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

Post-transcriptional Regulation of HTLV Gene Expression: Rex to the Rescue

Human T-lymphotropic virus type 1 (HTLV-1) and other members of the Deltaretrovirus genus code for a regulatory protein named Rex that binds to the Rex-responsive element present on viral mRNAs. Rex rescues viral mRNAs from complete splicing or degradation and guides them to the cytoplasm for translation. The activity of Rex is essential for expression of viral transcripts coding for the virion components and thus represents a potential target for virus eradication. We present an overview of the functional properties of the HTLV-1 and HTLV-2 Rex proteins (Rex-1 and Rex-2), outline mechanisms controlling Rex function, and discuss similarities and differences in the sequences of Rex coded by HTLV-1, -2, -3, and -4 that may influence their molecular anatomy and functional properties.

Comparative virology of HTLV-1 and HTLV-2

Human T cell leukemia virus type 1 (HTLV-1) was the first discovered human retrovirus and the etiologic agent of adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. Shortly after the discovery of HTLV-1, human T-cell leukemia virus type 2 (HTLV-2) was isolated from a patient with hairy cell leukemia. Despite possession of similar structural features to HTLV-1, HTLV-2 has not been definitively associated with lymphoproliferative disease. Since their discovery, studies have been performed with the goal of highlighting the differences between HTLV-1 and HTLV-2. A better understanding of these differences will shed light on the specific pathogenic mechanisms of HTLV-1 and lead to novel therapeutic targets. This review will compare and contrast the two oldest human retroviruses with regards to epidemiology, genomic structure, gene products, and pathobiology.

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.

Fine tuning of the temporal expression of HTLV-1 and HTLV-2

Human T-cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2) are delta retroviruses that share a common overall genetic organization, splicing pattern, and ability to infect and immortalize T-cells in vitro. However, HTLV-1 and HTLV-2 exhibit a clearly distinct pathogenic potential in infected patients. To find clues to the possible viral determinants of the biology of these viruses, recent studies investigated the timing of expression and the intracellular compartmentalization of viral transcripts in ex-vivo samples from infected patients. Results of these studies revealed a common overall pattern of expression of HTLV-1 and -2 with a two-phase kinetics of expression and a nuclear accumulation of minus-strand transcripts. Studies in cells transfected with HTLV-1 molecular clones demonstrated the strict Rex-dependency of this "two-phase" kinetics. These studies also highlighted interesting differences in the relative abundance of transcripts encoding the Tax and Rex regulatory proteins, and that of the accessory proteins controlling Rex expression and function, thus suggesting a potential basis for the different pathobiology of the two viruses.

Post-transcriptional regulation of CD83 expression by AUF1 proteins

Mature dendritic cells (DC), activated lymphocytes, mononuclear cells and neutrophils express CD83, a surface protein apparently necessary for effective DC-mediated activation of naïve T-cells and T-helper cells, thymic T-cell maturation and the regulation of B-cell activation and homeostasis. Although a defined ligand of CD83 remains elusive, the multiple cellular subsets expressing CD83, as well as its numerous potential implications in immunological processes suggest that CD83 plays an important regulatory role in the mammalian immune system. Lately, nucleocytoplasmic translocation of CD83 mRNA was shown to be mediated by direct interaction between the shuttle protein HuR and a novel post-transcriptional regulatory element (PRE) located in the CD83 transcript's coding region. Interestingly, this interaction commits the CD83 mRNA to efficient nuclear export through the CRM1 protein translocation pathway. More recently, the cellular phosphoprotein and HuR ligand ANP32B (APRIL) was demonstrated to be directly involved in this intracellular transport process by linking the CD83 mRNA:HuR ribonucleoprotein (RNP) complex with the CRM1 export receptor. Casein kinase II regulates this process by phosphorylating ANP32B. Here, we identify another RNA binding protein, AUF1 (hnRNP D) that directly interacts with CD83 PRE. Unlike HuR:PRE binding, this interaction has no impact on intracellular trafficking of CD83 mRNA-containing complexes; but it does regulate translation of CD83 mRNA. Thus, our data shed more light on the complex process of post-transcriptional regulation of CD83 expression. Interfering with this process may provide a novel strategy for inhibiting CD83, and thereby cellular immune activation.

Overexpression of the catalytically impaired Taspase1 T234V or Taspase1 D233A variants does not have a dominant negative effect in T(4;11) leukemia cells

Background

The chromosomal translocation t(4;11)(q21;q23) is associated with high-risk acute lymphoblastic leukemia of infants. The resulting AF4•MLL oncoprotein becomes activated by Taspase1 hydrolysis and is considered to promote oncogenic transcriptional activation. Hence, Taspase1’s proteolytic activity is a critical step in AF4•MLL pathophysiology. The Taspase1 proenzyme is autoproteolytically processed in its subunits and is assumed to assemble into an αββα-heterodimer, the active protease. Therefore, we investigated here whether overexpression of catalytically inactive Taspase1 variants are able to interfere with the proteolytic activity of the wild type enzyme in AF4•MLL model systems.

Methodology/Findings

The consequences of overexpressing the catalytically dead Taspase1 mutant, Taspase1^T234V, or the highly attenuated variant, Taspase1^D233A, on Taspase1’s processing of AF4•MLL and of other Taspase1 targets was analyzed in living cancer cells employing an optimized cell-based assay. Notably, even a nine-fold overexpression of the respective Taspase1 mutants neither inhibited Taspase1’s cis- nor trans-cleavage activity in vivo. Likewise, enforced expression of the α- or β-subunits showed no trans-dominant effect against the ectopically or endogenously expressed enzyme. Notably, co-expression of the individual α- and β-subunits did not result in their assembly into an enzymatically active protease complex. Probing Taspase1 multimerization in living cells by a translocation-based protein interaction assay as well as by biochemical methods indicated that the inactive Taspase1 failed to assemble into stable heterocomplexes with the wild type enzyme.

Conclusions

Collectively, our results demonstrate that inefficient heterodimerization appears to be the mechanism by which inactive Taspase1 variants fail to inhibit wild type Taspase1’s activity in trans. Our work favours strategies targeting Taspase1’s catalytic activity rather than attempts to block the formation of active Taspase1 dimers to interfere with the pathobiological function of AF4•MLL.

Nuclear export and expression of human T-cell leukemia virus type 1 tax/rex mRNA are RxRE/Rex dependent

Human T-cell leukemia virus type 1 (HTLV-1) is a complex retrovirus associated with the lymphoproliferative disease adult T-cell leukemia/lymphoma (ATL) and the neurodegenerative disorder tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). Replication of HTLV-1 is under the control of two major trans-acting proteins, Tax and Rex. Previous studies suggested that Tax activates transcription from the viral long terminal repeat (LTR) through recruitment of cellular CREB and transcriptional coactivators. Other studies reported that Rex acts posttranscriptionally and allows the cytoplasmic export of unspliced or incompletely spliced viral mRNAs carrying gag/pol and env only. As opposed to HIV's Rev-responsive element (RRE), the Rex-responsive element (RxRE) is present in all viral mRNAs in HTLV-1. However, based on indirect observations, it is believed that nuclear export and expression of the doubly spliced tax/rex RNA are Rex independent. In this study, we demonstrate that Rex does stimulate Tax expression, through nuclear-cytoplasmic export of the tax/rex RNA, even though a Rex-independent basal export mechanism exists. This effect was dependent upon the RxRE element and the RNA-binding activity of Rex. In addition, Rex-mediated export of tax/rex RNA was CRM1 dependent and inhibited by leptomycin B treatment. RNA immunoprecipitation (RNA-IP) experiments confirmed Rex binding to the tax/rex RNA in both transfected cells with HTLV-1 molecular clones and HTLV-1-infected T cells. Since both Rex and p30 interact with the tax/rex RNA and with one another, this may offer a temporal and dynamic regulation of HTLV-1 replication. Our results shed light on HTLV-1 replication and reveal a more complex regulatory network than previously anticipated.

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.

Requirements for mouse mammary tumor virus Rem signal peptide processing and function

Mouse mammary tumor virus (MMTV) encodes a Rev-like protein, Rem, which is involved in the nuclear export and expression of viral RNA. Previous data have shown that all Rev-like functions are localized to the 98-amino-acid signal peptide (SP) at the N terminus of MMTV Rem or envelope proteins. MMTV-SP uses endoplasmic reticulum-associated degradation (ERAD) for protein trafficking. Rem cleavage by signal peptidase in the ER is necessary for MMTV-SP function in a reporter assay, but many requirements for trafficking are not known. To allow detection and localization of both MMTV-SP and the C-terminal cleavage product, we prepared plasmids expressing green fluorescent protein (GFP) tags. N-terminal Rem tagging led to protein accumulation relative to untagged Rem and allowed signal peptidase cleavage but reduced its specific activity. C-terminal tagging also led to Rem accumulation yet dramatically reduced cleavage, GFP fluorescence, and activity relative to N-terminally tagged Rem (GFPRem). Substitutions of an invariant leucine at position 71 between the known RNA-binding and nuclear export sequences interfered with GFPRem accumulation and activity but not cleavage. Similarly, deletion of 100 or 150 C-terminal amino acids from GFPRem dramatically reduced both Rem and MMTV-SP levels and function. Removal of the entire C terminus (203 amino acids) restored both protein levels and activity of MMTV-SP. Only C-terminal GFP tagging, and not other modifications, appeared to trap Rem in the ER membrane. Thus, Rem conformation in both the ER lumen and cytoplasm determines cleavage, retrotranslocation, and MMTV-SP function. These mutants further characterize intermediates in Rem trafficking and have implications for all proteins affected by ERAD.

Converging strategies in expression of human complex retroviruses

The discovery of human retroviruses in the early 1980s revealed the existence of viral-encoded non-structural genes that were not evident in previously described animal retroviruses. Based on the absence or presence of these additional genes retroviruses were classified as ‘simple’ and ‘complex’, respectively. Expression of most of these extra genes is achieved through the generation of alternatively spliced mRNAs. The present review summarizes the genetic organization and expression strategies of human complex retroviruses and highlights the converging mechanisms controlling their life cycles.

Functional characterization of the HuR:CD83 mRNA interaction

Maturation of dendritic cells (DC) is characterized by expression of CD83, a surface protein that appears to be necessary for the effective activation of naïve T-cells and T-helper cells by DC. Lately it was shown that CD83 expression is regulated on the posttranscriptional level by interaction of the shuttle protein HuR with a novel posttranscriptional regulatory RNA element (PRE), which is located in the coding region of the CD83 transcript. Interestingly, this interaction commits the CD83 mRNA to efficient nuclear export via the CRM1 pathway. To date, however, the structural basis of this interaction, which potentially involves three distinct RNA recognition motifs (RRM1–3) in HuR and a complex three-pronged RNA stem-loop element in CD83 mRNA, has not been investigated in detail. In the present work we analyzed this interaction in vitro and in vivo using various HuR- and CD83 mRNA mutants. We are able to demonstrate that both, RRM1 and RRM2 are crucial for binding, whereas RRM3 as well as the HuR hinge region contributed only marginally to this protein∶RNA interaction. Furthermore, mutation of uridine rich patches within the PRE did not disturb HuR:CD83 mRNA complex formation while, in contrast, the deletion of specific PRE subfragments from the CD83 mRNA prevented HuR binding in vitro and in vivo. Interestingly, the observed inhibition of HuR binding to CD83 mRNA does not lead to a nuclear trapping of the transcript but rather redirected this transcript from the CRM1- towards the NXF1/TAP-specific nuclear export pathway. Thus, the presence of a functional PRE permits nucleocytoplasmic trafficking of the CD83 transcript via the CRM1 pathway.

Retroviral Rem protein requires processing by signal peptidase and retrotranslocation for nuclear function

Mouse mammary tumor virus (MMTV) is a complex murine retrovirus that encodes an HIV Rev-like export protein, Rem, from a doubly spliced version of envelope (Env) mRNA. Previously, the N-terminal 98-amino acid sequence of Rem, which is identical to Env signal peptide (SP), and full-length Rem were shown to be functional in a reporter assay that measures a postexport function. Here we show that MMTV-infected cells or cells transfected with rem or env cDNAs express SP, which is the active component in the reporter assay. Uncleaved Rem was partially glycosylated, but mutations in both glycosylation sites within the C terminus prevented Rem function. Mutations that reduced Rem or Env cleavage by signal peptidase greatly reduced SP levels and functional activity in the reporter assay and allowed accumulation of the uncleaved protein. Fluorescence microscopy revealed that GFP-tagged cleavage-site mutants are unstable and lack fluorescence compared with wild-type Rem, suggesting improper folding. Proteasome inhibitors allowed accumulation of uncleaved Rem relative to SP and increased reporter activity, consistent with SP retrotranslocation and proteasome escape before nuclear entry. Expression of a dominant-negative p97 ATPase did not alter levels of unprocessed Rem and SP but decreased reporter activity, suggesting p97-facilitated retrotranslocation of SP. Our results provide an example of a SP that is processed by signal peptidase and retrotranslocated to allow nuclear localization and function.

Analysis of nucleocytoplasmic trafficking of the HuR ligand APRIL and its influence on CD83 expression

Dendritic cells (DC) are the most potent antigen-presenting cells of the immune system and are able to sensitize even naïve T cells. Mature DC are characterized by expression of CD83, a surface molecule that is proposed to be involved in efficient T cell activation. It has been recently shown that CD83 mRNA is transported from the nucleus to the cytoplasm in a HuR- and CRM1-dependent manner. Therefore we here investigated the impact of two known protein ligands of HuR, pp32 and APRIL, on CD83 expression. Both pp32 (ANP32A) and APRIL (ANP32B) are shuttle proteins, and it has been reported earlier that these HuR ligands can act as adaptors that link HuR and the CRM1-specific nuclear export pathway. By employing RNA interference (RNAi) technology we demonstrate that pp32 is dispensable for CD83 expression, whereas APRIL contributes to the nuclear export and subsequent translation of CD83 mRNA. Furthermore, we have determined the nuclear import signal (NLS) as well as the nuclear export signal (NES) of human APRIL. Moreover, we analyzed the status of phosphorylation of endogenous APRIL and identified threonine 244 to be an as yet unrecognized phosphate acceptor. Finally, we were able to show that phosphorylation of this specific amino acid residue regulates the nuclear export of APRIL. In sum, we report here the signal sequences in APRIL that mediate its intracellular trafficking and provide evidence that this protein ligand of HuR is an important player in the post-transcriptional regulation of CD83 expression by affecting the nucleocytoplasmic translocation of CD83 mRNA.

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.

Nuclear export is evolutionarily conserved in CVC paired-like homeobox proteins and influences protein stability, transcriptional activation, and extracellular secretion

Homeodomain transcription factors control a variety of essential cell fate decisions during development. To understand the developmental regulation by these transcription factors, we describe here the molecular analysis of paired-like CVC homeodomain protein (PLC-HDP) trafficking. Complementary experimental approaches demonstrated that PLC-HDP family members are exported by the Crm1 pathway and contain an evolutionary conserved leucine-rich nuclear export signal. Importantly, inactivation of the nuclear export signal enhanced protein stability, resulting in increased transactivation of transfected reporters and decreased extracellular secretion. In addition, PLC-HDPs harbor a conserved active nuclear import signal that could also function as a protein transduction domain. In our study, we characterized PLC-HDPs as mobile nucleocytoplasmic shuttle proteins with the potential for unconventional secretion and intercellular transfer. Nucleocytoplasmic transport may thus represent a conserved control mechanism to fine-tune the transcriptional activity of PLC-HDPs prerequisite for regulating and maintaining the complex expression pattern during development.

Long-term infection with retroviral structural gene vector provides protection against bovine leukemia virus disease in rabbits

Bovine leukemia virus (BLV) infection of rabbits is a tractable model system to evaluate vaccination strategies against lymphotropic retroviruses, which represent a global human health problem. We have previously developed genetically simplified BLV structural gene vector (SGV) that replicates BLV structural and enzymatic genes independently of BLV regulatory and accessory genes. Results of a 20-month study in a rabbit model demonstrated that BLV SGV induces an antiviral immunological response and lacks pathogenicity. Here, these chronically infected-BLV SGV rabbits are assessed in a proof-of-principle study of preventative vaccination against challenge with pathogenic BLV. This study commences 24 months after BLV SGV inoculation and proceeds for an additional 20 months. The previously characterized BLV SGV rabbits and age-matched control rabbits were challenged with 1 x 10(8) fetal lamb kidney/BLV producer cells. BLV SGV rabbits seroconverted upon BLV challenge, but did not progress to BLV infection nor clinical disease. By contrast, naive rabbits became infected and succumbed to lymphotropic disease. Our findings provide proof-of-principle that chronic infection with BLV SGV induces protection against BLV infection. The data indicate that SGV based on HTLV or HIV is a promising approach against lymphotropic disease by human retroviruses.

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.

Human T lymphotropic virus type-1 p30II alters cellular gene expression to selectively enhance signaling pathways that activate T lymphocytes

Background

Human T-lymphotropic virus type-1 (HTLV-1) is a deltaretrovirus that causes adult T-cell leukemia/lymphoma and is implicated in a variety of lymphocyte-mediated disorders. HTLV-1 contains both regulatory and accessory genes in four pX open reading frames. pX ORF-II encodes two proteins, p13^II and p30^II, which are incompletely defined in the virus life cycle or HTLV-1 pathogenesis. Proviral clones of the virus with pX ORF-II mutations diminish the ability of the virus to maintain viral loads in vivo. Exogenous expression of p30^II differentially modulates CREB and Tax-responsive element-mediated transcription through its interaction with CREB-binding protein/p300 and represses tax/rex RNA nuclear export.

Results

Herein, we further characterized the role of p30^II in regulation of cellular gene expression, using stable p30^II expression system employing lentiviral vectors to test cellular gene expression with Affymetrix U133A arrays, representing ~33,000 human genes. Reporter assays in Jurkat T cells and RT-PCR in Jurkat and primary CD4+ T-lymphocytes were used to confirm selected gene expression patterns. Our data reveals alterations of interrelated pathways of cell proliferation, T-cell signaling, apoptosis and cell cycle in p30^II expressing Jurkat T cells. In all categories, p30^II appeared to be an overall repressor of cellular gene expression, while selectively increasing the expression of certain key regulatory genes.

Conclusions

We are the first to demonstrate that p30^II, while repressing the expression of many genes, selectively activates key gene pathways involved in T-cell signaling/activation. Collectively, our data suggests that this complex retrovirus, associated with lymphoproliferative diseases, relies upon accessory gene products to modify cellular environment to promote clonal expansion of the virus genome and thus maintain proviral loads in vivo.

Critical role of human T-lymphotropic virus type 1 accessory proteins in viral replication and pathogenesis

Human T-cell lymphotropic virus type 1 (HTLV-1) infection is associated with a diverse range of lymphoproliferative and neurodegenerative diseases, yet pathogenic mechanisms induced by the virus remain obscure. This complex retrovirus contains typical structural and enzymatic genes but also unique regulatory and accessory genes in four open reading frames (ORFs) of the pX region of the viral genome (pX ORFs I to IV). The regulatory proteins encoded by pX ORFs III and IV, Tax and Rex, respectively, have been extensively characterized. In contrast the contribution of the four accessory proteins p12(I), p27(I), p13(II), and p30(II), encoded by pX ORFs I and II, to viral replication and pathogenesis remained unclear. Proviral clones that are mutated in either pX ORF I or II, while fully competent in cell culture, are severely limited in their replicative capacity in a rabbit model. Emerging evidence indicates that the HTLV-1 accessory proteins are critical for establishment of viral infectivity, enhance T-lymphocyte activation, and potentially alter gene transcription and mitochondrial function. HTLV-1 pX ORF I expression is critical to the viral infectivity in resting primary lymphocytes, suggesting a role for p12(I) in lymphocyte activation. The endoplasmic reticulum and cis-Golgi localizing p12(I), encoded from pX ORF I, activates NFAT, a key T-cell transcription factor, through calcium-mediated signaling pathways and may lower the threshold of lymphocyte activation via the JAK/STAT pathway. In contrast p30(II) localizes to the nucleus and represses viral promoter activity, but may regulate cellular gene expression through p300/CBP or related coactivators of transcription. p13(II) targets mitochondrial proteins, where it alters the organelle morphology and may influence energy metabolism. Collectively, studies of the molecular functions of the HTLV-1 accessory proteins provide insight into strategies used by retroviruses that are associated with lymphoproliferative diseases.

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.

Qualitative highly divergent nuclear export signals can regulate export by the competition for transport cofactors in vivo

Nucleo-cytoplasmic transport of proteins is mediated by nuclear export signals, identified in various proteins executing heterologous biological functions. However, the molecular mechanism underlying the orchestration of export is only poorly understood. Using microinjection of defined recombinant export substrates, we now demonstrate that leucine-rich nuclear export signals varied dramatically in determining the kinetics of export in vivo. Thus, nuclear export signals could be kinetically classified which correlated with their affinities for CRM1-containing export complexes in vitro. Strikingly, cotransfection experiments revealed that proteins containing a fast nuclear export signal inhibited export and the biological activity of proteins harboring a slower nuclear export signal in vivo. The affinity for export complexes seems therefore predominantly controlled by the nuclear export signal itself, even in the context of the complete protein in vivo. Overexpression of FG-rich repeats of nucleoporins affected a medium nuclear export signal containing protein to the same extent as a fast nuclear export signal containing protein, indicating that nucleoporins appear not to contribute significantly to nuclear export signal-specific export regulation. Our results imply a novel mode for controlling the biological activity of shuttle proteins already by the composition of the nuclear export signal itself.

The adenovirus type 5 E1B-55K oncoprotein actively shuttles in virus-infected cells, whereas transport of E4orf6 is mediated by a CRM1-independent mechanism

The E1B-55K and E4orf6 proteins of adenovirus type 5 are involved in viral mRNA export. Here we demonstrate that adenovirus infection does not inhibit the function of the E1B-55K nuclear export signal and that E1B-55K also shuttles in infected cells. Even during virus infection, E1B-55K was exported by the leptomycin B-sensitive CRM1 pathway, whereas E4orf6 transport appeared to be mediated by an alternative mechanism. Our results strengthen the potential role of E1B-55K as the "driving force" for adenoviral late mRNA export.

Investigation of nucleo-cytoplasmic transport using UV-guided microinjection

Active nucleo-cytoplasmic transport is mediated by dynamic signal-mediated pathways. We investigated the effects of transcription inhibitors or fluorescent lectins on nuclear import mediated by nuclear localization signals (NLSs). Therefore, a novel experimental approach that allows the controlled sequential introduction of fluorescent substances into living cells was established. A microinjection system equipped with an UV-source enabled us to identify fluorescent-labeled cells for the subsequent introduction of additional fluorescent compounds, in order to study their interactions in vivo. Cells were initially labeled either by expression of autofluorescent proteins or by microinjection of fluorescent substances. Transcription inhibitors did not affect nuclear transport mediated by classical NLSs but inhibited import mediated by the M9-domain of hnRNPA1. Comparison of a mono- and bipartite NLS revealed that the bipartite signal was more active in import. Sequential injection of differentially labeled nuclear import and export substrates allowed monitoring of import and export simultaneously in the same living cell. The introduced experimental approach will also be useful to analyze a variety of biological processes in living mammalian cells.

Mutational definition of functional domains within the Rev homolog encoded by human endogenous retrovirus K

Nuclear export of the incompletely spliced mRNAs encoded by several complex retroviruses, including human immunodeficiency virus type 1 (HIV-1), is dependent on a virally encoded adapter protein, termed Rev in HIV-1, that directly binds both to a cis-acting viral RNA target site and to the cellular Crm1 export factor. Human endogenous retrovirus K, a family of ancient endogenous retroviruses that is not related to the exogenous retrovirus HIV-1, was recently shown to also encode a Crm1-dependent nuclear RNA export factor, termed K-Rev. Although HIV-1 Rev and K-Rev display little sequence identity, they share the ability not only to bind to Crm1 and to RNA but also to form homomultimers and shuttle between nucleus and cytoplasm. We have used mutational analysis to identify sequences in the 105-amino-acid K-Rev protein required for each of these distinct biological activities. While mutations in K-Rev that inactivate any one of these properties also blocked K-Rev-dependent nuclear RNA export, several K-Rev mutants were comparable to wild type when assayed for any of these individual activities yet nevertheless defective for RNA export. Although several nonfunctional K-Rev mutants acted as dominant negative inhibitors of K-Rev-, but not HIV-1 Rev-, dependent RNA export, these were not defined by their inability to bind to Crm1, as is seen with HIV-1 Rev. In total, this analysis suggests a functional architecture for K-Rev that is similar to, but distinct from, that described for HIV-1 Rev and raises the possibility that viral RNA export mediated by the approximately 25 million-year-old K-Rev protein may require an additional cellular cofactor that is not required for HIV-1 Rev function.

Corf, the Rev/Rex homologue of HTDV/HERV-K, encodes an arginine-rich nuclear localization signal that exerts a trans-dominant phenotype when mutated

Regulation of nucleo-cytoplasmic export of viral transcripts by a viral protein (Rev/Rex) is a characteristic feature in the replication cycle of complex retroviruses. We recently reported that the endogenous retrovirus family HTDV/HERV-K encodes a protein, Corf, that is a cellular Counterpart of Rev/Rex function and thus a new component of nucleo-cytoplasmic pathways. In HTDV/HERV-K-expressing cells, Corf is localized within the nucleoli. Here we describe the nuclear localization signal (NLS) of the Corf protein. Mutations in the NLS lead to cytoplasmic accumulation of the mutated protein and abrogate Corf function in a trans-dominant way.

The adenovirus type 5 E1B-55K oncoprotein is a highly active shuttle protein and shuttling is independent of E4orf6, p53 and Mdm2

The E1B-55K and E4orf6 oncoproteins of adenovirus type 5 are involved in the export of viral mRNAs. Previously, it was suggested that a complex composed of E1B-55K and E4orf6 serves as a nucleocytoplasmic transporter for viral mRNAs in which the E4orf6 protein directs both nuclear import and export. We now demonstrate that the E1B-55K protein itself shuttles efficiently in the absence of E4orf6. In addition, E1B-55K trafficking was independent of the defined shuttle proteins Mdm2 or p53, which interacts with E1B-55K. The identified N-terminal E1B-55K leucine-rich nuclear-export signal (NES) conferred rapid nuclear export even in a heterologous system in contrast to the postulated E4orf6NES. Interestingly, although shuttling was blocked by inhibitors of the CRM1 mediated export pathway, E1B-55K inhibited neither the activity nor the trafficking of the retroviral shuttle proteins HIV-1 Rev and HTLV-1 Rex. In contrast, Rev or Rex blocked the nuclear export of E1B-55K, most likely by competing for essential export factors. Our results provide new insights into the regulation of the adenovirus mRNA export system and the processes of adenovirus mediated transformation. Oncogene (2000) 19, 850 - 857.