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. 2009 Sep;5(9):e1000574.
doi: 10.1371/journal.ppat.1000574. Epub 2009 Sep 4.

HIV-1 Vpu neutralizes the antiviral factor Tetherin/BST-2 by binding it and directing its beta-TrCP2-dependent degradation

Bastien Mangeat  1 , Gustavo Gers-Huber, Martin Lehmann, Madeleine Zufferey, Jeremy Luban, Vincent Piguet
Affiliations

HIV-1 Vpu neutralizes the antiviral factor Tetherin/BST-2 by binding it and directing its beta-TrCP2-dependent degradation

Bastien Mangeat et al. PLoS Pathog. 2009 Sep.

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 beta-TrCP in a tri-molecular complex. This interaction leads to Vpu-mediated proteasomal degradation of tetherin in a beta-TrCP2-dependent manner. Accordingly, in conditions where Vpu-beta-TrCP2-tetherin interplay was not operative, including cells stably knocked down for beta-TrCP2 expression or cells expressing a dominant negative form of beta-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 beta-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.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Vpu depletes human tetherin but not murine tetherin, which parallels its ability to rescue virion release.
(A) Vpu counteracts human but not murine tetherin antiviral activity. 293T cells were transfected with an HIV-1 provirus either proficient or deficient for the Vpu gene, together with the indicated tetherin constructs. Viral output was then scored by titration of the resulting supernatant on HeLa indicator cells. (B) Vpu depletes human, but not murine, tetherin. Duplicate cell extracts from the above experiment were monitored for tetherin level (as detected with an anti-HA antibody). The viral p55 Gag protein was monitored to exclude variations of transfection efficiency. The depicted tetherin bands correspond to the heterogeneously glycosylated monomer of tetherin of around 30 kDa, but equivalent depletion could be observed for the 60 kDa dimeric form (data not shown). PCNA was used as a loading control. Note that all parts come from the same blot, but the detection of murine tetherin required longer exposure due to its lower expression. (C) Vpu depletes human tetherin in a dose dependent manner, in the absence of other viral components. Increasing doses of a Vpu-expressing plasmid was co-transfected with a Flag-tagged human tetherin in 293T cells (molar ratios of 1∶1 and 2∶1, indicated by + and ++ respectively). Steady state levels of tetherin were monitored by western blot analysis of duplicate cell extracts using an anti-Flag antibody. Transfection levels were assessed with a GFP plasmid, and actin was used as a loading control. All three sections of this figure are representative of five independent experiments performed in duplicate. Sizes of molecular weight markers are shown in kilodaltons.
Figure 2
Figure 2. The depletion of tetherin by Vpu correlates with its ability to block tetherin antiviral activity.
(A) The depletion of tetherin by Vpu correlates in a dose-dependent manner with its ability to block tetherin antiviral activity. 293T cells were transfected with an HIV-1 provirus deleted for Vpu, together with a fixed dose of human HA-tagged tetherin, and either without or with increasing doses of Vpu added in trans (molar ratios are indicated). Viral output was scored by titration of the supernatant on HeLa indicator cells. In parallel, the level of tetherin was monitored by western blotting and subsequently quantified by densitometry. Both the cellular content of tetherin and its antiviral activity were then plotted. The values obtained in the absence of Vpu were given the arbitrary score of 100%. The plot was generated from two independent experiments performed in duplicate. The extracts of duplicate samples were pooled for gel loading. Equal loading was controlled by monitoring PCNA, and the viral p24 protein was examined to exclude variations of transfection efficiency. (B) The depletion of tetherin by Vpu correlates across different time points with its ability to block tetherin antiviral activity. 293T cells were transfected with an HIV-1 provirus deleted for the Vpu gene, with or without a given dose of HA-tagged human tetherin, in the presence or the absence of Vpu added in trans. At indicated time points, the titer of the viral output was scored on HeLa indicator cells. In parallel, the level of tetherin was monitored by western blotting and subsequently quantified by densitometry. For each condition, both the cellular content of tetherin and its antiviral activity were plotted as a percent of the values obtained in parallel in the absence of Vpu, which were given the arbitrary score of 100%. The plot was generated from two independent experiments performed in duplicate. The extracts of duplicate samples were pooled for gel loading. Equal loading was controlled by monitoring actin, and the viral p55 Gag protein was examined to exclude variations of transfection efficiency. For both figures, Pearson coefficients of correlation and Student p values were computed for tetherin expression versus antiviral activity. Sizes of molecular weight markers are shown in kilodaltons.
Figure 3
Figure 3. β-TrCP interaction motif is required for Vpu-induced tetherin level reduction and for its ability to rescue virion release.
(A) β-TrCP interaction motif is required for Vpu-induced tetherin depletion. HIV-1 deleted for the Vpu gene was produced from 293T cells in the presence or absence of Flag-tagged tetherin. Where indicated, Vpu wild type, or mutated in one (Vpu S52A) or both serines (Vpu 2S/A) known to be required for β-TrCP interaction, was added in trans. The effect of these different Vpu constructs on tetherin protein level was monitored by western blotting. The extracts of duplicate samples were pooled for gel loading. Equal loading was controlled by monitoring PCNA, and the viral p55 Gag protein was examined to exclude variations of transfection efficiency. The depicted gel is representative of three independent experiments. Sizes of molecular weight markers are shown in kilodaltons. (B) β-TrCP interaction motif is required for Vpu-induced rescue of virion release. Titer of the viral output obtained during the above experiment was measured on HeLa indicator cells. The titer of the virus produced in the absence of either tetherin or Vpu was given the arbitrary score of 100%. The plot was generated from two independent experiments performed in duplicate.
Figure 4
Figure 4. β-TrCP-mediated tetherin degradation is required for Vpu to counteract IFN-α-induced tetherin.
(A) β-TrCP interaction motif is required for Vpu-induced depletion of endogenous tetherin. 293T cells were co-transfected with HIV-1 ΔVpu in addition to the indicated Vpu constructs (with a Vpu∶provirus molar ratio of 3∶1). Eighteen hours after transfection, cells were either left untreated or treated for 8 hours with 3000 units per ml of IFN-α to induce tetherin expression and, 20 hours after the end of this treatment, triplicate cell extracts were pooled and analyzed by western blotting to detect endogenous tetherin (left panel). In parallel, RIG-I upregulation was scored to exclude any alteration of IFN receptor signalling by Vpu. Monitoring the viral p55 Gag protein as well as GFP, which was also co-transfected, excluded transfection variations. PCNA served as a loading control. The effect of Vpu constructs on tetherin protein level was quantified by densitometry, with the level of tetherin in the absence of Vpu being given the arbitrary value of 100% (right panel). Sizes of molecular weight markers are shown in kilodaltons. (B) Vpu expression does not affect IFN-α-mediated tetherin mRNA upregulation. Total RNA was extracted and used to monitor tetherin mRNA level by real-time RT-PCR. Expression of the TBP cellular gene was scored in parallel and used as a normalizer. (C) β-TrCP interaction motif is required for Vpu counteraction of the antiviral activity of endogenous tetherin. The viral output obtained at the end of the above experiment was scored by titration on HeLa indicator cells. Titer of the virus produced in the absence of IFN, tetherin or Vpu was given the arbitrary value of 100%. Results from all three panels were generated from two independent experiments performed in duplicate and triplicate, respectively.
Figure 5
Figure 5. A β-TrCP dominant negative prevents Vpu-mediated tetherin degradation.
293T were transfected with HA-tagged tetherin with or without Vpu, in the presence of either a Flag-tagged dominant negative β-TrCP-ΔF or a Flag-tagged wild type β-TrCP1. The molar ratio of β-TrCP to Vpu to tetherin constructs was 2.5∶2∶1. A GFP plasmid was included to exclude variations in transfection efficiency. The resulting duplicate lysates were pooled for gel loading, and proteins levels were determined by western blotting. Actin served as a loading control. The depicted figure is representative of four independent experiments performed in duplicate. Sizes of molecular weight markers are shown in kilodaltons.
Figure 6
Figure 6. Vpu requires β-TrCP2 to deplete tetherin from cells and antagonize its antiviral action.
(A) Creation of 293T cell lines harboring stably downregulated β-TrCP1 and β-TrCP2 levels. Total RNA from 293T cell lines stably expressing the indicated shRNAmir constructs was extracted, and used to monitor β-TrCP1 and β-TrCP2 mRNA levels by real-time RT-PCR. The expression of the TBP cellular gene was used for normalization. The values of β-TrCP1 and β-TrCP2 measured in the presence of the control shRNAmir were given the arbitrary value of 100%. (B) Vpu requires β-TrCP2 to deplete tetherin from cells and antagonize its antiviral action. A Vpu-deleted HIV-1, or its Vpu-proficient counterpart, was transfected in duplicate in the indicated stable cell lines, in the presence of an HA-tagged tetherin plasmid (molar ratio of 2∶1 in favor of tetherin). The extracts of the duplicate samples were pooled for gel loading, and tetherin protein levels were monitored by western blotting (lower panel). Equal loading was controlled by monitoring PCNA, and the viral p55 Gag protein was examined to exclude variations of transfection efficiency. In parallel, titer of the viral output present in the supernatant was monitored on HeLa indicator cells (upper panel). Similar results were obtained by scoring the physical viral particle output by reverse transcription assay (data not shown). The western blot figure is assembled from the data of two gels performed in parallel, on both of which all relevant controls were present and gave identical results. The figure is representative of two independent experiments performed in duplicate. Sizes of molecular weight markers are shown in kilodaltons.
Figure 7
Figure 7. Vpu and β-TrCP co-immunoprecipitate with tetherin.
293T cells were transfected with the indicated Vpu and β-TrCP-ΔF constructs, in the presence or absence of HA-tetherin (with a molar ratio of 2∶1 in the favor of Vpu). Equal amounts of lysates were subjected to immunoprecipitation with an anti-HA resin and analyzed by western blotting. PCNA served as a loading control. The first left lane was cut and pasted from another position from the same scan of the same blot. The figure is representative of two independent experiments. Sizes of molecular weight markers are shown in kilodaltons.
Figure 8
Figure 8. Vpu induces proteasomal degradation of tetherin.
(A) The MG132 proteasomal inhibitor impedes Vpu-mediated depletion of tetherin. 293T cells were transfected in duplicate with an HA-tetherin construct in the presence or absence of Vpu (with a molar ratio of 2∶1 in favor of Vpu), and were either left untreated or treated for 12 hours with the proteasome inhibitor MG132. Duplicate lysates were then pooled for western blot analysis (left panel). Actin served as a loading control. The effect of MG132 on Vpu-mediated tetherin depletion was quantified by densitometry and a plot was generated from the results of three independent experiments performed in duplicate (right panel). The values obtained in the absence of Vpu were given the arbitrary value of 100%. (B) An ubiquitin mutant that blocks the formation of the polyubiquitin chains involved in proteasomal targeting impedes Vpu-mediated depletion of tetherin. 293T cells were transfected in duplicate with a Flag-tetherin construct in the presence or absence of Vpu. In addition, cells were co-transfected with a wild type or K48R mutant version of HA-tagged ubiquitin. The molar ratio of ubiquitin to Vpu to tetherin constructs was 2.5∶1.75∶1. Duplicate extracts from these cells were pooled and analyzed by western blotting (left panel). Ezrin was used as a loading control. The effect of the different ubiquitin constructs on Vpu-mediated tetherin depletion was quantified by densitometry, and a plot was generated from the results of three independent experiments performed in duplicate (right panel). The values obtained in the absence of Vpu were given the arbitrary value of 100%. Results were statistically significant as the p value, determined by the Student test, was lower than 0.05 for indicated pairs (*). Sizes of molecular weight markers are shown in kilodaltons.
Figure 9
Figure 9. Vpu-induced proteasomal degradation of tetherin involves ERAD.
(A) The ERAD pathway is involved in Vpu-mediated tetherin depletion. 293T cells were transfected in duplicate with 100 nM of either a non-silencing control siRNA, or of a siRNA pool targeting p97. Twenty-four hours later, these cells were transfected with Flag-tagged tetherin in the presence or absence of Vpu (with a molar ratio of 2∶1 in favor of Vpu). Duplicate cell lysates were pooled for western blot analysis (left panel). Actin served as a loading control. Note that both parts of the figure come from the same scan of the same blot. The effect of the different siRNAs on Vpu-mediated tetherin depletion was quantified by densitometry and a plot was generated from the results of two independent experiments performed in duplicate (right panel). The values obtained in the absence of Vpu were given the arbitrary value of 100%. (B) Vpu-mediated tetherin depletion does not require ubiquitination of tetherin cytosolic lysines. 293T cells were co-transfected in duplicate with or without Vpu in the presence of either HA-tagged wild type tetherin or its counterpart having both its cytosolic lysines K18 and K21 replaced with arginines (KcytoR tetherin). Duplicate extracts were pooled and analyzed by western blotting. PCNA served as a loading control. Vpu-mediated tetherin depletion was quantified by densitometry, and a plot was generated from the results of two independent experiments performed in duplicate (right panel). The values obtained in the absence of Vpu were given the arbitrary value of 100%. Sizes of molecular weight markers are shown in kilodaltons.
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