Involvement of the betaTrCP in the ubiquitination and stability of the HIV-1 Vpu protein

Beta-Transducin Repeats-Containing Proteins as an Anticancer Target

Beta-transducin repeat-containing proteins (β-TrCPs) are E3-ubiquitin-ligase-recognizing substrates and regulate proteasomal degradation. The degradation of β-TrCPs' substrates is tightly controlled by various external and internal signaling and confers diverse cellular processes, including cell cycle progression, apoptosis, and DNA damage response. In addition, β-TrCPs function to regulate transcriptional activity and stabilize a set of substrates by distinct mechanisms. Despite the association of β-TrCPs with tumorigenesis and tumor progression, studies on the mechanisms of the regulation of β-TrCPs' activity have been limited. In this review, we studied publications on the regulation of β-TrCPs themselves and analyzed the knowledge gaps to understand and modulate β-TrCPs' activity in the future.

Detection of the HIV-1 Accessory Proteins Nef and Vpu by Flow Cytometry Represents a New Tool to Study Their Functional Interplay within a Single Infected CD4+ T Cell

The HIV-1 Nef and Vpu accessory proteins are known to protect infected cells from antibody-dependent cellular cytotoxicity (ADCC) responses by limiting exposure of CD4-induced (CD4i) envelope (Env) epitopes at the cell surface. Although both proteins target the host receptor CD4 for degradation, the extent of their functional redundancy is unknown. Here, we developed an intracellular staining technique that permits the intracellular detection of both Nef and Vpu in primary CD4+ T cells by flow cytometry. Using this method, we show that the combined expression of Nef and Vpu predicts the susceptibility of HIV-1-infected primary CD4+ T cells to ADCC by HIV+ plasma. We also show that Vpu cannot compensate for the absence of Nef, thus providing an explanation for why some infectious molecular clones that carry a LucR reporter gene upstream of Nef render infected cells more susceptible to ADCC responses. Our method thus represents a new tool to dissect the biological activity of Nef and Vpu in the context of other host and viral proteins within single infected CD4+ T cells. IMPORTANCE HIV-1 Nef and Vpu exert several biological functions that are important for viral immune evasion, release, and replication. Here, we developed a new method allowing simultaneous detection of these accessory proteins in their native form together with some of their cellular substrates. This allowed us to show that Vpu cannot compensate for the lack of a functional Nef, which has implications for studies that use Nef-defective viruses to study ADCC responses.

Proteasomal Degradation Machinery: Favorite Target of HIV-1 Proteins

Proteasomal degradation pathways play a central role in regulating a variety of protein functions by controlling not only their turnover but also the physiological behavior of the cell. This makes it an attractive target for the pathogens, especially viruses which rely on the host cellular machinery for their propagation and pathogenesis. Viruses have evolutionarily developed various strategies to manipulate the host proteasomal machinery thereby creating a cellular environment favorable for their own survival and replication. Human immunodeficiency virus-1 (HIV-1) is one of the most dreadful viruses which has rapidly spread throughout the world and caused high mortality due to its high evolution rate. Here, we review the various mechanisms adopted by HIV-1 to exploit the cellular proteasomal machinery in order to escape the host restriction factors and components of host immune system for supporting its own multiplication, and successfully created an infection.

The ubiquitin-conjugating system: multiple roles in viral replication and infection

Through the combined action of ubiquitinating and deubiquitinating enzymes, conjugation of ubiquitin to a target protein acts as a reversible post-translational modification functionally similar to phosphorylation. Indeed, ubiquitination is more and more recognized as a central process for the fine regulation of many cellular pathways. Due to their nature as obligate intracellular parasites, viruses rely on the most conserved host cell machineries for their own replication. Thus, it is not surprising that members from almost every viral family are challenged by ubiquitin mediated mechanisms in different steps of their life cycle and have evolved in order to by-pass or exploit the cellular ubiquitin conjugating system to maximize their chance to establish a successful infection. In this review we will present several examples of the complex interplay that links viruses and the ubiquitin conjugation machinery, with a special focus on the mechanisms evolved by the human immunodeficiency virus to escape from cellular restriction factors and to exit from infected cells.

The ER quality control and ER associated degradation machineries are vital for viral pathogenesis

The endoplasmic reticulum (ER) is central to protein production and membrane lipid synthesis. The unfolded protein response (UPR) supports cellular metabolism by ensuring protein quality control in the ER. Most positive strand RNA viruses cause extensive remodeling of membranes and require active membrane synthesis to promote infection. How viruses interact with the cellular machinery controlling membrane metabolism is largely unknown. Furthermore, there is mounting data pointing to the importance of the UPR and ER associated degradation (ERAD) machineries in viral pathogenesis in eukaryotes emerging topic. For many viruses, the UPR is an early event that is essential for persistent infection and benefits virus replication. In addition, many viruses are reported to commandeer ER resident chaperones to contribute to virus replication and intercellular movement. In particular, calreticulin, the ubiquitin machinery, and the 26S proteasome are most commonly identified components of the UPR and ERAD machinery that also regulate virus infection. In addition, researchers have noted a link between UPR and autophagy. It is well accepted that positive strand RNA viruses use autophagic membranes as scaffolds to support replication and assembly. However this topic has yet to be explored using plant viruses. The goal of research on this topic is to uncover how viruses interact with this ER-related machinery and to use this information for designing novel strategies to boost immune responses to virus infection.

The ER quality control and ER associated degradation machineries are vital for viral pathogenesis

The endoplasmic reticulum (ER) is central to protein production and membrane lipid synthesis. The unfolded protein response (UPR) supports cellular metabolism by ensuring protein quality control in the ER. Most positive strand RNA viruses cause extensive remodeling of membranes and require active membrane synthesis to promote infection. How viruses interact with the cellular machinery controlling membrane metabolism is largely unknown. Furthermore, there is mounting data pointing to the importance of the UPR and ER associated degradation (ERAD) machineries in viral pathogenesis in eukaryotes emerging topic. For many viruses, the UPR is an early event that is essential for persistent infection and benefits virus replication. In addition, many viruses are reported to commandeer ER resident chaperones to contribute to virus replication and intercellular movement. In particular, calreticulin, the ubiquitin machinery, and the 26S proteasome are most commonly identified components of the UPR and ERAD machinery that also regulate virus infection. In addition, researchers have noted a link between UPR and autophagy. It is well accepted that positive strand RNA viruses use autophagic membranes as scaffolds to support replication and assembly. However this topic has yet to be explored using plant viruses. The goal of research on this topic is to uncover how viruses interact with this ER-related machinery and to use this information for designing novel strategies to boost immune responses to virus infection.

Ubiquitination of BST-2 protein by HIV-1 Vpu protein does not require lysine, serine, or threonine residues within the BST-2 cytoplasmic domain

The cellular protein BST-2/CD317/Tetherin has been shown to inhibit the release of HIV-1 and other enveloped viruses from infected cells. The HIV-1 accessory protein Vpu binds to both BST-2 and βTrCP, a substrate-recognition subunit for the SCF (Skip1-Cullin1-F-box protein) E3 ubiquitin ligase complex. This interaction leads to both the degradation of BST-2 and the enhancement of viral egress. Recently BST-2 was shown to be ubiquitinated in this process. Here we have confirmed the Vpu- and βTrCP-dependent multi/polyubiquitination of BST-2. Ubiquitinated BST-2 accumulated in cells treated with a lysosomal inhibitor but not a proteasomal inhibitor. Additionally, we observed that a BST-2 mutant deleted for its cytosolically exposed lysine residues is also ubiquitinated. Subsequent experiments suggested that Vpu promotes BST-2 ubiquitination upon amino acid residues bearing hydroxyl- but not thiol-bearing side chains. However, a BST-2 mutant bearing substitutions for its cytoplasmically exposed Ser, Thr, and Lys residues was still down-regulated, ubiquitinated, and degraded in a Vpu-dependent manner. Our results suggest that Vpu may target either the BST-2 cytoplasmic Tyr residues or the NH(2) terminus itself for ubiquitination.

Transmembrane domain determinants of CD4 Downregulation by HIV-1 Vpu

The transmembrane domains (TMDs) of integral membrane proteins do not merely function as membrane anchors but play active roles in many important biological processes. The downregulation of the CD4 coreceptor by the Vpu protein of HIV-1 is a prime example of a process that is dependent on specific properties of TMDs. Here we report the identification of Trp22 in the Vpu TMD and Gly415 in the CD4 TMD as critical determinants of Vpu-induced targeting of CD4 to endoplasmic reticulum (ER)-associated degradation (ERAD). The two residues participate in different aspects of ERAD targeting. Vpu Trp22 is required to prevent assembly of Vpu into an inactive, oligomeric form and to promote CD4 polyubiquitination and subsequent recruitment of the VCP-UFD1L-NPL4 dislocase complex. In the presence of a Vpu Trp22 mutant, CD4 remains integrally associated with the ER membrane, suggesting that dislocation from the ER into the cytosol is impaired. CD4 Gly415, on the other hand, contributes to CD4-Vpu interactions. We also identify two residues, Val20 and Ser23, in the Vpu TMD that mediate retention of Vpu and, by extension, CD4 in the ER. These findings highlight the exploitation of several TMD-mediated mechanisms by HIV-1 Vpu in order to downregulate CD4 and thus promote viral pathogenesis.

Misdirection of membrane trafficking by HIV-1 Vpu and Nef: Keys to viral virulence and persistence

The HIV-1 accessory protein Nef is well known for its manipulation of host cell endosomal trafficking. By linking transmembrane proteins to endosomal coats, Nef removes them from the surface of infected cells. Modulation of MHC proteins leads to viral evasion of cellular adaptive immunity, whereas modulation of receptors for the HIV envelope glycoprotein, including CD4, enhances viral infectivity. The other HIV-1 accessory proteins, Vif, Vpr and Vpu, share a mechanism of action distinct from Nef in that each interacts with a multi-subunit ubiquitin ligase complex to target cellular proteins for proteosomal degradation. However, newly uncovered functions and mechanistic aspects of Vpu likely involve endosomal trafficking: these include counteraction of the innate antiviral activity of the cellular transmembrane protein BST-2 (tetherin), as well as the removal of the lipid-antigen presenting protein CD1d and the natural killer cell ligand NTB-A from the cell surface. This review focuses on how Nef and Vpu interfere with normal intracellular membrane trafficking to facilitate the spread and virulence of HIV-1.

Inhibition of β-TrcP-dependent ubiquitination of p53 by HIV-1 Vpu promotes p53-mediated apoptosis in human T cells

HIV-1 viral protein U (Vpu) is involved in ubiquitination and degradation of BM stromal cell Ag 2 and surface receptor CD4 through their recruitment to SCF(β-TrcP) (Skp1/Cul1/F-box) ubiquitin ligase (SCF) complex. Here, we show that specific interaction of wild-type Vpu protein with SCF complex leads to inhibition of ubiquitination and proteasomal degradation of p53 protein in a β-TrcP-dependent manner. Successful interaction of SCF(β-TrcP) complex with β-TrcP binding motif (DS(52)GNES(56)) present in Vpu is essential because mutant Vpu possessing specific alanine substitutions (DA(52)GNEA(56)) in the β-TrcP binding motif not only failed to stabilize p53 protein but was also unable to inhibit ubiquitination of p53 protein. Furthermore, Vpu competes efficiently with the interaction of p53 protein with the β-TrcP subunit of the SCF complex and inhibits subsequent ubiquitination of p53 proteins in a dose-dependent manner. We also observed potent apoptotic activity in a p53 null cell line (H-1299) that was cotransfected with p53 and Vpu-expressing plasmids. Furthermore, MOLT-3 (human T-lymphoblast) cells when infected with vesicular stomatitis virus glycoprotein-pseudotypic HIV-1 possessing wild-type vpu gene exhibited maximum activation of p53/Bax proteins and p53-mediated cell death. These findings establish a novel function of Vpu in modulating the stability of p53 protein that correlates positively with apoptosis during late stages of HIV-1 infection.

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

Background

The 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.

Results

To 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.

Conclusions

These 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.

Vpu directs the degradation of the human immunodeficiency virus restriction factor BST-2/Tetherin via a {beta}TrCP-dependent mechanism

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 betaTrCP. To identify additional cellular betaTrCP-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 betaTrCP. 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 betaTrCP in this process was confirmed by demonstrating that in the absence of betaTrCP, 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 betaTrCP. 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.

Mutation of a single residue renders human tetherin resistant to HIV-1 Vpu-mediated depletion

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.

The ubiquitin system, disease, and drug discovery

The ubiquitin system of protein modification has emerged as a crucial mechanism involved in the regulation of a wide array of cellular processes. As our knowledge of the pathways in this system has grown, so have the ties between the protein ubiquitin and human disease. The power of the ubiquitin system for therapeutic benefit blossomed with the approval of the proteasome inhibitor Velcade in 2003 by the FDA. Current drug discovery activities in the ubiquitin system seek to (i) expand the development of new proteasome inhibitors with distinct mechanisms of action and improved bioavailability, and (ii) validate new targets. This review summarizes our current understanding of the role of the ubiquitin system in various human diseases ranging from cancer, viral infection and neurodegenerative disorders to muscle wasting, diabetes and inflammation. I provide an introduction to the ubiquitin system, highlight some emerging relationships between the ubiquitin system and disease, and discuss current and future efforts to harness aspects of this potentially powerful system for improving human health.

Republished from Current BioData's Targeted Proteins database (TPdb; ).

Ubiquitin ligases as therapeutic targets in HIV-1 infection

BACKGROUND

Introduction of highly active antiretroviral therapy has led to a profound reduction in human immunodeficiency virus (HIV) related mortality; although, the complete eradication of the virus from infected individuals has never been achieved. In addition, due to the high mutation and evolution rate, drug-resistant viruses are continuously emerging.

OBJECTIVE

Genetically more stable cellular pathways represent attractive targets for innovative antiviral strategies, especially the ubiquitin proteasome system, which regulates various steps in the HIV replication cycle.

METHODS

This review focuses on certain interactions of HIV and E3 ligases as a major player in the ubiquitin proteasome system.

RESULTS/CONCLUSION

Due to the importance in HIV replication, and together with the high substrate specificity, E3 ligases can be considered as bona fide targets to interfere with HIV infection.