Walleye dermal sarcoma virus Orf C is targeted to the mitochondria

Walleye dermal sarcomas are associated with the presence of a complex retrovirus, walleye dermal sarcoma virus (WDSV). These sarcomas develop and regress seasonally in naturally infected fish. In addition to gag, pol and env, WDSV contains three open reading frames (ORFs), designated orf a, orf b and orf c. orf c is located between the 5' long terminal repeat and gag. Developing tumours contain low levels of orf a and orf b transcripts, whereas regressing tumours contain high levels of genomic transcripts and virus particles. Orf C protein is encoded by the full-length, genomic transcript and can be detected in tumour extracts with anti-Orf C-specific antisera. To determine the subcellular location of WDSV Orf C, cultured cells were transfected with an expression vector encoding haemagglutinin-tagged Orf C and examined by immunofluorescence. Orf C was observed throughout the cytoplasm and accumulated in cytoplasmic organelles. Dual-antibody staining for Orf C and mitochondrial cytochrome c demonstrated colocalization of Orf C with mitochondria and loss of the normal distribution of mitochondria in the cytoplasm. Cells transiently expressing Orf C exhibited apoptotic morphology and increased levels of surface phosphatidylserine and were unable to retain MitoTracker Orange, a dye that accumulates in active mitochondria. These results imply a functional role for WDSV Orf C in an alteration of mitochondrial function that results in apoptosis contributing to tumour regression.

The promyelocytic leukemia protein does not mediate foamy virus latency in vitro

Spumaviruses, commonly called foamy viruses, are complex retroviruses that establish life-long persistent infections in the absence of accompanying pathology. Depending upon cell type, infection of cells in tissue culture cells can result in either lytic replication, persistence, or latency. The cellular factors that mediate foamy virus (FV) latency are poorly understood. In this study we show that the only known inhibitor of FV replication, the promyelocytic leukemia protein (PML), which binds the FV transactivator (Tas), does not play an important role in FV latency in vitro. We found no significant differences in PML levels in cells that supported lytic replication compared to those that were latently infected. Furthermore, endogenous PML levels did not change following exposure to phorbol myristate acetate (PMA), which induces FV replication. We demonstrated that FV replication proceeded in the presence of substantial levels of PML, both in fully permissive cells and during reactivation of latent FV. Endogenous PML did not efficiently colocalize with Tas, even after upregulation by alpha interferon (IFN-alpha) treatment. IFN-alpha did, however, partially suppress the reactivation of latent FV by PMA. Finally, depletion of endogenous PML by small interfering RNA did not promote activation of FV in cells that responded to PMA treatment. Taken together, these data indicate that endogenous PML does not play an important role in mediating FV latency.

Retroviral proteins that target the major histocompatibility complex class I

The human T-cell leukemia virus type 1 (HTLV-1) and human immunodeficiency virus type 1 (HIV-1) retroviruses are two evolutionary distinct human pathogens. HTLV-1 is the etiologic agent of two diverse diseases: adult T-cell leukemia/lymphoma, as well as the neurologic disorder tropical spastic paraparesis/HTLV-1-associated myelopathy. HTLV-1 is the only retrovirus known to be the etiologic agent of human cancer. HTLV-2, the other known oncovirus, is not apparently associated with human cancer. While HTLV-1 transforms T-cells in vitro, HIV kills CD4+ T-cells and is the etiological agent of human acquired immunodeficiency syndrome, characterized by a progressive loss of CD4+ cells, weakening of the immune system, and susceptibility to opportunistic infections and cancer. HTLV-1 and HIV-1 both cause lifelong infections, which suggests that they have evolved mechanism(s) to evade detection by the host's immune response; particularly to evade cytotoxic T-lymphocytes, which play a major role in cellular immunity against viruses and will be the focus of this review.

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.

Further characterization of equine foamy virus reveals unusual features among the foamy viruses

Foamy viruses (FVs) are nonpathogenic, widely spread complex retroviruses which have been isolated in nonhuman primates, cattle, cats, and more recently in horses. The equine foamy virus (EFV) was isolated from healthy horses and was characterized by molecular cloning and nucleotide sequence analysis. Here, to further characterize this new FV isolate, the location of the transcriptional cap and poly(A) addition sites as well as the main splice donor and acceptor sites were determined, demonstrating the existence of the specific subgenomic pol mRNA, one specific feature of FVs. Moreover, similar to what has been described for the human foamy virus (HFV), the prototype of FVs, a replication-defective EFV genome was identified during persistent infection. At the protein level, the use of specific antibodies allowed us to determine the size and the subcellular localization of EFV Gag, Env, and Tas, the viral transactivators. While EFV Gag was detected in both the cytoplasm and the nucleus, EFV Env mainly localized in the Golgi complex, in contrast to HFV Env, which is sequestered in the endoplasmic reticulum. In addition, electron microscopy analysis demonstrated that EFV budding occurs at the plasma membrane and not intracellularly, as is the case for primate FVs. Interestingly, EFV Tas was detected both in the nucleus and the cytoplasm of Tas-transfected cells, in contrast to the strict nuclear localization of other FV Tas but similar to the equine infectious anemia virus Tat gene product. Taken together, our results reveal that this new FV isolate exhibits remarkable features among FVs, bringing new insights into the biology of these unconventional retroviruses.

Envelope glycoprotein incorporation, not shedding of surface envelope glycoprotein (gp120/SU), Is the primary determinant of SU content of purified human immunodeficiency virus type 1 and simian immunodeficiency virus

Human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) particles typically contain small amounts of the surface envelope protein (SU), and this is widely believed to be due to shedding of SU from mature virions. We purified proteins from HIV-1 and SIV isolates using procedures which allow quantitative measurements of viral protein content and determination of the ratios of gag- and env-encoded proteins in virions. All of the HIV-1 and most of the SIV isolates examined contained low levels of envelope proteins, with Gag:Env ratios of approximately 60:1. Based on an estimate of 1,200 to 2,500 Gag molecules per virion, this corresponds to an average of between 21 and 42 SU molecules, or between 7 and 14 trimers, per particle. In contrast, some SIV isolates contained levels of SU at least 10-fold greater than SU from HIV-1 isolates. Quantification of relative amounts of SU and transmembrane envelope protein (TM) provides a means to assess the impact of SU shedding on virion SU content, since such shedding would be expected to result in a molar excess of TM over SU on virions that had shed SU. With one exception, viruses with sufficient SU and TM to allow quantification were found to have approximately equivalent molar amounts of SU and TM. The quantity of SU associated with virions and the SU:TM ratios were not significantly changed during multiple freeze-thaw cycles or purification through sucrose gradients. Exposure of purified HIV-1 and SIV to temperatures of 55 degrees C or greater for 1 h resulted in loss of most of the SU from the virus but retention of TM. Incubation of purified virus with soluble CD4 at 37 degrees C resulted in no appreciable loss of SU from either SIV or HIV-1. These results indicate that the association of SU and TM on the purified virions studied is quite stable. These findings suggest that incorporation of SU-TM complexes into the viral membrane may be the primary factor determining the quantity of SU associated with SIV and HIV-1 virions, rather than shedding of SU from mature virions.

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.

Intra- and intercellular trafficking of the foamy virus auxiliary bet protein

The Bet protein of foamy viruses (FVs) is an auxiliary protein encoded by the 3' end of the viral genome. Although its function during the viral replication cycle is still unknown, Bet seems to play a key role in the establishment and/or maintenance of viral persistence, representing the predominant viral protein detected during chronic infection. To clarify the function of this viral protein, the subcellular distribution of Bet from the prototypic human foamy virus (HFV) was examined. We report here that this protein is distributed in both the cytoplasm and the nucleus of HFV-infected or Bet-transfected cells. The nuclear targeting results from the presence of a bipartite nuclear localization signal at the C-terminal region, sufficient to direct heterologous reporter proteins to the nucleus. Since HFV Bet spreads between cells, we show here that the secreted protein targets the nuclei of recipient cells. HFV Bet follows an unconventional route to exit the cell since its secretion is not affected by brefeldin A, a drug which disrupts the trafficking between the endoplasmic reticulum and the Golgi complex. Finally, these inter- and intracellular movements were also observed for the equine foamy virus Bet protein, strongly suggesting that these remarkable features are conserved among FVs.

The murine endogenous retrovirus MIA14 encodes an active aspartic proteinase that is functionally similar to proteinases from D-type retroviruses

Murine intracisternal A-type particles (IAPs) are endogenous retroviruses showing sequence homologies to B/D- and avian C-type retroviruses and a gene expression strategy similar to that of D-type retroviruses. These viruses form immature particles in the endoplasmic reticulum and do not release extracellular virions, but are competent for retrotransposition within the virus-producing cell. It had been assumed that lack of polyprotein processing and maturation is due to a defect in the viral proteinase (PR), but recent experiments have shown that polyprotein processing occurs when assembly of the mouse IAP MIA14 is artificially directed to the plasma membrane. We have expressed and purified recombinant MIA14 PR and show that it undergoes N- and C-terminal autoprocessing at defined sites. Using peptide cleavage and inhibition assays and in vitro cleavage of recombinant HIV-1 and MIA14 Gag polyproteins, we show that MIA14 PR is a catalytically competent enzyme comparable in its efficiency to PRs from type D exogenous retroviruses. MIA14 PR is related to the PR of Mason-Pfizer monkey virus both functionally and with respect to its expression strategy, and is distinct from HIV-1 PR with respect to substrate specificity and catalytic efficiency. These findings reveal a functional and possibly evolutionary relationship between MIA14 and D-type retroviruses and imply that a functional PR may be relevant for intracellular retrotransposition even in the case of an endogenous retrovirus that does not produce extracellular virus.

Oncoviral bovine leukemia virus G4 and human T-cell leukemia virus type 1 p13(II) accessory proteins interact with farnesyl pyrophosphate synthetase

G4 and p13(II) are accessory proteins encoded by the X region of bovine leukemia virus and human T-cell leukemia virus type 1 (HTLV-1), respectively. Disruption of the G4 and p13(II) open reading frames interferes with viral spread in animal model systems, indicating that the corresponding proteins play a key role in viral replication. In addition, G4 is oncogenic in primary cell cultures and is absolutely required for efficient onset of leukemogenesis in sheep. To gain insight into the function of these proteins, we utilized the yeast two-hybrid system to identify protein partners of G4. Results revealed that G4 interacts with farnesyl pyrophosphate synthetase (FPPS), a protein involved in the mevalonate/squalene pathway and in synthesis of FPP, a substrate required for prenylation of Ras. The specificity of the interaction was verified by glutathione S-transferase (GST) pull-down assays and by coimmunoprecipitation experiments. Furthermore, confocal microscopy showed that the subcellular localization of G4 was profoundly affected by FPPS. The G4 protein itself was not prenylated, at least in rabbit reticulocyte lysate-based assays. The domain of G4 required for binding to FPPS was restricted to an amphipathic alpha-helix rich in arginine residues. Subtle mutation of this alpha-helix abrogated G4 oncogenic potential in vitro, providing a biological relevance for FPPS-G4 complex formation in cells. Finally, HTLV-1 p13(II) was also found to specifically interact with FPPS (in yeast as well as in GST pull-down assays) and to colocalize with G4 in mitochondria, suggesting a functional analogy between these oncoviral accessory proteins. Identification of FPPS as a molecular partner for p13(II) and G4 accessory proteins opens new prospects for treatment of retrovirus-induced leukemia.

Accumulation of beta-amyloid precursor protein in axons correlates with CNS expression of SIV gp41

Axonal damage represented by accumulation of beta-amyloid precursor protein (beta-APP) develops in numerous central nervous system (CNS) diseases including human immunodeficiency virus (HIV) infection. To study the underlying mechanisms of axonal damage associated with HIV CNS infection, the amount of axonal beta-APP immunostaining in the corpus callosum of 24 simian immunodeficiency virus (SIV)-infected macaques and 3 control macaques was measured by computerized image analysis. The amounts of beta-APP accumulation were then compared with time post-inoculation, extent and character of CNS inflammation, and viral load in the CNS measured by the amount of immunohistochemical staining for the viral transmembrane protein gp41. Significant increases over control values were present in 10 of 24 SIV-infected animals. SIV encephalitis was present in 9 of the 10 animals with elevated beta-APP Increases in beta-APP correlated most strongly with levels of SIV gp41 in the brain (p = 0.005), but significant associations with macrophage infiltration and microglial activation (p = 0.04) and infiltration by cytotoxic lymphocytes (p = 0.05) also were identified. These data demonstrate that beta-APP accumulation in the white matter of SIV-infected macaques develops during SIV infection in close correlation with levels of viral replication and may serve as a sensitive marker of neuronal/axonal damage mediated by viral proteins.

Envelope glycoprotein cytoplasmic domains from diverse lentiviruses interact with the prenylated Rab acceptor

Lentivirus envelope glycoproteins have unusually long cytoplasmic domains compared to those of other retroviruses. To identify cellular binding partners of the simian immunodeficiency virus (SIV) envelope transmembrane protein (gp41) cytoplasmic domain (CD), we performed a yeast two-hybrid screen of a phytohemagglutinin-activated human T-cell cDNA library with the SIV gp41 CD. The majority of positive clones (50 of 54) encoded the prenylated Rab acceptor (PRA1). PRA1 is a 21-kDa protein associated with Golgi membranes that binds to prenylated Rab proteins in their GTP-bound state. While the cellular function of PRA1 is presently unknown, this protein appears to participate in intracellular vesicular trafficking, based on its cellular localization and ability to bind multiple members of the Rab protein family. Mammalian two-hybrid assays confirmed the interaction between the SIV gp41 CD and PRA1. Furthermore, gp41 sequences important for PRA1 binding were mapped to a central leucine-rich, amphipathic alpha-helix in the SIV gp41 cytoplasmic tail. Although the human immunodeficiency virus (HIV-1) gp41 CD failed to interact with PRA1 in the yeast two-hybrid system, its interaction with PRA1 was significantly better than that of the SIV gp41 CD in mammalian two-hybrid assays. Interestingly, PRA1 also interacted with the Env CDs of HIV-2, bovine immunodeficiency virus, equine infectious anemia virus, and feline immunodeficiency virus. Thus, PRA1 associates with envelope glycoproteins from widely divergent lentiviruses.

Expression and functional properties of proteins encoded in the x-II ORF of HTLV-I

With the aim of identifying viral proteins that contribute to the distinctive properties of HTLV-I biology and pathogenicity, several laboratories have investigated the coding potential of the X region of the genome, which includes five partially overlapping open reading frames (ORFs). We and others have shown that, in addition to the essential regulatory proteins Rex and Tax, a number of accessory proteins encoded in the X region can be produced by alternative splicing and multicistronic translation. One X region ORF, termed X-II, produces two protein isoforms named Tof/p30II and p13II, which are expressed from a doubly- and singly-spliced mRNA, respectively. Initial functional analyses demonstrated that Tof/p30II is a nucleolar/nuclear protein that possesses a region capable of binding to RNA, and p13II is a mitochondrial protein that alters the morphology and function of this organelle. Together with data from other laboratories demonstrating the production of antibodies and CTL against x-II ORF products in HTLV-I infected subjects and the requirement of this ORF for efficient viral replication in vivo, these findings suggest that further characterization of Tof/p30II and p13II will yield insight into remaining undefined aspects of HTLV-I pathogenicity and replication.

Sustained multilineage gene persistence and expression in dogs transplanted with CD34(+) marrow cells transduced by RD114-pseudotype oncoretrovirus vectors

Previous studies have shown that the choice of envelope protein (pseudotype) can have a significant effect on the efficiency of retroviral gene transfer into hematopoietic stem cells. This study used a competitive repopulation assay in the dog model to evaluate oncoretroviral vectors carrying the envelope protein of the endogenous feline virus, RD114. CD34-enriched marrow cells were divided into equal aliquots and transduced with vectors produced by the RD114-pseudotype packaging cells FLYRD (LgGLSN and LNX) or by the gibbon ape leukemia virus (GALV)-pseudotype packaging cells PG13 (LNY). A total of 5 dogs were studied. One dog died because of infection before sustained engraftment could be achieved, and monitoring was discontinued after 9 months in another animal that had very low overall gene-marking levels. The 3 remaining animals are alive with follow-ups at 11, 22, and 23 months. Analyses of gene marking frequencies in peripheral blood and marrow by polymerase chain reaction revealed no significant differences between the RD114 and GALV-pseudotype vectors. The LgGLSN vector also contained the enhanced green fluorescent protein (GFP), enabling us to monitor proviral expression by flow cytometry. Up to 10% of peripheral blood cells expressed GFP shortly after transplantation and approximately 6% after the longest follow-up of 23 months. Flow cytometric analysis of hematopoietic subpopulations showed that most of the GFP-expressing cells were granulocytes, although GFP-positive lymphocytes and monocytes were also detected. In summary, these results show that RD114-pseudotype oncoretroviral vectors are able to transduce hematopoietic long-term repopulating cells and, thus, may be useful for human stem cell gene therapy.

Human T-lymphotropic virus type 1 p30(II) regulates gene transcription by binding CREB binding protein/p300

The highly conserved coadapters CREB binding protein (CBP) and p300 form complexes with CREB as well as other DNA binding transcription factors to modulate chromatin remodeling and thus transcription. Human T-lymphotropic virus type 1 (HTLV-1) transcription is controlled, in part, by the CREB/ATF family of transcription factors which bind promoter sequences and function as complexes with the viral oncogenic protein Tax. We have reported that the nuclear localizing protein p30(II) of HTLV-1 functions as a transcription factor, differentially modulates CREB-responsive promoters, and is critical for maintenance of proviral loads in rabbits. In this study, we tested whether p30(II) directly interacts with CBP/p300 to modulate gene transcription. Gal4(BD)-p30(II)-mediated transactivation was enhanced following exogenous expression of p300 and was competitively repressed by the p300 binding protein, adenovirus E1A, and E1ACR2 (mutated for retinoblastoma binding but retaining p300 binding). In contrast, E1ACR1 (mutated for p300 binding) failed to alter Gal4(BD)-p30(II)-mediated transactivation. In addition, Gal4(BD)-p30(II)-mediated transactivation was competitively inhibited by the cotransfection of CMV-p30(II)-HA and CMV-Tax but could be rescued by exogenous p300. Importantly, we demonstrate that p30(II) colocalizes with p300 in cell nuclei and directly binds to CBP/p300 in cells. Deletion mutants of CBP/p300 were used to localize the site critical for binding p30(II) to a highly conserved KIX region. DNA binding assays confirmed the interference of p30(II) with the assembly of CREB-Tax-p300/CBP multiprotein complexes on 21-bp repeat oligonucleotides in vitro. Collectively, our results demonstrate that CBP/p300 is a cellular protein target for HTLV-1 p30(II) and mediates its transcriptional effects in vivo.

SIV gp41 binds to membranes both in the monomeric and trimeric states: consequences for the neuropathology and inhibition of HIV infection

The viral envelope glycoprotein gp41 mediates membrane fusion in HIV/SIV infection. gp41 ectodomain (e-gp41, residues 27-149), which was shown to interact with phospholipid membranes, exists in an equilibrium between the monomeric and trimeric states. Here, we analyzed, by intrinsic Trp fluorescence and resonance energy transfer, whether SIV e-gp41-membrane interaction depends on the gp41 oligomeric state. We found that both gp41 monomers and trimers bind membranes, with the monomers' full binding being reached at substantially lower lipid to protein ratios. Furthermore, the different characteristics of the Trp fluorescence of monomers and trimers enabled us to detect binding of each form at concentrations at which both species were present. CD spectroscopy revealed that the secondary structure of gp41 monomers does not change upon membrane binding, suggesting that membrane-bound monomeric-gp41 is a possible target for DP-178, a potent peptide inhibitor of HIV infection. The consequences of the interaction between monomeric and trimeric gp41 with membranes in HIV/SIV infection, its inhibition, and its associated neuropathologies are discussed.

Cell-type-specific regulation of the two foamy virus promoters

The foamy virus (FV) genome contains two promoters, the canonical long terminal repeat (LTR) promoter, containing three consensus AP-1 binding sites, and an internal promoter (IP) within the env gene. We investigated the regulation of the two promoters in lytic and persistent infections and found that in the presence of a constitutive source of the viral transactivator protein Tas, transactivation of the LTR promoter and that of the IP differ. In lytic infections, both the LTR promoter and the IP are efficiently transactivated by Tas, while in persistent infections, the IP is efficiently transactivated by Tas, but the LTR promoter is not. Analysis of proteins expressed from the LTR promoter and the IP during infection indicated that IP transcription is more robust than that of the LTR promoter in persistently infected cells, while the opposite is true for lytically infected cells. Coculture experiments also showed that LTR promoter transcription is greatest in cells which support lytic replication. Replacement of much of the LTR promoter with the IP leads to increased viral replication in persistent but not lytic infections. We also found that the induction of persistently infected cells with phorbol 12-myristate 13-acetate (PMA) greatly enhanced viral replication and transcription from the SFVcpz(hu) (new name for human FV) LTR promoter. However, mutation of three consensus AP-1 binding sites in the FV LTR promoter did not affect viral replication in lytically or persistently infected cells, nor did the same mutations affect LTR promoter transactivation by Tas in PMA-treated cells. Our data indicate that differential regulation of transcription is important in the outcome of FV infection but is unlikely to depend on AP-1.

Experimental determination and calculations of redox potential descriptors of compounds directed against retroviral zinc fingers: Implications for rational drug design

A diverse set of electrophilic compounds that react with cysteine thiolates in retroviral nucleocapsid (NC) proteins and abolish virus infectivity has been identified. Although different in chemical composition, these compounds are all oxidizing agents that lead to the ejection of Zn(II) ions bound to conserved structural motifs (zinc fingers) present in retroviral NC proteins. The reactivity of a congeneric series of aromatic disulfides toward the NC protein of the human immunodeficiency virus type 1 (HIV-1), NCp7, has been characterized by HPLC separation of starting reagents from reaction products. We calculated the absolute redox potentials of these compounds in the gas phase and in aqueous solvent, using a density functional theory method and a continuum solvation model. Pulsed polarography experiments were performed and showed a direct correlation between calculated and experimentally determined redox propensities. A dependence between protein reactivity and redox potential for a specific compound was shown: Reaction with NCp7 did not take place below a threshold value of redox potential. This relationship permits the distinction between active and nonactive compounds targeted against NCp7, and provides a theoretical basis for a scale of reactivity with retroviral zinc fingers. Our results indicate that electrophilic agents with adequate thiophilicity to react with retroviral NC fingers can now be designed using known or calculated electrochemical properties. This may assist in the design of antiretroviral compounds with greater specificity for NC protein. Such electrophilic agents can be used in retrovirus inactivation with the intent of preparing a whole-killed virus vaccine formulation that exhibits unaffected surface antigenic properties.

Molecular biology and pathogenesis of the human T-cell leukaemia/lymphotropic virus Type-1 (HTLV-1)

Retroviruses are associated with a variety of diseases, including immunological and neurological disorders, and various forms of cancer. In humans, the Human T-cell Leukaemia/Lymphotropic virus type 1 (HTLV-1), which belongs to the Oncovirus family, is the aetiological agent of two diverse diseases: Adult T-cell leukaemia/lymphoma (ATLL) (Poiesz et al. 1980; Hinuma et al. 1981; Yoshida et al. 1982), as well as the neurological disorder tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM) (Gessain et al. 1985; Rodgers-Johnson et al. 1985; Osame et al. 1986). HTLV-1 is the only human retrovirus known to be the aetiological agent of cancer. A genetically related virus, HTLV-2, has been identified and isolated (Kalyanaraman et al. 1982). However, there has been no demonstration of a definitive aetiological role for HTLV-2 in human disease to date. Simian T-cell lymphotropic viruses types 1 and 2 (STLV-1 and -2) and bovine leukaemia virus (BLV) have also been classified in same group, Oncoviridae, based upon their similarities in genetic sequence and structure to HTLV-1 and -2 (Burny et al. 1988; Dekaban et al. 1995; Slattery et al. 1999). This article will focus on HTLV-1, reviewing its discovery, molecular biology, and its role in disease pathogenesis.

Domains in the simian immunodeficiency virus gp41 cytoplasmic tail required for envelope incorporation into particles

The mechanism by which lentivirus envelope (Env) glycoproteins are packaged into budding virions is poorly understood. Simian immunodeficiency virus (SIV) contains an Env protein with an unusually long cytoplasmic tail. To investigate the role of this domain in the incorporation of the SIV Env into virions, we generated a series of SIV Env mutants carrying small in-frame deletions within the cytoplasmic domain. The effects of these mutations on Env synthesis, processing, and association with Gag particles were analyzed by means of the vaccinia virus expression system. All of the mutant Env glycoproteins were synthesized and processed in a manner similar to that of the wild-type Env. However, deletions affecting domains C-terminal to residue 832 in the SIV Env protein significantly impaired Env incorporation into particles. Cell surface biotinylation assays showed that this phenotype could not be attributed to inefficient cell surface expression of the Env mutants. Furthermore, when the Env deletion mutants were tested for their ability to mediate virus entry in single-cycle infectivity assays, those mutations that impaired Env incorporation also caused a severe defect in virus infectivity. Our results suggest that domains in the C-terminal third of the SIV Env protein are required for Env incorporation into particles and Env-mediated virus entry.

Pathways to neuronal injury and apoptosis in HIV-associated dementia

Human immunodeficiency virus-1 (HIV-1) can induce dementia with alarming occurrence worldwide. The mechanism remains poorly understood, but discovery in brain of HIV-1-binding sites (chemokine receptors) provides new insights. HIV-1 infects macrophages and microglia, but not neurons, although neurons are injured and die by apoptosis. The predominant pathway to neuronal injury is indirect through release of macrophage, microglial and astrocyte toxins, although direct injury by viral proteins might also contribute. These toxins overstimulate neurons, resulting in the formation of free radicals and excitotoxicity, similar to other neurodegenerative diseases. Recent advances in understanding the signalling pathways mediating these events offer hope for therapeutic intervention.

Developmental regulation of transcription by a tissue-specific TAF homolog

Alternate forms of the general transcription machinery have been described in several tissues or cell types. However, the role of tissue-specific TBP-associated factors (TAF(II)s) and other tissue-specific transcription components in regulating differential gene expression during development was not clear. Here we show that the cannonball gene of Drosophila encodes a cell type-specific homolog of a more ubiquitously expressed component of the general transcription factor TFIID. cannonball is required in vivo for high level transcription of a set of stage- and tissue-specific target genes during male gametogenesis. Regulation of transcription by cannonball is absolutely required for spermatogenesis, as null mutations block meiotic cell cycle progression and result in a complete failure of spermatid differentiation. Our results demonstrate that cell type-specific TAF(II)s play an important role in developmental regulation of gene expression.

Interaction of human immunodeficiency virus type 1 Vpr with the HHR23A DNA repair protein does not correlate with multiple biological functions of Vpr

The virion-associated Vpr protein of human immunodeficiency virus type 1 (HIV-1) alters cell cycle progression from the G2 phase, influences the virus in vivo mutation rate, and participates in the nuclear translocation of viral DNA. While many Vpr-interacting proteins have been identified, the functional relevance of these interactions remains to be thoroughly documented. We have explored the contribution of the interaction of HIV-1 Vpr with HHR23A, a cellular protein implicated in DNA repair, to the known phenotypes of Vpr. The association of Vpr with HHR23A required the core region of Vpr, which encompasses the two alpha-helical structures of the protein. No binding of HHR23A was detected with the Vpr and Vpx proteins of other primate lentiviruses. HIV-1 Vpr variants containing single amino acid substitutions in each alpha-helix and deficient for binding to HHR23A were isolated. The functional characterization of these Vpr variants indicated that binding to HHR23A did not correlate with the ability of Vpr to induce cell cycle arrest, even though it was previously proposed that HHR23A is a mediator of the Vpr-induced G2 arrest. Also, the Vpr-HHR23A interaction did not influence the HIV-1 in vivo mutation rate. Finally, Vpr and HHR23A are both localized in the nucleus, but no correlation was observed between the nuclear targeting of Vpr and the interaction with HHR23A. Further analysis is needed to determine the functional role(s) of the Vpr-HHR23A association during the HIV-1 life cycle.

Lessons from HIV: movement of macromolecules inside the cell

Molecular biological investigations of HIV have made fundamental contributions to our understanding of eukaryotic biology. These studies elucidated new paradigms in transcription, RNA and protein export from the nucleus to the cytoplasm, cellular activation, morphology and vesicular trafficking.

Mitochondrial control of cell death induced by HIV-1-encoded proteins

In most examples of physiological or pathological cell death, mitochondrial membrane permeabilization (MMP) constitutes an early critical event of the lethal process. Signs of MMP that precede nuclear apoptosis include the translocation of cytochrome c and apoptosis-inducing factor (AIF) from mitochondria to an extra-mitochondrial localization, as well as the dissipation of the mitochondrial transmembrane potential. MMP also occurs in HIV-1-induced apoptosis. Different HIV-1 encoded proteins (Env, Vpr, Tat, PR) can directly or indirectly trigger MMP, thereby causing cell death. The gp120/gp41 Env complex constitutes an example for an indirect MMP inducer. Env expressed on the plasma membrane of HIV-1 infected (or Env-transfected) cells mediates cell fusion with CD4/CXCR4-expressing uninfected cells. After a cell type-dependent latency period, syncytia then undergo MMP and apoptosis. Vpr exemplifies a direct MMP inducer. Vpr binds to the adenine nucleotide translocator (ANT), a mitochondrial inner membrane protein which also interacts with apoptosis-regulatory proteins from the Bcl-2/Bax family. Binding of Vpr to ANT favors formation of a non-specific pore leading to MMP. The structural motifs of the Vpr protein involved in MMP are conserved among most pathogenic HIV-1 isolates and determine the cytotoxic effect of Vpr. These data suggest the possibility that viruses employ multiple strategies to regulate host cell apoptosis by targeting mitochondria.

Intracellular targeting of walleye dermal sarcoma virus Orf A (rv-cyclin)

Walleye dermal sarcoma virus (WDSV) induces tumors and allows or possibly directs tumor regression. WDSV encodes a putative cyclin homologue, Orf A, and six variant Orf A transcripts have been identified. Northern analysis indicated that a 3.3-kb transcript, encoding full-length Orf A, is the predominant transcript in developing, but not regressing, tumors. Three Orf A proteins, one full-length and two amino-truncated forms, were expressed in mammalian and piscine cells, and their intracellular locations were determined. The full-length form was nuclear and concentrated in interchromatin granule clusters, defined by colocalization with SC-35. The amino-truncated forms were cytoplasmic. Fusion of amino-terminal portions of Orf A to a heterologous protein demonstrated that residues 1-112 were necessary for nuclear localization. Mutation of aa K80 and/or E110 disrupted nuclear localization, suggesting a mechanism similar to that of cellular A- and D-type cyclins for its nuclear import.

Calmodulin binding properties of peptide analogues and fragments of the calmodulin-binding domain of simian immunodeficiency virus transmembrane glycoprotein 41

The calcium-regulatory protein calmodulin (CaM) can bind with high affinity to a region in the cytoplasmic C-terminal tail of glycoprotein 41 of simian immunodeficiency virus (SIV). The amino acid sequence of this region is (1)DLWETLRRGGRW(13)ILAIPRRIRQGLELT(28)L. In this work, we have used near- and far-uv CD, and fluorescence spectroscopy, to study the orientation of this peptide with respect to CaM. We have also studied biosynthetically carbon-13 methyl-Met calmodulin by (1)H, (13)C heteronuclear multiple quantum coherence NMR spectroscopy. Two Trp-substituted peptides, SIV-W3F and SIV-W12F, were utilized in addition to the intact SIV peptide. Two half-peptides, SIV-N (residues 1-13) and SIV-C (residues 13-28) were also synthesized and studied. The spectroscopic results obtained with the SIV-W3F and SIV-W12F peptides were generally consistent with those obtained for the native SIV peptide. Like the native peptide, these two analogues bind with an alpha-helical structure as shown by CD spectroscopy. Fluorescence intermolecular quenching studies suggested binding of Trp3 to the C-lobe of CaM. Our NMR results show that SIV-N can bind to both lobes of calcium-CaM, and that it strongly favors binding to the C-terminal hydrophobic region of CaM. The SIV-C peptide binds with relatively low affinity to both halves of the protein. These data reveal that the intact SIV peptide binds with its N-terminal region to the carboxy-terminal region of CaM, and this interaction initiates the binding of the peptide. This orientation is similar to that of most other CaM-binding domains.

The human T-cell leukemia virus type I (HTLV-I) X region encoded protein p13(II) interacts with cellular proteins

Interactions between the Human T-cell leukemia virus type I (HTLV-I) gene product p13(II) and cellular proteins were investigated using the yeast two-hybrid system. Variant forms of p13(II) were derived from two HTLV-I molecular clones, K30p and K34p, that differ in both virus production and in vivo and in vitro infectivity. Two nucleotide differences between the p13 from K30p (p13K30) and K34p (p13K34) result in a Trp-Arg substitution at amino acid 17 and the truncation of the 25 carboxyl-terminal residues of p13K34. A cDNA library from an HTLV-I-infected rabbit T-cell line was screened with p13K30 and p13K34 as bait. Products of two cDNA clones, C44 and C254, interacted with p13K34 but not with p13K30. Interactions were further confirmed using the GST-fusion protein coprecipitation assay. Sequence analysis of C44 and C254 cDNA clones revealed similarities to members of the nucleoside monophosphate kinase superfamily and actin-binding protein 280, respectively. Further analysis of the function of these two proteins and the consequence of their interaction with p13 may help elucidate a role for p13 in virus production, infectivity, or the pathogenesis of HTLV-I.

The p13II protein of HTLV type 1: comparison with mitochondrial proteins coded by other human viruses

In addition to the essential regulatory proteins Rex and Tax, the HTLV-1 genome encodes several accessory proteins of yet undefined function. One of these "orphan" proteins, named p13(II), was recently shown to be selectively targeted to mitochondria and to induce specific changes in mitochondrial morphology suggestive of altered inner membrane permeability and swelling. This represented the first report of a retroviral gene product targeted to mitochondria, and suggested that p13(II)-induced alterations in the function of this organelle may play a role in HTLV-1 replication and/or pathogenesis. The more recent findings that both Vpr and Tat of HIV-1 are targeted to mitochondria reinforces the proposed relevance of mitochondrial metabolism to the life cycle of retroviruses. Thus, p13(II), Vpr, and Tat can be added to the growing list of mitochondrial proteins produced by clinically important human viruses, including Epstein-Barr virus, human cytomegalovirus, and hepatitis B virus. Mitochondria are known to play a critical role by providing an amplification loop required for the execution of signaling pathways leading to programmed cell death. The functional consequences of the interactions between viral proteins and mitochondria described so far have been attributed to either the positive or negative control of apoptotic responses mediated by this organelle. Further analysis of the effects of p13(II) on mitochondrial function is likely to add to our understanding of the mechanisms underlying the development of HTLV-1-associated diseases.

In vitro and in vivo functional analysis of human T cell lymphotropic virus type 1 pX open reading frames I and II

Human T lymphotropic virus type 1 (HTLV-1) is a complex retrovirus containing regulatory and accessory genes encoded in four open reading frames (ORF I-IV) of the pX region. It is not clear what role pX ORFs I and II-encoded proteins have in the pathogenesis of the lymphoproliferative diseases associated with HTLV-1 infection. The conserved ORF I encodes for a hydrophobic 12-kDa protein, p12, (I) that contains four SH3 binding motifs (PXXP) that localizes to cellular endomembranes when overexpressed in cultured cells. Differential splicing of pX ORF II results in the production of two nuclear proteins, p13(II) and p30(II). p13(II) also localizes to mitochondria. p30(II) shares homology with the POU family of transcription factors. We have identified functional roles of pX ORF I and ORF II in establishment and maintenance of infection in a rabbit model. To functionally study p12(I) we have tested a proviral clone with selective ablation of ORF I (ACH.p12(I)) for its ability to infect quiescent peripheral blood mononuclear cells (PBMC). Our data indicate that T cells infected with the wild-type clone of HTLV-1 (ACH) are more efficient than ACH.p12(I) in infecting quiescent PBMC. These findings parallel our animal model data and suggest a role for p12(I) in the activation of quiescent lymphocytes, a prerequisite for effective viral replication in vivo. To test the ability of p30(II) to function as a transcription factor we have constructed p30(II) as a Gal4-fusion protein. When transfected with Gal4-driven luciferase reporter genes, the p30(II)-Gal4-fusion protein induces transcriptional activity up to 50-fold in both 293 and HeLa-Tat cells. These systems will be useful to identify molecular mechanisms that explain the functional role of pX ORF I and ORF II-encoded proteins in HTLV-1 replication.

The MHC class I heavy chain is a common target of the small proteins encoded by the 3' end of HTLV type 1 and HTLV type 2

Human T cell leukemia/lymphotropic virus types 1 and 2 are two immunologically and phylogenetically related retroviruses that differ in their pathogenicity in vivo. The overall genetic structure of HTLV-1 and -2 is similar. Each contains a unique region at the 3' end of the genome, designated the pX region. p12(I) is a membrane-associated protein encoded by the open reading frame I (ORF I) region of HTLV-1, which lies within the pX region. A corresponding protein, p10(I) is encoded by the ORF I region of HTLV-2 and an additional protein, p11(V), is encoded by ORF V, which overlaps the HTLV-2 ORF I region. As with HTLV-1, the small proteins encoded by the pX region of HTLV-2 appear to be dispensable for viral replication and cellular transformation in vitro. However, the small open reading frames of both viruses are important for viral replication in vivo, which suggests they may play an important role during the viral life cycle. This study was undertaken to investigate and compare the cellular targets of the p10(I), p11(V), and p12(I) putative proteins.

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.

Differential apoptosis effects of primate lentiviral Vpr and Vpx in mammalian cells

The growth inhibitory effects of Vpr and Vpx are species- and cell type-dependent. HIV-1, HIV-2 and SIV Vpr are primarily cytostatic in mammalian cells and HIV-1 Vpr has been reported to induce apoptosis in human cells. Our previous studies have shown that HIV-1, HIV-2 and SIV Vpr and Vpx have differential cytostatic and cytotoxic effects in the yeast cells [Zhang et al.: Virology, 230:103-112; 1997]. Here, we further examined the apoptosis function of HIV-1 Vpr in different species of mammalian cells and investigated if other primate lentiviral Vpr and Vpx exert similar functions. Our results show that none of the primate lentiviral Vpr or Vpx we tested induces apoptosis in nonhuman species of mammalian cells. However, HIV-1 Vpr, but not HIV-2 or SIV Vpr and/or Vpx, induced apoptosis in different types of human cell lines. Further, the apoptotic effect of HIV-1 Vpr can be distinguished from that of the human interferon-gamma, a known proapoptotic protein, that HIV-1 Vpr shows little to no paracrine and/or bystander effect. When coexpressed with Bcl-2 or Bcl-X(L), the apoptotic effect of HIV-1 Vpr became markedly attenuated. These results indicate that the apoptotic effect of HIV-1 Vpr is species-dependent and is intracellularly modulated by the Bcl-2 family of proteins. Our study also suggests that the proapoptotic function of HIV-1 Vpr is developmentally associated with human but not nonhuman primate species.

Biophysical characterization of gp41 aggregates suggests a model for the molecular mechanism of HIV-associated neurological damage and dementia

In human immunodeficiency virus (HIV)-infected individuals, the level of the HIV envelope protein gp41 in brain tissue is correlated with neurological damage and dementia. In this paper we show by biochemical methods and electron microscopy that the extracellular ectodomain of purified HIV and simian immunodeficiency virus gp41 (e-gp41) forms a mixture of soluble high molecular weight aggregate and native trimer at physiological pH. The e-gp41 aggregate is shown to be largely alpha-helical and relatively stable to denaturants. The high molecular weight form of e-gp41 is variable in size ranging from 7 to 70 trimers, which associate by interactions at the interior of the aggregate involving the loop that connects the N- and C-terminal helices of the e-gp41 core. The trimers are predominantly arranged with their long axes oriented radially, and the width of the high molecular weight aggregate corresponds to the length of two e-gp41 trimers (approximately 200 A). Using both light and electron microscopy combined with immunohistochemistry we show that HIV gp41 accumulates as an extracellular aggregate in the brains of HIV-infected patients diagnosed with dementia. We postulate that the high molecular weight aggregates of e-gp41 are responsible for HIV-associated neurological damage and dementia, consistent with known mechanisms of encephalopathy.

Neural stem cells as tools for understanding retroviral neuropathogenesis

The discovery within the past decade that neural stem cells (NSCs) from the developing and adult mammalian brain can be propagated, cloned, and genetically manipulated ex vivo for ultimate transfer back into the CNS has opened the door to a novel means for modifying the CNS environment for experimental and therapeutic purposes. While a great deal of interest has been focused on the properties and promise of this new technology, especially in regard to cellular replacement and gene therapy, this minireview will focus on the recent use of NSCs to study the neuropathogenesis of the murine oncornaviruses. In brief, the use of this NSC-based approach has provided a means for selective reconstitution within the brain, of specific retroviral life cycle events, in order to consider their contribution to the induction of neurodegeneration. Furthermore, by virtue of their ability to disseminate virus within the brain, NSCs have provided a reliable means for assessing the true neurovirulence potential of murine oncornaviruses by directly circumventing a restriction to virus entry into the CNS. Importantly, these experiments have demonstrated that the neurovirulence of oncornaviruses requires late virus life cycle events occurring specifically within microglia, the resident macrophages of the brain. This initial application of NSC biology to the analysis of oncornavirus-CNS interactions may serve as an example for how other questions in viral neuropathogenesis might be addressed in the future.

Alteration of substrate and inhibitor specificity of feline immunodeficiency virus protease

Feline immunodeficiency virus (FIV) protease is structurally very similar to human immunodeficiency virus (HIV) protease but exhibits distinct substrate and inhibitor specificities. We performed mutagenesis of subsite residues of FIV protease in order to define interactions that dictate this specificity. The I37V, N55M, M56I, V59I, and Q99V mutants yielded full activity. The I37V, N55M, V59I, and Q99V mutants showed a significant increase in activity against the HIV-1 reverse transcriptase/integrase and P2/nucleocapsid junction peptides compared with wild-type (wt) FIV protease. The I37V, V59I, and Q99V mutants also showed an increase in activity against two rapidly cleaved peptides selected by cleavage of a phage display library with HIV-1 protease. Mutations at Q54K, I98P, and L101I dramatically reduced activity. Mutants containing a I35D or I57G substitution showed no activity against either FIV or HIV substrates. FIV proteases all failed to cut HIV-1 matrix/capsid, P1/P6, P6/protease, and protease/reverse transcriptase junctions, indicating that none of the substitutions were sufficient to change the specificity completely. The I37V, N55M, M56I, V59I, and Q99V mutants, compared with wt FIV protease, all showed inhibitor specificity more similar to that of HIV-1 protease. The data also suggest that FIV protease prefers a hydrophobic P2/P2' residue like Val over Asn or Glu, which are utilized by HIV-1 protease, and that S2/S2' might play a critical role in distinguishing FIV and HIV-1 protease by specificity. The findings extend our observations regarding the interactions involved in substrate binding and aid in the development of broad-based inhibitors.

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.

A novel, non-nested reverse-transcriptase polymerase chain reaction (RT-PCR) test for the detection of the t(15;17) translocation: a comparative study of RT-PCR cytogenetics, and fluorescence In situ hybridization

BACKGROUND

The development of a rapid and simple reverse-transcription polymerase chain reaction (RT-PCR) assay is described that identifies the promyelocytic leukemia- retinoic acid receptor alpha (PML-RARa) hybrid messenger RNA (mRNA), a characteristic feature of acute promyelocytic leukemia (APL).

METHODS AND RESULTS

Randomly primed complementary (cDNA) is synthesized from leukocyte RNA and amplified in the presence of Taq Gold in 2 separate reaction tubes containing primer pairs specific for intron 3 (bcr 3, long [L] form mRNA transcript) and intron 6 (bcr 1, short [S] form)/exon 6 (bcr 2, variant [V] form) breakpoints in PML, respectively. The different sized products generated from each RNA transcript (S, L, or V forms) are readily and unambiguously distinguishable after agarose gel electrophoresis without the need for either nested PCR or hybridization. The sensitivity of the assay is 1 in 10,000 to 1 in 100,000. The separate amplification of a b2-microglobulin transcript controls for adequate RNA and cDNA preparation. The newly developed assay was used clinically for the evaluation of 78 patients with APL. It was rapid and more sensitive than cytogenetic karyotyping, both for the diagnosis of APL and the assessment of minimal residual disease (MRD) after therapy. RT-PCR detected PML-RARa mRNA in all cases positive for the t(15;17) translocation by cytogenetics. However, as many as 50% and 80% of the diagnostic specimens and the specimens for MRD assessment, respectively, that were positive by RT-PCR were negative by cytogenetics. The ratio of cases with L-form to S-form PML-RARa fusion transcript was 2:1, whereas 3 cases (10%) had fusion sites in exon 6 of the PML gene (V forms). In addition, approximately 50% of the patients were diagnosed morphologically with microgranular M3V-type leukemia, but no significant correlation with PML breakpoints was found.

CONCLUSION

The current assay is rapid, sensitive, and specific without using nested PCR or hybridization.

Monomer-trimer equilibrium of the ectodomain of SIV gp41: insight into the mechanism of peptide inhibition of HIV infection

The monomer-trimer equilibrium of the ectodomain of SIV gp41 (residues 27-149, e-gp41) has been characterized by analytical ultracentrifugation, circular dichroism (CD), and NMR spectroscopy. Based on analytical ultracentrifugation experiments performed at different rotor speeds and protein concentrations, the equilibrium association constant for the SIV e-gp41 trimer is 3.1 x 10(11) M(-2). The presence of intermolecular nuclear Overhauser effects in a mixture of 12C and 13C-labeled e-gp41 prepared under nondenaturing conditions unambiguously demonstrates that there is a dynamic equilibrium between the monomer and trimer. The CD spectra taken as a function of SIV e-gp41 concentration suggest that the helical content of the monomeric state does not change significantly relative to that of the trimeric state. The relevance of the monomer-trimer equilibrium is discussed with respect to gp41 function and the inhibitory properties of gp41 peptides.

Mitochondrial targeting of the p13II protein coded by the x-II ORF of human T-cell leukemia/lymphotropic virus type I (HTLV-I)

The X region of the HTLV-I genome contains four major open reading frames (ORFs), two of which, termed x-I and x-II, are of still undefined biological significance. By indirect immunofluorescence and dual labeling with marker proteins, we demonstrate that p13II, an 87-amino acid protein coded by the x-II ORF, is selectively targeted to mitochondria. Mutational analysis revealed that mitochondrial targeting of p13II is directed by an atypical 10-amino acid signal sequence that is not cleaved upon import and is able to target the Green Fluorescent Protein to mitochondria. Expression of p13II results in specific alterations of mitochondrial morphology and distribution from a typical string-like, dispersed network to round-shaped clusters, suggesting that p13II might interfere with processes relying on an intact mitochondrial architecture. Functional studies of mitochondria with the cationic fluorochrome tetramethylrhodamine revealed that a subpopulation of the cells with p13II-positive mitochondria show a disruption in the mitochondrial inner membrane potential (Apsi), an early event observed in cells committed to apoptosis. Taken together, these results suggest novel virus-cell interactions that might be important in HTLV-I replication and/or pathogenicity.

The cytoplasmic domain of HIV-1 gp41 interacts with the carboxyl-terminal region of alpha-catenin

To know the cellular protein interactions with the viral protein can give an insight into the molecular mechanisms of the virus life cycle. As the function of the cytoplasmic domain of human immunodeficiency virus type 1 (HIV-1) gp41 is not known clearly, we searched for a cellular protein that interacts with the cytoplasmic domain of the HIV-1 gp41 using the yeast two-hybrid assay system. Screening of HeLa cell cDNA library yielded alpha-catenin cDNA. The cytoplasmic domain of the HIV-1 gp41 and the simian immunodeficiency virus (SIV) gp41 were able to interact with the carboxyl-terminal region of alpha-catenin specifically. Mapping of the interaction sites revealed that the interaction between the domain containing the second helix structure from the carboxyl-terminus of HIV-1 gp41 and the carboxyl-terminal region of alpha-catenin was stronger than other domains of gp41.

Replication and budding of simian immunodeficiency virus in polarized epithelial cells

Simian immunodeficiency virus (SIV) infection of primates provides an important model for infection of humans by HIV. Since mucosal epithelium is likely to be an important portal of entry, we decided to study aspects of the interaction of SIV with epithelial cells. SIV was shown to produce virus efficiently in polarized epithelial cells (Vero C1008) transfected with SIVmac239 proviral DNA. The virus titer in the epithelial cell culture fluid reached 10(3) TCID50/ml at day 3 posttransfection. Initially after transfected epithelial cells were plated on a permeable membrane, virus budded at both the apical and the basolateral domains. However, after the cells formed a tight monolayer, 95-100% of the virus particles budded basolaterally, as assessed by release of p27 antigen into the fluid above and below the monolayer. This finding was confirmed by electron microscopy, which showed that the mature virus budded basolaterally in polarized cells. After introduction of the CD4 gene into Vero cells by a retrovirus vector, polarizable cells were able to be infected by cell-free SIVmac239 virus. The virus titer reached 10(4) TCID50/ml in culture fluid and virions also budded basolaterally, the same as the virus from transfected cells. Two viruses (SIVmac1A11 and SIVmac251) that contain truncated TMgp28 instead of TMgp41 also budded basolaterally. Furthermore, we found that HIV-1 with full-length or truncated TMgp41 also budded basolaterally.

A novel strategy of cell targeting based on tissue-specific expression of the ecotropic retrovirus receptor gene

Gene transfer into specific tissues or cell types is a key technique in the development of gene therapy. Modification of vector particles such that they selectively bind to the target cells has been attempted, but the limitation of this approach is the low transduction efficiency. Here, we show that a two-step gene transfer system can be used for efficient cell targeting. With this strategy, and using a high-titer adenoviral vector containing a tissue-specific promoter, we have engineered a system in which only target cells become susceptible to retrovirus-mediated transduction. In a model experiment, we constructed an adenoviral vector (Ad.AFPEcoRec) containing the ecotropic retrovirus receptor (EcoRec) gene under the control of the alpha-fetoprotein (AFP) promoter. A binding assay showed that after transduction with AD.AFPEcoRec, EcoRec molecules were efficiently expressed in AFP+HepG2 cells, but not in AFP-HeLa and AFP-HLE cells. The EcoRec-expressing HepG2 cells could be stably transduced with ecotropic retroviral vectors, whereas HeLa and HLE cells remained highly resistant to retrovirus-mediated gene transfer. The apparent titer on HepG2 cells was greater than 2 x 10(5) CFU/ml. Because various tissue-specific promoter/enhancer elements are available, the two-step system could be used as a general strategy for both ex vivo and in vivo targeted gene transfer.

Nuclear localization of the functional Bel 1 transactivator but not of the gag proteins of the feline foamy virus

Interactions between host cells and foamy or spumaretroviruses are different from those of other known retroviruses. Previous work has suggested that the Gag and high-affinity DNA-binding Bel 1 transactivator of human foamy virus are localized in the nuclei of infected cells. Using two independent detection methods, we show here that the functionally active Bel 1 transactivator protein of feline foamy virus is of nuclear localization. In contrast to that reported for the human foamy virus Gag protein, the cat foamy virus Gag proteins exclusively localized in the cytoplasm close to perinuclear regions.

Folding the ribonuclease H domain of Moloney murine leukemia virus reverse transcriptase requires metal binding or a short N-terminal extension

Reverse transcriptase (RT) is a modular enzyme carrying polymerase and ribonuclease H (RNase H) activities in separable domains. Retroviral replication requires both of these activities. The RNase H domain is responsible for hydrolysis of the RNA portion of RNA x DNA hybrids, and this activity requires the presence of divalent cations (Mg2+ or Mn2+) that bind its active site. This domain is a part of a large family of homologous RNase H enzymes of which the RNase HI protein from Escherichia coli is the best characterized. Although the isolated RNase H domain from human immunodeficiency virus RT is inactive, the Moloney murine leukemia virus (MMLV) domain is active in the absence of the polymerase domain, making functional studies more accessible. Using circular dichroism spectroscopy, we characterized the stability and folding of two different fragments of MMLV RT that retain RNase H activity. The smaller fragment corresponding to the 157 C-terminal residues of RT is predominantly unfolded in the absence of divalent cations, but folding can be induced by the addition of metal. The larger fragment corresponding to the 175 C-terminal residues, however, is stably folded in the absence of metal. Thus, an 18 residue N-terminal extension outside the region homologous to E. coli RNase HI is important for the structural stability of the RNase H domain of MMLV RT. Therefore, this region should be considered part of the RNase H domain.

Crystal structure of the simian immunodeficiency virus (SIV) gp41 core: conserved helical interactions underlie the broad inhibitory activity of gp41 peptides

The gp41 subunit of the envelope protein complex from human and simian immunodeficiency viruses (HIV and SIV) mediates membrane fusion during viral entry. The crystal structure of the HIV-1 gp41 ectodomain core in its proposed fusion-active state is a six-helix bundle. Here we have reconstituted the core of the SIV gp41 ectodomain with two synthetic peptides called SIV N36 and SIV C34, which form a highly helical trimer of heterodimers. The 2.2 A resolution crystal structure of this SIV N36/C34 complex is very similar to the analogous structure in HIV-1 gp41. In both structures, three N36 helices form a central trimeric coiled coil. Three C34 helices pack in an antiparallel orientation into highly conserved, hydrophobic grooves along the surface of this coiled coil. The conserved nature of the N36-C34 interface suggests that the HIV-1 and SIV peptides are functionally interchangeable. Indeed, a heterotypic complex between HIV-1 N36 and SIV C34 peptides is highly helical and stable. Moreover, as with HIV-1 C34, the SIV C34 peptide is a potent inhibitor of HIV-1 infection. These results identify conserved packing interactions between the N and C helices of gp41 and have implications for the development of C peptide analogs with broad inhibitory activity.

Derivation and functional characterization of a consensus DNA binding sequence for the tas transcriptional activator of simian foamy virus type 1

Although DNA binding sites specific for the Bel-1 and Tas transcriptional activators, encoded, respectively, by the human and simian foamy viruses, have been mutationally defined, they show little evident sequence identity. As a result, the sequence determinants for DNA binding by both Bel-1 and Tas have remained unclear. Here, we report the use of a novel in vivo randomization and selection strategy to identify a Tas DNA binding site consensus. This approach takes advantage of the fact that Tas can effectively activate gene expression in yeast cells via a Tas DNA binding site derived from the simian foamy virus type 1 (SFV-1) internal promoter. The defined Tas DNA binding site consensus extends over approximately 25 bp and contains a critical core sequence of approximately 5 bp. Positions adjacent to this core sequence, while clearly also subject to selection, show a significantly higher level of sequence variation. Surprisingly, the wild-type SFV-1 internal promoter Tas DNA binding site fails to conform to the consensus at several positions. Further analysis demonstrated that the consensus sequence bound Tas more effectively than did the wild-type sequence in vitro and could mediate an enhanced Tas response in vivo when substituted into the SFV-1 internal promoter context. These findings explain the limited sequence identity observed for mutationally defined Tas or Bel-1 response elements and should facilitate the identification of Tas DNA target sites located elsewhere in the SFV-1 genome.

Human immunodeficiency virus type 2 Vpx-Gag interaction

Incorporation of Vpx into human immunodeficiency virus type 2 (HIV-2) virus-like particles is mediated by the Gag polyprotein. We have identified residues 15 to 40 of Gag p6 and residues 73 to 89 of Vpx as being necessary for virion incorporation. In addition, we show enhanced in vitro binding of Vpx to a chimeric HIV-1/HIV-2 Gag construct containing residues 2 to 49 of HIV-2 p6 and demonstrate that the presence of residues 73 to 89 of Vpx allows for in vitro binding to HIV-2 Gag.

An evolutionarily conserved splice generates a secreted env-Bet fusion protein during human foamy virus infection

Foamy viruses (spumaretroviruses) represent a retroviral genus which exhibits unusual features relating it to pararetroviruses. Previously, we reported the existence of a protein species harboring Env, Bel, and Bet epitopes in human foamy virus (HFV)-infected cells (M. L. Giron, F. Rozain, M. C. Debons-Guillemin, M. Canivet, J. Périès, and R. Emanoil-Ravier, J. Virol. 67:3596-3600, 1993). Here, we identify this protein as a 160-kDa Env-Bet fusion glycoprotein (gp160) translated from an mRNA species harboring a highly conserved splice site which deletes the membrane anchor domain of Env and fuses the env open reading frame with that of bel1/bet. While gp160 and Bet proteins were both secreted into the supernatant, only Bet was taken up by recipient cells. Since Bet plays a key role in the switch from lytic to chronic infection, secretion of Bet and gp160, together with cellular uptake of Bet, could be highly relevant for both immune response and development of HFV infection in vivo.