Characterization of a novel serine/threonine kinase associated with nuclear bodies

The important roles of protein SUMOylation in the occurrence and development of leukemia and clinical implications

Leukemia is a severe malignancy of the hematopoietic system, which is characterized by uncontrolled proliferation and dedifferentiation of immature hematopoietic precursor cells in the lymphatic system and bone marrow. Leukemia is caused by alterations of the genetic and epigenetic regulation of processes underlying hematologic malignancies, including SUMO modification (SUMOylation). Small ubiquitin-like modifier (SUMO) proteins covalently or noncovalently conjugate and modify a large number of target proteins via lysine residues. SUMOylation is a small ubiquitin-like modification that is catalyzed by the SUMO-specific activating enzyme E1, the binding enzyme E2, and the ligating enzyme E3. SUMO is covalently linked to substrate proteins to regulate the cellular localization of target proteins and the interaction of target proteins with other biological macromolecules. SUMOylation has emerged as a critical regulatory mechanism for subcellular localization, protein stability, protein-protein interactions, and biological function and thus regulates normal life activities. If the SUMOylation process of proteins is affected, it will cause a cellular reaction and ultimately lead to various diseases, including leukemia. There is growing evidence showing that a large number of proteins are SUMOylated and that SUMOylated proteins play an important role in the occurrence and development of various types of leukemia. Targeting the SUMOylation of proteins alone or in combination with current treatments might provide powerful targeted therapeutic strategies for the clinical treatment of leukemia.

Pandemic influenza A viruses escape from restriction by human MxA through adaptive mutations in the nucleoprotein

The interferon-induced dynamin-like MxA GTPase restricts the replication of influenza A viruses. We identified adaptive mutations in the nucleoprotein (NP) of pandemic strains A/Brevig Mission/1/1918 (1918) and A/Hamburg/4/2009 (pH1N1) that confer MxA resistance. These resistance-associated amino acids in NP differ between the two strains but form a similar discrete surface-exposed cluster in the body domain of NP, indicating that MxA resistance evolved independently. The 1918 cluster was conserved in all descendent strains of seasonal influenza viruses. Introduction of this cluster into the NP of the MxA-sensitive influenza virus A/Thailand/1(KAN-1)/04 (H5N1) resulted in a gain of MxA resistance coupled with a decrease in viral replication fitness. Conversely, introduction of MxA-sensitive amino acids into pH1N1 NP enhanced viral growth in Mx-negative cells. We conclude that human MxA represents a barrier against zoonotic introduction of avian influenza viruses and that adaptive mutations in the viral NP should be carefully monitored.

Role of the SUMO-interacting motif in HIPK2 targeting to the PML nuclear bodies and regulation of p53

Homeodomain-interacting protein kinase 2 (HIPK2) is a key regulator of various transcription factors including p53 and CtBP in the DNA damage signaling pathway. PML-nuclear body (NB) is required for HIPK2-mediated p53 phosphorylation at Ser46 and induction of apoptosis. Although PML-NB targeting of HIPK2 has been shown, much is not clear about the molecular mechanism of HIPK2 recruitment to PML-NBs. Here we show that HIPK2 colocalizes specifically with PML-I and PML-IV. Mutational analysis showed that HIPK2 recruitment to PML-IV-NBs is mediated by the SUMO-interaction motifs (SIMs) of both PML-IV and HIPK2. Wild-type HIPK2 associated with SUMO-conjugated PML-IV at a higher affinity than with un-conjugated PML-IV, while the association of a HIPK2 SIM mutant with SUMO-modified PML-IV was impaired. In colony formation assays, HIPK2 strongly suppressed cell proliferation, but HIPK2 SIM mutants did not. In addition, activation and phosphorylation of p53 at the Ser46 residue were impaired by HIPK2 SIM mutants. These results suggest that SIM-mediated HIPK2 targeting to PML-NBs is crucial for HIPK2-mediated p53 activation and induction of apoptosis.

The Mx GTPase family of interferon-induced antiviral proteins

Mx proteins are interferon-induced members of the dynamin superfamily of large GTPases. They inhibit a wide range of viruses by blocking an early stage of the replication cycle. Studies in genetically defined mouse strains highlight their powerful action in early antiviral host defence.

Interferon-induced Mx proteins in antiviral host defense

Mx proteins are key components of the antiviral state induced by interferons in many species. They belong to the class of dynamin-like large guanosine triphosphatases (GTPases) known to be involved in intracellular vesicle trafficking and organelle homeostasis. Mx GTPases share structural and functional properties with dynamin, such as self-assembly and association with intracellular membranes. A unique property of some Mx GTPases is their antiviral activity against a wide range of RNA viruses, including influenza viruses and members of the bunyavirus family. These viruses are inhibited at an early stage in their life cycle, soon after host cell entry and before genome amplification. The mouse Mx1 GTPase accumulates in the cell nucleus where it associates with components of the PML nuclear bodies and inhibits influenza and Thogoto viruses known to replicate in the nucleus. The human MxA GTPase accumulates in the cytoplasm and is partly associated with a COP-I-positive subcompartment of the endoplasmic reticulum. This membrane compartment seems to provide an interaction platform that facilitates viral target recognition. In the case of bunyaviruses, MxA recognizes the viral nucleocapsid protein and interferes with its role in viral genome replication. In the case of Thogoto virus, MxA recognizes the viral nucleoprotein and prevents the incoming viral nucleocapsids from being transported into the nucleus, the site of viral transcription and replication. In both cases, GTP-binding and carboxy-terminal effector functions of MxA are required for target recognition. In general, Mx GTPases appear to detect viral infection by sensing nucleocapsid-like structures. As a consequence, these viral components are trapped and sorted to locations where they become unavailable for the generation of new virus particles.

The Interferon-Inducible GTPases

Mammalian cells respond to interferons (IFNs) secreted during infection by the transcriptional upregulation of as many as a thousand genes. This remarkable transition prepares cells and organisms for resistance to infection, and many IFN-regulated gene products are players in well-understood resistance programs. Oddly, however, many of the most abundantly induced proteins are GTPases whose functions are not well understood. Here we review the progress that has been made toward understanding the roles of individual GTPase families in disease resistance and the hints of common mechanisms that are now available.

Mx1 GTPase accumulates in distinct nuclear domains and inhibits influenza A virus in cells that lack promyelocytic leukaemia protein nuclear bodies

The interferon-induced murine Mx1 GTPase is a nuclear protein. It specifically inhibits influenza A viruses at the step of primary transcription, a process known to occur in the nucleus of infected cells. However, the exact mechanism of inhibition is still poorly understood. The Mx1 GTPase has previously been shown to accumulate in distinct nuclear dots that are spatially associated with promyelocytic leukaemia protein (PML) nuclear bodies (NBs), but the significance of this association is not known. Here it is reported that, in cells lacking PML and, as a consequence, PML NBs, Mx1 still formed nuclear dots. These dots were indistinguishable from the dots observed in wild-type cells, indicating that intact PML NBs are not required for Mx1 dot formation. Furthermore, Mx1 retained its antiviral activity against influenza A virus in these PML-deficient cells, which were fully permissive for influenza A virus. Nuclear Mx proteins from other species showed a similar subnuclear distribution. This was also the case for the human MxA GTPase when this otherwise cytoplasmic protein was translocated into the nucleus by virtue of a foreign nuclear localization signal. Human MxA and mouse Mx1 do not interact or form heterooligomers. Yet, they co-localized to a large degree when co-expressed in the nucleus. Taken together, these findings suggest that Mx1 dots represent distinct nuclear domains (‘Mx nuclear domains’) that are frequently associated with, but functionally independent of, PML NBs.

US11 of herpes simplex virus type 1 interacts with HIPK2 and antagonizes HIPK2-induced cell growth arrest

Homeodomain-interacting protein kinase 2 (HIPK2) is a nuclear serine/threonine kinase of the subfamily of dual-specificity Yak1-related kinase proteins. HIPK2 was first described as a homeodomain-interacting protein kinase acting as a corepressor for homeodomain transcription factors. More recently, it was reported that HIPK2 plays a role in p53-mediated cellular apoptosis and could also participate in the regulation of the cell cycle. US11 protein of herpes simplex virus type 1 is a multifunctional protein involved in the regulation of several processes related to the survival of cells submitted to environmental stresses by mechanisms that are not fully elucidated. In an attempt to better understand the multiple functions of US11, we identified cellular binding partners of this protein by using the yeast two-hybrid system. We report that US11 interacts with HIPK2 through the PEST domain of HIPK2 and that this interaction occurs also in human cells. This interaction modifies the subcellular distribution of HIPK2 and protects the cell against the HIPK2-induced cell growth arrest.

Homeodomain-interacting protein kinase-2 activity and p53 phosphorylation are critical events for cisplatin-mediated apoptosis

HIPK2 is a member of a novel family of nuclear serine-threonine kinases identified through their ability to interact with the Nkx-1.2 homeoprotein. The physiological role of these kinases is largely unknown, but we have recently reported on the involvement of HIPK2 in the induction of apoptosis of tumor cells after UV stress through p53 phosphorylation and transcriptional activation. Here, we demonstrate that the chemotherapeutic drug cisplatin increases HIPK2 protein expression and its kinase activity, and that HIPK2 is involved in cisplatin-dependent apoptosis. Indeed, induction of HIPK2 and of cell death by cisplatin are efficiently inhibited by the serine-threonine kinase inhibitor SB203580 or the transduction of HIPK2-specific RNA-interfering molecules. HIPK2 gene silencing efficiently reduces the p53-mediated transcriptional activation of apoptotic gene promoters as well as apoptotic cell death after treatment with cisplatin. These findings, along with the involvement of p53 phosphorylation at serine 46 (Ser46) in the transcriptional activation of apoptotic gene promoters, suggest a critical role for HIPK2 in triggering p53-dependent apoptosis in response to the antineoplastic drug cisplatin.

DYRK1A accumulates in splicing speckles through a novel targeting signal and induces speckle disassembly

The protein kinase DYRK1A is distributed throughout the nucleoplasm, accumulating in speckle-like regions. We have found that this punctuated nuclear distribution is determined by the contribution of several elements. Although the nuclear import is mediated by two distinct nuclear localization signals, one at the N-terminus and the other located in the linker region, between subdomains X and XI of the catalytic domain, the accumulation in speckles that are SC35 positive depends on a sequence motif that is located C-terminal to the kinase domain and comprises a histidine tail. A similar sequence is also responsible for the targeting of cyclin T1. Therefore the histidine-rich region represents a novel splicing speckle targeting signal. Moreover, overexpression of DYRK1A induces speckle disassembly. Such disassembly is DYRK1A activity specific, since the overexpression of a DYRK1A kinase inactive mutant, the paralogous DYRK1B or a chimeric protein DYRK1B that has been directed to the speckles via the DYRK1A targeting signal, leaves the SC35 speckle pattern untouched. Thus DYRK1A protein kinase may play a role in regulating the biogenesis of the splicing speckle compartment.

The homeodomain-interacting kinase PKM (HIPK-2) modifies ND10 through both its kinase domain and a SUMO-1 interaction motif and alters the posttranslational modification of PML

Homeodomain-interacting protein kinases (HIPK-1, -2, and -3) are a family of enzymes that have been implicated in the phosphorylation and repression of homeodomain-containing transcription factors. HIPK-2 has been found to interact with the SUMO-1-conjugating enzyme Ubc9 and can be covalently modified by SUMO-1. It has also been shown to interact with and phosphorylate p53 and to form punctate speckles in the nucleus of which a proportion colocalize with PML nuclear bodies (ND10). We have previously shown that the hamster equivalent of HIPK-2 (named PKM) interacts with the interferon-induced antiviral GTPase Mx1 and associates with ND10 in interferon-treated cells. Given the connections between the interferon response pathway, constituents of ND10, and SUMO-1-conjugated proteins, we have studied the effects of exogenously expressed PKM on endogenous ND10 proteins. We found that PKM induces structural changes in ND10 that can be attributed both to its kinase activity and to the presence of a functional SUMO-1 interaction motif in the C-terminal half of the protein. The changes in the localization of PML, Sp100, and hDaxx induced by exogenous PKM or fragments thereof correlate with changes in the posttranslationally modified species of PML. We propose that PKM is able to modify ND10 structure by inducing changes in the posttranslational modification of PML and by interacting with SUMO-1 modification pathways.

HIPK2 associates with RanBPM

Using the yeast two-hybrid system, we have identified the Ran-binding protein (RanBPM) as an interaction partner of homeodomain-interacting protein kinase 2 (HIPK2). RanBPM has been described as a centrosomal protein through which Ran regulates the centrosomal function. HIPK2 is mainly a nuclear protein, which among other functions represses transcription mediated by homeodomain containing transcription factors. Here, we show that overexpressed wildtype HIPK2 and a kinase defective mutant of HIPK2 directly interact with RanBPM in the nucleus of mammalian cells. Overexpressed wildtype RanBPM and a kinase defective mutant of HIPK2 co-localise with HIPK2 in defined nuclear structures. A carboxy- and an amino-terminal deletion of HIPK2 do not seem to be able to bind to RanBPM.

Identification and characterization of HIPK2 interacting with p73 and modulating functions of the p53 family in vivo

To study the biological role of p73 alpha, a member of the p53 tumor suppressor family, we performed a yeast two-hybrid screen of a human cDNA library. Using a p73 alpha fragment consisting of amino acids 49-636 as bait, we found that p73 alpha is functionally associated with the human homologue of mouse and hamster homeodomain-interacting protein kinase 2 (HIPK2). The hamster homologue, also known as haHIPK2 or PKM, was used for further characterization of interactions between HIPK2 and members of the p53 protein family. Systematic yeast two-hybrid assays indicated a physical interaction between the oligomerization domains of p73 alpha and p53 (amino acid regions 345-380 and 319-360, respectively) and amino acid region 812-907 of haHIPK2. This region of haHIPK2 includes a PEST sequence, an Ubc9-binding domain, and a partial speckle retention sequence and is identical to amino acid residues 846-941 of human HIPK2 (hHIPK2). The interaction was confirmed by glutathione S-transferase pull-down assays in vitro and immunoprecipitation assays in vivo. HIPK2 colocalized with p73 and p53 in nuclear bodies, as shown by confocal microscopy. Overexpression of HIPK2 stabilized the p53 protein and greatly increased the p73- and p53-induced transcriptional repression of multidrug-resistant and collagenase promoters in Saos2 cells but had little effect on the p73- or p53-mediated transcriptional activation of synthetic p53-responsive and p21WAF1 promoters. Stable expression of HIPK2 in U2OS cells enhanced the cisplatin response of sub-G(1) and G(2)/M populations, and it also increased the apoptotic response to cisplatin and adriamycin as demonstrated by fluorescence-activated cell sorter and 4',6-diamidino-2-phenylindole-staining analyses. HIPK2 potentiated the inhibition of colony formation by p73 and p53. These results suggest that physical interactions between HIPK2 and members of the p53 family may determine the roles of these proteins in cell cycle regulation and apoptosis.

Regulation of p53 activity by its interaction with homeodomain-interacting protein kinase-2

Transcriptional activity of p53, a central regulatory switch in a network controlling cell proliferation and apoptosis, is modulated by protein stability and post-translational modifications including phosphorylation and acetylation. Here we demonstrate that the human serine/threonine kinase homeodomain-interacting protein kinase-2 (HIPK2) colocalizes and interacts with p53 and CREB-binding protein (CBP) within promyelocytic leukaemia (PML) nuclear bodies. HIPK2 is activated by ultraviolet (UV) radiation and selectively phosphorylates p53 at Ser 46, thus facilitating the CBP-mediated acetylation of p53 at Lys 382, and promoting p53-dependent gene expression. Accordingly, the kinase function of HIPK2 mediates the increased expression of p53 target genes, which results in growth arrest and the enhancement of UV-induced apoptosis. Interference with HIPK2 expression by antisense oligonucleotides impairs UV-induced apoptosis. Our results imply that HIPK2 is a novel regulator of p53 effector functions involved in cell growth, proliferation and apoptosis.

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.

Cellular proteins localized at and interacting within ND10/PML nuclear bodies/PODs suggest functions of a nuclear depot

ND10, PML bodies or PODs have become the defining nuclear structure for a highly complex protein complement involved in cell activities such as aging, apoptosis, the cell cycle, stress response, hormone signaling, transcriptional regulation and development. ND10 are present in many but not all cell types and are not essential for cell survival. Here, we review the cellular proteins found in ND10, their few known interactions and their contribution to the ND10 structure per se and to functions elsewhere in the nucleus. The discrepancy between the functions of the ND10 proteins and the nonessential nature of the structure in which they are aggregated at their highest concentrations leads to the conclusion that the proteins function elsewhere. The regulated recruitment of specific proteins into ND10 as well as their controlled release upon external induced stress points to a regulated nuclear depot function for ND10. These nuclear depot functions seem important as nuclear defense against viral attack and other external insults.

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.

HIPK2 overexpression leads to stabilization of p53 protein and increased p53 transcriptional activity by decreasing Mdm2 protein levels

BACKGROUND

HIPK2 (homeodomain-interacting protein kinase 2) has been identified as a nuclear serine/threonine kinase. A central function of HIPK2 is repressing transcription of homeodomain containing transcription factors.

RESULTS AND CONCLUSIONS

We show here that HIPK2 activates transcription mediated by tumor suppressor p53 responsive promoter elements. Overexpression of HIPK2 leads to an increase of p53 protein expression or stability, which becomes enhanced further in the presence of the DNA damaging drug doxorubicin. The effects of HIPK2 on p53 are not observed with kinase deficient HIPK2 mutants. However, HIPK2 is not sufficient for phosphorylation of three crucial serine residues of p53, suggesting that HIPK2-induced p53 activation does not involve phosphorylation of p53. Instead, HIPK2 leads to a downregulation of p53-induced Mdm2 protein and this may lead to stabilization of p53. Overexpression of HIPK2 does not lead to a change of Mdm2 mRNA expression. The data suggest that HIPK2 plays a critical role in p53 mediated cellular responses by removing the p53 inhibitor protein Mdm2 via modification of the protein itself or its intracellular movement.

Isolation and characterization of cDNAs for the protein kinase HIPK2

HIPK2 (homeodomain-interacting protein kinase 2) is a CD95 binding partner in yeast. Its primary amino acid sequence is highly conserved between human and mouse. The highest HIPK2 mRNA expression is found in neuronal tissue. The HIPK2 gene is located on human chromosome 7q33-35 and the protein is mainly localized in the nucleus. HIPK2 has been described to play a role as a co-repressor for homeodomain transcription factors.

Human homeodomain-interacting protein kinase-2 (HIPK2) is a member of the DYRK family of protein kinases and maps to chromosome 7q32-q34

Here we identified the human serine/threonine kinase HIPK2 as a novel member of the DYRK kinase subfamily. Alignment of several DYRK family proteins including the kinases minibrain, MJAK, PKY, the Dictyostelium kinase YakA and Saccharomyces YAK1 allowed the identification of several evolutionary conserved DYRK consensus motifs within the kinase domain. A lysine residue conserved between all DYRK kinase family members was found to be essential for the kinase function of HIPK2. Human HIPK2 was mapped to chromosome 7q32-q34 and murine HIPK2 to chromosome 6B, the homologue to human chromosome 7.

Covalent modification of p73alpha by SUMO-1. Two-hybrid screening with p73 identifies novel SUMO-1-interacting proteins and a SUMO-1 interaction motif

Two-hybrid screening in yeast with p73alpha isolated SUMO-1 (small ubiquitin-like modifier 1), the enzyme responsible for its conjugation, Ubc-9, and a number of novel SUMO-1-interacting proteins, including thymine DNA glycosylase, PM-Scl75, PIASx, PKY, and CHD3/ZFH. A subset of these proteins contain a common motif, hhXSXS/Taaa, where h is a hydrophobic amino acid and a is an acidic amino acid, that is shown to interact with SUMO-1 in the two-hybrid system. We show here that p73alpha, but not p73beta, can be covalently modified by SUMO-1. The major SUMO-1-modified residue in p73alpha is the C-terminal lysine (Lys(627)). The sequence surrounding this lysine conforms to a consensus SUMO-1 modification site b(X)XXhKXE, where b is a basic amino acid. SUMO-1-modified p73 is more rapidly degraded by the proteasome than unmodified p73, although SUMO-1 modification is not required for p73 degradation. SUMO-1 modification does not affect the transcriptional activity of p73alpha on an RGC-luciferase reporter gene in SK-N-AS cells. Instead, SUMO-1 modification may alter the subcellular localization of p73, because SUMO-1-modified p73 is preferentially found in detergent-insoluble fractions. Alternatively, it may modulate the interaction of p73 with other proteins that are substrates for SUMO-1 modification or which interact with SUMO-1, such as those identified here.

The serine/threonine kinase HIPK2 interacts with TRADD, but not with CD95 or TNF-R1 in 293T cells

HIPK2 has been described as a homoedomain-interacting protein kinase with a nuclear localization. Here we describe that HIPK2 can also associate with TRADD, a protein that interacts with tumor necrosis factor receptor type 1 (TNF-R1). Under the conditions where HIPK2/TRADD association was found, no direct interaction of HIPK2 with CD95, TNF-R1, FADD or caspase-8 could be detected. Therefore, HIPK2 may play a role in TNF-R1 mediated signaling.

Replication but not transcription of simian virus 40 DNA is dependent on nuclear domain 10

DNA viruses from several families including herpes simplex virus type 1, adenovirus type 5, and simian virus 40 (SV40), start their transcription and replication adjacent to a specific nuclear domain, ND10. We asked whether a specific viral DNA sequence determines the location of these synthetic activities at such restricted nuclear sites. Partial and overlapping SV40 sequences were introduced into a beta-galactosidase expression vector, and the beta-galactosidase transcripts were localized by in situ hybridization. Transcripts derived from control plasmids were found throughout the nucleus and at highly concentrated sites but not at ND10. SV40 genomic segments supported ND10-associated transcription only when the origin and the coding sequence for the large T antigen were present. When the large T-antigen coding sequence was eliminated but the T antigen was constitutively expressed in COS-7 cells, the viral origin was sufficient to localize transcription and replication to ND10. Deletion analysis showed that only the large T-antigen binding site II (the core origin) was required but the T antigen was needed for detectable transcription at ND10. Large T antigen expressed from plasmids without the viral core origin did not bind or localize to ND10. Blocking of DNA replication prevented the accumulation of transcripts at ND10, indicating that only sites with replicating templates accumulated transcripts. Transcription at ND10 did not enhance total protein synthesis of plasmid transcripts. These findings suggest that viral transcription at ND10 may only be a consequence of viral genomes directed to ND10 for replication. Although plasmid transcription can take place anywhere in the nucleus, T-antigen-directed replication is apparently restricted to ND10.