Human immunodeficiency virus type 1 Vpr: oligomerization is an essential feature for its incorporation into virus particles

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.

HIV-1 Gag Recruits Oligomeric Vpr via Two Binding Sites in p6, but Both Mature p6 and Vpr Are Rapidly Lost upon Target Cell Entry

The p12 region of murine leukemia virus (MLV) Gag and the p6 region of HIV-1 Gag contain late domains required for virus budding. Additionally, the accessory protein Vpr is recruited into HIV particles via p6. Mature p12 is essential for early viral replication events, but the role of mature p6 in early replication is unknown. Using a proviral vector in which the gag and pol reading frames are uncoupled, we have performed the first alanine-scanning mutagenesis screens across p6 to probe its importance for early HIV-1 replication and to further understand its interaction with Vpr. The infectivity of our mutants suggests that, unlike p12, p6 is not important for early viral replication. Consistent with this, we observed that p6 is rapidly lost upon target cell entry in time course immunoblot experiments. By analyzing Vpr incorporation into p6 mutant virions, we identified that the 15-FRFG-18 and 41-LXXLF-45 motifs previously identified as putative Vpr-binding sites are important for Vpr recruitment but that the 34-ELY-36 motif also suggested to be a Vpr-binding site is dispensable. Additionally, disrupting Vpr oligomerization together with removing either binding motif in p6 reduced Vpr incorporation ∼25- to 50-fold more than inhibiting Vpr oligomerization alone and ∼10- to 25-fold more than deleting each p6 motif alone, implying that multivalency/avidity is important for the interaction. Interestingly, using immunoblotting and immunofluorescence, we observed that most Vpr is lost concomitantly with p6 during infection but that a small fraction remains associated with the viral capsid for several hours. This has implications for the function of Vpr in early replication. IMPORTANCE The p12 protein of MLV and the p6 protein of HIV-1 are both supplementary Gag cleavage products that carry proline-rich motifs that facilitate virus budding. Importantly, p12 has also been found to be essential for early viral replication events. However, while Vpr, the only accessory protein packaged into HIV-1 virions, is recruited via the p6 region of Gag, the function of both mature p6 and Vpr in early replication is unclear. Here, we have systematically mutated the p6 region of Gag and have studied the effects on HIV infectivity and Vpr packaging. We have also investigated what happens to p6 and Vpr during early infection. We show that, unlike p12, mature p6 is not required for early replication and that most of the mature p6 and the Vpr that it recruits are lost rapidly upon target cell entry. This has implications for the role of Vpr in target cells.

Distinct MCM10 Proteasomal Degradation Profiles by Primate Lentiviruses Vpr Proteins

Viral protein R (Vpr) is an accessory protein found in various primate lentiviruses, including human immunodeficiency viruses type 1 and 2 (HIV-1 and HIV-2) as well as simian immunodeficiency viruses (SIVs). Vpr modulates many processes during viral lifecycle via interaction with several of cellular targets. Previous studies showed that HIV-1 Vpr strengthened degradation of Mini-chromosome Maintenance Protein10 (MCM10) by manipulating DCAF1-Cul4-E3 ligase in proteasome-dependent pathway. However, whether Vpr from other primate lentiviruses are also associated with MCM10 degradation and the ensuing impact remain unknown. Based on phylogenetic analyses, a panel of primate lentiviruses Vpr/x covering main virus lineages was prepared. Distinct MCM10 degradation profiles were mapped and HIV-1, SIVmus and SIVrcm Vprs induced MCM10 degradation in proteasome-dependent pathway. Colocalization and interaction between MCM10 with these Vprs were also observed. Moreover, MCM10 2-7 interaction region was identified as a determinant region susceptible to degradation. However, MCM10 degradation did not alleviate DNA damage response induced by these Vpr proteins. MCM10 degradation by HIV-1 Vpr proteins was correlated with G₂/M arrest, while induction of apoptosis and oligomerization formation of Vpr failed to alter MCM10 proteolysis. The current study demonstrated a distinct interplay pattern between primate lentiviruses Vpr proteins and MCM10.

Vpr and Its Cellular Interaction Partners: R We There Yet?

Vpr is a lentiviral accessory protein that is expressed late during the infection cycle and is packaged in significant quantities into virus particles through a specific interaction with the P6 domain of the viral Gag precursor. Characterization of the physiologically relevant function(s) of Vpr has been hampered by the fact that in many cell lines, deletion of Vpr does not significantly affect viral fitness. However, Vpr is critical for virus replication in primary macrophages and for viral pathogenesis in vivo. It is generally accepted that Vpr does not have a specific enzymatic activity but functions as a molecular adapter to modulate viral or cellular processes for the benefit of the virus. Indeed, many Vpr interacting factors have been described by now, and the goal of this review is to summarize our current knowledge of cellular proteins targeted by Vpr.

HIV-1 Integrase-Targeted Short Peptides Derived from a Viral Protein R Sequence

HIV-1 integrase (IN) inhibitors represent a new class of highly effective anti-AIDS therapeutics. Current FDA-approved IN strand transfer inhibitors (INSTIs) share a common mechanism of action that involves chelation of catalytic divalent metal ions. However, the emergence of IN mutants having reduced sensitivity to these inhibitors underlies efforts to derive agents that antagonize IN function by alternate mechanisms. Integrase along with the 96-residue multifunctional accessory protein, viral protein R (Vpr), are both components of the HIV-1 pre-integration complex (PIC). Coordinated interactions within the PIC are important for viral replication. Herein, we report a 7-mer peptide based on the shortened Vpr (69–75) sequence containing a biotin group and a photo-reactive benzoylphenylalanyl residue, and which exhibits low micromolar IN inhibitory potency. Photo-crosslinking experiments have indicated that the peptide directly binds IN. The peptide does not interfere with IN-DNA interactions or induce higher-order, aberrant IN multimerization, suggesting a mode of action for the peptide that is distinct from clinically used INSTIs and developmental allosteric IN inhibitors. This compact Vpr-derived peptide may serve as a valuable pharmacological tool to identify a potential new pharmacologic site.

Virion encapsidated HIV-1 Vpr induces NFAT to prime non-activated T cells for productive infection

The majority of T cells encountered by HIV-1 are non-activated and do not readily allow productive infection. HIV-1 Vpr is highly abundant in progeny virions, and induces signalling and HIV-1 LTR transcription. We hence hypothesized that Vpr might be a determinant of non-activated T-cell infection. Virion-delivered Vpr activated nuclear factor of activated T cells (NFAT) through Ca²⁺ influx and interference with the NFAT export kinase GSK3β. This leads to NFAT translocation and accumulation within the nucleus and was required for productive infection of unstimulated primary CD4⁺ T cells. A mutagenesis approach revealed correlation of Vpr-mediated NFAT activation with its ability to enhance LTR transcription and mediate cell cycle arrest. Upon NFAT inhibition, Vpr did not augment resting T-cell infection, and showed reduced G2/M arrest and LTR transactivation. Altogether, Vpr renders unstimulated T cells more permissive for productive HIV-1 infection and stimulates activation of productively infected as well as virus-exposed T cells. Therefore, it could be involved in the establishment and reactivation of HIV-1 from viral reservoirs and might have an impact on the levels of immune activation, which are determinants of HIV-1 pathogenesis.

The ClusPro web server for protein-protein docking

The ClusPro server (https://cluspro.org) is a widely used tool for protein-protein docking. The server provides a simple home page for basic use, requiring only two files in Protein Data Bank (PDB) format. However, ClusPro also offers a number of advanced options to modify the search; these include the removal of unstructured protein regions, application of attraction or repulsion, accounting for pairwise distance restraints, construction of homo-multimers, consideration of small-angle X-ray scattering (SAXS) data, and location of heparin-binding sites. Six different energy functions can be used, depending on the type of protein. Docking with each energy parameter set results in ten models defined by centers of highly populated clusters of low-energy docked structures. This protocol describes the use of the various options, the construction of auxiliary restraints files, the selection of the energy parameters, and the analysis of the results. Although the server is heavily used, runs are generally completed in <4 h.

The ClusPro web server for protein-protein docking

The ClusPro server (https://cluspro.org) is a widely used tool for protein-protein docking. The server provides a simple home page for basic use, requiring only two files in Protein Data Bank (PDB) format. However, ClusPro also offers a number of advanced options to modify the search; these include the removal of unstructured protein regions, application of attraction or repulsion, accounting for pairwise distance restraints, construction of homo-multimers, consideration of small-angle X-ray scattering (SAXS) data, and location of heparin-binding sites. Six different energy functions can be used, depending on the type of protein. Docking with each energy parameter set results in ten models defined by centers of highly populated clusters of low-energy docked structures. This protocol describes the use of the various options, the construction of auxiliary restraints files, the selection of the energy parameters, and the analysis of the results. Although the server is heavily used, runs are generally completed in <4 h.

The HIV-1 Vpr Protein: A Multifaceted Target for Therapeutic Intervention

The human immunodeficiency virus type 1 (HIV-1) Vpr protein is an attractive target for antiretroviral drug development. The conservation both of the structure along virus evolution and the amino acid sequence in viral isolates from patients underlines the importance of Vpr for the establishment and progression of HIV-1 disease. While its contribution to virus replication in dividing and non-dividing cells and to the pathogenesis of HIV-1 in many different cell types, both extracellular and intracellular forms, have been extensively studied, its precise mechanism of action nevertheless remains enigmatic. The present review discusses how the apparently multifaceted interplay between Vpr and host cells may be due to the impairment of basic metabolic pathways. Vpr protein modifies host cell energy metabolism, oxidative status, and proteasome function, all of which are likely conditioned by the concentration and multimerization of the protein. The characterization of Vpr domains along with new laboratory tools for the assessment of their function has become increasingly relevant in recent years. With these advances, it is conceivable that drug discovery efforts involving Vpr-targeted antiretrovirals will experience substantial growth in the coming years.

Specific amino acids in HIV-1 Vpr are significantly associated with differences in patient neurocognitive status

Even in the era of combination antiretroviral therapies used to combat human immunodeficiency virus type 1 (HIV-1) infection, up to 50 % of well-suppressed HIV-1-infected patients are still diagnosed with mild neurological deficits referred to as HIV-associated neurocognitive disorders (HAND). The multifactorial nature of HAND likely involves the HIV-1 accessory protein viral protein R (Vpr) as an agent of neuropathogenesis. To investigate the effect of naturally occurring variations in Vpr on HAND in well-suppressed HIV-1-infected patients, bioinformatic analyses were used to correlate peripheral blood-derived Vpr sequences with patient neurocognitive performance, as measured by comprehensive neuropsychological assessment and the resulting Global Deficit Score (GDS). Our studies revealed unique associations between GDS and the presence of specific amino acid changes in peripheral blood-derived Vpr sequences [neuropsychological impairment Vpr (niVpr) variants]. Amino acids N41 and A55 in the Vpr sequence were associated with more pronounced neurocognitive deficits (higher GDS). In contrast, amino acids I37 and S41 were connected to measurably lower GDS. All niVpr variants were also detected in DNA isolated from HIV-1-infected brain tissues. The implication of these results is that niVpr variants alter the genesis and/or progression of HAND through differences in Vpr-mediated effects in the peripheral blood and/or the brain.

Molecular Mechanism of HIV-1 Vpr for Binding to Importin-α

Viral protein R (Vpr) is an accessory gene product of human immunodeficiency virus type 1 (HIV-1) that plays multiple important roles associated with viral replication. Structural studies using NMR have revealed that Vpr consists of three α-helices and contains flexible N- and C-termini. However, the molecular mechanisms associated with Vpr function have not been elucidated. To investigate Vpr multifunctionality, we performed an X-ray crystallographic study of Vpr complexes containing importin-α, a known Vpr binding partner present in host cells. Elucidation of the crystal structure revealed that the flexible C-terminus changes its conformation to a twisted β-turn via an induced-fit mechanism, enabling binding to a minor nuclear localization signal (NLS) site of importin-α. The Vpr C-terminus can also bind with major NLS sites of importin-α in an extended conformation in different ways. These results, which represent the first reported crystallographic analysis of Vpr, demonstrate the multifunctional aspects that enable Vpr interaction with a variety of cellular proteins.

Sensing of biomolecular interactions using fluorescence complementing systems in living cells

Sensing biomolecule interactions in living cells allows for a deeper understanding of the mechanisms governing biological processes, and has increasing significance for improvements in clinical diagnosis. It is now possible by using molecular biosensors. One method involving molecular biosensors is called molecular fluorescence complementation, usually referred to as BiFC (bimolecular fragment/fluorescence complementary/complementation) or TriFC (trimolecular fragment complementary/complementation). This complementation method is based on the principle that two non-fluorescent fragments of a fluorescent protein are brought into sufficient lyclose proximity, upon which they are reconstructed so that fluorescence is re-established. This process relies on the interaction between the two fusion partners, which normally are proteins. This method is simple, noninvasive, sensitive, and does not require specialized tools, hence being available to most standard laboratories. Here, we selectively describe three relevant examples, although many other molecular interactions have been shown to work with this method. Recent developments of this method include multicolor BiFC, which allows for simultaneous detection of multi-biomolecule interactions, RNA-protein interactions, far red and near infrared sensing systems for deep tissue imaging. Challenges in the utilization of this method are discussed. Given the current rate of technological advancements, we believe that fluorescence fragment complementing systems have the potential to be utilized across a wide range of areas, including in routine research and clinical diagnosis.

Functional antagonism of TMPRSS2-ERG splice variants in prostate cancer

The fusion between ERG coding sequences and the TMPRSS2 promoter is the most prevalent in prostate cancer (CaP). The presence of two main types of TMPRSS2-ERG fusion transcripts in CaP specimens, Type I and Type II, prompted us to hypothesize that the cumulative actions of different ERG variants may impact CaP development/progression. Using TMPRSS2-ERG3 (Type I) and TMPRSS2-ERG8 (Type II) expression vectors, we determined that the TMPRSS2- ERG8 encoded protein is deficient in transcriptional regulation compared to TMPRSS2-ERG3. Co-transfection of vectors resulted in decreased transcriptional regulation compared to TMPRSS2-ERG3 alone, suggesting transdominance of ERG8. Expression of exogenous ERG8 protein resulted in a decrease in endogenous ERG3 protein levels in TMPRSS2-ERG positive VCaP cells, with a concomitant decrease in C-MYC. Further, we showed a physical association between ERG3 and ERG8 in live cells by the bimolecular fluorescence complementation assay, providing a basis for the observed effects. Inhibitory effects of TMPRSS2-ERG8 on TMPRSS2- ERG3 were also corroborated by gene expression data from human prostate cancers, which showed a positive correlation between C-MYC expression and TMPRSS2-ERG3/TMPRSS2- ERG8 ratio. We propose that an elevated TMPRSS2-ERG3/TMPRSS2-ERG8 ratio results in elevated C-MYC in CaP, providing a strong rationale for the biomarker and therapeutic utility of ERG splice variants, along with C-MYC.

The HIVToolbox 2 web system integrates sequence, structure, function and mutation analysis

There is enormous interest in studying HIV pathogenesis for improving the treatment of patients with HIV infection. HIV infection has become one of the best-studied systems for understanding how a virus can hijack a cell. To help facilitate discovery, we previously built HIVToolbox, a web system for visual data mining. The original HIVToolbox integrated information for HIV protein sequence, structure, functional sites, and sequence conservation. This web system has been used for almost 40,000 searches. We report improvements to HIVToolbox including new functions and workflows, data updates, and updates for ease of use. HIVToolbox2, is an improvement over HIVToolbox with new functions. HIVToolbox2 has new functionalities focused on HIV pathogenesis including drug-binding sites, drug-resistance mutations, and immune epitopes. The integrated, interactive view enables visual mining to generate hypotheses that are not readily revealed by other approaches. Most HIV proteins form multimers, and there are posttranslational modification and protein-protein interaction sites at many of these multimerization interfaces. Analysis of protease drug binding sites reveals an anatomy of drug resistance with different types of drug-resistance mutations regionally localized on the surface of protease. Some of these drug-resistance mutations have a high prevalence in specific HIV-1 M subtypes. Finally, consolidation of Tat functional sites reveals a hotspot region where there appear to be 30 interactions or posttranslational modifications. A cursory analysis with HIVToolbox2 has helped to identify several global patterns for HIV proteins. An initial analysis with this tool identifies homomultimerization of almost all HIV proteins, functional sites that overlap with multimerization sites, a global drug resistance anatomy for HIV protease, and specific distributions of some DRMs in specific HIV M subtypes. HIVToolbox2 is an open-access web application available at [http://hivtoolbox2.bio-toolkit.com].

HIV-1 Vpr-a still "enigmatic multitasker"

Like other HIV-1 auxiliary proteins, Vpr is conserved within all the human (HIV-1, HIV-2) and simian (SIV) immunodeficiency viruses. However, Vpr and homologous HIV-2, and SIV Vpx are the only viral auxiliary proteins specifically incorporated into virus particles through direct interaction with the Gag precursor, indicating that this presence in the core of the mature virions is mainly required for optimal establishment of the early steps of the virus life cycle in the newly infected cell. In spite of its small size, a plethora of effects and functions have been attributed to Vpr, including induction of cell cycle arrest and apoptosis, modulation of the fidelity of reverse transcription, nuclear import of viral DNA in macrophages and other non-dividing cells, and transcriptional modulation of viral and host cell genes. Even if some more recent studies identified a few cellular targets that HIV-1 Vpr may utilize in order to perform its different tasks, the real role and functions of Vpr during the course of natural infection are still enigmatic. In this review, we will summarize the main reported functions of HIV-1 Vpr and their significance in the context of the viral life cycle.

The phosphorylation of HIV-1 Gag by atypical protein kinase C facilitates viral infectivity by promoting Vpr incorporation into virions

Background

Human immunodeficiency virus type 1 (HIV-1) Gag is the main structural protein that mediates the assembly and release of virus-like particles (VLPs) from an infected cell membrane. The Gag C-terminal p6 domain contains short sequence motifs that facilitate virus release from the plasma membrane and mediate incorporation of the viral Vpr protein. Gag p6 has also been found to be phosphorylated during HIV-1 infection and this event may affect virus replication. However, the kinase that directs the phosphorylation of Gag p6 toward virus replication remains to be identified. In our present study, we identified this kinase using a proteomic approach and further delineate its role in HIV-1 replication.

Results

A proteomic approach was designed to systematically identify human protein kinases that potently interact with HIV-1 Gag and successfully identified 22 candidates. Among this panel, atypical protein kinase C (aPKC) was found to phosphorylate HIV-1 Gag p6. Subsequent LC-MS/MS and immunoblotting analysis with a phospho-specific antibody confirmed both in vitro and in vivo that aPKC phosphorylates HIV-1 Gag at Ser487. Computer-assisted structural modeling and a subsequent cell-based assay revealed that this phosphorylation event is necessary for the interaction between Gag and Vpr and results in the incorporation of Vpr into virions. Moreover, the inhibition of aPKC activity reduced the Vpr levels in virions and impaired HIV-1 infectivity of human primary macrophages.

Conclusion

Our current results indicate for the first time that HIV-1 Gag phosphorylation on Ser487 is mediated by aPKC and that this kinase may regulate the incorporation of Vpr into HIV-1 virions and thereby supports virus infectivity. Furthermore, aPKC inhibition efficiently suppresses HIV-1 infectivity in macrophages. aPKC may therefore be an intriguing therapeutic target for HIV-1 infection.

Human immunodeficiency virus type 1 Vpr polymorphisms associated with progressor and nonprogressor individuals alter Vpr-associated functions

Following infection with Human immunodeficiency virus 1 (HIV-1) there is a remarkable variation in virus replication and disease progression. Both host and viral factors have been implicated in the observed differences in disease status. Here, we focus on understanding the contribution of HIV-1 viral protein R (Vpr) by evaluating the disease-associated Vpr polymorphism and its biological functions from HIV-1 positive rapid progressor (RP) and long-term nonprogressor (LTNP) subjects. Results presented here show distinct variation in phenotypes of Vpr alleles from LTNP and RP subjects. Most notably, the polymorphism of Vpr at R36W and L68M associated with RP shows higher levels of oligomerization, and increased virus replication, whereas R77Q exhibits poor replication kinetics. Interestingly, we did not observe correlation with cell cycle arrest function. Together these results indicate that polymorphisms in Vpr in part may contribute to altered virus replication kinetics leading to the observed differences in disease progression in LTNP and RP groups.

DOT2: Macromolecular docking with improved biophysical models

Computational docking is a useful tool for predicting macromolecular complexes, which are often difficult to determine experimentally. Here, we present the DOT2 software suite, an updated version of the DOT intermolecular docking program. DOT2 provides straightforward, automated construction of improved biophysical models based on molecular coordinates, offering checkpoints that guide the user to include critical features. DOT has been updated to run more quickly, allow flexibility in grid size and spacing, and generate an infinitive complete list of favorable candidate configurations. Output can be filtered by experimental data and rescored by the sum of electrostatic and atomic desolvation energies. We show that this rescoring method improves the ranking of correct complexes for a wide range of macromolecular interactions and demonstrate that biologically relevant models are essential for biologically relevant results. The flexibility and versatility of DOT2 accommodate realistic models of complex biological systems, improving the likelihood of a successful docking outcome.

Exosomes derived from HIV-1-infected cells contain trans-activation response element RNA

Exosomes are nano-sized vesicles produced by healthy and virus-infected cells. Exosomes derived from infected cells have been shown to contain viral microRNAs (miRNAs). HIV-1 encodes its own miRNAs that regulate viral and host gene expression. The most abundant HIV-1-derived miRNA, first reported by us and later by others using deep sequencing, is the trans-activation response element (TAR) miRNA. In this study, we demonstrate the presence of TAR RNA in exosomes from cell culture supernatants of HIV-1-infected cells and patient sera. TAR miRNA was not in Ago2 complexes outside the exosomes but enclosed within the exosomes. We detected the host miRNA machinery proteins Dicer and Drosha in exosomes from infected cells. We report that transport of TAR RNA from the nucleus into exosomes is a CRM1 (chromosome region maintenance 1)-dependent active process. Prior exposure of naive cells to exosomes from infected cells increased susceptibility of the recipient cells to HIV-1 infection. Exosomal TAR RNA down-regulated apoptosis by lowering Bim and Cdk9 proteins in recipient cells. We found 10(4)-10(6) copies/ml TAR RNA in exosomes derived from infected culture supernatants and 10(3) copies/ml TAR RNA in the serum exosomes of highly active antiretroviral therapy-treated patients or long term nonprogressors. Taken together, our experiments demonstrated that HIV-1-infected cells produced exosomes that are uniquely characterized by their proteomic and RNA profiles that may contribute to disease pathology in AIDS.

Development of a robust cell-based high-throughput screening assay to identify targets of HIV-1 viral protein R dimerization

Targeting protein-protein interactions (PPI) is an emerging field in drug discovery. Dimerization and PPI are essential properties of human immunodeficiency virus (HIV)-1 proteins, their mediated functions, and virus biology. Additionally, dimerization is required for the functional interaction of HIV-1 proteins with many host cellular components. In this study, a bimolecular fluorescence complementation (BiFC)-based screening assay was developed that can quantify changes in dimerization, using HIV-1 viral protein R (Vpr) dimerization as a "proof of concept." Results demonstrated that Venus Vpr (generated by BiFC Vpr constructs) could be competed off in a dose-dependent manner using untagged, full-length Vpr as a competitor molecule. The change in signal intensity was measured quantitatively through flow cytometry and fluorescence microscopy in a high content screening assay. High content imaging was used to screen a library of small molecules for an effect on Vpr dimerization. Among the tested molecules, a few of the small molecules demonstrate an effect on Vpr dimerization in a dose-dependent manner.

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.

Human immunodeficiency virus viral protein R as an extracellular protein in neuropathogenesis

Numerous studies published in the past two decades have identified the viral protein R (Vpr) as one of the most versatile proteins in the life cycle of human immunodeficiency virus type 1 (HIV-1). In this regard, more than a thousand Vpr molecules are present in extracellular viral particles. Subsequent to viral entry, Vpr participates in early replicative events by assisting in viral genome nuclear import and, during the viral life cycle, by shuttling between the nucleus and the cytoplasm to accomplish its functions within the context of other replicative functions. Additionally, several studies have implicated Vpr as a proapoptotic protein because it promotes formation of permeability transition pores in mitochondria, which in turn affects transmembrane potential and adenosine triphosphate synthesis. Recent studies have identified Vpr as a virion-free protein in the serum and cerebrospinal fluid of patients infected with HIV-1 whose plasma viremia directly correlates with the extracellular concentration of Vpr. These observations pointed to a new role for Vpr as an additional weapon in the HIV-1 arsenal, involving the use of an extracellular protein to target and possibly inhibit HIV-1-uninfected bystander cells to enable them to escape immune surveillance. In addition, extracellular Vpr decreases adenosine triphosphate levels and affects the intracellular redox balance in neurons, ultimately causing their apoptosis. Herein, we review the role of Vpr as an extracellular protein and its downstream effects on cellular metabolism, functionality, and survival, with particular emphasis on how extracellular Vpr-induced oxidative stress might aggravate HIV-1-induced symptoms, thus affecting pathogenesis and disease progression.

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.

Human polyomavirus JC small regulatory agnoprotein forms highly stable dimers and oligomers: implications for their roles in agnoprotein function

JC virus (JCV) encodes a small basic phosphoprotein from the late coding region called agnoprotein, which has been shown to play important regulatory roles in the viral replication cycle. In this study, we report that agnoprotein forms highly stable dimers and higher order oligomer complexes. This was confirmed by immunoblotting and mass spectrometry studies. These complexes are extremely resistant to strong denaturing agents, including urea and SDS. Central portion of the protein, amino acids spanning from 17 to 42 is important for dimer/oligomer formation. Removal of 17 to 42 aa region from the viral background severely affected the efficiency of the JCV replication. Extracts prepared from JCV-infected cells showed a double banding pattern for agnoprotein in vivo. Collectively, these findings suggest that agnoprotein forms functionally active homodimer/oligomer complexes and these may be important for its function during viral propagation and thus for the progression of PML.