Overexpression of PRMT6 does not suppress HIV-1 Tat transactivation in cells naturally lacking PRMT6

Background

Protein arginine methyltransferase 6 (PRMT6) can methylate the HIV-1 Tat, Rev and nucleocapsid proteins in a manner that diminishes each of their functions in in vitro assays, and increases the stability of Tat in human cells. In this study, we explored the relationship between PRMT6 and HIV-1 Tat by determining the domains in each protein required for interaction.

Methods

Through domain mapping and immunoprecipitation experiments, we determined that both the amino and carboxyl termini of PRMT6, and the activation domain within Tat are essential for interaction. Mutation of the basic domain of Tat did not affect the ability of PRMT6 to interact with Tat.

Results

We next used the A549 human alveolar adenocarcinoma cell line, which naturally expresses undetectable levels of PRMT6, as a model for testing the effects of PRMT6 on Tat stability, transactivation, and HIV-1 replication. As previously observed, steady state levels and the protein half-life of Tat were increased by the ectopic expression of PRMT6. However, no down regulation of Tat transactivation function was observed, even with over 300-fold molar excess of PRMT6 plasmid. We also observed no negative effect on HIV-1 infectivity when A549 producer cells overexpressed PRMT6.

Conclusions

We show that PRMT6 requires the activation domain, but surprisingly not the basic domain, of Tat for protein interaction. This interaction between Tat and PRMT6 may impact upon pathogenic effects attributed to Tat during HIV-1 infection other than its function during transactivation.

A mutant Tat protein provides strong protection from HIV-1 infection in human CD4+ T cells

Here we show potent inhibition of HIV-1 replication in a human T cell line and primary human CD4(+) cells by expressing a single antiviral protein. Nullbasic is a mutant form of the HIV-1 Tat protein that was previously shown to strongly inhibit HIV-1 replication in nonhematopoietic cell lines by targeting three steps of HIV-1 replication: reverse transcription, transport of viral mRNA, and trans-activation of HIV-1 gene expression. Here we investigated gene delivery of Nullbasic, using lentiviral and retroviral vectors. Although Nullbasic could be delivered by lentiviral vectors to target cells, transduction efficiencies were sharply reduced primarily because of negative effects on reverse transcription mediated by Nullbasic. However, Nullbasic did not inhibit transduction of HEK293T cells by a murine leukemia virus (MLV)-based retroviral vector. Therefore, MLV-based virus-like particles were used to transduce and express Nullbasic-EGFP or EGFP in Jurkat cells, a human leukemia T cell line, and Nullbasic-ZsGreen1 or ZsGreen1 in primary human CD4(+) cells. HIV-1 replication kinetics were similar in parental Jurkat and Jurkat-EGFP cells, but were strongly attenuated in Jurkat-Nullbasic-EGFP cells. Similarly, virus replication in primary CD4(+) cells expressing a Nullbasic-ZsGreen1 fusion protein was inhibited by approximately 8- to 10-fold. These experiments demonstrate the potential of Nullbasic, which has unique inhibitory activity, as an antiviral agent against HIV-1 infection.

Nullbasic, a potent anti-HIV tat mutant, induces CRM1-dependent disruption of HIV rev trafficking

Nullbasic, a mutant of the HIV-1 Tat protein, has anti-HIV-1 activity through mechanisms that include inhibition of Rev function and redistribution of the HIV-1 Rev protein from the nucleolus to the nucleoplasm and cytoplasm. Here we investigate the mechanism of this effect for the first time, establishing that redistribution of Rev by Nullbasic is not due to direct interaction between the two proteins. Rather, Nullbasic affects subcellular localization of cellular proteins that regulate Rev trafficking. In particular, Nullbasic induced redistribution of exportin 1 (CRM1), nucleophosmin (B23) and nucleolin (C23) from the nucleolus to the nucleus when Rev was coexpressed, but never in its absence. Inhibition of the Rev:CRM1 interaction by leptomycin B or a non-interacting RevM10 mutant completely blocked redistribution of Rev by Nullbasic. Finally, Nullbasic did not inhibit importin β- or transportin 1-mediated nuclear import, suggesting that cytoplasmic accumulation of Rev was due to increased export by CRM1. Overall, our data support the conclusion that CRM1-dependent subcellular redistribution of Rev from the nucleolus by Nullbasic is not through general perturbation of either nuclear import or export. Rather, Nullbasic appears to interact with and disrupt specific components of a Rev trafficking complex required for its nucleocytoplasmic shuttling and, in particular, its nucleolar accumulation.

Distribution of vaccine/antivirals and the 'least spread line' in a stratified population

We describe a prioritization scheme for an allocation of a sizeable quantity of vaccine or antivirals in a stratified population. The scheme builds on an optimal strategy for reducing the epidemic's initial growth rate in a stratified mass-action model. The strategy is tested on the EpiSims network describing interactions and influenza dynamics in the population of Utah, where the stratification we have chosen is by age (0-6, 7-13, 14-18, adults). No prior immunity information is available, thus everyone is assumed to be susceptible-this may be relevant, possibly with the exception of persons over 50, to the 2009 H1N1 influenza outbreak. We have found that the top priority in an allocation of a sizeable quantity of seasonal influenza vaccinations goes to young children (0-6), followed by teens (14-18), then children (7-13), with the adult share being quite low. These results, which rely on the structure of the EpiSims network, are compared with the current influenza vaccination coverage levels in the US population.

The HIV-1 Tat protein stimulates reverse transcription in vitro

The role of Tat in HIV-1 reverse transcription has been controversial largely because different studies have observed disparate effects of the Tat protein on reverse transcription. Studies of HIV-1 lacking a functional tat gene demonstrated a decrease in reverse transcription efficiency following infection of T-cells however, in vitro recombinant Tat(1-86) has been shown to inhibit RT activity. Here we show that 20-200 nM of both N-terminally histidine-tagged recombinant Tat(1-72) and Tat(1-86) stimulated reverse transcription by HIV-1 reverse transcriptase (RT) in vitro by 2-3 fold. However, both Tat species were efficient inhibitors of RT activity at 400 nM. The lower concentrations of Tat increased reverse transcription efficiency by facilitating multiple rounds of DNA synthesis, and this increase was either not seen or reduced when Tat proteins with multiply-mutated cysteine or basic domains were used. Tat-enhanced reverse transcription occurred in a RNA-independent manner, and required formation of a Tat-RT complex. Pull-down and immunoprecipitation experiments confirmed that Tat could interact with the RT p51 subunit, and mammalian two-hybrid experiments showed interaction between Tat and both the p51 and p66 subunits. Together these results provide evidence that Tat can stimulate reverse transcription through an interaction with RT.

Protein methylation is required to maintain optimal HIV-1 infectivity

Background:

Protein methylation is recognized as a major protein modification pathway regulating diverse cellular events such as protein trafficking, transcription, and signal transduction. More recently, protein arginine methyltransferase activity has been shown to regulate HIV-1 transcription via Tat. In this study, adenosine periodate (AdOx) was used to globally inhibit protein methyltransferase activity so that the effect of protein methylation on HIV-1 infectivity could be assessed.

Results:

Two cell culture models were used: HIV-1-infected CEM T-cells and HEK293T cells transfected with a proviral DNA plasmid. In both models, AdOx treatment of cells increased the levels of virion in culture supernatant. However, these viruses had increased levels of unprocessed or partially processed Gag-Pol, significantly increased diameter, and displayed reduced infectivity in a MAGI X4 assay. AdOx reduced infectivity equally in both dividing and non-dividing cells. However, infectivity was further reduced if Vpr was deleted suggesting virion proteins, other than Vpr, were affected by protein methylation. Endogenous reverse transcription was not inhibited in AdOx-treated HIV-1, and infectivity could be restored by pseudotyping HIV with VSV-G envelope protein. These experiments suggest that AdOx affects an early event between receptor binding and uncoating, but not reverse transcription.

Conclusion:

Overall, we have shown for the first time that protein methylation contributes towards maximal virus infectivity. Furthermore, our results also indicate that protein methylation regulates HIV-1 infectivity in a complex manner most likely involving the methylation of multiple viral or cellular proteins and/or multiple steps of replication.

Will Diverse Tat Interactions Lead to Novel Antiretroviral Drug Targets?

More than fifteen years following the description of Tat as a critical HIV gene expression regulatory protein, additional roles for Tat in HIV replication have been described, including reverse transcription. Tat achieves function through direct interaction with viral proteins, including reverse transcriptase, and numerous cellular proteins including cyclin T1, RNA polymerase II, protein kinase R (PKR), p300/CBP, and P/CAF. Despite our advanced knowledge of how Tat operates, this has not yet resulted in the discovery of effective agents capable of targeting various Tat functions. Nevertheless, Tat remains an attractive, virus-specific molecule and detailed understanding of specific protein interaction holds promise for future drug discovery.

Human immunodeficiency virus type 1 protease regulation of tat activity is essential for efficient reverse transcription and replication

The human immunodeficiency virus type 1 (HIV-1) Tat protein enhances reverse transcription, but it is not known whether Tat acts directly on the reverse transcription complex or through indirect mechanisms. Since processing of Tat by HIV protease (PR) might mask its presence and, at least in part, explain this lack of data, we asked whether Tat can be cleaved by PR. We used a rabbit reticulocyte lysate (RRL) system to make Tat and PR. HIV-1 PR is expressed as a Gag-Pol fusion protein, and a PR-inactivated Gag-Pol is also expressed as a control. We showed that Tat is specifically cleaved in the presence of PR, producing a protein of approximately 5 kDa. This result suggested that the cleavage site was located in or near the Tat basic domain (amino acids 49 to 57), which we have previously shown to be important in reverse transcription. We created a panel of alanine-scanning mutations from amino acids 45 to 54 in Tat and evaluated functional parameters, including transactivation, reverse transcription, and cleavage by HIV-1 PR. We showed that amino acids 49 to 52 (RKKR) are absolutely required for Tat function in reverse transcription, that mutation of this domain blocks cleavage by HIV-1 PR, and that other pairwise mutations in this region modulate reverse transcription and proteolysis in strikingly similar degrees. Mutation of Tat Y47G48 to AA also down-regulated Tat-stimulated reverse transcription but had little effect on transactivation or proteolysis by HIV PR, suggesting that Y47 is critical for reverse transcription. We altered the tat gene of the laboratory strain NL4-3 to Y47D and Y47N so that overlapping reading frames were not affected and showed that Y47D greatly diminished virus replication and conveyed a reverse transcription defect. We hypothesize that a novel, cleaved form of Tat is present in the virion and that it requires Y47 for its role in support of efficient reverse transcription.

Human immunodeficiency virus type 1 reverse transcription is stimulated by tat from other lentiviruses

The tat gene is required by HIV-1 for efficient reverse transcription and this function of Tat can be distinguished from its role in transcription by RNA polymerase II using tat point mutations that abrogate each function independently. The mechanism of Tat's role in reverse transcription, however, is not known, nor is it known whether this role is conserved among trans-activating factors in other retroviruses. Here we examine the abilities of heterologous viral trans-activating proteins from jembrana disease virus (jTat), HIV-2 (Tat2), and equine infectious anemia virus (eTat) to substitute for HIV-1 Tat (Tat1) and restore reverse transcription in HIV-1 carrying an inactivated tat gene. Natural endogenous reverse transcription assays showed that trans-activators from some retroviruses (Tat2 and jTat, but not eTat) could substitute for Tat1 in complementation of HIV-1 reverse transcription. Finally, we show that Y47 is critical for Tat1 to function in reverse transcription, but not HIV-1 gene expression. We mutated the homologous position in jTat to H62Y and found it did not improve its ability to stimulate reverse transcription, but an H62A mutation did inhibit jTat complementation. These data highlight the finding that the role of Tat in reverse transcription is not related to trans-activation and demonstrate that other tat genes conserve this function.

Characterization of RasGRP2, a plasma membrane-targeted, dual specificity Ras/Rap exchange factor

Ras proteins operate as molecular switches in signal transduction pathways downstream of tyrosine kinases and G-protein-coupled receptors. Ras is switched from the inactive GDP-bound state to the active GTP-bound state by guanine nucleotide exchange factors (GEFs). We report here the cloning and characterization of RasGRP2, a longer alternatively spliced form of the recently cloned RapGEF, CalDAG-GEFI. A unique feature of RasGRP2 is that it is targeted to the plasma membrane by a combination of N-terminal myristoylation and palmitoylation. In vivo, RasGRP2 selectively catalyzes nucleotide exchange on N- and Ki-Ras, but not Ha-Ras. RasGRP2 also catalyzes nucleotide exchange on Rap1, but this RapGEF activity is less potent than that associated with CalDAG-GEFI. The nucleotide exchange activity of RasGRP2 toward N-Ras is stimulated by diacylglycerol and inhibited by calcium. The effects of diacylglycerol and calcium are additive but are not accompanied by any detectable change in the subcellular localization of RasGRP2. In contrast, CalDAG-GEFI is localized predominantly to the cytosol and lacks Ras exchange activity in vivo. However, prolonged exposure to phorbol esters, or growth in serum, results in localization of CalDAG-GEFI to the cell membrane and restoration of Ras exchange activity. Expression of RasGRP2 or CalDAG-GEFI in NIH3T3 cells transfected with wild type N-Ras results in an accelerated growth rate but not morphologic transformation. Thus, under appropriate growth conditions, CalDAG-GEFI and RasGRP2 are dual specificity Ras and Rap exchange factors.

H-ras but not K-ras traffics to the plasma membrane through the exocytic pathway

Ras proteins must be localized to the inner surface of the plasma membrane to be biologically active. The motifs that effect Ras plasma membrane targeting consist of a C-terminal CAAX motif plus a second signal comprising palmitoylation of adjacent cysteine residues or the presence of a polybasic domain. In this study, we examined how Ras proteins access the cell surface after processing of the CAAX motif is completed in the endoplasmic reticulum (ER). We show that palmitoylated CAAX proteins, in addition to being localized at the plasma membrane, are found throughout the exocytic pathway and accumulate in the Golgi region when cells are incubated at 15 degrees C. In contrast, polybasic CAAX proteins are found only at the cell surface and not in the exocytic pathway. CAAX proteins which lack a second signal for plasma membrane targeting accumulate in the ER and Golgi. Brefeldin A (BFA) significantly inhibits the plasma membrane accumulation of newly synthesized, palmitoylated CAAX proteins without inhibiting their palmitoylation. BFA has no effect on the trafficking of polybasic CAAX proteins. We conclude that H-ras and K-ras traffic to the cell surface through different routes and that the polybasic domain is a sorting signal diverting K-Ras out of the classical exocytic pathway proximal to the Golgi. Farnesylated Ras proteins that lack a polybasic domain reach the Golgi but require palmitoylation in order to traffic further to the cell surface. These data also indicate that a Ras palmitoyltransferase is present in an early compartment of the exocytic pathway.

Dominant-negative caveolin inhibits H-Ras function by disrupting cholesterol-rich plasma membrane domains

The plasma membrane pits known as caveolae have been implicated both in cholesterol homeostasis and in signal transduction. CavDGV and CavKSY, two dominant-negative amino-terminal truncation mutants of caveolin, the major structural protein of caveolae, significantly inhibited caveola-mediated SV40 infection, and were assayed for effects on Ras function. We find that CavDGV completely blocked Raf activation mediated by H-Ras, but not that mediated by K-Ras. Strikingly, the inhibitory effect of CavDGV on H-Ras signalling was completely reversed by replenishing cell membranes with cholesterol and was mimicked by cyclodextrin treatment, which depletes membrane cholesterol. These results provide a crucial link between the cholesterol-trafficking role of caveolin and its postulated role in signal transduction through cholesterol-rich surface domains. They also provide direct evidence that H-Ras and K-Ras, which are targeted to the plasma membrane by different carboxy-terminal anchors, operate in functionally distinct microdomains of the plasma membrane.

Ras isoforms vary in their ability to activate Raf-1 and phosphoinositide 3-kinase

Ha-, N-, and Ki-Ras are ubiquitously expressed in mammalian cells and can all interact with the same set of effector proteins. We show here, however, that in vivo there are marked quantitative differences in the ability of Ki- and Ha-Ras to activate Raf-1 and phosphoinositide 3-kinase. Thus, Ki-Ras both recruits Raf-1 to the plasma membrane more efficiently than Ha-Ras and is a more potent activator of membrane-recruited Raf-1 than Ha-Ras. In contrast, Ha-Ras is a more potent activator of phosphoinositide 3-kinase than Ki-Ras. Interestingly, the ability of Ha-Ras to recruit Raf-1 to the plasma membrane is significantly increased when the Ha-Ras hypervariable region is shortened so that the spacing of the Ha-Ras GTPase domains from the inner surface of the plasma membrane mimicks that of Ki-Ras. Importantly, these data show for the first time that the activation of different Ras isoforms can have distinct biochemical consequences for the cell. The mutation of specific Ras isoforms in different human tumors can, therefore, also be rationalized.

14-3-3 facilitates Ras-dependent Raf-1 activation in vitro and in vivo

14-3-3 proteins complex with many signaling molecules, including the Raf-1 kinase. However, the role of 14-3-3 in regulating Raf-1 activity is unclear. We show here that 14-3-3 is bound to Raf-1 in the cytosol but is totally displaced when Raf-1 is recruited to the plasma membrane by oncogenic mutant Ras, in vitro and in vivo. 14-3-3 is also displaced when Raf-1 is targeted to the plasma membrane. When serum-starved cells are stimulated with epidermal growth factor, some recruitment of 14-3-3 to the plasma membrane is evident, but 14-3-3 recruitment correlates with Raf-1 dissociation and inactivation, not with Raf-1 recruitment. In vivo, overexpression of 14-3-3 potentiates the specific activity of membrane-recruited Raf-1 without stably associating with the plasma membrane. In vitro, Raf-1 must be complexed with 14-3-3 for efficient recruitment and activation by oncogenic Ras. Recombinant 14-3-3 facilitates Raf-1 activation by membranes containing oncogenic Ras but reduces the amount of Raf-1 that associates with the membranes. These data demonstrate that the interaction of 14-3-3 with Raf-1 is permissive for recruitment and activation by Ras, that 14-3-3 is displaced upon membrane recruitment, and that 14-3-3 may recycle Raf-1 to the cytosol. A model that rationalizes many of the apparently discrepant observations on the role of 14-3-3 in Raf-1 activation is proposed.

Detection of A-type and B-type Epstein-Barr virus in throat washings and lymphocytes

Peripheral blood lymphocytes, obtained from 94 individuals, were assayed for the presence and type of Epstein-Barr virus (EBV) using primers specific for A-type and B-type EBV in the polymerase chain reaction (PCR). Samples from 16 individuals (17%) were negative for EBV sequences. Of the remaining individuals A-type EBV was detected in 35%, B-type in 21%, and both A- and B-type EBV in 27%. Samples of throat washings were also collected from 33 healthy donors and the presence and type of EBV was determined using PCR. EBV was not detected in 12 donors. However, of those who were excreting EBV, A-type EBV was present in 11 donors (52%), B-type in 7 donors (33%), and both A- and B-types in the remaining 3 donors (14%). These results suggest that infection with B-type EBV and coinfections with both A- and B-type EBV are more common than previously thought.

Sequence variation of cytotoxic T cell epitopes in different isolates of Epstein-Barr virus

Previous results have identified two distinct cytotoxic T lymphocyte (CTL) epitopes encoded by Epstein-Barr virus (EBV), TETA (ORF BLRF3/BERF1 residues 329-353) and EENL (ORF BERF3/BERF4 residues 290-309). Measurement of the specificities of CTL clones (TETA-specific clone 13 and EENL-specific clone 7) directed against these epitopes indicated that the EENL epitope is conserved in all strains of EBV tested while the TETA epitope varied between individual virus strains. Sequencing of the DNA regions encoding these two CTL epitopes in different EBV isolates confirmed these interpretations and demonstrated that different TETA epitope sequences were encoded by B-type EBV strains and by the B95-8 isolate of EBV compared to the other A-type EBV strains. Titration of synthetic variants of the TETA epitope revealed that the epitope encoded by B95-8 was 15-fold less efficient as a T cell epitope than the sequence encoded by other A-type viral strains while the TETA variant encoded by the B-type strains displayed essentially no activity as a T cell epitope.

Subtypes of Epstein-Barr virus in human immunodeficiency virus-associated non-Hodgkin lymphoma

Biopsy samples obtained from 20 patients with human immunodeficiency virus (HIV)-associated non-Hodgkin lymphoma (NHL) were assessed for evidence of Epstein-Barr virus (EBV) and HIV sequences. DNA was extracted from formalin-fixed, paraffin-embedded NHL tissue and specific viral gene sequences were sought using the polymerase chain reaction (PCR). EBV sequences were found in 10 NHL samples (50%), with five tumors showing A-type and five B-type sequences. By serologic testing, 18 of 19 patients had antibodies to EBV, with 14 patients having antibodies to A-type EBV and 11 to B-type EBV. Serology confirmed the high prevalence of type B EBV in HIV-infected patients, but was not a reliable indicator of the EBV subtype present in the lymphomas. HIV sequences were present in biopsy tissue but at a level consistent with an origin from bystander HIV-infected cells. All 20 patients were negative by enzyme-linked immunosorbent assay for antibodies to human T-cell leukemia virus-type I. The high prevalence of type B EBV in these tumors is similar to the findings in endemic Burkitt's lymphoma, where 40% of the tumors have type B viral sequences. In normal populations, type B EBV is rarely found outside the nasopharynx. These studies support the hypothesis that EBV is an important cofactor in NHL in HIV-infected persons. The finding that B-type EBV is present in 25% of HIV-associated NHL suggests that this EBV subtype may be an important human pathogen with a wider geographic distribution than originally thought.

Coinfection with A- and B-type Epstein-Barr virus in human immunodeficiency virus-positive subjects

A possible cofactor in human immunodeficiency virus (HIV) infection is the Epstein-Barr virus (EBV), which is divided into two primary types that differ significantly in their transformation efficiency. The B-type EBV cell line is much more difficult to establish than the A-type. The extent of systemic B-type EBV infection was assessed in HIV-positive subjects and controls. Lymphoblastoid cell lines were established from 26 HIV-positive subjects and analyzed for the presence of A- or B-type EBV by Southern analysis and immunoblotting. Some 19% of HIV-positive persons were infected with B-type EBV, 69% with A-type, and 12% with both types. Analysis of the individual strains of EBV harbored by the HIV-positive subjects showed that HIV-induced immunosuppression had not led to increased susceptibility to repeated EBV infections. However, the occurrence of B-type infection in HIV-positive subjects was sixfold higher than that in the general community, indicating that HIV-induced immunodeficiency or HIV itself specifically enhanced the expression of the B-type EBV.

Expression of Epstein-Barr virus nuclear antigens 3, 4, and 6 are altered in cell lines containing B-type virus

A high proportion of HIV-positive sera were found to react with 130- and 180-kDa antigens which were present in the Jijoye cell line. The majority of the HIV-positive sera which detected these antigens also contained antibodies to Epstein-Barr virus (EBV) nuclear antigen 2B (EBNA2B) suggesting a relationship between B-type EBV strains and the expression of the 130K/180K antigens. Cell lines were established by infection of B lymphocytes with different A- and B-type strains of EBV. Incubation of these lines with sera from individuals infected with either A-type or B-type EBV strains demonstrated that the 130K and 180K antigens were only expressed by cell lines containing B-type virus. Sera from individuals infected with A-type EBV did not react with the 180K antigen in any cell lines but could detect EBNAs 3, 4, and 6 antigens in the A-type cell lines. Restriction enzyme analysis of the BamHI E region of the EBV genome revealed marked differences between the A and B types of the virus. These results demonstrate that expression of antigens encoded from the BamHI E region of EBV (EBNAs 3, 4, and 6) are altered in cell lines transformed by B-type strains of EBV.