Nucleolar targeting signal of human T-cell leukemia virus type I rex-encoded protein is essential for cytoplasmic accumulation of unspliced viral mRNA

Human Retrovirus Genomic RNA Packaging

Two non-covalently linked copies of the retrovirus genome are specifically recruited to the site of virus particle assembly and packaged into released particles. Retroviral RNA packaging requires RNA export of the unspliced genomic RNA from the nucleus, translocation of the genome to virus assembly sites, and specific interaction with Gag, the main viral structural protein. While some aspects of the RNA packaging process are understood, many others remain poorly understood. In this review, we provide an update on recent advancements in understanding the mechanism of RNA packaging for retroviruses that cause disease in humans, i.e., HIV-1, HIV-2, and HTLV-1, as well as advances in the understanding of the details of genomic RNA nuclear export, genome translocation to virus assembly sites, and genomic RNA dimerization.

Molecular Studies of HTLV-1 Replication: An Update

Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus discovered. Studies on HTLV-1 have been instrumental for our understanding of the molecular pathology of virus-induced cancers. HTLV-1 is the etiological agent of an adult T-cell leukemia (ATL) and can lead to a variety of neurological pathologies, including HTLV-1-associated-myelopathy/tropical spastic paraparesis (HAM/TSP). The ability to treat the aggressive ATL subtypes remains inadequate. HTLV-1 replicates by (1) an infectious cycle involving virus budding and infection of new permissive target cells and (2) mitotic division of cells harboring an integrated provirus. Virus replication initiates host antiviral immunity and the checkpoint control of cell proliferation, but HTLV-1 has evolved elegant strategies to counteract these host defense mechanisms to allow for virus persistence. The study of the molecular biology of HTLV-1 replication has provided crucial information for understanding HTLV-1 replication as well as aspects of viral replication that are shared between HTLV-1 and human immunodeficiency virus type 1 (HIV-1). Here in this review, we discuss the various stages of the virus replication cycle—both foundational knowledge as well as current updates of ongoing research that is important for understanding HTLV-1 molecular pathogenesis as well as in developing novel therapeutic strategies.

Discovery and characterization of auxiliary proteins encoded by type 3 simian T-cell lymphotropic viruses

Human T-cell lymphotropic virus type 1 (HTLV-1) and HTLV-2 encode auxiliary proteins that play important roles in viral replication, viral latency, and immune escape. The presence of auxiliary protein-encoding open reading frames (ORFs) in HTLV-3, the latest HTLV to be discovered, is unknown. Simian T-cell lymphotropic virus type 3 (STLV-3) is almost identical to HTLV-3. Given the lack of HTLV-3-infected cell lines, we took advantage of STLV-3-infected cells and of an STLV-3 molecular clone to search for the presence of auxiliary transcripts. Using reverse transcriptase PCR (RT-PCR), we first uncovered the presence of three unknown viral mRNAs encoding putative proteins of 5, 8, and 9 kDa and confirmed the presence of the previously reported RorfII transcript. The existence of these viral mRNAs was confirmed by using splice site-specific RT-PCR with ex vivo samples. We showed that p5 is distributed throughout the cell and does not colocalize with a specific organelle. The p9 localization is similar to that of HTLV-1 p12 and induced a strong decrease in the calreticulin signal, similarly to HTLV-1 p12. Although p8, RorfII, and Rex-3 share an N-terminal sequence that is predicted to contain a nucleolar localization signal (NoLS), only p8 is found in the nucleolus. The p8 location in the nucleolus is linked to a bipartite NoLS. p8 and, to a lesser extent, p9 repressed viral expression but did not alter Rex-3-dependent mRNA export. Using a transformation assay, we finally showed that none of the STLV-3 auxiliary proteins had the ability to induce colony formation, while both Tax-3 and antisense protein of HTLV-3 (APH-3) promoted cellular transformation. Altogether, these results complete the characterization of the newly described primate T-lymphotropic virus type 3 (PTLV-3).

IMPORTANCE

Together with their simian counterparts, HTLVs form the primate T-lymphotropic viruses. HTLVs arose from interspecies transmission between nonhuman primates and humans. HTLV-1 and HTLV-2 encode auxiliary proteins that play important roles in viral replication, viral latency, and immune escape. The presence of ORFs encoding auxiliary proteins in HTLV-3 or STLV-3 genomes was unknown. Using in silico analyses, ex vivo samples, or in vitro experiments, we have uncovered the presence of 3 previously unknown viral mRNAs encoding putative proteins and confirmed the presence of a previously reported viral transcript. We characterized the intracellular localization of the four proteins. We showed that two of these proteins repress viral expression but that none of them have the ability to induce colony formation. However, both Tax and the antisense protein APH-3 promote cell transformation. Our results allowed us to characterize 4 new retroviral proteins for the first time.

HTLV-1 and HTLV-2: highly similar viruses with distinct oncogenic properties

HTLV-1 and HTLV-2 share broad similarities in their overall genetic organization and expression pattern, but they differ substantially in their pathogenic properties. This review outlines distinctive features of HTLV-1 and HTLV-2 that might provide clues to explain their distinct clinical outcomes. Differences in the kinetics of viral mRNA expression, functional properties of the regulatory and accessory proteins, and interactions with cellular factors and signal transduction pathways are discussed.

Nuclear trafficking of retroviral RNAs and Gag proteins during late steps of replication

Retroviruses exploit nuclear trafficking machinery at several distinct stages in their replication cycles. In this review, we will focus primarily on nucleocytoplasmic trafficking events that occur after the completion of reverse transcription and proviral integration. First, we will discuss nuclear export of unspliced viral RNA transcripts, which serves two essential roles: as the mRNA template for the translation of viral structural proteins and as the genome for encapsidation into virions. These full-length viral RNAs must overcome the cell's quality control measures to leave the nucleus by co-opting host factors or encoding viral proteins to mediate nuclear export of unspliced viral RNAs. Next, we will summarize the most recent findings on the mechanisms of Gag nuclear trafficking and discuss potential roles for nuclear localization of Gag proteins in retrovirus replication.

NC-mediated nucleolar localization of retroviral gag proteins

The assembly and release of retrovirus particles from the cell membrane is directed by the Gag polyprotein. The Gag protein of Rous sarcoma virus (RSV) traffics through the nucleus prior to plasma membrane localization. We previously reported that nuclear localization of RSV Gag is linked to efficient packaging of viral genomic RNA, however the intranuclear activities of RSV Gag are not well understood. To gain insight into the properties of the RSV Gag protein within the nucleus, we examined the subnuclear localization and dynamic trafficking of RSV Gag. Restriction of RSV Gag to the nucleus by mutating its nuclear export signal (NES) in the p10 domain or interfering with CRM1-mediated nuclear export of Gag by leptomycin B (LMB) treatment led to the accumulation of Gag in nucleoli and discrete nucleoplasmic foci. Retention of RSV Gag in nucleoli was reduced with cis-expression of the 5' untranslated RU5 region of the viral RNA genome, suggesting the psi (Ψ) packaging signal may alter the subnuclear localization of Gag. Fluorescence recovery after photobleaching (FRAP) demonstrated that the nucleolar fraction of Gag was highly mobile, indicating that there was rapid exchange with Gag proteins in the nucleoplasm. RSV Gag is targeted to nucleoli by a nucleolar localization signal (NoLS) in the NC domain, and similarly, the human immunodeficiency virus type 1 (HIV-1) NC protein also contains an NoLS consisting of basic residues. Interestingly, co-expression of HIV-1 NC or Rev with HIV-1 Gag resulted in accumulation of Gag in nucleoli. Moreover, a subpopulation of HIV-1 Gag was detected in the nucleoli of HeLa cells stably expressing the entire HIV-1 genome in a Rev-dependent fashion. These findings suggest that the RSV and HIV-1 Gag proteins undergo nucleolar trafficking in the setting of viral infection.

The bovine immunodeficiency virus Rev protein: identification of a novel nuclear import pathway and nuclear export signal among retroviral Rev/Rev-like proteins

The Rev protein is essential for the replication of lentiviruses. Rev is a shuttling protein that transports unspliced and partially spliced lentiviral RNAs from the nucleus to the cytoplasm via the nucleopore. To transport these RNAs, the human immunodeficiency virus type 1 (HIV-1) Rev uses the karyopherin β family importin β and CRM1 proteins that interact with the Rev nuclear localization signal (NLS) and nuclear exportation signal (NES), respectively. Recently, we reported the presence of new types of bipartite NLS and nucleolar localization signal (NoLS) in the bovine immunodeficiency virus (BIV) Rev protein. Here we report the characterization of the nuclear import and export pathways of BIV Rev. By using an in vitro nuclear import assay, we showed that BIV Rev is transported into the nucleus by a cytosolic and energy-dependent importin α/β classical pathway. Results from glutathione S-transferase (GST) pulldown assays that showed the binding of BIV Rev with importins α3 and α5 were in agreement with those from the nuclear import assay. We also identified a leptomycin B-sensitive NES in BIV Rev, which indicates that the protein is exported via CRM1 like HIV-1 Rev. Mutagenesis experiments showed that the BIV Rev NES maps between amino acids 109 to 121 of the protein. Remarkably, the BIV Rev NES was found to be of the cyclic AMP (cAMP)-dependent protein kinase inhibitor (PKI) type instead of the HIV-1 Rev type. In summary, our data showed that the nuclear import mechanism of BIV Rev is novel among Rev proteins characterized so far in lentiviruses.

Human T lymphotropic virus type 1 regulatory and accessory gene transcript expression and export are not rex dependent

Human T lymphotropic virus type 1 (HTLV-1) requires regulated gene expression from unspliced and alternatively spliced transcripts for efficient replication and persistence. HTLV-1 Rex is known to facilitate cytoplasmic export of unspliced, gag/pol and incompletely spliced env mRNAs, but its contribution to the expression of other viral transcripts has not been experimentally assessed. In this study, we utilized HTLV-1 proviral clones, cellular fractionation, and real-time reverse transcriptase PCR to determine the role of Rex on the expression and export of all viral mRNAs. Our results indicate that the steady-state levels of the different viral mRNAs are modulated by Rex, which we attribute to a redistribution of completely spliced mRNAs toward incompletely spliced mRNAs. Furthermore, we confirmed the positive effect of Rex on the unspliced gag/pol mRNA and singly spliced env mRNA, resulting in increased cytoplasmic expression. However, the cytoplasmic export of the alternatively spliced HTLV-1 mRNAs encoding the accessory proteins and the antisense Hbz mRNA are independent of direct Rex regulation. This is consistent with the conclusion that viral mRNAs that contain the cis-acting repressive sequence (CRS) and/or a fully functional splice donor site require a Rex/RxRE interaction for efficient cytoplasmic expression.

Comparison of the Genetic Organization, Expression Strategies and Oncogenic Potential of HTLV-1 and HTLV-2

Human T cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2) are genetically related complex retroviruses that are capable of immortalizing human T-cells in vitro and establish life-long persistent infections in vivo. In spite of these apparent similarities, HTLV-1 and HTLV-2 exhibit a significantly different pathogenic potential. HTLV-1 is recognized as the causative agent of adult T-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). In contrast, HTLV-2 has not been causally linked to human malignancy, although it may increase the risk of developing inflammatory neuropathies and infectious diseases. The present paper is focused on the studies aimed at defining the viral genetic determinants of the pathobiology of HTLV-1 and HTLV-2 through a comparison of the expression strategies and functional properties of the different gene products of the two viruses.

Multifunctional conjugates to prepare nucleolar-targeting CdS quantum dots

We used a click reaction to synthesize a bidentate 1,2,3-triazole-based ligand, TA, for use in the preparation of aqueous CdS quantum dots (QDs). TA-conjugated CdS QDs exhibited two fluorescence emission peaks, one at 540 nm arising from CdS nanocrystals and the other at approximately 670 nm arising from TA-CdS QD complexes formed via surface coordination. Coordination between TA and CdS was verified by using X-ray photoelectron (N 1s) spectra as well as Raman and NMR spectra of TA-capped QDs. Electrochemical analysis revealed that the 1,2,3-triazole moities in TA form complexes with the Cd(II) ions. The aqueous QDs protected by TA were very stable at different ionic strengths and over a broad pH range, according to fluorescence analysis. The ethidium bromide exclusion assay demonstrated that the bidentate TA ligand interacts strongly with DNA. Fluorescent micrographs and TEM images of cancer cells stained with TA-capped QDs clearly showed that the TA ligand targeted CdS QDs to the nucleoli of cells. In contrast, thioglycolic acid-capped CdS QDs just stained the cell membranes and could not pass the cell membranes to reach the cell nucleus.

Characterization and prediction of protein nucleolar localization sequences

Although the nucleolar localization of proteins is often believed to be mediated primarily by non-specific retention to core nucleolar components, many examples of short nucleolar targeting sequences have been reported in recent years. In this article, 46 human nucleolar localization sequences (NoLSs) were collated from the literature and subjected to statistical analysis. Of the residues in these NoLSs 48% are basic, whereas 99% of the residues are predicted to be solvent-accessible with 42% in α-helix and 57% in coil. The sequence and predicted protein secondary structure of the 46 NoLSs were used to train an artificial neural network to identify NoLSs. At a true positive rate of 54%, the predictor’s overall false positive rate (FPR) is estimated to be 1.52%, which can be broken down to FPRs of 0.26% for randomly chosen cytoplasmic sequences, 0.80% for randomly chosen nucleoplasmic sequences and 12% for nuclear localization signals. The predictor was used to predict NoLSs in the complete human proteome and 10 of the highest scoring previously unknown NoLSs were experimentally confirmed. NoLSs are a prevalent type of targeting motif that is distinct from nuclear localization signals and that can be computationally predicted.

The bovine immunodeficiency virus rev protein: identification of a novel lentiviral bipartite nuclear localization signal harboring an atypical spacer sequence

The bovine immunodeficiency virus (BIV) Rev protein (186 amino acids [aa] in length) is involved in the nuclear exportation of partially spliced and unspliced viral RNAs. Previous studies have shown that BIV Rev localizes in the nucleus and nucleolus of infected cells. Here we report the characterization of the nuclear/nucleolar localization signals (NLS/NoLS) of this protein. Through transfection of a series of deletion mutants of BIV Rev fused to enhanced green fluorescent protein and fluorescence microscopy analyses, we were able to map the NLS region between aa 71 and 110 of the protein. Remarkably, by conducting alanine substitution of basic residues within the aa 71 to 110 sequence, we demonstrated that the BIV Rev NLS is bipartite, maps to aa 71 to 74 and 95 to 101, and is predominantly composed of arginine residues. This is the first report of a bipartite Rev (or Rev-like) NLS in a lentivirus/retrovirus. Moreover, this NLS is atypical, as the length of the sequence between the motifs composing the bipartite NLS, e.g., the spacer sequence, is 20 aa. Further mutagenesis experiments also identified the NoLS region of BIV Rev. It localizes mainly within the NLS spacer sequence. In addition, the BIV Rev NoLS sequence differs from the consensus sequence reported for other viral and cellular nucleolar proteins. In summary, we conclude that the nucleolar and nuclear localizations of BIV Rev are mediated via novel NLS and NoLS motifs.

Transcriptional and post-transcriptional gene regulation of HTLV-1

Adult T-cell leukemia (ATL) is an aggressive hematologic malignancy caused by human T-cell leukemia virus type I (HTLV-1). Tax, encoded by the HTLV-1 pX region, has been recognized by its pleiotropic actions to play a critical role in leukemogenesis. Three highly conserved 21-bp repeat elements located within the long terminal repeat, commonly referred to as Tax-responsive element 1 (TRE-1), are critical to Tax-mediated viral transcriptional activation through complex interaction with cyclic AMP-responsive element binding protein (CREB), CBP/p300 and PCAF. Tax has also been shown to activate transcription from a number of critical cellular genes through the NF-kappaB and serum-responsive factor pathways. Tax transactivation has been attributed to the protein's interaction with transcription factors, chromatin remodeling complexes, cell cycle and repair genes. In this review, we will discuss some of the latest findings on this fascinating viral activator and highlight its regulation of cellular factors including CREB, p300/CBP and their effect on RNA polymerase II and chromatin remodeling, as well as its role in cytoplasmic and nuclear function. We will highlight the possible contribution of each factor, discuss Tax's critical peptide domains and highlight its post-transcriptional modifications. It is quite obvious that, collectively, Tax's effects on a wide variety of cellular targets cooperate in promoting cell proliferation and leukemogenesis. In addition, the post-transcriptional effects of Rex play an important role in virus replication. Understanding these interactions at a molecular level will facilitate the targeted development of drugs to effectively inhibit or treat ATL.

Mutational analysis of bovine leukemia virus Rex: identification of a dominant-negative inhibitor

The Rex proteins of the delta-retroviruses act to facilitate the export of intron-containing viral RNAs. The Rex of bovine leukemia virus (BLV) is poorly characterized. To gain a better understanding of BLV Rex, we generated a reporter assay to measure BLV Rex function and used it to screen a series of point and deletion mutations. Using this approach, we were able to identify the nuclear export signal of BLV Rex. Further, we identified a dominant-negative form of BLV Rex. Protein localization analysis revealed that wild-type BLV Rex had a punctate nuclear localization and was associated with nuclear pores. In contrast, the dominant-negative BLV Rex mutation had a diffuse nuclear localization and no nuclear pore association. Overexpression of the dominant-negative BLV Rex altered the localization of the wild-type protein. This dominant-negative derivative of BLV Rex could be a useful tool to test the concept of intracellular immunization against viral infection in a large animal model.

Translational repression by thecis-acting element of structure-anchored repression (CAESAR) of humanctgf/ccn2mRNA

The cis-acting element of structure-anchored repression (CAESAR) is a post-transcriptional regulatory element of gene expression, which is located in the 3'-untranslated region (UTR) of the human ccn2 gene (ctgf/ccn2). In this report, the repression mechanism of CAESAR, as well as the structural requirement, was investigated. Removal of minor stem-loops from CAESAR resulted in proportional attenuation of the repressive function, whereas removal of the single bulge or modification of primary nucleotide sequence did not affect its functionality. In light of functional mechanism, CAESAR exerted no significant effects on stability or nuclear export of the cis-linked mRNA. However, this element significantly interfered with the association of such mRNA on ribosome and slowed down the translation process thereafter in vitro. A translation repression mechanism by RNA secondary structure to determine the basal ctgf/ccn2 expression level was uncovered herein.

Nuclear localization of Japanese encephalitis virus core protein enhances viral replication

Japanese encephalitis virus (JEV) core protein was detected in both the nucleoli and cytoplasm of mammalian and insect cell lines infected with JEV or transfected with the expression plasmid of the core protein. Mutation analysis revealed that Gly(42) and Pro(43) in the core protein are essential for the nuclear and nucleolar localization. A mutant M4243 virus in which both Gly(42) and Pro(43) were replaced by Ala was recovered by plasmid-based reverse genetics. In C6/36 mosquito cells, the M4243 virus exhibited RNA replication and protein synthesis comparable to wild-type JEV, whereas propagation in Vero cells was impaired. The mutant core protein was detected in the cytoplasm but not in the nucleus of either C6/36 or Vero cell lines infected with the M4243 virus. The impaired propagation of M4243 in mammalian cells was recovered by the expression of wild-type core protein in trans but not by that of the mutant core protein. Although M4243 mutant virus exhibited a high level of neurovirulence comparable to wild-type JEV in spite of the approximately 100-fold-lower viral propagation after intracerebral inoculation to 3-week-old mice of strain Jcl:ICR, no virus was recovered from the brain after intraperitoneal inoculation of the mutant. These results indicate that nuclear localization of JEV core protein plays crucial roles not only in the replication in mammalian cells in vitro but also in the pathogenesis of encephalitis induced by JEV in vivo.

The human T-cell leukemia virus Rex protein

A critical step in the life cycle of complex retroviruses, including HTLV-1 and HTLV-2 is the ability of these viruses to adopt a mechanism by which the genome-length unspliced mRNA as well as the partially spliced mRNAs are exported from the nucleus instead of being subjected to splicing or degradation. In HTLV, this is accomplished through the expression of the viral Rex, which recognizes a specific response element on the incompletely spliced mRNAs, stabilizes them, inhibits their splicing, and utilizes the CRM1-dependent cellular pathway for transporting them from the nucleus to the cytoplasm. Rex itself is regulated by phosphorylation, which implies that proper activation of the protein in response to certain cellular cues is an important tool for the virus to ensure that specific viral gene expression is allowed only when the host cell can provide the best conditions for virion production. Having such a critical role in HTLV life cycle, Rex is indispensable for efficient viral replication, infection and spread. Indeed, Rex is considered to regulate the switch between the latent and productive phases of the HTLV life cycle. Without a functional Rex, the virus would still produce regulatory and some accessory gene products; however, structural and enzymatic post-transcriptional gene expression would be severely repressed, essentially leading to non-productive viral replication. More detailed understanding of the exact molecular mechanism of action of Rex will thus allow for better design of therapeutic drugs against Rex function and ultimately HTLV replication. Herein we summarize the progress made towards understanding Rex function and its role in the HTLV life cycle.

T-Cell Control by Human T-Cell Leukemia/Lymphoma Virus Type 1

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Collective evidence from in vitro studies indicates that several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function, such as antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation may therefore be of importance, as also suggested by epidemiological data. Thus genetic and environmental factors together with the virus contribute to disease development. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells. The relevance of these laboratory findings is related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Seizing of T Cells by Human T-Cell Leukemia⧸Lymphoma Virus Type 1

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function. Viral proteins modulate the downstream effects of antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation are therefore important, as also suggested by epidemiological data. The ability of a given individual to respond to specific antigens is determined genetically. Thus, genetic and environmental factors, together with the virus, contribute to disease development. As in the case of other virus-associated cancers, HTLV-1-induced leukemia/lymphoma can be prevented by avoiding viral infection or by intervention during the asymptomatic phase with approaches able to interrupt the vicious cycle of virus-induced proliferation of a subset of T-cells. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells in vitro. The relevance of these laboratory findings will be related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Subcellular localization of the bovine leukemia virus R3 and G4 accessory proteins

Bovine leukemia virus (BLV) is a complex retrovirus that belongs to the Deltaretrovirus genus, which also includes Human T-cell leukemia virus type 1 (HTLV-1). Both viruses contain an X region coding for at least four proteins: Tax and Rex, which are involved in transcriptional and posttranscriptional regulation, respectively, and the accessory proteins R3 and G4 (for BLV) and p12(I), p13(II), and p30(II) (for HTLV-1). The present study was aimed at characterizing the subcellular localization of BLV R3 and G4. The results of immunofluorescence experiments on transfected HeLa Tat cells demonstrated that R3 is located in the nucleus and in cellular membranes, as previously reported for HTLV-1 p12(I). In contrast, G4, like p13(II), is localized both in the nucleus and in mitochondria. In addition, we have shown that G4 harbors a mitochondrial targeting signal consisting of a hydrophobic region and an amphipathic alpha-helix. Thus, despite a lack of significant primary sequence homology, R3 and p12(I) and G4 and p13(II) exhibit similar targeting properties, suggesting possible overlap in their functional properties.

Rat CRM1 is responsible for the poor activity of human T-cell leukemia virus type 1 Rex protein in rat cells

Rat models of human T-cell leukemia virus type 1 (HTLV-1)-related diseases such as adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis have been reported. However, these models do not completely reproduce human diseases partly because HTLV-1 replicates poorly in rats. We investigated here the possible reason for this. We found that the activity of Rex in rat cells is quite low compared to that in human cells. As Rex function depends largely on the CRM1 protein, whose human type (human CRM1 [hCRM1]) directly binds to Rex and exports it from the nucleus to the cytoplasm, we assessed whether rat CRM1 (rCRM1) could act as well as hCRM1 as a cofactor for Rex activity. We first cloned a cDNA encoding rCRM1 and found that both rCRM1 and hCRM1 could bind to and export Rex protein to the cytoplasm with similar efficiencies. However, unlike hCRM1, rCRM1 could hardly support Rex function because of its poor ability in inducing the Rex-Rex interaction required for RNA export into the cytoplasm. These observations suggest that the poor ability of rCRM1 to act as a cofactor for Rex function may be responsible for the poor replication of HTLV-1 in rats.

Phosphorylation of two serine residues regulates human T-cell leukemia virus type 2 Rex function

The function of the human T-cell leukemia virus (HTLV) Rex phosphoprotein is to increase the level of the viral structural and enzymatic gene products expressed from the incompletely spliced viral RNAs containing the Rex-responsive element. The phosphorylation of HTLV type 2 Rex (Rex-2), predominantly on serine residues, correlates with an altered conformation, as detected by a gel mobility shift, and is required for specific binding to its viral RNA target sequence. Thus, the phosphorylation state of Rex in the infected cell may be a switch that determines whether the virus exists in a latent or a productive state. A mutational analysis of Rex-2 that focused on serine and threonine residues was performed to identify regions or domains within Rex-2 important for function, with a specific emphasis on identifying Rex-2 phosphorylation mutants. We identified mutations near the carboxy terminus that disrupted a novel region or domain and abrogated Rex-2 function. Mutant M17 (with S151A and S153A mutations) displayed reduced phosphorylation that correlated with reduced function. Replacement of both serine residues 151 and 153 with phosphomimetic aspartic acid restored Rex-2 function and locked Rex-2 in a phosphorylated active conformation. A mutant containing threonine residues at positions 151 and 153 displayed a phenotype indistinguishable from that of wild-type Rex. Furthermore, this same mutant showed increased threonine phosphorylation and decreased serine phosphorylation, providing conclusive evidence that one or both of these residues are phosphorylated in vivo. Our results provide the first direct evidence that the phosphorylation of Rex-2 is important for function. Further understanding of HTLV Rex phosphorylation will provide insight into the regulatory control of HTLV replication and ultimately the pathobiology of HTLV.

Novel properties of the nucleolar targeting signal of human angiogenin

The polypeptide ligand angiogenin, a potent inducer of angiogenesis, localizes in the nucleus/nucleolus subsequent to endocytosis by relevant cell types. This study examines the kinetic properties of the nucleolar targeting signal (NTS) of angiogenin (IMRRRGL(35)) at the single cell level. We show that the NTS is sufficient to target green fluorescent protein (GFP), but not beta-galactosidase, to the nucleolus of rat hepatoma cells. Mutation of Arg(33) to Ala within the NTS abolishes targeting activity. Nuclear/nucleolar import conferred by the NTS of angiogenin is reduced by cytosolic factors as well as ATP and is independent of importins and Ran. The NTS also confers the ability to bind to nuclear/nucleolar components which is inhibited by ATP hydrolysis; nonhydrolysable GTP analogs prevent nuclear accumulation in the absence of an intact nuclear envelope through an apparent cytoplasmic retention mechanism. Since the lectin wheat germ agglutinin does not inhibit transport, we postulate a mechanism for angiogenin nuclear/nucleolar import involving passive diffusion of angiogenin through the nuclear pore and NTS-mediated nuclear/nucleolar retention, and with cytoplasmic retention modulating the process. This pathway is clearly distinct from that of conventional signal-mediated nuclear protein import.

Identification and characterization of SEB, a novel protein that binds to the acute undifferentiated leukemia-associated protein SET

SET, the translocation breakpoint-encoded protein in acute undifferentiated leukemia (AUL), is a 39-kDa nuclear phosphoprotein and has an inhibitory activity for protein phosphatase 2A (PP2A). SET is fused to a putative oncoprotein, CAN/NUP214, in AUL and is thought to play a key role in leukemogenesis by its nuclear localization, protein-protein interactions and PP2A inhibitory activity. Here, we describe the isolation and characterization of a novel cDNA encoding a protein with 1542 amino-acid residues that specifically interacts in a yeast two-hybrid system as well as in human cells with SET. This new protein, which we name SEB (SET-binding protein), is identified as a 170-kDa protein by immunoprecipitation with a specific antibody and is localized predominantly in the nucleus. SEB1238--1434 is determined as a SET-binding region that specifically binds to SET182--223. SEB also has an oncoprotein Ski homologous region (amino acids 654--858), six PEST sequences and three sequential PPLPPPPP repeats at the C-terminus. SEB mRNA is expressed ubiquitously in all human adult tissues and cells examined. The SEB gene locus is assigned to the chromosome 18q21.1 that contains candidate tumor suppressor genes associated with deletions in cancer and leukemia. Although the function of SEB is not known, we propose that SEB plays a key role in the mechanism of SET-related leukemogenesis and tumorigenesis, perhaps by suppressing SET function or by regulating the transforming activity of Ski in the nucleus.

The potential terminase subunit of human cytomegalovirus, pUL56, is translocated into the nucleus by its own nuclear localization signal and interacts with importin alpha

Human cytomegalovirus (HCMV) DNA-binding protein pUL56 is thought to be involved in the cleavage/packaging process of viral DNA and therefore needs to be transported into the nucleus. By using indirect immunofluorescence analysis, HCMV pUL56 (p130) was found to be localized predominantly in the nucleus of infected cells. Solitary expression of wild-type as well as epitope-tagged pUL56 also resulted in nuclear distribution after transfection, suggesting the presence of an endogenous nuclear localization signal (NLS). Deletion of a carboxy-terminal stretch of basic amino acids (aa 816-827) prevented nuclear translocation, indicating that the sequence RRVRATRKRPRR of HCMV pUL56 mediates nuclear targetting. The signal character of the NLS sequence was demonstrated by successful transfer of the NLS to a reporter protein chimera. Furthermore, sequential substitutions of pairs of amino acids by alanine in the context of the reporter protein as well as substitutions within the full-length pUL56 sequence indicated that residues at positions 7 and 8 of the NLS (R and K at positions 822 and 823 of pUL56) were essential for nuclear translocation. In order to identify the transport machinery involved, the potential of pUL56 to bind importin alpha (hSRP1 alpha) was examined. Clear evidence of a direct interaction of a carboxy-terminal portion as well as the NLS of pUL56 with hSRP1 alpha was provided by in vitro binding assays. In view of these findings, it is suggested that nuclear translocation of HCMV pUL56 is mediated by the importin-dependent pathway.

Novel nucleolar protein, midnolin, is expressed in the mesencephalon during mouse development

Using the gene trap method and the selection of embryonic stem cells in vitro, we have identified several novel genes involved in mouse development. The detailed analysis of one of these, named midnolin (midbrain nucleolar protein), is reported here. Expression of the midnolin gene is developmentally regulated: it is strongly expressed at the mesencephalon (midbrain) of the embryo in day 12.5 (E12.5) mice. The midnolin encodes a protein of 508 amino acids (aa), which contains a Ubiquitin-like domain. The intracellular distribution of the midnolin was studied by using midnolin-green fluorescent protein (GFP) fusion proteins. Midnolin was found to be localized in the nucleus and nucleolus, but not in the cytoplasm. The nucleolar localization signal was determined to be a 28aa peptide (440-QQKRLRRKARRDARGPYHWTPSRKAGRS-467) located at the C-terminal region of the midnolin. Our results suggest that midnolin is involved in regulation of genes related to neurogenesis in the nucleolus.

Initiation of translation by non-AUG codons in human T-cell lymphotropic virus type I mRNA encoding both Rex and Tax regulatory proteins

Human T-cell lymphotropic virus type I (HTLV-I) double-spliced mRNA exhibits two GUG and two CUG codons upstream to, and in frame with, the sequences encoding Rex and Tax regulatory proteins, respectively. To verify whether these GUG and CUG codons could be used as additional initiation codons of translation, two chimeric constructs were built for directing the synthesis of either Rex-CAT or Tax-CAT fusion proteins. In both cases, the CAT reporter sequence was inserted after the Tax AUG codon and in frame with either the Rex or Tax AUG codon. Under transient expression of these constructs, other proteins of higher molecular mass were synthesized in addition to the expected Rex-CAT and Tax-CAT proteins. The potential non-AUG initiation codons were exchanged for either an AUG codon or a non-initiation codon. This allowed us to demonstrate that the two GUG codons in frame with the Rex coding sequence, and only the second CUG in frame with the Tax coding sequence, were used as additional initiation codons. In HTLV-I infected cells, two Rex and one Tax additional proteins were detected that exhibited molecular mass compatible with the use of the two GUG and the second CUG as additional initiation codons of translation. Comparison of the HTLV-I proviral DNA sequence with that of other HTLV-related retroviruses revealed a striking conservation of the three non-AUG initiation codons, strongly suggesting their use for the synthesis of additional Rex and Tax proteins.

Titration of cellular export factors, but not heteromultimerization, is the molecular mechanism of trans-dominant HTLV-1 rex mutants

The HTLV-1 Rex protein is an essential shuttle protein required for nuclear export of unspliced and incompletely-spliced viral RNAs. Several trans-dominant (TD) mutant Rex proteins have been reported, however, the mechanism of trans-dominance is not known. We compared TD Rex mutants and found that a natural occurring Rex mutant, Rexp21, lacking the RNA binding domain, was highly TD and inhibited also HIV-1 Rev function. Using fusions to the green fluorescent protein (GFP) we observed that Rexp21-GFP displayed a cytoplasmic localization but was actively shuttling between the nucleus and the cytoplasm in live human cells. The presence of Rexp21-GFP inhibited the nuclear export of Rex and HIV-1 Rev as assayed by cotransfection and microinjection experiments. However, Rex-GFP or Rexp21-GFP did not form heteromultimers with nuclear Rex mutants in vivo. In contrast, shuttling was essential for trans-dominance. Thus, we propose that TD Rex mutants do not function by retaining WT Rex in the nucleus by protein-protein interactions, as demonstrated for Rev, but to titrate factors essential for Rex/Rev export. Our findings demonstrate differences between the regulatory proteins Rex and Rev and implicate a novel strategy to generate highly TD Rex mutants also applicable to other proteins.

Multimer formation is not essential for nuclear export of human T-cell leukemia virus type 1 Rex trans-activator protein

The Rex trans-regulatory protein of human T-cell leukemia virus type 1 (HTLV-1) is required for the nuclear export of incompletely spliced and unspliced viral mRNAs and is therefore essential for virus replication. Rex is a nuclear phosphoprotein that directly binds to its cis-acting Rex response element RNA target sequence and constantly shuttles between the nucleus and cytoplasm. Moreover, Rex induces nuclear accumulation of unspliced viral RNA. Three protein domains which mediate nuclear import-RNA binding, nuclear export, and Rex oligomerization have been mapped within the 189-amino-acid Rex polypeptide. Here we identified a different region in the carboxy-terminal half of Rex which is also required for biological activity. In inactive mutants with mutations that map within this region, as well as in mutants that are deficient in Rex-specific multimerization, Rex trans activation could be reconstituted by fusion to a heterologous leucine zipper dimerization interface. The intracellular trafficking capabilities of wild-type and mutant Rex proteins reveal that biologically inactive and multimerization-deficient Rex mutants are still efficiently translocated from the nucleus to the cytoplasm. This observation indicates that multimerization is essential for Rex function but is not required for nuclear export. Finally, we are able to provide an improved model of the HTLV-1 Rex domain structure.

The nucleolus is the site of Borna disease virus RNA transcription and replication

Borna disease virus (BDV) is a neurotropic nonsegmented negative-strand RNA virus with limited homology to rhabdoviruses and paramyxoviruses. A distinguishing feature of BDV is that it replicates in the nucleus of infected cells. Strand-specific probes used for in situ hybridization of infected rat brain showed that there was differential localization of positive- and negative-strand RNAs within the nucleus of neurons. Within nuclei, sense-strand RNAs were preferentially localized within nucleolar regions while genomic-sense RNAs were found in both nucleolar and nonnucleolar regions. These results suggested a role for the nucleolus in BDV replication. Nucleoli isolated from persistently infected neuroblastoma cells contained both genomic and antigenomic BDV RNA species as well as an enrichment of the 39/38-kDa and gp18 BDV proteins. Since the nucleolus is the site of rRNA transcription, we examined BDV transcription in the presence of inhibitors of RNA polymerase I. Inhibition of RNA polymerase I did not affect levels of BDV transcription.

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.

Modulation of HTLV-I gene expression by HIV-1 Rev through an alternative RxRE-independent pathway mediated by the RU5 portion of the 5'-LTR

The 5'-RU5 portion of human T-lymphocyte virus type I (HTLV-I) long terminal repeat (LTR) had been reported to contain cis-acting elements for the controlled viral gene expression by the rex gene product. In this study, the human immunodeficiency virus type I (HIV-1) Rev protein was found to enhance gene expression, acting through the 5'-RU5 portion of HTLV-I, while the Rex-responsive element (RxRE)-mediated activation by Rev was reconfirmed to be negative. This positive action of HIV-1 Rev on HTLV-I gene expression seemed to be distinct from the widely accepted Rex or Rev function to facilitate the nuclear export of RxRE-containing unspliced viral mRNAs, since a trans-dominant, nuclear export-deficient mutant (RevM10) still retained the RU5-mediated effector function. Analyses of the functional aspects of Tat/Rev fusion proteins on the HTLV-I RU5 suggested a specific interaction of Rev and RU5, but lacked evidence for the binding of Rev to the RU5 at the RNA level. These results suggest an answer to the controversy regarding a Rex-like function occasionally observed with HIV-1 Rev and its related proteins. It may also be suggested that particular care should be taken when such a trans-dominant Rev mutant is considered to be used as a genetic therapy against HIV-I infection, in individuals infected with both HIV-I and HTLV-1.

The HIV-1 Tat nuclear localization sequence confers novel nuclear import properties

The different classes of conventional nuclear localization sequences (NLSs) resemble one another in that NLS-dependent nuclear protein import is energy-dependent and mediated by the cytosolic NLS-binding importin/karyopherin subunits and monomeric GTP-binding protein Ran/TC4. Based on analysis of the nuclear import kinetics mediated by the NLS of the human immunodeficiency virus accessory protein Tat using in vivo and in vitro nuclear transport assays and confocal laser scanning microscopy, we report a novel nuclear import pathway. We demonstrate that the Tat-NLS, not recognized by importin 58/97 subunits as shown using an enzyme-linked immunosorbent assay-based binding assay, is sufficient to target the 476-kDa heterologous beta-galactosidase protein to the nucleus in ATP-dependent but cytosolic factor-independent fashion. Excess SV40 large tumor antigen (T-ag) NLS-containing peptide had no significant effect on the nuclear import kinetics implying that the Tat-NLS was able to confer nuclear accumulation through a pathway distinct from conventional NLS-dependent pathways. Nucleoplasmic accumulation of the Tat-NLS-beta-galactosidase fusion protein, in contrast to that of a T-ag-NLS-containing fusion protein, also occurred in the absence of an intact nuclear envelope, implying that the Tat-NLS conferred binding to nuclear components. This is in stark contrast to known NLSs such as those of T-ag which confer nuclear entry rather than retention. Significantly, the ability to accumulate in the nucleus in the absence of an intact nuclear envelope was blocked in the absence of ATP, as well as by nonhydrolyzable ATP and GTP analogs, demonstrating that ATP is required to effect release from a complex with insoluble cytoplasmic components. Taken together, the results demonstrate that, dependent on ATP for release from cytoplasmic retention, the Tat-NLS is able to confer nuclear entry and binding to nuclear components. These unique properties indicate that Tat accumulates in the nucleus through a novel import pathway.

Protein B23 is an important human factor for the nucleolar localization of the human immunodeficiency virus protein Tat

Nucleolar shuttle protein B23 was found to bind to human immunodeficiency virus protein Tat, and this binding required the nucleolar localization motif of Tat. A fusion protein containing the B23 binding domain and beta-galactosidase caused mislocalization of Tat to the cytoplasm and inhibited the transactivation activity of Tat. These data suggest that B23 is a human factor necessary for the nucleolar localization of Tat.

The human T-cell lymphotropic virus type 1 Tof protein contains a bipartite nuclear localization signal that is able to functionally replace the amino-terminal domain of Rex

The X region of human T-cell lymphotropic virus type 1 (HTLV-1) encodes two nucleolar/nuclear proteins, the posttranscriptional regulator of mRNA expression Rex and a protein of unknown function named Tof. To gain insight into the possible biological role of Tof, we investigated the mechanism governing its intracellular trafficking and identified its nucleolar/nuclear localization signal (NLS). Mutational analysis of Tof revealed that its NLS was located between amino acids 71 and 98 and contained two arginine-rich domains that functioned in an interdependent manner. Studies of Tof-Rex hybrid proteins showed that the Tof NLS could functionally replace the NLS of Rex at the level of nuclear targeting. As the NLS of Rex is known to mediate its interaction with its RNA target, the Rex-responsive element (RXRE), we tested whether the NLS of Tof could replace that of Rex in mediating activation of a RXRE-containing mRNA. Results showed that the NLS of Tof was indeed able to mediate activation of RXRE-containing mRNAs, suggesting that Tof itself may function as a regulator of RNA expression and utilization. A comparison of their compartmentalization in response to actinomycin D treatment indicated that Tof did not share Rex's shuttling pathway. Expression of Tof from its natural multiply spliced mRNA required the presence of Rex, suggesting that Tof may regulate viral or cellular mRNA expression during the later stages of viral replication.

Enhancement of nucleocytoplasmic export of HTLV-1 Rex mRNA through cis and trans interactions of the mRNA with the complex of Rex protein and Rex-responsive element

p27 rex of HTLV-1 promotes nucleocytoplasmic export of viral mRNAs through binding of the Rex-response element (RexRE) present at the 3' end of the viral transcripts in cis with respect to the ORFs of the viral mRNAs. We have found that expression of the RexRE in trans, as a separate RNA, still allows Rex protein to promote export of viral mRNAs lacking the RexRE. The data suggest the formation of a ternary complex between Rex protein, RexRE and upstream elements of viral mRNA and hence the existence of secondary sites of interaction between Rex protein and viral RNAs.

Intracellular compartmentalization of the glycoprotein B of herpesvirus Simian agent 8 expressed with a baculovirus vector in insect cells

The intracellular localization of the glycoprotein B of herpesvirus simian agent 8 expressed with a baculovirus system in insect cells was studied. Cell fractionation and immunoprecipitation revealed that gB is present in microsomal as well as in nuclear membranes. Both fractions contain oligomers, probably dimers, of gB with endoglycosidase-H sensitive, mannose-rich carbohydrates. Nuclear transport of gB was further analysed by immuno electron microscopy of recombinant baculovirus-infected cells. The glycoprotein is present both in the outer and the inner nuclear membrane as well as in cytoplasmic structures and at the cell surface. This study precludes the possibility that glycosylation and/or oligomerisation of SA8 gB are responsible for nuclear targeting.

Human T-cell leukemia virus type 2 Rex inhibits pre-mRNA splicing in vitro at an early stage of spliceosome formation

The Rex protein is an essential regulator of RNA expression in human T-cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2) that promotes the accumulation of full-length and partially spliced viral transcripts in the cytoplasm. Rex-mediated regulation correlates with specific binding to a cognate RNA recognition element which overlaps the 5' splice site in the viral long terminal repeat. It has been unclear whether Rex directly affects splicing or only nuclear-to-cytoplasmic transport of viral mRNA. We demonstrate that HTLV-2 Rex is a potent inhibitor of splicing in vitro at an early step in spliceosome assembly. Inhibition requires phosphorylation of Rex and the ability of Rex to bind to the Rex response element. Direct inhibition of early spliceosome assembly by Rex may account for differential accumulation of unspliced transcripts and represents a novel mechanism of retroviral gene regulation.

C23 interacts with B23, a putative nucleolar-localization-signal-binding protein

The human protein C23 (nucleolin) is a major nucleolar protein. Its interactions with other proteins were studied with the two-hybrid system which identified nucleolar protein B23 (nucleophosmin) as being associated with C23. Both proteins were co-immunoprecipitated from HeLa cell nuclear extract by either monoclonal anti-C23 or monoclonal anti-B23. Binding studies utilizing deletion mutants indicated that the binding of C23 and B23 involves specific motifs. In addition to an approximately 46-amino-acid-binding domain in B23 (amino acids 194-239), amino acids 540-628 of C23 were required for binding; this region of C23 is required for the nucleolar localization. In addition, nucleolar protein p120 was also found to be co-immunoprecipitated with B23. A fragment of p120 containing a functional nucleolar localization signal bound to the truncated binding domain of B23, as did C23. These results suggest that the interaction of C23 and B23 may represent a nucleolar-targeting mechanism in which B23 acts as a nucleolar-localization signal-binding protein.

Nucleolar accumulation of poly (A)+ RNA in heat-shocked yeast cells: implication of nucleolar involvement in mRNA transport

Transport of mRNA from the nucleus to the cytoplasm plays an important role in gene expression in eukaryotic cells. In wild-type Schizosaccharomyces pombe cells poly(A)+ RNA is uniformly distributed throughout the nucleoplasm and cytoplasm. However, we found that a severe heat shock blocks mRNA transport in S. pombe, resulting in the accumulation of bulk poly(A)+ RNA, as well as a specific intron-less transcript, in the nucleoli. Pretreatment of cells with a mild heat shock, which induces heat shock proteins, before a severe heat shock protects the mRNA transport machinery and allows mRNA transport to proceed unimpeded. In heat-shocked S. pombe cells, the nucleolar region condensed into a few compact structures. Interestingly, poly(A)+ RNA accumulated predominantly in the condensed nucleolar regions of the heat-shocked cells. These data suggest that the yeast nucleolus may play a role in mRNA transport in addition to its roles in rRNA synthesis and preribosome assembly.

The 5' coding sequence of IL-2 receptor alpha chain mRNA mediates mRNA stabilization by HTLV-1 Rex

The interleukin-2 receptor alpha chain (IL-2R alpha) is a T-cell growth factor receptor and is known to be induced in helper T cells by infection with human T-cell leukaemia virus type-1 (HTLV-1). The Rex protein of HTLV-1 has been shown to stabilize IL-2R alpha mRNA. Although the 3' untranslated region of many RNA has been regarded as a key element for stabilization, we found that the first 300 bases of the IL-2R alpha protein coding sequence were necessary for stabilization of the mRNA. As the first 201 bases were not sufficient for this effect, we conclude that the bases at position 201-300 downstream of the AUG start are important for stabilization.

Nucleolar accumulation of poly (A)+ RNA in heat-shocked yeast cells: implication of nucleolar involvement in mRNA transport

Transport of mRNA from the nucleus to the cytoplasm plays an important role in gene expression in eukaryotic cells. In wild-type Schizosaccharomyces pombe cells poly(A)+ RNA is uniformly distributed throughout the nucleoplasm and cytoplasm. However, we found that a severe heat shock blocks mRNA transport in S. pombe, resulting in the accumulation of bulk poly(A)+ RNA, as well as a specific intron-less transcript, in the nucleoli. Pretreatment of cells with a mild heat shock, which induces heat shock proteins, before a severe heat shock protects the mRNA transport machinery and allows mRNA transport to proceed unimpeded. In heat-shocked S. pombe cells, the nucleolar region condensed into a few compact structures. Interestingly, poly(A)+ RNA accumulated predominantly in the condensed nucleolar regions of the heat-shocked cells. These data suggest that the yeast nucleolus may play a role in mRNA transport in addition to its roles in rRNA synthesis and preribosome assembly.

Binding of human prothymosin alpha to the leucine-motif/activation domains of HTLV-I Rex and HIV-1 Rev

Rex of human T-cell leukemia virus type I (HTLV-I) and Rev of human immunodeficiency virus 1 (HIV-1) are post-transcriptional regulators of viral gene expression. By means of affinity chromatography, we purified an 18-kDa cellular protein that bound to the conserved leucine-motif/activation domain of HTLV-I Rex or HIV-1 Rev. The protein that was purified through a Rev-affinity column was found to bind to Rex immunoprecipitated with anti-Rex IgG from an HTLV-I-producing cell line. We analyzed the purified approximately 18-kDa protein biochemically and identified it as prothymosin alpha. The binding activity of prothymosin alpha to Rev or Rex was completely abolished when the epsilon-amino groups of its lysine residues were chemically modified by N-succinimidyl-3-(4-hydroxy-3,5-diodo- phenyl)propionate. The functional relationship between the nuclear protein prothymosin alpha and Rex-Rev is discussed.

Occurrence and role of an early antigen and evidence for transforming ability of porcine circovirus

By means of indirect immunofluorescence assay (IFA) using natural swine immune serum and hyperimmune serum from rabbits infected with porcine circovirus (PCV), a PCV antigen was detected present prior to the onset of viral and cellular DNA synthesis in nucleoli of cells of synchronized and growth stimulated infected PS cell cultures grown for more than 12 h in the presence of hydroxyurea. The number of cells containing specifically fluorescing nucleoli increased with increasing time of growth in the presence of hydroxyurea. The concomitant increase in the number of cells containing virus structural (VS) antigen in the nuclei and the increase in the amount of replicative (RF) DNA and accompanying 5 S DNA after release from the hydroxyurea block suggest that EA is involved in induction of PCV DNA replication. Primary pig kidney cell cultures persistently infected with PCV survived mock-infected control cultures for 16 passages. They had lost contact inhibition and formed cell colonies in soft agar at a ratio of 0.1 to 0.4%. Cell lines derived from agar colonies showed properties of transformed cells e.g. low requirement for serum growth factors, ability to overgrow a continuous cell layer, anchorage independence of growth. In transformed cells stimulated to growth and grown in the presence of hydroxyurea, non-structural viral antigen visible by IFA in nucleoli and VS antigen located in the cytoplasm were expressed. Contrary to virus bound nuclear VS antigen in productive infection, accumulation of cytoplasmatic VS antigen was independent of DNA synthesis and caused cell destruction, thus limiting growth of cell layers and colonies in soft agar.

Mutations in nucleolar proteins lead to nucleolar accumulation of polyA+ RNA in Saccharomyces cerevisiae

Synthesis of mRNA and rRNA occur in the chromatin-rich nucleoplasm and the nucleolus, respectively. Nevertheless, we here report that a Saccharomyces cerevisiae gene, MTR3, previously implicated in mRNA transport, codes for a novel essential 28-kDa nucleolar protein. Moreover, in mtr3-1 the accumulated polyA+ RNA actually colocalizes with nucleolar antigens, the nucleolus becomes somewhat disorganized, and rRNA synthesis and processing are inhibited. A strain with a ts conditional mutation in RNA polymerase I also shows nucleolar accumulation of polyA+ RNA, whereas strains with mutations in the nucleolar protein Nop1p do not. Thus, in several mutant backgrounds, when mRNA cannot be exported i concentrates in the nucleolus. mRNA may normally encounter nucleolar components before export and proteins such as Mtr3p may be critical for export of both mRNA and ribosomal subunits.

Adenoviral E1B-55kDa protein inhibits yeast mRNA export and perturbs nuclear structure

The mechanisms of export of RNA from the nucleus are poorly understood; however, several viral proteins modulate nucleocytoplasmic transport of mRNA. Among these are the adenoviral proteins E1B-55kDa and E4-34kDa. Late in infection, these proteins inhibit export of host transcripts and promote export of viral mRNA. To investigate the mechanism by which these proteins act, we have expressed them in Saccharomyces cerevisiae. Overexpression of either or both proteins has no obvious effect on cell growth. By contrast, overexpression of E1B-55kDa bearing a nuclear localization signal (NLS) dramatically inhibits cell growth. In this situation, the NLS-E1B-55kDa protein is localized to the nuclear periphery, fibrous material is seen in the nucleoplasm, and poly(A)+ RNA accumulates in the nucleus. Simultaneous overexpression of E4-34kDa bearing or lacking an NLS does not modify these effects. We discuss the mechanisms of selective mRNA transport.

GTP gamma S restores nucleophosmin (NPM) localization to nucleoli of GTP-depleted HeLa cells

Previous studies showed that localization of nucleophosmin/B23 (NPM) to nucleoli requires adequate cellular GTP levels (Finch et al., J Biol Chem 268, 5823-5827, 1993). In order to study whether hydrolysis of GTP plays a role in NPM localization, we introduced a nonhydrolyzable GTP analog into HeLa cells. Cells were first depleted of GTP with the IMP dehydrogenase inhibitor, mycophenolic acid (MA), to induce translocation of NPM from the nucleoli to the nucleoplasm. Non-hydrolyzable GTP analogs were then introduced into cells by electroporation. We found that introduction of the non-hydrolyzable analog, GTP gamma S, was effective in restoring NPM localization to nucleoli. Cells incubated in medium containing G-nucleotides without electroporation showed no effect. To reduce the possibility that cells use guanine from degraded nucleotide to supplement GTP pools via salvage pathways, experiments were also performed in the presence of (6-mercaptopurine) 6MP, a competitive inhibitor of the salvage enzyme, HGPRT (hypoxanthine guanine phosphoribosyl transferase), in addition to MA. Under these conditions, introduction of GTP gamma S still effectively restored the localization of NPM into nucleoli. This study demonstrates that electroporation can be used effectively to introduce nucleotides into cultured cells without excessive loss of viability. Our results also indicate that the GTP dependent localization of NPM to the nucleoli may not require GTP hydrolysis.