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Mol Med. 2009 October 24: doi: 10.2119/molmed.2009.00133. | PMCID: PMC2765407 |
Integration of HIV-1 DNA is regulated by interplay between viral Rev and cellular LEDGF/p75 proteins Aviad Levin,1 Joseph Rosenbluh,1 Zvi Hayouka,2 Assaf Friedler,2 and Abraham Loyter1* 1 Department of Biological Chemistry, The Alexander Silberman Institute of Life Sciences and 2 Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel The present work describes a new-not yet described-interaction between the Human immunodeficiency virus type 1 (HIV-1) Rev protein and the cellular lens epithelium-derived growth factor p75 (LEDGF/p75) protein in-vitro and in virus-infected cells. Here we show, for the first time, that formation of a Rev-LEDGF/p75 complex is a crucial step in regulating the viral cDNA integration. Co-immunoprecipitation experiments at various times after virus infection revealed that first an integrase(IN)-LEDGF/p75 complex is formed, which is then replaced by a Rev-LEDGF/p75 and Rev-IN complexes. This was supported by in-vitro experiments showing that Rev promotes dissociation of the IN-LEDGF/p75 complex. Combination of the viral IN and the cellular LEDGF/p75 is required for proper integration of the viral cDNA into the host chromosomal DNA. Our findings suggest a new mechanism demonstrating that integration of HIV-1 cDNA is regulated by an interplay between viral Rev and the host-cell LEDGF/p75 proteins. Integration of the HIV genome into the host-cell chromosome is a central event in the viral replication cycle ( 1). The viral integrase enzyme (IN), which integrates the viral cDNA into the host chromosome, is one of the key components of the pre-integration complex (PIC) ( 2). In addition to the IN, the cellular protein lens epithelium-derived growth factor (LEDGF/p75), was also shown to be required for promoting integration of the viral DNA ( 3, 4). The contribution of LEDGF/p75 to the tethering of IN to the host chromatin, and thus to the whole integration process, has been demonstrated by the dramatic decrease in HIV-1 replication in siRNA LEDGF/p75-knockdown cells ( 5– 8). Only a small percentage of the cDNA molecules become part of the chromosomal DNA, leaving the large majority as unintegrated copies ( 9, 10). In some cases, the level of non-integrated HIV DNA can reach up to 99% of total viral DNA ( 10). A quantitative estimation of integration revealed that out of a total of 15 to 21 copies of cDNA, only one or two molecules are integrated per cell ( 9). Why the number of integration events is that low remains an enigma. It may be that, in addition to the stimulatory LEDGF/p75 protein, the integration process is subjected to inhibition by another regulatory factor. Our recent data suggest that this factor may be the viral Rev protein ( 11, 12). The Rev protein is known to promote nuclear export of singly spliced and unspliced viral RNA in the late phase of viral infection ( 13). However, it was well established that Rev is also transcribed during the early phase of infection before integration of the viral DNA, namely from unintegrated DNA which has the capacity to synthesize all classes of viral transcripts ( 14– 18). Recently it has also been clearly demonstrated that the amount of Rev transcribed from the unintegrated DNA can reach up to 70% of that transcribed by the integrated DNA ( 18). We have recently proposed that an early transcribed Rev interacts with IN, thereby decreasing the degree of integration ( 16). The ability of Rev to inhibit the integration of cDNA in-vivo was further confirmed by experiments showing that infection of cultured cells by Rev-deficient HIV results in a relatively high number of integration events (about 10 integrations/cell) ( 16). On the other hand, practically no integration was observed when Rev-expressing cells were infected with WT HIV ( 16). Thus, in addition to its function in the late phase of replication, Rev may play a central role in inhibiting/regulating the integration process ( 16). In the present work, we studied the functional relations between the stimulatory LEDGF/p75 and the inhibitory Rev proteins. Based on in-vitro quantitative experiments as well as on co-immunoprecipitation (co-IP) of virus-infected cells, we suggest that Rev can remove the LEDGF/p75 protein from its association with IN, resulting in the formation of two complexes: the non-active Rev-IN ( 11) and the—as yet unobserved—complex between viral Rev and the cellular LEDGF/p75. We conclude that integration of viral cDNA is regulated by the interplay between the viral IN and Rev proteins and the cellular LEDGF/p75 protein. (For further experimental details see Supplementary Materials and Methods). Cells Monolayer adherent HEK293T cells, HEK293T cells overexpressing Rev (Rev10 + cells) and HeLa MAGI cells (TZM-bl) ( 19, 20) were grown in Dulbecco’s modified Eagle’s Medium (DMEM). The T-lymphocyte cell lines Sup-T1 and H9 were grown in RPMI 1640 medium. Cells other than the Rev10 + cells were provided by the NIH Reagent Program, Division of AIDS, NIAID, NIH, Bethesda, MD, USA. Cells were incubated at 37°C in a 5% CO 2 atmosphere. All media were supplemented with 10% (v/v) fetal calf serum, 0.3 g/l L-glutamine, 100 U/ml penicillin and 100 U/ml streptomycin (Biological Industries, Beit Haemek, Israel). HeLaP4/shp75Cl15 cells, a generous gift of Prof. Z. Debyser (Molecular Medicine, K.U. Leuven, Flanders, Belgium), were grown as described in ( 4). Rev10 + and LEDGF/p75-knockdown Rev-expressing cells were generated by transfection into HEK293T and HeLaP4/shp75Cl15 cells, respectively ( 21) with pcDNA3.1 plasmid bearing the full Rev. Viruses WT HIV-1 (HXB2 ( 22)) and ΔEnv ( 23), as well as the IN mutant D64N D116N ( 24), were generated by transfection into HEK293T cells ( 21) of the virus-containing plasmid or co-transfected with a plasmid containing VSV-G ( 11). ΔRev pLAIY47H2 ( 25) and Rev M10 ( 26) HIVs were generated by transfection into Rev10 + cells. Viruses were harvested and stored as described in ( 11). The pLAIY47H2 ( 25) viruses were a generous gift from Prof. B. Berkhout (Department of Human Retrovirology, Academic Medical Center, University of Amsterdam, The Netherlands), and the IN mutant D64N D116N virus was a generous gift from Prof. A. Engelman (Department of Cancer Immunology and AIDS Dana-Farber Cancer Institute and Division of AIDS, Harvard Medical School, Boston, MA, USA). Infection of cultured cells Cultured lymphocytes were infected exactly as described in ( 11). Cultured HEK293T cells, Rev10 + cells and HeLa MAGI cells (TZM-bl) were grown for 24 h before infection, then the medium was discarded and cells were incubated at different multiplicity of infections (MOI) with the indicated virus for 2 h at 37°C. Cells were washed three times with PBS and incubated in DMEM. Peptide synthesis and purification Peptides were synthesized on an Applied Biosystems (ABI) 433A peptide synthesizer and purification was performed on a Gilson HPLC using a reverse-phase C8 semi-preparative column (ACE, Advanced Chromatography Technologies, USA) as described in ( 11). Protein expression and purification Expression and purification of histidine-tagged Rev-GFP was performed as previously described ( 27). The histidine-tagged IN and LEDGF/p75 expression vectors were a generous gift from Prof. A. Engelman and their expression and purification were performed essentially as described previously ( 28, 29). GST-Tat was expressed and purified as described previously ( 30). ELISA-based binding assays Protein-peptide, protein-protein and protein-DNA binding was estimated using an ELISA-based binding assay exactly as described previously ( 31). Briefly, Maxisorp plates (Nunc) were incubated at room temperature for 2 h with 200 ml of 10 μg/ml synthetic peptide/recombinant protein in carbonate buffer. After incubation, the solution was removed, the plates were washed three times with PBS, and 200 μl of 10% BSA (Sigma) in PBS (w/v) was added for 2h at room temperature. After rewashing with PBS, tested BSA-biotinilited (Bb), peptide or protein (alone or biotinilated) or biotinilated DNA were added for further incubation for 1h at room temperature. Following three washes with PBS, the concentration of bound molecules was estimated after the addition of streptavidin-horseradish peroxidase (HRP) conjugate (Sigma), as described previously ( 32), or of anti-GFP mouse antibody (Santa Cruz) which was then interacted with rabbit anti-mouse IgG antibody conjugated to HRP. The enzymatic activity of HRP was estimated by monitoring the product’s optical density (OD) at 490 nm using an ELISA plate reader (Tecan Sunrise Swizerland). Each measurement was performed in duplicate. For dissociation from and binding to a complex after binding of the first protein to the Maxisorp plate, the binding partner was incubated for 1 h at room temperature and after three washes with PBS, the dissociated component was added and its binding to the complex, as well the amount of remaining bound complex, were estimated separately as described above. In-vitro IN activity assay Quantitative determination of IN activity was performed exactly as described previously ( 12) using a previously described assay system ( 33, 34). Plasmids construction All of the plasmids used in this study were constructed exactly as described in ( 12). The coding sequences for full-length HIV-1 IN, Rev, LEDGF/p75 and Tat were amplified by PCR and inserted in-frame into the corresponding sites of the GN- and GC-linkers ( 12). Bimolecular fluorescence complementation (BiFC) The above-described plasmids were transformed into the yeast strain EGY48 (Clontech) and the subsequent steps, including visualization by confocal microscope (MRC 1024 confocal imaging system, Bio-Rad), were as previously described ( 35) ( 12). Study of in-vivo protein-protein interactions by co-IP The co-IP experiments were conducted essentially as described previously ( 36) with several modifications. Briefly, cells were infected with a MOI of 15 for the indicated viruses. Cells were harvested at different times PI, washed three times in PBS and lysed by the addition of PBS containing 1% (v/v) Triton X-100 for whole-cell lysate. Cytoplasm, nuclei and PIC were isolated as described below. Half of the lysate or the isolated fraction was subjected to SDS-PAGE and immunoblotted with either a monoclonal anti-Rev antibody (α-Rev) ( 37) or antiserum raised against IN amino acids 276–288 (α-IN) (NIH AIDS Research & Reference Reagent Program catalog number 758), or anti-LEDGF/p75 (α-LEDGF/p75) (R&D Systems) or anti-actin (α-actin) antibody (Santa Cruz), and the complementary HRP-conjugated secondary antibodies (Jackson). The remaining lysate or isolated fractions were incubated for 1 h at 4°C with either the α-Rev, α-IN, α-LEDGF/p75 or α-actin antibodies. Following a 3-h incubation with protein G-agarose beads (Santa Cruz) at 4°C, the samples were washed three times with PBS containing 1% (v/v) Nonidet P-40. SDS buffer was added to the samples and after boiling and subjecting to SDS-PAGE, the membranes were immunoblotted with either α-Rev, α-IN, α-LEDGF/p75 or α-actin antibodies, and the complementary HRP-conjugated secondary antibodies. When peptides were used, cells were incubated with 150 μM of the indicated peptide for 2 h prior to infection. Quantitative estimation of the bands was performed by Image Gauge V3.46 software (Fujifilm). Isolation of cytoplasm, nuclei and PIC from infected cells The various fractions were obtained from virus-infected cells essentially as described previously ( 38) with several modifications. Briefly, cells were harvested and washed twice in buffer A (20 mM Hepes pH 7.3, 150 mM KCl, 5 mM MgCl 2, 1 mM DTT and 0.1 mM PMSF). Cells were then suspended in 200 μl of buffer A with 0.025% (w/v) digitonin and incubated at room temperature for 10 min. Cells were centrifuged for 3 min at 1000 g at room temperature. The supernatant was then centrifuged at 8000 g and separated into supernatant (cytoplasm) and pellet (nuclei) and stored at − 70°C. For PIC isolation, an equal volume of buffer B (20 mM Hepes pH 7.4, 5 mM MgCl 2, 1 mM DTT and 0.1 mM PMSF) was added to the cytoplasm fraction. Samples were incubated for 10 min at room temperature and then centrifuged for 10 min at 2000 g. The supernatant was discarded and the pellet, containing the PIC aggregates, was stored at − 70°C. Cytoplasm, nuclei and PICfraction Analysis Cytoplasm and nuclei fractions were analyzed by western blot as described above. For detection of fraction specific protein ant actin antibody (Santa Cruz)and anti histone H3 antibody (abcam) were used. For the anlysis of the PIC a total viral DNA was estimated by real time PCR as described below as well as integration of the PIC fraction in-vitro as described at ( 39). Quantitative analysis of copy numbers of HIV-1 DNA integrated into the cellular genome The integration reaction, as well as the integration events, were performed exactly as described previously ( 11). Quantitation of total viral DNA Total viral DNA was estimated using SYBR green real-time quantitative PCR 12 h PI, exactly as described in ( 40). Quantitative estimation of HIV-1 infection by determination of extracellular p24 The amount of p24 protein was estimated in the cell medium exactly as described previously ( 12). Immunostaining HeLaP4/shp75Cl15 cells were grown on chamber slides (Nunc), then infected with ΔRev HIV-1 at a MOI of 25. Cells were fixed 16 h PI exactly as described previously ( 41) and immunostained essentially as described previously ( 41) with some modifications. Briefly, after fixation, cells were blocked with 5% IgG-free BSA (Jackson) in PBS for 60 min. For detection of HIV-1 IN and Rev and the host LEDGF/p75, the cells were incubated with 1:50 rabbit α-IN (NIH AIDS Research & Reference Reagent Program catalog number 758), 1:50 rat α-Rev ( 37) and 1:100 goat α-LEDGF/p75 (R&D Systems) at room temperature for 60 min each. Cells were washed five times with PBS + 0.05% (v/v) Tween 20 between antibodies. Then the cells were incubated with the following secondary antibodies: Cy2-conjugated anti-rat, Cy3-conjugated anti-rabbit and Cy5-conjugated anti-goat (Jackson) (all diluted 1:100) at room temperature for 60 min each, with five washes with PBS + 0.05% Tween 20 between antibodies. For detection of DNA, cells were stained with DAPI according to the manufacturer's protocol. Slides were prepared with Mounting Media (Bio-Rad) and immunofluorescent cells were detected with an Olympus confocal microscope. Statistic analysis p < 0.05, calculated from at least 3 repetitions for Real time analysis p < 0.01, ± stand for standard deviation. Viral Rev and cellular LEDGF/p75 proteins interact under in-vitro conditions and in yeast cells Both the host cell’s LEDGF/p75 and the viral Rev have been found to interact with the viral IN protein, affecting its biological function ( 3, 4, 11, 16). Using an ELISA-based system, a Rev-LEDGF/p75 interaction was also observed with an apparent Kd in the low nanomolar range, similar to that observed for the IN-LEDGF/p75 and Rev-IN interactions (). Due to the relatively low solubility ( 42) of recombinant Rev, we have used—in our in-vitro experiments—a Rev-GFP conjugate ( 27) (). Specificity of the Rev-LEDGF/p75 interaction could be inferred from the observation that GFP alone fails to interact with LEDGF/p75 (). The Rev and LEDGF/p75 proteins were also able to interact within the intracellular environment of yeast cells, as shown by the bimolecular fluorescence complementation (BiFC) experiments depicted in . As can be seen restoration of fluorescence was observed only in samples bearing two known interacting proteins such as IN-IN ( 1), Rev–Rev ( 13), Rev–IN ( 12) and IN-LEDGF/p75 ( 8) (), indicating the specificity of the Rev-LEDGF/p75 interaction. Furthermore the fact that restoration of fluorescence demonstrates specific protein-protein interaction is strengthen by the control experiments showing that no fluorescence was restored when cells were transformed with the linker plasmids containing only the half GFP (GN or GC) or when such linkers were co-expressed with Rev, IN or LEDGF/p75 conjugated to the second half of the GFP (). Also no interaction was observed when a different early expressed protein namely Tat ( 15, 17) was co-expressed with either one of those proteins (). Rev promotes in-vitro dissociation of IN-LEDGF/p75 complex The results in show that addition of the Rev-GFP conjugate to pre-formed IN-LEDGF/p75 complex promotes removal of the bound LEDGF/p75. This disruption of the complex was probably due to the ability of Rev to interact with both LEDGF/p75 () and IN ( 12). GFP does not interact with either LEDGF/p75 () or IN ( 11). Specificity of Rev's binding ability could also be inferred from the observation that the HIV-1 Tat protein failed to promote the IN-LEDGF/p75 dissociation (). On the other hand, LEDGF/p75 did not promote dissociation of the Rev-IN complex (), probably due to its lower ability to bind that complex (). Our binding experiments, revealed slightly higher affinity of the Rev protein to the IN-LEDGF/p75 complex than to IN alone (), while LEDGF/p75 showed lower affinity to the Rev-IN complex (). For characterization of the domains mediating the Rev-LEDGF/p75 interaction see supplementary data and supplementary Figure S1. Rev promotes dissociation of the IN-LEDGF/p75 complex in virus-infected cells Western blot analysis revealed the presence of IN, LEDGF/p75 and Rev within the cytoplasm and the viral PIC in infected cells at 6h and 16h post infection (PI) (). These results demonstrate that Rev expressed from unintegrated DNA can be translocated into the PIC (see also ( 16)). Analysis of the cytoplasm, nuclei and PIC fractions) revealed IN enzymatic activity of the PIC fraction and the presence of actin only in the cytoplasm and chromatin only in the nuclei fractions (see supplementary data and supplementary Figure S2). It also should be noted that the western blot analysis revealed () that at early stages of viral infection (at 6h and 10h PI) the LEDGF/p75 protein is present in all these three fractions but predominantly is seen in the nuclei. At later periods it is only localized within the nuclei due to its karyophilic properties.( 43). The cytoplasm and PIC presence of LEDGF/p75 at early stages of infection may be due to its interaction with the IN protein which is localized at these stages in cytoplasm and within the PIC. This is indicated by the co-IP experiments demonstrating the presence of IN-LEDGF/p75 complexes in these fractions (). At the later time of infection the increasing amounts of Rev, expressed from unitegrated viral cDNA, promote dissociation of the IN-LEDGF/p75 complex as is evident from the co-IP experiment (). It is our assumption that disruption of this complex may allow translocation of LEDGF/p75 into the nuclei of the infected cells. Our co-IP experiments () revealed that a Rev-LEDGF/p75 complex is present also in virus-infected cells. As can be seen Rev-LEDGF/p75 and IN-LEDGF/p75 complexes were present, in almost the same amounts at 10–16 h PI, but not at 6 hrs PI (). At 20 h PI, the amount of IN-LEDGF/p75 complex was reduced, in parallel with an increase in the Rev-LEDGF/p75 complex ( and Supplementary Table 1). In virus-infected Rev-expressing cells (Rev10+), the IN-LEDGF/p75 complex could not be detected (). The presence of overexpressed Rev may either induce the dissociation or disrupt the formation of the IN-LEDGF/p75 complex, confirming the results obtained in-vitro (). On the other hand, the formation of Rev-IN and Rev-LEDGF/p75 complexes was not affected in Rev10+ cells, as demonstrated by the co-IP experiments (). Only IN-LEDGF/p75 complex was detected in cells infected with the ΔRev virus (). Correspondingly, only Rev-IN complex was observed in LEDGF/p75-knockdown cells (). As expected, no complex between IN and either LEDGF/p75 or Rev was detected in LEDGF/p75-knockdown cells infected with the ΔRev virus (), supporting the validity of our experimental system. Our experiments also demonstrated that the formation of either Rev-LEDGF/p75 or Rev-IN complexes does not require the presence of a functional Rev as was demonstrated following infection with a Rev-M10 mutated HIV (( 26) and not shown). Specific disruption of the Rev, IN and LEDGF/p75 interaction by peptides derived from the interaction domains The use of peptides in the co-IP experiments verified the specificity of the domains, which mediate the Rev-LEDGF/p75, Rev-IN and IN-LEDGF/p75 interactions, as were characterized by the in-vitro binding studies (see also supplementary data and supplementary Figure S1). The IN-binding Rev-derived peptides, Rev 13–23 and Rev 53–67 (( 12) and supplementary Figure S1), failed to disrupt the Rev-LEDGF/p75 and the IN-LEDGF/p75 complexes but were able to disrupt the Rev-IN complex (). Recently, we reported the selection of two IN-derived peptides—IN 66–80 and IN 118–128, designated INr-1 and 2, respectively—which specifically interact with the Rev protein and are able to promote dissociation of the Rev-IN complex in-vitro ( 11). As can be seen in , indeed these two peptides prevented co-IP of Rev and IN in the HIV-infected cells. Similar to the IN binding Rev derived, these peptides failed to disrupt the Rev-LEDGF/p75 or IN-LEDGF/p75 complexes (). On the other hand, the LEDGF/p75-binding Rev-derived peptides, Rev 35–50 and Rev 75–84 selected in the present work ( supplementary Figure S1), were able to specifically prevent co-IP of the Rev-LEDGF/p75 complex (). The LEDGF/p75 derived peptides, LEDGF 361–370 and LEDGF 402–411, which were shown to mediate the binding of LEDGF/p75 to both IN ( 44) and Rev ( supplementary Figure S1), disrupted formation of the IN-LEDGF/p75 and Rev-LEDGF/p75 complexes but not of the Rev-IN complex (). Integration of viral cDNA is regulated by the interplay between viral Rev and host-cell LEDGF/p75 proteins A) In-Vitro As previously shown ( 11), the interaction of the recombinant Rev(−GFP) protein with IN causes partial inhibition of IN’s enzymatic activity in-vitro (), The presence of Rev significantly blocked the interaction between the viral IN and its DNA substrate (). The apparent Kd of the IN-DNA interaction increased from about 30 nM to 185 nM in the presence of Rev (). On the other hand, and as has been observed previously ( 45), the LEDGF/p75 protein stimulated IN activity (). The addition of both Rev and LEDGF/p75 to IN, regardless of the order of addition, caused further inhibition of integration, above that observed with Rev(−GFP) alone (). This is probably due to the increased affinity of the inhibitory Rev protein to IN in the presence of LEDGF/p75 (). The addition of LEDGF/p75 slightly reduced the affinity of the Rev-IN complex to DNA (). Thus it is clear that even in the presence of excess LEDGF/p75, the inhibitory effect of Rev prevails may be due to direct effect on the IN activity regardless of its affinity to DNA. From these results, as well as from previous ones ( 11, 16), it appears that both the inhibitory Rev and the stimulatory LEDGF/p75 affect the enzymatic activity of IN in-vitro. B) In-Vivo To further clarify the relations between the inhibitory Rev and the stimulatory LEDGF/p75, we infected cells lacking the LEDGF/p75 protein with a ΔRev HIV. This resulted in four integration events/cell (). In contrast, practically no integration was observed when LEDGF/p75-knockdown cells with overexpressed Rev were infected by the wt HIV (). Similarly, almost no integration was observed when both types of LEDGF/p75-knockdown cells (+/− Rev expression) were infected by WT HIV (). Fluorescence immunostaining clearly demonstrated the presence of IN within the intranuclear space of the LEDGF/p75-knockdown cells infected by the ΔRev virus, indicating nuclear import of IN in the absence of LEDGF/p75 and Rev proteins (). To confirm that the viruses used in the present work are indeed infective and to study the correlation between infection and cDNA intracellular integration by the viruses used, HeLa P4 cells were infected with the following viruses: WT, ΔRev, Rev M10, and the IN-defective virus IN D64N D116N HIV (see Materials and methods and , F and G). As expected, viral p24 was detected only in systems in which both the integration process occurred and a functional Rev was present (). The different degrees of integration were not due to variations in the amount of viral cDNA present within the infected cells (). Also, as evidenced by the results depicted in , the addition of AZT completely prevented integration, as well as the appearance of viral DNA, clearly demonstrating the requirement for reverse-transcribed DNA for the observed infection. Disruption of the IN-LEDGF/p75 interaction by Rev inhibits the integration process Our present results clearly demonstrate for the first time that, in addition the previously described interaction with viral IN ( 6, 8), the LEDGF/p75 protein also interacts with the viral Rev. Formation of a Rev-LEDGF/p75 complex was demonstrated in-vitro using recombinant proteins, as well as by BiFC assay in yeast cells and by co-IP experiments in virus-infected cells. Using LEDGF/p75-derived peptides, we clearly show that the same protein domains can mediate binding of LEDGF/p75 to either IN and Rev (For a summary of the domains which mediate the formation of the various complexes and their ability of peptides bearing these domains to disrupt these complexes see ). Therefore, it is not surprising that the Rev protein is able to induce dissociation of the IN-LEDGF/p75 complex in-vitro and in virus infected cells. Interference with the formation of the IN-LEDGF/p75 complex in-vivo was especially evident in viral Rev10 + cells: due to the presence of overexpressed Rev no IN-LEDGF/p75 complex could be detected by the co-IP experiments. It is our assumption that in these cells, Rev interacts with LEDGF/p75 as well as with IN, thereby preventing the IN-LEDGF/p75 interaction and consequently the integration process (see the proposed model in ). The central role of Rev in regulating the integration process was also evident when comparing the results obtained using LEDGF/p75-deficient cells infected with WT and ΔRev viruses (). A novel view of the HIV integration process Our view of the integration process of HIV-1 cDNA, which is based on the present as well as on previous results ( 3, 4, 11, 16) is summarized in . From all these results, it appears that the integration process is regulated by interplay between the viral IN, Rev and the host-cell LEDGF/p75. IN-LEDGF/p75 complex was observed in the present work in the cytoplasm, nuclei as well as in the PIC of HIV-infected cells at early stages after infection, confirming previous observations ( 46, 47). In this early period, very little, if any, Rev-IN complex could be detected. Based on these observations, it appears that after infection, a IN-LEDGF/p75(-DNA) complex is formed either within the viral PIC or within the host cell nucleus. Potentially most, if not all, the viral cDNA of the nuclei localized IN-LEDGF/p75(-DNA) complexes should be integrated into host chromosomal DNA. Indeed, as is demonstrated in the present work and was observed before ( 16), about 10 integration events/cell were obtained in the absence of Rev as for example following infection with a ΔRev virus. On the other hand, only about 1–2 integration events/cell were obtained following infection with a wt virus ( 9, 16). These results as well as of a previous work ( 16) clearly indicate that this limited number of integration is due to the ability of Rev to interact with both, the IN and LEDGF/p75 proteins. It appears that the combination of these two interactions is required to completely block any further integration beyond the 1–2 integration events/cells. It is our assumption that this limited integration occurs—by IN-LEDGF/p75(-DNA) complexes—prior to the accumulation of sufficient amount of inhibitory Rev transcribed from unintegrated viral DNA ( 16, 18). At a later period of infection, increasing amounts of Rev-IN and Rev-LEDGF/p75 complexes could be observed with a concomitant decrease in the IN-LEDGF/p75 complex reaching a phase where none of these complexes could be detected in the PIC. This may be due to the fact most of the PIC contents had already been translocated into the nuclei of the infected cells. The view that viral DNA integration is controlled by interaction of Rev with both IN and LEDGF is further supported by the experiments showing that almost no integration is observed in LEDGF knockdown cells infected with a wt HIV ( 3, 4) while as many as 4 integrations/cell were observed following infection of the same cells with a ΔRev virus. Our previous ( 11, 16) and present results strongly suggest that the viral Rev protein is an IN inhibitor that promotes termination of the integration process and/or prevents it. In addition, our in-vitro and co-IP experiments suggest that the viral Rev protein may either interfere with the IN-LEDGF/p75 interaction or displace the LEDGF/p75 protein from the already formed IN-LEDGF/p75 complex by interacting with both IN and LEDGF/p75. From the in-vitro experiments, it appears that the affinities of LEDGF/p75 to IN and to Rev are almost the same, exhibiting a Kd of about 20 nM (). This may indicate that in the presence of high excess Rev, a Rev-LEDGF/p75 and Rev-IN complexes—rather than an IN-LEDGF/p75 complex—are formed. The same domain mediates binding of LEDGF/p75 to Rev and to IN (), making Rev a potent competitive inhibitor of IN-LEDGF/p75 binding. Moreover, the Kd of the Rev-IN interaction is 13 nM, almost the same as the affinity of Rev to LEDGF/p75 (19 nM). However, the binding affinity of Rev to the IN-LEDGF/p75 complex is relatively high, with a Kd of about 7 nM, while the affinity of LEDGF/p75 to the Rev-IN complex is somewhat lower, showing a Kd of 27 nM (), favoring the formation of an inactive Rev-IN complex once LEDGF/p75 is removed ( and ). These results indicate that relative amounts of these three partners as well as time of their appearance are crucial for their activity in regulating integration of the viral DNA. Evidently, detailed information regarding their intracellular concentrations will allow a better understanding of the above-mentioned interactions and regulation of integration. A model summarizing our view of the sequence of events leading to limitation of HIV-1 cDNA integration and of the involvement of LEDGF/p75 and Rev in regulating the integration events is presented in . Also attempts are currently being made in our laboratory to obtain mutants of HIV-1 bearing which either fail to interact with IN or with LEDGF/p75 leaving all it other biological activities intact. Such virus will be of tremendous help for getting a better understanding of the suggested interplay between the Rev, IN and LEDGF/p75 proteins. Figure S1: Identification and characterization of Rev-LEDGF/p75 interacting domains (A) LEDGF/p75 protein was added, in increasing amounts, to the following plate-bound peptides: Rev 13–23 (  ), Rev 35–50 (  ), Rev 53–67 (  ) and Rev 75–84 (  ) and the amount of bound LEDGF/p75 was estimated. (B) Rev-GFP and GFP proteins were added to plate-bound LEDGF 361–370 (  ,  respectively) and LEDGF 402–411 (  ,  respectively) peptides and the amount of Rev or GFP bound proteins was estimated. (C) Increasing amounts of the following peptides: LEDGF 361–370 ( ), LEDGF 402–411 ( ) and INr-1 (IN 66–80) ( 8) (  , control), were mixed with 100 nM LEDGF/p75 and the binding of LEDGF/p75 to plate-bound Rev-GFP was estimated. (D) Increasing amounts of the following peptides: Rev 35–50 ( ), Rev 75–84 ( ) and Rev 13–23 ( 18) (  , control), were mixed with 100 nM Rev-GFP and the binding of Rev-GFP to plate-bound LEDGF/p75 was estimated (E). Same as in (C) except that the peptides were added to an already plate-bound Rev(−GFP)-LEDGF/p75 complex. (F) Same as in (D) except that the peptides were added to an already plate-bound Rev(−GFP)-LEDGF/p75 complex. (G) Binding of Rev 35–50 peptide to plate-bound LEDGF 361–370 ( ) and LEDGF 402–411 ( ) peptides, and binding of Rev 75–84 peptide to plate-bound LEDGF 361–370 ( ) and LEDGF 402–411 ( ) peptides. (H) Apparent Kd as calculated according to the results obtained in A, B and G. All other details are described in Materials and methods. Figure S2: Cytoplasm, Nuclei and PIC fraction analysis The different cells fractions of HIV-1 infected wt and over expressing Rev cells were isolated as described in Material and Methods and at different times after infection. The amount of viral cDNA (A) and the viral IN enzymatic activity (B) were estimated by real time PCR as described in Material and Methods. Western blot analysis (C) for detection of actin and histone H3 was performed as described in Materials and Methods. Supplementary Table 1 Quantitative estimation of the co-immunoprecipitation (co-IP) (see ) bands was performed by Image Gauge V3.46 software (Fujifilm) | Supplementary Table 1A |
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| Time | Fraction | IP with | Anti-Rev | Anti-IN | Anti-LEDGF |
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| 6 h PI | Nucleus | Actin | 0.51 | 0 | 1.06 | | Rev | 0.16 | 0 | 0 | | IN | 0.81 | 5.98 | 22.82 | | LEDGF | 0.69 | 5.94 | 41.93 | | Cytoplasm | Actin | 0.31 | 0 | 0 | | Rev | 28.37 | 5.85 | 12.96 | | IN | 15.66 | 30.35 | 16.02 | | LEDGF | 2.57 | 3.72 | 26.2 | | PIC | Actin | 1.77 | 0 | 0 | | Rev | 3.89 | 0.98 | 1.27 | | IN | 5.22 | 32.83 | 16.15 | | LEDGF | 1.97 | 11.81 | 26.49 | | 10 hrs PI | Nucleus | Actin | 1.63 | 0 | 0.02 | | Rev | 54.67 | 6.03 | 24.08 | | IN | 26.56 | 31.26 | 42.07 | | LEDGF | 27.79 | 36.53 | 49.1 | | Cytoplasm | Actin | 1.59 | 0.02 | 0.14 | | Rev | 56.08 | 31.13 | 14.96 | | IN | 57.3 | 30.42 | 16.48 | | LEDGF | 20.83 | 7.23 | 28.77 | | PIC | Actin | 1.11 | 0 | 0 | | Rev | 34.47 | 7.09 | 17.7 | | IN | 29.51 | 31.77 | 15.87 | | LEDGF | 12.48 | 34.09 | 25.06 | | 16 h PI | Nucleus | Actin | 6.08 | 0 | 0.11 | | Rev | 68.14 | 37.09 | 47.01 | | IN | 65.88 | 32.12 | 42.87 | | LEDGF | 66.91 | 31.93 | 41.43 | | Cytoplasm | Actin | 6.67 | 3.25 | 5.23 | | Rev | 59.36 | 27.46 | 15.82 | | IN | 68.39 | 32.79 | 11.57 | | LEDGF | 23.18 | 8.81 | 12.37 | | PIC | Actin | 8.4 | 0.06 | 0.44 | | Rev | 10.41 | 2.78 | 3.07 | | IN | 8.12 | 2.45 | 3.02 | | LEDGF | 8.59 | 2.51 | 2.91 | | 20 h PI | Nucleus | Actin | 6.25 | 0.24 | 0.34 | | Rev | 68.64 | 0.11 | 39.76 | | IN | 65.5 | 5.98 | 14.38 | | LEDGF | 64.85 | 32.42 | 41.6 | | Cytoplasm | Actin | 7.48 | 30.82 | 2.47 | | Rev | 65.05 | 29.68 | 10.14 | | IN | 63.06 | 33.18 | 4.26 | | LEDGF | 29.89 | 3.88 | 18.72 | | PIC | Actin | 7.24 | 0.06 | 1.33 | | Rev | 8.14 | 0 | 0 | | IN | 9.03 | 0 | 0 | | LEDGF | 6.72 | 0 | 0 |
| Supplementary Table 1B |
|---|
| Times | IP with: | Anti-Rev | Anti-IN | Anti-LEDGF | Anti-Actin |
|---|
| 6 h PI | Lysate | 81.54 | 10.82 | 56.75 | 30.4 | | Actin | 0.38 | 8 | 0.11 | 36.53 | | Rev | 81.28 | 11.13 | 58.53 | 0 | | IN | 19.62 | 11.04 | 0.07 | 0 | | LEDGF | 80.19 | 8 | 54.67 | 0 | | 10 h PI | Lysate | 73.66 | 10.71 | 50.14 | 30.23 | | Actin | 0.36 | 8 | 0.03 | 35.3 | | Rev | 83.29 | 11.3 | 60.31 | 0 | | IN | 21.54 | 11.26 | 0.03 | 0 | | LEDGF | 83.2 | 8 | 60.28 | 0 | | 16 h PI | Lysate | 78.48 | 21.74 | 56.76 | 32.83 | | Actin | 0.79 | 8 | 0 | 38.12 | | Rev | 77.82 | 20.94 | 53 | 0.09 | | IN | 36.82 | 21.39 | 0 | 0 | | LEDGF | 80.61 | 8 | 57.83 | 0 | | 20 h PI | Lysate | 81.65 | 22.29 | 56.97 | 32.8 | | Actin | 1.26 | 8 | 0 | 36.57 | | Rev | 80.72 | 22.22 | 59.18 | 0 | | IN | 40.87 | 22.58 | 0.02 | 0 | | LEDGF | 91.07 | 8.01 | 64.68 | 0 |
| Supplementary Table 1C |
|---|
| Times | IP with: | Anti-Rev | Anti-IN | Anti-LEDGF | Anti-Actin |
|---|
| 6 h PI | Lysate | 0.24 | 19.2 | 53 | 44.75 | | Actin | 0.25 | 7 | 0.15 | 47.47 | | Rev | 0.25 | 7 | 0 | 0.22 | | IN | 0.25 | 20.89 | 11.08 | 0 | | LEDGF | 0.26 | 20.82 | 56.39 | 0 | | 10 h PI | Lysate | 0.26 | 35.17 | 52.96 | 40.66 | | Actin | 0.26 | 7 | 0 | 45.36 | | Rev | 0.26 | 7 | 0 | 0.23 | | IN | 0.26 | 40.35 | 28.13 | 0 | | LEDGF | 0.24 | 40.25 | 61.34 | 0 | | 16 h PI | Lysate | 0.24 | 77.95 | 57.47 | 42.78 | | Actin | 0.24 | 7 | 0 | 48.8 | | Rev | 0.25 | 7 | 0.03 | 0.95 | | IN | 0.26 | 79.73 | 57.77 | 0 | | LEDGF | 0.26 | 80.29 | 59.35 | 0 | | 20 h PI | Lysate | 0.25 | 80.86 | 60.75 | 43.68 | | Actin | 0.25 | 7 | 0 | 44.36 | | Rev | 0.25 | 7.02 | 0 | 0.71 | | IN | 0.26 | 81.83 | 62 | 0 | | LEDGF | 0.25 | 88.43 | 67.87 | 0 |
| Supplementary Table 1D |
|---|
| Times | IP with: | Anti-Rev | Anti-IN | Anti-LEDGF | Anti-Actin |
|---|
| 6 h PI | Lysate | 17.82 | 10.37 | 0.24 | 23.62 | | Actin | 0.01 | 0 | 0.28 | 29.1 | | Rev | 17.59 | 11.95 | 0.27 | 0.11 | | IN | 17.2 | 11.34 | 0.28 | 0 | | LEDGF | 0 | 0 | 0.27 | 0 | | 10 h PI | Lysate | 58.52 | 10.28 | 0.28 | 24.82 | | Actin | 0 | 0 | 0.28 | 27.71 | | Rev | 69.27 | 12.46 | 0.27 | 0.01 | | IN | 19.55 | 12.31 | 0.28 | 0 | | LEDGF | 0 | 0 | 0.28 | 0 | | 16 h PI | Lysate | 64.1 | 11.44 | 0.28 | 25.78 | | Actin | 0 | 0 | 0.27 | 30.13 | | Rev | 62.33 | 11.08 | 0.28 | 0.03 | | IN | 17.18 | 11.21 | 0.28 | 0 | | LEDGF | 0.01 | 0 | 0.28 | 0 | | 20 h PI | Lysate | 66.58 | 11.88 | 0.28 | 27.04 | | Actin | 0 | 0 | 0.28 | 28.74 | | Rev | 66.81 | 11.72 | 0.28 | 0.01 | | IN | 19.99 | 12.23 | 0.27 | 0 | | LEDGF | 0 | 0 | 0.27 | 0 |
| Supplementary Table 1E |
|---|
| Times | IP with: | Anti-Rev | Anti-IN | Anti-LEDGF | Anti-Actin |
|---|
| 6 h PI | Lysate | 0.37 | 10.3 | 0.46 | 30.97 | | Actin | 0.37 | 0 | 0.48 | 33.59 | | Rev | 0.37 | 0.04 | 0.48 | 0.01 | | IN | 0.38 | 11.19 | 0.49 | 0 | | LEDGF | 0.41 | 0 | 0.48 | 0 | | 10 h PI | Lysate | 0.38 | 10.36 | 0.46 | 29.43 | | Actin | 0.43 | 0 | 0.55 | 31.89 | | Rev | 0.43 | 0 | 0.56 | 0.18 | | IN | 0.42 | 12.14 | 0.52 | 0 | | LEDGF | 0.44 | 0 | 0.56 | 0 | | 16 h PI | Lysate | 0.39 | 11.42 | 0.50 | 29.54 | | Actin | 0.38 | 0 | 0.49 | 32.69 | | Rev | 0.38 | 0 | 0.48 | 2.1 | | IN | 0.37 | 10.99 | 0.49 | 0 | | LEDGF | 0.38 | 0 | 0.50 | 0 | | 20 h PI | Lysate | 0.38 | 12.08 | 0.56 | 30.39 | | Actin | 0.38 | 0 | 0.56 | 29.63 | | Rev | 0.40 | 0 | 0.55 | 2.9 | | IN | 0.41 | 12.21 | 0.55 | 0 | | LEDGF | 0.40 | 0 | 0.56 | 0 |
| Supplementary Table 1F |
|---|
| Times | Peptide | IP with: | Anti-Rev | Anti-IN | Anti-LEDGF |
|---|
| 6 h PI | INr-1 & 2 | Rev | 26.42 | 0.01 | 7.86 | | IN | 0.83 | 17.02 | 19.95 | | LEDGF | 0 | 17.44 | 42.58 | | Rev 13–23 & 53–67 | Rev | 18.08 | 0 | 6.48 | | IN | 0 | 16.68 | 21.52 | | LEDGF | 0 | 15.04 | 46.15 | | Rev 35–50 & 75–84 | Rev | 91.13 | 6.6 | 0.04 | | IN | 24.39 | 39.75 | 21.74 | | LEDGF | 0 | 16.12 | 52.39 | | LEDGF 361–370 & 402–411 | Rev | 92.76 | 6.73 | 0 | | IN | 19.41 | 38.63 | 0.36 | | LEDGF | 0.02 | 0.01 | 52.63 | | 10 h PI | INr-1 & 2 | Rev | 16.46 | 1.3 | 19.81 | | IN | 0.02 | 41.06 | 46.03 | | LEDGF | 19.65 | 36.47 | 49.39 | | Rev 13–23 & 53–67 | Rev | 23.6 | 0.02 | 20.75 | | IN | 0 | 41.59 | 46.67 | | LEDGF | 18.95 | 36.51 | 58.28 | | Rev 35–50 & 75–84 | Rev | 111.49 | 39.95 | 0.02 | | IN | 97.09 | 41.2 | 50.18 | | LEDGF | 0.01 | 40.46 | 55.05 | | LEDGF 361–370 & 402–411 | Rev | 108.23 | 41.8 | 0.05 | | IN | 109.81 | 35.24 | 0.02 | | LEDGF | 0 | 0 | 51.15 | | 16 h PI | INr-1 & 2 | Rev | 18.23 | 0 | 47.29 | | IN | 0.01 | 30.04 | 12.37 | | LEDGF | 85.94 | 11.41 | 50.78 | | Rev 13–23 & 53–67 | Rev | 18.75 | 0.03 | 51.58 | | IN | 0.01 | 31.07 | 12.14 | | LEDGF | 83.08 | 11.79 | 40.16 | | Rev 35–50 & 75–84 | Rev | 90.12 | 29.11 | 0.93 | | IN | 83.24 | 35.53 | 31.41 | | LEDGF | 0.01 | 10.23 | 53.27 | | LEDGF 361–370 & 402–411 | Rev | 91.76 | 32.44 | 0 | | IN | 86.63 | 33.35 | 0.3 | | LEDGF | 0 | 0 | 52.81 | | 20 h PI | INr-1 & 2 | Rev | 15.81 | 0 | 45.57 | | IN | 0 | 28.8 | 12.26 | | LEDGF | 86.09 | 10.08 | 50.96 | | Rev 13–23 & 53–67 | Rev | 17.18 | 0 | 56.01 | | IN | 0 | 31.7 | 15.35 | | LEDGF | 90.36 | 12.14 | 53.33 | | Rev 35–50 & 75–84 | Rev | 80.62 | 31.64 | 0.01 | | IN | 77.85 | 34.89 | 34.09 | | LEDGF | 1.09 | 11.33 | 50.3 | | LEDGF 361–370 & 402–411 | Rev | 92.01 | 32.36 | 0 | | IN | 86.65 | 38.89 | 0 | | LEDGF | 0 | 0.01 | 55.22 |
Materials and Methods Cells Monolayer adherent HEK293T cells, HEK293T cells overexpressing Rev (Rev10 + cells) and HeLa MAGI cells (TZM-bl) ( 1, 2) were grown in Dulbecco’s modified Eagle’s Medium (DMEM). The T-lymphocyte cell lines Sup-T1 and H9 were grown in RPMI 1640 medium. Cells other than the Rev10 + cells were provided by the NIH Reagent Program, Division of AIDS, NIAID, NIH, Bethesda, MD, USA. Cells were incubated at 37°C in a 5% CO 2 atmosphere. All media were supplemented with 10% (v/v) fetal calf serum, 0.3 g/l L-glutamine, 100 U/ml penicillin and 100 U/ml streptomycin (Biological Industries, Beit Haemek, Israel). HeLaP4/shp75Cl15 cells (LEDGF/P75 knockdown cell line), a generous gift of Prof. Z. Debyser (Molecular Medicine, K.U. Leuven, Flanders, Belgium), were grown as described in ( 3). Rev10 + and LEDGF/p75-knockdown Rev-expressing cells were generated by transfection into HEK293T and HeLaP4/shp75Cl15 cells, respectively ( 4) with pcDNA3.1 plasmid bearing the full Rev. Viruses WT HIV-1 (HXB2 ( 5)) and ΔEnv ( 6), as well as the IN mutant D64N D116N ( 7), were generated by transfection into HEK293T cells ( 4) of the virus-containing plasmid or co-transfected with a plasmid containing VSV-G ( 8). ΔRev pLAIY47H2 ( 9) and Rev M10 ( 10) HIVs were generated by transfection into Rev10 + cells. Viruses were harvested and stored as described in ( 8). The pLAIY47H2 ( 9) viruses were a generous gift from Prof. B. Berkhout (Department of Human Retrovirology, Academic Medical Center, University of Amsterdam, The Netherlands), and the IN mutant D64N D116N virus was a generous gift from Prof. A. Engelman (Department of Cancer Immunology and AIDS Dana-Farber Cancer Institute and Division of AIDS, Harvard Medical School, Boston, MA, USA). Infection of cultured cells Cultured lymphocytes were infected exactly as described in ( 8). Briefly, Cultured lymphocytes (1 × 10 5) were centrifuged for 5 min at 2000 rpm and after removal of the supernatant, the cells were resuspended in 0.2 to 0.5 ml of RPMI 1640 medium containing virus at the indicated multiplicity of infection (MOI). Following absorption for 2 h at 37°C, the cells were washed to remove unbound virus and then incubated at the same temperature in RPMI 1640 medium. Cultured HEK293T cells, Rev10 + cells and HeLa MAGI cells (TZM-bl) were grown for 24 h before infection, then the medium was discarded and cells were incubated at different multiplicity of infections (MOI) with the indicated virus for 2 h at 37°C. Cells were washed three times with PBS and incubated in DMEM. Virus stock titration and normalization Quantitative titration of HIV-1 was carried out using the MAGI assay, as described by Kimpton and Emerman ( 2). Briefly, TZM-b1 cells were grown in 96-well plates at 1 × 10 4 cells per well. the cells were infected with 50 μl of serially diluted virus (wild-type, ΔRev or Rev M10 HIV-1) as described ( 2). Two days post-infection (PI), cultured cells were fixed and β-galactosidase was estimated exactly as described previously ( 2). Blue cells were counted under a light microscope at 200X magnification. In the case of IN mutant viruses (D64N D116N) the amount of viral RNA was estimated by real time revers transcription PCR as described in ( 11). This was also proformed to the other HIV-1 viruses and was used to normalize the titer of the IN mutant virus. Peptide synthesis and purification Peptides were synthesized on an Applied Biosystems (ABI) 433A peptide synthesizer and purification was performed on a Gilson HPLC using a reverse-phase C8 semi-preparative column (ACE, Advanced Chromatography Technologies, USA) as described in ( 8). Protein expression and purification Expression and purification of histidine-tagged Rev-GFP conjugate was performed as previously described ( 12). The histidine-tagged IN and LEDGF/p75 expression vectors were a generous gift from Prof. A. Engelman and their expression and purification were performed essentially as described previously ( 13, 14). GST-Tat was expressed and purified as described previously ( 15). ELISA-based binding assays Protein-peptide, protein-protein and protein-DNA binding was estimated using an ELISA-based binding assay exactly as described previously ( 16). Briefly, Maxisorp plates (Nunc) were incubated at room temperature for 2 h with 200 ml of 10 μg/ml synthetic peptide/recombinant protein in carbonate buffer (0.05 M Na 2CO 3/0.05 M NaHCO 3, pH 9.6). After incubation, the solution was removed, the plates were washed three times with PBS, and 200 μl of 10% BSA (Sigma) in PBS (w/v) was added for 2h at room temperature. After rewashing with PBS, tested BSA-biotinilated (Bb), peptide or protein (alone or biotinilated) or biotinilated DNA were added for further incubation for 1h at room temperature. Following three washes with PBS, the concentration of bound molecules was estimated after the addition of streptavidin-horseradish peroxidase (HRP) conjugate (Sigma), as described previously ( 17), or of anti-GFP mouse antibody (Santa Cruz) followed by rabbit anti-mouse IgG antibody conjugated to HRP. The enzymatic activity of HRP was estimated by monitoring the product’s optical density (OD) at 490 nm using an ELISA plate reader (Tecan Sunrise Swizerland ). Each measurement was performed in duplicate. For dissociation from and binding to a complex after binding of the first protein to the Maxisorp plate, the binding partner was incubated for 1 h at room temperature and after three washes with PBS, the dissociated component was added and its binding to the complex, as well the amount of remaining bound complex, were estimated separately as described above. In-vitro IN activity assay Quantitative determination of IN activity was performed exactly as described previously ( 18) using a previously described assay system ( 19, 20). Briefly, the oligonucleotide substrate consisted of one oligo (5′-ACTGCTAGAGATTTTCCACACTGACTAAAAGGGTC-3') labeled with biotin at the 3′ end and the other oligo (5′-GACCCTTTTAGTCAGTGTGGAAAATCTCTAGCAGT-3') labeled with digoxigenin at the 5′ end. When inhibition was studied, the IN was preincubated with the peptide or protein for 15 min prior to addition of the DNA substrate. The entire IN reaction was followed by immunosorbent assay on avidin-coated plates as described previously ( 18, 20). Plasmid construction All of the plasmids used in this study were constructed using PCR cloning techniques with the highfidelity enzyme Platinum Pfx DNA polymerase (Invitrogen). Clones were subjected to automated DNA sequencing. For the bimolecular fluorescence complementation (BiFC) experiments, the yeast multicopy shuttle vectors pRS423 (with HIS3 as the selective marker) and pRS426 (with URA3 as the selective marker), both with the ADH1 promoter, were used as the cloning plasmids (a kind gift from Dr. D. Engelberg, Alexander Silberman Institute, The Hebrew University of Jerusalem). The DNA-coding region of the two yeast green fluorescent protein (GFP) fragments ( 21), namely the N terminus (GN) containing GFP amino acids 1–154, and theCterminus (GC) containing GFP amino acids 155–239, were cloned into pRS423 and pRS426, respectively. A linker consisting of (GGS) 5 was used to separate the inserted genes. The final vectors were termed GN-linker (cloned into pRS423) and GC-linker (cloned into pRS426). The coding sequences of full-length HIV-1 IN, Rev, LEDGF/p75 and Tat were individually amplified by PCR and inserted in-frame into the corresponding sites of the GN-linker and GC-linker to allow reunition of the two GFP halves into a functional protein upon protein-protein interaction as described in ( 18). Bimolecular fluorescence complementation (BiFC) The above-described plasmids were transformed into the yeast strain EGY48 (Clontech) and the cells were grown on yeast nitrogen base medium lacking histidine and uracil. After 48 h at 30 °C, the plates were transferred to 23 °C for 2 to 3 days and the appearance of fluorescence in yeast cells was visualized by confocal microscope (MRC 1024 confocal imaging system, Bio-Rad), were as previously described ( 22) ( 18). Study of in-vivo protein-protein interactions by co-IP The co-IP experiments were conducted essentially as described previously ( 23) with several modifications. Briefly, cells were infected with a MOI of 15 for the indicated viruses. Cells were harvested at different times post infection (PI), washed three times in PBS and lysed by the addition of PBS containing 1% (v/v) Triton X-100 for whole-cell lysate. Cytoplasmic, nuclei and PIC fractions were isolated as described below. Half of the lysate or the isolated fraction was subjected to SDS-PAGE and immunoblotted with either a monoclonal anti-Rev antibody (α-Rev) ( 24) or antiserum raised against IN amino acids 276–288 (α-IN) (NIH AIDS Research & Reference Reagent Program catalog number 758), or anti-LEDGF/p75 (α-LEDGF/p75) (R&D Systems) or anti-actin (α-actin) antibody (Santa Cruz), and the complementary HRP-conjugated secondary antibodies (Jackson). The remaining lysate or isolated fractions were incubated for 1 h at 4°C with either the α-Rev, α-IN, α-LEDGF/p75 or α-actin antibodies. Following a 3-h incubation with protein G-agarose beads (Santa Cruz) at 4°C, the samples were washed three times with PBS containing 1% (v/v) Nonidet P-40. SDS buffer was added to the samples and after boiling and subjecting to SDS-PAGE, the membranes were immunoblotted with either α-Rev, α-IN, α-LEDGF/p75 or α-actin antibodies, and the complementary HRP-conjugated secondary antibodies. When peptides were used, cells were incubated with 150 μM of the indicated peptide for 2 h prior to infection. Quantitative estimation of the bands was performed by Image Gauge V3.46 software (Fujifilm). Isolation of cytoplasm, nuclei and PIC from infected cells The various fractions were obtained from virus-infected cells essentially as described previously ( 25) with several modifications. Briefly, cells were harvested and washed twice in buffer A (20 mM Hepes pH 7.3, 150 mM KCl, 5 mM MgCl 2, 1 mM DTT and 0.1 mM PMSF). Cells were then suspended in 200 μl of buffer A with 0.025% (w/v) digitonin and incubated at room temperature for 10 min. Cells were centrifuged for 3 min at 1000 g at room temperature. The supernatant was then centrifuged at 8000 g and separated into supernatant (Cytoplasmic fraction) and pellet (nuclei fraction) and stored at −70°C. For PIC isolation, an equal volume of buffer B (20 mM Hepes pH 7.4, 5 mM MgCl 2, 1 mM DTT and 0.1 mM PMSF) was added to the cytoplasm fraction. Samples were incubated for 10 min at room temperature and then centrifuged for 10 min at 2000 g. The supernatant was discarded and the pellet, containing the PIC aggregates, was stored at −70°C. Cytoplasm, nuclei and PICfraction Analysis Cytoplasm and nuclei fractions were analyzed by western blot as described above. For detection of fraction specific protein ant actin antibody (Santa Cruz) and anti histone H3 antibody (abcam) were used. For the anlysis of the PIC a total viral DNA was estimated by real time PCR as described below as well as integration of the PIC fraction in-vitro as described at ( 26). Quantitative analysis of copy numbers of HIV-1 DNA integrated into the cellular genome (integration events) The integration reaction, as well as the integration events, were performed exactly as described previously ( 8). Briefly, Integrated HIV-1 sequences were amplified by two PCR replication steps using the HIV-1 LTR-specific primer (LTR-TAG-F 5’-ATGCCACGTAAGCGAAACTCTGGCTAACTAGGGAACCCACTG-3’) and Alu-targeting primers (first-Alu-F 5’-AGCCTCCCGAGTAGCTGGGA-3’ and first-Alu-R 5’-TTACAGGCATGAGCCACCG-3’) ( 27). Alu-LTR fragments were amplified from 10 ng of total cell DNA in a 25-μl reaction mixture containing 1X PCR buffer, 3.5 mM MgCl 2, 200 μM dNTPs, 300 nM primers, and 0.025 units/μl of Taq polymerase. The first-round PCR cycle conditions were as follows: a DNA denaturation and polymerase activation step of 10 min at 95°C and then 12 cycles of amplification (95°C for 15 s, 60°C for 30 s, 72°C for 5 min). During the second-round PCR, the first-round PCR product could be specifically amplified by using the tag-specific primer (tag-F 5’-ATGCCACGTAAGCGAAACTC-3’) and the LTR primer (LTR-R 5’-AGGCAAGCTTTATTGAGGCTTAAG-3’) designed by PrimerExpress (Applied Biosystems) using default settings. The second-round PCR was performed on 1/25th of the first-round PCR product in a mixture containing 300 nM of each primer, 12.5 μl of 2X SYBR Green master mixture (Applied Biosystems) at a final volume of 25 μl, run on an ABI PRIZM 7700 (Applied Biosystems). The second-round PCR cycles began with DNA denaturation and a polymerase-activation step (95°C for 10 min), followed by 40 cycles of amplification (95°C for 15 s, 60°C for 60 s). For generation of a standard calibration curve, the SVC21 plasmid containing the full-length HIV-1 HXB2 viral DNA was used as a template. In the first-round PCR, the LTR-TAG-F and LTR-R primers were used and the second-round PCR was performed using the tag-F and LTR-R primers. The standard linear curve was in the range of 5 ng to 0.25 fg ( R = 0.99). DNA samples were assayed with quadruplets of each sample. For further experimental details see ref ( 18). The cell equivalents in the sample DNA were calculated based on amplification of the 18S gene by real-time PCR as described in ( 28). Quantitation of total viral DNA Total viral DNA was estimated using SYBR green real-time quantitative PCR 12 h PI, exactly as described in ( 29). Briefly, DNA samples (1 μg of DNA) were added to 95 μl containing 1×Hot-Rescue Real Time PCR Kit-SG (Diatheva s.r.l, Fano, Italy), and 100 nM of each PBS (primer-binding site) primer: F5 (5′ primer, 5′-TAGCAGTGGCGCCCGA-3′) and R5 (3′ primer, 5’-TCTCTCTCCTTCTAGCCTCCGC-3’). All amplification reactions were carried out using an ABI Prism 7700 Sequence Detection System (Applied Biosystems): One cycle at 95 °C for 10 min, followed by 45 cycles of 15 s at 95 °C and 35 s at 68 °C. In each PCR run, three replicates were performed. Quantitative estimation of HIV-1 infection by determination of extracellular p24 The amount of p24 protein was estimated in the cell medium using an ELISA based capture assay kit (SAIC, AIDS Vaccine Program, Frederick, MD), according to the manufacturer’s instructions and as described previously ( 18). Immunostaining HeLaP4/shp75Cl15 cells were grown on chamber slides (Nunc), then infected with ΔRev HIV-1 at a MOI of 25. Cells were fixed 16 h PI exactly as described previously ( 30) and immunostained essentially as described previously ( 30) with some modifications. Briefly, after fixation, cells were blocked with 5% IgG-free BSA (Jackson) in PBS for 60 min. For detection of HIV-1 IN and Rev and the host LEDGF/p75, the cells were incubated with 1:50 rabbit α-IN (NIH AIDS Research & Reference Reagent Program catalog number 758), 1:50 rat α-Rev ( 24) and 1:100 goat α-LEDGF/p75 (R&D Systems) at room temperature for 60 min each. Cells were washed five times with PBS + 0.05% (v/v) Tween 20 between antibodies. Then the cells were incubated with the following secondary antibodies: Cy2-conjugated anti-rat, Cy3-conjugated anti-rabbit and Cy5-conjugated anti-goat (Jackson) (all diluted 1:100) at room temperature for 60 min each, with five washes with PBS + 0.05% Tween 20 between antibodies. For detection of DNA, cells were stained with DAPI according to the manufacturer's protocol. Slides were prepared with Mounting Media (Bio-Rad) and immunofluorescent cells were detected with an Olympus confocal microscope. Statistic analysis p < 0.05, calculated from at least 3 repetitions for Real time analysis p < 0.01, ± stand for standard deviation. Results Characterization of the domains mediating the Rev-LEDGF/p75 interaction We used a Rev-derived peptide library (AIDS Research and Reference Reagent Program, Division of AIDS, NIAID, NIH: HIV-1 Consensus B Rev (15-mer) Peptides - Complete Set) to determine the sequences in Rev that specifically interact with the LEDGF/p75 protein. Two peptides, bearing residues 35 to 50 and 75 to 84 of the Rev protein, were found to interact with LEDGF/p75 ( Figure S1A and H). Rev 35–50 and Rev 75–84 bear the Rev arginine-rich motif (ARM) and nuclear export signal (NES), respectively ( 31). In addition, two LEDGF/p75-derived peptides (LEDGF 361–370 and LEDGF 402–411), which were previously found by us to interact with IN ( 32), also interacted with Rev-GFP ( Figure S1B and H). It may be suggested that the peptides Rev 35–50, Rev 75–84, LEDGF 361–370 and LEDGF 402–411 represent the binding domains between Rev and LEDGF/p75. Support for this view was obtained from the results depicted in Figure S1C–F, showing that both groups of peptides were able to compete for binding and promote dissociation of the Rev-LEDGF/p75 interaction. Moreover, a mutual interaction between these two groups of peptides was observed ( Figure S1G and H). IN enzymatic activity of PIC isolated from HIV-1 infected cells The results in Figure S2A and B show that the PIC fraction obtained from a wt infected cells bears-as expected-viral cDNA as well as an enzymatic active IN. Optimal activity was observed in PIC obtained from infected cell between 10–16h PI. Both viral cDNA and IN enzymatic activity were diminished in PIC obtained from cells after 20h PI probably due to intracellular degradation processes. On the other hand in PIC isolated from infected cells with over expressing Rev, the amount of cDNA was the same as that observed as in wt cells but no IN enzymatic activity was detected due probably to the Rev inhibitory effect. Characterization of the Cytoplasm and Nuclei fraction of HIV infected cells Our western blot analysis clearly has indicated that no cross contamination has been occurred between the cytoplasm and nuclei fraction obtained from viral infected cells. This is evident from the results showing the presence of histone H3 protein in the nuclei but not in the cytoplasm fraction and on the other hand the presence of actin molecules in the cytoplasm but not in the nuclei fraction ( Figure S2C). References 1. Derdeyn CA, et al. Sensitivity of human immunodeficiency virus type 1 to the fusion inhibitor T-20 is modulated by coreceptor specificity defined by the V3 loop of gp120. J Virol. 2000;74:8358–8367. [PubMed] 2. Kimpton J, Emerman M. Detection of replication-competent and pseudotyped human immunodeficiency virus with a sensitive cell line on the basis of activation of an integrated beta-galactosidase gene. J Virol. 1992;66:2232–2239. [PubMed] 3. Vandekerckhove L, et al. Transient and stable knockdown of the integrase cofactor LEDGF/p75 reveals its role in the replication cycle of human immunodeficiency virus. J Virol. 2006;80:1886–1896. [PubMed] 4. Cullen BR. Use of eukaryotic expression technology in the functional analysis of cloned genes. Methods Enzymol. 1987;152:684–704. [PubMed] 5. Ratner L, et al. Complete nucleotide sequence of the AIDS virus, HTLV-III. Nature. 1985;313:277–284. [PubMed] 6. Gummuluru S, Kinsey CM, Emerman M. An in vitro rapid-turnover assay for human immunodeficiency virus type 1 replication selects for cell-to-cell spread of virus. J Virol. 2000;74:10882–10891. [PubMed] 7. Nakajima N, Lu R, Engelman A. Human immunodeficiency virus type 1 replication in the absence of integrase-mediated dna recombination: definition of permissive and nonpermissive T-cell lines. J Virol. 2001;75:7944–7955. [PubMed] 8. Levin A, et al. Peptides derived from HIV-1 integrase that bind Rev stimulate viral genome integration. PLoS ONE. 2009;4:e 4155. 9. Verhoef K, Koper M, Berkhout B. Determination of the minimal amount of Tat activity required for human immunodeficiency virus type 1 replication. Virology. 1997;237:228–236. [PubMed] 10. Bahner I, et al. Comparison of trans-dominant inhibitory mutant human immunodeficiency virus type 1 genes expressed by retroviral vectors in human T lymphocytes. J Virol. 1993;67:3199–3207. [PubMed] 11. Pizzato M, et al. A one-step SYBR Green I-based product-enhanced reverse transcriptase assay for the quantitation of retroviruses in cell culture supernatants. J Virol Methods. 2009;156:1–7. [PubMed] 12. Fineberg K, et al. Inhibition of nuclear import mediated by the Rev-arginine rich motif by RNA molecules. Biochemistry. 2003;42:2625–2633. [PubMed] 13. Jenkins TM, Engelman A, Ghirlando R, Craigie R. A soluble active mutant of HIV-1 integrase: involvement of both the core and carboxyl-terminal domains in multimerization. J Biol Chem. 1996;271:7712–7718. [PubMed] 14. Turlure F, Maertens G, Rahman S, Cherepanov P, Engelman A. A tripartite DNA-binding element, comprised of the nuclear localization signal and two AT-hook motifs, mediates the association of LEDGF/p75 with chromatin in vivo. Nucleic Acids Res. 2006;34:1653–1675. [PubMed] 15. Rhim H, Echetebu CO, Herrmann CH, Rice AP. Wild-type and mutant HIV-1 and HIV-2 Tat proteins expressed in Escherichia coli as fusions with glutathione S-transferase. J Acquir Immune Defic Syndr. 1994;7:1116–1121. [PubMed] 16. Rosenbluh J, et al. Positively charged peptides can interact with each other, as revealed by solid phase binding assays. Anal Biochem. 2006;352:157–168. [PubMed] 17. Melchior F, Paschal B, Evans J, Gerace L. Inhibition of nuclear protein import by nonhydrolyzable analogues of GTP and identification of the small GTPase Ran/TC4 as an essential transport factor. J Cell Biol. 1993;123:1649–1659. [PubMed] 18. Rosenbluh J, et al. Interaction between HIV-1 Rev and Integrase Proteins: A BASIS FOR THE DEVELOPMENT OF ANTI-HIV PEPTIDES. J Biol Chem. 2007;282:15743–15753. [PubMed] 19. Craigie R, Mizuuchi K, Bushman FD, Engelman A. A rapid in vitro assay for HIV DNA integration. Nucleic Acids Res. 1991;19:2729–2734. [PubMed] 20. Hwang Y, Rhodes D, Bushman F. Rapid microtiter assays for poxvirus topoisomerase, mammalian type IB topoisomerase and HIV-1 integrase: application to inhibitor isolation. Nucleic Acids Res. 2000;28:4884–4892. [PubMed] 21. Magliery TJ, et al. Detecting protein-protein interactions with a green fluorescent protein fragment reassembly trap: scope and mechanism. J Am Chem Soc. 2005;127:146–157. [PubMed] 22. Kass G, et al. Permeabilized mammalian cells as an experimental system for nuclear import of geminiviral karyophilic proteins and of synthetic peptides derived from their nuclear localization signal regions. J Gen Virol. 2006;87:2709–2720. [PubMed] 23. Iordanskiy S, et al. Heat shock protein 70 protects cells from cell cycle arrest and apoptosis induced by human immunodeficiency virus type 1 viral protein R. J Virol. 2004;78:9697–9704. [PubMed] 24. Kramer-Hammerle S, et al. Identification of a novel Rev-interacting cellular protein. BMC Cell Biol. 2005;6:20. [PubMed] 25. Zhang J, Scadden DT, Crumpacker CS. Primitive hematopoietic cells resist HIV-1 infection via p21. J Clin Invest. 2007;117:473–481. [PubMed] 26. Iordanskiy S, Berro R, Altieri M, Kashanchi F, Bukrinsky M. Intracytoplasmic maturation of the human immunodeficiency virus type 1 reverse transcription complexes determines their capacity to integrate into chromatin. Retrovirology. 2006;3:4. [PubMed] 27. Yamamoto N, et al. Analysis of human immunodeficiency virus type 1 integration by using a specific, sensitive and quantitative assay based on real-time polymerase chain reaction. Virus Genes. 2006;32:105–113. [PubMed] 28. Field FJ, Born E, Murthy S, Mathur SN. Polyunsaturated fatty acids decrease the expression of sterol regulatory element-binding protein-1 in CaCo-2 cells: effect on fatty acid synthesis and triacylglycerol transport. Biochem J. 2002;368:855–864. [PubMed] 29. Casabianca A, et al. Fast and sensitive quantitative detection of HIV DNA in whole blood leucocytes by SYBR green I real-time PCR assay. Mol Cell Probes. 2007;21:368–378. [PubMed] 30. Levin A, Kutznetova L, Kahana R, Rubinstein-Guini M, Stram Y. Highly effective inhibition of Akabane virus replication by siRNA genes. Virus Res. 2006;120:121–127. [PubMed] 31. Pollard VW, Malim MH. The HIV-1 Rev protein. Annu Rev Microbiol. 1998;52:491–532. [PubMed] 32. Hayouka Z, et al. Inhibiting HIV-1 integrase by shifting its oligomerization equilibrium. Proc Natl Acad Sci U S A. 2007;104:8316–8321. [PubMed] This work was supported by the Israeli Science Foundation (AL) and by a starting grant from the European Research Council (ERC) (to AF). 1. Esposito D, Craigie R. HIV integrase structure and function. Adv Virus Res. 1999;52:319–333. [PubMed] 2. Sherman MP, Greene WC. Slipping through the door: HIV entry into the nucleus. Microbes Infect. 2002;4:67–73. [PubMed] 3. Llano M, et al. An essential role for LEDGF/p75 in HIV integration. Science. 2006;314:461–464. [PubMed] 4. Vandekerckhove L, et al. Transient and stable knockdown of the integrase cofactor LEDGF/p75 reveals its role in the replication cycle of human immunodeficiency virus. J Virol. 2006;80:1886–1896. [PubMed] 5. Emiliani S, et al. Integrase mutants defective for interaction with LEDGF/p75 are impaired in chromosome tethering and HIV-1 replication. J Biol Chem. 2005;280:25517–25523. [PubMed] 6. Llano M, et al. LEDGF/p75 determines cellular trafficking of diverse lentiviral but not murine oncoretroviral integrase proteins and is a component of functional lentiviral preintegration complexes. J Virol. 2004;78:9524–9537. [PubMed] 7. Maertens G, Cherepanov P, Debyser Z, Engelborghs Y, Engelman A. Identification and characterization of a functional nuclear localization signal in the HIV-1 integrase interactor LEDGF/p75. J Biol Chem. 2004;279:33421–33429. [PubMed] 8. Maertens G, et al. LEDGF/p75 is essential for nuclear and chromosomal targeting of HIV-1 integrase in human cells. J Biol Chem. 2003;278:33528–33539. [PubMed] 9. Butler SL, Hansen MS, Bushman FD. A quantitative assay for HIV DNA integration in vivo. Nat Med. 2001;7:631–634. [PubMed] 10. Chun TW, et al. Quantification of latent tissue reservoirs and total body viral load in HIV-1 infection. Nature. 1997;387:183–188. [PubMed] 11. Levin A, et al. Peptides derived from HIV-1 integrase that bind Rev stimulate viral genome integration. PLoS ONE. 2009;4:e 4155. 12. Rosenbluh J, et al. Interaction between HIV-1 Rev and integrase proteins: a basis for the development of anti-HIV peptides. J Biol Chem. 2007;282:15743–15753. [PubMed] 13. Pollard VW, Malim MH. The HIV-1 Rev protein. Annu Rev Microbiol. 1998;52:491–532. [PubMed] 14. Wu Y. HIV-1 gene expression: lessons from provirus and non-integrated DNA. Retrovirology. 2004;1:13. [PubMed] 15. Wu Y, Marsh JW. Early transcription from nonintegrated DNA in human immunodeficiency virus infection. J Virol. 2003;77:10376–10382. [PubMed] 16. Levin A, et al. Novel Regulation of HIV-1 Replication and Pathogenicity: Rev Inhibition of Integration. In press Protein Engineering Design and Selection. 2009 doi: 10.1093/protein/gzp060. 17. Kelly J, et al. Human macrophages support persistent transcription from unintegrated HIV-1 DNA. Virology. 2008;372:300–312. [PubMed] 18. Iyer SR, Yu D, Biancotto A, Margolis LB, Wu Y. Measurement of human immunodeficiency virus type 1 preintegration transcription by using Rev-dependent Rev-CEM cells reveals a sizable transcribing DNA population comparable to that from proviral templates. J Virol. 2009;83:8662–8673. [PubMed] 19. Derdeyn CA, et al. Sensitivity of human immunodeficiency virus type 1 to the fusion inhibitor T-20 is modulated by coreceptor specificity defined by the V3 loop of gp120. J Virol. 2000;74:8358–8367. [PubMed] 20. Kimpton J, Emerman M. Detection of replication-competent and pseudotyped human immunodeficiency virus with a sensitive cell line on the basis of activation of an integrated beta-galactosidase gene. J Virol. 1992;66:2232–2239. [PubMed] 21. Cullen BR. Use of eukaryotic expression technology in the functional analysis of cloned genes. Methods Enzymol. 1987;152:684–704. [PubMed] 22. Ratner L, et al. Complete nucleotide sequence of the AIDS virus, HTLV-III. Nature. 1985;313:277–284. [PubMed] 23. Gummuluru S, Kinsey CM, Emerman M. An in vitro rapid-turnover assay for human immunodeficiency virus type 1 replication selects for cell-to-cell spread of virus. J Virol. 2000;74:10882–10891. [PubMed] 24. Nakajima N, Lu R, Engelman A. Human immunodeficiency virus type 1 replication in the absence of integrase-mediated dna recombination: definition of permissive and nonpermissive T-cell lines. J Virol. 2001;75:7944–7955. [PubMed] 25. Verhoef K, Koper M, Berkhout B. Determination of the minimal amount of Tat activity required for human immunodeficiency virus type 1 replication. Virology. 1997;237:228–236. [PubMed] 26. Bahner I, et al. Comparison of trans-dominant inhibitory mutant human immunodeficiency virus type 1 genes expressed by retroviral vectors in human T lymphocytes. J Virol. 1993;67:3199–3207. [PubMed] 27. Fineberg K, et al. Inhibition of nuclear import mediated by the Rev-arginine rich motif by RNA molecules. Biochemistry. 2003;42:2625–2633. [PubMed] 28. Jenkins TM, Engelman A, Ghirlando R, Craigie R. A soluble active mutant of HIV-1 integrase: involvement of both the core and carboxyl-terminal domains in multimerization. J Biol Chem. 1996;271:7712–7718. [PubMed] 29. Turlure F, Maertens G, Rahman S, Cherepanov P, Engelman A. A tripartite DNA-binding element, comprised of the nuclear localization signal and two AT-hook motifs, mediates the association of LEDGF/p75 with chromatin in vivo. Nucleic Acids Res. 2006;34:1653–1675. [PubMed] 30. Rhim H, Echetebu CO, Herrmann CH, Rice AP. Wild-type and mutant HIV-1 and HIV-2 Tat proteins expressed in Escherichia coli as fusions with glutathione S-transferase. J Acquir Immune Defic Syndr. 1994;7:1116–1121. [PubMed] 31. Rosenbluh J, et al. Positively charged peptides can interact with each other, as revealed by solid phase binding assays. Anal Biochem. 2006;352:157–168. [PubMed] 32. Melchior F, Paschal B, Evans J, Gerace L. Inhibition of nuclear protein import by nonhydrolyzable analogues of GTP and identification of the small GTPase Ran/TC4 as an essential transport factor. J Cell Biol. 1993;123:1649–1659. [PubMed] 33. Craigie R, Mizuuchi K, Bushman FD, Engelman A. A rapid in vitro assay for HIV DNA integration. Nucleic Acids Res. 1991;19:2729–2734. [PubMed] 34. Hwang Y, Rhodes D, Bushman F. Rapid microtiter assays for poxvirus topoisomerase, mammalian type IB topoisomerase and HIV-1 integrase: application to inhibitor isolation. Nucleic Acids Res. 2000;28:4884–4892. [PubMed] 35. Kass G, et al. Permeabilized mammalian cells as an experimental system for nuclear import of geminiviral karyophilic proteins and of synthetic peptides derived from their nuclear localization signal regions. J Gen Virol. 2006;87:2709–2720. [PubMed] 36. Iordanskiy S, et al. Heat shock protein 70 protects cells from cell cycle arrest and apoptosis induced by human immunodeficiency virus type 1 viral protein R. J Virol. 2004;78:9697–9704. [PubMed] 37. Kramer-Hammerle S, et al. Identification of a novel Rev-interacting cellular protein. BMC Cell Biol. 2005;6:20. [PubMed] 38. Zhang J, Scadden DT, Crumpacker CS. Primitive hematopoietic cells resist HIV-1 infection via p21. J Clin Invest. 2007;117:473–481. [PubMed] 39. Iordanskiy S, Berro R, Altieri M, Kashanchi F, Bukrinsky M. Intracytoplasmic maturation of the human immunodeficiency virus type 1 reverse transcription complexes determines their capacity to integrate into chromatin. Retrovirology. 2006;3:4. [PubMed] 40. Casabianca A, et al. Fast and sensitive quantitative detection of HIV DNA in whole blood leucocytes by SYBR green I real-time PCR assay. Mol Cell Probes. 2007;21:368–378. [PubMed] 41. Levin A, Kutznetova L, Kahana R, Rubinstein-Guini M, Stram Y. Highly effective inhibition of Akabane virus replication by siRNA genes. Virus Res. 2006;120:121–127. [PubMed] 42. Watts NR, et al. Three-dimensional structure of HIV-1 Rev protein filaments. J Struct Biol. 1998;121:41–52. [PubMed] 43. Singh DP, Kimura A, Chylack LT, Jr, Shinohara T. Lens epithelium-derived growth factor (LEDGF/p75) and p52 are derived from a single gene by alternative splicing. Gene. 2000;242:265–273. [PubMed] 44. Hayouka Z, et al. Inhibiting HIV-1 integrase by shifting its oligomerization equilibrium. Proc Natl Acad Sci U S A. 2007;104:8316–8321. [PubMed] 45. Pandey KK, Sinha S, Grandgenett DP. Transcriptional coactivator LEDGF/p75 modulates human immunodeficiency virus type 1 integrase-mediated concerted integration. J Virol. 2007;81:3969–3979. [PubMed] 46. Engelman A, Cherepanov P. The lentiviral integrase binding protein LEDGF/p75 and HIV-1 replication. PLoS Pathog. 2008;4:e1000046. [PubMed] 47. Poeschla EM. Integrase, LEDGF/p75 and HIV replication. Cell Mol Life Sci. 2008;65:1403–1424. [PubMed]
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