Cellular distribution and karyophilic properties of matrix, integrase, and Vpr proteins from the human and simian immunodeficiency viruses

Infections by human and simian immunodeficiency viruses (HIV and SIV) are independent of host cell division since the preintegration complex (PIC), containing the viral DNA, is able to undergo active nuclear import after viral entry. In order to clarify the mechanisms responsible for nuclear import of the PIC, we have analyzed the subcellular distribution and the karyophilic properties of its viral components, matrix protein (MA), integrase (IN), Vpr, and Vpx. Although MA has been reported to contain a nuclear localization signal, the MA/GFP fusions are excluded from the nucleus and associated with cellular membranes. In contrast, both HIV-1 and SIV IN and Vpr localize in the nucleus of transfected cells. Interestingly, only Vpx from SIVsm virus accumulate in the nucleus while SIVsm Vpr is uniformly distributed throughout nucleus and cytoplasm. Coexpression of MA, Vpr, and IN does not induce any change in their respective intracellular localizations. Finally, we confirm the karyophilic properties of HIV-1 IN and Vpr using an in vitro nuclear import assay. These results indicate that the viral proteins IN and Vpr, which are strongly associated with the viral DNA within PIC, may participate in the nuclear import of the HIV PIC.     

Targeting of Chrolamphenicol Acetyltransferase to Human Immunodeficiency Virus Particles via Vpr and Vpx

Vpr and Vpx are the auxiliary proteins of human immunodeficiency viruses (HIVs) selectively incorporated into mature viral particles. We showed that the bacterial chloramphenicol acetyltransferase (CAT) fused to the N-terminus of HIV-1 Vpr, HIV-2 Vpr, or HIV-2 Vpx was incorporated into mature virions in a type-selective manner. By using chimeric proteins between HIV-1 Vpr and HIV-2 Vpx, we found that the N-terminal side of these proteins was mainly important for type-selective virion incorporation. The C-terminal arginine-rich region of HIV-1 Vpr was also found to transport CAT fusion proteins into virions but without any type selectivity. Furthermore, the corresponding regions of HIV-2 Vpr and HIV-2 Vpx had no such activity. This region of HIV-1 Vpr may interact nonspecifically with viral genomic RNA. Collectively, Vpr and Vpx may provide a means to introduce foreign proteins and other molecules into HIV virions for therapeutic purposes.     

The Role of Vpr in HIV-1 Disease Progression is Independent of Its G2 Arrest Induction Function

Vpr (viral protein R) is a vital HIV-1 accessory protein with multiple functions in the viral life cycle, including nuclear import of preintegration complex, induction of apoptosis and G2 cell cycle arrest. The cell cycle perturbation activity of Vpr requires activation of the ATR (Ataxia-Telangiectasia and Rad3-related) pathway and the integrity of Vpr C-terminal motif that is crucial for chromatin binding. Recent studies also demonstrated Vpr as one of the viral factors that influence HIV disease progression, as mutations in Vpr were overrepresented in some cohorts of long-term nonprogressors (LTNP). The LTNP-associated mutations of Vpr are frequently observed in the C-terminal domain. This raises the question whether the LTNP phenotype of Vpr is the result of the loss its ability to induce G2 arrest. Here we report that the LTNP-associated mutants of Vpr function normally in the induction of G2 arrest. No defects in ATR activation and direct binding to chromatin are observed. These mutants also show similar levels of apoptosis induction as wild-type Vpr. These data differentiate the LTNP-associated mutations of Vpr with those defective in inducing G2 arrest. We propose that the G2 arrest function of Vpr is separated from the LTNP phenotype, and the role of Vpr in HIV disease progression may involve other functions of Vpr.     

Two putative alpha-helical domains of human immunodeficiency virus type 1 Vpr mediate nuclear localization by at least two mechanisms

To identify the domains of Vpr that are involved nuclear localization, we transfected HeLa cells with a panel of expression vectors that encode mutant Vpr protein with deletions or substitutions within putative domains. Immunofluorescence staining of transfected cells revealed that wild-type Vpr was localized predominantly in the nucleus and the nuclear envelope and certainly in the cytoplasm. Introduction of substitutions or deletions within alphaH1 or alphaH2 resulted, by contrast, in diffuse expression over the entire cell. In addition, double mutations within both of these alpha-helical domains led to the complete absence of Vpr from nuclei. Next, we prepared HeLa cells that express chimeric proteins which consist of the alphaH1 and alphaH2 domains fused individually with green fluorescent protein (GFP) and a Flag tag and extracted them with digitonin and Triton X-100 prior to fixation. Flag-alphaH1-GFP was detected in the nucleus but not in the cytoplasm, while Flag-alphaH2-GFP was retained predominantly in the nucleus and in a small amount in the cytoplasm. The immunostaining patterns were almost eliminated by substitutions in each chimeric protein. Thus, it appeared that the two alpha-helical domains might be involved in nuclear import by binding to certain cellular factors. Taken together, our data suggest that the two putative alpha-helical domains mediate the nuclear localization of Vpr by at least two mechanisms.     

Vpr Stimulates Viral Expression and Induces Cell Killing in Human Immunodeficiency Virus Type 1-Infected Dividing Jurkat T Cells

In this study we investigated the effects of Vpr during human immunodeficiency virus (HIV) infection of proliferating Jurkat T cells by using a vesicular stomatitis virus envelope G glycoprotein pseudotyped HIV superinfection system. We observe that the expression of Vpr results in a severe reduction in the life span of HIV type 1 (HIV-1)-infected dividing T cells in culture. In agreement with a recent report (S. A. Stewart, B. Poon, J. B. M. Jowett, and I. S. Chen, J. Virol. 71:5579–5592, 1997), we show that events characteristic of apoptotic cell death are involved in the Vpr-mediated cytopathic effects. Our results also show that infection with viruses expressing the wild-type vpr gene results in an increase in viral gene expression and production. Interestingly, the effects of Vpr on cell viability and on viral gene expression both correlate with the ability of the protein to induce a cell cycle arrest in the G₂/M phase. Mutagenesis analyses show that the C terminus of Vpr is essential for these biological activities. Although the role of Vpr is currently associated with the infection of nondividing cells, our results suggest that Vpr can also directly increase viral replication in vivo in infected dividing T cells. Furthermore, these in vitro observations suggest that Vpr-mediated cytotoxic effects could contribute to the CD4⁺ depletion associated with AIDS progression.     

Nuclear targeting of adenovirus type 2 requires CRM1-mediated nuclear export

Incoming adenovirus type 2 (Ad2) and Ad5 shuttle bidirectionally along microtubules, biased to the microtubule-organizing center by the dynein/dynactin motor complex. It is unknown how the particles reach the nuclear pore complex, where capsids disassemble and viral DNA enters the nucleus. Here, we identified a novel link between nuclear export and microtubule-mediated transport. Two distinct inhibitors of the nuclear export factor CRM1, leptomycin B (LMB) and ratjadone A (RJA) or CRM1-siRNAs blocked adenovirus infection, arrested cytoplasmic transport of viral particles at the microtubule-organizing center or in the cytoplasm and prevented capsid disassembly and nuclear import of the viral genome. In mitotic cells where CRM1 is in the cytoplasm, adenovirus particles were not associated with microtubules but upon LMB treatment, they enriched at the spindle poles implying that CRM1 inhibited microtubule association of adenovirus. We propose that CRM1, a nuclear factor exported by CRM1 or a protein complex containing CRM1 is part of a sensor mechanism triggering the unloading of the incoming adenovirus particles from microtubules proximal to the nucleus of interphase cells.     

Exploitation of microtubule cytoskeleton and dynein during parvoviral traffic toward the nucleus

Canine parvovirus (CPV), a model virus for the study of parvoviral entry, enters host cells by receptor-mediated endocytosis, escapes from endosomal vesicles to the cytosol, and then replicates in the nucleus. We examined the role of the microtubule (MT)-mediated cytoplasmic trafficking of viral particles toward the nucleus. Immunofluorescence and immunoelectron microscopy showed that capsids were transported through the cytoplasm into the nucleus after cytoplasmic microinjection but that in the presence of MT-depolymerizing agents, viral capsids were unable to reach the nucleus. The nuclear accumulation of capsids was also reduced by microinjection of an anti-dynein antibody. Moreover, electron microscopy and light microscopy experiments demonstrated that viral capsids associate with tubulin and dynein in vitro. Coprecipitation studies indicated that viral capsids interact with dynein. When the cytoplasmic transport process was studied in living cells by microinjecting fluorescently labeled capsids into the cytoplasm of cells containing fluorescent tubulin, capsids were found in close contact with MTs. These results suggest that intact MTs and the motor protein dynein are required for the cytoplasmic transport of CPV capsids and contribute to the accumulation of the capsid in the nucleus.     

Nuclear import, virion incorporation, and cell cycle arrest/differentiation are mediated by distinct functional domains of human immunodeficiency virus type 1 Vpr.

The vpr gene product of human immunodeficiency virus type 1 (HIV-1) is a virion-associated protein that is essential for efficient viral replication in monocytes/macrophages. Vpr is primarily localized in the nucleus when expressed in the absence of other viral proteins. Vpr is packaged efficiently into viral particles through interactions with the p6 domain of the Gag precursor polyprotein p55gag. We developed a panel of expression vectors encoding Vpr molecules mutated in the amino-terminal helical domain, leucine-isoleucine (LR) domain, and carboxy-terminal domain to map the different functional domains and to define the interrelationships between virion incorporation, nuclear localization, cell cycle arrest, and differentiation functions of Vpr. We observed that substitution mutations in the N-terminal domain of Vpr impaired both nuclear localization and virion packaging, suggesting that the helical structure may play a vital role in modulating both of these biological properties. The LR domain was found to be involved in the nuclear localization of Vpr. In contrast, cell cycle arrest appears to be largely controlled by the C-terminal domain of Vpr. The LR and C-terminal domains do not appear to be essential for virion incorporation of Vpr. Interestingly, we found that two Vpr mutants harboring single amino acid substitutions (A30L and G75A) retained the ability to translocate to the nucleus but were impaired in the cell cycle arrest function. In contrast, mutation of Leu68 to Ser resulted in a protein that localizes in the cytoplasm while retaining the ability to arrest host cell proliferation. We speculate that the nuclear localization and cell cycle arrest functions of Vpr are not interrelated and that these functions are mediated by separable putative functional domains of Vpr.     

Vpr-mediated incorporation of UNG2 into HIV-1 particles is required to modulate the virus mutation rate and for replication in macrophages

Human immunodeficiency virus type 1 is able to infect nondividing cells, such as macrophages, and the viral Vpr protein has been shown to participate in this process. Here, we investigated the impact of the recruitment into virus particles of the nuclear form of uracil DNA glycosylase (UNG2), a cellular DNA repair enzyme, on the virus mutation rate and on replication in macrophages. We demonstrate that the interaction of Vpr with UNG2 led to virion incorporation of a catalytically active enzyme that is directly involved with Vpr in modulating the virus mutation rate. The lack of UNG in virions during virus replication in primary monocyte-derived macrophages further exacerbated virus mutant frequencies to an 18-fold increase compared with the 4-fold increase measured in actively dividing cells. Because the presence of UNG is also critical for efficient infection of macrophages, these observations extend the role of Vpr to another early step of the virus life cycle, e.g. viral DNA synthesis, that is essential for replication of human immunodeficiency virus type 1 in nondividing cells.     

Functional and structural characterization of synthetic HIV-1 Vpr that transduces cells, localizes to the nucleus, and induces G2 cell cycle arrest

Human immunodeficiency virus (HIV) Vpr contributes to nuclear import of the viral pre-integration complex and induces G(2) cell cycle arrest. We describe the production of synthetic Vpr that permitted the first studies on the structure and folding of the full-length protein. Vpr is unstructured at neutral pH, whereas under acidic conditions or upon addition of trifluorethanol it adopts alpha-helical structures. Vpr forms dimers in aqueous trifluorethanol, whereas oligomers exist in pure water. (1)H NMR spectroscopy allows the signal assignment of N- and C-terminal amino acid residues; however, the central section of the molecule is obscured by self-association. These findings suggest that the in vivo folding of Vpr may require structure-stabilizing interacting factors such as previously described interacting cellular and viral proteins or nucleic acids. In biological studies we found that Vpr is efficiently taken up from the extracellular medium by cells in a process that occurs independent of other HIV-1 proteins and appears to be independent of cellular receptors. Following cellular uptake, Vpr is efficiently imported into the nucleus of transduced cells. Extracellular addition of Vpr induces G(2) cell cycle arrest in dividing cells. Together, these findings raise the possibility that circulating forms of Vpr observed in HIV-infected patients may exert biological effects on a broad range of host target cells.     

Evidence that an interaction between EB1 and p150(Glued) is required for the formation and maintenance of a radial microtubule array anchored at the centrosome

EB1 is a microtubule tip-associated protein that interacts with the APC tumor suppressor protein and components of the dynein/dynactin complex. We have found that the C-terminal 50 and 84 amino acids (aa) of EB1 were sufficient to mediate the interactions with APC and dynactin, respectively. EB1 formed mutually exclusive complexes with APC and dynactin, and a direct interaction between EB1 and p150(Glued) was identified. EB1-GFP deletion mutants demonstrated a role for the N-terminus in mediating the EB1-microtubule interaction, whereas C-terminal regions contributed to both its microtubule tip localization and a centrosomal localization. Cells expressing the last 84 aa of EB1 fused to GFP (EB1-C84-GFP) displayed profound defects in microtubule organization and centrosomal anchoring. EB1-C84-GFP expression severely inhibited microtubule regrowth, focusing, and anchoring in transfected cells during recovery from nocodazole treatment. The recruitment of gamma-tubulin and p150(Glued) to centrosomes was also inhibited. None of these effects were seen in cells expressing the last 50 aa of EB1 fused to GFP. Furthermore, EB1-C84-GFP expression did not induce Golgi apparatus fragmentation. We propose that a functional interaction between EB1 and p150(Glued) is required for microtubule minus end anchoring at centrosomes during the assembly and maintenance of a radial microtubule array.     

Mutational analysis of cell cycle arrest, nuclear localization and virion packaging of human immunodeficiency virus type 1 Vpr.

Human immunodeficiency virus type 1 Vpr is a virion-associated, regulatory protein that is required for efficient viral replication in monocytes/macrophages. The protein is believed to act in conjunction with the Gag matrix protein to allow import of the viral preintegration complex in nondividing cells. In cells, Vpr localizes to the nucleus. Recently, we showed that Vpr prevents the activation of p34cdc2-cyclin B. This results in arrest of Vpr-expressing cells in the G2/M phase of the cell cycle. Here, we use a panel of expression vectors encoding Vpr molecules mutated in the amino-terminal alpha-helical region, the central hydrophobic region, or the carboxy-terminal basic region to define the functional domains of the protein. The results showed cell cycle arrest was largely controlled by the carboxy-terminal basic domain of the protein. In contrast, the amino-terminal alpha-helical region of Vpr was required for nuclear localization and packaging into virions. The carboxy terminus appeared to be unnecessary for nuclear localization. In the alpha-helical region, mutation of Ala-30 to Pro resulted in a protein that localized to the cytoplasm. Surprisingly, fusion of Vpr to luciferase resulted in a molecule that failed to localize to the nucleus. In addition, we show that simian immunodeficiency virus Vpr, but not Vpx, induces G2 arrest. We speculate that Vpr has two sites for interaction with cellular factors: one in the alpha-helical region that specifies nuclear localization and one in the carboxy-terminal domain that is required for Cdc2 inhibition.