellular communications.

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.

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.

Sp100 isoform-specific regulation of human adenovirus 5 gene expression

Promyelocytic leukemia nuclear bodies (PML-NBs) are nuclear structures that accumulate intrinsic host factors to restrict viral infections. To ensure viral replication, these must be limited by expression of viral early regulatory proteins that functionally inhibit PML-NB-associated antiviral effects. To benefit from the activating capabilities of Sp100A and simultaneously limit repression by Sp100B, -C, and -HMG, adenoviruses (Ads) employ several features to selectively and individually target these isoforms. Ads induce relocalization of Sp100B, -C, and -HMG from PML-NBs prior to association with viral replication centers. In contrast, Sp100A is kept at the PML tracks that surround the newly formed viral replication centers as designated sites of active transcription. We concluded that the host restriction factors Sp100B, -C, and -HMG are potentially inactivated by active displacement from these sites, whereas Sp100A is retained to amplify Ad gene expression. Ad-dependent loss of Sp100 SUMOylation is another crucial part of the virus repertoire to counteract intrinsic immunity by circumventing Sp100 association with HP1, therefore limiting chromatin condensation. We provide evidence that Ad selectively counteracts antiviral responses and, at the same time, benefits from PML-NB-associated components which support viral gene expression by actively recruiting them to PML track-like structures. Our findings provide insights into novel strategies for manipulating transcriptional regulation to either inactivate or amplify viral gene expression.

IMPORTANCE

We describe an adenoviral evasion strategy that involves isoform-specific and active manipulation of the PML-associated restriction factor Sp100. Recently, we reported that the adenoviral transactivator E1A targets PML-II to efficiently activate viral transcription. In contrast, the PML-associated proteins Daxx and ATRX are inhibited by early viral factors. We show that this concept is more intricate and significant than originally believed, since adenoviruses apparently take advantage of specific PML-NB-associated proteins and simultaneously inhibit antiviral measures to maintain the viral infectious program. Specifically, we observed Ad-induced relocalization of the Sp100 isoforms B, C, and HMG from PML-NBs juxtaposed with viral replication centers. In contrast, Sp100A is retained at Ad-induced PML tracks that surround the newly formed viral replication centers, acting as designated sites of active transcription. The host restriction factors Sp100B, -C, and -HMG are potentially inactivated by active displacement from these sites, whereas Sp100A is retained to amplify Ad gene expression.

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.

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.

Poly(A)-binding protein acts in translation termination via eukaryotic release factor 3 interaction and does not influence [PSI(+)] propagation

Recent studies of translational control suggest that translation termination may not be simply the end of synthesizing a protein but rather be involved in modulating both the translation efficiency and stability of a given transcript. Using recombinant eukaryotic release factor 3 (eRF3) and cellular extracts, we have shown for Saccharomyces cerevisiae that yeast eRF3 and Pab1p can interact. This interaction, mediated by the N+M domain of eRF3 and amino acids 473 to 577 of Pab1p, was demonstrated to be direct by the two-hybrid approach. We confirmed that a genetic interaction exists between eRF3 and Pab1p and showed that Pab1p overexpression enhances the efficiency of termination in SUP35 (eRF3) mutant and [PSI(+)] cells. This effect requires the interaction of Pab1p with eRF3. These data further strengthen the possibility that Pab1p has a role in coupling translation termination events with initiation of translation. Several lines of evidence indicate that Pab1p does not influence [PSI(+)] propagation. First, "[PSI(+)]-no-more" mutations do not affect eRF3-Pab1p two-hybrid interaction. Second, overexpression of PAB1 does not cure the [PSI(+)] phenotype or solubilize detectable amounts of eRF3. Third, prion-curing properties of overexpressed HSP104p, which is required for formation and maintenance of [PSI(+)], were not modified by excess Pab1p.

Sp100 interacts with ETS-1 and stimulates its transcriptional activity

The cell nucleus is highly organized into distinct domains that spatially separate physiological processes. One of these domains, the Sp100-promyelocytic leukemia protein nuclear body (NB), is implicated in pathological processes, such as cancer and viral infection, yet its functions remain poorly understood. We show here that Sp100 interacts physically and functionally with ETS-1 and that NB morphology is affected by ETS-1. ETS-1 is a member of the ets family of transcription factors, which are key mediators of physiological and pathological processes. We have found that Sp100 interacts with two regions of ETS-1 (domains A+B and D+E+F). ETS-1 alters NBs while remaining localized throughout the nucleus, apparently by recruitment of the core component Sp100 away from the NBs. Sp100 strongly increases ETS-1 activation of natural and ets-focused promoters, through a mechanism involving the activation (C) domain of ETS-1 in addition to the interaction domains. Sp100 acts as a novel coactivator that potentiates the activator function of ETS-1. Our results provide an important new connection between nuclear structures and an important regulator of gene expression.

Anti-multiple nuclear dots (anti-MND) and anti-SP100 antibodies in hepatic and rheumatological disorders

Multiple nuclear dots pattern has been described in primary biliary cirrhosis and, less often, in rheumatological disorders. Sp100 is the major antigen of multiple nuclear dots. We evaluated prevalence and diagnostic significance of multiple nuclear dots and anti-Sp100 reactivity both in hepatic and rheumatological diseases. A series of 283 consecutive liver patients (89 primary biliary cirrhosis, 12 primary sclerosing cholangitis, 85 autoimmune hepatitis, 97 hepatitis C virus-related chronic liver disease) and of 89 consecutive rheumatological cases were evaluated. Presence of multiple nuclear dots was assessed by indirect immunofluorescence on HEp-2 cells, anti-Sp100 reactivity by ELISA with recombinant protein. Multiple nuclear dots were detected in 20 patients (7%) with liver disease (of whom 15 with primary biliary cirrhosis), and in eight patients (9%) with rheumatological disorders. Anti-Sp100 was detected in 45 liver patients (16%), of whom 30 with primary biliary cirrhosis, but in only two with rheumatological disorders (2%) (P =0.0004). The concordance between multiple nuclear dots and anti-Sp100 in liver and rheumatological patients was 90% and 25% (P=0.0018), respectively. Among 89 consecutive patients with primary biliary cirrhosis, multiple nuclear dots and anti-Sp100 were present in 17% and 34%, respectively (P=0.0152). Anti-Sp100 positivity was associated with older age and higher gamma-globulin levels. Multiple nuclear dots are similarly observed in liver and rheumatological patients. In contrast, anti-Sp100 is more frequent in liver patients and is significantly more often detected in primary biliary cirrhosis, of which it can be regarded as a highly specific serological marker. The antigenic target of multiple nuclear dots in most rheumatological patients is other than Sp100.

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.

Upc2p and Ecm22p, dual regulators of sterol biosynthesis in Saccharomyces cerevisiae

Sterol levels affect the expression of many genes in yeast and humans. We found that the paralogous transcription factors Upc2p and Ecm22p of yeast were sterol regulatory element (SRE) binding proteins (SREBPs) responsible for regulating transcription of the sterol biosynthetic genes ERG2 and ERG3. We defined a 7-bp SRE common to these and other genes, including many genes involved in sterol biosynthesis. Upc2p and Ecm22p activated ERG2 expression by binding directly to this element in the ERG2 promoter. Upc2p and Ecm22p may thereby coordinately regulate genes involved in sterol homeostasis in yeast. Ecm22p and Upc2p are members of the fungus-specific Zn[2]-Cys[6] binuclear cluster family of transcription factors and share no homology to the analogous proteins, SREBPs, that are responsible for transcriptional regulation by sterols in humans. These results suggest that Saccharomyces cerevisiae and human cells regulate sterol synthesis by different mechanisms.

Common properties of nuclear body protein SP100 and TIF1alpha chromatin factor: role of SUMO modification

The SP100 protein, together with PML, represents a major constituent of the PML-SP100 nuclear bodies (NBs). The function of these ubiquitous subnuclear structures, whose integrity is compromised in pathological situations such as acute promyelocytic leukemia (APL) or DNA virus infection, remains poorly understood. There is little evidence for the occurrence of actual physiological processes within NBs. The two NB proteins PML and SP100 are covalently modified by the ubiquitin-related SUMO-1 modifier, and recent work indicates that this modification is critical for the regulation of NB dynamics. In exploring the functional relationships between NBs and chromatin, we have shown previously that SP100 interacts with members of the HP1 family of nonhistone chromosomal proteins and that a variant SP100 cDNA encodes a high-mobility group (HMG1/2) protein. Here we report the isolation of a further cDNA, encoding the SP100C protein, that contains the PHD-bromodomain motif characteristic of chromatin proteins. We further show that TIF1alpha, a chromatin-associated factor with homology to both PML and SP100C, is also modified by SUMO-1. Finally, in vitro experiments indicate that SUMO modification of SP100 enhances the stability of SP100-HP1 complexes. Taken together, our results suggest an association of SP100 and its variants with the chromatin compartment and, further, indicate that SUMO modification may play a regulatory role in the functional interplay between the nuclear bodies and chromatin.

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.

Interactions of herpes simplex virus type 1 with ND10 and recruitment of PML to replication compartments

Many of the events required for productive herpes simplex virus type 1 (HSV-1) infection occur within globular nuclear domains called replication compartments, whose formation appears to depend on interactions with cellular nuclear domains 10 (ND10). We have previously demonstrated that the formation of HSV-1 replication compartments involves progression through several stages, including the disruption of intact ND10 (stage I to stage II) and the formation of PML-associated prereplicative sites (stage III) and replication compartments (stage IV) (J. Burkham, D. M. Coen, and S. K. Weller, J. Virol. 72:10100-10107, 1998). In this paper, we show that some, but not all, PML isoforms are recruited to stage III foci and replication compartments. Genetic experiments showed that the recruitment of PML isoforms to stage III prereplicative sites and replication compartments requires the localization of the HSV-1 polymerase protein (UL30) to these foci but does not require polymerase catalytic activity. We also examined the stages of viral infection under conditions affecting ND10 integrity. Treatment with factors that increase the stability of ND10, arsenic trioxide and the proteasome inhibitor MG132, inhibited viral disruption of ND10, formation of replication compartments, and production of progeny virus. These results strengthen the previously described correlation between ND10 disruption and productive viral infection.

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.

Two new members of the emerging KDWK family of combinatorial transcription modulators bind as a heterodimer to flexibly spaced PuCGPy half-sites

Initially recognized as a HeLa factor essential for parvovirus DNA replication, parvovirus initiation factor (PIF) is a site-specific DNA-binding complex consisting of p96 and p79 subunits. We have cloned and sequenced the human cDNAs encoding each subunit and characterized their products expressed from recombinant baculoviruses. The p96 and p79 polypeptides have 40% amino acid identity, focused particularly within a 94-residue region containing the sequence KDWK. This motif, first described for the Drosophila homeobox activator DEAF-1, identifies an emerging group of metazoan transcriptional modulators. During viral replication, PIF critically regulates the viral nickase, but in the host cell it probably modulates transcription, since each subunit is active in promoter activation assays and the complex binds to previously described regulatory elements in the tyrosine aminotransferase and transferrin receptor promoters. Within its recognition site, PIF binds coordinately to two copies of the tetranucleotide PuCGPy, which, remarkably, can be spaced from 1 to 15 nucleotides apart, a novel flexibility that we suggest may be characteristic of the KDWK family. Such tetranucleotides are common in promoter regions, particularly in activating transcription factor/cyclic AMP response element-binding protein (ATF/CREB) and E-box motifs, suggesting that PIF may modulate the transcription of many genes.

Structural and functional heterogeneity of nuclear bodies

The nuclear body is a cellular structure that appears to be involved in the pathogenesis of acute promyelocytic leukemia and viral infection. In addition, the nuclear body is a target of autoantibodies in patients with the autoimmune disease primary biliary cirrhosis. Although the precise function of the nuclear body in normal cellular biology is unknown, this structure may have a role in the regulation of gene transcription. In a previous investigation, we identified a leukocyte-specific, gamma interferon (IFN-gamma)-inducible autoantigen designated Sp140. The objectives of the present study were to investigate the cellular location of Sp140 with respect to the nuclear-body components PML and Sp100 and to examine the potential role of Sp140 in the regulation of gene transcription. We used adenovirus-mediated gene transfer to express Sp140 in human cells and observed that the protein colocalized with PML and Sp100 in resting cells and associated with structures containing PML during mitosis. In cells infected with the adenovirus expressing Sp140 and incubated with IFN-gamma, the number of PML-Sp100 nuclear bodies per cell increased but immunoreactive Sp140 was not evenly distributed among the nuclear bodies. Sp140 associated with a subset of IFN-gamma-induced PML-Sp100 nuclear bodies. To examine the potential effect of Sp140 on gene transcription, a plasmid encoding Sp140 fused to the DNA-binding domain of GAL4 was cotransfected into COS cells with a chloramphenicol acetyltransferase (CAT) reporter gene containing five GAL4-binding sites and a simian virus 40 enhancer region. The GAL4-Sp140 fusion protein increased the expression of the reporter gene. In contrast, Sp100 fused to the GAL4 DNA-binding domain inhibited CAT activity in transfected mammalian cells. The results of this study demonstrate that Sp140 associates with a subset of PML-Sp100 nuclear bodies in IFN-gamma-treated cells and that Sp140 may activate gene transcription. Taken together, these observations suggest that the nuclear bodies within a cell may be heterogeneous with respect to both composition and function.

ND10 protein PML is recruited to herpes simplex virus type 1 prereplicative sites and replication compartments in the presence of viral DNA polymerase

Herpes simplex virus type 1 (HSV-1) infection results in the disruption of ND10 (also called nuclear bodies, PODs, or PML-associated bodies), which are nuclear matrix domains of unknown function present in mammalian cells. After ND10 disruption, viral transcription and DNA replication occur in globular nuclear domains called replication compartments. In this report we define four stages of infection by using antibodies to ICP8 (also called SSB and UL29) and the ND10 antigen PML. Immediately after infection, cells contain intact ND10 as detected by staining for PMLs (stage I); within 1 hour, however, ND10 are disrupted and cells begin to exhibit diffuse staining for the major viral DNA binding protein, ICP8 (stage II). After all ND10 have been disrupted, foci which resemble but are not equivalent to ND10 appear, containing both PML and ICP8 (stage III). Cells infected with mutants defective in the helicase-primase or origin binding protein are unable to form stage III foci. Cells infected with a mutant that is null for the polymerase catalytic subunit, however, form stage III-like ICP8 foci which do not contain PML. Thus, stage III foci recruit the cellular PML protein in the presence but not the absence of HSV polymerase. PML was recruited to stage III foci in some but not all cells infected with a mutant defective in the polymerase accessory protein, UL42. Thus, UL42 is not required for the recruitment of PML to viral foci. In wild-type infection, stage III cells are quickly replaced by cells containing replication compartments (stage IV). PML and ICP8 staining are both observed within replication compartments, indicating a potential role for PML in HSV-1 replication. Models for the role of ND10 proteins in the formation of replication compartments are discussed.

Spa2p interacts with cell polarity proteins and signaling components involved in yeast cell morphogenesis

The yeast protein Spa2p localizes to growth sites and is important for polarized morphogenesis during budding, mating, and pseudohyphal growth. To better understand the role of Spa2p in polarized growth, we analyzed regions of the protein important for its function and proteins that interact with Spa2p. Spa2p interacts with Pea2p and Bud6p (Aip3p) as determined by the two-hybrid system; all of these proteins exhibit similar localization patterns, and spa2Delta, pea2Delta, and bud6Delta mutants display similar phenotypes, suggesting that these three proteins are involved in the same biological processes. Coimmunoprecipitation experiments demonstrate that Spa2p and Pea2p are tightly associated with each other in vivo. Velocity sedimentation experiments suggest that a significant portion of Spa2p, Pea2p, and Bud6p cosediment, raising the possibility that these proteins form a large, 12S multiprotein complex. Bud6p has been shown previously to interact with actin, suggesting that the 12S complex functions to regulate the actin cytoskeleton. Deletion analysis revealed that multiple regions of Spa2p are involved in its localization to growth sites. One of the regions involved in Spa2p stability and localization interacts with Pea2p; this region contains a conserved domain, SHD-II. Although a portion of Spa2p is sufficient for localization of itself and Pea2p to growth sites, only the full-length protein is capable of complementing spa2 mutant defects, suggesting that other regions are required for Spa2p function. By using the two-hybrid system, Spa2p and Bud6p were also found to interact with components of two mitogen-activated protein kinase (MAPK) pathways important for polarized cell growth. Spa2p interacts with Ste11p (MAPK kinase [MEK] kinase) and Ste7p (MEK) of the mating signaling pathway as well as with the MEKs Mkk1p and Mkk2p of the Slt2p (Mpk1p) MAPK pathway; for both Mkk1p and Ste7p, the Spa2p-interacting region was mapped to the N-terminal putative regulatory domain. Bud6p interacts with Ste11p. The MEK-interacting region of Spa2p corresponds to the highly conserved SHD-I domain, which is shown to be important for mating and MAPK signaling. spa2 mutants exhibit reduced levels of pheromone signaling and an elevated level of Slt2p kinase activity. We thus propose that Spa2p, Pea2p, and Bud6p function together, perhaps as a complex, to promote polarized morphogenesis through regulation of the actin cytoskeleton and signaling pathways.

Interaction of SP100 with HP1 proteins: a link between the promyelocytic leukemia-associated nuclear bodies and the chromatin compartment

The PML/SP100 nuclear bodies (NBs) were first described as discrete subnuclear structures containing the SP100 protein. Subsequently, they were shown to contain the PML protein which is part of the oncogenic PML-RARalpha hybrid produced by the t(15;17) chromosomal translocation characteristic of acute promyelocytic leukemia. Yet, the physiological role of these nuclear bodies remains unknown. Here, we show that SP100 binds to members of the heterochromatin protein 1 (HP1) families of non-histone chromosomal proteins. Further, we demonstrate that a naturally occurring splice variant of SP100, here called SP100-HMG, is a member of the high mobility group-1 (HMG-1) protein family and may thus possess DNA-binding potential. Both HP1 and SP100-HMG concentrate in the PML/SP100 NBs, and overexpression of SP100 leads to enhanced accumulation of endogenous HP1 in these structures. When bound to a promoter, SP100, SP100-HMG and HP1 behave as transcriptional repressors in transfected mammalian cells. These observations present molecular evidence for an association between the PML/SP100 NBs and the chromatin nuclear compartment. They support a model in which the NBs may play a role in certain aspects of chromatin dynamics.

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.

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.

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.

Arsenic-induced PML targeting onto nuclear bodies: implications for the treatment of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is associated with the t(15;17) translocation, which generates a PML/RAR alpha fusion protein between PML, a growth suppressor localized on nuclear matrix-associated bodies, and RAR alpha, a nuclear receptor for retinoic acid (RA). PML/RAR alpha was proposed to block myeloid differentiation through inhibition of nuclear receptor response, as does a dominant negative RAR alpha mutant. In addition, in APL cells, PML/RAR alpha displaces PML and other nuclear body (NB) antigens onto nuclear microspeckles, likely resulting in the loss of PML and/or NB functions. RA leads to clinical remissions through induction of terminal differentiation, for which the respective contributions of RAR alpha (or PML/RAR alpha) activation, PML/RAR alpha degradation, and restoration of NB antigens localization are poorly determined. Arsenic trioxide also leads to remissions in APL patients, presumably through induction of apoptosis. We demonstrate that in non-APL cells, arsenic recruits the nucleoplasmic form of several NB antigens onto NB, but induces the degradation of PML only, identifying a powerful tool to approach NB function. In APL cells, arsenic targets PML and PML/RAR alpha onto NB and induces their degradation. Thus, RA and arsenic target RAR alpha and PML, respectively, but both induce the degradation of the PML/RAR alpha fusion protein, which should contribute to their therapeutic effects. The difference in the cellular events triggered by these two agents likely stems from RA-induced transcriptional activation and arsenic effects on NB proteins.

Prolonged gene expression and cell survival after infection by a herpes simplex virus mutant defective in the immediate-early genes encoding ICP4, ICP27, and ICP22

Very early in infection, herpes simplex virus (HSV) expresses four immediate-early (IE) regulatory proteins, ICP4, ICP0, ICP22, and ICP27. The systematic inactivation of sets of the IE proteins in cis, and the subsequent phenotypic analysis of the resulting mutants, should provide insights into how these proteins function in the HSV life cycle and also into the specific macromolecular events that are altered or perturbed in cells infected with virus strains blocked very early in infection. This approach may also provide a rational basis to assess the efficacy and safety of HSV mutants for use in gene transfer experiments. In this study, we generated and examined the phenotype of an HSV mutant simultaneously mutated in the ICP4, ICP27, and ICP22 genes of HSV. Unlike mutants deficient in ICP4 (d120), ICP4 and ICP27 (d92), and ICP4 and ICP22 (d96), mutants defective in ICP4, ICP27, and ICP22 (d95) were visually much less toxic to Vero and human embryonic lung cells. Cells infected with d95 at a multiplicity of infection of 10 PFU per cell retained a relatively normal morphology and expressed genes from the viral and cellular genomes for at least 3 days postinfection. The other mutant backgrounds were too toxic to allow examination of gene expression past 1 day postinfection. However, when cell survival was measured by the capacity of the infected cells to form colonies, d95 inhibited colony formation similarly to d92. This apparent paradox was reconciled by the observation that host cell DNA synthesis was inhibited in cells infected with d120, d92, d96, and d95. In addition, all of the mutants exhibited pronounced and distinctive alterations in nuclear morphology, as determined by electron microscopy. The appearance of d95-infected cells deviated from that of uninfected cells in that large circular structures formed in the nucleus. d95-infected cells abundantly expressed ICP0, which accumulated in fine punctate structures in the nucleus at early times postinfection and coalesced or grew to the large circular objects that were revealed by electron microscopy. Therefore, while the abundant accumulation of ICPO in the absence of ICP4, ICP22, and ICP27 may allow for prolonged gene expression, cell survival is impaired, in part, as a result of the inhibition of cellular DNA synthesis.

A highly amplified mouse gene is homologous to the human interferon-responsive Sp100 gene encoding an autoantigen associated with nuclear dots

In human cells, three proteins are currently known to colocalize in di screte nuclear domains (designated nuclear dots): Sp100, a transcription-activating protein autoantigenic primarily in patients with primary biliary cirrhosis; PML, a tumor suppressor protein involved in development of acute promyelocytic leukemia; and NDP52, a protein of unknown function. Here we report sequence similarities between the Sp100 protein and a putative protein encoded by a highly amplified mouse gene which is visible as an inherited homogeneously staining region (HSR) on chromosome 1 of some mouse populations. By in situ hybridization, the Sp100 gene was mapped to locus 2q37, the syntenic region of the HSR on mouse chromosome 1. Unlike the highly amplified mouse gene, Sp100 was found to be a single-copy gene and showed no restriction fragment length polymorphisms. Sequence similarities in the promoter regions and similar exon-intron organizations of the two genes were revealed. As for Sp100, steady-state levels of the mRNAs of the HSR-encoded genes could be greatly increased by interferon (IFN) treatment. As in human cells, IFN treatment led to an enlargement in both size and number of nuclear dots in mouse cells as visualized by immunofluorescence staining with autoimmune sera from patients with primary biliary cirrhosis. These data indicate that a gene located in the inherited HSR of mice, designated mSp100, is homologous to the human Sp100 gene, has a similar gene organization, and responds similarly to IFN treatment.

Targeting of adenovirus E1A and E4-ORF3 proteins to nuclear matrix-associated PML bodies

The PML protein was first identified as part of a fusion product with the retinoic acid receptor alpha (RAR alpha), resulting from the t(15;17) chromosomal translocation associated with acute promyelocytic leukemia (APL). It has been previously demonstrated that PML, which is tightly bound to the nuclear matrix, concentrates in discrete subnuclear compartments that are disorganized in APL cells due to the expression of the PML-RAR alpha hybrid. Here we report that adenovirus infection causes a drastic redistribution of PML from spherical nuclear bodies into fibrous structures. The product encoded by adenovirus E4-ORF3 is shown to be responsible for this reorganization and to colocalize with PML into these fibers. In addition, we demonstrate that E1A oncoproteins concentrate in the PML domains, both in infected and transiently transfected cells, and that this association requires the conserved amino acid motif (D)LXCXE, common to all viral oncoproteins that bind pRB or the related p107 and p130 proteins. The SV-40 large T antigen, another member of this oncoprotein family is also found in close association with the PML nuclear bodies. Taken together, the present data indicate that the subnuclear domains containing PML represent a preferential target for DNA tumor viruses, and therefore suggest a more general involvement of the PML nuclear bodies in oncogenic processes.

Molecular characterization of NDP52, a novel protein of the nuclear domain 10, which is redistributed upon virus infection and interferon treatment

The nuclear domain (ND)10 also described as POD or Kr bodies is involved in the development of acute promyelocytic leukemia and virus-host interactions. Immunofluorescence analysis using a variety of human autoimmune sera and monoclonal antibodies showed a typical dot like nuclear staining for ND10, suggesting that this structure consists of several proteins. Two of the ND10 proteins, Sp100 and PML are genetically characterized and show homology with several transcription factors. Here we describe NDP52, an additional novel protein of the ND10. We raised a new mAb C8A2, that specifically recognizes NDP52. Immunofluorescence analysis using this mAb showed a typical nuclear dot staining as it was described for ND10. Isolation and sequencing of the corresponding cDNA revealed that NDP52 has a predicted molecular mass of 52 kD. The deduced amino acid sequence exhibits an extended central coiled coil domain containing a leucine zipper motif. The COOH terminus of NDP52 shows homology with LIM domains, that have recently been described to mediate protein interactions, which let NDP52 appear as a suitable candidate for mediating interactions between ND10 proteins. In vivo, NDP52 is transcribed in all human tissues analyzed. Furthermore, we show that NDP52 colocalizes with the ND10 protein PML and can be redistributed upon viral infection and interferon treatment. These data suggest that ND10 proteins play an important role in the viral life cycle.

Two short autoepitopes on the nuclear dot antigen are similar to epitopes encoded by the Epstein-Barr virus

To understand the relationship between antibodies present in patients with anti-nuclear dot (ND) autoimmune disease and the proteins they recognize, epitopes that react with the autoantibodies were mapped. A panel of fusion proteins containing different portions of the ND protein were overproduced in Escherichia coli. Immunoblot analysis with anti-ND antibodies revealed that most (10 of 12) sera recognize two major autoepitopes that are each a maximum of 8 amino acids long. The other two sera recognize one of the two epitopes. In addition to the short linear autoepitopes, a conformational epitope appears to be present on the ND antigen. Each of the two linear epitope sequences shares sequence similarities with those of several viral proteins found in the databases. Furthermore, two fusion proteins containing short Epstein-Barr virus (EBV) protein sequences that are similar to the ND epitopes were recognized by the human autoimmune sera, indicating that the autoepitopes are present in EBV protein sequences. Our results are consistent with the hypothesis that ND autoimmune disease might be associated with EBV infections.

Evaluation of colocalization interactions between the IE110, IE175, and IE63 transactivator proteins of herpes simplex virus within subcellular punctate structures

A number of previous studies have implied that three herpes simplex virus-encoded nuclear transactivator proteins, IE175 (ICP4), IE110 (ICP0), and IE63 (ICP27), may cooperate in transcriptional and posttranscriptional stimulation of viral gene expression. Using double-label immunofluorescence assays (IFA) in transient expression assays, we have examined the intracellular localization of these three proteins in DNA-transfected cells. The IE110 protein on its own forms spherical punctate domains within the nucleus, whereas the IE175 and IE63 proteins alone give uniform and speckled diffuse patterns, respectively. In infected cells, the IE110 punctate granules have been shown to correspond to novel preexisting subnuclear structures referred to as ND10 domains or PODs that contain a variety of cellular proteins, including SP100 and the PML proto-oncogene product. Cotransfection experiments with wild-type nuclear forms of both IE175 and IE110 provided direct evidence for partial redistribution of IE175 into the same punctate granules that contained IE110. Surprisingly, nuclear forms of IE110 were found to move a cytoplasmic form of IE175 into nuclear punctate structures, and a cytoplasmic form of IE110 was able to retain nuclear forms of IE175 in cytoplasmic punctate structures. Therefore, the punctate characteristic of IE110 appeared to both dominate the interactions and override the normal nuclear localization signals. The domains responsible for the interaction mapped to between codons 518 and 768 in 1E110 and to between codons 835 and 1029 in IE175. Importantly, a truncated nuclear form of the 1,298-amino-acid IE175 protein, which lacked the C-terminal domain beyond codon 834, was found to be excluded from the IE110 punctate granules. Cotransfection of nuclear or cytoplasmic IE110 with a truncated nuclear form of IE63 also led to partial redistribution of IE63 into either nuclear or cytoplasmic punctate granules containing IE110. Both the IE63-IE110 and IE175-IE110 colocalization interactions were demonstrated in Vero cells but not in 293 cells. Consequently, they differ from IE110 self-interactions, which correlate with in vitro dimerization and occur efficiently in both cell types. These interactions may help to explain the altered promoter target specificity and synergism observed when IE175 is cotransfected with IE110 in transactivation studies.

Association of nuclear matrix antigens with exon-containing splicing complexes

mAbs raised against the human nuclear matrix (anti-NM)1 mAbs have been used to investigate the role of nuclear matrix antigens in pre-mRNA processing. The three anti-NM mAbs used in this study recognize antigens that are highly localized to nuclear matrix speckles. Surprisingly, all three of these mAbs preferentially immunoprecipitate splicing complexes containing exon sequences. The anti-NM mAbs efficiently immunoprecipitate the exon product complex but not complexes containing the lariat product after the second step of splicing. Two of the anti-NM mAbs completely inhibit pre-mRNA splicing in vitro. However, none of the anti-NM mAbs appear to recognize factors stably associated with splicing snRNPs. The three anti-NM mAbs predominantly react with distinct high molecular weight antigens, which belong to a class of nuclear proteins that selectively precipitate with Ser-Arg protein-splicing factors in the presence of high Mg2+ concentrations. Immunological, biochemical, and cell biological data indicate that two of the NM antigens are related to the defined set of Ser-Arg proteins. The results suggest the existence of an extended Ser-Arg family as a component of the nuclear matrix.

HSV-1 IE protein Vmw110 causes redistribution of PML

Herpes simplex virus immediate-early protein Vmw110 is required for fully efficient viral gene expression and reactivation from latency. At early times of viral infection, Vmw110 localizes to discrete nuclear structures (known as ND10, PODs or Kr bodies) which contain several cellular proteins, including PML. Interestingly, the unregulated growth of promyelocytic leukaemia cells is correlated with disruption of the normal state of ND10. In this paper we show that: (i) Vmw110 affects the distribution of PML in the cell; (ii) Vmw110 proteins lacking a functional RING finger zinc-binding domain cause the production of striking abnormal cytoplasmic and nuclear structures, some of which contain PML and other ND10 antigens; (iii) a mutant form of Vmw110 which is confined to the cytoplasm appears to result in cytoplasmic PML in some cells; (iv) normal interaction with the nuclear structures requires the C-terminal portion of Vmw110; (v) the C-terminal portion of Vmw110, when linked to a heterologous protein, disrupts the normal distribution of PML. The results suggest that, in normal cells, the PML protein migrates between nucleus and cytoplasm. These observations present an unexpected link between processes involved in the control of cell growth and viral infection and latency.

NHP6A and NHP6B, which encode HMG1-like proteins, are candidates for downstream components of the yeast SLT2 mitogen-activated protein kinase pathway

The yeast SLK1 (BCK1) gene encodes a mitogen-activated protein kinase (MAPK) activator protein which functions upstream in a protein kinase cascade that converges on the MAPK Slt2p (Mpk1p). Dominant alleles of SLK1 have been shown to bypass the conditional lethality of a protein kinase C mutation, pkc1-delta, suggesting that Pkc1p may regulate Slk1p function. Slk1p has an important role in morphogenesis and growth control, and deletions of the SLK1 gene are lethal in a spa2-delta mutant background. To search for genes that interact with the SLK1-SLT2 pathway, a synthetic lethal suppression screen was carried out. Genes which in multiple copies suppress the synthetic lethality of slk1-1 spa2-delta were identified, and one, the NHP6A gene, has been extensively characterized. The NHP6A gene and the closely related NHP6B gene were shown previously to encode HMG1-like chromatin-associated proteins. We demonstrate here that these genes are functionally redundant and that multiple copies of either NHP6A or NHP6B suppress slk1-delta and slt2-delta. Strains from which both NHP6 genes were deleted (nhp6-delta mutants) share many phenotypes with pkc1-delta, slk1-delta, and slt2-delta mutants. nhp6-delta cells display a temperature-sensitive growth defect that is rescued by the addition of 1 M sorbitol to the medium, and they are sensitive to starvation. nhp6-delta strains also exhibit a variety of morphological and cytoskeletal defects. At the restrictive temperature for growth, nhp6-delta mutant cells contain elongated buds and enlarged necks. Many cells have patches of chitin staining on their cell surfaces, and chitin deposition is enhanced at the necks of budded cells. nhp6-delta cells display a defect in actin polarity and often accumulate large actin chunks. Genetic and phenotypic analysis indicates that NHP6A and NHP6B function downstream of SLT2. Our results indicate that the Slt2p MAPK pathway in Saccharomyces cerevisiae may mediate its function in cell growth and morphogenesis, at least in part, through high-mobility group proteins.

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.

NHP6A and NHP6B, which encode HMG1-like proteins, are candidates for downstream components of the yeast SLT2 mitogen-activated protein kinase pathway

The yeast SLK1 (BCK1) gene encodes a mitogen-activated protein kinase (MAPK) activator protein which functions upstream in a protein kinase cascade that converges on the MAPK Slt2p (Mpk1p). Dominant alleles of SLK1 have been shown to bypass the conditional lethality of a protein kinase C mutation, pkc1-delta, suggesting that Pkc1p may regulate Slk1p function. Slk1p has an important role in morphogenesis and growth control, and deletions of the SLK1 gene are lethal in a spa2-delta mutant background. To search for genes that interact with the SLK1-SLT2 pathway, a synthetic lethal suppression screen was carried out. Genes which in multiple copies suppress the synthetic lethality of slk1-1 spa2-delta were identified, and one, the NHP6A gene, has been extensively characterized. The NHP6A gene and the closely related NHP6B gene were shown previously to encode HMG1-like chromatin-associated proteins. We demonstrate here that these genes are functionally redundant and that multiple copies of either NHP6A or NHP6B suppress slk1-delta and slt2-delta. Strains from which both NHP6 genes were deleted (nhp6-delta mutants) share many phenotypes with pkc1-delta, slk1-delta, and slt2-delta mutants. nhp6-delta cells display a temperature-sensitive growth defect that is rescued by the addition of 1 M sorbitol to the medium, and they are sensitive to starvation. nhp6-delta strains also exhibit a variety of morphological and cytoskeletal defects. At the restrictive temperature for growth, nhp6-delta mutant cells contain elongated buds and enlarged necks. Many cells have patches of chitin staining on their cell surfaces, and chitin deposition is enhanced at the necks of budded cells. nhp6-delta cells display a defect in actin polarity and often accumulate large actin chunks. Genetic and phenotypic analysis indicates that NHP6A and NHP6B function downstream of SLT2. Our results indicate that the Slt2p MAPK pathway in Saccharomyces cerevisiae may mediate its function in cell growth and morphogenesis, at least in part, through high-mobility group proteins.