NMR structure of the (1-51) N-terminal domain of the HIV-1 regulatory protein Vpr

Computational Insights into the Conformational Dynamics of HIV-1 Vpr in a Lipid Bilayer for Ion Channel Modeling

HIV-1 Vpr is a multifunctional accessory protein consisting of 96 amino acids that play a critical role in viral pathogenesis. Among its diverse range of activities, Vpr can create a cation-selective ion channel within the plasma membrane. However, the oligomeric state of this channel has not yet been elucidated. In this study, we investigated the conformational dynamics of Vpr helices to model the ion channel topology. First, we employed a series of multiscale simulations to investigate the specific structure of monomeric Vpr in a membrane model. During the lipid bilayer self-assembly coarse grain simulation, the C-terminal helix (residues 56-77) effectively formed the transmembrane region, while the N-terminal helix exhibited an amphipathic nature by associating horizontally with a single leaflet. All-atom molecular dynamics (MD) simulations of full-length Vpr inside a phospholipid bilayer show that the C-terminal helix remains very stable inside the bilayer core in a vertical orientation. Subsequently, using the predicted C-terminal helix orientation and conformation, various oligomeric states (ranging from tetramer to heptamer) possibly forming the Vpr ion channel were built and further evaluated. Among these models, the pentameric form exhibited consistent stability in MD simulations and displayed a compatible conformation for a water-assisted ion transport mechanism. This study provides structural insights into the ion channel activity of the Vpr protein and the foundation for developing therapeutics against HIV-1 Vpr-related conditions.

A Pilot Investigation of the Association Between Vpr Amino Acid Substitutions and Peripheral Immune Marker Levels in People With Human Immunodeficiency Virus: Implications for Neurocognitive Impairment

Background

Subtype-specific amino acid variations in viral proteins of human immunodeficiency virus type 1 (HIV-1) influence disease progression. Furthermore, Vpr sequence variation correlates with chronic inflammation, a central mechanism in HIV-1 (neuro)pathogenesis. Nevertheless, no clinical study has investigated the link between Vpr sequence variation and peripheral inflammation in people with HIV (PWH). The aim of this pilot study was to ascertain whether specific Vpr amino acid variants were associated with immune markers in PWH.

Methods

We included a unique cohort of 48 treatment-naive South African PWH to determine the association between blood-derived Vpr sequence variation and peripheral immune marker levels using Sanger sequencing and enzyme-linked immunosorbent assay analysis, respectively.

Results

Our findings indicate that among the many neuropathogenic Vpr amino acid variants and immune markers examined, after applying Bonferroni corrections (P = .05/3) and adjusting for sex and locality, soluble urokinase plasminogen activator receptor (suPAR) was nearing significance for higher levels in participants with the G41 amino acid variant compared to those with the S41 variant (P = .035). Furthermore, amino acid variations at position 41 (between G41 and S41) exhibited a significant association with suPAR (adjusted R2 = 0.089, β = .386 [95% confidence interval, .125-3.251]; P = .035).

Conclusions

These findings suggest that Vpr amino acid sequence variations might contribute to dysregulated inflammation, which could explain the observed association between specific Vpr variants and HIV-1 (neuro)pathogenesis found in prior research. These Vpr variants merit further investigation to fully understand their roles in HIV-1 pathogenesis and neuropathogenesis.

Evidence of Divergent Amino Acid Usage in Comparative Analyses of R5- and X4-Associated HIV-1 Vpr Sequences

Vpr is an HIV-1 accessory protein that plays numerous roles during viral replication, and some of which are cell type dependent. To test the hypothesis that HIV-1 tropism extends beyond the envelope into the vpr gene, studies were performed to identify the associations between coreceptor usage and Vpr variation in HIV-1-infected patients. Colinear HIV-1 Env-V3 and Vpr amino acid sequences were obtained from the LANL HIV-1 sequence database and from well-suppressed patients in the Drexel/Temple Medicine CNS AIDS Research and Eradication Study (CARES) Cohort. Genotypic classification of Env-V3 sequences as X4 (CXCR4-utilizing) or R5 (CCR5-utilizing) was used to group colinear Vpr sequences. To reveal the sequences associated with a specific coreceptor usage genotype, Vpr amino acid sequences were assessed for amino acid diversity and Jensen-Shannon divergence between the two groups. Five amino acid alphabets were used to comprehensively examine the impact of amino acid substitutions involving side chains with similar physiochemical properties. Positions 36, 37, 41, 89, and 96 of Vpr were characterized by statistically significant divergence across multiple alphabets when X4 and R5 sequence groups were compared. In addition, consensus amino acid switches were found at positions 37 and 41 in comparisons of the R5 and X4 sequence populations. These results suggest an evolutionary link between Vpr and gp120 in HIV-1-infected patients.

Defining the roles for Vpr in HIV-1-associated neuropathogenesis

It is increasingly evident that the human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) has a unique role in neuropathogenesis. Its ability to induce G2/M arrest coupled with its capacity to increase viral gene transcription gives it a unique role in sustaining viral replication and aiding in the establishment and maintenance of a systemic infection. The requirement of Vpr for HIV-1 infection and replication in cells of monocytic origin (a key lineage of cells involved in HIV-1 neuroinvasion) suggests an important role in establishing and sustaining infection in the central nervous system (CNS). Contributions of Vpr to neuropathogenesis can be expanded further through (i) naturally occurring HIV-1 sequence variation that results in functionally divergent Vpr variants; (ii) the dual activities of Vpr as a intracellular protein delivered and expressed during HIV-1 infection and as an extracellular protein that can act on neighboring, uninfected cells; (iii) cell type-dependent consequences of Vpr expression and exposure, including cell cycle arrest, metabolic dysregulation, and cytotoxicity; and (iv) the effects of Vpr on exosome-based intercellular communication in the CNS. Revealing that the effects of this pleiotropic viral protein is an essential part of a greater understanding of HIV-1-associated pathogenesis and potential approaches to treating and preventing disease caused by HIV-1 infection.

Expression and purification of soluble HIV-2 viral protein R (Vpr) using a sandwich-fusion protein strategy

Viral accessory proteins of the human immunodeficiency virus (HIV), including virus protein R (Vpr), are crucial for the efficient replication of the virus in the host organism. While functional data are available for HIV-1 Vpr, there is a paucity of data describing the function and structure of HIV-2 Vpr. In this report, the construction of a His6-MBP-intein1-Vpr-intein2-Cyt b5-His6 fusion protein is presented. Unlike previous research efforts where only microgram quantities of HIV-1 Vpr could be produced, this construct enabled soluble milligram yields via an Escherichia coli over-expression system. Straightforward protein purification of HIV-2 Vpr was achieved by standard chromatography routines and autocatalytic intein cleavage. Preliminary structural studies by circular dichroism (CD) and NMR spectroscopy revealed that the protein is stable in the presence of micellar concentrations of the detergent DPC and adopts an α-helix secondary structure.

Viral factors in non-progression

Research has undergone considerable development in understanding a small subset of human immunodeficiency virus type 1 (HIV-1)-infected, therapy-naive individuals who maintain a favorable course of infection surviving for longer periods of time. Although, viral, host genetic, and immunological factors have been analyzed in many previous studies in order to delineate mechanisms that contribute to non-progressive HIV disease, there appears to be a no clear cut winner and the non-progressive HIV disease in <1% of HIV-infected individuals appears to be a complex interplay between viral and host factors. Therefore, it is important to review them separately to signify their potential contribution to non-progressive HIV disease. With respect to virological features, genomic sequencing of HIV-1 strains derived from long-term non-progressors has shown that some individuals are infected with attenuated strains of HIV-1 and harbor mutations from single nucleotide polymorphisms to large deletions in HIV-1 structure, regulatory, and accessory genes. The elucidation of functional attributes of defective/attenuated HIV strains may provide better understanding of viral pathogenesis and the discovery of new therapeutic strategies against HIV. This review mainly focuses on the defects in viral genes that possibly guide non-progressive HIV disease.

His-Trp cation-π interaction and its structural role in an α-helical dimer of HIV-1 Vpr protein

Vpr is a multifunctional accessory protein of HIV-1 virus and was previously proposed to assume an antiparallel helical dimer with the third helices HIII of different subunits facing each other. In this study, we have examined the structure and stability of the antiparallel dimer by using a fragment peptide, Vpr52-80, spanning the HIII region. The present analyses of fluorescence, circular dichroism, and UV absorption spectra have shown that a cation-π interaction takes place between protonated His71 and Trp54 located near the opposite ends of the two antiparallel helices. The cation-π interaction induces a small elongation of the HIII helix, an increase in thermal stability of the helical dimer, and a modification of the helix arrangement to produce a more compact form. The His71-Trp54 cation-π interaction may be utilized in stabilizing and tuning the dimeric structure of Vpr to achieve proper interactions with other proteins.

Delineating HIV-associated neurocognitive disorders using transgenic models: the neuropathogenic actions of Vpr

HIV-associated neurocognitive disorders (HAND) represent a constellation of neurological disabilities defined by neuropsychological impairments, neurobehavioral abnormalities and motor deficits. To gain insights into the mechanisms underlying the development of these disabilities, several transgenic models have been developed over the past two decades, which have provided important information regarding the cellular and molecular factors contributing to the neuropathogenesis of HAND. Herein, we concentrate on the neuropathogenic effects of HIV-1 Vpr expressed under the control of c-fms, resulting transgene expression in myeloid cells in both the central and peripheral nervous systems. Vpr's actions, possibly through its impact on cell cycle machinery, in brain culminate in neuronal and astrocyte injury and death through apoptosis involving activation of caspases-3, -6 and -9 depending on the individual target cell type. Indeed, these outcomes are also induced by soluble Vpr implying Vpr's effects stem from direct interaction with target cells. Remarkably, in vivo transgenic Vpr expression induces a neurodegenerative phenotype defined by neurobehavioral deficits and neuronal loss in the absence of frank inflammation. Implantation of another viral protein, hepatitis C virus (HCV) core, into Vpr transgenic animals' brains stimulated neuroinflammation and amplified the neurodegenerative disease phenotype, thereby recapitulating HCV's putative neuropathogenic actions. The availability of different transgenic models to study HIV neuropathogenesis represents exciting and innovative approaches to understanding disease mechanisms and perhaps developing new therapeutic strategies in the future.

Vpr-host interactions during HIV-1 viral life cycle

Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) is a multifunctional viral protein that plays important role at multiple stages of the HIV-1 viral life cycle. Although the molecular mechanisms underlying these activities are subject of ongoing investigations, overall, these activities have been linked to promotion of viral replication and impairment of anti-HIV immunity. Importantly, functional defects of Vpr have been correlated with slow disease progression of HIV-infected patients. Vpr is required for efficient viral replication in non-dividing cells such as macrophages, and it promotes, to some extent, viral replication in proliferating CD4+ T cells. The specific activities of Vpr include modulation of fidelity of viral reverse transcription, nuclear import of the HIV-1 pre-integration complex, transactivation of the HIV-1 LTR promoter, induction of cell cycle G2 arrest and cell death via apoptosis. In this review, we focus on description of the cellular proteins that specifically interact with Vpr and discuss their significance with regard to the known Vpr activities at each step of the viral life cycle in proliferating and non-proliferating cells.

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.

HIV-1 Vpr: mechanisms of G2 arrest and apoptosis

Since the first isolation of HIV-1 from a patient with generalized lymphadenopathy in 1983, great progress has been made in understanding the viral life cycle and the functional nuances of each of the nine genes encoded by HIV-1. Considerable attention has been paid to four small HIV-1 open reading frames, vif, vpr, vpu and nef. These genes were originally termed "accessory" because their deletion failed to completely disable viral replication in vitro. More than twenty years after the cloning and sequencing of HIV-1, a great deal of information is available regarding the multiple functions of the accessory proteins and it is well accepted that, collectively, these gene products modulate the host cell biology to favor viral replication, and that they are largely responsible for the pathogenesis of HIV-1. Expression of Vpr, in particular, leads to cell cycle arrest in G(2), followed by apoptosis. Here we summarize our current understanding of Vpr biology with a focus on Vpr-induced G(2) arrest and apoptosis.

Molecular mimicry in inducing DNA damage between HIV-1 Vpr and the anticancer agent, cisplatin

The human immunodeficiency virus type 1 (HIV-1) viral protein R (vpr) gene is an evolutionarily conserved gene among the primate lentiviruses. Several functions are attributed to Vpr including the ability to cause cell death, cell cycle arrest, apoptosis and DNA damage. The Vpr domain responsible for DNA damage as well as the mechanism(s) through which Vpr induces this damage is unknown. Using site-directed mutagenesis, we identified the helical domain II within Vpr (aa 37-50) as the region responsible for causing DNA damage. Interestingly, Vpr Delta(37-50) failed to cause cell cycle arrest or apoptosis, to induce Ku70 or Ku80 and to suppress tumor growth, but maintained its capability to activate the HIV-1 LTR, to localize to the nucleus and to promote nonhomologous end-joining. In addition, our cytogenetic data indicated that helical domain II induced chromosomal aberrations, which mimicked those induced by cisplatin, an anticancer agent. This novel molecular mimicry function of Vpr might lead to its potential therapeutic use as a tumor suppressor.

Proline 35 of human immunodeficiency virus type 1 (HIV-1) Vpr regulates the integrity of the N-terminal helix and the incorporation of Vpr into virus particles and supports the replication of R5-tropic HIV-1 in human lymphoid tissue ex vivo

Mutational analysis of the four conserved proline residues in human immunodeficiency virus type 1 (HIV-1) Vpr reveals that only Pro-35 is required for efficient replication of R5-tropic, but not of X4-tropic, viruses in human lymphoid tissue (HLT) cultivated ex vivo. While Vpr-mediated apoptosis and G(2) cell cycle arrest, as well as the expression and subcellular localization of Vpr, were independent, the capacity for encapsidation of Vpr into budding virions was dependent on Pro-35. (1)H nuclear magnetic resonance data suggest that mutation of Pro-35 causes a conformational change in the hydrophobic core of the molecule, whose integrity is required for the encapsidation of Vpr, and thus, Pro-35 supports the replication of R5-tropic HIV-1 in HLT.

Inhibition of the activities of reverse transcriptase and integrase of human immunodeficiency virus type-1 by peptides derived from the homologous viral protein R (Vpr)

Shortly after infection by human immunodeficiency virus (HIV), two complexes are formed in a stepwise manner in the cytoplasm of infected cells: the reverse transcription complex that later becomes the preintegration complex. Both complexes include, in addition to cellular proteins, viral RNA or DNA and several proteins, such as reverse transcriptase (RT), integrase (IN), and viral protein R (Vpr). These proteins are positioned in close spatial proximity within these complexes, enabling mutual interactions between the proteins. Physical in vitro interactions between RT and IN that affect their enzymatic activities were already reported. Moreover, we found recently that HIV-1 RT-derived peptides bind and inhibit HIV-1 IN and that an IN-derived peptide binds and inhibits HIV-1 RT. Additionally, HIV-1 Vpr and its C-terminal domain affected in vitro the integration activity of HIV-1 IN. Here, we describe the associations of Vpr-derived peptides with RT and IN. Of a peptide library that spans the 96-residue-long Vpr protein, three partially overlapping peptides, derived from the C-terminal domain, bind both enzymes. Two of these peptides inhibit both RT and IN. Another peptide, derived from the Vpr N-terminal domain, binds IN and inhibits its activities, without binding and affecting RT. Interestingly, two sequential C-terminal peptides (derived from residues 57-71 and 61-75 of full-length Vpr) are the most effective inhibitors of both enzymes. The data and the molecular modeling presented suggest that RT and IN are inhibited as a result of steric hindrance or conformational changes of their active sites, whereas a second mechanism of blocking its dimerization state could be also attributed to the inhibition of IN.

Development and characterization of ten monoclonal anti-Vpr antibodies

HIV-1 Vpr is a 96-amino acid auxiliary protein that performs numerous activities during viral infection. In the present study, 10 antibodies were generated after mice immunization with either the N- or the C-terminus domain of Vpr, respectively, Vpr(1-51) and Vpr(52-96). ELISA and immunoblot experiments using pure synthetic overlapping Vpr peptides suggested that these anti-Vpr antibodies could be classified into five groups and that they recognized conformational or linear Vpr epitopes. Further analysis revealed the effect of C-terminal arginine mutations on the antibody binding. Two of the antibodies precipitated Vpr expressed after transfection of a Vpr-encoding vector in human cells. More importantly, one of them was able to detect Vpr in HIV-1-infected U1 cells and in HIV-1-infected human PBMC. Surface plasmon resonance experiments demonstrated that some of these antibodies prevented the interaction between Vpr and one of its cellular partners, the adenine nucleotide translocator. Thus, these anti-Vpr monoclonal antibodies may be useful to any laboratory working on the molecular mechanism of HIV-1 infection.

Interaction between the HIV-1 Protein Vpr and the Adenine Nucleotide Translocator

The HIV-1 protein Vpr circulates in the serum of seropositive individuals and in the cerebrospinal fluid of AIDS patients with neurological disorders. Vpr triggers apoptosis of numerous cell types after extracellular addition, vpr gene transfer or in the context of viral infection. Moreover, in vivo, transgenic mice over-expressing Vpr have enhanced T lymphocytes apoptosis. In previous studies, we suggested that the Vpr apoptotic activities were because of its binding to the adenine nucleotide translocator (ANT), a mitochondrial ATP/ADP antiporter. To specify this interaction, fragments of both proteins were synthesized and used in biochemical and biophysical experiments. We demonstrate here that in vitro, the (27-51) and (71-82) Vpr peptides bind to a region encompassing the first ANT intermembrane space loop and part of its second and third transmembrane helices. Computational analysis using a docking program associated to dynamic simulations enabled us to construct a three-dimensional model of the Vpr-ANT complex. In this model, the N-terminus of Vpr plunges in the ANT cavity whereas the Vpr C-terminal extremity is located at the surface of the ANT allowing possible interactions with a third partner. These results could be used to design molecules acting as pro-apoptotic Vpr analogs or as apoptosis inhibitors preventing the Vpr-ANT interaction.

Roles of HIV-1 auxiliary proteins in viral pathogenesis and host-pathogen interactions

Active host-pathogen interactions take place during infection of human immunodeficiency virus type 1 (HIV-1). Outcomes of these interactions determine the efficiency of viral infection and subsequent disease progression. HIV-infected cells respond to viral invasion with various defensive strategies such as innate, cellular and humoral immune antiviral mechanisms. On the other hand, the virus has also developed various offensive tactics to suppress these host cellular responses. Among many of the viral offensive strategies, HIV-1 viral auxiliary proteins (Tat, Rev, Nef, Vif, Vpr and Vpu) play important roles in the host-pathogen interaction and thus have significant impacts on the outcome of HIV infection. One of the best examples is the interaction of Vif with a host cytidine deaminase APOBEC3G. Although specific roles of other auxiliary proteins are not as well described as Vif-APOBEC3G interaction, it is the goal of this brief review to summarize some of the preliminary findings with the hope to stimulate further discussion and investigation in this exhilarating area of research.

Synthetic Vpr protein activates activator protein-1, c-Jun N-terminal kinase, and NF-kappaB and stimulates HIV-1 transcription in promonocytic cells and primary macrophages

The human immunodeficiency virus (HIV) Vpr protein plays a critical role in AIDS pathogenesis, especially by allowing viral replication within nondividing cells such as mononuclear phagocytes. Most of the data obtained so far have been in experiments with endogenous Vpr protein; therefore the effects of extracellular Vpr protein remain largely unknown. We used synthetic Vpr protein to activate nuclear transcription factors activator protein-1 (AP-1) and NF-kappaB in the promonocytic cell line U937 and in primary macrophages. Synthetic HIV-1 Vpr protein activated AP-1, c-Jun N-terminal kinase, and MKK7 in both U937 cells and primary macrophages. Synthetic Vpr activated NF-kappaB in primary macrophages and to a lesser extent in U937 cells. Because synthetic Vpr activated AP-1 and NF-kappaB, which bind to the HIV-1 long terminal repeat, we investigated the effect of synthetic Vpr on HIV-1 replication. We observed that synthetic Vpr stimulated HIV-1 long terminal repeat in U937 cells and enhanced viral replication in chronically infected U1 promonocytic cells. Similarly, synthetic Vpr stimulated HIV-1 replication in acutely infected primary macrophages. Activation of transcription factors and enhancement of viral replication in U937 cells and primary macrophages were mediated by both the N-terminal and the C-terminal moieties of synthetic Vpr. Therefore, our results suggest that extracellular Vpr could fuel the progression of AIDS via stimulation of HIV-1 provirus present in such cellular reservoirs as mononuclear phagocytes in HIV-infected patients.

The C-terminal domain of the HIV-1 regulatory protein Vpr adopts an antiparallel dimeric structure in solution via its leucine-zipper-like domain

HIV-1 Vpr is a highly conserved accessory protein that is involved in many functions of the virus life cycle. Vpr facilitates the entry of the HIV pre-integration complex through the nuclear pore, induces G2 cell cycle arrest, regulates cell apoptosis, increases transcription from the long terminal repeat and enhances viral replication. Vpr contains a Leu/Ile-rich domain (amino acids 60-81) in its C-terminal part, which is critical for dimerization. The sequence comprising residues 52-96 is implicated in properties of the protein such as DNA interaction and apoptosis via interaction with the adenine nucleotide translocator. To understand the specific interactions of Vpr-(52-96), the ability of this peptide to dimerize via a leucine-zipper mechanism has been investigated, by NMR and fluorescence spectroscopy. In contrast with results from a study performed in the presence of trifluoroethanol, our results, obtained in 30% (v/v) [2H]acetonitrile, show that Vpr-(52-96) in solution still forms an a-helix spanning residues 53-75, but dimerizes in an antiparallel orientation, through hydrophobic interactions between leucine and isoleucine residues and stacking between His71 and Trp54. Moreover, to demonstrate the physiological relevance of the dimer structure, fluorescence spectroscopy experiments have been performed in a Mes buffer, which confirmed the formation of the dimer in aqueous solution and highlighted the spatial proximity between Trp54 and His71. Surprisingly, the leucine-zipper structure shown in the present work for Vpr-(52-96) mimics the structure of full-length Vpr-(1-96), and this could explain why some of the properties of Vpr-(52-96) and Vpr-(1-96) are identical, while some are even enhanced for Vpr-(52-96), particularly in the case of DNA transfection experiments.

The Vpr protein from HIV-1: distinct roles along the viral life cycle

The genomes of human and simian immunodeficiency viruses (HIV and SIV) encode the gag, pol and env genes and contain at least six supplementary open reading frames termed tat, rev, nef, vif, vpr, vpx and vpu. While the tat and rev genes encode regulatory proteins absolutely required for virus replication, nef, vif, vpr, vpx and vpu encode for small proteins referred to "auxiliary" (or "accessory"), since their expression is usually dispensable for virus growth in many in vitro systems. However, these auxiliary proteins are essential for viral replication and pathogenesis in vivo. The two vpr- and vpx-related genes are found only in members of the HIV-2/SIVsm/SIVmac group, whereas primate lentiviruses from other lineages (HIV-1, SIVcpz, SIVagm, SIVmnd and SIVsyk) contain a single vpr gene. In this review, we will mainly focus on vpr from HIV-1 and discuss the most recent developments in our understanding of Vpr functions and its role during the virus replication cycle.

Analysis of HIV-1 Vpr determinants responsible for cell growth arrest in Saccharomyces cerevisiae

Background

The HIV-1 genome encodes a well-conserved accessory gene product, Vpr, that serves multiple functions in the retroviral life cycle, including the enhancement of viral replication in nondividing macrophages, the induction of G2 cell-cycle arrest, and the modulation of HIV-1-induced apoptosis. We previously reported the genetic selection of a panel of di-tryptophan (W)-containing peptides capable of interacting with HIV-1 Vpr and inhibiting its cytostatic activity in Saccharomyces cerevisiae (Yao, X.-J., J. Lemay, N. Rougeau, M. Clément, S. Kurtz, P. Belhumeur, and E. A. Cohen, J. Biol. Chem. v. 277, p. 48816–48826, 2002). In this study, we performed a mutagenic analysis of Vpr to identify sequence and/or structural determinants implicated in the interaction with di-W-containing peptides and assessed the effect of mutations on Vpr-induced cytostatic activity in S. cerevisiae.

Results

Our data clearly shows that integrity of N-terminal α-helix I (17–33) and α-helix III (53–83) is crucial for Vpr interaction with di-W-containing peptides as well as for the protein-induced cytostatic effect in budding yeast. Interestingly, several Vpr mutants, mainly in the N- and C-terminal domains, which were previously reported to be defective for cell-cycle arrest or apoptosis in human cells, still displayed a cytostatic activity in S. cerevisiae and remained sensitive to the inhibitory effect of di-W-containing peptides.

Conclusions

Vpr-induced growth arrest in budding yeast can be effectively inhibited by GST-fused di-W peptide through a specific interaction of di-W peptide with Vpr functional domain, which includes α-helix I (17–33) and α-helix III (53–83). Furthermore, the mechanism(s) underlying Vpr-induced cytostatic effect in budding yeast are likely to be distinct from those implicated in cell-cycle alteration and apoptosis in human cells.

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.

Structural characterization of the HIV-1 Vpr N terminus: evidence of cis/trans-proline isomerism

The 96-residue human immunodeficiency virus (HIV) accessory protein Vpr serves manifold functions in the retroviral life cycle including augmentation of viral replication in non-dividing host cells, induction of G2 cell cycle arrest, and modulation of HIV-induced apoptosis. Using a combination of dynamic light scattering, circular dichroism, and NMR spectroscopy the N terminus of Vpr is shown to be a unique domain of the molecule that behaves differently from the C-terminal domain in terms of self-association and secondary structure folding. Interestingly, the four highly conserved proline residues in the N terminus are predicted to have a high propensity for cis/trans isomerism. Thus the high resolution structure and folding of a synthetic N-terminal peptide (Vpr1-40) and smaller fragments thereof have been investigated. 1H NMR data indicate Vpr1-40 possesses helical structure between residues 17-32, and for the first time, this helix, which is bound by proline residues, was observed even in aqueous solution devoid of any detergent supplements. In addition, NMR data revealed that all of the proline residues undergo a cis/ trans isomerism to such an extent that approximately 40% of all Vpr molecules possess at least one proline in a cis conformation. This phenomenon of cis/trans isomerism, which is unprecedented for HIV-1 Vpr, not only provides an explanation for the molecular heterogeneity observed in the full-length molecule but also indicates that in vivo the folding and function of Vpr should depend on a cis/trans-proline isomerase activity, particularly as two of the proline residues in positions 14 and 35 show considerable amounts of cis isomers. This prediction correlates well with our recent observation (Zander, K., Sherman, M. P., Tessmer, U., Bruns, K., Wray, V., Prechtel, A. T., Schubert, E., Henklein, P., Luban, J., Neidleman, J., Greene, W. C., and Schubert, U. (2003) J. Biol. Chem. 278, 43170-43181) of a functional interaction between the major cellular isomerase cyclophilin A and Vpr, both of which are incorporated into HIV-1 virions.

Cyclophilin A interacts with HIV-1 Vpr and is required for its functional expression

Viral protein R (Vpr) of human immunodeficiency virus, type 1 (HIV-1) is the major virion-associated accessory protein that affects a number of biological functions in the retroviral life cycle, including promotion of the transport of the preintegration complex into the nucleus and the induction of G2 host cell cycle arrest. Our recent investigation of the conformational heterogeneity of the proline residues in the N terminus of Vpr suggested a functional interaction between Vpr and a host peptidylprolyl cis/trans isomerase (PPIase) that might regulate the cis/trans interconversion of the imidic bond within the conserved proline residues of Vpr in vivo. Using surface plasmon resonance spectroscopy, Far Western blot, and pulldown experiments a physical interaction of Vpr with the major host PPIase cyclophilin A (CypA) is now demonstrated. The interaction domain involves the N-terminal region of Vpr including an essential role for proline in position 35. The CypA inhibitor cyclosporin A and non-immunosuppressive PPIase inhibitors such as NIM811 and sanglifehrin A block expression of Vpr without affecting pre- or post-translational events such as transcription, intracellular transport, or virus incorporation of Vpr. Similarly to CypA inhibition, Vpr expression is also reduced in HIV-1 infected CypA-/- knock-out T cells. This study thus shows that in addition to the interaction between CypA and HIV-1 capsid occurring during early steps in virus replication, CypA is also important for the de novo synthesis of Vpr and that in the absence of CypA activity, the Vpr-mediated cell cycle arrest is completely lost in HIV-1-infected T cells.

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.

Antibody fragments selected by phage display against the nuclear localization signal of the HIV-1 Vpr protein inhibit nuclear import in permeabilized and intact cultured cells

The HIV-1 Vpr protein harbors a nuclear localization signal in its N-terminal domain. A peptide bearing this domain and which is designated VprN has been used as a target to screen a phage display single chain Fv (scFv) library. Here we report the isolation of anti-VprN scFv fragments from this library. The purified scFv fragments were able to bind the VprN peptide in an ELISA-based system and to inhibit VprN-mediated nuclear import in permeabilized as well as in intact microinjected cells. Furthermore, the anti-VprN scFv fragments recognized the full-length recombinant Vpr protein and inhibited its nuclear import. The same scFv fragments did not inhibit nuclear import mediated by the nuclear localization signal of the SV40 large T-antigen demonstrating a specific effect. The use of the described inhibitory anti-VprN scFv fragments to study nuclear import of viral karyophilic proteins and their therapeutic potential is discussed.

Docking of HIV-1 Vpr to the nuclear envelope is mediated by the interaction with the nucleoporin hCG1

The HIV-1 genome contains several genes coding for auxiliary proteins, including the small Vpr protein. Vpr affects the integrity of the nuclear envelope and participates in the nuclear translocation of the preintegration complex containing the viral DNA. Here, we show by photobleaching experiments performed on living cells expressing a Vpr-green fluorescent protein fusion that the protein shuttles between the nucleus and the cytoplasm, but a significant fraction is concentrated at the nuclear envelope, supporting the hypothesis that Vpr interacts with components of the nuclear pore complex. An interaction between HIV-1 Vpr and the human nucleoporin CG1 (hCG1) was revealed in the yeast two-hybrid system, and then confirmed both in vitro and in transfected cells. This interaction does not involve the FG repeat domain of hCG1 but rather the N-terminal region of the protein. Using a nuclear import assay based on digitonin-permeabilized cells, we demonstrate that hCG1 participates in the docking of Vpr at the nuclear envelope. This association of Vpr with a component of the nuclear pore complex may contribute to the disruption of the nuclear envelope and to the nuclear import of the viral DNA.

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.

HIV-1 Vpr displays natural protein-transducing properties: implications for viral pathogenesis

The 14-kDa Vpr protein of human immunodeficiency virus type 1 (HIV-1) serves multiple functions in the retroviral life cycle, including the enhancement of viral replication in nondividing macrophages, the induction of G2 cell-cycle arrest in proliferating T lymphocytes, and the modulation of HIV-1-induced apoptosis. Extracellular Vpr has been detected in the sera and cerebral spinal fluid of HIV-infected patients. However, it is not known whether such forms of Vpr are biologically active. Vpr contains a carboxy-terminal basic amino acid rich segment stretch that is homologous to domains that mediate the energy- and receptor-independent cellular uptake of polypeptides by a process termed protein transduction. Similar functional protein-transducing domains are present in HIV-1 Tat, herpes simplex virus-1 DNA-binding protein VP22, and the Drosophila antennapedia homeotic transcription factor. We now demonstrate effective transduction of biologically active, synthetic Vpr (sVpr) as well as the Vpr-beta-galactosidase fusion protein. However, in contrast to other transducing proteins, Vpr transduction is not enhanced by protein denaturation, and Vpr's carboxy-terminal basic domain alone is not sufficient for its transduction across biological membranes. In contrast, the full-length Vpr protein effectively transduces a broad array of cells, leading to dose-dependent G2 cell-cycle arrest and apoptosis. Addition of Vpr into the extracellular medium also rescues the replication of Vpr-deficient strains of HIV-1 in human macrophage cultures. Native Vpr may thus be optimized for protein transduction, a feature that might enhance and extend the pathological effects of HIV infection.

Functional conservation of HIV-1 Vpr and variability in a mother-child pair of long-term non-progressors

Increasing evidence suggests that HIV-1 Vpr is required in vivo for viral pathogenesis. Since Vpr displays multiple activities, little is known about which Vpr-specific activities are conserved in naturally occurring viruses or how natural mutations in Vpr might modulate viral pathogenesis in HIV-infected individuals. The goals of this study were to evaluate the functional variability of Vpr in naturally occurring viruses. The Vpr-specific activities of nuclear localization, induction of cell cycle G2 arrest and cell death were compared between viruses isolated from the fast progressing AIDS patients and a mother-child pair of long-term non-progressors (LTNPs). Wild-type Vpr activities were found in all of the viruses that were isolated from the fast progressing AIDS patients except for the truncated Vpr(IIIB) which lacked these activities. In contrast, defective Vpr were readily detected in viral populations isolated, over an 11-year period, from the mother-child pair. Sequence analyses indicated that these Vpr carried unique amino acid substitutions that frequently interrupted a highly conserved domain containing an N-terminal alpha-helix-turn-alpha-helix. Thus, Vpr activities are generally conserved in naturally occurring viruses. The functionally defective Vpr identified in the mother-child pair of LTNPs are likely to be unique and may possibly contribute to the slow disease progression.

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.

Structure of human immunodeficiency virus type 1 Vpr(34-51) peptide in micelle containing aqueous solution

Human immunodeficiency virus type 1 protein R (HIV-1 Vpr) promotes nuclear entry of viral nucleic acids in nondividing cells, causes G(2) cell cycle arrest and is involved in cellular differentiation and cell death. 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 is clearly dependent on its localization in or at membranes. We focused our structural studies on the secondary structure of a peptide consisting of residues 34-51 of HIV-1 Vpr. This part of Vpr plays an important role in Vpr oligomerization, contributes to cell cycle arrest activity, and is essential for virion incorporation and binding to HHR23A, a protein involved in DNA repair. Employing NMR spectroscopy we found this part of Vpr to be almost completely alpha helical in the presence of micelles, as well as in trifluoroethanol containing and methanol/chloroform solvent. Our results provide structural data suggesting residues 34-51 of Vpr to contain an amphipathic, leucine-zipper-like alpha helix, which serves as a basis for oligomerization of Vpr and its interactions with cellular and viral factors involved in subcellular localization and virion incorporation of Vpr.

A novel inducible expression system to study transdominant mutants of HIV-1 Vpr

In this study, an episomal system for ecdysone-inducible gene expression was developed. Human embryonic kidney 293 cells (293VE) expressing a heterodimer of modified ecdysone and retinoid X receptors and the Epstein-Barr nuclear antigen-1 were screened. Plasmids containing the EBV replication origin, oriP, and the ecdysone-response element could replicate and persist in 293VE cells to inducibly express luciferase or Vpr. The induction level, tested with luciferase reporter plasmid, varied among cell lines from 254- to 2056-fold. In one highly inducible cell line, HIV-1 Vpr was expressed well and caused G2 cell cycle arrest in the presence of the inducer, while in the absence of the inducer, no Vpr protein or cell cycle arrest could be detected. Using different selection markers, HIV-1 Vpr was coexpressed with Vpr mutants defective in phosphorylation at Ser79 and G2 cell cycle arrest activity. These Vpr mutants were transdominant to wild-type Vpr for G2 cell cycle arrest activity, but did not alter wild-type Vpr phosphorylation. It is likely that the transdominant mutants and wild-type Vpr compete for a downstream target(s) of G2 cell cycle arrest.

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 analysis of the simian immunodeficiency virus Vpx protein: identification of packaging determinants and a novel nuclear targeting domain

The vpx gene products of human immunodeficiency virus type 2 (HIV-2) and of the closely related simian immunodeficiency viruses from sooty mangabeys (SIVsm) and macaques (SIVmac) comprise a 112-amino-acid virion-associated protein that is critical for efficient virus replication in nondividing cells such as macrophages. When expressed in the absence of other viral proteins, Vpx localizes to the nuclear membrane as well as to the nucleus; however, in the context of virus replication Vpx is packaged into virions via interaction with the p6 domain of the Gag precursor polyprotein (p55(gag)). To identify the domains essential for virion incorporation and nuclear localization, site-directed mutations were introduced into the vpx gene of SIVsmPBj1.9 and functionally analyzed. Our results show that (i) mutation of two highly conserved L74 and I75 residues impaired both virion incorporation and nuclear localization of Vpx; (ii) substitution of conserved H82, G86, C87, P103, and P106 residues impaired Vpx nuclear localization but not virion incorporation; (iii) mutations of conserved Y66, Y69, and Y71 residues impaired virion incorporation but not the translocation of Vpx to the nucleus; and (iv) a mutation at E30 (predicted to disrupt an N-terminal alpha-helix) had no effect on either virion incorporation or nuclear localization of Vpx. Importantly, mutations in Vpx which impaired nuclear localization also reduced virus replication in macaque macrophages, suggesting an important role of the carboxyl terminus of Vpx in nuclear translocation of the viral preintegration complex. Analyzing this domain in greater detail, we identified a 26-amino-acid (aa 60 to 85) fragment that was sufficient to mediate the transport of a heterologous protein (green fluorescent protein [GFP]) to the nucleus. Taken together, these results indicate that virion incorporation and nuclear localization are encoded by two partially overlapping domains in the C-terminus of Vpx (aa 60 to 112). The identification of a novel 26-amino-acid nuclear targeting domain provides a new tool to investigate the nuclear import of the HIV-2/SIV preintegration complex.

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.

Human immunodeficiency virus type 1 Vpr-mediated G(2) cell cycle arrest: Vpr interferes with cell cycle signaling cascades by interacting with the B subunit of serine/threonine protein phosphatase 2A

The Vpr protein of primate lentiviruses arrests cell cycling at the G(2)/M phase through an inactivation of cyclin B-p34(cdc2) and its upstream regulator cdc25. We provide here biochemical and functional evidence demonstrating that human immunodeficiency virus type 1 (HIV-1) Vpr mediates G(2) arrest by forming a complex with protein phosphatase 2A (PP2A), an upstream regulator of cdc25. Vpr associates with PP2A through a specific interaction with the B55 regulatory subunit. This interaction is necessary but not sufficient for G(2) arrest. Interestingly, we found that Vpr association with B55-containing PP2A targets the enzymatic complex to the nucleus and, importantly, enhances the recruitment and dephosphorylation of the cdc25 substrate. Our data suggest that Vpr mediates G(2) arrest by enhancing the nuclear import of PP2A and by positively modulating its catalytic activity towards active phosphorylated nuclear cdc25.

Efficient DNA transfection mediated by the C-terminal domain of human immunodeficiency virus type 1 viral protein R

Viral protein R (Vpr) of human immunodeficiency virus type 1 is produced late in the virus life cycle and is assembled into the virion through binding to the Gag protein. It is known to play a significant role early in the viral life cycle by facilitating the nuclear import of the preintegration complex in nondividing cells. Vpr is also able to interact with nucleic acids, and we show here that it induces condensation of plasmid DNA. We have explored the possibility of using these properties in DNA transfection experiments. We report that the C-terminal half of the protein (Vpr(52-96)) mediates DNA transfection in a variety of human and nonhuman cell lines with efficiencies comparable to those of the best-known transfection agents. Compared with polylysine, a standard polycationic transfection reagent, Vpr(52-96) was 10- to 1,000-fold more active. Vpr(52-96)-DNA complexes were able to reach the cell nucleus through a pH-independent mechanism. These observations possibly identify an alternate pathway for DNA transfection.

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.

Transdominant activity of human immunodeficiency virus type 1 Vpr with a mutation at residue R73

The 96-amino-acid-long human immunodeficiency virus type 1 virion-encoded accessory protein Vpr is of particular interest, as this protein, which is found in association with viral particles, can exert a regulatory effect on both virus replication and host cell function. Evidently, Vpr, through interaction with several host regulatory proteins, can modulate transcription from the viral long terminal repeat promoter. Expression of Vpr in cells results in deregulation of cell proliferation during the cell cycle pathway at the G(2) stage. Vpr has unique structural features consisting of multiple functional domains. In this study, we have focused on the leucine/isoleucine-rich domain near the carboxyl terminus of Vpr at residue 73 (arginine) and have demonstrated that alterations at this residue result in ablation of transcriptional activity of Vpr and its ability to block cell cycle events at the G(2) stage. Interestingly, substitution mutations at R73 have resulted in a peptide with dominant negative activities on wild-type functions in transcription and host proliferation events. Results from in vitro and in vivo protein-protein interaction studies have revealed that functionally inactive mutant Vpr can be associated with wild-type protein, presumably through the N-terminal regions of the protein which have been shown to be important for Vpr oligomerization. Thus, it is likely that complexation of the mutant Vpr with wild-type protein functionally inactivates Vpr. The importance of these findings in light of the development of therapeutic strategies is discussed.