Nuclear transport of adenovirus DNA polymerase is facilitated by interaction with preterminal protein

Adenovirus Terminal Protein Contains a Bipartite Nuclear Localisation Signal Essential for Its Import into the Nucleus

Adenoviruses contain dsDNA covalently linked to a terminal protein (TP) at the 5′end. TP plays a pivotal role in replication and long-lasting infectivity. TP has been reported to contain a nuclear localisation signal (NLS) that facilitates its import into the nucleus. We studied the potential NLS motifs within TP using molecular and cellular biology techniques to identify the motifs needed for optimum nuclear import. We used confocal imaging microscopy to monitor the localisation and nuclear association of GFP fusion proteins. We identified two nuclear localisation signals, PV(R)6VP and MRRRR, that are essential for fully efficient TP nuclear entry in transfected cells. To study TP–host interactions further, we expressed TP in Escherichia coli (E. coli). Nuclear uptake of purified protein was determined in digitonin-permeabilised cells. The data confirmed that nuclear uptake of TP requires active transport using energy and shuttling factors. This mechanism of nuclear transport was confirmed when expressed TP was microinjected into living cells. Finally, we uncovered the nature of TP binding to host nuclear shuttling proteins, revealing selective binding to Imp β, and a complex of Imp α/β but not Imp α alone. TP translocation to the nucleus could be inhibited using selective inhibitors of importins. Our results show that the bipartite NLS is required for fully efficient TP entry into the nucleus and suggest that this translocation can be carried out by binding to Imp β or Imp α/β. This work forms the biochemical foundation for future work determining the involvement of TP in nuclear delivery of adenovirus DNA.

Identification of peptide domains involved in the subcellular localization of the feline coronavirus 3b protein

Feline coronavirus (FCoV) has been identified as the aetiological agent of feline infectious peritonitis (FIP), a highly fatal systemic disease in cats. FCoV open reading frame 3 (ORF3) encodes accessory proteins 3a, 3b and 3 c. The FCoV 3b accessory protein consists of 72 amino acid residues and localizes to nucleoli and mitochondria. The present work focused on peptide domains within FCoV 3b that drive its intracellular trafficking. Transfection of different cell types with FCoV 3b fused to enhanced green fluorescent protein (EGFP) or 3×FLAG confirmed localization of FCoV 3b in the mitochondria and nucleoli. Using serial truncated mutants, we showed that nucleolar accumulation is controlled by a joint nucleolar and nuclear localization signal (NoLS/NLS) in which the identified overlapping pat4 motifs (residues 53–57) play a critical role. Mutational analysis also revealed that mitochondrial translocation is mediated by N-terminal residues 10–35, in which a Tom20 recognition motif (residues 13–17) and two other overlapping hexamers (residues 24–30) associated with mitochondrial targeting were identified. In addition, a second Tom20 recognition motif was identified further downstream (residues 61–65), although the mitochondrial translocation evoked by these residues seemed less efficient as a diffuse cytoplasmic distribution was also observed. Assessing the spatiotemporal distribution of FCoV 3b did not provide convincing evidence of dynamic shuttling behaviour between the nucleoli and the mitochondria.

Misdelivery at the Nuclear Pore Complex-Stopping a Virus Dead in Its Tracks

Many viruses deliver their genomes into the host cell’s nucleus before they replicate. While onco-retroviruses and papillomaviruses tether their genomes to host chromatin upon mitotic breakdown of the nuclear envelope, lentiviruses, such as human immunodeficiency virus, adenoviruses, herpesviruses, parvoviruses, influenza viruses, hepatitis B virus, polyomaviruses, and baculoviruses deliver their genomes into the nucleus of post-mitotic cells. This poses the significant challenge of slipping a DNA or RNA genome past the nuclear pore complex (NPC) embedded in the nuclear envelope. Quantitative fluorescence imaging is shedding new light on this process, with recent data implicating misdelivery of viral genomes at nuclear pores as a bottleneck to virus replication. Here, we infer NPC functions for nuclear import of viral genomes from cell biology experiments and explore potential causes of misdelivery, including improper virus docking at NPCs, incomplete translocation, virus-induced stress and innate immunity reactions. We conclude by discussing consequences of viral genome misdelivery for viruses and host cells, and lay out future questions to enhance our understanding of this phenomenon. Further studies into viral genome misdelivery may reveal unexpected aspects about NPC structure and function, as well as aid in developing strategies for controlling viral infections to improve human health.

Multiple roles of genome-attached bacteriophage terminal proteins

Protein-primed replication constitutes a generalized mechanism to initiate DNA or RNA synthesis in linear genomes, including viruses, gram-positive bacteria, linear plasmids and mobile elements. By this mechanism a specific amino acid primes replication and becomes covalently linked to the genome ends. Despite the fact that TPs lack sequence homology, they share a similar structural arrangement, with the priming residue in the C-terminal half of the protein and an accumulation of positively charged residues at the N-terminal end. In addition, various bacteriophage TPs have been shown to have DNA-binding capacity that targets TPs and their attached genomes to the host nucleoid. Furthermore, a number of bacteriophage TPs from different viral families and with diverse hosts also contain putative nuclear localization signals and localize in the eukaryotic nucleus, which could lead to the transport of the attached DNA. This suggests a possible role of bacteriophage TPs in prokaryote-to-eukaryote horizontal gene transfer.

Nuclear and nucleoid localization are independently conserved functions in bacteriophage terminal proteins

Bacteriophage terminal proteins (TPs) prime DNA replication and become covalently linked to the DNA 5'-ends. In addition, they are DNA-binding proteins that direct early organization of phage DNA replication at the bacterial nucleoid and, unexpectedly, contain nuclear localization signals (NLSs), which localize them to the nucleus when expressed in mammalian cells. In spite of the lack of sequence homology among the phage TPs, these three properties share some common features, suggesting a possible evolutionary common origin of TPs. We show here that NLSs of three different phage TPs, Φ29, PRD1 and Cp-1, are mapped within the protein region required for nucleoid targeting in bacteria, in agreement with a previously proposed common origin of DNA-binding domains and NLSs. Furthermore, previously reported point mutants of Φ29 TP with no nuclear localization still can target the bacterial nucleoid, and Cp-1 TP contains two independent NLSs, only one of them required for nucleoid localization. Altogether, our results show that nucleoid and nucleus localization sequence requirements partially overlap, but they can be uncoupled, suggesting that conservation of both features could have a common origin but, at the same time, they have been independently conserved during evolution.

Functional eukaryotic nuclear localization signals are widespread in terminal proteins of bacteriophages

A number of prokaryotic proteins have been shown to contain nuclear localization signals (NLSs), although its biological role remains sometimes unclear. Terminal proteins (TPs) of bacteriophages prime DNA replication and become covalently linked to the genome ends. We predicted NLSs within the TPs of bacteriophages from diverse families and hosts and, indeed, the TPs of Φ29, Nf, PRD1, Bam35, and Cp-1, out of seven TPs tested, were found to localize to the nucleus when expressed in mammalian cells. Detailed analysis of Φ29 TP led us to identify a bona fide NLS within residues 1-37. Importantly, gene delivery into the eukaryotic nucleus is enhanced by the presence of Φ29 TP attached to the 5' DNA ends. These findings show a common feature of TPs from diverse bacteriophages targeting the eukaryotic nucleus and suggest a possible common function by facilitating the horizontal transfer of genes between prokaryotes and eukaryotes.

The Attachment, Internalization, and Time-Dependent, Intracellular Distribution of Clostridium difficile Toxin A in Porcine Intestinal Explants

Toxin A (TcdA), secreted by toxigenic strains of Clostridium difficile, produces lesions typical of C. difficile–associated disease (CDAD) in susceptible mammal species. Porcine colon explants maintained for 2 hours with TcdA developed severe lesions characterized by cell swelling, swelling of mitochondria and other organelles, distension of cytoplasmic vesicles, expansion of paracellular spaces, apoptosis, and necrosis. Severity of lesions was proportional to the dosage of toxin. No lesions were present in uninoculated control tissues after 2 hours. Receptor-mediated endocytosis is the keystone event in the pathogenesis of the toxin, and susceptibility of a given species is thought to depend on the presence of receptors in intestinal epithelial cells. The fate of TcdA applied to viable colon explants was determined by transmission electron microscopy in an anti-toxin-labeled gold assay. At 5 minutes postinoculation, the presence of TcdA was indicated at the membrane of microvilli or in the cytoplasm of epithelial cells. TcdA was also indirectly observed within endosomes or attached at their margin. A 30-minute inoculation period was associated with many more gold particles labeling structures inside the cell, although some were still attached to microvilli. Within the cell, most TcdA was associated with mitochondria of epithelial cells, but some gold particles decorated the nuclei. Endothelial cells of the lamina propria had evidence of TcdA at both their lumenal and basal aspects, as well as in the cytoplasm and, occasionally, nuclei. Gold particles also labeled the lumen of such vessels as well as leucocytes in blood vessels and the lamina propria.

Adenovirus: a blueprint for non-viral gene delivery

Although adenoviral vectors may not find a direct clinical role in gene therapy, an understanding of the mechanisms of DNA delivery that adenoviruses use is of vital importance to the design of next-generation non-viral gene delivery systems. Adenoviruses overcome a series of biological barriers, including endosomal escape, intracellular trafficking, capsid dissociation, and nuclear import of DNA, to deliver their genome to the host cell nucleus. The understanding of these processes at the molecular level is progressing and is set to inform the design of synthetic gene delivery systems.

DNA-tumor virus entry--from plasma membrane to the nucleus

DNA-tumor viruses comprise enveloped and non-enveloped agents that cause malignancies in a large variety of cell types and tissues by interfering with cell cycle control and immortalization. Those DNA-tumor viruses that replicate in the nucleus use cellular mechanisms to transport their genome and newly synthesized viral proteins into the nucleus. This requires cytoplasmic transport and nuclear import of their genome. Agents that employ this strategy include adenoviruses, hepadnaviruses, herpesviruses, and likely also papillomaviruses, and polyomaviruses, but not poxviruses which replicate in the cytoplasm. Here, we discuss how DNA-tumor viruses enter cells, take advantage of cytoplasmic transport, and import their DNA genome through the nuclear pore complex into the nucleus. Remarkably, nuclear import of incoming genomes does not necessarily follow the same pathways used by the structural proteins of the viruses during the replication and assembly phases of the viral life cycle. Understanding the mechanisms of DNA nuclear import can identify new pathways of cell regulation and anti-viral therapies.

FHY1 mediates nuclear import of the light-activated phytochrome A photoreceptor

The phytochrome (phy) family of photoreceptors is of crucial importance throughout the life cycle of higher plants. Light-induced nuclear import is required for most phytochrome responses. Nuclear accumulation of phyA is dependent on two related proteins called FHY1 (Far-red elongated HYpocotyl 1) and FHL (FHY1 Like), with FHY1 playing the predominant function. The transcription of FHY1 and FHL are controlled by FHY3 (Far-red elongated HYpocotyl 3) and FAR1 (FAr-red impaired Response 1), a related pair of transcription factors, which thus indirectly control phyA nuclear accumulation. FHY1 and FHL preferentially interact with the light-activated form of phyA, but the mechanism by which they enable photoreceptor accumulation in the nucleus remains unsolved. Sequence comparison of numerous FHY1-related proteins indicates that only the NLS located at the N-terminus and the phyA-interaction domain located at the C-terminus are conserved. We demonstrate that these two parts of FHY1 are sufficient for FHY1 function. phyA nuclear accumulation is inhibited in the presence of high levels of FHY1 variants unable to enter the nucleus. Furthermore, nuclear accumulation of phyA becomes light- and FHY1-independent when an NLS sequence is fused to phyA, strongly suggesting that FHY1 mediates nuclear import of light-activated phyA. In accordance with this idea, FHY1 and FHY3 become functionally dispensable in seedlings expressing a constitutively nuclear version of phyA. Our data suggest that the mechanism uncovered in Arabidopsis is conserved in higher plants. Moreover, this mechanism allows us to propose a model explaining why phyA needs a specific nuclear import pathway.

Nucleolar translocalization of GRA10 of Toxoplasma gondii transfectionally expressed in HeLa cells

Toxoplasma gondii GRA10 expressed as a GFP-GRA10 fusion protein in HeLa cells moved to the nucleoli within the nucleus rapidly and entirely. GRA10 was concentrated specifically in the dense fibrillar component of the nucleolus morphologically by the overlap of GFP-GRA10 transfection image with IFA images by monoclonal antibodies against GRA10 (Tg378), B23 (nucleophosmin) and C23 (nucleolin). The nucleolar translocalization of GRA10 was caused by a putative nucleolar localizing sequence (NoLS) of GRA10. Interaction of GRA10 with TATA-binding protein associated factor 1B (TAF1B) in the yeast two-hybrid technique was confirmed by GST pull-down assay and immunoprecipitation assay. GRA10 and TAF1B were also co-localized in the nucleolus after co-transfection. The nucleolar condensation of GRA10 was affected by actinomycin D. Expressed GFP-GRA10 was evenly distributed over the nucleoplasm and the nucleolar locations remained as hollows in the nucleoplasm under a low dose of actinomycin D. Nucleolar localizing and interacting of GRA10 with TAF1B suggested the participation of GRA10 in rRNA synthesis of host cells to favor the parasitism of T. gondii.

Cell polarity protein Par3 complexes with DNA-PK via Ku70 and regulates DNA double-strand break repair

The partitioning-defective 3 (Par3), a key component in the conserved Par3/Par6/aPKC complex, plays fundamental roles in cell polarity. Herein we report the identification of Ku70 and Ku80 as novel Par3-interacting proteins through an in vitro binding assay followed by liquid chromatography-tandem mass spectrometry. Ku70/Ku80 proteins are two key regulatory subunits of the DNA-dependent protein kinase (DNA-PK), which plays an essential role in repairing double-strand DNA breaks (DSBs). We determined that the nuclear association of Par3 with Ku70/Ku80 was enhanced by gamma-irradiation (IR), a potent DSB inducer. Furthermore, DNA-PKcs, the catalytic subunit of DNA-PK, interacted with the Par3/Ku70/Ku80 complex in response to IR. Par3 over-expression or knockdown was capable of up- or downregulating DNA-PK activity, respectively. Moreover, the Par3 knockdown cells were found to be defective in random plasmid integration, defective in DSB repair following IR, and radiosensitive, phenotypes similar to that of Ku70 knockdown cells. These findings identify Par3 as a novel component of the DNA-PK complex and implicate an unexpected link of cell polarity to DSB repair.

Locating proteins in the cell using TargetP, SignalP and related tools

Determining the subcellular localization of a protein is an important first step toward understanding its function. Here, we describe the properties of three well-known N-terminal sequence motifs directing proteins to the secretory pathway, mitochondria and chloroplasts, and sketch a brief history of methods to predict subcellular localization based on these sorting signals and other sequence properties. We then outline how to use a number of internet-accessible tools to arrive at a reliable subcellular localization prediction for eukaryotic and prokaryotic proteins. In particular, we provide detailed step-by-step instructions for the coupled use of the amino-acid sequence-based predictors TargetP, SignalP, ChloroP and TMHMM, which are all hosted at the Center for Biological Sequence Analysis, Technical University of Denmark. In addition, we describe and provide web references to other useful subcellular localization predictors. Finally, we discuss predictive performance measures in general and the performance of TargetP and SignalP in particular.

Adenovirus core protein pVII is translocated into the nucleus by multiple import receptor pathways

Adenoviruses are nonenveloped viruses with an approximately 36-kb double-stranded DNA genome that replicate in the nucleus. Protein VII, an abundant structural component of the adenovirus core that is strongly associated with adenovirus DNA, is imported into the nucleus contemporaneously with the adenovirus genome shortly after virus infection and may promote DNA import. In this study, we evaluated whether protein VII uses specific receptor-mediated mechanisms for import into the nucleus. We found that it contains potent nuclear localization signal (NLS) activity by transfection of cultured cells with protein VII fusion constructs and by microinjection of cells with recombinant protein VII fusions. We identified three NLS-containing regions in protein VII by deletion mapping and determined important NLS residues by site-specific mutagenesis. We found that recombinant protein VII and its NLS-containing domains strongly and specifically bind to importin alpha, importin beta, importin 7, and transportin, which are among the most abundant cellular nuclear import receptors. Moreover, these receptors can mediate the nuclear import of protein VII fusions in vitro in permeabilized cells. Considered together, these data support the hypothesis that protein VII is a major NLS-containing adaptor for receptor-mediated import of adenovirus DNA and that multiple import pathways are utilized to promote efficient nuclear entry of the viral genome.

Enhancing instance-based classification with local density: a new algorithm for classifying unbalanced biomedical data

MOTIVATION

Classification is an important data mining task in biomedicine. In particular, classification on biomedical data often claims the separation of pathological and healthy samples with highest discriminatory performance for diagnostic issues. Even more important than the overall accuracy is the balance of a classifier, particularly if datasets of unbalanced class size are examined.

RESULTS

We present a novel instance-based classification technique which takes both information of different local density of data objects and local cluster structures into account. Our method, which adopts the basic ideas of density-based outlier detection, determines the local point density in the neighborhood of an object to be classified and of all clusters in the corresponding region. A data object is assigned to that class where it fits best into the local cluster structure. The experimental evaluation on biomedical data demonstrates that our approach outperforms most popular classification methods.

AVAILABILITY

The algorithm LCF is available for testing under http://biomed.umit.at/upload/lcfx.zip.

Interactor-mediated nuclear translocation and retention of protein phosphatase-1

Protein Ser/Thr phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that controls numerous cellular processes by the dephosphorylation of key regulatory proteins. PP1 is expressed in various cellular compartments but is most abundant in the nucleus. We have examined the determinants for the nuclear localization of enhanced green fluorescent protein-tagged PP1 in COS1 cells. Our studies show that PP1gamma(1) does not contain a functional nuclear localization signal and that its nuclear accumulation does not require Sds22, which has previously been implicated in the nuclear accumulation of PP1 in yeast (Peggie, M. W., MacKelvie, S. H., Bloecher, A., Knatko, E. V., Tatchell, K., and Stark, M. J. R. (2002) J. Cell Sci. 115, 195-206). However, the nuclear targeting of PP1 isoforms was alleviated by the mutation of their binding sites for proteins that interact via an RVXF motif. Moreover, one of the mutants with a cytoplasmic accumulation and decreased affinity for RVXF motifs (PP1gamma(1)-F257A) could be re-targeted to the nucleus by the overexpression of nuclear interactors (NIPP1 (nuclear inhibitor of PP1) and PNUTS (PP1 nuclear targeting subunit)) with a functional RVXF motif. Also, the addition of a synthetic RVXF-containing peptide to permeabilized cells resulted in the loss of nuclear enhanced green fluorescent protein-PP1gamma(1). Finally, NIPP1(-/-) mouse embryos showed a nuclear hyperphosphorylation on threonine, consistent with a role for NIPP1 in the nuclear targeting and/or retention of PP1. Our data suggest that both the nuclear translocation and the nuclear retention of PP1 depend on its binding to interactors with an RVXF motif.

Identification of a nuclear targeting signal in YopM from Yersinia spp

YopM is a type III secretion effector from Yersinia which contributes to pathogenicity but whose action still remains unclear. It is an acidic, leucine-rich repeats (LRR) containing protein which migrates to the nucleus of target cells in spite of the fact that it does not contain any classical nuclear localization signal (NLS). Using a yeast approach, we observed that the three first LRRs (LRR1-3) and the 32 C-terminal residues of YopM (YopMC-ter) act as NLSs in yeast. Furthermore, by transfection of HEK293T cells, we observed that YopMC-ter could direct large recombinant EGFP-LexA-AD proteins into the nucleus of mammalian cells confirming that it contains a NLS. Critical residues for nuclear targeting were identified by site-directed mutagenesis in YopMC-ter. In addition, we show that YopMC-ter NLS is crucial for the nuclear targeting of an EGFP-YopM fusion protein.

Adenovirus DNA replication

Replication of the adenovirus genome is catalysed by adenovirus DNA polymerase in which the adenovirus preterminal protein acts as a protein primer. DNA polymerase and preterminal protein form a heterodimer which, in the presence of the cellular transcription factors NFI/CTFI and NFIII/Oct-1, binds to the origin of DNA replication. DNA replication is initiated by DNA polymerase mediated transfer of dCMP onto preterminal protein. Further DNA synthesis is catalysed by DNA polymerase in a strand displacement mechanism which also requires adenovirus DNA binding protein. Here, we discuss the role of individual proteins in this process as revealed by biochemical analysis, mutagenesis and molecular modelling.

The abortive infection of Syrian hamster cells with human adenovirus type 12

Human adenovirus type 12 (Ad12) induces undifferentiated tumors in newborn Syrian hamsters, and this tumor model has been investigated in detail in our laboratory. One of the characteristics of the Ad12-hamster cell system is a strictly abortive infection cycle. In this chapter, we summarize previous and more recent results of studies on the interaction of Ad12 with the nonpermissive BHK21 hamster cell line. The block of Ad12 replication lies before viral DNA replication and late gene transcription which cannot be detected with the most sensitive techniques. Ad12 adsorption, cellular uptake and transport of the viral DNA to the nucleus are less efficient in the nonpermissive hamster cells than in permissive human cells. However, most of the early functions of the Ad12 genome are expressed in BHK21 cells, though at a low level. In the downstream region, the first exon, of the major late promoter (MLP) of Ad12 DNA, a mitigator element of 33 nucleotide pairs in length has been identified which contributes to the inactivity of the MLP in hamster cells and its markedly decreased activity in human cells. The E1 functions of Ad2 or Ad5 are capable of partly complementing the Ad12 deficiencies in hamster cells in that Ad12 viral DNA replication and late gene transcription can proceed, e.g. in a BHK hamster cell line, BHK297-C131,which carries in an integrated form and constitutively expresses the E1 region of Ad5 DNA. Nevertheless, the late Ad12 mRNAs, which are synthesized in this system with the authentic nucleotide sequence, fail to be translated to structural viral proteins. Hence, infectious virions are not produced in the partly complementing system. Probably there is also a translational block for late Ad12 mRNAs in hamster cells. We have recently shown that the overexpression of the Ad12 preterminal protein (pTP) gene or of the E1A gene facilitates the synthesis of full-length, authentic Ad12 DNA in BHK21 cells infected with Ad12. Apparently the pTP has a hitherto unknown function in eliciting full cycles of Ad12 DNA replication even in nonpermissive BHK21 cells when sufficient levels of Ad12 pTP are produced. We pursue the possibility that the completely abortive infection cycle of Ad12 in hamster cells ensures the survival of Ad12-induced hamster tumor cells which all carry, integrated in their genomes, multiple copies of Ad12 DNA. In this way, the viral genomes are immortalized and expanded in a huge number of tumor cells.

DNA binding properties of the adenovirus DNA replication priming protein pTP

The precursor terminal protein pTP is the primer for the initiation of adenovirus (Ad) DNA replication and forms a heterodimer with Ad DNA polymerase (pol). Pol can couple dCTP to pTP directed by the fourth nucleotide of the viral genome template strand in the absence of other replication proteins, which suggests that pTP/pol binding destabilizes the origin or stabilizes an unwound state. We analyzed the contribution of pTP to pTP/pol origin binding using various DNA oligonucleotides. We show that two pTP molecules bind cooperatively to short DNA duplexes, while longer DNA fragments are bound by single pTP molecules as well. Cooperative binding to short duplexes is DNA sequence independent and most likely mediated by protein/protein contacts. Furthermore, we observed that pTP binds single-stranded (ss)DNA with a minimal length of approximately 35 nt and that random ssDNA competed 25-fold more efficiently than random duplex DNA for origin binding by pTP. Remarkably, short DNA fragments with two opposing single strands supported monomeric pTP binding. pTP did not stimulate, but inhibited strand displacement by the Ad DNA binding and unwinding protein DBP. These observations suggest a mechanism in which the ssDNA affinity of pTP stabilizes Ad pol on partially unwound origin DNA.

Roles of cytoskeletal and junctional plaque proteins in nuclear signaling

Cytoplasmic junctional plaque proteins play an important role at intercellular junctions. They link transmembrane cell adhesion molecules to components of the cytoskeleton, thereby playing an important role in the control of many cellular processes. Recent studies on the subcellular distribution of some plaque proteins have revealed that a number of these proteins are able to localize in the nucleus. This dual location indicates that in addition to promoting adhesive interactions, plaque proteins may also play a direct role in nuclear processes, and in particular in the transfer of signals from the membrane to the nucleus. Therefore, translocation of plaque proteins into the nucleus in response to extracellular signals could represent a novel and direct mechanism by which signals can be transmitted from the plasma membrane to the nucleus. This could allow cells to respond to changing environmental conditions in a rapid and efficient way. In addition, conditional sequestration of karyophilic proteins at the sites of cell-cell and cell-substratum adhesion may represent a general mechanism for the regulation of nucleocytoplasmic transport.

Role of conserved residues in the activity of adenovirus preterminal protein

Preterminal protein (pTP) is a component of the preinitiation complex which forms at the adenovirus origin of DNA replication and acts as the protein primer during DNA synthesis. In order to determine the role of various regions of the molecule a series of 18 mutations was introduced into conserved motifs of pTP which were predicted to be surface exposed, and the mutants expressed in insect cells using a baculovirus expression system. Their ability to initiate DNA replication was assessed and the effect the mutations have on the individual interactions which contribute to the formation of the pre-initiation complex was determined. Classes of mutants could be identified which were unable to bind DNA or interact with the adenovirus DNA polymerase, but one class of mutants retained these activities and yet failed to initiate DNA replication. These mutants therefore identify regions of pTP required for different aspects of adenovirus DNA replication.

Different dynamics in nuclear entry of subunits of the repair/transcription factor TFIIH

We report here the different ways in which four subunits of the basal transcription/repair factor TFIIH (XPB, XPD, p62 and p44) and the damage recognition XPC repair protein can enter the nucleus. We examined their nuclear localization by transiently expressing the gene products tagged with the enhanced green fluorescent protein (EGFP) in transfected 3T3 cells. In agreement with the identification of more than one putative nuclear localization signal (NLS) in their protein sequences, XPB, XPC, p62 and p44 chimeras were rapidly sorted to the nucleus. In contrast, the XPD-EGFP chimeras appeared mainly localized in the cytoplasm, with a minor fraction of transfectants showing the EGFP-based fluorescence also in the nucleus. The ability of the XPD chimeras to enter the nucleus was confirmed by western blotting on fractionated cell extracts and by functional complementation of the repair defect in the UV5 rodent cells, mutated in the XPD homologous gene. By deletion mutagenesis, we were unable to identify any sequence specific for nuclear localization. In particular, deletion of the putative NLS failed to affect subcellular localization and, conversely, the C-terminal part of XPD containing the putative NLS showed no specific nuclear accumulation. These findings suggest that the nuclear entry of XPD depends on its complexation with other proteins in the cytoplasm, possibly other components of the TFIIH complex.

Viral entry into the nucleus

Because many viruses replicate in the nucleus of their host cells, they must have ways of transporting their genome and other components into and out of this compartment. For the incoming virus particle, nuclear entry is often one of the final steps in a complex transport and uncoating program. Typically, it involves recognition by importins (karyopherins), transport to the nucleus, and binding to nuclear pore complexes. Although all viruses take advantage of cellular signals and factors, viruses and viral capsids vary considerably in size, structure, and in how they interact with the nuclear import machinery. Influenza and adenoviruses undergo extensive disassembly prior to genome import; herpesviruses release their genome into the nucleus without immediate capsid disassembly. Polyoma viruses, parvoviruses, and lentivirus preintegration complexes are thought to enter in intact form, whereas the corresponding complexes of onco-retroviruses have to wait for mitosis because they cannot infect interphase nuclei.

Development of DNA-based radiopharmaceuticals carrying Auger-electron emitters for antigene radiotherapy

PURPOSE

Antigene radiotherapy (AR) is based on targeting localized radiodamage to specific sites in the genome by using sequence-specific triplex-forming oligonucleotides (TFO) to carry Auger-electron-emitters (A-Ettr) such as Iodine-125 (125I) to the target gene sequence. The radiodecay of an A-Ettr produces a cascade of low-energy electrons and creates a highly positively-charged daughter atom; delivered by a TFO, it should produce double-strand breaks (dsb) localized to the specific DNA target sequence. The result should be a "knock-out" of the targeted gene.

METHODS AND MATERIALS

As a model, we used the MDR1 gene amplified nearly 100 times in the human KB-V1 carcinoma cell line. Chemically modified TFO complementary to the polypurine/polypyrimidine region of the MDR1 gene were synthesized and radiolabeled with 125I-dCTP by the primer extension method. Purified plasmid and genomic DNA and extracted nuclei were treated with 125I-TFO and analyzed for sequence-specific cleavage by electrophoresis in agarose gel and Southern hybridization.

RESULTS

We created 125I-TFO that could effectively recognize, bind, and cleave the target sequence in plasmid and genomic DNA. We showed that these 125I-TFO in nanomolar concentrations were able to cleave the target MDR1 gene sequence in a natural environment, i.e., within the eucaryotic nucleus.

CONCLUSION

125I-TFO can effectively introduce sequence-specific dsb to a target within the MDR1 gene, both in purified DNA and inside intact nuclei. Chemically modified TFO conjugated with nuclear localization signal appear to be a promising delivery vehicle for future in vivo trials of AR.

Cytoplasmic retention of HIV-1 regulatory protein Vpr by protein-protein interaction with a novel human cytoplasmic protein VprBP

Vpr is an HIV-1 auxiliary regulatory protein packaged in the virion. It has been shown to enhance the nuclear transport of the HIV-1 pre-integration complex, activate transcription of cellular and viral promoters, and arrest the cell cycle at the G2/M check-point. We previously identified a cellular protein of 180 kDa (RIP) that interacted with HIV-1 Vpr specifically. We now rename this cellular protein as Vpr-binding protein, or VprBP. In this report, we describe the cloning of the VprBP cDNA that encodes 1507 aa residues and is identical to the previously cloned cDNA KIAA0800. We demonstrate that Vpr specifically interacts with recombinantly expressed VprBP in vitro as well as in vivo. Furthermore, Vpr interacts with the cellular endogenous VprBP in the context of the HIV-1 life cycle. Mutational analysis of VprBP suggests that the Vpr binding domain is located within the C-terminal half of VprBP, which has a Pro-rich domain and several Phe-x-x-Phe repeats. Subcellular fractionation studies show that both the endogenous VprBP and the adenovirus-expressed VprBP are distributed predominantly in the cytoplasmic fraction. Consistent with previous reports, the adenovirus-expressed Vpr is distributed in both the cytoplasmic and the nuclear fractions. However, when VprBP and Vpr are expressed together, Vpr is found almost exclusively in the cytoplasm. Expression of VprBP does not affect the nuclear transport of the adenoviral nuclear protein, pTP. VprBP expressed in insect cells also blocks the nuclear transport of a Vpr-GFP fusion protein, and VprBP mutants incapable of interacting with Vpr fail to block Vpr-GFP nuclear transport. We hypothesize that Vpr interaction with VprBP may cause changes in the host cell cytoplasm that affect HIV-1 pathogenesis as well as HIV-1 replication.

Targeting the human mdr1 gene by 125I-labeled triplex-forming oligonucleotides

Antigene radiotherapy is our approach to targeting specific sites in the genome by combining the highly localized DNA damage produced by the decay of Auger electron emitters, such as 125I, with the sequence-specific action of triplex-forming oligonucleotides (TFO). As a model, we used the multidrug resistance gene (mdr1) overexpressed and amplified nearly 100 times in the human KB-V1 carcinoma cell line. Phosphodiester pyrrazolopyrimidine dG (PPG)-modified TFO complementary to the polypurine-polypyrimidine region of the mdr1 gene were synthesized and labeled with 125I-dCTP at the C5 position of two cytosines by the primer extension method. 125I-TFO were delivered into KB-V1 cells with several delivery systems. DNA from the 125I-TFO-treated cells was recovered and analyzed for sequence-specific cleavage in the mdr1 target by Southern hybridization. Experiments with plasmid DNA containing the mdr1 polypurine-polypyrimidine region and with purified genomic DNA confirmed the ability of the designed 125I-TFO to bind to and introduce double-strand breaks into the target sequence. We showed that 125I-TFO in nanomolar concentrations can recognize and cleave a target sequence in the mdr1 gene in situ, that is, within isolated nuclei and intact digitonin-permeabilized cells. Our results demonstrate the ability of 125I-TFO to target specific sequences in their natural environment, that is, within the eukaryotic nucleus. The nearly 100-fold amplification of the mdr1 gene in KB-V1 cells affords a very useful cell culture model for evaluation of methods to produce sequence-specific DNA double-strand breaks for gene-specific radiotherapy.

LOT1 is a growth suppressor gene down-regulated by the epidermal growth factor receptor ligands and encodes a nuclear zinc-finger protein

We previously reported cloning the rLot1 gene, and its human homolog (hLOT1), through analysis of differential gene expression in normal and malignant rat ovarian surface epithelial cells. Both human and rat ovarian carcinoma cell lines exhibited lost or decreased expression of this gene. Interestingly, the LOT1 gene localized at band q25 of human chromosome 6 which is a frequent site for LOH in many solid tumors including ovarian cancer. In this report we have further characterized the potential role of LOT1 in malignant transformation and developed evidence that the gene is a novel target of growth factor signaling pathway. Assays using transient transfections showed that LOT1 is a nuclear protein and may act as a transcription factor. In vitro and in vivo studies involving ovarian cancer cell lines revealed that expression of LOT1 is directly associated with inhibition of cellular proliferation and induction of morphological transformations. Additionally, we show that in normal rat ovarian surface epithelial cells Lot1 gene expression is responsive to growth factor stimulation. Its mRNA is strongly down-regulated by epidermal growth factor receptor (EGFR) ligands, namely EGF and TGF-alpha. Blocking the ligand-activated EGFR signal transduction pathway by the specific EGF receptor inhibitor, tyrphostin AG1478, and the MEK inhibitor, PD098059, restores the normal level of Lot1 gene expression. It appears that the regulation of Lot1 gene is unique to these ligands, as well as the growth promoting agent TPA, since other factors either did not affect Lot1 expression, or the effect was modest and transient. Altogether, the results suggest that Lot1 expression is primarily mediated via EGF receptor or a related pathway and it may regulate the growth promoting signals as a zinc-finger motif containing nuclear transcription factor.

Mechanism of DNA replication in eukaryotic cells: cellular host factors stimulating adenovirus DNA replication

Replication of adenovirus (Ad) DNA depends on interactions between three viral and three cellular proteins. Human transcription factors NFI and Oct-1 recruit the Ad DNA polymerase to the origin of DNA replication as a complex with the Ad protein primer pTP. High affinity and specificity DNA binding to recognition sites in this origin by the transcription factors stimulate and stabilize pre-initiation complex formation to compensate for the low binding specificity of the pTP/pol complex. In this review, we discuss the properties of NFI and Oct-1 and the mechanism by which they enhance initiation of DNA replication. We propose a model that describes the dynamics of initiation and elongation as well as the assembly and disassembly of the pre-initiation complex.

Novel low molecular weight microtubule-associated protein-2 isoforms contain a functional nuclear localization sequence

Known high and low molecular weight (LMW) MAP2 protein isoforms result from alternative splicing of the MAP2 gene. Contrary to previous reports that MAP2 is neural-specific, we recently identified MAP2 mRNA and protein in somatic and germ cells of rat testis, and showed the predominant testicular isoform is LMW. Although cytoplasmic in neural tissue, MAP2 appeared predominantly nuclear in germ cells using immunohistochemistry. We sought to determine whether this unexpected localization was due to the inclusion of exon 10 within novel LMW MAP2 isoforms. Normally excluded from the LMW MAP2c, exon 10 harbors a putative CcN motif, comprising a nuclear localization sequence (NLS) flanked by regulatory phosphorylation sites for protein kinase CK2 and cdc2 kinase. Characterization of MAP2 mRNA in adult and immature brain and testis, by reverse transcriptase-polymerase chain reaction/Southern analysis and Northern blot, identified novel LMW forms containing exons 10 and 11, previously detected only in high molecular weight MAP2a and 2b. The MAP2 NLS targeted a large heterologous protein to the nucleus, as demonstrated using bacterially expressed MAP2-CcN-beta-galactosidase fusion protein and an in vitro nuclear import assay. Antibodies raised against the fusion protein produced a testicular immunohistochemical staining pattern correlating with MAP2 protein distribution in the nucleus of most germ cells, and precipitated both approximately 70-kDa and >220-kDa proteins recognized by the commercial MAP2-specific HM2 monoclonal antibody, supporting our hypothesis of a novel LMW MAP2 isoform. These results demonstrate the presence of a functional NLS in MAP2 and indicate that novel LMW MAP2 isoforms may be targeted to the nucleus in both neural and non-neuronal tissues.

Regulation of steroid receptor subcellular trafficking

Cellular responses to external signals often reflect alterations in gene expression. The activation of cell surface hormone or growth factor receptors upon the binding of appropriate ligands mobilizes signal transduction cascades that can ultimately impact the activity of defined sets of transcription factors. The interpretation of hormonal signals can also be initiated intracellularly, as is the case for steroid hormone receptors. In addition to recognizing specific hormones, steroid hormone receptors also function as transcription factors and directly transduce hormonal signals to activation or repression of unique target genes. The delivery of activated steroid receptors to high-affinity genomic sites must be efficient to account for the rapidity and selectivity of many transcriptional responses to steroid hormones. Thus, the signal transduction capacity of steroid hormone receptors will be affected by the efficiency of receptor trafficking both between different subcellular compartments (i.e., the cytoplasm and nucleus) and within a specific compartment (i.e., the nucleus). This article will highlight the recent advances in our understanding of subcellular and subnuclear trafficking of steroid receptors.

How do animal DNA viruses get to the nucleus?

Genome and pre-genome replication in all animal DNA viruses except poxviruses occurs in the cell nucleus (Table 1). In order to reproduce, an infecting virion enters the cell and traverses through the cytoplasm toward the nucleus. Using the cell's own nuclear import machinery, the viral genome then enters the nucleus through the nuclear pore complex. Targeting of the infecting virion or viral genome to the multiplication site is therefore an essential process in productive viral infection as well as in latent infection and transformation. Yet little is known about how infecting genomes of animal DNA viruses reach the nucleus in order to reproduce. Moreover, this nuclear locus for viral multiplication is remarkable in that the sizes and composition of the infectious particles vary enormously. In this article, we discuss virion structure, life cycle to reproduce infectious particles, viral protein's nuclear import signal, and viral genome nuclear targeting.

Nucleotide sequence and transcription map of porcine adenovirus type 3

The complete nucleotide sequence of porcine adenovirus type 3 was determined and a transcriptional map for the genome was constructed. The size of the genome is 34094 bp in length with an unusually high G + C content (63.7%), the highest thus far reported for any adenovirus. Overall organization of the genome is similar to that for previously sequenced adenoviral DNAs, but there also were distinct differences. The late regions genes are organized into six families, instead of five as they are in human adenovirus type 2. In contrast to bovine adenovirus type 3 and ovine adenovirus, which lack virion-associated RNA genes, the nucleotide sequence analysis of the viral genome indicates that it encodes one short VA RNA species. With the exception of the fiber and a 33-kDa nonstructural protein, the predicted amino acid sequences of the open reading frames in the late regions and the E2 region and IVa2 exhibited a high level of homology, whereas the deduced amino acid sequences of ORFs in E1, E3, and E4 regions, and the pIX showed a lesser homology with the corresponding proteins of other adenoviruses. The proteins V, VII, and IX are unusually long, and the protein VII lacks the consensus protease cleavage site. Genomic and cDNA sequence analysis has identified promoters, cap sites, intron-exon boundaries, polyadenylation signals, and polyadenylation sites in the viral genome.

Virus assembly and disassembly: the adenovirus cysteine protease as a trigger factor

Viruses are efficient carriers of genetic material between cells. They specifically recognise a target cell and utilise host functions for genome delivery to the replication site. A mature viral capsid emerging from an infected cell serves at least three distinct functions. It enables virus egress from the infected cell, protects the extracellular genome against chemical and physical stress and mediates virus entry into a non-infected cell. How can a virus particle be stably assembled in an infected cell and moments later-after passing through the extracellular milieu-be disintegrated by a non-infected cell? In this review I discuss how adenovirus, a DNA virus, recruits cellular and viral factors and makes use of its own cysteine protease to regulate capsid assembly and disassembly. Copyright 1998 John Wiley & Sons, Ltd.

Characterization of novel nuclear targeting and apoptosis-inducing domains in FAS associated factor 1

FAS associated factor 1 (FAF1) has been described as an unusual protein that binds to the intracellular portion of the apoptosis signal transducing receptor FAS/Apo-1 and potentiates apoptosis in L-cells. By means of mRNA differential display we have identified the avian homologue (qFAF) as a fibroblast growth factor-inducible gene in pluripotent cells from E0 quail embryos during mesoderm induction in vitro. Later during embryonic development, qFAF expression is ubiquitous. We confirm that qFAF is associated with FAS, and show that it is phosphorylated on serine residues and localized to the nucleus. By in vitro mutagenesis we have delimited a novel nuclear targeting domain to a short 35 amino acid α-helical region in the amino-terminal half of the protein. The nuclear function of qFAF remains unclear. However, a probably dominant negative deletion mutant of qFAF causes apoptosis of transfected cells. This function resides in the carboxy-terminal domain of qFAF which shares remarkable sequence homologies with a putative ubiquitin conjugating enzyme from Caenorhabditis elegans. Our data indicate a complex function for FAF, which may be executed during FAS signalling and/or in the ubiquitination pathway, and may be essential for cell differentiation and survival.

Characterization of bovine adenovirus type 3 early region 2B

We have determined the nucleotide sequence of a 6999 base pair region of bovine adenovirus-3 covering map units 9.0 to 29.17, which contained the adenovirus homologs of IVa2 protein and the DNA replication proteins, precursor of terminal protein and DNA polymerase proteins. Analysis of the sequence for cis-acting elements suggests that transcripts of DNA polymerase and precursor of terminal protein are 3' co-terminal. In addition, this region also contains major late promoter sequence. The sequence to the left of IVa2 contains the ORF of pIX with a potential TATA box immediately upstream and two polyadenylation consensus signals immediately downstream of the ORF.

Signals mediating nuclear targeting and their regulation: application in drug delivery

The recent progress with respect to understanding the signals mediating the transport of proteins in both directions through the NPC, and cellular proteins interacting with these signals to effect the transport process has made possible a number of advances in terms of the use of this information in a clinical setting. In particular, our knowledge of the mechanism of regulation of the process, and of how we may exploit the cellular transport machinery itself in a therapeutic situation, especially where there may be transport pathways specific to particular viruses, has advanced considerably. In this context, this review expounds current understanding of the signals conferring targeting to the nucleus, and their practical and potential use in delivering molecules of interest to the nucleus in a clinical context. It also deals with targeting signals conferring nuclear protein export/ shuttling between nuclear and cytoplasmic compartments as well as with those conferring nuclear or cytoplasmic retention, and with the specific mechanisms regulating the activity of these signals, and in particular those regulating signal-dependent nuclear protein import. Detailed understanding of the processes of signal-mediated nuclear protein import/export and its regulation enables the considered application and optimization of approaches to target molecules of interest, such as plasmid DNA or toxic molecules, efficiently to the nucleus according to need in a clinical or research context, and enhance the expression or efficiency of their action, respectively. The use of nuclear targeting signals in this context is reviewed, and future possibilities in terms of the application of our growing understanding of nuclear transport and its regulation are discussed.

Search for a nuclear localization signal in the prion protein

Spongiform transmissible encephalopathies are neurodegenerative diseases characterized by the accumulation, in infected brains, of a pathological form of a normal host-encoded protein called PrP. Previous data have shown that PrP could interact with cytosolic factors, including nuclear molecules, emphasizing the possible function of such interactions. Moreover, in infected cells, PrP is observed not only at the plasma membrane but also in the nuclear compartment. The N-terminal extremity of the mature PrP has been thought to harbor a nuclear localization signal reminiscent of the nuclear localization signal of the simian virus 40 large T antigen. By designing a fusion protein between the putative nuclear localization signal of PrP and the green fluorescent protein, we have shown that the N-terminal sequence of PrP is not efficient in targeting the protein in the nuclear compartment. This implies new insights regarding the way by which PrP could, however, reach the nuclear compartment.

Identification of a novel cytoplasmic protein that specifically binds to nuclear localization signal motifs

Active transport of proteins into the nucleus is mediated by interaction between the classical nuclear localization signals (NLSs) of the targeted proteins and the NLS receptor (importin) complex. This nuclear transport system is highly regulated and conserved in eukaryotes and is essential for cell survival. Using a fragment of BRCA1 containing the two NLS motifs as a bait for yeast two-hybrid screening, we have isolated four clones, one of which is importin alpha. Here we characterize one of the other clones identified, BRAP2, which is a novel gene and expressed as a 2-kilobase mRNA in human mammary epithelial cells and some but not all tissues of mice. The isolated full-length cDNA encodes a novel protein containing 600 amino acid residues with pI 6.04. Characteristic motifs of C2H2 zinc fingers and leucine heptad repeats are present in the middle and C-terminal regions of the protein, respectively. BRAP2 also shares significant homology with a hypothetical protein from yeast Saccharomyces cerevisiae, especially in the zinc finger region. Antibodies prepared against the C-terminal region of BRAP2 fused to glutathione S-transferase specifically recognize a cellular protein with a molecular size of 68 kDa, consistent with the size of the in vitro translated protein. Cellular BRAP2 is mainly cytoplasmic and binds to the NLS motifs of BRCA1 with similar specificity to that of importin alpha in both two-hybrid assays in yeast and glutathione S-transferase pull-down assays in vitro. Other motifs such as the SV40 large T antigen NLS motif and the bipartite NLS motif found in mitosin are also recognized by BRAP2. Similarly, the yeast homolog of BRAP2 also binds to these NLS motifs in vitro. These results imply that BRAP2 may function as a cytoplasmic retention protein and play a role in regulating transport of nuclear proteins.

The complete nucleotide sequence of the egg drop syndrome virus: an intermediate between mastadenoviruses and aviadenoviruses

The complete nucleotide sequence of an avian adenovirus, the egg drop syndrome (EDS) virus, was determined. The total genome length is 33,213 nucleotides, resulting in a molecular weight of 21.9 x 10(6). The GC content is only 42.5%. Between map units 3.5 and 76.9, the distribution of open reading frames with homology to known genes is similar to that reported for other mammalian and avian adenoviruses. However, no homologies to adenovirus genes such as E1A, pIX, pV, and E3 could be found. Outside this region, several open reading frames were identified without any obvious homology to known adenovirus proteins. In the region organized similarly as other adenoviral genomes, most homologies were found to an ovine adenovirus (OAV strain 287). The highest level of amino acid identity was found for the hexon proteins of EDS and OAV. The virus-associated RNA (VA RNA) was identified thanks to the homology with the VA RNA of fowl adenovirus serotype 1 (FAV1). Similarities with FAV1 were also found in the fiber protein. Our results demonstrate that the avian EDS virus represents an intermediate between mammalian and avian adenoviruses. The nucleotide sequence and genomic organization of the EDS virus reflect the heterogeneity of the aviadenovirus genus and the Adenoviridae family.

DNA sequence analysis of an avian adenovirus terminal protein precursor

Nucleotide sequence of the genomic region between map units 25 and 31 of the fowl adenovirus serotype 10 (FAV 10) was determined and analyzed. An open reading frame (ORF) running from right to left (that is on /-strand) of 1806 nucleotides in length was found. This ORF encoded a polypeptide of 602 amino acids with a molecular weight (M[r]) of approximately 70.4 kilo-Daltons. The genomic location of the ORF was determined to be between map units 25.5 and 29.5, similar to the genomic position of the human adenovirus (HAV) terminal protein precursor (pTP). From its size, approximate genomic location and direction of transcription, this ORF was suspected to be the FAV10 homologue of the pTP. Amino acid sequence comparison with the HAV2 pTP revealed an amino acid sequence similarity of 32.4% but was 51 amino acids shorter in length. A potential proteolytic cleavage site was identified which would create a post-cleavage terminal protein of 316 amino acids, again comparable to the 322 amino acids of the post-cleavage TP of HAV.

Early region 4 modulates adenovirus DNA replication by two genetically separable mechanisms

Three viral proteins, all products of early region 2 (E2), participate directly in adenovirus DNA replication. Three products of early region 4 (E4) also affect viral DNA synthesis: the product of E4 ORF4 inhibits viral DNA accumulation, while the products of E4 ORFs 3 and 6 antagonize that effect of ORF4 expression. Because no E4 products are required for DNA synthesis, these proteins probably act indirectly. The E4 ORF3, 4, and 6 proteins all participate in aspects of the regulation of viral gene expression. To determine whether they modulate DNA replication by effects on expression of viral replication proteins, we examined E2 expression in E4 mutant-infected cells. In cells infected by ORF3-, 6- mutants, expression of ORF4 substantially depressed the steady-state levels of replication proteins and E2 mRNAs, reduced E2 transcription rates, and profoundly inhibited viral DNA replication. Thus, in the absence of E4 ORFs 3 and 6, ORF4 acts as a transcriptional regulator of E2 expression, and reduced replication protein levels largely account for the inhibition of DNA replication by ORF4. Cells infected by viruses that express ORFs 3 and 6 in addition to ORF4 accumulated much larger quantities of viral DNA than did cells infected by the ORF3-, 6-, 4+ mutant. Increased DNA accumulation was not accompanied by a comparable increase in E2 expression. Therefore, the ORF3 and 6 products counteract the ORF4-induced reduction of DNA replication by a mechanism other than reversing the inhibitory effect of ORF4 on E2 expression. The effect of ORF4 on E2 expression is consistent with its ability to regulate levels of the transcription factor AP-1 (Müller et al., 1992, J. Virol. 66, 5867-5878); the mechanism by which ORFs 3 and 6 enhance replication is unknown.

Completion of the DNA sequence of mouse adenovirus type 1: sequence of E2B, L1, and L2 (18-51 map units)

The DNA sequence of 9991 nt, corresponding to 18-51 map units of mouse adenovirus type 1 (MAV-1), was determined, completing the sequence of the Larsen strain of MAV-1. The length of the complete MAV-1 genome is 30,946 nucleotides, consistent with previous experimental estimates. The 18-51 map unit region encodes early region 2B proteins necessary for adenoviral replication as well as late region L1 and L2 structural and packaging proteins. Sequence comparison in this region with human adenoviruses indicates broad similarities, including colinear preservation of all recognized open reading frames (ORFs), with highest amino acid identity occurring in the DNA polymerase and polypeptide III (penton base subunit) ORFs. Virus-associated (VA) RNA is not encoded in the region where VA RNAs are found in the human adenoviruses, between E2B and L1, nor is it encoded anywhere in the entire MAV-1 genome. The MAV-1 polypeptide III lacks the arginine-glycine-aspartic acid (RGD) motif which is involved in an association with cell-surface integrins. Only one RGD sequence is found in an identified coding region in the entire MAV-1 genome. Similar to the porcine adenovirus, this RGD sequence is found in the C-terminus of the MAV-1 fiber protein.

The cytokine interleukin-5 (IL-5) effects cotransport of its receptor subunits to the nucleus in vitro

Interleukin (IL)-5 is central in regulating eosinophilia in allergic disease and parasitic infections. We have recently shown that human (h) IL-5 both possesses a functional nuclear localization signal capable of targeting a heterologous protein to the nucleus and localises to the nucleus of intact receptor-expressing cells. In this study, the extracellular domains of the hIL-5 alpha- and beta-receptor subunits were expressed in baculovirus, fluorescently labelled and assayed for nuclear targeting in vitro in the absence and presence of IL-5. The beta-subunit, which lacks IL-5 binding activity, only accumulated in the nucleus in the presence of both the hIL-5 binding alpha-subunit and hIL-5. The IL-5-binding alpha-subunit showed similar results. IL-5 thus effected nuclear transport of its alpha- and beta-receptor subunits apparently through a 'piggy back' mechanism, raising the possibility that IL-5's nuclear signalling role may be to cotarget its receptor subunits to the nucleus. This is the first demonstration of nuclear protein piggy back transport in vitro.