Functional role of human immunodeficiency virus type 1 vpu

Understanding how transmembrane domains regulate interactions between human BST-2 and the SARS-CoV-2 accessory protein ORF7a

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID, replicates at intracellular membranes. Bone marrow stromal antigen 2 (BST-2; tetherin) is an antiviral response protein that inhibits transport of viral particles after budding within infected cells. RNA viruses such as SARS-CoV-2 use various strategies to disable BST-2, including use of transmembrane 'accessory' proteins that interfere with BST-2 oligomerization. ORF7a is a small, transmembrane protein present in SARS-CoV-2 shown previously to alter BST-2 glycosylation and function. In this study, we investigated the structural basis for BST-2 ORF7a interactions, with a particular focus on transmembrane and juxtamembrane interactions. Our results indicate that transmembrane domains play an important role in BST-2 ORF7a interactions and mutations to the transmembrane domain of BST-2 can alter these interactions, particularly single-nucleotide polymorphisms in BST-2 that result in mutations such as I28S. Using molecular dynamics simulations, we identified specific interfaces and interactions between BST-2 and ORF7a to develop a structural basis for the transmembrane interactions. Differences in glycosylation are observed for BST-2 transmembrane mutants interacting with ORF7a, consistent with the idea that transmembrane domains play a key role in their heterooligomerization. Overall, our results indicate that ORF7a transmembrane domain interactions play a key role along with extracellular and juxtamembrane domains in modulating BST-2 function.

An Observational Study of Genetic Diversity of HIV-1 vpu in Rapid Progressors in India

BACKGROUND

The genetic diversity in HIV-1 genes affects viral pathogenesis in HIV-1 positive patients. Accessory genes of HIV-1, including vpu, are reported to play a critical role in HIV pathogenesis and disease progression. Vpu has a crucial role in CD4 degradation and virus release. The sequence heterogeneity in the vpu gene may affect disease progression in patients, therefore, the current study was undertaken to identify the role of vpu in patients defined as rapid progressors.

OBJECTIVE

The objective of the study was to identify the viral determinants present on vpu that may be important in disease progression in rapid progressors.

METHODS

Blood samples were collected from 13 rapid progressors. DNA was isolated from PBMCs and vpu was amplified using nested PCR. Both strands of the gene were sequenced using an automated DNA Sequencer. The characterization and analysis of vpu was done using various bioinformatics tools.

RESULTS

The analysis revealed that all sequences had intact ORF and sequence heterogeneity was present across all sequences and distributed all over the gene. The synonymous substitutions, however, were higher than nonsynonymous substitutions. The phylogenetic tree analysis showed an evolutionary relationship with previously published Indian subtype C sequences. Comparatively, the cytoplasmic tail(77 - 86) showed the highest degree of variability in these sequences as determined by Entropy- one tool.

CONCLUSION

The study showed that due to the robust nature of the protein, the biological activity of the protein was intact and sequence heterogeneity may promote disease progression in the study population.

Viroporins: Structure, function, and their role in the life cycle of SARS-CoV-2

Viroporins are indispensable for viral replication. As intracellular ion channels they disturb pH gradients of organelles and allow Ca²⁺ flux across ER membranes. Viroporins interact with numerous intracellular proteins and pathways and can trigger inflammatory responses. Thus, they are relevant targets in the search for antiviral drugs. Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) underlies the world-wide pandemic of COVID-19, where an effective therapy is still lacking despite impressive progress in the development of vaccines and vaccination campaigns. Among the 29 proteins of SARS-CoV-2, the E- and ORF3a proteins have been identified as viroporins that contribute to the massive release of inflammatory cytokines observed in COVID-19. Here, we describe structure and function of viroporins and their role in inflammasome activation and cellular processes during the virus replication cycle. Techniques to study viroporin function are presented, with a focus on cellular and electrophysiological assays. Contributions of SARS-CoV-2 viroporins to the viral life cycle are discussed with respect to their structure, channel function, binding partners, and their role in viral infection and virus replication. Viroporin sequences of new variants of concern (α–ο) of SARS-CoV-2 are briefly reviewed as they harbour changes in E and 3a proteins that may affect their function.

HIV-1 propagation is highly dependent on basal levels of the restriction factor BST2

BST2 is an interferon-inducible antiviral host protein antagonized by HIV-1 Vpu that entraps nascent HIV-1 virions on the cell surface. Unexpectedly, we find that HIV-1 lacking Nef can revert to full replication competence simply by losing the ability to antagonize BST2. Using gene editing together with cell sorting, we demonstrate that even the propagation of wild-type HIV-1 is strikingly dependent on BST2, including in primary human cells. HIV-1 propagation in BST2^−/− populations can be fully rescued by exogenous BST2 irrespective of its capacity to signal and even by an artificial BST2-like protein that shares its virion entrapment activity but lacks sequence homology. Counterintuitively, our results reveal that HIV-1 propagation is critically dependent on basal levels of virion tethering by a key component of innate antiviral immunity.

Role of Viral Protein U (Vpu) in HIV-1 Infection and Pathogenesis

Human immunodeficiency virus (HIV)-1 and HIV-2 originated from cross-species transmission of simian immunodeficiency viruses (SIVs). Most of these transfers resulted in limited spread of these viruses to humans. However, one transmission event involving SIVcpz from chimpanzees gave rise to group M HIV-1, with M being the principal strain of HIV-1 responsible for the AIDS pandemic. Vpu is an HIV-1 accessory protein generated from Env/Vpu encoded bicistronic mRNA and localized in cytosolic and membrane regions of cells capable of being infected by HIV-1 and that regulate HIV-1 infection and transmission by downregulating BST-2, CD4 proteins levels, and immune evasion. This review will focus of critical aspects of Vpu including its zoonosis, the adaptive hurdles to cross-species transmission, and future perspectives and broad implications of Vpu in HIV-1 infection and dissemination.

Unconventional viral gene expression mechanisms as therapeutic targets

Unlike the human genome that comprises mostly noncoding and regulatory sequences, viruses have evolved under the constraints of maintaining a small genome size while expanding the efficiency of their coding and regulatory sequences. As a result, viruses use strategies of transcription and translation in which one or more of the steps in the conventional gene-protein production line are altered. These alternative strategies of viral gene expression (also known as gene recoding) can be uniquely brought about by dedicated viral enzymes or by co-opting host factors (known as host dependencies). Targeting these unique enzymatic activities and host factors exposes vulnerabilities of a virus and provides a paradigm for the design of novel antiviral therapies. In this Review, we describe the types and mechanisms of unconventional gene and protein expression in viruses, and provide a perspective on how future basic mechanistic work could inform translational efforts that are aimed at viral eradication.

Design, synthesis and biological evaluation of novel 2-(5-aryl-1H-imidazol-1-yl) derivatives as potential inhibitors of the HIV-1 Vpu and host BST-2 protein interaction

Novel ethyl 2-(5-aryl-1H-imidazol-1-yl)-acetates 17 and propionates 18, together with their acetic acid 19 and acetohydrazide 20 derivatives, were designed and synthesized using TosMIC chemistry. Biological evaluation of these newly synthesized scaffolds in the HIV-1 Vpu- Host BST-2 ELISA assay identified seven hits (17a, 17b, 17c, 17g, 18a, 20f and 20g) with greater than 50% inhibitory activity. These hits were validated in the HIV-1 Vpu- Host BST-2 AlphaScreen™ and six of the seven compounds were found to have comparable percentage inhibitory activities to those of the ELISA assay. Compounds 17b and 20g, with consistent percentage inhibitory activities across the two assays, had IC₅₀ values of 11.6 ± 1.1 μM and 17.6 ± 0.9 μM in a dose response AlphaScreen™ assay. In a cell-based HIV-1 antiviral assay, compound 17b exhibited an EC₅₀ = 6.3 ± 0.7 μM at non-toxic concentrations (CC₅₀ = 184.5 ± 0.8 μM), whereas compound 20g displayed antiviral activity roughly equivalent to its toxicity (CC₅₀ = 159.5 ± 0.9 μM). This data suggests that compound 17b, active in both cell-based and biochemical assays, provides a good starting point for the design of possible lead compounds for prevention of HIV-1 Vpu and host BST-2 protein binding in new anti-HIV therapeutics.

Plasma Membrane-Associated Restriction Factors and Their Counteraction by HIV-1 Accessory Proteins

The plasma membrane is a site of conflict between host defenses and many viruses. One aspect of this conflict is the host's attempt to eliminate infected cells using innate and adaptive cell-mediated immune mechanisms that recognize features of the plasma membrane characteristic of viral infection. Another is the expression of plasma membrane-associated proteins, so-called restriction factors, which inhibit enveloped virions directly. HIV-1 encodes two countermeasures to these host defenses: The membrane-associated accessory proteins Vpu and Nef. In addition to inhibiting cell-mediated immune-surveillance, Vpu and Nef counteract membrane-associated restriction factors. These include BST-2, which traps newly formed virions at the plasma membrane unless counteracted by Vpu, and SERINC5, which decreases the infectivity of virions unless counteracted by Nef. Here we review key features of these two antiviral proteins, and we review Vpu and Nef, which deplete them from the plasma membrane by co-opting specific cellular proteins and pathways of membrane trafficking and protein-degradation. We also discuss other plasma membrane proteins modulated by HIV-1, particularly CD4, which, if not opposed in infected cells by Vpu and Nef, inhibits viral infectivity and increases the sensitivity of the viral envelope glycoprotein to host immunity.

Comparative Genetic Variability in HIV-1 Subtype C vpu Gene in Early Age Groups of Infants

OBJECTIVE

Identifying the genetic variability in vertically transmitted viruses in early infancy is important to understand the disease progression. Being important in HIV-1 disease pathogenesis, vpu gene, isolated from young infants was investigated to understand the viral characteristics.

METHOD

Blood samples were obtained from 80 HIV-1 positive infants, categorized in two age groups; acute (<6 months) and early (>6-18 months). A total of 77 PCR positive samples, amplified for vpu gene, were sequenced and analyzed.

RESULTS

73 isolates belonged to subtype C. Analysis of heterogeneity of amino acid sequences in infant groups showed that in the sequences of acute age group both insertions and deletions were present while in the early age group only deletions were present. In the acute age group, a deletion of 3 residues (RAE) in the first alfa helix in one sequence and insertions of 1-2 residues (DM, GH, G and H) in the second alfa helix in 4 sequences were observed. In the early age group, deletion of 2 residues (VN) in the cytoplasmic tail region in 2 sequences was observed. Length of the amino terminal was observed to be gradually increasing with the increasing age of the infants. Protein Variation Effect Analyzer software showed that deleterious mutations were more in the acute than the early age group. Entropy analysis revealed that heterogeneity of the residues was comparatively higher in the sequences of acute than the early age group.

CONCLUSION

Mutations observed in the helixes may affect the conformation and lose the ability to degrade CD4 receptors. Heterogeneity was decreasing with the increasing ages of the infants, indicating positive selection for robust virion survival.

Beyond Channel Activity: Protein-Protein Interactions Involving Viroporins

Viroporins are short polypeptides encoded by viruses. These small membrane proteins assemble into oligomers that can permeabilize cellular lipid bilayers, disrupting the physiology of the host to the advantage of the virus. Consequently, efforts during the last few decades have been focused towards the discovery of viroporin channel inhibitors, but in general these have not been successful to produce licensed drugs. Viroporins are also involved in viral pathogenesis by engaging in critical interactions with viral proteins, or disrupting normal host cellular pathways through coordinated interactions with host proteins. These protein-protein interactions (PPIs) may become alternative attractive drug targets for the development of antivirals. In this sense, while thus far most antiviral molecules have targeted viral proteins, focus is moving towards targeting host proteins that are essential for virus replication. In principle, this largely would overcome the problem of resistance, with the possibility of using repositioned existing drugs. The precise role of these PPIs, their strain- and host- specificities, and the structural determination of the complexes involved, are areas that will keep the fields of virology and structural biology occupied for years to come. In the present review, we provide an update of the efforts in the characterization of the main PPIs for most viroporins, as well as the role of viroporins in these PPIs interactions.

Type I Interferon Responses by HIV-1 Infection: Association with Disease Progression and Control

Human immunodeficiency virus type 1 (HIV-1) is the causative agent of acquired immunodeficiency syndrome and its infection leads to the onset of several disorders such as the depletion of peripheral CD4⁺ T cells and immune activation. HIV-1 is recognized by innate immune sensors that then trigger the production of type I interferons (IFN-Is). IFN-Is are well-known cytokines eliciting broad anti-viral effects by inducing the expression of anti-viral genes called interferon-stimulated genes (ISGs). Extensive in vitro studies using cell culture systems have elucidated that certain ISGs such as APOBEC3G, tetherin, SAM domain and HD domain-containing protein 1, MX dynamin-like GTPase 2, guanylate-binding protein 5, and schlafen 11 exert robust anti-HIV-1 activity, suggesting that IFN-I responses triggered by HIV-1 infection are detrimental for viral replication and spread. However, recent studies using animal models have demonstrated that at both the acute and chronic phase of infection, the role of IFN-Is produced by HIV or SIV infection in viral replication, spread, and pathogenesis, may not be that straightforward. In this review, we describe the pluses and minuses of HIV-1 infection stimulated IFN-I responses on viral replication and pathogenesis, and further discuss the possibility for therapeutic approaches.

Making Sense of Multifunctional Proteins: Human Immunodeficiency Virus Type 1 Accessory and Regulatory Proteins and Connections to Transcription

Viruses are completely dependent upon cellular machinery to support replication and have therefore developed strategies to co-opt cellular processes to optimize infection and counter host immune defenses. Many viruses, including human immunodeficiency virus type 1 (HIV-1), encode a relatively small number of genes. Viruses with limited genetic content often encode multifunctional proteins that function at multiple stages of the viral replication cycle. In this review, we discuss the functions of HIV-1 regulatory (Tat and Rev) and accessory (Vif, Vpr, Vpu, and Nef) proteins. Each of these proteins has a highly conserved primary activity; however, numerous additional activities have been attributed to these viral proteins. We explore the possibility that HIV-1 proteins leverage their multifunctional nature to alter host transcriptional networks to elicit a diverse set of cellular responses. Although these transcriptional effects appear to benefit the virus, it is not yet clear whether they are strongly selected for during viral evolution or are a ripple effect from the primary function. As our detailed knowledge of these viral proteins improves, we will undoubtedly uncover how the multifunctional nature of these HIV-1 regulatory and accessory proteins, and in particular their transcriptional functions, work to drive viral pathogenesis.

Palmitoylation of Sindbis Virus TF Protein Regulates Its Plasma Membrane Localization and Subsequent Incorporation into Virions

Palmitoylation is a reversible, posttranslational modification that helps target proteins to cellular membranes. The alphavirus small membrane proteins 6K and TF have been reported to be palmitoylated and to positively regulate budding. 6K and TF are isoforms that are identical in their N termini but unique in their C termini due to a -1 ribosomal frameshift during translation. In this study, we used cysteine (Cys) mutants to test differential palmitoylation of the Sindbis virus 6K and TF proteins. We modularly mutated the five Cys residues in the identical N termini of 6K and TF, the four additional Cys residues in TF's unique C terminus, or all nine Cys residues in TF. Using these mutants, we determined that TF palmitoylation occurs primarily in the N terminus. In contrast, 6K is not palmitoylated, even on these shared residues. In the C-terminal Cys mutant, TF protein levels increase both in the cell and in the released virion compared to the wild type. In viruses with the N-terminal Cys residues mutated, TF is much less efficiently localized to the plasma membrane, and it is not incorporated into the virion. The three Cys mutants have minor defects in cell culture growth but a high incidence of abnormal particle morphologies compared to the wild-type virus as determined by transmission electron microscopy. We propose a model where the C terminus of TF modulates the palmitoylation of TF at the N terminus, and palmitoylated TF is preferentially trafficked to the plasma membrane for virus budding.

IMPORTANCE

Alphaviruses are a reemerging viral cause of arthritogenic disease. Recently, the small 6K and TF proteins of alphaviruses were shown to contribute to virulence in vivo Nevertheless, a clear understanding of the molecular mechanisms by which either protein acts to promote virus infection is missing. The TF protein is a component of budded virions, and optimal levels of TF correlate positively with wild-type-like particle morphology. In this study, we show that the palmitoylation of TF regulates its localization to the plasma membrane, which is the site of alphavirus budding. Mutants in which TF is not palmitoylated display drastically reduced plasma membrane localization, which effectively prevents TF from participating in budding or being incorporated into virus particles. Investigation of the regulation of TF will aid current efforts in the alphavirus field searching for approaches to mitigate alphaviral disease in humans.

Hepatitis E virus ORF3 is a functional ion channel required for release of infectious particles

Hepatitis E virus (HEV) is the leading cause of enterically transmitted viral hepatitis globally. Of HEV's three ORFs, the function of ORF3 has remained elusive. Here, we demonstrate that via homophilic interactions ORF3 forms multimeric complexes associated with intracellular endoplasmic reticulum (ER)-derived membranes. HEV ORF3 shares several structural features with class I viroporins, and the function of HEV ORF3 can be maintained by replacing it with the well-characterized viroporin influenza A virus (IAV) matrix-2 protein. ORF3's ion channel function is further evidenced by its ability to mediate ionic currents when expressed in Xenopus laevis oocytes. Furthermore, we identified several positions in ORF3 critical for its formation of multimeric complexes, ion channel activity, and, ultimately, release of infectious particles. Collectively, our data demonstrate a previously undescribed function of HEV ORF3 as a viroporin, which may serve as an attractive target in developing direct-acting antivirals.

Synthesis and Evaluation of Some Symmetrical Phosphate Dimer Derivatives of 3′-Modified Nucleosides as Potential anti-HIV Agents

Novel symmetrical nucIeotide-(5′,5′)-dimers of 3′-O-acetylthymidine, 3′-O-methylthymidine, 3′-O-ethylthymidine, 3′-O-n-propylthymidine and 3′-azido-3′-deoxythymidine (AZT) were synthesized as membrane soluble pro-drugs. These were prepared using phosphorodichloridate chemistry and were characterised by spectroscopic and analytical data. In-vitro evaluation of the derivatives in cells acutely infected with the human immunodeficiency virus (HIV-1) demonstrated a range of activities. These derivatives were generally found to display poor inhibition of HIV proliferation. Derivatives containing AZT moieties were found to be potent, but such compounds were less active than the parent nucleoside. The data indicated that the AZT-containing compounds act primarily via the release of the free nucleoside. However, in some cases, the dimers of certain inactive nucleoside analogues were found to be active. In these cases, release of the nucleoside alone cannot account for the activity.

Targeting the Channel Activity of Viroporins

Since the discovery that certain small viral membrane proteins, collectively termed as viroporins, can permeabilize host cellular membranes and also behave as ion channels, attempts have been made to link this feature to specific biological roles. In parallel, most viroporins identified so far are virulence factors, and interest has focused toward the discovery of channel inhibitors that would have a therapeutic effect, or be used as research tools to understand the biological roles of viroporin ion channel activity. However, this paradigm is being shifted by the difficulties inherent to small viral membrane proteins, and by the realization that protein-protein interactions and other diverse roles in the virus life cycle may represent an equal, if not, more important target. Therefore, although targeting the channel activity of viroporins can probably be therapeutically useful in some cases, the focus may shift to their other functions in following years. Small-molecule inhibitors have been mostly developed against the influenza A M2 (IAV M2 or AM2). This is not surprising since AM2 is the best characterized viroporin to date, with a well-established biological role in viral pathogenesis combined the most extensive structural investigations conducted, and has emerged as a validated drug target. For other viroporins, these studies are still mostly in their infancy, and together with those for AM2, are the subject of the present review.

The role of BST-2/Tetherin in host protection and disease manifestation

Host cells respond to viral infections by activating immune response genes that are not only involved in inflammation, but may also predispose cells to cancerous transformation. One such gene is BST‐2, a type II transmembrane protein with a unique topology that endows it tethering and signaling potential. Through this ability to tether and signal, BST‐2 regulates host response to viral infection either by inhibiting release of nascent viral particles or in some models inhibiting viral dissemination. However, despite its antiviral functions, BST‐2 is involved in disease manifestation, a function linked to the ability of BST‐2 to promote cell‐to‐cell interaction. Therefore, modulating BST‐2 expression and/or activity has the potential to influence course of disease.

Intrinsic host restrictions to HIV-1 and mechanisms of viral escape

To replicate in their hosts, viruses have to navigate the complexities of the mammalian cell, co-opting mechanisms of cellular physiology while defeating restriction factors that are dedicated to halting their progression. Primate lentiviruses devote a relatively large portion of their coding capacity to counteracting restriction factors by encoding accessory proteins dedicated to neutralizing the antiviral function of these intracellular inhibitors. Research into the roles of the accessory proteins has revealed the existence of previously undetected intrinsic defenses, provided insight into the evolution of primate lentiviruses as they adapt to new species and uncovered new targets for the development of therapeutics. This Review discusses the biology of the restriction factors APOBEC3, SAMHD1 and tetherin and the viral accessory proteins that counteract them.

In virio SHAPE analysis of tRNA(Lys3) annealing to HIV-1 genomic RNA in wild type and protease-deficient virus

Background

tRNA^Lys3 annealing to the viral RNA of human immunodeficiency virus type-1 (HIV-1) is an essential step in the virus life cycle, because this tRNA serves as the primer for initiating reverse transcription. tRNA^Lys3 annealing to viral RNA occurs in two steps. First, Gag promotes annealing of tRNA^Lys3 to the viral RNA during cytoplasmic HIV-1 assembly. Second, mature nucleocapsid (NCp7), produced from the processing of Gag by viral protease during viral budding from the cell, remodels the annealed complex to form a more stable interaction between the viral RNA and tRNA^Lys3, resulting in a more tightly bound and efficient primer for reverse transcription.

Results

In this report, we have used in virio SHAPE analysis of both the 5´-untranslated region in HIV-1 RNA and the annealed tRNA^Lys3 to determine structural differences of the annealed complex that occur between protease-negative (Pr-) and wild type viruses. Our results indicate that the weaker binding of tRNA^Lys3 annealed by Gag in Pr- virions reflects both missing interactions of tRNA^Lys3 with viral RNA regions in the upper PBS stem, and a weaker interaction with the internal stem-loop found within the unannealed primer binding site in viral RNA.

Conclusions

We propose secondary structure models for the tRNA^Lys3/viral RNA annealed complexes in PR- and wild type viruses that support the two-step annealing model by showing that Gag promotes a partial annealing of tRNA^Lys3 to HIV-1 viral RNA, followed by a more complete annealing by NCp7.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0171-7) contains supplementary material, which is available to authorized users.

Bacteria-based analysis of HIV-1 Vpu channel activity

HIV-1 Vpu is a small, single-span membrane protein with two attributed functions that increase the virus' pathogenicity: degradation of CD4 and inactivation of BST-2. Vpu has also been shown to posses ion channel activity, yet no correlation has been found between this attribute and Vpu's role in viral release. In order to gain further insight into the channel activity of Vpu we devised two bacteria-based assays that can examine this function in detail. In the first assay Vpu was over-expressed, such that it was deleterious to bacterial growth due to membrane permeabilization. In the second and more sensitive assay, the channel was expressed at low levels in K⁺ transport deficient bacteria. Consequently, Vpu expression enabled the bacteria to grow at otherwise non permissive low K⁺ concentrations. Hence, Vpu had the opposite impact on bacterial growth in the two assays: detrimental in the former and beneficial in the latter. Furthermore, we show that channel blockers also behave reciprocally in the two assays, promoting growth in the first assay and hindering it in the second assay. Taken together, we investigated Vpu's channel activity in a rapid and quantitative approach that is amenable to high-throughput screening, in search of novel blockers.

Human immunodeficiency virus-1 (HIV-1)-mediated apoptosis: new therapeutic targets

HIV has posed a significant challenge due to the ability of the virus to both impair and evade the host’s immune system. One of the most important mechanisms it has employed to do so is the modulation of the host’s native apoptotic pathways and mechanisms. Viral proteins alter normal apoptotic signaling resulting in increased viral load and the formation of viral reservoirs which ultimately increase infectivity. Both the host’s pro- and anti-apoptotic responses are regulated by the interactions of viral proteins with cell surface receptors or apoptotic pathway components. This dynamic has led to the development of therapies aimed at altering the ability of the virus to modulate apoptotic pathways. These therapies are aimed at preventing or inhibiting viral infection, or treating viral associated pathologies. These drugs target both the viral proteins and the apoptotic pathways of the host. This review will examine the cell types targeted by HIV, the surface receptors exploited by the virus and the mechanisms whereby HIV encoded proteins influence the apoptotic pathways. The viral manipulation of the hosts’ cell type to evade the immune system, establish viral reservoirs and enhance viral proliferation will be reviewed. The pathologies associated with the ability of HIV to alter apoptotic signaling and the drugs and therapies currently under development that target the ability of apoptotic signaling within HIV infection will also be discussed.

Role of the OB-fold of RNA helicase A in the synthesis of HIV-1 RNA

RNA helicase A (RHA), a DExD/H protein, contains a stretch of repeated arginine and glycine-glycine (RGG) residues and an oligonucleotide/oligosaccharide-binding fold (OB-fold) at the C-terminus. RHA has been reported to function as a transcriptional cofactor. This study shows the role of RGG and OB-fold domains of RHA in the activation of transcription and splicing of HIV-1 RNA. RHA stimulates HIV-1 transcription by enhancing the occupancy of RNA polymerase II on the proviral DNA. Deletion of RGG or both RGG and OB-fold does not change the transcriptional activity of RHA, nor does the stability of viral RNA. However, deletion of both RGG and OB-fold rather than deletion of RGG only results in less production of multiply spliced 6D RNAs. The results suggest that the OB-fold is involved in modulating HIV-1 RNA splicing in the context of some HIV-1 strains while it is dispensable for the activation of HIV-1 transcription.

The role of A-kinase anchoring protein 95-like protein in annealing of tRNALys3 to HIV-1 RNA

Background

RNA helicase A (RHA), a DExH box protein, promotes annealing of tRNA^Lys3, a primer for reverse transcription, to HIV-1 RNA and assembles into virus particles. A-kinase anchoring protein 95-like protein (HAP95) is a binding partner of RHA. The role of HAP95 in the annealing of tRNA^Lys3 was examined in this study.

Results

HAP95 associates with the reverse transcriptase region of Pol protein of HIV-1. Decreasing endogenous HAP95 in HIV-1-producing 293T cells by siRNA reduces the amount of tRNA^Lys3 annealed on viral RNA. This defect was further deteriorated by knockdown of RHA in the same cells, suggesting a cooperative effect between these two proteins. Biochemical assay in vitro using purified GST-tagged HAP95 shows that HAP95 may inhibit the activity of RHA.

Conclusion

The results support a hypothesis that HAP95 may transiently block RHA’s activity to protect the annealed tRNA^Lys3 on viral RNA in the cells from removing by RHA during the packaging of RHA into virus particles, thus facilitating the annealing of tRNA^Lys3 to HIV-1 RNA.

Different activities of the conserved lysine residues in the double-stranded RNA binding domains of RNA helicase A in vitro and in the cell

BACKGROUND

RNA helicase A regulates a variety of RNA metabolism processes including HIV-1 replication and contains two double-stranded RNA binding domains (dsRBD1 and dsRBD2) at the N-terminus. Each dsRBD contains two invariant lysine residues critical for the binding of isolated dsRBDs to RNA. However, the role of these conserved lysine residues was not tested in the context of enzymatically active full-length RNA helicase A either in vitro or in the cells.

METHODS

The conserved lysine residues in each or both of dsRBDs were substituted by alanine in the context of full-length RNA helicase A. The mutant RNA helicase A was purified from mammalian cells. The effects of these mutations were assessed either in vitro upon RNA binding and unwinding or in the cell during HIV-1 production upon RNA helicase A-RNA interaction and RNA helicase A-stimulated viral RNA processes.

RESULTS

Unexpectedly, the substitution of the lysine residues by alanine in either or both of dsRBDs does not prevent purified full-length RNA helicase A from binding and unwinding duplex RNA in vitro. However, these mutations efficiently inhibit RNA helicase A-stimulated HIV-1 RNA metabolism including the accumulation of viral mRNA and tRNA(Lys3) annealing to viral RNA. Furthermore, these mutations do not prevent RNA helicase A from binding to HIV-1 RNA in vitro as well, but dramatically reduce RNA helicase A-HIV-1 RNA interaction in the cells.

CONCLUSIONS

The conserved lysine residues of dsRBDs play critical roles in the promotion of HIV-1 production by RNA helicase A.

GENERAL SIGNIFICANCE

The conserved lysine residues of dsRBDs are key to the interaction of RNA helicase A with substrate RNA in the cell, but not in vitro.

Viral miniproteins

Many viruses encode short transmembrane proteins that play vital roles in virus replication or virulence. Because many of these proteins are less than 50 amino acids long and not homologous to cellular proteins, their open reading frames were often overlooked during the initial annotation of viral genomes. Some of these proteins oligomerize in membranes and form ion channels. Other miniproteins bind to cellular transmembrane proteins and modulate their activity, whereas still others have an unknown mechanism of action. Based on the underlying principles of transmembrane miniprotein structure, it is possible to build artificial small transmembrane proteins that modulate a variety of biological processes. These findings suggest that short transmembrane proteins provide a versatile mechanism to regulate a wide range of cellular activities, and we speculate that cells also express many similar proteins that have not yet been discovered.

HIV accessory proteins versus host restriction factors

Primate immunodeficiency viruses, including HIV-1, are characterized by the presence of accessory genes such as vif, vpr, vpx, vpu, and nef. Current knowledge indicates that none of the primate lentiviral accessory proteins has enzymatic activity. Instead, these proteins interact with cellular ligands to either act as adapter molecules to redirect the normal function of host factors for virus-specific purposes or to inhibit a normal host function by mediating degradation or causing intracellular mislocalization/sequestration of the factors involved. This review aims at providing an update of our current understanding of how Vif, Vpu, and Vpx control the cellular restriction factors APOBEC3G, BST-2, and SAMHD1, respectively.

Qualitative computational bioanalytics: assembly of viral channel-forming peptides around mono and divalent ions

A fine-grained docking protocol was used to generate a bundle-like structure of the bitopic membrane protein Vpu from HIV-1. Vpu is a type I membrane protein with 81 amino acids. It is proposed that Vpu forms ion- and substrate-conducting bundles, which are located at the plasma membrane in the infected cell. The Vpu1-32 peptide that includes the transmembrane domain (TMD) is assembled into homo-pentameric bundles around prepositioned Na, K, Ca or Cl ions. For bundles with the lowest energy, the TMDs generate a hydrophobic pore. Bundles in which Ser-24 faces the pore have higher energy. The tilt of the helices in the lowest energy bundles is larger than bundles with serines facing the pore. Left-handed bundles are lowest in energy where the ions are located at the serines.

Roles of the linker region of RNA helicase A in HIV-1 RNA metabolism

RNA helicase A (RHA) promotes multiple steps in HIV-1 production including transcription and translation of viral RNA, annealing of primer tRNA^Lys3 to viral RNA, and elevating the ratio of unspliced to spliced viral RNA. At its amino terminus are two double-stranded RNA binding domains (dsRBDs) that are essential for RHA-viral RNA interaction. Linking the dsRBDs to the core helicase domain is a linker region containing 6 predicted helices. Working in vitro with purified mutant RHAs containing deletions of individual helices reveals that this region may regulate the enzyme's helicase activity, since deletion of helix 2 or 3 reduces the rate of unwinding RNA by RHA. The biological significance of this finding was then examined during HIV-1 production. Deletions in the linker region do not significantly affect either RHA-HIV-1 RNA interaction in vivo or the incorporation of mutant RHAs into progeny virions. While the partial reduction in helicase activity of mutant RHA containing a deletion of helices 2 or 3 does not reduce the ability of RHA to stimulate viral RNA synthesis, the promotion of tRNA^Lys3 annealing to viral RNA is blocked. In contrast, deletion of helices 4 or 5 does not affect the ability of RHA to promote tRNA^Lys3 annealing, but reduces its ability to stimulate viral RNA synthesis. Additionally, RHA stimulation of viral RNA synthesis results in an increased ratio of unspliced to spliced viral RNA, and this increase is not inhibited by deletions in the linker region, nor is the pattern of splicing changed within the ∼ 4.0 kb or ∼ 1.8 kb HIV-1 RNA classes, suggesting that RHA's effect on suppressing splicing is confined mainly to the first 5′-splice donor site. Overall, the differential responses to the mutations in the linker region of RHA reveal that RHA participates in HIV-1 RNA metabolism by multiple distinct mechanisms.

HIV-1 Vpu - an ion channel in search of a job

Vpu is a small membrane protein encoded by HIV-1 and some SIV isolates. The protein is best known for its ability to degrade CD4 and to enhance the release of progeny virions from infected cells. However, Vpu also promotes host-cell apoptosis by deregulating the NFκB signaling pathway and it assembles into cation-conducting membrane pores. This review summarizes our current understanding of these various functions of Vpu with particular emphasis on recent progress in the Vpu field. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

The antiviral activities of tetherin

Tetherin (BST2/CD317) has emerged as a key host cell defense molecule, inhibiting the release and spread of diverse enveloped virions from infected cells. In this chapter, I review the molecular and cellular basis for tetherin's antiviral activities and the function of virally encoded countermeasures that disrupt its function. I further describe recent advances in our understanding of tetherin's associated role in viral pattern recognition and the evidence for its role in limiting viral pathogenesis in vivo.

Simian-Human immunodeficiency viruses expressing chimeric subtype B/C Vpu proteins demonstrate the importance of the amino terminal and transmembrane domains in the rate of CD4(+) T cell loss in macaques

Previously, we reported that simian-human immunodeficiency viruses expressing either the lab adapted subtype B (SHIV(KU-1bMC33)) or subtype C (SHIV(SCVpu)) Vpu proteins of human immunodeficiency virus type 1 (HIV-1) had different rates of CD4(+) T cell loss following inoculation into macaques. In this study, we have generated SHIVs that express either the subtype B or subtype C N-terminal (NTD) and transmembrane (TMD) domains and the opposing cytoplasmic domain (SHIV(VpuBC), SHIV(VpuCB)). In culture systems, SHIV(VpuBC) replicated faster than SHIV(VpuCB) while both proteins exhibited similar ability to down-modulate CD4 surface expression. Following inoculation into macaques, SHIV(VpuBC) resulted in rapid CD4(+) T cell loss similar to the parental SHIV(KU-1bMC33), while the rate of CD4(+) T cell loss in those inoculated with SHIV(VpuCB) was intermediate of SHIV(SCVpu) and SHIV(KU-1bMC33). These results emphasize the importance of the Vpu NTD/TMD region in the rate of CD4(+) T cell loss in the pathogenic X4 SHIV/macaque model.

Two retroviruses packaged in one cell line can combined inhibit the replication of HIV-1 in TZM-bl cells

The cellular protein tetherin tethers the HIV-1 viral particles on the cellular membrane to inhibit the replication of HIV-1. However, the HIV-1 accessory protein Vpu counteracts the antiviral function of tetherin. In this study, two retroviral vector plasmids were constructed. One inhibited the vpu gene expression; the other one over-expressed the tetherin. Both retroviral vector plasmids could be packaged in the packaging cell line PT67 to obtain the corresponding retroviruses. The retroviral vector plasmids' functions of tetherin over-expression or vpu-RNAi were detected at the cell level. Retroviral vector plasmids were transfected to PT67 cells at different ratios from 0T3V to 3T0V, and then mixed retroviruses were harvested. The antiviral functions of mixed retroviruses were detected in HIV-1 infected TZM-bl cells. The results showed that packaged mixed retroviruses could repress the replication of HIV-1 in TZM-bl cells.

Sequences within RNA coding for HIV-1 Gag p17 are efficiently targeted to exosomes

HIV budding requires the interaction with cell factors involved in the biogenesis of exosomes. This implies the possibility that viral products undergo exosome incorporation. While this has been already described for both Gag and Nef HIV-1 proteins, no conclusive results on HIV genome have been produced so far. Here, we report that unspliced, but not single or double spliced, HIV-1 RNA species are incorporated in exosomes. Deletion mutant analysis indicated that the presence of a stretch of sequences within the 5' end of the Gag p17 open reading frame is sufficient for HIV-1 RNA exosome incorporation. These sequences were found associating with exosomes also out of the HIV-1 context, thus indicating that the diversion towards the vesicular compartment can occur without need of additional HIV-1 sequences. Finally, the incorporation of genomic HIV-1 RNA in exosomes significantly increased when producer cells express HIV-1 defective for viral genome packaging. Manipulating infected cells to favour the selective incorporation in exosomes of genomic HIV-1 RNA might have therapeutic implications.

Viroporins: structure and biological functions

Viroporins are small virally encoded hydrophobic proteins that oligomerize in the membrane of host cells, leading to the formation of hydrophilic pores. This activity modifies several cellular functions, including membrane permeability, Ca²⁺ homeostasis, membrane remodelling and glycoprotein trafficking.

A classification scheme for viroporins is proposed on the basis of their structure and membrane topology. Thus, class I and class II viroporins are defined according to the number of transmembrane domains in the protein (one and two, respectively), and subclasses are defined according to their orientation in the membrane.

The main function of viroporins during viral replication is to participate in virion morphogenesis and release from host cells. In addition, some viroporins are involved in viral entry and genome replication.

The structure and activity of several viroporins, such as picornavirus protein 2B (P2B), influenza A virus matrix protein 2 (M2), hepatitis C virus p7 and HIV-1 viral protein U (Vpu), have been analysed in detail.

New members of this expanding family of viral proteins have been described, from both RNA and DNA viruses. In addition to having a common general structure, all of these new viroporins have the ability to increase membrane permeability.

Viroporins represent ideal targets to block viral replication and the spread of infection. Although a number of selective inhibitors of viroporin ion channels have been analysed in detail, optimized screening systems promise to provide new and more potent antiviral compounds in the near future.

Viroporins belong to a growing family of virally encoded proteins that form aqueous channels in the membranes of host cells. Here, Carrasco and colleagues review the structure and diverse biological functions of these proteins during the viral life cycle, as well as their potential as antiviral therapeutic targets.

Viroporins are small, hydrophobic proteins that are encoded by a wide range of clinically relevant animal viruses. When these proteins oligomerize in host cell membranes, they form hydrophilic pores that disrupt a number of physiological properties of the cell. Viroporins are crucial for viral pathogenicity owing to their involvement in several diverse steps of the viral life cycle. Thus, these viral proteins, which include influenza A virus matrix protein 2 (M2), HIV-1 viral protein U (Vpu) and hepatitis C virus p7, represent ideal targets for therapeutic intervention, and several compounds that block their pore-forming activity have been identified. Here, we review recent studies in the field that have advanced our knowledge of the structure and function of this expanding family of viral proteins.

Dimerization of the transmembrane domain of human tetherin in membrane mimetic environments

Tetherin/Bst-2 is a cell surface protein that can act as a restriction factor against a number of enveloped viruses, including HIV-1. It acts by tethering new virus particles to the host cell membrane, promoting their internalization and degradation. Tetherin is a type II membrane protein, with an N-terminal transmembrane domain, an extracellular coiled-coil domain, and a C-terminal GPI anchor. This double membrane anchor is important for anti-HIV activity, as is dimerization of the coiled-coil domain, but despite recent crystal structures of the coiled-coil ectodomains of human and mouse tetherin, the topology of tetherin with respect to host and viral membranes has yet to be determined. The tetherin transmembrane domain is also thought to mediate interactions with the HIV-1 encoded integral membrane protein Vpu, which is an antagonist of tetherin, through direct binding to the transmembrane region of Vpu. Using a combination of SDS-PAGE, size exclusion chromatography, and pyrene excimer fluorescence, we show that in the absence of the coiled-coil domain the transmembrane domain of human tetherin forms parallel homodimers in membrane mimetic environments. Transmembrane domain dimerization does not require disulfide bond formation and is favored in TFE, SDS micelles, and POPC liposomes. This observation has implications for functional models of tetherin, suggesting that both transmembrane domains in the dimeric molecule are inserted into the same lipid bilayer, rather than into opposing membranes.

Protein intrinsic disorder as a flexible armor and a weapon of HIV-1

Many proteins and protein regions are disordered in their native, biologically active states. These proteins/regions are abundant in different organisms and carry out important biological functions that complement the functional repertoire of ordered proteins. Viruses, with their highly compact genomes, small proteomes, and high adaptability for fast change in their biological and physical environment utilize many of the advantages of intrinsic disorder. In fact, viral proteins are generally rich in intrinsic disorder, and intrinsically disordered regions are commonly used by viruses to invade the host organisms, to hijack various host systems, and to help viruses in accommodation to their hostile habitats and to manage their economic usage of genetic material. In this review, we focus on the structural peculiarities of HIV-1 proteins, on the abundance of intrinsic disorder in viral proteins, and on the role of intrinsic disorder in their functions.

Mechanism of function of viral channel proteins and implications for drug development

Viral channel-forming proteins comprise a class of viral proteins which, similar to their host companions, are made to alter electrochemical or substrate gradients across lipid membranes. These proteins are active during all stages of the cellular life cycle of viruses. An increasing number of proteins are identified as channel proteins, but the precise role in the viral life cycle is yet unknown for the majority of them. This review presents an overview about these proteins with an emphasis on those with available structural information. A concept is introduced which aligns the transmembrane domains of viral channel proteins with those of host channels and toxins to give insights into the mechanism of function of the viral proteins from potential sequence identities. A summary of to date investigations on drugs targeting these proteins is given and discussed in respect of their mode of action in vivo.

Antibody-mediated enhancement of HIV-1 and HIV-2 production from BST-2/tetherin-positive cells

BST-2/CD317/HM1.24/tetherin is a B-cell antigen overexpressed on the surface of myeloma cell lines and on neoplastic plasma cells of patients with multiple myeloma. Antibodies to BST-2 are in clinical trial for the treatment of multiple myeloma and are considered for the treatment of solid tumors with high BST-2 antigen levels. Functionally, BST-2 restricts the secretion of retroviruses, including human immunodeficiency virus type 1, as well as members of the herpesvirus, filovirus, and arenavirus families, presumably by tethering nascent virions to the cell surface. Here we report that BST-2 antibody treatment facilitates virus release from BST-2(+) cells by interfering with the tethering activity of BST-2. BST-2 antibodies were unable to release already tethered virions and were most effective when added early during virus production. BST-2 antibody treatment did not affect BST-2 dimerization and did not reduce the cell surface expression of BST-2. Interestingly, BST-2 antibody treatment reduced the nonspecific shedding of BST-2 and limited the encapsidation of BST-2 into virions. Finally, flotation analyses indicate that BST-2 antibodies affect the distribution of BST-2 within membrane rafts. Our data suggest that BST-2 antibody treatment may enhance virus release by inducing a redistribution of BST-2 at the cell surface, thus preventing it from accumulating at the sites of virus budding.

New insights into HIV assembly and trafficking

Assembly and release of human immunodeficiency virus type 1 (HIV-1) particles is mediated by the viral Gag polyprotein precursor. Gag is synthesized in the cytosol and rapidly translocates to membrane to orchestrate particle production. The cell biology of HIV-1 Gag trafficking is currently one of the least understood aspects of HIV-1 replication. In this review, we highlight the current understanding of the cellular machinery involved in Gag trafficking and virus assembly.

Some human immunodeficiency virus type 1 Vpu proteins are able to antagonize macaque BST-2 in vitro and in vivo: Vpu-negative simian-human immunodeficiency viruses are attenuated in vivo

Human immunodeficiency virus type 1 (HIV-1) Vpu enhances the release of viral particles from infected cells by targeting BST-2/tetherin, a cellular protein inhibiting virus release. The widely used HIV-1(NL4-3) Vpu functionally inactivates human BST-2 but not murine or monkey BST-2, leading to the notion that Vpu antagonism is species specific. Here we investigated the properties of the CXCR4-tropic simian-human immunodeficiency virus DH12 (SHIV(DH12)) and the CCR5-tropic SHIV(AD8), each of which carries vpu genes derived from different primary HIV-1 isolates. We found that virion release from infected rhesus peripheral blood mononuclear cells was enhanced to various degrees by the Vpu present in both SHIVs. Transfer of the SHIV(DH12) Vpu transmembrane domain to the HIV-1(NL4-3) Vpu conferred antagonizing activity against macaque BST-2. Inactivation of the SHIV(DH12) and SHIV(AD8) vpu genes impaired virus replication in 6 of 8 inoculated rhesus macaques, resulting in lower plasma viral RNA loads, slower losses of CD4(+) T cells, and delayed disease progression. The expanded host range of the SHIV(DH12) Vpu was not due to adaptation during passage in macaques but was an intrinsic property of the parental HIV-1(DH12) Vpu protein. These results demonstrate that the species-specific inhibition of BST-2 by HIV-1(NL4-3) Vpu is not characteristic of all HIV-1 Vpu proteins; some HIV-1 isolates encode a Vpu with a broader host range.

The role of BST2/tetherin in feline retrovirus infection

Pathogenic retroviral infections of mammals have induced the evolution of cellular anti-viral restriction factors and have shaped their biological activities. This intrinsic immunity plays an important role in controlling viral replication and imposes a barrier to viral cross-species transmission. Well-studied examples of such host restriction factors are TRIM5α, an E3 ubiquitin ligase that binds incoming retroviral capsids in the cytoplasm via its C-terminal PRY/SPRY (B30.2) domain and targets them for proteasomal degradation, and APOBEC3 proteins, cytidine deaminases that induce hypermutation and impair viral reverse transcription. Tetherin (BST-2, CD317) is an interferon-inducible transmembrane protein that potently inhibits the release of nascent retrovirus particles in single-cycle replication assays. However, whether the primary biological activity of tetherin in vivo is that of a restriction factor remains uncertain as recent studies on human tetherin suggest that it is unable to prevent spreading infection of human immunodeficiency virus type 1 (HIV-1). The feline tetherin homologue resembles human tetherin in amino acid sequence, protein topology and anti-viral activity. Transiently expressed feline tetherin displays potent inhibition of feline immunodeficiency virus (FIV) and HIV-1 particle release. However, stable ectopic expression of feline tetherin in a range of feline cell lines has no inhibitory effect on the growth of either primary or cell culture-adapted strains of FIV. By comparing and contrasting the activities of the felid and primate tetherins against their respective immunodeficiency-causing lentiviruses we may gain insight into the contribution of tetherins to the control of lentiviral replication and the evolution of lentiviral virulence.

Polarity changes in the transmembrane domain core of HIV-1 Vpu inhibits its anti-tetherin activity

Tetherin (BST-2/CD317) is an interferon-inducible antiviral protein that restricts the release of enveloped viruses from infected cells. The HIV-1 accessory protein Vpu can efficiently antagonize this restriction. In this study, we analyzed mutations of the transmembrane (TM) domain of Vpu, including deletions and substitutions, to delineate amino acids important for HIV-1 viral particle release and in interactions with tetherin. The mutants had similar subcellular localization patterns with that of wild-type Vpu and were functional with respect to CD4 downregulation. We showed that the hydrophobic binding surface for tetherin lies in the core of the Vpu TM domain. Three consecutive hydrophobic isoleucine residues in the middle region of the Vpu TM domain, I15, I16 and I17, were important for stabilizing the tetherin binding interface and determining its sensitivity to tetherin. Changing the polarity of the amino acids at these positions resulted in severe impairment of Vpu-induced tetherin targeting and antagonism. Taken together, these data reveal a model of specific hydrophobic interactions between Vpu and tetherin, which can be potentially targeted in the development of novel anti-HIV-1 drugs.

Infection by agnoprotein-negative mutants of polyomavirus JC and SV40 results in the release of virions that are mostly deficient in DNA content

BACKGROUND

Human polyomavirus JC (JCV) is the etiologic agent of a brain disease, known as progressive multifocal leukoencephalopathy (PML). The JCV genome encodes a small multifunctional phospho-protein, agnoprotein, from the late coding region of the virus, whose regulatory functions in viral replication cycle remain elusive. In this work, the functional role of JCV and SV40 agnoproteins in virion release was investigated using a point mutant (Pt) of each virus, where the ATG codon of agnoprotein was mutated to abrogate its expression.

RESULTS

Analysis of both viral protein expression and replication using Pt mutant of each virus revealed that both processes were substantially down-regulated in the absence of agnoprotein compared to wild-type (WT) virus. Complementation studies in cells, which are constitutively expressing JCV agnoprotein and transfected with the JCV Pt mutant genome, showed an elevation in the level of viral DNA replication near to that observed for WT. Constitutive expression of large T antigen was found to be not sufficient to compensate the loss of agnoprotein for efficient replication of neither JCV nor SV40 in vivo. Examination of the viral release process for both JCV and SV40 Pt mutants showed that viral particles are efficiently released from the infected cells in the absence of agnoprotein but were found to be mostly deficient in viral DNA content.

CONCLUSIONS

The results of this study provide evidence that agnoprotein plays an important role in the polyomavirus JC and SV40 life cycle. Infection by agnoprotein-negative mutants of both viruses results in the release of virions that are mostly deficient in DNA content.

Antiviral inhibition of enveloped virus release by tetherin/BST-2: action and counteraction

Tetherin (BST2/CD317) has been recently recognized as a potent interferon-induced antiviral molecule that inhibits the release of diverse mammalian enveloped virus particles from infected cells. By targeting an immutable structure common to all these viruses, the virion membrane, evasion of this antiviral mechanism has necessitated the development of specific countermeasures that directly inhibit tetherin activity. Here we review our current understanding of the molecular basis of tetherin's mode of action, the viral countermeasures that antagonize it, and how virus/tetherin interactions may affect viral transmission and pathogenicity.

Identification of Residues in the BST-2 TM Domain Important for Antagonism by HIV-1 Vpu Using a Gain-of-Function Approach

The HIV-1 Vpu protein enhances the release of viral particles from the cell-surface in a cell-type specific manner. In the absence of Vpu, nascent virions remain tethered to the cell-surface in restricted cell-types. Recently, the human host factor BST-2/CD317/tetherin was found to be responsible for the inhibition of virus release. It was also reported that HIV-1 Vpu can target human BST-2 but is unable to interfere with the function of murine or simian BST-2. We performed a gain-of-function study to determine which of the differences between human and rhesus BST-2 account for the differential sensitivity to Vpu. We transferred human BST-2 sequences into rhesus BST-2 and assessed the resulting chimeras for inhibition of HIV-1 virus release and sensitivity to Vpu. We found that rhesus BST-2 carrying the transmembrane (TM) domain of human BST-2 is susceptible to HIV-1 Vpu. Finally, a single-amino-acid change in the rhesus BST-2 TM domain was sufficient to confer Vpu sensitivity.

Insights into cellular factors that regulate HIV-1 replication in human cells

Retroviruses integrate into the host cell's chromosome. Accordingly, many aspects of the life cycle of retroviruses like HIV-1 are intimately linked to the functions of cellular proteins and RNAs. In this review, we discuss in brief recent genomewide screens for the identification of cellular proteins that assist HIV-1 replication in human cells. We also review findings for other cellular moieties that help or restrict the viral life cycle.

Cell-to-cell transmission of retroviruses: Innate immunity and interferon-induced restriction factors

It has been known for some time that retroviruses can disseminate between immune cells either by conventional cell-free transmission or by directed cell-to-cell spread. Over the past few years there has been increasing interest in how retroviruses may use cell-to-cell spread to promote more rapid infection kinetics and circumvent humoral immunity. Effective humoral immune responses are intimately linked with innate immunity and the interplay between retroviruses and innate immunity is a rapidly expanding area of research that has been advanced considerably by the identification of cellular restriction factors that provide barriers to retroviral infection. The effect of innate immunity and restriction factors on retroviral cell-to-cell spread has been comparatively little studied; however recent work suggests this maybe changing. Here I will review some recent advances in what is a budding area of retroviral research.

Budding capability of the influenza virus neuraminidase can be modulated by tetherin

We have determined that, in addition to its receptor-destroying activity, the influenza virus neuraminidase is capable of efficiently forming virus-like particles (VLPs) when expressed individually from plasmid DNA. This observation applies to both human subtypes of neuraminidase, N1 and N2. However, it is not found with every strain of influenza virus. Through gain-of-function and loss-of-function analyses, a critical determinant within the neuraminidase ectodomain was identified that contributes to VLP formation but is not sufficient to accomplish release of plasmid-derived VLPs. This sequence lies on the plasma membrane-proximal side of the neuraminidase globular head. Most importantly, we demonstrate that the antiviral restriction factor tetherin plays a role in determining the strain-specific limitations of release competency. If tetherin is counteracted by small interfering RNA knockdown or expression of the HIV anti-tetherin factor vpu, budding and release capability is bestowed upon an otherwise budding-deficient neuraminidase. These data suggest that budding-competent neuraminidase proteins possess an as-yet-unidentified means of counteracting the antiviral restriction factor tetherin and identify a novel way in which the influenza virus neuraminidase can contribute to virus release.