β-TrCP is dispensable for Vpu's ability to overcome the CD317/Tetherin-imposed restriction to HIV-1 release

BackgroundThe cellular transmembrane protein CD317/BST-2/HM1.24/Tetherin restricts HIV-1 infection by physically tethering mature virions to the surface of infected cells. HIV-1 counteracts this restriction by expressing the accessory protein Vpu, yet the mechanism of this antagonism is incompletely understood. β-TrCP is the substrate recognition domain of an E3 ubiquitin ligase complex that interacts with the di-serine motif S52/S56 in the cytoplasmic tail of Vpu to target the CD4 receptor for proteasomal degradation. Recently, it has been suggested that β-TrCP is also critically involved in Vpu's ability to overcome the CD317-mediated virion release block.ResultsTo test this model, we analyzed the consequences of several experimental strategies to interfere with the Vpu-β-TrCP protein-protein interaction. Under these conditions, we studied effects of Vpu on expression and localization of CD317 and CD4, as well as on its ability to promote HIV-1 release. Our results demonstrate a strict requirement for Vpu's di-serine motif for degradation of CD4 and also CD317, reduction of cell surface exposure of CD317, and HIV-1 release enhancement. We further show a critical role of β-TrCP2, but not of the structurally related β-TrCP1 isoform, for Vpu-mediated degradation of both receptors. Most importantly, Vpu remained active in downregulating CD317 from the cell surface and in overcoming the HIV-1 release restriction in β-TrCP-depleted cells.ConclusionsThese results demonstrate that β-TrCP is not strictly required for Vpu's ability to counteract the CD317-imposed virion release block and support the relevance of cell surface down-modulation of the restriction factor as a central mechanism of Vpu antagonism. Moreover, we propose the existence of a critical, yet to be identified cellular factor that interacts with Vpu via its di-serine motif to alter the trafficking of the restriction factor.

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title: Broad-Spectrum Inhibition of Retroviral and Filoviral Particle Release by Tetherin
abstract: The expression of many putative antiviral genes is upregulated when cells encounter type I interferon (IFN), but the actual mechanisms by which many IFN-induced gene products inhibit virus replication are poorly understood. A recently identified IFN-induced antiretroviral protein, termed tetherin (previously known as BST-2 or CD317), blocks the release of nascent human immunodeficiency virus type 1 (HIV-1) particles from infected cells, and an HIV-1 accessory protein, Vpu, acts as a viral antagonist of tetherin. Here, we show that tetherin is capable of blocking not only the release of HIV-1 particles but also the release of particles assembled using the major structural proteins of a variety of prototype retroviruses, including members of the alpharetrovirus, betaretrovirus, deltaretrovirus, lentivirus, and spumaretrovirus families. Moreover, we show that the release of particles assembled using filovirus matrix proteins from Marburg virus and Ebola virus is also sensitive to inhibition by tetherin. These findings indicate that tetherin is a broadly specific inhibitor of enveloped particle release, and therefore, inhibition is unlikely to require specific interactions with viral proteins. Nonetheless, tetherin colocalized with nascent virus-like particles generated by several retroviral and filoviral structural proteins, indicating that it is present at, or recruited to, sites of particle assembly. Overall, tetherin is potentially active against many enveloped viruses and likely to be an important component of the antiviral innate immune defense.

title: Tetherin inhibits retrovirus release and is antagonized by HIV-1 Vpu
abstract: Human cells possess an antiviral activity that inhibits the release of retrovirus particles, and other enveloped virus particles, and is antagonized by the HIV-1 accessory protein, Vpu. This antiviral activity can be constitutively expressed or induced by interferon-α, and it consists of protein-based tethers, which we term ‘tetherins’, that cause retention of fully formed virions on infected cell surfaces. Using deductive constraints and gene expression analyses, we identify CD317 (also called BST2 or HM1.24), a membrane protein of previously unknown function, as a tetherin. Specifically, CD317 expression correlated with, and induced, a requirement for Vpu during HIV-1 and murine leukaemia virus particle release. Furthermore, in cells where HIV-1 virion release requires Vpu expression, depletion of CD317 abolished this requirement. CD317 caused retention of virions on cell surfaces and, after endocytosis, in CD317-positive compartments. Vpu co-localized with CD317 and inhibited these effects. Inhibition of Vpu function and consequent mobilization of tetherin’s antiviral activity is a potential therapeutic strategy in HIV/AIDS.

title: Inhibition of Lassa and Marburg Virus Production by Tetherin
abstract: Recently, tetherin has been identified as an effective cellular factor that prevents the release of human immunodeficiency virus type 1. Here, we show that the production of virus-like particles induced by viral matrix proteins of Lassa virus or Marburg virus was markedly inhibited by tetherin and that N-linked glycosylation of tetherin was dispensable for this antiviral activity. Our data also suggest that viral matrix proteins or one or more components that originate from host cells are targets of tetherin but that viral surface glycoproteins are not. These results suggest that tetherin inhibits the release of a wide variety of enveloped viruses from host cells by a common mechanism.

title: Tetherin-mediated restriction of filovirus budding is antagonized by the Ebola glycoprotein
abstract: Mammalian cells employ numerous innate cellular mechanisms to inhibit viral replication and spread. Tetherin, also known as Bst-2 or CD317, is a recently identified, IFN-induced, cellular response factor that blocks release of HIV-1 and other retroviruses from infected cells. The means by which tetherin retains retroviruses on the cell surface, as well as the mechanism used by the HIV-1 accessory protein Vpu to antagonize tetherin function and promote HIV-1 release, are unknown. Here, we document that tetherin functions as a broadly acting antiviral factor by demonstrating that both human and murine tetherin potently inhibit the release of the filovirus, Ebola, from the surface of cells. Expression of the Ebola glycoprotein (GP) antagonized the antiviral effect of human and murine tetherin and facilitated budding of Ebola particles, as did the HIV-1 Vpu protein. Conversely, Ebola GP could substitute for Vpu to promote HIV-1 virion release from tetherin-expressing cells, demonstrating a common cellular target for these divergent viral proteins. Ebola GP efficiently coimmunoprecipitated with tetherin, suggesting that the viral glycoprotein directly interferes with this host antiviral factor. These results demonstrate that tetherin is a cellular antiviral factor that restricts budding of structurally diverse enveloped viruses. Additionally, Ebola has evolved a highly effective strategy to combat this antiviral response elicited in the host during infection.

title: Molecular Mechanism of BST2/Tetherin Downregulation by K5/MIR2 of Kaposi's Sarcoma-Associated Herpesvirus
abstract: are viral members of the membrane-associated RING-CH (MARCH) ubiquitin ligase family and contribute to viral immune evasion by directing the conjugation of ubiquitin to immunostimulatory transmembrane proteins. In a quantitative proteomic screen for novel host cell proteins downregulated by viral immunomodulators, we previously observed that K5, as well as the human immunodeficiency virus type 1 (HIV-1) immunomodulator VPU, reduced steady-state levels of bone marrow stromal cell antigen 2 (BST2; also called CD317 or tetherin), suggesting that BST2 might be a novel substrate of K5 and VPU. Recent work revealed that in the absence of VPU, HIV-1 virions are tethered to the plasma membrane in BST2-expressing HeLa cells. By targeting BST2, K5 might thus similarly overcome an innate antiviral host defense mechanism. Here we establish that despite its type II transmembrane topology and carboxy-terminal glycosylphosphatidylinositol (GPI) anchor, BST2 represents a bona fide target of K5 that is downregulated during primary infection by and reactivation of KSHV. Upon exit of the protein from the endoplasmic reticulum, lysines in the short amino-terminal domain of BST2 are ubiquitinated by K5, resulting in rapid degradation of BST2. Ubiquitination of BST2 is required for degradation, since BST2 lacking cytosolic lysines was K5 resistant and ubiquitin depletion by proteasome inhibitors restored BST2 surface expression. Thus, BST2 represents the first type II transmembrane protein targeted by K5 and the first example of a protein that is both ubiquitinated and GPI linked. We further demonstrate that KSHV release is decreased in the absence of K5 in a BST2-dependent manner, suggesting that K5 contributes to the evasion of intracellular antiviral defense programs.

title: The RING-CH Ligase K5 Antagonizes Restriction of KSHV and HIV-1 Particle Release by Mediating Ubiquitin-Dependent Endosomal Degradation of Tetherin
abstract: Tetherin (CD317/BST2) is an interferon-induced membrane protein that inhibits the release of diverse enveloped viral particles. Several mammalian viruses have evolved countermeasures that inactivate tetherin, with the prototype being the HIV-1 Vpu protein. Here we show that the human herpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV) is sensitive to tetherin restriction and its activity is counteracted by the KSHV encoded RING-CH E3 ubiquitin ligase K5. Tetherin expression in KSHV-infected cells inhibits viral particle release, as does depletion of K5 protein using RNA interference. K5 induces a species-specific downregulation of human tetherin from the cell surface followed by its endosomal degradation. We show that K5 targets a single lysine (K18) in the cytoplasmic tail of tetherin for ubiquitination, leading to relocalization of tetherin to CD63-positive endosomal compartments. Tetherin degradation is dependent on ESCRT-mediated endosomal sorting, but does not require a tyrosine-based sorting signal in the tetherin cytoplasmic tail. Importantly, we also show that the ability of K5 to substitute for Vpu in HIV-1 release is entirely dependent on K18 and the RING-CH domain of K5. By contrast, while Vpu induces ubiquitination of tetherin cytoplasmic tail lysine residues, mutation of these positions has no effect on its antagonism of tetherin function, and residual tetherin is associated with the trans-Golgi network (TGN) in Vpu-expressing cells. Taken together our results demonstrate that K5 is a mechanistically distinct viral countermeasure to tetherin-mediated restriction, and that herpesvirus particle release is sensitive to this mode of antiviral inhibition.

title: Infectious Lassa Virus, but Not Filoviruses, Is Restricted by BST-2/Tetherin
abstract: Bone marrow stromal antigen 2 (BST-2/tetherin) is a cellular membrane protein that inhibits the release of HIV-1. We show for the first time, using infectious viruses, that BST-2 also inhibits egress of arenaviruses but has no effect on filovirus replication and spread. Specifically, infectious Lassa virus (LASV) release significantly decreased or increased in human cells in which BST-2 was either stably expressed or knocked down, respectively. In contrast, replication and spread of infectious Zaire ebolavirus (ZEBOV) and Lake Victoria marburgvirus (MARV) were not affected by these conditions. Replication of infectious Rift Valley fever virus (RVFV) and cowpox virus (CPXV) was also not affected by BST-2 expression. Elevated cellular levels of human or murine BST-2 inhibited the release of virus-like particles (VLPs) consisting of the matrix proteins of multiple highly virulent NIAID Priority Pathogens, including arenaviruses (LASV and Machupo virus [MACV]), filoviruses (ZEBOV and MARV), and paramyxoviruses (Nipah virus). Although the glycoproteins of filoviruses counteracted the antiviral activity of BST-2 in the context of VLPs, they could not rescue arenaviral (LASV and MACV) VLP release upon BST-2 overexpression. Furthermore, we did not observe colocalization of filoviral glycoproteins with BST-2 during infection with authentic viruses. None of the arenavirus-encoded proteins rescued budding of VLPs in the presence of BST-2. Our results demonstrate that BST-2 might be a broad antiviral factor with the ability to restrict release of a wide variety of human pathogens. However, at least filoviruses, RVFV, and CPXV are immune to its inhibitory effect.

title: Interferon-Induced Cell Membrane Proteins, IFITM3 and Tetherin, Inhibit Vesicular Stomatitis Virus Infection via Distinct Mechanisms
abstract: Tetherin and IFITM3 are recently identified interferon-induced cellular proteins that restrict infections by retroviruses and filoviruses and of influenza virus and flaviviruses, respectively. In our efforts to further explore their antiviral activities against other viruses and determine their antiviral mechanisms, we found that the two antiviral proteins potently inhibit the infection of vesicular stomatitis virus (VSV), a prototype member of the Rhabdoviridae family. Taking advantage of this well-studied virus infection system, we show that although both tetherin and IFITM3 are plasma membrane proteins, tetherin inhibits virion particle release from infected cells, while IFITM3 disrupts an early event after endocytosis of virion particles but before primary transcription of incoming viral genomes. Furthermore, we demonstrate that both the N-terminal 21 amino acid residues and C-terminal transmembrane region of IFITM3 are required for its antiviral activity. Collectively, our work sheds light on the mechanisms by which tetherin and IFITM3 restrict infection with rhabdoviruses and possibly other pathogenic viruses.

title: The interferon-induced protein BST-2 restricts HIV-1 release and is downregulated from the cell surface by the viral Vpu protein.
abstract: The HIV-1 accessory protein Vpu counteracts a host factor that restricts virion release from infected cells. Here we show that the interferon-induced cellular protein BST-2/HM1.24/CD317 is such a factor. BST-2 is downregulated from the cell surface by Vpu, and BST-2 is specifically expressed in cells that support the vpu phenotype. Exogenous expression of BST-2 inhibits HIV-1 virion release, while suppression of BST-2 relieves the requirement for Vpu. Downregulation of BST-2 requires both the transmembrane/ion channel domain and conserved serines in the cytoplasmic domain of Vpu. Endogenous BST-2 colocalizes with the HIV-1 structural protein Gag in endosomes and at the plasma membrane, suggesting that BST-2 traps virions within and on infected cells. The unusual structure of BST-2, which includes a transmembrane domain and a lumenal GPI anchor, may allow it to retain nascent enveloped virions on cellular membranes, providing a mechanism of viral restriction counteracted by a specific viral accessory protein.

title: Tetherin Inhibits HIV-1 Release by Directly Tethering Virions to Cells
abstract: Tetherin is an interferon-induced protein whose expression blocks the release of HIV-1 and other enveloped viral particles. The underlying mechanism by which tetherin functions and whether it directly or indirectly causes virion retention are unknown. Here, we elucidate the mechanism by which tetherin exerts its antiviral activity. We demonstrate, through mutational analyses and domain replacement experiments, that tetherin configuration rather than primary sequence is critical for antiviral activity. These findings allowed the design of a completely artificial protein, lacking sequence homology with native tetherin, that nevertheless mimicked its antiviral activity. We further show that tetherin is incorporated into HIV-1 particles as a parallel homodimer using either of its two membrane anchors. These results indicate that tetherin functions autonomously and directly and that infiltration of virion envelopes by one or both of tetherin's membrane anchors is necessary, and likely sufficient, to tether enveloped virus particles that bud through the plasma membrane.

title: Quantitative Membrane Proteomics Reveals New Cellular Targets of Viral Immune Modulators
abstract: Immunomodulators of pathogens frequently affect multiple cellular targets, thus preventing recognition by different immune cells. For instance, the K5 modulator of immune recognition (MIR2) from Kaposi sarcoma–associated herpesvirus prevents activation of cytotoxic T cells, natural killer cells, and natural killer T cells by downregulating major histocompatibility complex (MHC) class I molecules, the MHC-like molecule CD1, the cell adhesion molecules ICAM-1 and PECAM, and the co-stimulatory molecule B7.2. K5 belongs to a family of viral- and cellular-membrane-spanning RING ubiquitin ligases. While a limited number of transmembrane proteins have been shown to be targeted for degradation by this family, it is unknown whether additional targets exist. We now describe a quantitative proteomics approach to identify novel targets of this protein family. Using stable isotope labeling by amino acids, we compared the proteome of plasma, Golgi, and endoplasmic reticulum membranes in the presence and absence of K5. Mass spectrometric protein identification revealed four proteins that were consistently underrepresented in the plasma membrane of K5 expression cells: MHC I (as expected), bone marrow stromal antigen 2 (BST-2, CD316), activated leukocyte cell adhesion molecule (ALCAM, CD166) and Syntaxin-4. Downregulation of each of these proteins was independently confirmed by immunoblotting with specific antibodies. We further demonstrate that ALCAM is a bona fide target of both K5 and the myxomavirus homolog M153R. Upon exiting the endoplasmic reticulum, ALCAM is ubiquitinated in the presence of wild-type, but not RING-deficient or acidic motif–deficient, K5, and is targeted for lysosomal degradation via the multivesicular body pathway. Since ALCAM is the ligand for CD6, a member of the immunological synapse of T cells, its removal by viral immune modulators implies a role for CD6 in the recognition of pathogens by T cells. The unbiased global proteome analysis therefore revealed novel immunomodulatory functions of pathogen proteins.

title: HIV-1 antagonism of CD317 is species specific and involves Vpu-mediated proteasomal degradation of the restriction factor.
abstract: Mammals encode proteins that inhibit viral replication at the cellular level. In turn, certain viruses have evolved genes that can functionally counteract these intrinsic restrictions. Human CD317 (BST-2/HM1.24/tetherin) is a restriction factor that blocks release of human immunodeficiency virus type 1 (HIV-1) from the cell surface and can be overcome by HIV-1 Vpu. Here, we show that mouse and rat CD317 potently inhibit HIV-1 release but are resistant to Vpu. Interspecies chimeras reveal that the rodent-specific resistance and human-specific sensitivity to Vpu antagonism involve all three major structural domains of CD317. To promote virus release, Vpu depletes cellular pools of human CD317, but not of the rodent orthologs, by accelerating its degradation via the 20S proteasome. Thus, HIV-1 Vpu suppresses the expression of the CD317 antiviral factor in human cells, and the species-specific resistance to this suppression may guide the development of small animal models of HIV infection.

title: Mutation of a Single Residue Renders Human Tetherin Resistant to HIV-1 Vpu-Mediated Depletion
abstract: The recently identified restriction factor tetherin/BST-2/CD317 is an interferon-inducible trans-membrane protein that restricts HIV-1 particle release in the absence of the HIV-1 countermeasure viral protein U (Vpu). It is known that Tantalus monkey CV1 cells can be rendered non-permissive to HIV-1 release upon stimulation with type 1 interferon, despite the presence of Vpu, suggesting species-specific sensitivity of tetherin proteins to viral countermeasures such as Vpu. Here we demonstrate that Tantalus monkey tetherin restricts HIV-1 by nearly two orders of magnitude, but in contrast to human tetherin the Tantalus protein is insensitive to HIV-1 Vpu. We have investigated tetherin's sensitivity to Vpu using positive selection analyses, seeking evidence for evolutionary conflict between tetherin and viral countermeasures. We provide evidence that tetherin has undergone positive selection during primate evolution. Mutation of a single amino acid (showing evidence of positive selection) in the trans-membrane cap of human tetherin to that in Tantalus monkey (T45I) substantially impacts on sensitivity to HIV-1 Vpu, but not on antiviral activity. Finally, we provide evidence that cellular steady state levels of tetherin are substantially reduced by Vpu, and that the T45I mutation abrogates this effect. This study provides evidence that tetherin is important in protecting mammals against viral infection, and that the HIV-1 Vpu–mediated countermeasure is specifically adapted to act against human tetherin. It also emphasizes the power of selection analyses to illuminate the molecular details of host–virus interactions. This work suggests that tetherin binding agents might protect it from viral encoded countermeasures and thus make powerful antivirals.

title: HIV-1 Vpu Neutralizes the Antiviral Factor Tetherin/BST-2 by Binding It and Directing Its Beta-TrCP2-Dependent Degradation
abstract: Host cells impose a broad range of obstacles to the replication of retroviruses. Tetherin (also known as CD317, BST-2 or HM1.24) impedes viral release by retaining newly budded HIV-1 virions on the surface of cells. HIV-1 Vpu efficiently counteracts this restriction. Here, we show that HIV-1 Vpu induces the depletion of tetherin from cells. We demonstrate that this phenomenon correlates with the ability of Vpu to counteract the antiviral activity of both overexpressed and interferon-induced endogenous tetherin. In addition, we show that Vpu co-immunoprecipitates with tetherin and β-TrCP in a tri-molecular complex. This interaction leads to Vpu-mediated proteasomal degradation of tetherin in a β-TrCP2-dependent manner. Accordingly, in conditions where Vpu-β-TrCP2-tetherin interplay was not operative, including cells stably knocked down for β-TrCP2 expression or cells expressing a dominant negative form of β-TrCP, the ability of Vpu to antagonize the antiviral activity of tetherin was severely impaired. Nevertheless, tetherin degradation did not account for the totality of Vpu-mediated counteraction against the antiviral factor, as binding of Vpu to tetherin was sufficient for a partial relief of the restriction. Finally, we show that the mechanism used by Vpu to induce tetherin depletion implicates the cellular ER-associated degradation (ERAD) pathway, which mediates the dislocation of ER membrane proteins into the cytosol for subsequent proteasomal degradation. In conclusion, we show that Vpu interacts with tetherin to direct its β-TrCP2-dependent proteasomal degradation, thereby alleviating the blockade to the release of infectious virions. Identification of tetherin binding to Vpu provides a potential novel target for the development of drugs aimed at inhibiting HIV-1 replication.

title: Vpu Antagonizes BST-2–Mediated Restriction of HIV-1 Release via β-TrCP and Endo-Lysosomal Trafficking
abstract: The interferon-induced transmembrane protein BST-2/CD317 (tetherin) restricts the release of diverse enveloped viruses from infected cells. The HIV-1 accessory protein Vpu antagonizes this restriction by an unknown mechanism that likely involves the down-regulation of BST-2 from the cell surface. Here, we show that the optimal removal of BST-2 from the plasma membrane by Vpu requires the cellular protein β-TrCP, a substrate adaptor for a multi-subunit SCF E3 ubiquitin ligase complex and a known Vpu-interacting protein. β-TrCP is also required for the optimal enhancement of virion-release by Vpu. Mutations in the DSGxxS β-TrCP binding-motif of Vpu impair both the down-regulation of BST-2 and the enhancement of virion-release. Such mutations also confer dominant-negative activity, consistent with a model in which Vpu links BST-2 to β-TrCP. Optimal down-regulation of BST-2 from the cell surface by Vpu also requires the endocytic clathrin adaptor AP-2, although the rate of endocytosis is not increased; these data suggest that Vpu induces post-endocytic membrane trafficking events whose net effect is the removal of BST-2 from the cell surface. In addition to its marked effect on cell-surface levels, Vpu modestly decreases the total cellular levels of BST-2. The decreases in cell-surface and intracellular BST-2 are inhibited by bafilomycin A1, an inhibitor of endosomal acidification; these data suggest that Vpu induces late endosomal targeting and partial degradation of BST-2 in lysosomes. The Vpu-mediated decrease in surface expression is associated with reduced co-localization of BST-2 and the virion protein Gag along the plasma membrane. Together, the data support a model in which Vpu co-opts the β-TrCP/SCF E3 ubiquitin ligase complex to induce endosomal trafficking events that remove BST-2 from its site of action as a virion-tethering factor.

title: HIV-1 Accessory Protein Vpu Internalizes Cell-surface BST-2/Tetherin through Transmembrane Interactions Leading to Lysosomes*
abstract: Bone marrow stromal antigen 2 (BST-2, also known as tetherin) is a recently identified interferon-inducible host restriction factor that can block the production of enveloped viruses by trapping virus particles at the cell surface. This antiviral effect is counteracted by the human immunodeficiency virus type 1 (HIV-1) accessory protein viral protein U (Vpu). Here we show that HIV-1 Vpu physically interacts with BST-2 through their mutual transmembrane domains and leads to the degradation of this host factor via a lysosomal, not proteasomal, pathway. The degradation is partially controlled by a cellular protein, β-transducin repeat-containing protein (βTrCP), which is known to be required for the Vpu-induced degradation of CD4. Importantly, targeting of BST-2 by Vpu occurs at the plasma membrane followed by the active internalization of this host protein by Vpu independently of constitutive endocytosis. Thus, the primary site of action of Vpu is the plasma membrane, where Vpu targets and internalizes cell-surface BST-2 through transmembrane interactions, leading to lysosomal degradation, partially in a βTrCP-dependent manner. Also, we propose the following configuration of BST-2 in tethering virions to the cell surface; each of the dimerized BST-2 molecules acts as a bridge between viral and cell membranes.

title: Antagonism of CD317 Restriction of Human Immunodeficiency Virus Type 1 (HIV-1) Particle Release and Depletion of CD317 Are Separable Activities of HIV-1 Vpu
abstract: Vpu antagonizes human immunodeficiency virus type 1 (HIV-1) particle release inhibition by CD317/BST-2/Tetherin. Whether this Vpu activity strictly requires cellular depletion of the restriction factor is unclear. Here, we characterized CD317 variants with mutations in putative sorting or ubiquitination motifs. All mutants still potently impaired release of Vpu-defective HIV-1 and remained sensitive to Vpu-mediated release enhancement. Importantly, this virological antagonism correlated with surface downregulation of CD317 mutants by Vpu, while intracellular pools of these mutants, which were consistently depleted of the wild-type protein, were highly variable or even enhanced. Thus, Vpu can efficiently antagonize virion tethering in the absence of CD317 degradation.

title: CD317/Tetherin Is Enriched in the HIV-1 Envelope and Downregulated from the Plasma Membrane upon Virus Infection
abstract: is a host factor that restricts the release of human immunodeficiency virus type 1 (HIV-1) by trapping virions at the plasma membrane of certain producer cells. It is antagonized by the HIV-1 accessory protein Vpu. Previous light microscopy studies localized CD317 to the plasma membrane and the endosomal compartment and showed Vpu induced downregulation. In the present study, we performed quantitative immunoelectron microscopy of CD317 in cells producing wild-type or Vpu-defective HIV-1 and in control cells. Double-labeling experiments revealed that CD317 localizes to the plasma membrane, to early and recycling endosomes, and to the trans-Golgi network. CD317 largely relocated to endosomes upon HIV-1 infection, and this effect was partly counteracted by Vpu. Unexpectedly, CD317 was enriched in the membrane of viral buds and cell-associated and cell-free viruses compared to the respective plasma membrane, and this enrichment was independent of Vpu. These results suggest that the tethering activity of CD317 critically depends on its density at the cell surface and appears to be less affected by its density in the virion membrane.

title: Human immunodeficiency virus type 1 Vpu protein regulates the formation of intracellular gp160-CD4 complexes
abstract: Intracellular transport and processing of the human immunodeficiency virus type 1 (HIV-1) envelope precursor glycoprotein, gp160, proceeds via the endoplasmic reticulum and Golgi complex and involves proteolytic processing of gp160 into the mature virion components, gp120 and gp41. We found that coexpression of gp160 and human CD4 in HeLa cells severely impaired gp120 production due to the formation of intracellular gp160-CD4 complexes. This CD4-mediated inhibition of gp160 processing was alleviated by coexpression of the HIV-1-encoded Vpu protein. The coexpression of Vpu and CD4 in the presence of gp160 resulted in increased degradation of CD4. Although the precise mechanism(s) responsible for the Vpu effect is presently unclear, our findings suggest that Vpu may destabilize intracellular gp160-CD4 complexes.

title: A novel human WD protein, h-beta TrCp, that interacts with HIV-1 Vpu connects CD4 to the ER degradation pathway through an F-box motif.
abstract: HIV-1 Vpu interacts with CD4 in the endoplasmic reticulum and triggers CD4 degradation, presumably by proteasomes. Human beta TrCP identified by interaction with Vpu connects CD4 to this proteolytic machinery, and CD4-Vpu-beta TrCP ternary complexes have been detected by coimmunoprecipitation. beta TrCP binding to Vpu and its recruitment to membranes require two phosphoserine residues in Vpu essential for CD4 degradation. In beta TrCP, WD repeats at the C terminus mediate binding to Vpu, and an F box near the N terminus is involved in interaction with Skp1p, a targeting factor for ubiquitin-mediated proteolysis. An F-box deletion mutant of beta TrCP had a dominant-negative effect on Vpu-mediated CD4 degradation. These data suggest that beta TrCP and Skp1p represent components of a novel ER-associated protein degradation pathway that mediates CD4 proteolysis.

title: The Nef Protein of Human Immunodeficiency Virus Establishes Superinfection Immunity by a Dual Strategy to Downregulate Cell-Surface CCR5 and CD4
abstract: Viruses frequently render cells refractory to subsequent infection with the same virus. This state of superinfection immunity counteracts potentially detrimental consequences for the infected cell and facilitates high-level replication and viral spread in the host.



Here, we show that human immunodeficiency virus (HIV) employs its early gene product Nef to efficiently interfere with superinfection at the viral-entry step. In this context, we identify the downregulation of cell-surface CCR5, the major HIV coreceptor, as a novel and highly conserved activity of Nef. Nef targets the CCR5 coreceptor and the HIV binding receptor CD4 via distinct cellular machineries to enhance the endocytosis rate of both HIV receptor components and to accelerate their degradation. Functionally, these genetically separable actions by Nef synergized to efficiently protect cells from HIV superinfection at the level of fusion of the viral envelope with the plasma membrane.



HIV has evolved two independent activities for Nef to downregulate the receptor complex and to facilitate its efficient replication and spread. This evasion strategy likely represents a mechanism by which the pathogenicity factor Nef elevates viral replication in vivo and thus promotes AIDS pathogenesis.

title: Control of meiotic and mitotic progression by the F box protein beta-Trcp1 in vivo.
abstract: SCF ubiquitin ligases, composed of three major subunits, Skp1, Cul1, and one of many F box proteins (Fbps), control the proteolysis of important cellular regulators. We have inactivated the gene encoding the Fbp beta-Trcp1 in mice. beta-Trcp1(-/-) males show reduced fertility correlating with an accumulation of methaphase I spermatocytes. beta-Trcp1(-/-) MEFs display a lengthened mitosis, centrosome overduplication, multipolar metaphase spindles, and misaligned chromosomes. Furthermore, cyclin A, cyclin B, and Emi1, an inhibitor of the anaphase promoting complex, are stabilized in mitotic beta-Trcp1(-/-) MEFs. Indeed, we demonstrate that Emi1 is a bona fide substrate of beta-Trcp1. In contrast, stabilization of beta-catenin and IkappaBalpha, two previously reported beta-Trcp1 substrates, does not occur in the absence of beta-Trcp1 and instead requires the additional silencing of beta-Trcp2 by siRNA. Thus, beta-Trcp1 regulates the timely order of meiotic and mitotic events.

title: Impaired degradation of inhibitory subunit of NF-kappa B (I kappa B) and beta-catenin as a result of targeted disruption of the beta-TrCP1 gene.
abstract: beta-TrCP1 (also known as Fbw1a or FWD1) is the F-box protein component of an Skp1/Cul1/F-box (SCF)-type ubiquitin ligase complex. Although biochemical studies have suggested that beta-TrCP1 targets inhibitory subunit of NF-kappa B(I kappa B) proteins and beta-catenin for ubiquitylation, the physiological role of beta-TrCP1 in mammals has remained unclear. We have now generated mice deficient in beta-TrCP1 and shown that the degradation of I kappa B alpha and I kappa B beta is reproducibly, but not completely, impaired in the cells of these animals. The nuclear translocation and DNA-binding activity of NF-kappa B as well as the ability of this transcription factor to activate a luciferase reporter gene were also inhibited in beta-TrCP1-/- cells compared with those apparent in wild-type cells. The subcellular localization of beta-catenin was altered markedly in beta-TrCP1-/- cells. Furthermore, the rate of proliferation was reduced and both cell size and the percentage of polyploid cells were increased in embryonic fibroblasts derived from beta-TrCP1-/- mice compared with the corresponding wild-type cells. These results suggest that beta-TrCP1 contributes to, but is not absolutely required for, the degradation of I kappa B and beta-catenin and the consequent regulation of the NF-kappa B and Wnt signaling pathways, respectively. In addition, they implicate beta-TrCP1 in the maintenance of ploidy during cell-cycle progression.

title: Silencing of both beta-TrCP1 and HOS (beta-TrCP2) is required to suppress human immunodeficiency virus type 1 Vpu-mediated CD4 down-modulation.
abstract: The human immunodeficiency virus type 1 (HIV-1) Vpu protein interacts with CD4 within the endoplasmic reticula of infected cells and targets CD4 for degradation through interaction with beta-TrCP1. Mammals possess a homologue of beta-TrCP1, HOS, which is also named beta-TrCP2. We show by coimmunoprecipitation experiments that beta-TrCP2 binds Vpu and is able to induce CD4 down-modulation as efficiently as beta-TrCP1. In two different cell lines, HeLa CD4+ and Jurkat, Vpu-mediated CD4 down-modulation could not be reversed through the individual silencing of endogenous beta-TrCP1 or beta-TrCP2 but instead required the two genes to be silenced simultaneously.

title: Vpu serine 52 dependent counteraction of tetherin is required for HIV-1 replication in macrophages, but not in ex vivo human lymphoid tissue
abstract: BackgroundThe human immunodeficiency virus type 1 (HIV-1) Vpu protein degrades CD4 and counteracts a restriction factor termed tetherin (CD317; Bst-2) to enhance virion release. It has been suggested that both functions can be genetically separated by mutation of a serine residue at position 52. However, recent data suggest that the S52 phosphorylation site is also important for the ability of Vpu to counteract tetherin. To clarify this issue, we performed a comprehensive analysis of HIV-1 with a mutated casein kinase-II phosphorylation site in Vpu in various cell lines, primary blood lymphocytes (PBL), monocyte-derived macrophages (MDM) and ex vivo human lymphoid tissue (HLT).ResultsWe show that mutation of serine 52 to alanine (S52A) entirely disrupts Vpu-mediated degradation of CD4 and strongly impairs its ability to antagonize tetherin. Furthermore, casein-kinase II inhibitors blocked the ability of Vpu to degrade tetherin. Overall, Vpu S52A could only overcome low levels of tetherin, and its activity decreased in a manner dependent on the amount of transiently or endogenously expressed tetherin. As a consequence, the S52A Vpu mutant virus was unable to replicate in macrophages, which express high levels of this restriction factor. In contrast, HIV-1 Vpu S52A caused CD4+ T-cell depletion and spread efficiently in ex vivo human lymphoid tissue and PBL, most likely because these cells express comparably low levels of tetherin.ConclusionOur data explain why the effect of the S52A mutation in Vpu on virus release is cell-type dependent and suggest that a reduced ability of Vpu to counteract tetherin impairs HIV-1 replication in macrophages, but not in tissue CD4+ T cells.

title: The two biological activities of human immunodeficiency virus type 1 Vpu protein involve two separable structural domains
abstract: The human immunodeficiency virus type 1 (HIV-1) Vpu protein is an integral membrane phosphoprotein that induces CD4 degradation in the endoplasmic reticulum and enhances virus release from the cell surface. CD4 degradation is specific, requires phosphorylation of Vpu, and involves the interaction between Vpu and the CD4 cytoplasmic domain. In contrast, regulation of virus release is less specific and not restricted to HIV-1 and may be mechanistically-distinct from CD4 degradation. We show here that a mutant of Vpu, Vpu35, lacking most of its cytoplasmic domain has residual biological activity for virus release but is unable to induce CD4 degradation. This finding suggests that the N terminus of Vpu encoding the transmembrane (TM) anchor represents an active domain important for the regulation of virus release but not CD4 degradation. To better define the functions of Vpu's TM anchor and cytoplasmic domain, we designed a mutant, VpuRD, containing a scrambled TM sequence with a conserved amino acid composition and alpha-helical structure. The resulting protein was integrated normally into membranes, was able to form homo-oligomers, and exhibited expression levels, protein stability, and subcellular localization similar to those of wild-type Vpu. Moreover, VpuRD was capable of binding to CD4 and to induce CD4 degradation with wild-type efficiency, confirming proper membrane topology and indicating that the alteration of the Vpu TM domain did not interfere with this function of Vpu. However, VpuRD was unable to enhance the release of virus particles from infected or transfected cells, and virus encoding VpuRD had replication characteristics in T cells indistinguishable from those of a Vpu-deficient HIV-1 isolate. Mutation of the phosphorylation sites in VpuRD resulted in a protein which was unable to perform either function of Vpu. The results of our experiments suggest that the two biological activities of Vpu operate via two distinct molecular mechanisms and involve two different structural domains of the Vpu protein.

title: Vpu Directs the Degradation of the Human Immunodeficiency Virus Restriction Factor BST-2/Tetherin via a βTrCP-Dependent Mechanism
abstract: The primary roles attributed to the human immunodeficiency virus type 1 (HIV-1) Vpu protein are the degradation of the viral receptor CD4 and the enhancement of virion release. With regard to CD4 downregulation, Vpu has been shown to act as an adapter linking CD4 with the ubiquitin-proteasome machinery via interaction with the F-box protein βTrCP. To identify additional cellular βTrCP-dependent Vpu targets, we performed quantitative proteomics analyses using the plasma membrane fraction of HeLa cells expressing either wild-type Vpu or a Vpu mutant (S52N/S56N) that does not bind βTrCP. One cellular protein, BST-2 (CD317), was consistently underrepresented in the membrane proteome of cells expressing wild-type Vpu compared to the proteome of cells expressing the Vpu mutant. To verify the biological relevance of this phenotype for HIV pathogenesis, we showed that in T cells infected with HIV-1, BST-2 downregulation occurred in a Vpu-dependent manner. Recently, BST-2 has been identified as the interferon-inducible cellular factor Tetherin, which restricts HIV virion release in the absence of Vpu. We address here the unresolved mechanism of Vpu-mediated BST-2 downregulation. Our data show that the presence of wild-type Vpu reduced cell surface and total steady-state BST-2 levels, whereas that of the mutant Vpu had no effect. In addition, treatment of cells with the lysosome acidification inhibitor concanamycin A, but not treatment with the proteasome inhibitor MG132, reduced BST-2 downregulation by wild-type Vpu, thereby suggesting that the presence of Vpu leads to the degradation of BST-2 via an endosome-lysosome degradation pathway. The importance of βTrCP in this process was confirmed by demonstrating that in the absence of βTrCP, BST-2 levels were restored despite the presence of Vpu. Taken together, these data support the hypothesis that, in similarity to its role in CD4 degradation, Vpu acts as an adapter molecule linking BST-2 to the cellular ubiquitination machinery via βTrCP. However, in contrast to the proteasome-dependent degradation of CD4, which occurs in the endoplasmic reticulum, Vpu appears to interact with BST-2 in the trans-Golgi network or in early endosomes, leading to lysosomal degradation of BST-2. Via this action, Vpu could counter the tethering function of BST-2, resulting in enhanced HIV-1 virion release. Interestingly, although HIV-2 does not express Vpu, an isolate known to exhibit enhanced viral egress can downregulate surface BST-2 by an as-yet-unknown mechanism that does not appear to involve degradation. Understanding the molecular mechanisms of both Vpu-dependent and -independent mediated antagonism of BST-2 will be critical for therapeutic strategies that exploit this novel viral function.

title: BST-2 Diminishes HIV-1 Infectivity
abstract: Bone marrow stromal cell antigen 2 (BST-2, also known as tetherin/CD317/HM1.24) inhibits the release of human immunodeficiency virus type 1 (HIV-1) and other enveloped viruses by tethering virus particles to the cell surface. In this study, we provide evidence not only that the yield of cell-free HIV-1 particles is significantly reduced by BST-2 but also that the infectivity of these progeny virions is severely impaired. The lowered virion infectivity is due to the accumulation of pr55 Gag precursor and the p40Gag intermediates as well as to the loss of a mature core in the majority of HIV-1 particles. These data suggest that, in addition to impeding the release of HIV-1 particles from host cells, BST-2 may also interfere with the activation of viral protease and, as a result, impairs viral Gag processing as well as maturation of HIV-1 particles.

title: Molecular dissection of the interactions among IkappaBalpha, FWD1, and Skp1 required for ubiquitin-mediated proteolysis of IkappaBalpha.
abstract: The SCF complex containing Skp1, Cul1, and the F-box protein FWD1 (the mouse homologue of Drosophila Slimb and Xenopus beta-TrCP) functions as the ubiquitin ligase for IkappaBalpha. FWD1 associates with Skp1 through the F-box domain and also recognizes the conserved DSGXXS motif of IkappaBalpha. The structural requirements for the interactions of FWD1 with IkappaBalpha and with Skp1 have now been investigated further. The D31A mutation (but not the G33A mutation) in the DSGXXS motif of IkappaBalpha abolished the binding of IkappaBalpha to FWD1 and its subsequent ubiquitination without affecting the phosphorylation of IkappaBalpha. The IkappaBalpha mutant D31E still exhibited binding to FWD1 and underwent ubiquitination. These results suggest that, in addition to site-specific phosphorylation at Ser(32) and Ser(36), an acidic amino acid at position 31 is required for FWD1-mediated ubiquitination of IkappaBalpha. Deletion analysis of Skp1 revealed that residues 61-143 of this protein are required for binding to FWD1. On the other hand, the highly conserved residues Pro(149), Ile(160), and Leu(164) in the F-box domain of FWD1 were dispensable for binding to Skp1. Together, these data delineate the structural requirements for the interactions among IkappaBalpha, FWD1, and Skp1 that underlie substrate recognition by the SCF ubiquitin ligase complex.

title: Stability of Homologue of Slimb F-box Protein Is Regulated by Availability of Its Substrate*
abstract: The homologue of Slimb (HOS) F-box protein is a receptor of the Skp1-Cullin1-F-box protein (SCFHOS) E3 ubiquitin ligase, which mediates ubiquitination and degradation of β-catenin and the inhibitor of NFκB, IκB. We found that HOS itself is an unstable protein that undergoes ubiquitination and degradation in a 26 S proteasome-dependent manner. A HOS mutant lacking the F-box that is deficient in binding to the core SCF components underwent ubiquitination less efficiently and was more stable than the wild type protein. Furthermore, ubiquitination and degradation of HOS was impaired in ts41 cells, in which the activities of Cullin-based ligases were decreased because the NEDD8 pathway was abrogated. Whereas HOS was directly ubiquitinated within the SCFHOS complex in vitro, the addition of phosphorylated IκBα inhibited this ubiquitination. Increasing cellular levels of HOS substrate (phosphorylated IκBα) by activating IκB kinase inhibited HOS ubiquitination and led to stabilization of HOS, indicating that interaction between HOS and its substrate might protect HOS from proteolysis. Taken together, our data suggest that proteolysis of HOS depends on its interaction with active components of the SCF complex and that HOS stability is regulated by a bound substrate. These findings may define a mechanism for maintaining activities of specific SCF complexes based on availability of a particular substrate.

title: HOS, a human homolog of Slimb, forms an SCF complex with Skp1 and Cullin1 and targets the phosphorylation-dependent degradation of IkappaB and beta-catenin.
abstract: SCF E3 ubiquitin ligases mediate ubiquitination and proteasome-dependent degradation of phosphorylated substrates. We identified a human F-box/WD40 repeats protein (HOS), which is homologous to Slimb/h betaTrCP. Being a part of SCF complex with Skp1 and Cullin1, HOS specifically interacted with the phosphorylated IkappaB and beta-catenin, targeting these proteins for proteasome-dependent degradation in vivo. This targeting required Cullin1 as expression of a mutant Cullin1 abrogated the degradation of IkappaB and of beta-catenin. Mutant HOS which lacks the F-box blocked TNF alpha-induced degradation of IkappaB as well as GSK3beta-mediated degradation of beta-catenin. This mutant also inhibited NF-kappaB transactivation and increased the beta-catenin-dependent transcription activity of Tcf. These results demonstrate that SCF(HOS) E3 ubiquitin ligase regulate both NF-kappaB and beta-catenin signaling pathways.

title: The human immunodeficiency virus type 1 encoded Vpu protein is phosphorylated by casein kinase-2 (CK-2) at positions Ser52 and Ser56 within a predicted alpha-helix-turn-alpha-helix-motif.
abstract: The human immunodeficiency virus type 1 (HIV-1) encoded Vpu is a small integral membrane phosphoprotein that functions in the enhancement of viral particle release and has more recently been shown to cause degradation of CD4 at the endoplasmic reticulum. We have demonstrated earlier that Vpu is phosphorylated by the ubiquitous casein kinase-2 (CK-2) in HIV-1 infected cells. The phosphoacceptor sites targeted by CK-2 in Vpu, however, have not been demonstrated and it was unclear whether Vpu was phosphorylated at one or more of its four serine residues. In this study we characterized the CK-2 phosphoacceptor sites in Vpu using recombinant CK-2 for in vitro phosphorylation of recombinant Vpu protein as well as synthetic peptides of Vpu. Phosphorylation of both Ser52 and Ser56 was demonstrated by in vitro phosphorylation using three 54-residue peptides comprising the entire hydrophilic part of Vpu and containing single serine to asparagine transitions in either position 52 or 56. The Km values of CK-2 to these peptides were established, revealing a preferential phosphorylation of Ser56. The Km values are: Ser56 = 31 microM; Ser 52 = 156 microM; wild type = 27 microM. In addition, we studied phosphorylation of Vpu by endogenous CK-2 following in vitro translation in rabbit reticulocyte lysate of wild-type Vpu or a mutant, Vpum2/6, carrying serine to asparagine changes at amino acid positions 52 and 56. The in vivo phosphorylation of Vpu was studied in transiently transfected human embryonic kidney (293) cells. In this system, the mutant Vpum2/6 was not phosphorylated, indicating that the seryl residues of Vpu at amino acid positions 52 and 56, but not those at positions 23 and 61, are phosphorylated by CK-2. The two CK-2 phosphorylation sites are conserved in all known Vpu sequences and represent the consensus Ser52GlyAsn(Glu/Asp)Ser(Glu/Asp)Gly(Glu/Asp)59. Prediction of the secondary structure revealed a conserved alpha-helix-turn-alpha-helix motif for the hydrophilic C-terminal part of Vpu. A structural model for Vpu is proposed in which the membrane anchor precedes a region comprising two amphipathic alpha-helices of opposed polarity, joined by a strongly acidic turn that protrudes into the cytoplasm and contains the CK-2 phosphorylation sites. Possible functional and structural homologies of Vpu to the membrane channel-forming M2 protein of influenza A viruses are discussed.

title: Human-immunodeficiency-virus-type-1-encoded Vpu protein is phosphorylated by casein kinase II.
abstract: Vpu as a human-immunodeficiency-virus-type-1-encoded 81-amino-acid integral-membrane protein was expressed in Escherichia coli using the inducible ptrc promoter of an ATG fusion vector. Recombinant Vpu is associated with membranes of E. coli and could be partially solubilized by detergents. Recombinant Vpu was phosphorylated in vitro with purified porcine casein kinase II (CKII) as well as with a CKII-related protein kinase found in cytoplasmic extracts of human and hamster cells. Recombinant Vpu associated with E. coli membranes has turned out to be the best substrate for in vitro phosphorylation with CKII. This reaction can be inhibited by heparin and the ATP analogue 5,6-dichloro-1-(beta-D-ribofuranosyl)benzimidazole (DRB), both known to be potent inhibitors of CKII. Radiolabelled gamma ATP and gamma GTP were used as phosphate donors in vitro phosphorylation of recombinant Vpu. In vivo phosphorylation of Vpu in HIV-1-infected H9 cells was also inhibited by DRB. We concluded therefrom that the Vpu protein is phosphorylated by the ubiquitous CKII in HIV-1-infected human host cells. Two seryl residues in the sequence of Vpu (position 52 and 56) correspond to the consensus S/TXXD/E for CKII. These potential phosphorylation sites are located within a well-conserved dodecapeptide of Vpu (residues 47-58), which is found in different HIV-1 strains as well as in a Vpu-like protein of SIVCPZ. Monoclonal and polyclonal antibodies directed against two different epitopes of Vpu were used for immunoprecipitation of Vpu from HIV-1-infected cells and for detection of Vpu in Western blot analyses. Vpu from HIV-1-infected cells as well as recombinant Vpu expressed in E. coli were determined by SDS/PAGE using 6 M urea to be 9 kDa, which corresponds to the calculated molecular mass of Vpu.

title: Human immunodeficiency virus type 1 Vpu protein induces degradation of CD4 in vitro: the cytoplasmic domain of CD4 contributes to Vpu sensitivity
abstract: CD4 is an integral membrane glycoprotein which functions as the human immunodeficiency virus (HIV) receptor for infection of human host cells. We have recently demonstrated that Vpu, an HIV type 1 (HIV-1) encoded integral membrane phosphoprotein, induces rapid degradation of CD4 in the endoplasmic reticulum. In this report, we describe an in vitro model system that allowed us to define important parameters for Vpu-dependent CD4 degradation. The rate of CD4 decay in rabbit reticulocyte lysate was approximately one-third of that observed previously in tissue culture experiments in the presence of Vpu (40 versus 12 min) and required no other HIV-1 encoded proteins. Degradation was contingent on the presence of microsomal membranes in the assay and the coexpression of Vpu and CD4 in the same membrane compartment. By using the in vitro degradation assay, the effects of specific mutations in CD4, including C-terminal truncations and glycosylation mutants, were analyzed. The results of these experiments indicate that Vpu has the capacity to induce degradation of glycosylated as well as nonglycosylated membrane-associated CD4. Truncation of 13 C-terminal amino acids of CD4 did not affect the ability of Vpu to induce its degradation. However, the removal of 32 amino acids from the C-terminus of CD4 completely abolished sensitivity to Vpu. This suggests that Vpu targets specific sequences in the cytoplasmic domain of CD4 to induce its degradation. We also analyzed the effects of mutations in Vpu on its biological activity in the in vitro CD4 degradation assay. The results of these experiments suggest that sequences critical for this function of Vpu are located in its hydrophilic C-terminal domain.

title: Species-Specific Activity of HIV-1 Vpu and Positive Selection of Tetherin Transmembrane Domain Variants
abstract: Tetherin/BST-2/CD317 is a recently identified antiviral protein that blocks the release of nascent retrovirus, and other virus, particles from infected cells. An HIV-1 accessory protein, Vpu, acts as an antagonist of tetherin. Here, we show that positive selection is evident in primate tetherin sequences and that HIV-1 Vpu appears to have specifically adapted to antagonize variants of tetherin found in humans and chimpanzees. Tetherin variants found in rhesus macaques (rh), African green monkeys (agm) and mice were able to inhibit HIV-1 particle release, but were resistant to antagonism by HIV-1 Vpu. Notably, reciprocal exchange of transmembrane domains between human and monkey tetherins conferred sensitivity and resistance to Vpu, identifying this protein domain as a critical determinant of Vpu function. Indeed, differences between hu-tetherin and rh-tetherin at several positions in the transmembrane domain affected sensitivity to antagonism by Vpu. Two alterations in the hu-tetherin transmembrane domain, that correspond to differences found in rh- and agm-tetherin proteins, were sufficient to render hu-tetherin completely resistant to HIV-1 Vpu. Interestingly, transmembrane and cytoplasmic domain sequences in primate tetherins exhibit variation at numerous codons that is likely the result of positive selection, and some of these changes coincide with determinants of HIV-1 Vpu sensitivity. Overall, these data indicate that tetherin could impose a barrier to viral zoonosis as a consequence of positive selection that has been driven by ancient viral antagonists, and that the HIV-1 Vpu protein has specialized to target the transmembrane domains found in human/chimpanzee tetherin proteins.

title: Functional Interaction of Human Immunodeficiency Virus Type 1 Vpu and Gag with a Novel Member of the Tetratricopeptide Repeat Protein Family
abstract: Viral protein U (Vpu) is a protein encoded by human immunodeficiency virus type 1 (HIV-1) that promotes the degradation of the virus receptor, CD4, and enhances the release of virus particles from cells. We isolated a cDNA that encodes a novel cellular protein that interacts with Vpu in vitro, in vivo, and in yeast cells. This Vpu-binding protein (UBP) has a molecular mass of 41 kDa and is expressed ubiquitously in human tissues at the RNA level. UBP is a novel member of the tetratricopeptide repeat (TPR) protein family containing four copies of the 34-amino-acid TPR motif. Other proteins that contain TPR motifs include members of the immunophilin superfamily, organelle-targeting proteins, and a protein phosphatase. UBP also interacts directly with HIV-1 Gag protein, the principal structural component of the viral capsid. However, when Vpu and Gag are coexpressed, stable interaction between UBP and Gag is diminished. Furthermore, overexpression of UBP in virus-producing cells resulted in a significant reduction in HIV-1 virion release. Taken together, these data indicate that UBP plays a role in Vpu-mediated enhancement of particle release.

title: Structural and functional characterization of human SGT and its interaction with Vpu of the human immunodeficiency virus type 1.
abstract: The small glutamine-rich tetratricopeptide repeat protein (SGT) belongs to a family of cochaperones that interacts with both Hsp70 and Hsp90 via the so-called TPR domain. Here, we present the crystal structure of the TPR domain of human SGT (SGT-TPR), which shows that it contains typical features found in the structures of other TPR domains. Previous studies show that full-length SGT can bind to both Vpu and Gag of human immunodeficiency virus type 1 (HIV-1) and the overexpression of SGT in cells reduces the efficiency of HIV-1 particle release. We show that SGT-TPR can bind Vpu and reduce the amount of HIV-1 p24, which is the viral capsid, secreted from cells transfected with the HIV-1 proviral construct, albeit at a lower efficiency than full-length SGT. This indicates that the TPR domain of SGT is sufficient for the inhibition of HIV-1 particle release but the N- and/or C-terminus also have some contributions. The SGT binding site in Vpu was also identified by using peptide array and confirmed by GST pull-down assay.

title: Human Immunodeficiency Virus Type 1 Vpu Protein Interacts with CD74 and Modulates Major Histocompatibility Complex Class II Presentation
abstract: The human immunodeficiency virus type 1 (HIV-1) Vpu accessory protein is a transmembrane protein that down regulates CD4 expression and promotes the release of new virions. We screened a human leukocyte-specific yeast two-hybrid expression library to discover novel Vpu-interacting cellular proteins. The major histocompatibility complex class II (MHC II) invariant chain, also called Ii or CD74, was found to be one such protein. We show direct binding of Vpu and CD74 by using a yeast two-hybrid assay and coimmunoprecipitation from HIV-1-infected cells. The cytoplasmic region of Vpu was found to interact with the 30-amino-acid cytoplasmic tail of CD74. Human monocytic U937 cells infected with wild-type or Vpu-defective HIV-1 and transfected cells showed that Vpu down modulated the surface expression of mature MHC II molecules. The reduction in cell surface mature MHC II molecules correlated with decreased antigen presentation to T cells in culture. Thus, the Vpu protein also contributes to viral persistence by attenuating immune responses during HIV infection. This report further exemplifies the rich diversity and redundancy shown by HIV in immune evasion.

title: Mutual functional destruction of HIV-1 Vpu and host TASK-1 channel.
abstract: Sequence analysis predicted significant structural homology between the HIV-1 accessory protein Vpu and the N-terminal region of TASK-1, a mammalian background K(+) channel. If the homology resulted from molecular piracy during HIV-1 evolution, these two proteins may have important functional interactions. Here we demonstrate that TASK and Vpu physically interact in cultured cells and in AIDS lymphoid tissues. The functional consequences were potentially destructive for both components: Vpu abolished TASK-1 current, while overexpressing TASK led to a marked impairment of Vpu's ability to enhance viral particle release. Further, the first 40 amino acids of TASK-1 (part of the homology to Vpu) were capable of enhancing HIV-1 particle release. This virus-host interaction may influence HIV-1/AIDS progression, as well as electrical signaling in infected host tissues.

title: Species-Specific Activity of SIV Nef and HIV-1 Vpu in Overcoming Restriction by Tetherin/BST2
abstract: Tetherin, also known as BST2, CD317 or HM1.24, was recently identified as an interferon-inducible host–cell factor that interferes with the detachment of virus particles from infected cells. HIV-1 overcomes this restriction by expressing an accessory protein, Vpu, which counteracts tetherin. Since lentiviruses of the SIVsmm/mac/HIV-2 lineage do not have a vpu gene, this activity has likely been assumed by other viral gene products. We found that deletion of the SIVmac239 nef gene significantly impaired virus release in cells expressing rhesus macaque tetherin. Virus release could be restored by expressing Nef in trans. However, Nef was unable to facilitate virus release in the presence of human tetherin. Conversely, Vpu enhanced virus release in the presence of human tetherin, but not in the presence of rhesus tetherin. In accordance with the species-specificity of Nef in mediating virus release, SIV Nef downregulated cell-surface expression of rhesus tetherin, but did not downregulate human tetherin. The specificity of SIV Nef for rhesus tetherin mapped to four amino acids in the cytoplasmic domain of the molecule that are missing from human tetherin, whereas the specificity of Vpu for human tetherin mapped to amino acid differences in the transmembrane domain. Nef alleles of SIVsmm, HIV-2 and HIV-1 were also able to rescue virus release in the presence of both rhesus macaque and sooty mangabey tetherin, but were generally ineffective against human tetherin. Thus, the ability of Nef to antagonize tetherin from these Old World primates appears to be conserved among the primate lentiviruses. These results identify Nef as the viral gene product of SIV that opposes restriction by tetherin in rhesus macaques and sooty mangabeys, and reveal species-specificity in the activities of both Nef and Vpu in overcoming tetherin in their respective hosts.

title: Antagonism to and Intracellular Sequestration of Human Tetherin by the Human Immunodeficiency Virus Type 2 Envelope Glycoprotein
abstract: Tetherin (CD317/BST-2), an interferon-induced membrane protein, restricts the release of nascent retroviral particles from infected cell surfaces. While human immunodeficiency virus type 1 (HIV-1) encodes the accessory gene vpu to overcome the action of tetherin, the lineage of primate lentiviruses that gave rise to HIV-2 does not. It has been previously reported that the HIV-2 envelope glycoprotein has a Vpu-like function in promoting virus release. Here we demonstrate that the HIV-2 Rod envelope glycoprotein (HIV-2 Rod Env) is a tetherin antagonist. Expression of HIV-2 Rod Env, but not that of HIV-1 or the closely related simian immunodeficiency virus (SIV) SIVmac1A11, counteracts tetherin-mediated restriction of Vpu-defective HIV-1 in a cell-type-specific manner. This correlates with the ability of the HIV-2 Rod Env to mediate cell surface downregulation of tetherin. Antagonism requires an endocytic motif conserved across HIV/SIV lineages in the gp41 cytoplasmic tail, but specificity for tetherin is governed by extracellular determinants in the mature Env protein. Coimmunoprecipitation studies suggest an interaction between HIV-2 Rod Env and tetherin, but unlike studies with Vpu, we found no evidence of tetherin degradation. In the presence of HIV-2 Rod Env, tetherin localization is restricted to the trans-Golgi network, suggesting Env-mediated effects on tetherin trafficking sequester it from virus assembly sites on the plasma membrane. Finally, we recapitulated these observations in HIV-2-infected CD4+ T-cell lines, demonstrating that tetherin antagonism and sequestration occur at physiological levels of Env expression during virus replication.

title: Nef proteins from simian immunodeficiency viruses are tetherin antagonists.
abstract: The tetherin/BST2/CD317 protein blocks the release of HIV-1 and other enveloped viruses by inducing tethering of nascent particles to infected cell surfaces. The HIV-1 Vpu protein antagonizes the antiviral activity of human but not monkey tetherins and many simian immunodeficiency viruses (SIVs) do not encode Vpu. Here, we show that the apparently "missing" antitetherin activity in SIVs has been acquired by several SIV Nef proteins. Specifically, SIV(MAC)/SIV(SMM), SIV(AGM), and SIV(BLU) Nef proteins can suppress tetherin activity. Notably, tetherin antagonism by SIV Nef proteins is species specific, is genetically separable from other Nef activities, and is most evident with simian rather than human tetherin proteins. Accordingly, a critical determinant of sensitivity to SIV(MAC) Nef in the tetherin cytoplasmic tail is variable in nonhuman primate tetherins and deleted in human tetherin, likely due to selective pressures imposed by viral antagonists, perhaps including Nef proteins.

title: Susceptibility of Rat-Derived Cells to Replication by Human Immunodeficiency Virus Type 1
abstract: Progress in developing a small animal model of human immunodeficiency virus type 1 (HIV-1) disease would greatly facilitate studies of transmission, pathogenesis, host immune responses, and antiviral strategies. In this study, we have explored the potential of rats as a susceptible host. In a single replication cycle, rat cell lines Rat2 and Nb2 produced infectious virus at levels 10- to 60-fold lower than those produced by human cells. Rat-derived cells supported substantial levels of early HIV-1 gene expression, which was further enhanced by overexpression of human cyclin T1. Rat cells displayed quantitative, qualitative, and cell-type-specific limitations in the late phase of the HIV-1 replication cycle including relative expression levels of HIV-1 Gag proteins, intracellular Gag processing, and viral egress. Nb2 cells were rendered permissive to HIV-1 R5 viruses by coexpression of human CD4 and CCR5, indicating that the major restriction on HIV-1 replication was at the level of cellular entry. We also found that primary rat lymphocytes, macrophages, and microglia expressed considerable levels of early HIV-1 gene products following infection with pseudotyped HIV-1. Importantly, primary rat macrophages and microglia, but not lymphocytes, also expressed substantial levels of HIV-1 p24 CA and produced infectious virions. Collectively, these results identify the rat as a promising candidate for a transgenic small animal model of HIV-1 infection and highlight pertinent cell-type-specific restrictions that are features of this species.

title: Effects of signal peptide exchange on HIV‐1 glycoprotein expression and viral infectivity in mammalian cells
abstract: In certain cell systems, exchange of the human immunodeficiency virus (HIV) Env signal peptide (SP) sequence with that of heterologous SPs has been shown to increase gp120 transport and secretion. Here we demonstrate that exchange of the HIV‐Env‐SP with those from erythropoietin or tissue plasminogen activator in the proviral context does not increase wild‐type membrane‐bound Env expression or incorporation into released virions. In fact, virion infectivities were decreased. These infectivity decreases were largely due to effects on Env transport and/or function and only to a minor extent to cis effects as a result of the sequence exchanges themselves. Thus, in fact, it is not advantageous to employ heterologous SPs to achieve high‐level expression of functional cell surface membrane‐ or virion‐associated HIV‐Env.

title: Contribution of Vpu, Env, and Nef to CD4 Down-Modulation and Resistance of Human Immunodeficiency Virus Type 1-Infected T Cells to Superinfection
abstract: Human immunodeficiency virus type 1 (HIV-1) utilizes Vpu, Env, and Nef to down-modulate its primary CD4 receptor from the cell surface, and this function seems to be critical for the pathogenesis of AIDS. The physiological relevance of CD4 down-modulation, however, is currently not well understood. In the present study, we analyzed the kinetics of CD4 down-modulation and the susceptibility of HIV-1-infected T cells to superinfection using proviral HIV-1 constructs containing individual and combined defects in vpu, env, and nef and expressing red or green fluorescent proteins. T cells infected with HIV-1 mutants containing functional nef genes expressed low surface levels of CD4 from the first moment that viral gene expression became detectable. In comparison, Vpu and Env had only minor to moderate effects on CD4 during later stages of infection. Consistent with these quantitative differences, Nef inhibited superinfection more efficiently than Vpu and Env. Notably, nef alleles from AIDS patients were more effective in preventing superinfection than those derived from a nonprogressor of HIV-1 infection. Our data suggest that protection against X4-tropic HIV-1 superinfection involves both CD4-independent and CD4-dependent mechanisms of HIV-1 Nef. X4 was effectively down-regulated by simian immunodeficiency virus and HIV-2 but not by HIV-1 Nef proteins. Thus, maximal protection seems to involve an as-yet-unknown mechanism that is independent of CD4 or coreceptor down-modulation. Finally, we demonstrate that superinfected primary T cells show enhanced levels of apoptosis. Accordingly, one reason that HIV-1 inhibits CD4 surface expression and superinfection is to prevent premature cell death in order to expand the period of effective virus production.

title: Codon optimization of the HIV-1 vpu and vif genes stabilizes their mRNA and allows for highly efficient Rev-independent expression.
abstract: Two HIV-1 accessory proteins, Vpu and Vif, are notoriously difficult to express autonomously in the absence of the viral Tat and Rev proteins. We examined whether the codon bias observed in the vpu and vif genes relative to highly expressed human genes contributes to the Rev dependence and low expression level outside the context of the viral genome. The entire vpu gene as well as the 5' half of the vif gene were codon optimized and the resulting open reading frames (ORFs) (vphu and hvif, respectively) were cloned in autonomous expression vectors under the transcriptional control of the CMV promoter. Codon optimization efficiently removed the expression block observed in the native genes and allowed high levels of Rev- and Tat-independent expression of Vpu and Vif. Most of the higher protein levels detected are accounted for by enhanced steady-state levels of the mRNA encoding the optimized species. Nuclear run-on experiments show for the first time that codon optimization has no effect on the rate of transcriptional initiation or elongation of the vphu mRNA. Instead, optimization of the vpu gene was found to stabilize the vphu mRNA in the nucleus and enhance its export to the cytoplasm. This was achieved by allowing the optimized mRNA to use a new CRM I-independent nuclear export pathway. This work provides a better understanding of the molecular mechanisms underlying the process of codon optimization and introduces novel tools to study the biological functions of the Vpu and Vif proteins independently of other viral proteins.

title: An F‐box protein, FWD1, mediates ubiquitin‐dependent proteolysis of β‐catenin
abstract: β‐catenin plays an essential role in the Wingless/Wnt signaling cascade and is a component of the cadherin cell adhesion complex. Deregulation of β‐catenin accumulation as a result of mutations in adenomatous polyposis coli (APC) tumor suppressor protein is believed to initiate colorectal neoplasia. β‐catenin levels are regulated by the ubiquitin‐dependent proteolysis system and β‐catenin ubiquitination is preceded by phosphorylation of its N‐terminal region by the glycogen synthase kinase‐3β (GSK‐3β)/Axin kinase complex. Here we show that FWD1 (the mouse homologue of Slimb/βTrCP), an F‐box/WD40‐repeat protein, specifically formed a multi‐molecular complex with β‐catenin, Axin, GSK‐3β and APC. Mutations at the signal‐induced phosphorylation site of β‐catenin inhibited its association with FWD1. FWD1 facilitated ubiquitination and promoted degradation of β‐catenin, resulting in reduced cytoplasmic β‐catenin levels. In contrast, a dominant‐negative mutant form of FWD1 inhibited the ubiquitination process and stabilized β‐catenin. These results suggest that the Skp1/Cullin/F‐box protein FWD1 (SCFFWD1)–ubiquitin ligase complex is involved in β‐catenin ubiquitination and that FWD1 serves as an intracellular receptor for phosphorylated β‐catenin. FWD1 also links the phosphorylation machinery to the ubiquitin–proteasome pathway to ensure prompt and efficient proteolysis of β‐catenin in response to external signals. SCFFWD1 may be critical for tumor development and suppression through regulation of β‐catenin protein stability.

title: Rodent Cells Support Key Functions of the Human Immunodeficiency Virus Type 1 Pathogenicity Factor Nef
abstract: After infection with human immunodeficiency virus (HIV), progression toward immunodeficiency is governed by a complex interplay of viral and host determinants. The viral accessory protein Nef is a key factor for the development of AIDS. Strains of HIV and simian immunodeficiency virus that lack functional nef genes either do not induce AIDS or do so only after a significant delay. The validity of a transgenic-small-animal model for de novo infection by HIV will depend on its ability to recapitulate the actions of critical factors of viral pathogenicity, such as Nef. We assessed the ability of rat, mouse, and hamster cells to support key effector functions of Nef. In cell lines from rodents, the subcellular distribution of wild-type HIV type 1 strain SF2 Nef and mutants was comparable to that in human cells. Nef downregulated human CD4 from the cell surface, was associated with p21-activated kinase activity, and enhanced the infectivity of HIV-1 virions. Importantly, these Nef-induced effects, as well as the downregulation of rat CD4 and major histocompatibility complex class I molecules, could also be demonstrated in primary T lymphocytes and macrophages from human CD4-transgenic rats. Thus, HIV-1 Nef exerts key functions in rodent cells. In line with our ongoing efforts to establish a transgenic-rat model of HIV disease, these results indicate that important aspects of viral pathogenesis could be addressed in a transgenic-rodent model permissive for de novo infection and that such a model would be valuable for evaluating the function of Nef in vivo.

title: Aqueous extracts from peppermint, sage and lemon balm leaves display potent anti-HIV-1 activity by increasing the virion density
abstract: BackgroundAqueous extracts from leaves of well known species of the Lamiaceae family were examined for their potency to inhibit infection by human immunodeficiency virus type 1 (HIV-1).ResultsExtracts from lemon balm (Melissa officinalis L.), peppermint (Mentha × piperita L.), and sage (Salvia officinalis L.) exhibited a high and concentration-dependent activity against the infection of HIV-1 in T-cell lines, primary macrophages, and in ex vivo tonsil histocultures with 50% inhibitory concentrations as low as 0.004%. The aqueous Lamiaceae extracts did not or only at very high concentrations interfere with cell viability. Mechanistically, extract exposure of free virions potently and rapidly inhibited infection, while exposure of surface-bound virions or target cells alone had virtually no antiviral effect. In line with this observation, a virion-fusion assay demonstrated that HIV-1 entry was drastically impaired following treatment of particles with Lamiaceae extracts, and the magnitude of this effect at the early stage of infection correlated with the inhibitory potency on HIV-1 replication. Extracts were active against virions carrying diverse envelopes (X4 and R5 HIV-1, vesicular stomatitis virus, ecotropic murine leukemia virus), but not against a non-enveloped adenovirus. Following exposure to Lamiaceae extracts, the stability of virions as well as virion-associated levels of envelope glycoprotein and processed Gag protein were unaffected, while, surprisingly, sucrose-density equilibrium gradient analyses disclosed a marked increase of virion density.ConclusionAqueous extracts from Lamiaceae can drastically and rapidly reduce the infectivity of HIV-1 virions at non-cytotoxic concentrations. An extract-induced enhancement of the virion's density prior to its surface engagement appears to be the most likely mode of action. By harbouring also a strong activity against herpes simplex virus type 2, these extracts may provide a basis for the development of novel virucidal topical microbicides.

title: Molecular cloning and chromosomal mapping of a bone marrow stromal cell surface gene, BST2, that may be involved in pre-B-cell growth.
abstract: Bone marrow stromal cells regulate B-cell growth and development through their surface molecules and cytokines. In this study, we generated a mAb, RS38, that recognized a novel human membrane protein, BST-2, expressed on bone marrow stromal cell lines and synovial cell lines. We cloned a cDNA encoding BST-2 from a rheumatoid arthritis-derived synovial cell line. BST-2 is a 30- to 36-kDa type II transmembrane protein, consisting of 180 amino acids. The BST-2 gene (HGMW-approved symbol BST2) is located on chromosome 19p13.2. BST-2 is expressed not only on certain bone marrow stromal cell lines but also on various normal tissues, although its expression pattern is different from that of another bone marrow stromal cell surface molecule, BST-1. BST-2 surface expression on fibroblast cell lines facilitated the stromal cell-dependent growth of a murine bone marrow-derived pre-B-cell line, DW34. The results suggest that BST-2 may be involved in pre-B-cell growth.