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Formation of mobile chromatin-associated nuclear foci containing HIV-1 Vpr and VPRBP is critical for the induction of G2 cell cycle arrest.

ABSTRACT: HIV-1 Viral protein R (Vpr) induces a cell cycle arrest at the G2/M phase by activating the ATR DNA damage/stress checkpoint. Recently, we and several other groups showed that Vpr performs this activity by recruiting the DDB1-CUL4A (VPRBP) E3 ubiquitin ligase. While recruitment of this E3 ubiquitin ligase complex has been shown to be required for G2 arrest, the subcellular compartment where this complex forms and functionally acts is unknown. Herein, using immunofluorescence and confocal microscopy, we show that Vpr forms nuclear foci in several cell types including HeLa cells and primary CD4+ T-lymphocytes. These nuclear foci contain VPRBP and partially overlap with DNA repair foci components such as gamma-H2AX, 53BP1 and RPA32. While treatment with the non-specific ATR inhibitor caffeine or depletion of VPRBP by siRNA did not inhibit formation of Vpr nuclear foci, mutations in the C-terminal domain of Vpr and cytoplasmic sequestration of Vpr by overexpression of Gag-Pol resulted in impaired formation of these nuclear structures and defective G2 arrest. Consistently, we observed that G2 arrest-competent sooty mangabey Vpr could form these foci but not its G2 arrest-defective paralog Vpx, suggesting that formation of Vpr nuclear foci represents a critical early event in the induction of G2 arrest. Indeed, we found that Vpr could associate to chromatin via its C-terminal domain and that it could form a complex with VPRBP on chromatin. Finally, analysis of Vpr nuclear foci by time-lapse microscopy showed that they were highly mobile and stable structures. Overall, our results suggest that Vpr recruits the DDB1-CUL4A (VPRBP) E3 ligase to these nuclear foci and uses these mobile structures to target a chromatin-bound cellular substrate for ubiquitination in order to induce DNA damage/replication stress, ultimately leading to ATR activation and G2 cell cycle arrest.

SUBMITTER: Belzile JP

PROVIDER: S-EPMC2932712 | biostudies-literature | 2010 Sep

REPOSITORIES: biostudies-literature

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Formation of mobile chromatin-associated nuclear foci containing HIV-1 Vpr and VPRBP is critical for the induction of G2 cell cycle arrest.

Belzile Jean-Philippe JP Abrahamyan Levon G LG Gérard Francine C A FC Rougeau Nicole N Cohen Eric A EA

PLoS pathogens 20100902 9

HIV-1 Viral protein R (Vpr) induces a cell cycle arrest at the G2/M phase by activating the ATR DNA damage/stress checkpoint. Recently, we and several other groups showed that Vpr performs this activity by recruiting the DDB1-CUL4A (VPRBP) E3 ubiquitin ligase. While recruitment of this E3 ubiquitin ligase complex has been shown to be required for G2 arrest, the subcellular compartment where this complex forms and functionally acts is unknown. Herein, using immunofluorescence and confocal microsc ...[more]

PMID: 20824083

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Project description:Vpr is an accessory protein of human immunodeficiency virus type 1 (HIV-1) with multiple functions. The induction of G2 arrest by Vpr plays a particularly important role in efficient viral replication because the transcriptional activity of the HIV-1 long terminal repeat is most active in G2 phase. The regulation of apoptosis by Vpr is also important for immune suppression and pathogenesis during HIV infection. However, it is not known whether Vpr-induced apoptosis depends on the ability of Vpr to induce G2 arrest, and the dynamics of Vpr-induced G2 arrest and apoptosis have not been visualized. We performed time-lapse imaging to examine the temporal relationship between Vpr-induced G2 arrest and apoptosis using HeLa cells containing the fluorescent ubiquitination-based cell cycle indicator2 (Fucci2). The dynamics of G2 arrest and subsequent long-term mitotic cell rounding in cells transfected with the Vpr-expression vector were visualized. These cells underwent nuclear mis-segregation after prolonged mitotic processes and then entered G1 phase. Some cells subsequently displayed evidence of apoptosis after prolonged mitotic processes and nuclear mis-segregation. Interestingly, Vpr-induced apoptosis was seldom observed in S or G2 phase. Likewise, visualization of synchronized HeLa/Fucci2 cells infected with an adenoviral vector expressing Vpr clearly showed that Vpr arrests the cell cycle at G2 phase, but does not induce apoptosis at S or G2 phase. Furthermore, time-lapse imaging of HeLa/Fucci2 cells expressing SCAT3.1, a caspase-3-sensitive fusion protein, clearly demonstrated that Vpr induces caspase-3-dependent apoptosis. Finally, to examine whether the effects of Vpr on G2 arrest and apoptosis were reversible, we performed live-cell imaging of a destabilizing domain fusion Vpr, which enabled rapid stabilization and destabilization by Shield1. The effects of Vpr on G2 arrest and subsequent apoptosis were reversible. This study is the first to characterize the dynamics of the morphological changes that occur during Vpr-induced G2 arrest and apoptosis.

Project description:The accessory gene vpr, common to all primate lentiviruses, induces potent G2/M arrest in cycling cells. A recent study showed that human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) mediates this through activation of the SLX4/MUS81/EME1 exonuclease complex that forms part of the Fanconi anemia DNA repair pathway. To confirm these observations, we have examined the G2/M arrest phenotypes of a panel of simian immunodeficiency virus (SIV) Vpr proteins. We show that SIV Vpr proteins differ in their ability to promote cell cycle arrest in human cells. While this is dependent on the DCAF1/DDB1/CUL4 ubiquitin ligase complex, interaction with human DCAF1 does not predict G2/M arrest activity of SIV Vpr in human cells. In all cases, SIV Vpr-mediated cell cycle arrest in human cells correlated with interaction with human SLX4 (huSLX4) and could be abolished by small interfering RNA (siRNA) depletion of any member of the SLX4 complex. In contrast, all but one of the HIV/SIV Vpr proteins tested, including those that lacked activity in human cells, were competent for G2/M arrest in grivet cells. Correspondingly, here cell cycle arrest correlated with interaction with the grivet orthologues of the SLX4 complex, suggesting a level of host adaptation in these interactions. Phylogenetic analyses strongly suggest that G2/M arrest/SLX4 interactions are ancestral activities of primate lentiviral Vpr proteins and that the ability to dysregulate the Fanconi anemia DNA repair pathway is an essential function of Vpr in vivo.The Vpr protein of HIV-1 and related viruses is essential for the virus in vivo. The ability of Vpr to block the cell cycle at mitotic entry is well known, but the importance of this function for viral replication is unclear. Recent data have shown that HIV-1 Vpr targets the Fanconi anemia DNA repair pathway by interacting with and activating an endonuclease complex, SLX4/MUS81/EME1, that processes interstrand DNA cross-links. Here we show that the ability of a panel of SIV Vpr proteins to mediate cell cycle arrest correlates with species-specific interactions with the SLX4 complex in human and primate cells. The results of these studies suggest that the SLX4 complex is a conserved target of primate lentiviral Vpr proteins and that the ability to dysregulate members of the Fanconi anemia DNA repair pathway is essential for HIV/SIV replication in vivo.

Project description:Human immunodeficiency virus type 1 (HIV-1) Vpr induces cell cycle G(2) arrest in fission yeast (Schizosaccharomyces pombe) and mammalian cells, suggesting the cellular pathway(s) targeted by Vpr is conserved among eukaryotes. Our previous studies in fission yeast demonstrated that Vpr induces G(2) arrest in part through inhibition of Cdc25, a Cdc2-specific phosphatase that promotes G(2)/M transition. The goal of this study was to further elucidate molecular mechanism underlying the inhibitory effect of Vpr on Cdc25. We show here that, similar to the DNA checkpoint controls, expression of vpr promotes subcellular relocalization of Cdc25 from nuclear to cytoplasm and thereby prevents activation of Cdc2 by Cdc25. Vpr-induced nuclear exclusion of Cdc25 appears to depend on the serine/threonine phosphorylation of Cdc25 and the presence of Rad24/14-3-3 protein, since amino acid substitutions of the nine possible phosphorylation sites of Cdc25 with Ala (9A) or deletion of the rad24 gene abolished nuclear exclusion induced by Vpr. Interestingly, Vpr is still able to promote Cdc25 nuclear export in mutants defective in the checkpoints (rad3 and chk1/cds1), the kinases that are normally required for Cdc25 phosphorylation and nuclear exclusion of Cdc25, suggesting that others kinase(s) might modulate phosphorylation of Cdc25 for the Vpr-induced G(2) arrest. We report here that this kinase is Srk1. Deletion of the srk1 gene blocks the nuclear exclusion of Cdc25 caused by Vpr. Overexpression of srk1 induces cell elongation, an indication of cell cycle G(2) delay, in a similar fashion to Vpr; however, no additive effect of cell elongation was observed when srk1 and vpr were coexpressed, indicating Srk1 and Vpr are likely affecting the cell cycle G(2)/M transition through the same cellular pathway. Immunoprecipitation further shows that Vpr and Srk1 are part of the same protein complex. Consistent with our findings in fission yeast, depletion of the MK2 gene, a human homologue of Srk1, either by small interfering RNA or an MK2 inhibitor suppresses Vpr-induced cell cycle G(2) arrest in mammalian cells. Collectively, our data suggest that Vpr induces cell cycle G(2) arrest at least in part through a Srk1/MK2-mediated mechanism.

Dataset Information

Formation of mobile chromatin-associated nuclear foci containing HIV-1 Vpr and VPRBP is critical for the induction of G2 cell cycle arrest.

Publications

Formation of mobile chromatin-associated nuclear foci containing HIV-1 Vpr and VPRBP is critical for the induction of G2 cell cycle arrest.

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