Premature transcription termination complex proteins PCF11 and WDR82 silence HIV-1 expression in latently infected cells.

Postintegration transcriptional silencing of HIV-1 leads to the establishment of a pool of latently infected cells. In these cells, mechanisms controlling RNA Polymerase II (RNAPII) pausing and premature transcription termination (PTT) remain to be explored. Here, we found that the cleavage and polyadenylation (CPA) factor PCF11 represses HIV-1 expression independently of the other subunits of the CPA complex or the polyadenylation signal located at the 5' LTR. We show that PCF11 interacts with the RNAPII-binding protein WDR82. Knock-down of PCF11 or WDR82 reactivated HIV-1 expression in latently infected cells. To silence HIV-1 transcription, PCF11 and WDR82 are specifically recruited at the promoter-proximal region of the provirus in an interdependent manner. Codepletion of PCF11 and WDR82 indicated that they act on the same pathway to repress HIV expression. These findings reveal PCF11/WDR82 as a PTT complex silencing HIV-1 expression in latently infected cells.

DNA topoisomerase 1 represses HIV-1 promoter activity through its interaction with a guanine quadruplex present in the LTR sequence.

BACKGROUND: Once integrated in the genome of infected cells, HIV-1 provirus is transcribed by the cellular transcription machinery. This process is regulated by both viral and cellular factors, which are necessary for an efficient viral replication as well as for the setting up of viral latency, leading to a repressed transcription of the integrated provirus. RESULTS: In this study, we examined the role of two parameters in HIV-1 LTR promoter activity. We identified DNA topoisomerase1 (TOP1) to be a potent repressor of this promoter and linked this repression to its catalytic domain. Additionally, we confirmed the folding of a Guanine quadruplex (G4) structure in the HIV-1 promoter and its repressive effect. We demonstrated a direct interaction between TOP1 and this G4 structure, providing evidence of a functional relationship between the two repressive elements. Mutations abolishing G4 folding affected TOP1/G4 interaction and hindered G4-dependent inhibition of TOP1 catalytic activity in vitro. As a result, HIV-1 promoter activity was reactivated in a native chromatin environment. Lastly, we noticed an enrichment of predicted G4 sequences in the promoter of TOP1-repressed cellular genes. CONCLUSIONS: Our results demonstrate the formation of a TOP1/G4 complex on the HIV-1 LTR promoter and its repressive effect on the promoter activity. They reveal the existence of a new mechanism of TOP1/G4-dependent transcriptional repression conserved between viral and human genes. This mechanism contrasts with the known property of TOP1 as global transcriptional activator and offers new perspectives for anti-cancer and anti-viral strategies.

Biological and Structural Analyses of New Potent Allosteric Inhibitors of HIV-1 Integrase.

HIV-1 integrase-LEDGF allosteric inhibitors (INLAIs) share the binding site on the viral protein with the host factor LEDGF/p75. These small molecules act as molecular glues promoting hyper-multimerization of HIV-1 IN protein to severely perturb maturation of viral particles. Herein, we describe a new series of INLAIs based on a benzene scaffold that display antiviral activity in the single digit nanomolar range. Akin to other compounds of this class, the INLAIs predominantly inhibit the late stages of HIV-1 replication. A series of high-resolution crystal structures revealed how these small molecules engage the catalytic core and the C-terminal domains of HIV-1 IN. No antagonism was observed between our lead INLAI compound BDM-2 and a panel of 16 clinical antiretrovirals. Moreover, we show that compounds retained high antiviral activity against HIV-1 variants resistant to IN strand transfer inhibitors and other classes of antiretroviral drugs. The virologic profile of BDM-2 and the recently completed single ascending dose phase I trial (ClinicalTrials.gov identifier: NCT03634085) warrant further clinical investigation for use in combination with other antiretroviral drugs. Moreover, our results suggest routes for further improvement of this emerging drug class.

YTHDC1 regulates distinct post-integration steps of HIV-1 replication and is important for viral infectivity.

BACKGROUND: The recent discovery of the role of m6A methylation in the regulation of HIV-1 replication unveiled a novel layer of regulation for HIV gene expression. This epitranscriptomic modification of HIV-1 RNAs is under the dynamic control of specific writers and erasers. In addition, cytoplasmic readers of the m6A mark are recruited to the modified viral RNAs and regulate HIV-1 replication. Yet, little is known about the effects of m6A writers and readers on the biogenesis of HIV-1 RNAs. RESULTS: We showed that the METTL3/14 m6A methyltransferase complex and the m6A YTHDF2 cytoplasmic writer down regulates the abundance of HIV-1 RNAs in infected cells. We also identified the m6A nuclear writer YTHDC1 as a novel regulator of HIV-1 transcripts. In HIV-1 producer cells, we showed that knocking down YTHDC1 increases the levels of unspliced and incompletely spliced HIV-1 RNAs, while levels of multiply spliced transcripts remained unaffected. In addition, we observed that depletion of YTHDC1 has no effect on the nuclear cytoplasmic distribution of viral transcripts. YTHDC1 binds specifically to HIV-1 transcripts in a METTL3-dependent manner. Knocking down YTHDC1 reduces the expression of Env and Vpu viral proteins in producer cells and leads to the incorporation of unprocessed Env gp160 in virus particles, resulting in the decrease of their infectivity. CONCLUSIONS: Our findings indicate that, by controlling HIV-1 RNA biogenesis and protein expression, the m6A nuclear reader YTHDC1 is required for efficient production of infectious viral particles.

Dynamic nanopore long-read sequencing analysis of HIV-1 splicing events during the early steps of infection.

BACKGROUND: Alternative splicing is a key step in Human Immunodeficiency Virus type 1 (HIV-1) replication that is tightly regulated both temporally and spatially. More than 50 different transcripts can be generated from a single HIV-1 unspliced pre-messenger RNA (pre-mRNA) and a balanced proportion of unspliced and spliced transcripts is critical for the production of infectious virions. Understanding the mechanisms involved in the regulation of viral RNA is therefore of potential therapeutic interest. However, monitoring the regulation of alternative splicing events at a transcriptome-wide level during cell infection is challenging. Here we used the long-read cDNA sequencing developed by Oxford Nanopore Technologies (ONT) to explore in a quantitative manner the complexity of the HIV-1 transcriptome regulation in infected primary CD4+ T cells. RESULTS: ONT reads mapping to the viral genome proved sufficiently long to span all possible splice junctions, even distant ones, and to be assigned to a total of 150 exon combinations. Fifty-three viral RNA isoforms, including 14 new ones were further considered for quantification. Relative levels of viral RNAs determined by ONT sequencing showed a high degree of reproducibility, compared favourably to those produced in previous reports and highly correlated with quantitative PCR (qPCR) data. To get further insights into alternative splicing regulation, we then compiled quantifications of splice site (SS) usage and transcript levels to build "splice trees", a quantitative representation of the cascade of events leading to the different viral isoforms. This approach allowed visualizing the complete rewiring of SS usages upon perturbation of SS D2 and its impact on viral isoform levels. Furthermore, we produced the first dynamic picture of the cascade of events occurring between 12 and 24 h of viral infection. In particular, our data highlighted the importance of non-coding exons in viral RNA transcriptome regulation. CONCLUSION: ONT sequencing is a convenient and reliable strategy that enabled us to grasp the dynamic of the early splicing events modulating the viral RNA landscape in HIV-1 infected cells.

Structure-function analyses unravel distinct effects of allosteric inhibitors of HIV-1 integrase on viral maturation and integration.

Recently, a new class of HIV-1 integrase (IN) inhibitors with a dual mode of action, called IN-LEDGF/p75 allosteric inhibitors (INLAIs), was described. Designed to interfere with the IN-LEDGF/p75 interaction during viral integration, unexpectedly, their major impact was on virus maturation. This activity has been linked to induction of aberrant IN multimerization, whereas inhibition of the IN-LEDGF/p75 interaction accounts for weaker antiretroviral effect at integration. Because these dual activities result from INLAI binding to IN at a single binding site, we expected that these activities co-evolved together, driven by the affinity for IN. Using an original INLAI, MUT-A, and its activity on an Ala-125 (A125) IN variant, we found that these two activities on A125-IN can be fully dissociated: MUT-A-induced IN multimerization and the formation of eccentric condensates in viral particles, which are responsible for inhibition of virus maturation, were lost, whereas inhibition of the IN-LEDGF/p75 interaction and consequently integration was fully retained. Hence, the mere binding of INLAI to A125 IN is insufficient to promote the conformational changes of IN required for aberrant multimerization. By analyzing the X-ray structures of MUT-A bound to the IN catalytic core domain (CCD) with or without the Ala-125 polymorphism, we discovered that the loss of IN multimerization is due to stabilization of the A125-IN variant CCD dimer, highlighting the importance of the CCD dimerization energy for IN multimerization. Our study reveals that affinity for the LEDGF/p75-binding pocket is not sufficient to induce INLAI-dependent IN multimerization and the associated inhibition of viral maturation.

Argonaute proteins regulate HIV-1 multiply spliced RNA and viral production in a Dicer independent manner.

Argonaute (Ago) proteins associate with microRNAs (miRNAs) to form the core of the RNA-induced silencing complex (RISC) that mediates post-transcriptional gene silencing of target mRNAs. As key players in anti-viral defense, Ago proteins are thought to have the ability to interact with human immunodeficiency virus type 1 (HIV-1) RNA. However, the role of this interaction in regulating HIV-1 replication has been debated. Here, we used high throughput sequencing of RNA isolated by cross-linking immunoprecipitation (HITS-CLIP) to explore the interaction between Ago2 and HIV-1 RNA in infected cells. By only considering reads of 50 nucleotides length in our analysis, we identified more than 30 distinct binding sites for Ago2 along the viral RNA genome. Using reporter assays, we found four binding sites, located near splice donor sites, capable of repressing Luciferase gene expression in an Ago-dependent manner. Furthermore, inhibition of Ago1 and Ago2 levels in cells expressing HIV-1 led to an increase of viral multiply spliced transcripts and to a strong reduction in the extracellular CAp24 level. Depletion of Dicer did not affect these activities. Our results highlight a new role of Ago proteins in the control of multiply spliced HIV-1 transcript levels and viral production, independently of the miRNA pathway.

The HIV-1 integrase-LEDGF allosteric inhibitor MUT-A: resistance profile, impairment of virus maturation and infectivity but without influence on RNA packaging or virus immunoreactivity.

BACKGROUND: HIV-1 Integrase (IN) interacts with the cellular co-factor LEDGF/p75 and tethers the HIV preintegration complex to the host genome enabling integration. Recently a new class of IN inhibitors was described, the IN-LEDGF allosteric inhibitors (INLAIs). Designed to interfere with the IN-LEDGF interaction during integration, the major impact of these inhibitors was surprisingly found on virus maturation, causing a reverse transcription defect in target cells. RESULTS: Here we describe the MUT-A compound as a genuine INLAI with an original chemical structure based on a new type of scaffold, a thiophene ring. MUT-A has all characteristics of INLAI compounds such as inhibition of IN-LEDGF/p75 interaction, IN multimerization, dual antiretroviral (ARV) activities, normal packaging of genomic viral RNA and complete Gag protein maturation. MUT-A has more potent ARV activity compared to other INLAIs previously reported, but similar profile of resistance mutations and absence of ARV activity on SIV. HIV-1 virions produced in the presence of MUT-A were non-infectious with the formation of eccentric condensates outside of the core. In studying the immunoreactivity of these non-infectious virions, we found that inactivated HIV-1 particles were captured by anti-HIV-specific neutralizing and non-neutralizing antibodies (b12, 2G12, PGT121, 4D4, 10-1074, 10E8, VRC01) with efficiencies comparable to non-treated virus. Autologous CD4+ T lymphocyte proliferation and cytokine induction by monocyte-derived dendritic cells (MDDC) pulsed either with MUT-A-inactivated HIV or non-treated HIV were also comparable. CONCLUSIONS: Although strongly defective in infectivity, HIV-1 virions produced in the presence of the MUT-A INLAI have a normal protein and genomic RNA content as well as B and T cell immunoreactivities comparable to non-treated HIV-1. These inactivated viruses might form an attractive new approach in vaccine research in an attempt to study if this new type of immunogen could elicit an immune response against HIV-1 in animal models.

SAMHD1 is the dendritic- and myeloid-cell-specific HIV-1 restriction factor counteracted by Vpx.

The primate lentivirus auxiliary protein Vpx counteracts an unknown restriction factor that renders human dendritic and myeloid cells largely refractory to HIV-1 infection. Here we identify SAMHD1 as this restriction factor. SAMHD1 is a protein involved in Aicardi-Goutières syndrome, a genetic encephalopathy with symptoms mimicking congenital viral infection, that has been proposed to act as a negative regulator of the interferon response. We show that Vpx induces proteasomal degradation of SAMHD1. Silencing of SAMHD1 in non-permissive cell lines alleviates HIV-1 restriction and is associated with a significant accumulation of viral DNA in infected cells. Concurrently, overexpression of SAMHD1 in sensitive cells inhibits HIV-1 infection. The putative phosphohydrolase activity of SAMHD1 is probably required for HIV-1 restriction. Vpx-mediated relief of restriction is abolished in SAMHD1-negative cells. Finally, silencing of SAMHD1 markedly increases the susceptibility of monocytic-derived dendritic cells to infection. Our results demonstrate that SAMHD1 is an antiretroviral protein expressed in cells of the myeloid lineage that inhibits an early step of the viral life cycle.

Identification of low molecular weight nuclear complexes containing integrase during the early stages of HIV-1 infection.

BACKGROUND: HIV-1 replication requires integration of its reverse transcribed viral cDNA into a host cell chromosome. The DNA cutting and joining reactions associated to this key step are catalyzed by the viral protein integrase (IN). In infected cells, IN binds the viral cDNA, together with viral and cellular proteins, to form large nucleoprotein complexes. However, the dynamics of IN complexes formation is still poorly understood. RESULTS: Here, we characterized IN complexes during the early stages of T-lymphocyte infection. We found that following viral entry into the host cell, IN was rapidly targeted to proteasome-mediated degradation. Interactions between IN and cellular cofactors LEDGF/p75 and TNPO3 were detected as early as 6 h post-infection. Size exclusion chromatography of infected cell extracts revealed distinct IN complexes in vivo. While at 2 h post-infection the majority of IN eluted within a high molecular weight complex competent for integration (IN complex I), IN was also detected in a low molecular weight complex devoid of full-length viral cDNA (IN complex II, ~440 KDa). At 6 h post-infection the relative proportion of IN complex II increased. Inhibition of reverse transcription or integration did not alter the elution profile of IN complex II in infected cells. However, in cells depleted for LEDGF/p75 IN complex II shifted to a lower molecular weight complex (IN complex III, ~150 KDa) containing multimers of IN. Notably, cell fractionation experiments indicated that both IN complex II and III were exclusively nuclear. Finally, IN complex II was not detected in cells infected with a virus harboring a mutated IN defective for LEDGF/p75 interaction and tetramerization. CONCLUSIONS: Our findings indicate that, shortly after viral entry, a significant portion of DNA-free IN that is distinct from active pre-integration complexes accumulates in the nucleus.

Dual inhibition of HIV-1 replication by integrase-LEDGF allosteric inhibitors is predominant at the post-integration stage.

BACKGROUND: LEDGF/p75 (LEDGF) is the main cellular cofactor of HIV-1 integrase (IN). It acts as a tethering factor for IN, and targets the integration of HIV in actively transcribed gene regions of chromatin. A recently developed class of IN allosteric inhibitors can inhibit the LEDGF-IN interaction. RESULTS: We describe a new series of IN-LEDGF allosteric inhibitors, the most active of which is Mut101. We determined the crystal structure of Mut101 in complex with IN and showed that the compound binds to the LEDGF-binding pocket, promoting conformational changes of IN which explain at the atomic level the allosteric effect of the IN/LEDGF interaction inhibitor on IN functions. In vitro, Mut101 inhibited both IN-LEDGF interaction and IN strand transfer activity while enhancing IN-IN interaction. Time of addition experiments indicated that Mut101 behaved as an integration inhibitor. Mut101 was fully active on HIV-1 mutants resistant to INSTIs and other classes of anti-HIV drugs, indicative that this compound has a new mode of action. However, we found that Mut101 also displayed a more potent antiretroviral activity at a post-integration step. Infectivity of viral particles produced in presence of Mut101 was severely decreased. This latter effect also required the binding of the compound to the LEDGF-binding pocket. CONCLUSION: Mut101 has dual anti-HIV-1 activity, at integration and post-integration steps of the viral replication cycle, by binding to a unique target on IN (the LEDGF-binding pocket). The post-integration block of HIV-1 replication in virus-producer cells is the mechanism by which Mut101 is most active as an antiretroviral. To explain this difference between Mut101 antiretroviral activity at integration and post-integration stages, we propose the following model: LEDGF is a nuclear, chromatin-bound protein that is absent in the cytoplasm. Therefore, LEDGF can outcompete compound binding to IN in the nucleus of target cells lowering its antiretroviral activity at integration, but not in the cytoplasm where post-integration production of infectious viral particles takes place.

Structural basis for HIV-1 DNA integration in the human genome, role of the LEDGF/P75 cofactor.

Integration of the human immunodeficiency virus (HIV-1) cDNA into the human genome is catalysed by integrase. Several studies have shown the importance of the interaction of cellular cofactors with integrase for viral integration and infectivity. In this study, we produced a stable and functional complex between the wild-type full-length integrase (IN) and the cellular cofactor LEDGF/p75 that shows enhanced in vitro integration activity compared with the integrase alone. Mass spectrometry analysis and the fitting of known atomic structures in cryo negatively stain electron microscopy (EM) maps revealed that the functional unit comprises two asymmetric integrase dimers and two LEDGF/p75 molecules. In the presence of DNA, EM revealed the DNA-binding sites and indicated that, in each asymmetric dimer, one integrase molecule performs the catalytic reaction, whereas the other one positions the viral DNA in the active site of the opposite dimer. The positions of the target and viral DNAs for the 3' processing and integration reaction shed light on the integration mechanism, a process with wide implications for the understanding of viral-induced pathologies.

Impairment of human immunodeficiency virus type-1 integrase SUMOylation correlates with an early replication defect.

HIV-1 integrase (IN) orchestrates the integration of the reverse transcribed viral cDNA into the host cell genome and participates also in other steps of HIV-1 replication. Cellular and viral factors assist IN in performing its multiple functions, and post-translational modifications contribute to modulate its activities. Here, we show that HIV-1 IN is modified by SUMO proteins and that phylogenetically conserved SUMOylation consensus motifs represent major SUMO acceptor sites. Viruses harboring SUMOylation site IN mutants displayed a replication defect that was mapped during the early stages of infection, before integration but after reverse transcription. Because SUMOylation-defective IN mutants retained WT catalytic activity, we hypothesize that SUMOylation might regulate the affinity of IN for co-factors, contributing to efficient HIV-1 replication.

TASOR epigenetic repressor cooperates with a CNOT1 RNA degradation pathway to repress HIV.

The Human Silencing Hub (HUSH) complex constituted of TASOR, MPP8 and Periphilin recruits the histone methyl-transferase SETDB1 to spread H3K9me3 repressive marks across genes and transgenes in an integration site-dependent manner. The deposition of these repressive marks leads to heterochromatin formation and inhibits gene expression, but the underlying mechanism is not fully understood. Here, we show that TASOR silencing or HIV-2 Vpx expression, which induces TASOR degradation, increases the accumulation of transcripts derived from the HIV-1 LTR promoter at a post-transcriptional level. Furthermore, using a yeast 2-hybrid screen, we identify new TASOR partners involved in RNA metabolism including the RNA deadenylase CCR4-NOT complex scaffold CNOT1. TASOR and CNOT1 synergistically repress HIV expression from its LTR. Similar to the RNA-induced transcriptional silencing complex found in fission yeast, we show that TASOR interacts with the RNA exosome and RNA Polymerase II, predominantly under its elongating state. Finally, we show that TASOR facilitates the association of RNA degradation proteins with RNA polymerase II and is detected at transcriptional centers. Altogether, we propose that HUSH operates at the transcriptional and post-transcriptional levels to repress HIV proviral expression.

Transportin-SR2 imports HIV into the nucleus.

BACKGROUND: The human immunodeficiency virus type 1 (HIV-1) and other lentiviruses have the capacity to infect nondividing cells like macrophages. This requires import of the preintegration complex (PIC) through the nuclear pore. Although many cellular and viral determinants have been proposed, the mechanism leading to nuclear import is not yet understood. RESULTS: Using yeast two-hybrid and pull-down, we identified and validated transportin-SR2 (TRN-SR2) as a bona fide binding partner of HIV-1 integrase. We confirmed the biological relevance of this interaction by RNAi. Depletion of TRN-SR2 interfered with the replication of HIV-1 and HIV-2 but not MoMLV in HeLaP4 cells. Knockdown of TRN-SR2 in primary macrophages likewise interfered with HIV-1 replication. Using Q-PCR, we pinpoint this block in replication to the early steps of the viral lifecycle. A reduction in 2-LTR formation suggests a block in PIC nuclear import upon siRNA-mediated knockdown. Different lines of evidence clearly proved that the late steps of viral replication are not affected. In an in vivo nuclear-import assay using labeled HIV-1 particles, the defect in nuclear import after depletion of TRN-SR2 was directly visualized. In comparison with control cell lines, the great majority of siRNA-treated cells did not contain any PIC in the nucleus. CONCLUSION: Our data clearly demonstrate that TRN-SR2 is the nuclear-import factor of HIV.

Mutations affecting interaction of integrase with TNPO3 do not prevent HIV-1 cDNA nuclear import.

BACKGROUND: Integration of human immunodeficiency virus type 1 (HIV-1) into a host cell chromosome is an essential step under the control of the viral integrase (IN). Although this enzyme is necessary and sufficient to catalyze the integration reaction in vitro, cellular cofactors are involved in the process in vivo. The chromatin-associated factor LEDGF/p75 interacts with IN and promotes integration to transcription units of the host genome. HIV-1 IN also binds the karyopherin TNPO3, however the significance of this interaction during viral replication remains to be explored. RESULTS: Here we present a functional analysis of IN mutants impaired for LEDGF/p75 and TNPO3 interaction. Among them, IN W131A and IN Q168L, that were previously identified to be deficient for LEDGF/p75 interaction, were also partially impaired for TNPO3 binding. We observed that mutations abolishing IN ability to form tetramers resulted in a severe reduction in LEDGF/p75 binding. In sharp contrast, no correlation could be found between the ability of IN to multimerize and TNPO3 interaction. Most of the mutant viruses were essentially impaired for the integration step whereas the amount of 2-LTR circles, reflecting the nuclear import of the viral DNA, was not significantly affected. CONCLUSION: Our functional analysis of HIV-1 IN mutants reveals distinct structural basis for TNPO3 interaction and suggests that the interaction between IN and TNPO3 is not a major determinant of nuclear import but could take place at a nuclear step prior to integration.

A non-proteolytic role for ubiquitin in Tat-mediated transactivation of the HIV-1 promoter.

The human immunodeficiency virus type 1 (HIV-1) encodes a potent transactivator, Tat, which functions through binding to a short leader RNA, called transactivation responsive element (TAR). Recent studies suggest that Tat activates the HIV-1 long terminal repeat (LTR), mainly by adapting co-activator complexes, such as p300, PCAF and the positive transcription elongation factor P-TEFb, to the promoter. Here, we show that the proto-oncoprotein Hdm2 interacts with Tat and mediates its ubiquitination in vitro and in vivo. In addition, Hdm2 is a positive regulator of Tat-mediated transactivation, indicating that the transcriptional properties of Tat are stimulated by ubiquitination. Fusion of ubiquitin to Tat bypasses the requirement of Hdm2 for efficient transactivation, supporting the notion that ubiquitin has a non-proteolytic function in Tat-mediated transactivation.

Yeast two-hybrid detection of integrase-host factor interactions.

Here we describe methods developed based on systematic yeast two-hybrid screenings that allowed us to identify several binding partners of HIV-1 integrase. We have developed an efficient strategy to perform large comprehensive screenings with different highly complex cDNA libraries derived both random- and oligo-dT primed reactions. A very efficient mating procedure was used for screening in yeast, allowing genetic saturation of positive clones. This importantly leads with confidence to the determination of the regions within the participating proteins responsible for the interactions. Several additional tools were used that allowed us to assess the specificity of the interactions detected, including rebound screens with cellular co-factors as baits performed against a library of random fragments of HIV-1 proviral DNA. For some of the identified cell factors, we have generated and characterized loss of affinity mutants of integrase, which, when combined with viral functional assays, validated the involvement of human lens epithelium-derived growth factor (LEDGF/p75) in the integration step of the HIV-1 replication cycle. All tolled, our studies identified LEDGF/p75, Transportin-SR2 (TNPO3), von Hippel-Lindau binding protein 1 (VBP1), and sucrose non-fermenting 5 (SNF5) as cellular binding partners of HIV-1 integrase.