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.

Ubiquitination of BST-2 Protein by HIV-1 Vpu Protein Does Not Require Lysine, Serine, or Threonine Residues within the BST-2 Cytoplasmic Domain

“…2E). In general, our results corroborate those reported in several recent manuscripts (27,32,35), which have each found that the two cytosolic lysines have no phenotype with respect to viral egress or surface down-regulation by Vpu. However, as described above, disagreement remains regarding the contribution that these lysines make toward Vpu-dependent ubiquitination.…”

“…Note that in both cell lines, egress of the vpu2/6 virus displays a consistent intermediate phenotype that we and others have previously observed. Although BST-2 is not degraded in the presence of Vpu2/6, several labs including our own have shown that BST-2 binds to wild type Vpu and Vpu2/6 equally well (27,40,47). Therefore, the Vpu2/6 partial egress phenotype likely results from the transient sequestration of BST-2 within an intracellular compartment.…”

“…The Vpu-dependent down-regulation of CD4 and BST-2 share several common mechanistic features. For example, surface and total cellular levels of both proteins are diminished in the presence of Vpu (25)(26)(27). Additionally, CD4 and BST-2 have each been found in complexes with Vpu and ␤TrCP, and ␤TrCP is required for the Vpu-dependent degradation of CD4 and BST-2 (28,29).…”

Tetherin antagonism by Vpu protects HIV-infected cells from antibody-dependent cell-mediated cytotoxicity

“…Tetherin antagonism by Vpu also depends in part on the recruitment of βTrCP-2, an adaptor protein for an E3 ubiquitin ligase involved in targeting tetherin for degradation (24,25,30,31). Hence, substitutions in a conserved DSGxxS motif in the cytoplasmic tail of Vpu (S52,56N) that prevent the recruitment of βTrCP-2 partially impair tetherin antagonism by preventing its degradation, but not its endosomal sequestration (25,(30)(31)(32)(33). These substitutions are also known to abrogate CD4 down-regulation, but do not affect Vpu-mediated down-modulation of NTB-A (23).…”

Oligomerization state and supramolecular structure of the HIV‐1 Vpu protein transmembrane segment in phospholipid bilayers

“…38 Vpu interferes with tetherin by targeting tetherin for degradation, in a manner similar to that by which Vpu downregulates CD4. 39,40 Sequence variations in the TM domain of tetherin that affect sensitivity to Vpu have been identified. 41,42 Formation of Vpu/tetherin hetero-oligomers through direct interactions between their TM domains may therefore be essential for Vpu function, and these hetero-oligomers may be structurally related to Vpu homo-oligomers.…”