Functional dissection of the human spumaretrovirus transactivator identifies distinct classes of dominant-negative mutants

Nuclear import of prototype foamy virus transactivator Bel1 is mediated by KPNA1, KPNA6 and KPNA7

Bel1, a transactivator of the prototype foamy virus (PFV), plays pivotal roles in the replication of PFV. Previous studies have demonstrated that Bel1 bears a nuclear localization signal (NLS); however, its amino acid sequence remains unclear and the corresponding adaptor importins have not yet been identified. In this study, we inserted various fragments of Bel1 into an EGFP-GST fusion protein and investigated their subcellular localization by fluorescence microscopy. We found that the 215PRQKRPR221 fragment, which accords with the consensus sequence K(K/R)X(K/R) of the monopartite NLS, directed the nuclear translocation of Bel1. Point mutation experiments revealed that K218, R219 and R221 were essential for the nuclear localization of Bel1. The results of GST pull-down assay revealed that the Bel1 peptide 215-221, which bears the NLS, interacted with the nucleocytoplasmic transport receptors, karyopherin alpha 1 (importin alpha 5) (KPNA1), karyopherin alpha 6 (importin alpha 7) (KPNA6) and karyopherin alpha 7 (importin alpha 8) (KPNA7). Finally, in vitro nuclear import assays demonstrated that KPNA1, KPNA6 or KPNA7, along with other necessary nuclear factors, caused Bel1 to localize to the nucleus. Thus, the findings of our study indicate that KPNA1, KPNA6 and KPNA7 are involved in Bel1 nuclear distribution.

N-Myc interactor inhibits prototype foamy virus by sequestering viral Tas protein in the cytoplasm

Foamy viruses (FVs) are complex retroviruses that establish lifelong persistent infection without evident pathology. However, the roles of cellular factors in FV latency are poorly understood. This study revealed that N-Myc interactor (Nmi) could inhibit the replication of prototype foamy virus (PFV). Overexpression of Nmi reduced PFV replication, whereas its depletion by small interfering RNA increased PFV replication. The Nmi-mediated impairment of PFV replication resulted from the diminished transactivation by PFV Tas of the viral long terminal repeat (LTR) and an internal promoter (IP). Nmi was determined to interact with Tas and abrogate its function by sequestration in the cytoplasm. In addition, human and bovine Nmi proteins were found to inhibit the replication of bovine foamy virus (BFV) and PFV. Together, these results indicate that Nmi inhibits both human and bovine FVs by interfering with the transactivation function of Tas and may have a role in the host defense against FV infection.

IMPORTANCE

From this study, we report that the N-Myc interactor (Nmi), an interferon-induced protein, can interact with the regulatory protein Tas of the prototype foamy virus and sequester it in the cytoplasm. The results of this study suggest that Nmi plays an important role in maintaining foamy virus latency and may reveal a new pathway in the interferon-mediated antiviral barrier against viruses. These findings are important for understanding virus-host relationships not only with FVs but potentially for other retroviruses as well.

Residues R(199)H(200) of prototype foamy virus transactivator Bel1 contribute to its binding with LTR and IP promoters but not its nuclear localization

Prototype foamy virus encodes a transactivator called Bel1 that enhances viral gene transcription and is essential for PFV replication. Nuclear localization of Bel1 has been reported to rely on two proximal basic motifs R(199)H(200) and R(221)R(222)R(223) that likely function together as a bipartite nuclear localization signal. In this study, we report that mutating R(221)R(222)R(223), but not R(199)H(200), relocates Bel1 from the nucleus to the cytoplasm, suggesting an essential role for R(221)R(222)R(223) in the nuclear localization of Bel1. Although not affecting the nuclear localization of Bel1, mutating R(199)H(200) disables Bel1 from transactivating PFV promoters. Results of EMSA reveal that the R(199)H(200) residues are vital for the binding of Bel1 to viral promoter DNA. Moreover, mutating R(199)H(200) in Bel1 impairs PFV replication to a much greater extent than mutating R(221)R(222)R(223). Collectively, our findings suggest that R(199)H(200) directly participate in Bel1 binding to viral promoter DNA and are indispensible for Bel1 transactivation activity.

Coactivators p300 and PCAF physically and functionally interact with the foamy viral trans-activator

Background

Foamy virus Bel1/Tas trans-activators act as key regulators of gene expression and directly bind to Bel1 response elements (BRE) in both the internal and the 5'LTR promoters leading to strong transcriptional trans-activation. Cellular coactivators interacting with Bel1/Tas are unknown to date.

Results

Transient expression assays, co-immunoprecipitation experiments, pull-down assays, and Western blot analysis were used to demonstrate that the coactivator p300 and histone acetyltransferase PCAF specifically interact with the retroviral trans-activator Bel1/Tas in vivo. Here we show that the Bel1/Tas-mediated trans-activation was enhanced by the coactivator p300, histone acetyltransferases PCAF and SRC-1 based on the crucial internal promoter BRE. The Bel1/Tas-interacting region was mapped to the C/H1 domain of p300 by co-immunoprecipitation and pull-down assays. In contrast, coactivator SRC-1 previously reported to bind to the C-terminal domain of p300 did not directly interact with the Bel1 protein but nevertheless enhanced Bel1/Tas-mediated trans-activation. Cotransfection of Bel1/Tas and p300C with an expression plasmid containing the C/H1domain partially inhibited the p300C-driven trans-activation.

Conclusions

Our data identify p300 and PCAF as functional partner molecules that directly interact with Bel1/Tas. Since the acetylation activities of the three coactivators reside in or bind to the C-terminal regions of p300, a C/H1 expression plasmid was used as inhibitor. This is the first report of a C/H1 domain-interacting retroviral trans-activator capable of partially blocking the strong Bel1/Tas-mediated activation of the C-terminal region of coactivator p300. The potential mechanisms and functional roles of the three histone and factor acetyltransferases p300, PCAF, and SRC-1 in Bel1/Tas-mediated trans-activation are discussed.

Foamy virus transactivation and gene expression

An overview of the pattern and mechanisms of spuma or foamy virus (FV) gene expression is presented. FVs are complex retroviruses with respect to their genetic outfit and the elements used to control and regulate expression of the viral genome. The increased insight into transcriptional and posttranscriptional mechanisms has revealed that the FVs are distinct, unconventional retroviruses clearly apart from the orthoretroviruses. Although less characterized than the orthoretroviruses, FVs have several unique features that are important for construction and assembly of FV-based vectors for targeted gene delivery and vaccination purposes. Some of these distinguishing features are directly related to the FV-specific mechanisms of gene expression and include (1) the presence of an internal, functional active second transcription unit for expression of the nonstructural genes, (2) the utilization of a subgenomic, spliced transcript for Pol protein expression, and (3) distinct but not yet understood mechanisms for the nuclear exit of defined transcripts and thus an additional level of posttranscriptional control of gene expression. Finally, the interactions of the viral transactivator not only with both viral promoters but also with regulatory elements controlling the expression of defined cellular genes are an important issue with respect to vector development and the apparent apathogenicity of FVs in their natural hosts.

Bel1-mediated transactivation of the spumaretroviral internal promoter is repressed by nuclear factor I

Gene expression of the internal and long terminal repeat promoters of the spuma retrovirus is specifically activated by the transactivator Bel1, the key regulator of viral gene expression. Bel1 directly binds to and activates DNA target sites of viral promoters and those of distinct cellular genes. To determine the contribution of cellular transcription factors to viral transactivation, the viral internal promoter (IP) was analyzed by transient expression, electrophoretic mobility shift assays), and supershifts. Here we report that Bel1-mediated transactivation of the full-length and shortened versions of the Bel1 response element (BRE) were repressed by nuclear factor I (NFI). Electrophoretic mobility shift assays using nuclear extracts from transfected 293T cells revealed that different DNA-protein complexes consisting of DNA target sites of NFI and Bel1 proteins were formed. The specificity of the repressor and transactivator DNA binding was shown by NFI- and Bel1-specific antibodies that led to supershifts of the different nuclear protein-oligodeoxynucleotide complexes. The specificity of the complexes was confirmed by using unlabeled, shortened, and mutated IP.BRE oligodeoxynucleotides in competition experiments with the authentic IP.BRE. Cotransfection of the infectious spumavirus DNA genome with a human NFI-X1 expression plasmid into cell cultures greatly reduced the expression of viral structural and Bel1 proteins. These data demonstrate the relevance of NFI-mediated repression of Bel1-driven transactivation in vivo.

Identification and functional characterization of an intragenic DNA binding site for the spumaretroviral trans-activator in the human p57Kip2 gene

Expression of the human cyclin-dependent protein kinase inhibitor p57(Kip2) gene was previously shown to be specifically and strongly activated by the retroviral trans-activator Bel1 of human foamy virus by means of expression profiling, Northern, and Western blot analysis. Here we report that Bel1-mediated trans-activation was conferred by a Bel1 response element (BRE) located in the second exon of p57(Kip2). The intragenic Kip2-BRE was capable of trans-activating the luciferase reporter gene upon cotransfection with Bel1. In electrophoretic mobility shift assays using 293T nuclear extracts or a purified glutathione S-transferase (GST) small middle dotBel1 fusion protein, we identified the 55-nucleotide-long Kip2-BRE site that mainly consists of three direct repeats of 14-mers partially homologous to a functionally active BRE in the viral internal promoter. The specificity of the transactivator-DNA binding was shown by using mutated and shortened Kip2-BRE oligodeoxynucleotides in competition experiments with the authentic viral internal promoter and by Bel1-specific antibody that led to a supershift of the nuclear protein small middle dotKip2-BRE and GST small middle dotBel1 small middle dotKip2-BRE complex. The data indicate that Bel1 can directly bind to BRE sites. The cellular Kip2-BRE can be used to predict those human genes that are directly or indirectly activated by the Bel1 trans-activator.

Human foamy virus Bel 1 transactivator/estrogen receptor fusion proteins allow inducible transactivation of both human foamy virus promoters

Recombinant plasmids that express human foamy virus (HFV) Bel 1 transactivator and human estrogen receptor (ER) fusion proteins were constructed. The HFV bel 1 gene was inserted up- and downstream of the ER gene. Recombinant Bel 1-ER and ER-Bel 1 fusion proteins were expressed in eukaryotic cells. In the absence of estrogen, the ER moiety of the fusion proteins suppressed Bel 1-mediated transactivation as measured in CAT reporter gene-based transactivation assays. However, transactivation of the HFV LTR and the HFV internal promoter by Bel 1-ER and ER-Bel 1 fusion proteins was recovered in the presence of estrogen. Thus, the transactivation function of the Bel 1 moiety of the chimeric Bel 1-ER fusion proteins can be efficiently, specifically, and intentionally activated and inactivated by simply adding the low-molecular weight effector estrogen.

Induction of cellular genes is mediated by the Bel1 transactivator in foamy virus-infected human cells

To gain insight into human foamy virus (HFV; also called spumaretrovirus)-induced alterations of cellular genes, the expression profiles of defined genes in HFV-infected primary human cells were analyzed by cDNA array assays. Several distinct cellular genes activated by HFV infection were identified; the identities of the cellular genes were confirmed by RNA blot analyses. Compared with mock-infected controls, the concentrations of cellular Kip2, Egr-1, COUP-TF1, insulin-like growth factor II (IGF-II), and EphB3 mRNAs were significantly increased in HFV-infected cells and showed a gene-specific and time-dependent induction. Immunoblot analyses with antibodies against some of the cellular gene products revealed increased levels of the corresponding proteins. To investigate mechanisms of HFV-induced alterations in cellular gene expression, the capacity of known HFV genes to increase expression of defined cellular genes was analyzed by transient expression experiments. Plasmids that encode the HFV Bel1 transcriptional transactivator were necessary and sufficient to strongly increase expression of p57Kip2, IGF-II, and EphB3 genes in 293T cells. Potential mechanisms and consequences of activation of cellular genes during HFV infection and Bel1 transactivation of the Kip2 gene are discussed.

Characterization of provirus clones of simian foamy virus type 1

We have cloned proviral DNA of simian foamy virus type 1 (SFV-1) from linear unintegrated DNA (pSFV-1). Transfection of pSFV-1 induces cytopathology in several cell lines with supernatants from the transfected cell culture containing infectious viral particles. Electron microscopy of the transfected cells revealed foamy virus particles. Deletion analysis of pSFV-1 indicated that the transcriptional transactivator (tas) gene located between env and the long terminal repeat is critical for virus replication, whereas the second open reading frame (ORF-2) in this region is dispensable. Although the tas and ORF-2 regions of foamy viruses have significantly diverged, the results presented here suggested that the gene products have similar functions. Recombinant pSFV-1 containing the cat gene was able to transduce the heterologous gene, indicating the utility of SFV-1 as a vector. An infectious clone of SFV-1 which is distantly related to the human foamy virus will provide a means to understand the biology of this unique group of viruses.

Deletion analysis of both the long terminal repeat and the internal promoters of the human foamy virus

Deletion analyses of the long terminal repeat (LTR) and internal promoters (IP) of human foamy virus (HFV) showed that a negative acting element resides in the U5 region of the 5' LTR reducing reporter gene expression tenfold. The basal activity of the IP was higher than that obtained with LTR promoter constructs and strongly elevated in permissive BHK-21 cells whereas semi-permissive COS-7 cells showed low basal activity. Since the basal activity of the IP is critical for initiating HFV gene expression by providing Bel 1 transactivator early after infection, the basal activity of the IP may be the crucial factor that contributes to whether cells are permissive for HFV infection or not. Deletion mutagenesis allowed to define the minimal IP region. A region strongly transactivated by Bel 1 extends from -136 to +58 relative to the cap site of the IP. The major Bel 1 response element of the IP required for transactivation is located upstream of the cap site between -136 and -88 relative to the internal cap site. A DNA fragment reported to be protected by recombinant Bel 1 was deleted with marginal reduction of Bel 1 transactivation. HFV gene expression directed by the IP and LTR promoters is thus multiply regulated by positive and negative acting response elements in cis and their binding partners in trans.

Expression and maturation of human foamy virus Gag precursor polypeptides

In this report, we address the processing of the Gag polypeptides of human foamy virus previously reported to be atypical. In the cytoplasm or the nucleus of infected cells as well as in free virus particles, two Gag precursor polypeptides were identified at approximately 72 and 68 kDa, p72 giving rise to p68 by a maturation process. Efficient maturation of Gag precursors was observed only in two situations: (i) during the early steps of virus adsorption and (ii) under experimental conditions, including treatment with DNase I, known to dissociate actin polymers associated with high ionic strength and ionic detergents. Rather than being a defective viral protease function, an association of Gag precursors with a cytoskeleton network might be responsible for the low rate of Gag protein maturation through inhibition of their cleavage by the protease.

The human foamy virus Bel-1 transcription factor is a sequence-specific DNA binding protein

The Bel-1 transcriptional transactivator encoded by human foamy virus (HFV) can efficiently activate gene expression directed by both the HFV long terminal repeat (LTR) and internal (Int) promoter elements. By DNA footprinting and gel retardation analysis, we demonstrate that Bel-1 can specifically bind to discrete sites in both the LTR and Int promoter elements in vitro. However, transactivation of the HFV LTR by Bel-1 was observed to require not only the promoter-proximal Bel-1 binding site identified in vitro but also additional promoter-distal sequences. These data suggest that Bel-1 binding is necessary but not sufficient for efficient transactivation of Bel-1-responsive promoters in mammalian cells and therefore raise the possibility that Bel-1 function may require the action of a cellular DNA binding protein(s). Importantly, these data demonstrate that Bel-1 is unique among retroviral regulatory proteins in being a sequence-specific DNA binding protein.

The transcriptional transactivator of simian foamy virus 1 binds to a DNA target element in the viral internal promoter

The transcriptional transactivator (Tas) of simian foamy virus type 1 strongly augments gene expression directed by both the promoter in the viral long terminal repeat and the newly discovered internal promoter located within the env gene. A region of 121 bp, located immediately 5' to the TATA box in the internal promoter, is required for transactivation by Tas. The present study aimed to identify the precise Tas-responsive target(s) in this region and to determine the role of Tas in transcriptional regulation. By analysis of both clustered-site mutations and hybrid promoters in transient expression assays in murine and simian cells, two separate sequence elements within this 121-bp region were shown to be Tas-dependent transcriptional enhancers. These targets, each < 30 bp in length and displaying no apparent sequence homology one to the other, are designated the promoter-proximal and promoter-distal elements. By means of the gel electrophoresis mobility-shift assays, using purified glutathione S-transferase-Tas fusion protein expressed in Escherichia coli, the target proximal to the TATA box exhibited strong binding to glutathione S-transferase-Tas, whereas the distal element appears not to bind. In addition, footprint analysis revealed that 26 bp in the promoter proximal element was protected by glutathione S-transferase-Tas from DNase I. We propose a model for transactivation of the simian foamy virus type 1 internal promoter in which Tas interacts directly with the proximal target element positioned immediately 5' to the TATA box. In this model, Tas attached to this element is presumed to interact with a component(s) of the cellular RNA polymerase II initiation complex and thereby enhance transcription directed by the viral internal promoter.

Molecular biology of the human foamy virus

Foamy viruses also known as spumaretroviruses are complex retroviruses infecting cell lines with no apparent specific cellular tropism and induce the formation of multinucleated cells with numerous vacuoles. Far less well characterized than oncoviruses and lentiviruses, this class of viruses is thought to be innocuous in vivo. However, several important discoveries on foamy viruses brought new insights in the field of retrovirology.

Unexpected replication pathways of foamy viruses

Foamy viruses make up a distinct subgroup of retroviruses. They are widely distributed among nonhuman primates, felines, and bovines. In their natural hosts and in cases of rare zoonotic transmissions to humans foamy viruses cause persistent and apparently benign infections. While foamy viruses are not of medical importance in causing human or animal diseases, they may become valuable tools for somatic gene transfer in the future. However, a better understanding of the molecular biology of this virus group is a prerequisite for the development of foamy virus vectors. In this respect, recent research has revealed major differences between the foamy virus and the general retroviral replication strategies and some similarities to hepadnaviruses.

Human foamy virus Bel1 transactivator contains a bipartite nuclear localization determinant which is sensitive to protein context and triple multimerization domains

The Bel1 protein of human foamy virus is a 300-amino-acid nuclear regulatory protein which transactivates the gene expression directed by the homologous long terminal repeat and the human immunodeficiency virus type 1 long terminal repeat. While previous reports suggested that the single basic domain of Bel1 from residues 211 to 222 and/or 209 to 226 is necessary and sufficient for efficient nuclear localization (L. K. Venkatesh, C. Yang, P. A. Theodorakis, and G. Chinnandurai, J. Virol. 67:161-169, 1993; F. He, J. D. Sun, E. D. Garrett, and B. R. Cullen, J. Virol. 67:1896-1904, 1993), our recent data showed that another basic domain, from amino acid residues 199 to 200, is also required for nuclear localization of Bel1 (C. W. Lee, C. Jun, K. J. Lee, and Y. C. Sung, J. Virol. 68:2708-2719, 1994). To clarify this discrepancy, we constructed various bel1-lacZ chimeric constructs and several linker insertion mutants and determined their subcellular localization. When the region of Bel1 containing basic domains was placed at an internal site of the lacZ gene, the nuclear localization signal (NLS) of Bel1 consisted of two discontinuous basic regions separated by an intervening sequence. Moreover, insertion of specific amino acids between two basic regions disrupted the activity of the Bel1 NLS. On the other hand, Bel1 residues 199 and 200 were not required to direct the Bel1-beta-galactosidase chimeric protein to the nucleus when the Bel1 NLS was appended to the amino terminus of beta-galactosidase. These results indicate that the function of the Bel1 NLS is sensitive to the protein context within which the sequence is present. In addition, we demonstrated that the Bel1 protein forms a multimeric complex in the nuclei of mammalian cells by using a sensitive in vivo protein-protein interaction assay. Mutational analyses revealed that the regions which mediate multimer formation map to three domains of Bel1, i.e., residues 1 to 31, 42 to 82, and 82 to 111. Furthermore, our results show that the region of Bel1 from residues 202 to 226 prevents Bel1 from forming a multimeric complex.

The human foamy virus internal promoter is required for efficient gene expression and infectivity

The human foamy or spumaretrovirus (HFV) is a complex retrovirus that codes for the three retroviral genes gag, pol, and env and the regulatory and accessory bel genes. A particular feature of HFV gene expression was recently described: not only does the HFV provirus contain the classical retroviral long terminal repeat promoter, a second functionally active promoter is present in the env gene upstream of the bel genes (M. Löchelt, W. Muranyi, and R. M. Flügel, 1993, Proc. Natl. Acad. Sci. USA 90, 7317-7321). Both, the HFV long terminal repeat promoter I and internal promoter II depend upon the HFV transcriptional transactivator Bel 1 for efficient gene expression. The internal promoter directs the synthesis of functionally active Bel 1 transactivator and Bet proteins that are expressed early after HFV infection. In this report, it is shown that mutation of the promoter II TATA box resulted in HFV proviral clones with a reduction in infectivity by a factor of approximately 100. Gene expression by promoter II TATA box mutant HFV proviruses was reduced. HFV proviruses with the mutated promoter II TATA box used cryptic start sites of transcription upstream of the original promoter II TATA box, resulting in an inefficient and less accurate transcriptional initiation. The reduced HFV structural gene expression by the mutated HFV proviruses was relieved by providing Bel 1 protein in trans. This demonstrates that HFV promoter II-directed Bel 1 expression is important for producing the high levels of Bel 1 that increases virus replication.

Phylogenesis and genetic complexity of the nonhuman primate retroviridae

The three known groups of nonhuman primate retroviruses (simian immunodeficiency virus, simian T cell lymphotropic/leukemic virus type I, and simian foamy virus) are thought to have equivalent human counterparts. This is clearly the case with human immunodeficiency virus types 1 and 2, the causative agents of acquired immunodeficiency syndrome, and with human T cell lymphotropic/leukemia virus type I (HTLV-I), which causes T cell leukemia and a progressive form of myelopathy (tropical spastic paraparesis/HTLV-I-associated myelopathy), and HTLV-II. However, the presence of spumaviruses (foamy viruses) in humans remains uncertain. Data accumulated in the last 5 years suggest the possibility that the human retroviruses are indeed the result of transmission of simian retroviruses to humans. In this article we attempt to parallel the genetic features of the simian retroviridae with their human counterparts and argue for the possibility of horizontal transmission of these viruses from monkeys to humans.

Characterization of the internal promoter of simian foamy viruses

Simian and human foamy viruses (HFV and SFV), genetically related members of the spumavirus genus of retroviruses, have complex genome structures which encode the gag, pol, and env genes for virion proteins as well as additional open reading frames. One of these open reading frames is a viral transactivator, encoded by genes designated taf for SFV and bel-1 for HFV, which augments transcription directed by the long terminal repeat (LTR) through cis-acting targets in the U3 domain of the LTR. Recently, an internal transcriptional promoter has been identified in sequences within the 3' end of the HFV env gene (M. Lochelt, W. Muranyi, and R. M. Flugel, Proc. Natl. Acad. Sci. 90:7317-7321, 1993). We have demonstrated by using transient expression assays in several tissue culture cell lines and by analyzing viral transcripts in infected cells that SFV-1 from a rhesus macaque and SFV-3 from an African green monkey also encode an internal promoter in the env gene. Transcription directed by the internal promoters of SFV-1 and SFV-3 is activated by the taf-1 and taf-3 gene products, respectively, in several cell types. The importance of a TATA box for the SFV-1 internal promoter was established by site-specific mutagenesis, and the 5' ends of transcripts initiating in the internal promoter have been determined. cis-acting sequences in the SFV-1 env gene required for the response to taf-1 are contained within a 121-bp element located 5' to the TATA box in the internal promoter. This taf-1-responsive element in the internal promoter functions in a position- and orientation-independent fashion in a heterologous promoter and thus has the properties of an enhancer which depends on taf-1 activity. Alignments reveal that the SFV-1 internal promoter and the SFV-1 LTR have little sequence relatedness. Cross-transactivation studies show that the transactivators of SFV-1 and HFV function on the internal promoter and LTR of the homologous virus but not on the heterologous virus. In summary, the genomes of simian and human foamy viruses direct viral transcription through both the promoter in the LTR and an internal promoter within the env gene, and each promoter contains unique enhancer-like elements regulated by the viral transactivator.

The Bel1 protein of human foamy virus contains one positive and two negative control regions which regulate a distinct activation domain of 30 amino acids

The Bel1 transactivator is essential for the replication of human foamy virus (HFV). To define the functional domains of HFV Bel1, we generated random missense mutations throughout the entire coding sequence of Bel1. Functional analyses of 24 missense mutations have revealed the presence of at least two functional domains in Bel1. One domain corresponds to a basic amino acid-rich motif which acts as a bipartite nuclear targeting sequence. A second, central domain corresponds to a presumed effector region which, when mutated, leads to dominant-negative mutants and/or lacks transactivating ability. In addition, deletion analyses and domain-swapping experiments further showed that Bel1 protein contains a strong carboxy-terminal activation domain. The activating region is also capable of functioning as a transcription-activating domain in yeast cells, although it does not bear any significant sequence homology to the well-characterized acidic activation domain which is known to function only in yeast and mammalian cells. We also demonstrated that the regions of Bel1 from residues 1 to 76 and from residues 153 to 225 repressed transcriptional activation exerted by the Bel1 activation domain. In contrast, the region from residues 82 to 150 appears to overcome an inhibitory effect. These results indicate that Bel1 contains one positive and two negative regulatory domains that modulate a distinct activation domain of Bel1. These regulatory domains of Bel1 cannot affect the function of the VP16 activation domain, suggesting that these domains specifically regulate the activation domain of Bel1. Furthermore, in vivo competition experiments showed that the positive regulatory domain acts in trans. Thus, our results demonstrate that Bel1-mediated transactivation appears to undergo a complex regulatory pathway which provides a novel mode of regulation for a transcriptional activation domain.

Transcriptional mapping of the 3' end of the bovine syncytial virus genome

The bovine syncytial virus, a member of the retroviral subfamily Spumavirinae, causes a persistent, asymptomatic infection in cattle. Nucleotide sequence analysis of the viral genome revealed two overlapping reading frames in the 3' region, traditionally occupied by accessory-function genes in other complex retroviruses. In order to analyze the transcripts from the accessory-gene region, we designed oligonucleotide primers complementary to sequences within the 5' and 3' long terminal repeats (LTRs) for use with the PCR. Southern blot analysis of amplification products revealed eight major cDNA bands. Eleven distinct cDNA clones were subsequently isolated and characterized. The initial splice donor in each clone is located 49 bp downstream from the mRNA cap site in the 5' LTR. The primary splice acceptor site was located 17 bp upstream from the proximal 3' open reading frame known as BF-ORF1. A second major splice acceptor was localized to a region upstream of the second open reading frame, BF-ORF2. Clones were identified which spliced directly to each of these sites. Additional splice donor and acceptor sites within BF-ORF1 and BF-ORF2 and the 3' LTR were variously used to generate a complex array of multiply spliced transcripts. Each of these transcripts remained in frame and coded for a potential protein product.

The human foamy virus internal promoter directs the expression of the functional Bel 1 transactivator and Bet protein early after infection

The human foamy virus or spumaretrovirus (HFV) is a complex retrovirus that has the capacity to code not only for the three retroviral genes gag, pol, and env but, in addition, for at least three bel genes. The HFV provirus contains two different and functionally active promoters: the classical retroviral promoter in the 5' long terminal repeat and a recently identified second promoter in the env gene upstream of the bel genes. Both promoter/enhancers are strongly dependent on the HFV transcriptional transactivator protein Bel 1. Here we report that the internal promoter directs the synthesis of viral transcripts that code for functionally active Bel 1 and for Bet proteins that appeared early after HFV infection. The viral mRNAs of the internal promoter have a 112-nucleotide-long leader exon and were spliced predominantly at the first splice donor site in the 5' untranslated region. The data were obtained by transient expression assays, transactivation experiments, and RNA analyses of transcripts derived from HFV-infected cells. The results provide strong evidence for the crucial role the internal promoter plays during HFV infection in generating bel-specific transcripts.

Transcriptional trans activators of human and simian foamy viruses contain a small, highly conserved activation domain

The Bel-1 protein of human foamy virus is a potent transcriptional trans activator of its homologous long terminal repeat promoter element. Here, we demonstrate that Bel-1 can also efficiently activate gene expression when targeted to a heterologous promoter by fusion to the DNA-binding motif of the yeast GAL4 protein. Analysis of a series of deletion mutants of Bel-1 generated in this hybrid protein context suggests the presence of a single transcription activation domain that is fully contained within a discrete, approximately 30-amino-acid segment located proximal to the Bel-1 carboxy terminus. Although this short motif can be shown to function effectively in eukaryotic cells of mammalian, avian, and fungal origin, it does not bear any evident sequence homology to the known classes of eukaryotic activation domain. However, this Bel-1 activation domain was found to be fully conserved, in terms of both biological activity and location, in the distantly related Taf trans activator of simian foamy virus type 1.

Expression of the human foamy virus bel-1 transactivator in insect cells

The human foamy virus (HFV) bel-1 transactivator protein was expressed in insect cells by a recombinant baculovirus. For the generation of the recombinant baculovirus, Acbel-1, the bel-1 gene of an HFV mutant was used, that bears truncations in the bel-1 overlapping bel-2 open reading frame. Acbel-1 infected Sf9 cells produced high amounts of recombinant protein of the same electrophoretic mobility (36 kD) as bel-1 expressed in mammalian cells. The baculovirus expressed bel-1 protein was readily identified by a polyclonal rabbit serum directed against bel-1 in immunoblot assay. As in mammalian cells, bel-1 was predominantly localized to the nucleus of Acbel-1 infected insect cells. The baculovirus expressed bel-1 protein will be of use to determine the action of this novel viral transactivator more precisely.

Functional analysis of human foamy virus accessory reading frames

Foamy viruses belong to the retroviruses which possess a complex genome structure. The human foamy virus (HFV) isolate bears three open reading frames (the so-called bel genes) in the 3' region of the genome which have been reported to give rise to possibly six different proteins via alternative splicing (W. Muranyi and R. M. Flügel, J. Virol. 65:727-735, 1991). In order to analyze the requirements of these proteins for HFV replication in vitro, we constructed a set of single and combinatory bel gene mutants of an infectious molecular clone of HFV. The mutant which lacked the transacting activator, bel-1, was found to be replication incompetent. All other mutants replicated equally well and gave rise to comparable titers of infectious cell-free virus. When HFV proviruses were put under the control of a heterologous promoter (simian virus 40), none of the accessory gene products was found to be required for expression of structural (gag) proteins. There was no evidence for a posttranscriptional regulatory protein that is present in other complex retroviruses.

Functional domains of the simian foamy virus type 1 transcriptional transactivator (Taf)

The genome of simian foamy virus type 1 encodes a transcriptional transactivator (Taf) that dramatically elevates gene expression directed by the viral long terminal repeat. In this report, we describe the functional domains of simian foamy virus type 1 Taf. Several taf mutants and fusion proteins of Taf and the DNA-binding domain of the Saccharomyces cerevisiae transcriptional transactivator GAL4 were used in this study. Taf contains two potent activation domains. One of the activation domains is located at the amino terminus (positions 1 to 48, with position 1 representing the initiator amino acid methionine) and contains several acidic amino acids. The second activation domain was mapped to a region at the carboxy terminus (positions 277 to 300). These two domains activate gene expression directed by the viral long terminal repeat independently of each other. No significant amino acid sequence homology between the activation domains is noted. Thus, Taf belongs in part to the family of acidic transcriptional transactivators. The activation domain at the carboxy terminus is conserved among foamy virus transactivators but is not related to other known transcriptional activators. Therefore, the mechanism of gene activation by the carboxy terminus of Taf may be novel. In addition, a potential binding domain rich in basic amino acids (positions 179 to 222) and a highly conserved sequence among foamy virus transactivators (positions 93 to 109) were found to be critical for Taf activity.

The carboxy-terminal transcription enhancement region of the human spumaretrovirus transactivator contains discrete determinants of the activator function

The bel1 gene of human spumaretrovirus (HSRV) encodes a 300-amino-acid nuclear protein termed Bel1 that is a potent activator of transcription from the cognate long terminal repeat (LTR). Bel1 can also efficiently activate the human immunodeficiency virus type 1 (HIV-1) LTR. We have previously shown that the amino-terminal 227-residue region (minimal activator region) of Bel1 can activate the HSRV LTR at low levels and that two distinct domains within the carboxy-terminal 73 residues, from residues 255 to 266 and 272 to 300, that bear little sequence homology can independently enhance the activity of the minimal activator domain (L. K. Venkatesh, C. Yang, P. A. Theodorakis, and G. Chinnadurai, J. Virol. 67:161-169, 1993). We now report on the further characterization of these two transcriptional enhancement regions. Mutational analysis of the region comprising residues 255 to 266 indicates that a cluster of leucine residues is critical to the function of this region. Also, residues 273 to 287, which are identical in sequence to a 15-amino-acid segment near the carboxy terminus of the simian foamy virus transcriptional activator Taf, can independently enhance the activity of the minimal activator region. To delineate the region(s) of Bel1 that could function autonomously as an activator domain, we tested the activity of chimeric proteins comprising either wild-type or functionally defective forms of Bel1 fused to the DNA binding domain, Gal4(1-147), of the yeast transcriptional activator Gal4 on a synthetic promoter comprising Gal4 DNA binding sites linked to the adenovirus E1B TATA box (minimal promoter). Gal4-Bel1 was found to activate basal transcription from the E1B TATA box at least 35-fold, and the region responsible for this activation function was localized to the carboxy-terminal 73 amino acids. When the transcriptional enhancement regions were tested for autonomous activator function as Gal4(1-147) chimeras, residues 272 to 300, but not 255 to 266, were found to activate transcription efficiently when targeted to the E1B TATA motif and also to HSRV and HIV-1 LTRs. The highly conserved region between amino acids 273 and 287 alone was found to activate transcription efficiently when targeted to the HSRV LTR but not to the E1B TATA box or the HIV-1 LTR. Thus, our results demonstrate that the carboxy-terminal 29-amino-acid region (residues 272 to 300) contributes to Bel1 transactivation by functioning as an autonomous activator of TATA motif-directed transcription in a manner similar to that of other modular transcriptional activators.(ABSTRACT TRUNCATED AT 400 WORDS)