The putative alpha helix 2 of human immunodeficiency virus type 1 Vpr contains a determinant which is responsible for the nuclear translocation of proviral DNA in growth-arrested cells

HIV-1 Vpr Functions in Primary CD4+ T Cells

HIV-1 encodes four accesory proteins in addition to its structural and regulatory genes. Uniquely amongst them, Vpr is abundantly present within virions, meaning it is poised to exert various biological effects on the host cell upon delivery. In this way, Vpr contributes towards the establishment of a successful infection, as evidenced by the extent to which HIV-1 depends on this factor to achieve full pathogenicity in vivo. Although HIV infects various cell types in the host organism, CD4+ T cells are preferentially targeted since they are highly permissive towards productive infection, concomitantly bringing about the hallmark immune dysfunction that accompanies HIV-1 spread. The last several decades have seen unprecedented progress in unraveling the activities Vpr possesses in the host cell at the molecular scale, increasingly underscoring the importance of this viral component. Nevertheless, it remains controversial whether some of these advances bear in vivo relevance, since commonly employed cellular models significantly differ from primary T lymphocytes. One prominent example is the "established" ability of Vpr to induce G2 cell cycle arrest, with enigmatic physiological relevance in infected primary T lymphocytes. The objective of this review is to present these discoveries in their biological context to illustrate the mechanisms whereby Vpr supports HIV-1 infection in CD4+ T cells, whilst identifying findings that require validation in physiologically relevant models.

Proteasomal Degradation Machinery: Favorite Target of HIV-1 Proteins

Proteasomal degradation pathways play a central role in regulating a variety of protein functions by controlling not only their turnover but also the physiological behavior of the cell. This makes it an attractive target for the pathogens, especially viruses which rely on the host cellular machinery for their propagation and pathogenesis. Viruses have evolutionarily developed various strategies to manipulate the host proteasomal machinery thereby creating a cellular environment favorable for their own survival and replication. Human immunodeficiency virus-1 (HIV-1) is one of the most dreadful viruses which has rapidly spread throughout the world and caused high mortality due to its high evolution rate. Here, we review the various mechanisms adopted by HIV-1 to exploit the cellular proteasomal machinery in order to escape the host restriction factors and components of host immune system for supporting its own multiplication, and successfully created an infection.

Nuclear translocation as a novel target for anti-HIV drugs

During recent years, remarkable progress has been achieved in the treatment of patients infected with HIV. This progress involves not only the improvement of previously known drugs but also the introduction of new classes of anti-HIV agents. Currently, drugs targeting virus entry, reverse transcription, integration and maturation are either in clinical use or in the late stages of clinical development. Nonetheless, the high mutation rate of the virus and toxicity of the drugs, which become problematic during prolonged treatment regimens characteristic of anti-HIV therapy, drive the necessity to produce new drugs that will allow physicians to keep the virus at bay in patients on lifelong anti-HIV therapy. Ideally, such drugs would target a new step in the HIV life cycle, thus avoiding crossresistance with older compounds. One such new target for anti-HIV therapy is nuclear translocation--a process critical for HIV replication. In this article, the authors will review recent literature on the mechanisms of HIV nuclear import and will describe compounds that inhibit this step of HIV replication.

Effect on HIV-1 gene expression, Tat-Vpr interaction and cell apoptosis by natural variants of HIV-1 Tat exon 1 and Vpr from Northern India

BACKGROUND

Since HIV-1 Tat and Vpr genes are involved in promoter transactivation, apoptosis, etc, we carried out studies to find out nature and extent of natural variation in the two genes from seropositive patients from Northern India and determined their functional implications.

METHODS

HIV-1 tat exon 1 and vpr were amplified from the genomic DNA isolated from the blood of HIV-1 infected individuals using specific primers by Polymerase Chain reaction (PCR) and subjected to extensive genetic analysis (CLUSTAL W, Simplot etc). Their expression was monitored by generating myc fusion clones. Tat exon 1 and Vpr variants were co-transfected with the reporter gene construct (LTR-luc) and their transactivation potential was monitored by measuring luciferase activity. Apoptosis and cell cycle analysis was done by Propidium Iodide (PI) staining followed by FACS.

RESULTS

Exon 1 of tat was amplified from 21 samples and vpr was amplified from 16 samples. One of the Tat exon 1 variants showed phylogenetic relatedness to subtype B & C and turned out to be a unique recombinant. Two of the Vpr variants were B/C/D recombinants. These natural variations were found to have no impact on the stability of Tat and Vpr. These variants differed in their ability to transactivate B LTR and C LTR promoters. B/C recombinant Tat showed better co-operative interaction with Vpr. B/C/D recombination in Vpr was found to have no effect on its co-operativity with Tat. Recombinant Tat (B/C) induced more apoptosis than wild type B and C Tat. The B/C/D recombination in Vpr did not affect its G2 arrest induction potential but reduced its apoptosis induction ability.

CONCLUSIONS

Extensive sequence and region-specific variations were observed in Tat and Vpr genes from HIV-1 infected individuals from Northern India. These variations have functional implications & therefore important for the pathogenicity of virus.

The HIV-1 protein Vpr targets the endoribonuclease Dicer for proteasomal degradation to boost macrophage infection

The HIV-1 protein Vpr enhances macrophage infection, triggers G2 cell cycle arrest, and targets cells for NK-cell killing. Vpr acts through the CRL4(DCAF1) ubiquitin ligase complex to cause G2 arrest and trigger expression of NK ligands. Corresponding ubiquitination targets have not been identified. UNG2 and SMUG1 are the only known substrates for Vpr-directed depletion through CRL4(DCAF1). Here we identify the endoribonuclease Dicer as a target of HIV-1 Vpr-directed proteasomal degradation through CRL4(DCAF1). We show that HIV-1 Vpr inhibits short hairpin RNA function as expected upon reduction of Dicer levels. Dicer inhibits HIV-1 replication in T cells. We demonstrate that Dicer also restricts HIV-1 replication in human monocyte-derived macrophages (MDM) and that reducing Dicer expression in MDMs enhances HIV-1 infection in a Vpr-dependent manner. Our results support a model in which Vpr complexes with human Dicer to boost its interaction with the CRL4(DCAF1) ubiquitin ligase complex and its subsequent degradation.

Epstein-Barr virus protein kinase BGLF4 targets the nucleus through interaction with nucleoporins

BGLF4 of Epstein-Barr virus (EBV) encodes a serine/threonine protein kinase that phosphorylates multiple viral and cellular substrates to optimize the cellular environment for viral DNA replication and the nuclear egress of viral nucleocapsids. BGLF4 is expressed predominantly in the nucleus at early and late stages of virus replication, while a small portion of BGLF4 is distributed in the cytoplasm at the late stage of virus replication and packaged into the virion. Here, we analyzed systematically the functional domains crucial for nuclear localization of BGLF4 and found that both the N and C termini play important modulating roles. Analysis of amino acid substitution mutants revealed that the C terminus of BGLF4 does not contain a conventional nuclear localization signal (NLS). Additionally, deletion of the C-terminal putative helical regions at amino acids 386 to 393 and 410 to 419 diminished the nuclear translocation of BGLF4, indicating that the secondary structure of the C terminus is important for the localization of BGLF4. The green fluorescent protein-fused wild-type or C-terminal helical regions of BGLF4 associate with phenylalanine/glycine repeat-containing nucleoporins (Nups) in nuclear envelope fractionation. Both coimmunoprecipitation and in vitro pull-down assays further demonstrated that BGLF4 binds to Nup62 and Nup153. Remarkably, nuclear import assay with permeabilized HeLa cells demonstrated that BGLF4 translocated into nucleus independent of cytosolic factors. Data presented here suggest that BGLF4 employs a novel mechanism through direct interactions with nucleoporins for its nuclear targeting.

Exposed hydrophobic residues in human immunodeficiency virus type 1 Vpr helix-1 are important for cell cycle arrest and cell death

The human immunodeficiency virus type 1 (HIV-1) accessory protein viral protein R (Vpr) is a major determinant for virus-induced G2/M cell cycle arrest and cytopathicity. Vpr is thought to perform these functions through the interaction with partner proteins. The NMR structure of Vpr revealed solvent exposed hydrophobic amino acids along helices 1 and 3 of Vpr, which could be putative protein binding domains. We previously showed that the hydrophobic patch along helix-3 was important for G2/M blockade and cytopathicity. Mutations of the exposed hydrophobic residues along helix-1 were found to reduce Vpr-induced cell cycle arrest and cell death as well. The levels of toxicity during virion delivery of Vpr correlated with G2/M arrest. Thus, the exposed hydrophobic amino acids in the amino-terminal helix-1 are important for the cell cycle arrest and cytopathicity functions of Vpr.

HIV protein sequence hotspots for crosstalk with host hub proteins

HIV proteins target host hub proteins for transient binding interactions. The presence of viral proteins in the infected cell results in out-competition of host proteins in their interaction with hub proteins, drastically affecting cell physiology. Functional genomics and interactome datasets can be used to quantify the sequence hotspots on the HIV proteome mediating interactions with host hub proteins. In this study, we used the HIV and human interactome databases to identify HIV targeted host hub proteins and their host binding partners (H2). We developed a high throughput computational procedure utilizing motif discovery algorithms on sets of protein sequences, including sequences of HIV and H2 proteins. We identified as HIV sequence hotspots those linear motifs that are highly conserved on HIV sequences and at the same time have a statistically enriched presence on the sequences of H2 proteins. The HIV protein motifs discovered in this study are expressed by subsets of H2 host proteins potentially outcompeted by HIV proteins. A large subset of these motifs is involved in cleavage, nuclear localization, phosphorylation, and transcription factor binding events. Many such motifs are clustered on an HIV sequence in the form of hotspots. The sequential positions of these hotspots are consistent with the curated literature on phenotype altering residue mutations, as well as with existing binding site data. The hotspot map produced in this study is the first global portrayal of HIV motifs involved in altering the host protein network at highly connected hub nodes.

HIV-1 accessory protein Vpr: relevance in the pathogenesis of HIV and potential for therapeutic intervention

The HIV protein, Vpr, is a multifunctional accessory protein critical for efficient viral infection of target CD4⁺ T cells and macrophages. Vpr is incorporated into virions and functions to transport the preintegration complex into the nucleus where the process of viral integration into the host genome is completed. This action is particularly important in macrophages, which as a result of their terminal differentiation and non-proliferative status, would be otherwise more refractory to HIV infection. Vpr has several other critical functions including activation of HIV-1 LTR transcription, cell-cycle arrest due to DCAF-1 binding, and both direct and indirect contributions to T-cell dysfunction. The interactions of Vpr with molecular pathways in the context of macrophages, on the other hand, support accumulation of a persistent reservoir of HIV infection in cells of the myeloid lineage. The role of Vpr in the virus life cycle, as well as its effects on immune cells, appears to play an important role in the immune pathogenesis of AIDS and the development of HIV induced end-organ disease. In view of the pivotal functions of Vpr in virus infection, replication, and persistence of infection, this protein represents an attractive target for therapeutic intervention.

HIV-1 viral protein r: from structure to function

The viral protein r (Vpr) of HIV-1 binds several host proteins leading to pleiotropic functions, such as G2/M cell cycle arrest, apoptosis induction and gene transactivation. Vpr is encapsidated through the Gag C-terminus into the nascent viral particles, suggesting that Vpr plays several important functions in the early stages of the viral lifecycle. In this regard, Vpr interacts with nucleic acids and membranes to facilitate the preintegration complex migration and incorporation into the nucleus of nondividing cells. Thus, Vpr has to recruit several host and viral factors to promote its functions during HIV-1 pathogenesis. This article focuses on its interacting partners by giving an overview of the functional outcome of the different Vpr complexes, as well as the structural determinants of Vpr required for its binding properties.

Strategies to inhibit viral protein nuclear import: HIV-1 as a target

Nuclear import is a critical step in the life cycle of HIV-1. During the early (preintegration) stages of infection, HIV-1 has to transport its preintegration complex into the nucleus for integration into the host cell chromatin, while at the later (postintegration) stages viral regulatory proteins Tat and Rev need to get into the nucleus to stimulate transcription and regulate splicing and nuclear export of subgenomic and genomic RNAs. Given such important role of nuclear import in HIV-1 life cycle, this step presents an attractive target for antiviral therapeutic intervention. In this review, we describe the current state of our understanding of the interactions regulating nuclear import of the HIV-1 preintegration complex and describe current approaches to inhibit it. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Importin alpha3 interacts with HIV-1 integrase and contributes to HIV-1 nuclear import and replication

HIV-1 employs the cellular nuclear import machinery to actively transport its preintegration complex (PIC) into the nucleus for integration of the viral DNA. Several viral karyophilic proteins and cellular import factors have been suggested to contribute to HIV-1 PIC nuclear import and replication. However, how HIV interacts with different cellular machineries to ensure efficient nuclear import of its preintegration complex in dividing and nondividing cells is still not fully understood. In this study, we have investigated different importin alpha (Impalpha) family members for their impacts on HIV-1 replication, and we demonstrate that short hairpin RNA (shRNA)-mediated Impalpha3 knockdown (KD) significantly impaired HIV infection in HeLa cells, CD4(+) C8166 T cells, and primary macrophages. Moreover, quantitative real-time PCR analysis revealed that Impalpha3-KD resulted in significantly reduced levels of viral 2-long-terminal repeat (2-LTR) circles but had no effect on HIV reverse transcription. All of these data indicate an important role for Impalpha3 in HIV nuclear import. In an attempt to understand how Impalpha3 participates in HIV nuclear import and replication, we first demonstrated that the HIV-1 karyophilic protein integrase (IN) was able to interact with Impalpha3 both in a 293T cell expression system and in HIV-infected CD4(+) C8166 T cells. Deletion analysis suggested that a region (amino acids [aa] 250 to 270) in the C-terminal domain of IN is involved in this viral-cellular protein interaction. Overall, this study demonstrates for the first time that Impalpha3 is an HIV integrase-interacting cofactor that is required for efficient HIV-1 nuclear import and replication in both dividing and nondividing cells.

The functions of the HIV1 protein Vpr and its action through the DCAF1.DDB1.Cullin4 ubiquitin ligase

Among the proteins encoded by human and simian immunodeficiency viruses (HIV and SIV) at least three, Vif, Vpu and Vpr, subvert cellular ubiquitin ligases to block the action of anti-viral defenses. This review focuses on Vpr and its HIV2/SIV counterparts, Vpx and Vpr, which all engage the DDB1.Cullin4 ubiquitin ligase complex through the DCAF1 adaptor protein. Here, we discuss the multiple functions that have been linked to Vpr expression and summarize the current knowledge on the role of the ubiquitin ligase complex in carrying out a subset of these activities.

Lentiviral nuclear import: a complex interplay between virus and host

Although the capacity to infect non-dividing cells is a hallmark of lentiviruses, nuclear import is still barely understood. More than 100 research papers have been dedicated to this topic during the last 15 years, yet, more questions have been raised than answers. The signal-facilitating translocation of the viral preintegration complex (PIC) through the nuclear pore complex (NPC) remains unknown. It is clear, however, that nuclear import is the result of a complex interplay between viral and cellular components. In this review, we discuss the current knowledge on nuclear import. We focus on the controversies and pitfalls and discuss the interplay between virus and host.

HIV-1 matrix protein: a mysterious regulator of the viral life cycle

Significant progress has been achieved in the last few years concerning the human immunodeficiency virus (HIV-1) life cycle, mostly in the fields of cellular receptors for the virus, virus assembly and budding of virus particles from the cell surface. Meanwhile, some aspects, such as postentry events, virus maturation and the regulatory role of individual viral proteins remain poorly defined. This review summarizes some recent findings concerning the role of Gag Pr55 and its proteolytic processing in the HIV-1 life cycle with particular emphasis on the functions of matrix protein p17 (MA), the protein which plays a key role in regulation of the early and late steps of viral morphogenesis. Based on our recent observations, the possibility is discussed that two subsets of MA exist, one cleaved from the Gag precursor in the host cell (cMA), and the other cleaved in the virions (vMA). It is suggested that two MA fractions possess diverse functions and are involved in different stages of virus morphogenesis as key regulators of the viral life cycle.

Activation of JNK-dependent pathway is required for HIV viral protein R-induced apoptosis in human monocytic cells: involvement of antiapoptotic BCL2 and c-IAP1 genes

Human immunodeficiency virus (HIV) accessory protein viral protein R (Vpr) plays a key role in virus replication and induces cell cycle arrest and apoptosis in various cell types including T cells and neuronal and tumor cells following infection with Vpr-expressing HIV isolates or exposure to the extracellular Vpr protein. The C-terminal Vpr peptide encompassing amino acids 52-96 (Vpr-(52-96)) is required for exerting the apoptotic effects, whereas the N-terminal Vpr-(1-45) peptide is responsible for virus transcription. We demonstrate that Vpr-(52-96) induced apoptosis in human promonocytic THP-1 cells and primary monocytes through the mitochondrial pathway in a caspase-dependent manner. To understand the regulation of Vpr-induced apoptosis, we investigated the signaling pathways, particularly the MAPKs, and the transcription factors involved. Although both Vpr-(52-96) and Vpr-(1-45) peptides induced phosphorylation of all the three members of the MAPKs, Vpr-(52-96)-activated JNK selectively induced apoptosis in monocytic cells through the mitochondrial pathway as determined by using JNK inhibitors SP60025, dexamethasone, curcumin, and JNK-specific small interfering RNAs. Furthermore Vpr-(52-96)-induced apoptosis was mediated by inhibition of downstream antiapoptotic Bcl2 and c-IAP1 genes whose expression could be restored following pretreatment with JNK-specific inhibitors. Overall the results suggest that Vpr-(52-96)-activated JNK plays a key role in inducing apoptosis through the down-regulation of antiapoptotic Bcl2 and c-IAP1 genes.

Equine infectious anemia virus Gag p9 function in early steps of virus infection and provirus production

We have previously reported that serial truncation of the Gag p9 protein of equine infectious anemia virus (EIAV) revealed a progressive loss in replication phenotypes in transfected cells, such that a proviral mutant (E32) expressing the N-terminal 31 amino acids of p9 produced infectious virus particles similarly to parental provirus, while a proviral mutant (K30) with two fewer amino acids produced replication-defective virus particles, despite containing apparently normal levels of processed Gag and Pol proteins (C. Chen, F. Li, and R. C. Montelaro, J. Virol. 75:9762-9760, 2001). Based on these observations, we sought in the current study to identify the precise defect in K30 virion infection of permissive equine dermal (ED) cells. The results of these experiments clearly demonstrated that K30 virions entered target ED cells and produced early (minus-strand strong-stop) and late (Gag) viral DNA products as efficiently as did the replication-competent E32 mutant and parental EIAV(UK) viruses. However, in contrast to the replication-competent E32 mutant and parental viruses, infection with K30 mutant virus failed to produce detectable two-long-terminal-repeat DNA circles, stable integrated provirus, virus-specific Gag mRNA expression, or intracellular viral protein expression. Taken together, these data demonstrate that the K30 mutant is defective in the ability to produce sufficient nuclear viral DNA to establish a productive infection in ED cells. Thus, these observations indicate for the first time that the EIAV Gag p9 protein performs a critical role in viral DNA production and processing to provirus during EIAV infection, in addition to its previously defined role in viral budding mediated by the p9 L domain.

Importin-alpha promotes passage through the nuclear pore complex of human immunodeficiency virus type 1 Vpr

Viral protein R (Vpr) of human immunodeficiency virus type 1 has potent karyophilic properties, but details of the mechanism by which it enters the nucleus remain to be clarified. We reported previously that two regions, located between residues 17 and 34 (alphaH1) and between residues 46 and 74 (alphaH2), are indispensable for the nuclear localization of Vpr. Here, we reveal that a chimeric protein composed of the nuclear localization signal of Vpr, glutathione S-transferase, and green fluorescent protein was localized at the nuclear envelope and then entered the nucleus upon addition of importin-alpha. An in vitro transport assay using a series of derivatives of importin-alpha demonstrated that the carboxyl terminus was required for this nuclear import process. We also showed that Vpr interacts with importin-alpha through alphaH1 and alphaH2; only the interaction via alphaH1 is indispensable for the nuclear entry of Vpr. These observations indicate that importin-alpha functions as a mediator for the nuclear entry of Vpr.

Nuclear localization of Vpr is crucial for the efficient replication of HIV-1 in primary CD4+ T cells

The human immunodeficiency virus type 1 (HIV-1) accessory protein Vpr appears to make a substantial contribution to the replication of HIV-1 in established T cell lines when HIV-1 is present at very low multiplicities of infection. However, the role of Vpr in viral replication in primary CD4+ T cells remains to be clarified. In this study, we generated a panel of viruses that encoded mutant forms of Vpr that lacked either the ability to accumulate in the nucleus and induce G2 arrest or the ability to induce apoptosis, which has been shown to occur independently of G2 arrest of the cell cycle. We demonstrate here that the nuclear localization of Vpr and consequent G2 arrest but not the induction of apoptosis by Vpr are important for viral replication in primary CD4+ T cells at both high and low multiplicities of infection. Viruses that encoded mutant forms of Vpr that failed to be imported into the nucleus in the presence of cytoplasmic extracts from primary CD4+ T cells in an in vitro nuclear import assay replicated at drastically reduced rates. Thus, Vpr might be a key regulator of the viral nuclear import process during infection in primary CD4+ T cells. By contrast, a mutant form of Vpr that exhibited diffuse cytosolic staining exclusively in an immunofluorescence assay of HeLa cells and was not imported into nucleus by the cytosol from HeLa cells was effectively imported into the nucleus by cytosol from primary CD4+ T cells. This Vpr mutant virus replicated well in primary CD4+ T cells, indicating that cellular factors in primary CD4+ T cells are indispensable for the accumulation of Vpr in the nucleus and, thus, for viral replication. Our results suggest that the nuclear import of Vpr might be a good target in efforts to block the early stages of replication of HIV-1.

Heat-shock protein 70 exerts opposing effects on Vpr-dependent and Vpr-independent HIV-1 replication in macrophages

HIV-1 viral protein R (Vpr) shuttles between the nucleus and the cytoplasm and is believed to contribute to the process of nuclear translocation of the viral preintegration complex, thus facilitating HIV-1 replication in macrophages. In this report, we demonstrate that Hsp70, a heat-shock protein contributing to cellular stress responses, inhibits nuclear translocation of HIV-1 Vpr. In macrophages, Hsp70 is induced shortly after HIV-1 infection. Recombinant Hsp70 or a mild heat shock diminished replication of the wild-type HIV-1, suggesting that Hsp70 might function as an innate antiviral factor. Surprisingly, Hsp70 stimulated nuclear import and replication in macrophages of the Vpr-deficient HIV-1 construct. This finding suggests that Hsp70 and Vpr may function in a similar manner when expressed separately, but they neutralize each other's activity when present together. Consistent with this interpretation, Hsp70 coprecipitated with Vpr from HIV-1–infected cells.

Effect of HIV-1 Vpr on cell cycle regulators

Cell cycle is one of the most complex processes in the life of a dividing cell. It involves numerous regulatory proteins, which direct the cell through a specific sequence of events for the production of two daughter cells. Cyclin-dependent kinases (cdks), which complex with the cyclin proteins, are the main players in the cell cycle. They can regulate the progression of the cells through different stages regulated by several proteins including p53, p21(WAF1), p19, p16, and cdc25. Downstream targets of cyclin-cdk complexes include pRB and E2F. A cell cycle can be altered to the advantage of many viral agents, most notably polyomaviruses, papillomaviruses, adenoviruses, and retroviruses. In addition, viral protein R (Vpr) is a protein encoded by the human immunodeficiency virus type 1 (HIV-1). HIV-1, the causative agent of acquired immunodeficiency syndrome (AIDS), is a member of the lentivirus class of retroviruses. This accessory protein plays an important role in the regulation of the cell cycle by causing G(2) arrest and affecting cell cycle regulators. Vpr prevents infected cells from proliferating, and collaborates with the matrix protein (MA) to enable HIV-1 to enter the nucleus of nondividing cells. Studies from different labs including ours showed that Vpr affects the functions of cell cycle proteins, including p53 and p21(WAF1). Thus, the replication of HIV-1, and ultimately its pathogenesis, are intrinsically tied to cell-cycle control.

Assessment of the role of the central DNA flap in human immunodeficiency virus type 1 replication by using a single-cycle replication system

In this study, reverse transcriptase (RT)- and integrase (IN)-defective human immunodeficiency virus type 1 (HIV-1) was transcomplemented with Vpr-RT-IN fusion proteins to delineate pol sequences important for HIV-1 replication. Our results reveal that a 194-bp sequence encompassing the 3'end of the IN gene and containing the central DNA flap is necessary and sufficient for efficient HIV-1 single-cycle replication in dividing and nondividing cells. Furthermore, we show that the central DNA flap enhances HIV-1 single-round replication by five- to sevenfold, primarily by facilitating nuclear import of proviral DNA. In agreement with previous reports, our data support a functional role of the central DNA flap during the early stages of HIV-1 infection.

Partner Molecules of Accessory Protein Vpr of the Human Immunodeficiency Virus Type 1

Vpr (Viral protein-R) of the Human Immunodeficiency Virus type-1 is a 14-kDa virion-associated protein, conserved in HIV-1, -2 and the Simian Immunodeficiency Virus (SIV). Vpr is incorporated into the virion, travels to the nucleus, and has multiple activities including promoter activation, cell cycle arrest at the G2/M transition and apoptosis induction. Through these activities, Vpr is thought to influence not only viral replication but also numerous host cell functions. These functions may be categorized in three groups depending on the domains of Vpr that support them: (1) functions mediated by the amino terminal portion of Vpr, like virion packaging; (2) functions mediated by the carboxyl terminal portion such as cell cycle arrest; and (3) functions that depend on central alpha-helical structures such as transcriptional activation, apoptosis and subcellular shuttling. Association of these activities to specific regions of the Vpr molecule appears to correlate to the host/viral molecules that interact with corresponding portion of Vpr. They include Gag, host transcription factors/coactivators such as SP1, the glucocorticoid receptor, p300/CREB-binding protein and TFIIB, apoptotic adenine nucleotide translocator, cyclophilin A and 14-3-3 proteins. The properties of Vpr molecule has made it difficult to assess its function and determine the true cellular interactors. Further studies on Vpr function are needed to fully assess the function of this important early regulatory molecule of HIV and other lentiviruses.

HIV-1 Vpr: Genetic Diversity and Functional Features from the Perspective of Structure

RNA viruses are well known for the enormous genetic variation. Retroviruses share this feature with other RNA viruses, and human immunodeficiency virus type 1 (HIV-1) has been extensively investigated in this regard. Based on the DNA sequence analysis, HIV-1 has been classified into three groups; M, N, and O, with viral subtypes in each group. While the genetic variation between viral isolates has been documented throughout the genome, specifically, the env gene exhibits high variation. Analysis of the env gene from the sequential samples from HIV-1-infected patients reveals variation in the range of 1% per year. The variation observed in individual HIV-1 genes in the form of changes at the nucleotide level, as expected, should result in one of the possible scenarios: (1) no change in the amino acid, (2) conservative change in the amino acid, (3) nonconservative change in the amino acid, and (4) premature stop codon resulting in a truncated protein. Hence, it is likely that the variation may impact on the function of the protein, depending on the nature of the mutation. The goal of this review is to summarize the polymorphisms in Vpr using the available sequence information and discuss their effects on the functions of Vpr from the point of view of its structure. The data generated by several groups provide a base for understanding the consequences of natural polymorphisms in specific regions of the Vpr molecule. However, it is also clear that secondary changes (second site or compensatory mutations) may modify the effect of a specific mutation and a comprehensive analysis is needed to delineate the role of specific residues in Vpr molecule. This is an area which, we hope, will attract investigators for further studies, and may provide information for understanding the molecular basis of Vpr functions.

A hard way to the nucleus

As a member of the Retrovirus family, human immunodeficiency virus (HIV), a causative agent of AIDS, replicates by integrating its genome into the host cell's nuclear DNA. However, in contrast to most retroviruses that depend on mitotic dissolution of the nuclear envelope to gain access to the host cell's genome, the HIV pre-integration complex can enter the nucleus of the target cell during the interphase. Such capacity greatly enhances HIV replication and allows the virus to productively infect terminally differentiated nonproliferating cells, such as macrophages. Infection of macrophages is a critical factor in the pathogenesis of diseases caused by HIV-1 and other lentiviruses. The mechanisms responsible for this unusual feature of HIV have enticed researchers since the early 90s, when the first characterization of the HIV-1 pre-integration complex was reported. Several viral factors, including matrix protein, integrase, viral protein R, and central DNA flap, have been proposed as regulators of HIV-1 nuclear import, only to be later shown as nonessential for this process. As a result, after more than a decade of intense research, there is still no consensus on which HIV-1 and cellular proteins control this critical step in HIV-1 replication. In this review, we will discuss recent advances and suggest possible solutions to the controversial issue of HIV-1 nuclear import.

Nuclear export of Vpr is required for efficient replication of human immunodeficiency virus type 1 in tissue macrophages

Retroviruses must gain access to the host cell nucleus for subsequent replication and viral propagation. Human immunodeficiency virus type 1 (HIV-1) and other primate lentiviruses are distinguished from the gammaretroviruses by their ability to infect nondividing cells such as macrophages, an important viral reservoir in vivo. Rather than requiring nuclear membrane breakdown during cell division, the HIV-1 preintegration complex (PIC) enters the nucleus by traversing the central aqueous channel of the limiting nuclear pore complex. The HIV-1 PIC contains three nucleophilic proteins, matrix, integrase, and Vpr, all of which have been implicated in nuclear targeting. The mechanism by which Vpr can display such nucleophilic properties and yet also be available for incorporation into virions assembling at the plasma membrane is unresolved. We recently characterized Vpr as a nucleocytoplasmic shuttling protein that contains two novel nuclear import signals and an exportin-1-dependent nuclear export signal (NES). We now demonstrate that mutation of this NES impairs the incorporation of Vpr into newly formed virions. Furthermore, we find that the Vpr NES is required for efficient HIV replication in tissue macrophages present in human spleens and tonsils. These findings underscore how the nucleocytoplasmic shuttling of Vpr not only contributes to nuclear import of the HIV-1 PIC but also enables Vpr to be present in the cytoplasm for incorporation into virions, leading to enhancement of viral spread within nondividing tissue macrophages.

The cationic amphipathic alpha-helix of HIV-1 viral protein R (Vpr) binds to nucleic acids, permeabilizes membranes, and efficiently transfects cells

Viral protein R (Vpr) is a small protein of 96 amino acids that is conserved among the lentiviruses human immunodeficiency virus type 1 (HIV-1), HIV-2, and simian immunodeficiency virus. We recently sought to determine whether the karyophilic properties of Vpr, as well as its ability to bind nucleic acids, could be used to deliver DNA into cells. We have found that the C-terminal domain of Vpr-(52-96) is able to efficiently transfect various cell lines. Here, we show that the shortest active sequence for gene transfer corresponds to the domain that adopts a alpha-helix conformation. DNA binding studies and permeabilization assays performed on cells demonstrated that the peptides that are efficient in transfection condense plasmid DNA and are membranolytic. Electron microscopy studies and transfection experiments performed in the presence of inhibitors of the endocytic processes indicated that the major entry pathway of Vpr-DNA complexes is through endocytosis. Taken together, the results show that the cationic C-terminal alpha-helix of Vpr has DNA-condensing as well as membrane-destabilizing capabilities, both properties that are indispensable for efficient DNA transfection.

NMR structure of the HIV-1 regulatory protein VPR

The human immunodeficiency virus type 1 (HIV-1) genome encodes a highly conserved regulatory gene product, Vpr (96 residues, 14kDa), which is incorporated into virions. In the infected cells, Vpr, expressed late in the virus cycle, is believed to function in the early phases of HIV-1 replication, such as nuclear migration of pre-integration complex, transcription of the proviral genome, viral multiplication by blocking cells in G2 phase and regulation of apoptosis phenomenon. Vpr has a critical role in long term AIDS disease by inducing infection in non-dividing cells such as monocytes and macrophages. To gain insight into the structure-function relationships of Vpr, the (1-96)Vpr protein was synthesized with 22 labeled amino acids. Its 3D structure was analyzed in the presence of CD(3)CN and in pure water at low pH and refined by restrained simulated annealing. The structure of the protein is characterized by three well-defined alpha-helices: 17-33, 38-50 and 56-77 surrounded by flexible N and C-terminal domains. In contrast to the structure obtained in the presence of TFE, the three alpha-helices are folded around a hydrophobic core constituted of Leu, Ile, Val and aromatic residues as illustrated by numerous long range NOEs. This structure accounts for the interaction of Vpr with different targets.

Characterization of pol, vif, vpr, and vpu genes of HIV type 1 in AIDS patients with high viral load and stable CD4+ T cell counts on combination therapy

The success of combination therapy has also led to AIDS patients who exhibit elevated viral load without a corresponding decline in CD4+ T cells. In this study, we characterized changes in the pol gene and accessory genes vif, vpr, and vpu of HIV-1 isolated from the plasma of patients receiving highly active antiretroviral therapy. From each patient three sequences were obtained and compared with the sequence of HIV-1 from nontreated patients, revealing many substitutions that were similar in most cases. Protease and reverse transcriptase genes showed many mutations that were due to antiviral drugs. Premature termination was observed in the vif gene of one patient, leading to a protein truncated after 187 amino acids. In another patient the entire vpr open reading frame was missing, with no synthesis of Vpu protein because the 5' end of the gene was missing, including the start codon. In the same patient, the Vif protein was also truncated because of the deletion of 100 nucleotides at the 3' end of the vif gene.

NMR structure of the HIV-1 regulatory protein Vpr in H2O/trifluoroethanol. Comparison with the Vpr N-terminal (1-51) and C-terminal (52-96) domains

The human immunodeficiency virus type 1, HIV-1, genome encodes a highly conserved regulatory gene product, Vpr (96 amino acids), which is incorporated into virions in quantities equivalent to those of the viral Gag protein. In infected cells, Vpr is believed to function during the early stages of HIV-1 replication (such as transcription of the proviral genome and migration of preintegration nuclear complex), blocks cells in G2 phase and triggers apoptosis. Vpr also plays a critical role in long-term AIDS disease by inducing viral infection in nondividing cells such as monocytes and macrophages. To gain deeper insight of the structure-function relationship of Vpr, the intact protein (residues 1-96) was synthesized. Its three-dimensional structure was analysed using circular dichroism and two-dimensional 1H- and 15N-NMR and refined by restrained molecular dynamics. In addition, 15N relaxation parameters (T1, T2) and heteronuclear 1H-15N NOEs were measured. The structure of the protein is characterized by a well-defined gamma turn(14-16)-alpha helix(17-33)-turn(34-36), followed by a alpha helix(40-48)-loop(49-54)-alpha helix(55-83) domain and ends with a very flexible C-terminal sequence. This structural determination of the whole intact Vpr molecule provide insights into the biological role played by this protein during the virus life cycle, as such amphipathic helices are believed to be involved in protein-lipid bilayers, protein-protein and/or protein-nucleic acid interactions.

Intracellular trafficking of retroviral genomes during the early phase of infection: viral exploitation of cellular pathways

Retroviruses enter cells through specific cell-surface receptors and then embark on a journey that ultimately leads to the establishment of the integrated proviral DNA. The steps of the journey include the reverse transcription of the viral RNA into DNA, the trafficking of the viral protein-DNA complex through the cytoplasm, the entry of the complex into the nucleus, and the insertion of the linear viral DNA into the host genome. All these steps are likely to involve specific interactions of viral proteins with host machinery. Our knowledge of the details of these interactions is very limited but is rapidly expanding, and should provide a deeper understanding of the pathways and components used by the different classes of retroviruses. This knowledge in turn should enable the development of better and more efficient retroviral vectors for use in gene therapy protocols in vivo.

Vpr is preferentially targeted by CTL during HIV-1 infection

The HIV-1 accessory proteins Vpr, Vpu, and Vif are essential for viral replication, and their cytoplasmic production suggests that they should be processed for recognition by CTLs. However, the extent to which these proteins are targeted in natural infection, as well as precise CTL epitopes within them, remains to be defined. In this study, CTL responses against HIV-1 Vpr, Vpu, and Vif were analyzed in 60 HIV-1-infected individuals and 10 HIV-1-negative controls using overlapping peptides spanning the entire proteins. Peptide-specific IFN-gamma production was measured by ELISPOT assay and flow-based intracellular cytokine quantification. HLA class I restriction and cytotoxic activity were confirmed after isolation of peptide-specific CD8(+) T cell lines. CD8(+) T cell responses against Vpr, Vpu, and Vif were found in 45%, 2%, and 33% of HIV-1-infected individuals, respectively. Multiple CTL epitopes were identified in functionally important regions of HIV-1 Vpr and Vif. Moreover, in infected individuals in whom the breadth of HIV-1-specific responses was assessed comprehensively, Vpr and p17 were the most preferentially targeted proteins per unit length by CD8(+) T cells. These data indicate that despite the small size of these proteins Vif and Vpr are frequently targeted by CTL in natural HIV-1 infection and contribute importantly to the total HIV-1-specific CD8(+) T cell responses. These findings will be important in evaluating the specificity and breadth of immune responses during acute and chronic infection, and in the design and testing of candidate HIV vaccines.

Characterization of a late entry event in the replication cycle of human immunodeficiency virus type 2

Certain human cell lines and primary macrophage cultures are restricted to infection by some primary isolates of human immunodeficiency virus type 2 (HIV-2), although early steps of the viral life cycle such as fusion at the plasma membrane and reverse transcription are fully supported. The late postintegration events, transcription, translation, assembly, budding, and maturation into infectious virions are functional in restrictive cells. Apart from primary macrophages, the restrictive cell types are actively dividing, and nuclear import of preintegration complexes (PICs) is not required for infection. We therefore postulate that the PICs are trapped in a cellular compartment, preventing subsequent steps in the replication cycle that lead to integration of the provirus. To test this we showed that HIV-2 particles pseudotyped with vesicular stomatitis virus envelope G protein, which delivers HIV into an endocytic compartment, could overcome the block to infection. We suggest that delivery of the viral core into an appropriate cellular compartment is a critical step during the entry process of HIV.

Analysis of apoptosis induced by HIV-1 Vpr and examination of the possible role of the hHR23A protein

The HIV-1 Vpr protein induces apoptosis of cells, the mechanism of which is unknown. To clarify how this function may be related to other Vpr functions, we simultaneously assessed the effects of multiple point mutations upon various Vpr properties. Our data suggest that induction of arrest by Vpr may be unnecessary for induction of apoptosis. This is exemplified by a C-terminal mutant, R80A, that does not arrest cells, yet induces low but significant levels of apoptosis. We also show that mutation of Vpr at both of its nuclear localization sequences (within its alpha-helices and the overlapping leucine zipper-like domain) does not affect induction of either apoptosis or cell cycle arrest. This indicates that neither sequence is essential for these two functions of Vpr. It further suggests that multimerization of Vpr, which maps to residues 60 and 67 within the leucine-rich region, is unnecessary for initiation of apoptosis and arrest. We previously found that the Vpr-binding protein, hHR23A, can partially alleviate induction of arrest. We now show that overexpression of hHR23A itself causes apoptosis of cells. Mutation of its C-terminal UBA( 2 ) domain that is responsible for binding Vpr disrupts the apoptotic effect. This suggests that Vpr may induce apoptosis through a pathway involving hHR23A.

Nucleocytoplasmic shuttling by human immunodeficiency virus type 1 Vpr

Human immunodeficiency virus type 1 (HIV-1) is capable of infecting nondividing cells such as macrophages because the viral preintegration complex is able to actively traverse the limiting nuclear pore due to the redundant and possibly overlapping nuclear import signals present in Vpr, matrix, and integrase. We have previously recognized the presence of at least two distinct and novel nuclear import signals residing within Vpr that, unlike matrix and integrase, bypass the classical importin alpha/beta-dependent signals and do not require energy or a RanGTP gradient. We now report that the carboxy-terminal region of Vpr (amino acids 73 to 96) contains a bipartite nuclear localization signal (NLS) composed of multiple arginine residues. Surprisingly, when the leucine-rich Vpr(1-71) fragment, previously shown to harbor an NLS, or full-length Vpr is fused to the C terminus of a green fluorescent protein-pyruvate kinase (GFP-PK) chimera, the resultant protein is almost exclusively detected in the cytoplasm. However, the addition of leptomycin B (LMB), a potent inhibitor of CRM1-dependent nuclear export, produces a shift from a cytoplasmic localization to a nuclear pattern, suggesting that these Vpr fusion proteins shuttle into and out of the nucleus. Studies of nuclear import with GFP-PK-Vpr fusion proteins in the presence of LMB reveals that both of the leucine-rich alpha-helices are required for effective nuclear uptake and thus define a unique NLS. Using a modified heterokaryon analysis, we have localized the Vpr nuclear export signal to the second leucine-rich helix, overlapping a portion of the amino-terminal nuclear import signal. These studies thus define HIV-1 Vpr as a nucleocytoplasmic shuttling protein.

Solution structure of human immunodeficiency virus type 1 Vpr(13-33) peptide in micelles

Human immunodeficiency virus type 1 protein R (HIV-1 Vpr) promotes nuclear entry of viral nucleic acids in nondividing cells, causes G2 cell cycle arrest and is involved in cellular differentiation and cell death. Also, Vpr subcellular localization is as variable as its functions. It is known that, consistent with its role in nuclear transport, Vpr localizes to the nuclear envelope of human cells. Further, a reported ion channel activity of Vpr obviously is dependent on its localization in or at membranes. We focused our structural studies on the secondary structure of a peptide consisting of residues 13-33 of HIV-1 Vpr in micelles. Employing nuclear magnetic resonance and circular dichroism spectroscopy we found this part of Vpr, known to be essential for nuclear localization, to be almost completely alpha helical. Our results provide structural data suggesting residues 13-33 of Vpr to form an amphipathic, leucine-zipper-like alpha helix that serves as a basis for interactions with a variety of viral and cellular factors.

Functional role of residues corresponding to helical domain II (amino acids 35 to 46) of human immunodeficiency virus type 1 Vpr

Vpr, encoded by the human immunodeficiency virus type 1 genome, contains 96 amino acids and is a multifunctional protein with features which include cell cycle arrest at G(2), nuclear localization, participation in transport of the preintegration complex, cation channel activity, oligomerization, and interaction with cellular proteins, in addition to its incorporation into the virus particles. Recently, structural studies based on nuclear magnetic resonance and circular dichroism spectroscopy showed that Vpr contains a helix (HI)-turn-helix (HII) core at the amino terminus and an amphipathic helix (HIII) in the middle region. Though the importance of helical domains HI and HIII has been defined with respect to Vpr functions, the role of helical domain HII is not known. To address this issue, we constructed a series of mutants in which the HII domain was altered by deletion, insertion, and/or substitution mutagenesis. To enable the detection of Vpr, the sequence corresponding to the Flag epitope (DYKDDDDK) was added, in frame, to the Vpr coding sequences. Mutants, expressed through the in vitro transcription/translation system and in cells, showed an altered migration corresponding to deletions in Vpr. Substitution mutational analysis of residues in HII showed reduced stability for VprW38S-FL, VprL42G-FL, and VprH45W-FL. An assay involving cotransfection of NLDeltaVpr proviral DNA and a Vpr expression plasmid was employed to analyze the virion incorporation property of Vpr. Mutant Vpr containing deletions and specific substitutions (VprW38S-FL, VprL39G-FL, VprL42G-FL, VprG43P-FL, and VprI46G-FL) exhibited a negative virion incorporation phenotype. Further, mutant Vpr-FL containing deletions also failed to associate with wild-type Vpr, indicating a possible defect in the oligomerization feature of Vpr. Subcellular localization studies indicated that mutants VprDelta35-50-H-FL, VprR36W-FL, VprL39G-FL, and VprI46G-FL exhibited both cytoplasmic and nuclear localization, unlike other mutants and control Vpr-FL. While wild-type Vpr registered cell cycle arrest at G(2), mutant Vpr showed an intermediary effect with the exception of VprDelta35-50 and VprDelta35-50-H. These results suggest that residues in the HII domain are essential for Vpr functions.

Functional and structural characterization of synthetic HIV-1 Vpr that transduces cells, localizes to the nucleus, and induces G2 cell cycle arrest

Human immunodeficiency virus (HIV) Vpr contributes to nuclear import of the viral pre-integration complex and induces G(2) cell cycle arrest. We describe the production of synthetic Vpr that permitted the first studies on the structure and folding of the full-length protein. Vpr is unstructured at neutral pH, whereas under acidic conditions or upon addition of trifluorethanol it adopts alpha-helical structures. Vpr forms dimers in aqueous trifluorethanol, whereas oligomers exist in pure water. (1)H NMR spectroscopy allows the signal assignment of N- and C-terminal amino acid residues; however, the central section of the molecule is obscured by self-association. These findings suggest that the in vivo folding of Vpr may require structure-stabilizing interacting factors such as previously described interacting cellular and viral proteins or nucleic acids. In biological studies we found that Vpr is efficiently taken up from the extracellular medium by cells in a process that occurs independent of other HIV-1 proteins and appears to be independent of cellular receptors. Following cellular uptake, Vpr is efficiently imported into the nucleus of transduced cells. Extracellular addition of Vpr induces G(2) cell cycle arrest in dividing cells. Together, these findings raise the possibility that circulating forms of Vpr observed in HIV-infected patients may exert biological effects on a broad range of host target cells.

Dual role of the HIV-1 vpr protein in the modulation of the apoptotic response of T cells

We investigated the effect of vpr, physiologically expressed during the course of an acute HIV-1 infection, on the response of infected cells to apoptotic stimuli as well as on the HIV-induced apoptosis. At 48 h after infection, Jurkat cells exhibited a lower susceptibility to undergo apoptosis with respect to uninfected cells. This effect was not observed following infection with either a vpr-mutated virus or a wild-type strain in the presence of antisense oligodeoxynucleotides targeted at vpr mRNA. Single-cell analysis, aimed at simultaneously identifying apoptotic and infected cells, revealed that resistance to apoptosis correlated with productive infection. Notably, vpr-dependent protection from induced apoptosis was also observed in HIV-1-infected PBMC. In contrast, at later stages of infection, a marked increase in the number of cells spontaneously undergoing apoptosis was detected in infected cultures. This virus-induced apoptosis involved vpr expression and predominantly occurred in productively infected cells. These results indicate that HIV-1 vpr can exert opposite roles in the regulation of apoptosis, which may depend on the level of its intracellular expression at different stages of HIV-1 infection. The dual function of vpr represents a novel mechanism in the complex strategy evolved by HIV to influence the turnover of T lymphocytes leading to either viral persistence or virus release and spreading.

Two putative alpha-helical domains of human immunodeficiency virus type 1 Vpr mediate nuclear localization by at least two mechanisms

To identify the domains of Vpr that are involved nuclear localization, we transfected HeLa cells with a panel of expression vectors that encode mutant Vpr protein with deletions or substitutions within putative domains. Immunofluorescence staining of transfected cells revealed that wild-type Vpr was localized predominantly in the nucleus and the nuclear envelope and certainly in the cytoplasm. Introduction of substitutions or deletions within alphaH1 or alphaH2 resulted, by contrast, in diffuse expression over the entire cell. In addition, double mutations within both of these alpha-helical domains led to the complete absence of Vpr from nuclei. Next, we prepared HeLa cells that express chimeric proteins which consist of the alphaH1 and alphaH2 domains fused individually with green fluorescent protein (GFP) and a Flag tag and extracted them with digitonin and Triton X-100 prior to fixation. Flag-alphaH1-GFP was detected in the nucleus but not in the cytoplasm, while Flag-alphaH2-GFP was retained predominantly in the nucleus and in a small amount in the cytoplasm. The immunostaining patterns were almost eliminated by substitutions in each chimeric protein. Thus, it appeared that the two alpha-helical domains might be involved in nuclear import by binding to certain cellular factors. Taken together, our data suggest that the two putative alpha-helical domains mediate the nuclear localization of Vpr by at least two mechanisms.

Two nuclear localization signals in the HIV-1 matrix protein regulate nuclear import of the HIV-1 pre-integration complex

Replication of HIV-1 in non-dividing and slowly proliferating cell populations depends on active import of the viral pre-integration complex (PIC) into the cell nucleus. While it is commonly accepted that this process is mediated by an interaction between the HIV-1 PIC and the cellular nuclear import machinery, controversial results have been reported concerning the mechanisms of this interaction. Here, we demonstrate that a recently identified nuclear localization signal within the HIV-1 matrix protein (MA), MA NLS-2, together with previously described MA NLS-1, mediates nuclear import of the HIV-1 PIC. Inactivation of both MA NLSs precluded nuclear translocation of MA and rendered the virus defective in nuclear import and replication in non-dividing macrophage cultures, even when functional Vpr and integrase (IN), two more viral proteins implicated in HIV-1 nuclear import, were present. Taken together, these results indicate that Vpr does not function as an independent nuclear import factor and demonstrate that HIV-1 MA, by virtue of its two nuclear localization signals, regulates HIV-1 nuclear import.

Gene transfer by lentiviral vectors is limited by nuclear translocation and rescued by HIV-1 pol sequences

Gene-transfer vectors based on lentiviruses are distinguished by their ability to transduce non-dividing cells. The HIV-1 proteins Matrix, Vpr and Integrase have been implicated in the nuclear import of the viral genome in non-dividing cells. Here we show that a sequence within pol is also required in cis. It contains structural elements previously associated with the progress of reverse transcription in target cells. We restored these elements in cis within late-generation lentiviral vectors. The new vector transduced to a much higher efficiency several types of human primary cells, when both growing and growth-arrested, including haematopoietic stem cells assayed by long-term repopulation of NOD/SCID mice. On in vivo administration into SCID mice, the vector induced higher plasma levels of human clotting factor IX (F.IX) than non-modified vector. Our results indicate that nuclear translocation of the genome is a rate-limiting step in lentiviral infection of both dividing and non-dividing cells, and that it depends on protein and nucleic acid sequence determinants. Full rescue of this step in lentivirus-based vectors improves performance for gene-therapy applications.

Interactions of the C-terminus of viral protein R with nucleic acids are modulated by its N-terminus

The basic viral protein R (Vpr) performs several functions during the human immunodeficiency virus HIV-1 retroviral cycle, including G2 mitosis arrest and nuclear import of the preintegration complex allowing lentivirus to replicate in nondividing cells. Accordingly, this protein was found in the nucleus of infected cells. In the virus, Vpr is incorporated through interaction with both nucleocapsid protein 7 (NCp7) and p6, two small proteins encoded by the C-terminal part of the Gag precursor. NCp7 is also involved in genomic RNA encapsidation during the budding process suggesting a possible interaction of Vpr with nucleic acids, either directly or via the NCp7 intermediate. Gel shift experiments were carried out with RNA and DNA using synthetic Vpr and peptide derivatives. The results show that Vpr binds to nucleic-acid inducing aggregates. This process, which requires the C-terminal basic domain of the protein (in particular the helical 70-80 domain), is regulated by the N-terminal region of Vpr. Moreover, NCp7 was shown to enhance RNA recognition by Vpr, a feature that could be required for Vpr encapsidation and during nuclear import of the preintegration complex.

Pathogenesis of HIV-1 infection within bone marrow cells

Mononuclear phagocytic cells and CD4+ T lymphocytes represent the major targets for infection by HIV-1 in vivo. The most severe pathogenic features associated with HIV-1 infection can be attributed to malfunction or premature death of these cells that are of hematopoietic origin. Patients with acquired immunodeficiency syndrome (AIDS), suffer from many hematologic disorders, particularly those persons with long-term infection of HIV-1. These disorders include anemia, lymphocytopenia, thrombocytopenia and neutropenia. The mechanisms that lead to the induction of these disorders are multi-factorial. However, sufficient evidence has accumulated which suggests that HIV-1 infection of cells within the microenvironment of the bone marrow can lead to the induction of hematopoietic deficits. Most studies indicate that marrow-derived hematopoietic stem cells cannot be infected by HIV-1 until they undergo modest differentiation in order to express the appropriate receptors to enable virus entry and subsequent replication. Some cells within the mixed environment of the marrow stroma appear to support HIV-1 replication however. These cells include marrow microvascular endothelial cells, sometimes referred to as blanket cells, stromal fibroblasts, as well as mononuclear phagocytes. Our recent experiments suggest that the HIV-1 accessory protein, Vpr, plays some role in the activation of marrow-derived mononuclear phagocytes which appears to result in premature phagocytosis of non-adherent marrow cells present in the in vitro cultures. This phenomenon could account, in part, for the induction of cytopenias that are typical of individuals infected by HIV-1.

Lentivirus replication and regulation

Lentiviruses are associated with chronic diseases of the hematological and neurological systems in animals and man. In particular, human immunodeficiency virus type 1 (HIV-1) is the etiological agent of the global AIDS epidemic. The genomes of lentiviruses are complex, encoding a number of regulatory and accessory proteins not found in other retroviruses. This complexity is reflected in their replication cycle, which reveals intricate regulatory pathways and unique mechanisms for viral persistence. In this review, we highlight some of these unique features for HIV-1, with particular focus on the transcriptional and posttranscriptional control of gene expression. Although our understanding of the biology of HIV-1 is far from complete, the knowledge gained thus far has already led to novel strategies for both virus intervention and exploiting the lentiviruses for therapeutic applications.

Epitope-tagging approach to determine the stoichiometry of the structural and nonstructural proteins in the virus particles: amount of Vpr in relation to Gag in HIV-1

We used an epitope-tagging approach to determine the ratio of Gag (structural) to Vpr (nonstructural) in the virus particles directed by human immunodeficiency virus type 1. For this purpose, chimeric Gag and Vpr expression plasmids were constructed with the Flag epitope (DYKDDDDK), and the sequences corresponding to the chimeric protein were introduced into human immunodeficiency virus type 1 proviral DNA (NL4-3) to determine the ratio in the virus particles when these proteins are expressed in cis. In addition, NL4-3 DNA was modified to disrupt Vpr synthesis to determine the extent of incorporation of Vpr-FL when it is expressed in trans through a heterologous promoter. The analysis of virus particles generated by transfection of proviral DNA into RD cells indicated that (1) the ratio of Gag to Vpr in virus particles, when Vpr-FL is expressed in cis (in the context of proviral DNA), is in the range of 150-200:1 (14-18 molecules of Vpr per virion) and (2) the expression of Vpr-FL in trans showed efficient incorporation with a Gag to Vpr ratio of 5-7:1 (392-550 molecules of Vpr). These results suggest that the presence of the same epitope on different viral proteins may provide an accurate comparison of these proteins in the virus particles.