Viral Protein U (Vpu)-Mediated Enhancement of Human Immunodeficiency Virus Type 1 Particle Release Depends on the Rate of Cellular Proliferation

109

Primate lentiviruses code for a protein that stimulates virus production. In human immunodeficiency virus type 1 (HIV-1), the activity is provided by the accessory protein, Vpu, while in HIV-2 and simian immunodeficiency virus it is a property of the envelope (Env) glycoprotein. Using a group of diverse retroviruses and cell types, we have confirmed the functional equivalence of the two proteins. However, despite these similarities, the two proteins have markedly different functional domains. While the Vpu activity is associated primarily with its membrane-spanning region, we have determined that the HIV-2 Env activity requires both the cytoplasmic tail and ectodomain of the protein, with the membrane-spanning domain being less important. Within the Env cytoplasmic tail, we further defined the necessary sequence as a membrane-proximal tyrosine-based motif. Providing the two Env regions separately as distinct CD8 chimeric proteins did not increase virus release. This suggests that the two domains must be either contained within a single protein or closely associated within a multiprotein oligomer, such as the Env trimer, in order to function. Finally, we observed that wild-type levels of incorporation of the HIV-2 Env into budding viruses were not required for this activity.

Section: Resultsmentioning

confidence: 99%

Functional Domains within the Human Immunodeficiency Virus Type 2 Envelope Protein Required To Enhance Virus Production

Abada¹,

Noble

Cannon

2005

109

“…In vitro, EVRs typically boost steady-state levels of virus production from cell lines by 4-to 10-fold and this activity has been observed in various different human cell types, including HeLa and HEp-2 cells, various T-cell lines, primary blood mononuclear cells, and macrophages (2,8,6,7,10,14,17,21,39,44,45,46,54). EVRs are able to stimulate the budding of heterologous viral particles from human cells (1), and it has been suggested that they counteract a natural human restriction factor that acts to inhibit retrovirus budding (21,47).…”

mentioning

confidence: 99%

Recruitment of the Adaptor Protein 2 Complex by the Human Immunodeficiency Virus Type 2 Envelope Protein Is Necessary for High Levels of Virus Release

Noble

Abada²,

Nunez-Iglesias

et al. 2006

The envelope (Env) protein of human immunodeficiency virus type 2 (HIV-2) and the HIV-1 Vpu protein stimulate the release of retroviral particles from human cells that restrict virus production, an activity that we call the enhancement of virus release (EVR). We have previously shown that two separate domains in the HIV-2 envelope protein are required for this activity: a glycine-tyrosine-x-x-hydrophobic (GYxx) motif in the cytoplasmic tail and an unmapped region in the ectodomain of the protein. We here report that the cellular partner of the GYxx motif is the adaptor protein complex AP-2. The mutation of this motif or the depletion of AP-2 by RNA interference abrogated EVR activity and changed the cellular distribution of the Env from a predominantly punctate pattern to a more diffuse distribution. Since the L domain of equine infectious anemia virus (EIAV) contains a Yxx motif that interacts with AP-2, we used both wild-type and L domain-defective particles of HIV-1 and EIAV to examine whether the HIV-2 Env EVR function was analogous to L domain activity. We observed that the production of all particles was stimulated by HIV-2 Env or Vpu, suggesting that the L domain and EVR activities play independent roles in the release of retroviruses. Interestingly, we found that the cytoplasmic tail of the murine leukemia virus (MLV) Env could functionally substitute for the HIV-2 Env tail, but it did so in a manner that did not require a Yxx motif or AP-2. The cellular distribution of the chimeric HIV-2/MLV Env was significantly less punctate than the wild-type Env, although confocal analysis revealed an overlap in the steady-state locations of the two proteins. Taken together, these data suggest that the essential GYxx motif in the HIV-2 Env tail recruits AP-2 in order to direct Env to a cellular pathway or location that is necessary for its ability to enhance virus release but that an alternate mechanism provided by the MLV Env tail can functionally substitute.

“…This effect is caused by Vpu-mediated sequestration of ␤-TrCP, resulting in upregulation of the transcriptional repressor Snail. We have previously shown that the effects of Vpu on particle release depend on cell confluence (17). Since complexes of ␤-catenin associated with Ecadherin are more apparent at higher levels of cell confluence, we determined the effect of Vpu on the ␤-catenin interaction with E-cadherin at different cell densities.…”

Section: Discussionmentioning

confidence: 99%

“…However, there are high levels of E-cadherin in macrophages and dendritic cells, in which these effects of Vpu are likely to be important for pathogenesis (6,23,25,37,62). It has been reported that Vpu can mediate up to a 1,000-fold enhancement of particle release levels in macrophages (2,17). Langerhans cells (LC) express high levels of E-cadherin, which is important for mediating the adhesion of LC to keratinocytes.…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Modulation of β-Catenin and E-Cadherin Interaction by Vpu Increases Human Immunodeficiency Virus Type 1 Particle Release

Salim

Ratner

2008

Self Cite

Vpu (viral protein U) is a 17-kDa human immunodeficiency virus type 1 (HIV-1) accessory protein that enhances the release of particles from the surfaces of infected cells. Vpu recruits ␤-transducin repeat-containing protein (␤-TrCP) and mediates proteasomal degradation of CD4. By sequestering ␤-TrCP away from other cellular substrates, Vpu leads to the stabilization of ␤-TrCP substrates such as ␤-catenin, IB␣, ATF4, and Cdc25A, but not of other substrates such as Emi1. This study shows that in addition to stabilizing ␤-catenin, Vpu leads to the depression of both total and ␤-catenin-associated E-cadherin levels through ␤-TrCP-dependent stabilization of the transcriptional repressor Snail. We showed that both downregulation of overall E-cadherin levels and dissociation of E-cadherin from ␤-catenin result in enhanced viral release. By contrast, the overexpression of E-cadherin or the prevention of the dissociation of E-cadherin from ␤-catenin results in depressed levels of virus release. Since E-cadherin is expressed only in dendritic cells and macrophages, and not in T cells, our data suggest that the HIV-1 vpu gene may have evolved to counteract different restrictions to assembly in different cells.