Interaction of moloney murine leukemia virus capsid with Ubc9 and PIASy mediates SUMO-1 addition required early in infection

The Involvement of Ubiquitination and SUMOylation in Retroviruses Infection and Latency

Retroviruses, especially the pathogenic human immunodeficiency virus type 1 (HIV-1), have severely threatened human health for decades. Retroviruses can form stable latent reservoirs via retroviral DNA integration into the host genome, and then be temporarily transcriptional silencing in infected cells, which makes retroviral infection incurable. Although many cellular restriction factors interfere with various steps of the life cycle of retroviruses and the formation of viral latency, viruses can utilize viral proteins or hijack cellular factors to evade intracellular immunity. Many post-translational modifications play key roles in the cross-talking between the cellular and viral proteins, which has greatly determined the fate of retroviral infection. Here, we reviewed recent advances in the regulation of ubiquitination and SUMOylation in the infection and latency of retroviruses, focusing on both host defense- and virus counterattack-related ubiquitination and SUMOylation system. We also summarized the development of ubiquitination- and SUMOylation-targeted anti-retroviral drugs and discussed their therapeutic potential. Manipulating ubiquitination or SUMOylation pathways by targeted drugs could be a promising strategy to achieve a "sterilizing cure" or "functional cure" of retroviral infection.

Viruses, SUMO, and immunity: the interplay between viruses and the host SUMOylation system

The conjugation of small ubiquitin-like modifier (SUMO) proteins to substrates is a well-described post-translational modification that regulates protein activity, subcellular localization, and protein-protein interactions for a variety of downstream cellular activities. Several studies describe SUMOylation as an essential post-translational modification for successful viral infection across a broad range of viruses, including RNA and DNA viruses, both enveloped and un-enveloped. These viruses include but are not limited to herpes viruses, human immunodeficiency virus-1, and coronaviruses. In addition to the SUMOylation of viral proteins during infection, evidence shows that viruses manipulate the SUMO pathway for host protein SUMOylation. SUMOylation of host and viral proteins greatly impacts host innate immunity through viral manipulation of the host SUMOylation machinery to promote viral replication and pathogenesis. Other post-translational modifications like phosphorylation can also modulate SUMO function. For example, phosphorylation of COUP-TF interacting protein 2 (CTIP2) leads to its SUMOylation and subsequent proteasomal degradation. The SUMOylation of CTIP2 and subsequent degradation prevents CTIP2-mediated recruitment of a multi-enzymatic complex to the HIV-1 promoter that usually prevents the transcription of integrated viral DNA. Thus, the "SUMO switch" could have implications for CTIP2-mediated transcriptional repression of HIV-1 in latency and viral persistence. In this review, we describe the consequences of SUMO in innate immunity and then focus on the various ways that viral pathogens have evolved to hijack the conserved SUMO machinery. Increased understanding of the many roles of SUMOylation in viral infections can lead to novel insight into the regulation of viral pathogenesis with the potential to uncover new targets for antiviral therapies.

Dynein Light-Chain Dynlrb2 Is Essential for Murine Leukemia Virus Traffic and Nuclear Entry

Murine leukemia virus (MLV) requires the infected cell to divide to access the nucleus to integrate into the host genome. It has been determined that MLV uses the microtubule and actin network to reach the nucleus at the early stages of infection. Several studies have shown that viruses use the dynein motor protein associated with microtubules for their displacement. We have previously reported that dynein light-chain roadblock type 2 (Dynlrb2) knockdown significantly decreases MLV infection compared to nonsilenced cells, suggesting a functional association between this dynein light chain and MLV preintegration complex (PIC). In this study, we aimed to determine if the dynein complex Dynlrb2 subunit plays an essential role in the retrograde transport of MLV. For this, an MLV mutant containing the green fluorescent protein (GFP) fused to the viral protein p12 was used to assay the PIC localization and speed in cells in which the expression of Dynlrb2 was modulated. We found a significant decrease in the arrival of MLV PIC to the nucleus and a reduced net speed of MLV PICs when Dynlrb2 was knocked down. In contrast, an increase in nuclear localization was observed when Dynlrb2 was overexpressed. Our results suggest that Dynlrb2 plays an essential role in MLV retrograde transport. IMPORTANCE Different viruses use different components of cytoplasmic dynein complex to traffic to their replication site. We have found that murine leukemia virus (MLV) depends on dynein light-chain Dynlrb2 for infection, retrograde traffic, and nuclear entry. Our study provides new information regarding the molecular requirements for retrograde transport of MLV preintegration complex and demonstrates the essential role of Dynlrb2 in MLV infection.

Post-Translational Modifications of Retroviral HIV-1 Gag Precursors: An Overview of Their Biological Role

Protein post-translational modifications (PTMs) play key roles in eukaryotes since they finely regulate numerous mechanisms used to diversify the protein functions and to modulate their signaling networks. Besides, these chemical modifications also take part in the viral hijacking of the host, and also contribute to the cellular response to viral infections. All domains of the human immunodeficiency virus type 1 (HIV-1) Gag precursor of 55-kDa (Pr55^Gag), which is the central actor for viral RNA specific recruitment and genome packaging, are post-translationally modified. In this review, we summarize the current knowledge about HIV-1 Pr55^Gag PTMs such as myristoylation, phosphorylation, ubiquitination, sumoylation, methylation, and ISGylation in order to figure out how these modifications affect the precursor functions and viral replication. Indeed, in HIV-1, PTMs regulate the precursor trafficking between cell compartments and its anchoring at the plasma membrane, where viral assembly occurs. Interestingly, PTMs also allow Pr55^Gag to hijack the cell machinery to achieve viral budding as they drive recognition between viral proteins or cellular components such as the ESCRT machinery. Finally, we will describe and compare PTMs of several other retroviral Gag proteins to give a global overview of their role in the retroviral life cycle.

Post-translational modifications inducing proteasomal degradation to counter HIV-1 infection

Post-translational modifications (PTMs) are integral to regulating a wide variety of cellular processes in eukaryotic cells, such as regulation of protein stability, alteration of celluar location, protein activity modulation, and regulation of protein interactions. HIV-1, like other eukaryotic viruses, and its infected host exploit the proteasomal degradation system for their respective proliferation and survival, using various PTMs, including but not limited to ubiquitination, SUMOylation, NEDDylation, interferon-stimulated gene (ISG)ylation. Essentially all viral proteins within the virions -- and in the HIV-1-infected cells -- interact with their cellular counterparts for this degradation, utilizing ubiquitin (Ub), and the Ub-like (Ubl) modifiers less frequently, to eliminate the involved proteins throughout the virus life cycle, from the entry step to release of the assembled virus particles. Such interplay is pivotal for, on the one hand, the cell to restrict proliferation of the infecting virus, and on the other, for molecular counteraction by the virus to overcome this cellular protein-imposed restriction. Recent reports indicate that not only viral/cellular proteins but also viral/viral protein interactions play vital roles in regulating viral protein stability. We hence give an overview of the molecular processes of PTMs involved in proteasomal degradation of the viral and cellular proteins, and the viral/viral and viral/cellular protein interplay in restriction and competition for HIV-1 vs. host cell survival. Insights in this realm could open new avenues for developing therapeutics against HIV-1 via targeting specific steps of the proteasome degradation pathway during the HIV-1 life cycle.

Novel signal transducer and activator of transcription 1 mutation disrupts small ubiquitin-related modifier conjugation causing gain of function

BACKGROUND

Sumoylation is a posttranslational reversible modification of cellular proteins through the conjugation of small ubiquitin-related modifier (SUMO) and comprises an important regulator of protein function.

OBJECTIVE

We sought to characterize the molecular mechanism of a novel mutation at the SUMO motif on signal transducer and activator of transcription 1 (STAT1).

METHODS

STAT1 sequencing and functional characterization were performed in transfection experiments by using immunoblotting and immunoprecipitation in STAT1-deficient cell lines. Transcriptional response and target gene activation were also investigated in PBMCs.

RESULTS

We identified a novel STAT1 mutation (c.2114A>T, p.E705V) within the SUMO motif (702IKTE705) in a patient with disseminated Rhodococcus species infection, Norwegian scabies, chronic mucocutaneous candidiasis, hypothyroidism, and esophageal squamous cell carcinoma. The mutation is located in the tail segment and is predicted to disrupt STAT1 sumoylation. Immunoprecipitation experiments performed in transfected cells confirmed absent STAT1 sumoylation for E705V, whereas it was present in wild-type (WT) STAT1 cells, as well as the loss-of-function mutants L706S and Y701C. Furthermore, stimulation with IFN-γ led to enhanced STAT1 phosphorylation, enhanced transcriptional activity, and target gene expression in the E705V-transfected compared with WT-transfected cells. Computer modeling of WT and mutant STAT1 molecules showed variations in the accessibility of the phosphorylation site Y701, which corresponded to the loss-of-function and gain-of-function variants.

CONCLUSION

This is the first report of a mutation in the STAT1 sumoylation motif associated with clinical disease. These data reinforce sumoylation as a key posttranslational regulatory modification of STAT1 and identify a novel mechanism for gain-of-function STAT1 disease in human subjects.

Protein Inhibitor of Activated STAT2 Restricts HCV Replication by Modulating Viral Proteins Degradation

Hepatitis C virus (HCV) replication in cells is controlled by many host factors. In this report, we found that protein inhibitor of activated STAT2 (PIAS2), which is a small ubiquitin-like modifier (SUMO) E3 ligase, restricted HCV replication. During infection, HCV core, NS3 and NS5A protein expression, as well as the viral assembly and budding efficiency were enhanced when endogenous PIAS2 was knocked down, whereas exogenous PIAS2 expression decreased HCV core, NS3, and NS5A protein expression and the viral assembly and budding efficiency. PIAS2 did not influence the viral entry, RNA replication, and protein translation steps of the viral life cycle. When expressed together with SUMO1, PIAS2 reduced the HCV core, NS3 and NS5A protein levels expressed from individual plasmids through the proteasome pathway in a ubiquitin-independent manner; the stability of these proteins in the HCV infectious system was enhanced when PIAS2 was knocked down. Furthermore, we found that the core was SUMOylated at amino acid K78, and PIAS2 enhanced the SUMOylation level of the core.

Viral Interplay with the Host Sumoylation System

Viruses have evolved elaborate means to regulate diverse cellular pathways in order to create a cellular environment that facilitates viral survival and reproduction. This includes enhancing viral macromolecular synthesis and assembly, as well as preventing antiviral responses, including intrinsic, innate, and adaptive immunity. There are numerous mechanisms by which viruses mediate their effects on the host cell, and this includes targeting various cellular post-translational modification systems, including sumoylation. The wide-ranging impact of sumoylation on cellular processes such as transcriptional regulation, apoptosis, stress response, and cell cycle control makes it an attractive target for viral dysregulation. To date, proteins from both RNA and DNA virus families have been shown to be modified by SUMO conjugation, and this modification appears critical for viral protein function. More interestingly, members of the several viral families have been shown to modulate sumoylation, including papillomaviruses, adenoviruses, herpesviruses, orthomyxoviruses, filoviruses, and picornaviruses. This chapter will focus on mechanisms by which sumoylation both impacts human viruses and is used by viruses to promote viral infection and disease.

Covalent conjugation of the equine infectious anemia virus Gag with SUMO

The conjugation of small ubiquitin-like modifier (SUMO) to the target protein, namely, SUMOylation, is involved in the regulation of many important biological events including host-pathogen interaction. Some viruses have evolved to exploit the host SUMOylation machinery to modify their own protein. Retroviral Gag protein plays critical roles in the viral life cycle. The HIV-1 p6 and the Moloney murine leukemia virus CA have been reported to be conjugated with SUMO. In this study, we report for the first time, to our knowledge, the covalent conjugation of equine infectious anemia virus (EIAV) Gag with SUMO. The C-terminal p9 domain of Gag is a main target for SUMOylation and SUMO is attached to multiple sites of p9, including K30 whose mutation abolished p9 SUMOylation completely. The SUMOylation of p9, but not the p9-K30 mutant, was also detected in equine fibroblastic cells ATCC^® CCL-57™. Ubc9 and its C93 residue are indispensable for the SUMOylation of p9. Using confocal microscopy, it is found that EIAV Gag localizes primarily, if not exclusively, in the cytoplasm of the cell and the co-localization of EIAV Gag with Ubc9 was observed. Our findings that EIAV Gag is SUMOylated at p9-K30, together with previous findings on the defects of p9-K30 mutant in viral DNA translocation from cytoplasm to the nucleus, suggests that SUMOylation of Gag may be involved in such functions.

Dynein Regulators Are Important for Ecotropic Murine Leukemia Virus Infection

During the early steps of infection, retroviruses must direct the movement of the viral genome into the nucleus to complete their replication cycle. This process is mediated by cellular proteins that interact first with the reverse transcription complex and later with the preintegration complex (PIC), allowing it to reach and enter the nucleus. For simple retroviruses, such as murine leukemia virus (MLV), the identities of the cellular proteins involved in trafficking of the PIC in infection are unknown. To identify cellular proteins that interact with the MLV PIC, we developed a replication-competent MLV in which the integrase protein was tagged with a FLAG epitope. Using a combination of immunoprecipitation and mass spectrometry, we established that the microtubule motor dynein regulator DCTN2/p50/dynamitin interacts with the MLV preintegration complex early in infection, suggesting a direct interaction between the incoming viral particles and the dynein complex regulators. Further experiments showed that RNA interference (RNAi)-mediated silencing of either DCTN2/p50/dynamitin or another dynein regulator, NudEL, profoundly reduced the efficiency of infection by ecotropic, but not amphotropic, MLV reporters. We propose that the cytoplasmic dynein regulators are a critical component of the host machinery needed for infection by the retroviruses entering the cell via the ecotropic envelope pathway.

IMPORTANCE

Retroviruses must access the chromatin of host cells to integrate the viral DNA, but before this crucial event, they must reach the nucleus. The movement through the cytoplasm-a crowded environment where diffusion is slow-is thought to utilize retrograde transport along the microtubule network by the dynein complex. Different viruses use different components of this multisubunit complex. We found that the preintegration complex of murine leukemia virus (MLV) interacts with the dynein complex and that regulators of this complex are essential for infection. Our study provides the first insight into the requirements for retrograde transport of the MLV preintegration complex.

The activity of the glucocorticoid receptor is regulated by SUMO conjugation to FKBP51

FK506-binding protein 51 (FKBP51) regulates the activity of the glucocorticoid receptor (GR), and is therefore a key mediator of the biological actions of glucocorticoids. However, the understanding of the molecular mechanisms that govern its activity remains limited. Here, we uncover a novel regulatory switch for GR activity by the post-translational modification of FKBP51 with small ubiquitin-like modifier (SUMO). The major SUMO-attachment site, lysine 422, is required for FKBP51-mediated inhibition of GR activity in hippocampal neuronal cells. Importantly, impairment of SUMO conjugation to FKBP51 impacts on GR-dependent neuronal signaling and differentiation. We demonstrate that SUMO conjugation to FKBP51 is enhanced by the E3 ligase PIAS4 and by environmental stresses such as heat shock, which impact on GR-dependent transcription. SUMO conjugation to FKBP51 regulates GR hormone-binding affinity and nuclear translocation by promoting FKBP51 interaction within the GR complex. SUMOylation-deficient FKBP51 fails to interact with Hsp90 and GR thus facilitating the recruitment of the closely related protein, FKBP52, which enhances GR transcriptional activity. Moreover, we show that the modification of FKBP51 with SUMO modulates its binding to Hsp90. Our data establish SUMO conjugation as a novel regulatory mechanism in the Hsp90 cochaperone activity of FKBP51 with a functional impact on GR signaling in a neuronal context.

A putative SUMO interacting motif in the B30.2/SPRY domain of rhesus macaque TRIM5α important for NF-κB/AP-1 signaling and HIV-1 restriction

TRIM5α from the rhesus macaque (TRIM5αRh) is a restriction factor that shows strong activity against HIV-1. TRIM5αRh binds specifically to HIV-1 capsid (CA) through its B30.2/PRYSPRY domain shortly after entry of the virus into the cytoplasm. Recently, three putative SUMO interacting motifs (SIMs) have been identified in the PRYSPRY domain of human and macaque TRIM5α. However, structural modeling of this domain suggested that two of them were buried in the hydrophobic core of the protein, implying that interaction with SUMO was implausible, while the third one was not relevant to restriction. In light of these results, we re-analyzed the TRIM5αRh PRYSPRY sequence and identified an additional putative SIM ((435)VIIC(438)) which we named SIM4. This motif is exposed at the surface of the PRYSPRY domain, allowing potential interactions with SUMO or SUMOylated proteins. Introducing a double mutation in SIM4 (V435K, I436K) did not alter stability, unlike mutations in SIM1. SIM4-mutated TRIM5αRh failed to bind HIV-1CA and lost the ability to restrict this virus. Accordingly, SIM4 undergoes significant variation among primates and substituting this motif with naturally occurring SIM4 variants affected HIV-1 restriction by TRIM5αRh, suggesting a direct role in capsid recognition. Interestingly, SIM4-mutated TRIM5αRh also failed to activate NF-κB and AP-1-mediated transcription. Although there is no direct evidence that SIM4 is involved in direct interaction with SUMO or a SUMOylated protein, mutating this motif strongly reduced co-localization of TRIM5αRh with SUMO-1 and with PML, a SUMOylated nuclear protein. In conclusion, this new putative SIM is crucial for both direct interaction with incoming capsids and for NF-κB/AP-1 signaling. We speculate that the latter function is mediated by interactions of SIM4 with a SUMOylated protein involved in the NF-κB/AP-1 signaling pathways.

Endogenous TRIM5α Function Is Regulated by SUMOylation and Nuclear Sequestration for Efficient Innate Sensing in Dendritic Cells

During retroviral infection, viral capsids are subject to restriction by the cellular factor TRIM5α. Here, we show that dendritic cells (DCs) derived from human and non-human primate species lack efficient TRIM5α-mediated retroviral restriction. In DCs, endogenous TRIM5α accumulates in nuclear bodies (NB) that partly co-localize with Cajal bodies in a SUMOylation-dependent manner. Nuclear sequestration of TRIM5α allowed potent induction of type I interferon (IFN) responses during infection, mediated by sensing of reverse transcribed DNA by cGAS. Overexpression of TRIM5α or treatment with the SUMOylation inhibitor ginkgolic acid (GA) resulted in enforced cytoplasmic TRIM5α expression and restored efficient viral restriction but abrogated type I IFN production following infection. Our results suggest that there is an evolutionary trade-off specific to DCs in which restriction is minimized to maximize sensing. TRIM5α regulation via SUMOylation-dependent nuclear sequestration adds to our understanding of how restriction factors are regulated.

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Primate dendritic cells (DCs) lack efficient TRIM5α-mediated retroviral restriction

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In DCs TRIM5α is sequestered in the nucleus in a SUMOylation-dependent manner

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TRIM5α nuclear sequestration allows DC sensing of retroviral DNA by cGAS

Mutations of Conserved Residues in the Major Homology Region Arrest Assembling HIV-1 Gag as a Membrane-Targeted Intermediate Containing Genomic RNA and Cellular Proteins

The major homology region (MHR) is a highly conserved motif that is found within the Gag protein of all orthoretroviruses and some retrotransposons. While it is widely accepted that the MHR is critical for assembly of HIV-1 and other retroviruses, how the MHR functions and why it is so highly conserved are not understood. Moreover, consensus is lacking on when HIV-1 MHR residues function during assembly. Here, we first addressed previous conflicting reports by confirming that MHR deletion, like conserved MHR residue substitution, leads to a dramatic reduction in particle production in human and nonhuman primate cells expressing HIV-1 proviruses. Next, we used biochemical analyses and immunoelectron microscopy to demonstrate that conserved residues in the MHR are required after assembling Gag has associated with genomic RNA, recruited critical host factors involved in assembly, and targeted to the plasma membrane. The exact point of inhibition at the plasma membrane differed depending on the specific mutation, with one MHR mutant arrested as a membrane-associated intermediate that is stable upon high-salt treatment and other MHR mutants arrested as labile, membrane-associated intermediates. Finally, we observed the same assembly-defective phenotypes when the MHR deletion or conserved MHR residue substitutions were engineered into Gag from a subtype B, lab-adapted provirus or Gag from a subtype C primary isolate that was codon optimized. Together, our data support a model in which MHR residues act just after membrane targeting, with some MHR residues promoting stability and another promoting multimerization of the membrane-targeted assembling Gag oligomer.

IMPORTANCE The retroviral Gag protein exhibits extensive amino acid sequence variation overall; however, one region of Gag, termed the major homology region, is conserved among all retroviruses and even some yeast retrotransposons, although the reason for this conservation remains poorly understood. Highly conserved residues in the major homology region are required for assembly of retroviruses; however, when these residues are required during assembly is not clear. Here, we used biochemical and electron microscopic analyses to demonstrate that these conserved residues function after assembling HIV-1 Gag has associated with genomic RNA, recruited critical host factors involved in assembly, and targeted to the plasma membrane but before Gag has completed the assembly process. By revealing precisely when conserved residues in the major homology region are required during assembly, these studies resolve existing controversies and set the stage for future experiments aimed at a more complete understanding of how the major homology region functions.

SUMO Ubc9 enzyme as a viral target

Viruses alter specific host cell targets to counteract possible defense mechanisms aimed at eliminating infectivity and viral propagation. The SUMO conjugating enzyme Ubc9 functions as a hub for protein sumoylation, whilst also providing an interactive surface for sumoylated proteins through noncovalent interactions. The targeting of Ubc9 by viruses and viral proteins is thus highly beneficial for the disruption of both protein modification and protein-protein interaction mechanisms with which proteins increase their functional repertoire in cells. This review explores some of the clever mechanisms adopted by viruses to deregulate Ubc9, influence effector pathways and positively impact viral persistence consequently.

Viral manipulation of cellular protein conjugation pathways: The SUMO lesson

Small ubiquitin-like modifier (SUMO)ylation is a key post-translational modification mechanism that controls the function of a plethora of proteins and biological processes. Given its central regulatory role, it is not surprising that it is widely exploited by viruses. A number of viral proteins are known to modify and/or be modified by the SUMOylation system to exert their function, to create a cellular environment more favorable for virus survival and propagation, and to prevent host antiviral responses. Since the SUMO pathway is a multi-step cascade, viral proteins engage with it at many levels, to advance and favor each stage of a typical infection cycle: replication, viral assembly and immune evasion. Here we review the current knowledge on the interplay between the host SUMO system and viral lifecycle.

cAMP-stimulated phosphorylation of diaphanous 1 regulates protein stability and interaction with binding partners in adrenocortical cells

DIAPH1, the RhoA effector protein, forms a complex in adrenocortical cells and is phosphorylated by a cAMP/PKA-dependent pathway. Phosphorylation differentially modulates protein–protein interactions, regulates the stability of the protein, and facilitates sumoylation.

Diaphanous homologue 1 (DIAPH1) is a Rho effector protein that coordinates cellular dynamics by regulating microfilament and microtubule function. We previously showed that DIAPH1 plays an integral role in regulating the production of cortisol by controlling the rate of mitochondrial movement, by which activation of the adrenocorticotropin (ACTH)/cAMP signaling pathway stimulates mitochondrial trafficking and promotes the interaction between RhoA and DIAPH1. In the present study we use mass spectrometry to identify DIAPH1 binding partners and find that DIAPH1 interacts with several proteins, including RhoA, dynamin-1, kinesin, β-tubulin, β-actin, oxysterol-binding protein (OSBP)–related protein 2 (ORP2), and ORP10. Moreover, DIAPH1 is phosphorylated in response to dibutyryl cAMP (Bt₂cAMP) at Thr-759 via a pathway that requires extracellular signal-related kinase (ERK). Alanine substitution of Thr-759 renders DIAPH1 more stable and attenuates the interaction between DIAPH1 and kinesin, ORP2, and actin but has no effect on the ability of the protein to interact with RhoA or β-tubulin. Finally, overexpression of a DIAPH1 T759A mutant significantly decreases the rate of Bt₂cAMP-stimulated mitochondrial movement. Taken together, our findings establish a key role for phosphorylation in regulating the stability and function of DIAPH1.

Inhibition of retroviral replication by members of the TRIM protein family

The TRIM protein family is emerging as a central component of mammalian antiviral innate immunity. Beginning with the identification of TRIM5α as a mammalian post-entry restriction factor against retroviruses, to the repeated observation that many TRIMs ubiquitinate and regulate signaling pathways, the past decade has witnessed an intense research effort to understand how TRIM proteins influence immunity. The list of viral families targeted directly or indirectly by TRIM proteins has grown to include adenoviruses, hepadnaviruses, picornaviruses, flaviviruses, orthomyxoviruses, paramyxoviruses, herpesviruses, rhabdoviruses and arenaviruses. We have come to appreciate how, through intense bouts of positive selection, some TRIM genes have been honed into species-specific restriction factors. Similarly, in the case of TRIMCyp, we are beginning to understand how viruses too have mutated to evade restriction, suggesting that TRIM and viruses have coevolved for millions of years of primate evolution. Recently, TRIM5α returned to the limelight when it was shown to trigger the expression of antiviral genes upon recognition of an incoming virus, a paradigm shift that demonstrated that restriction factors make excellent pathogen sensors. However, it remains unclear how many of ~100 human TRIM genes are antiviral, despite the expression of many of these genes being upregulated by interferon and upon viral infection. TRIM proteins do not conform to one type of antiviral mechanism, reflecting the diversity of viruses they target. Moreover, the cofactors of restriction remain largely enigmatic. The control of retroviral replication remains an important medical subject and provides a useful backdrop for reviewing how TRIM proteins act to repress viral replication.

Covalent NEDD8 conjugation increases RCAN1 protein stability and potentiates its inhibitory action on calcineurin

Similar to ubiquitin, regulatory roles for NEDD8 (neural precursor cell-expressed developmentally down-regulated 8) are being clarified during cell growth, signal transduction, immune response, and development. However, NEDD8 targets and their functional alterations are not well known. Regulator of calcineurin 1 (RCAN1/DSCR1P1) is located near the Down syndrome critical region on the distal part of chromosome 21, and its gene product is an endogenous inhibitor of calcineurin signaling. RCAN1 is modified by ubiquitin and consequently undergoes proteasomal degradation. Here we report that NEDD8 is conjugated to RCAN1 (RCAN1-1S) via three lysine residues, K96, K104, and K107. Neddylation enhances RCAN1 protein stability without affecting its cellular location. In addition, we found that neddylation significantly inhibits proteasomal degradation of RCAN1, which may underlie the ability of NEDD8 to enhance RCAN1 stability. Furthermore, neddylation increases RCAN1 binding to calcineurin, which potentiates its inhibitory activity toward downstream NFAT signaling. The present study provides a new regulatory mechanism of RCAN1 function and highlights an important role for diverse RCAN1-involved cellular physiology.

Contribution of SUMO-interacting motifs and SUMOylation to the antiretroviral properties of TRIM5α

Recent findings suggested that the SUMO-interacting motifs (SIMs) present in the human TRIM5α (TRIM5α(hu)) protein play an important role in the ability of TRIM5α(hu) to restrict N-MLV. Here we explored the role of SIMs in the ability of rhesus TRIM5α (TRIM5α(rh)) to restrict HIV-1, and found that TRIM5α(rh) SIM mutants IL376KK (SIM1mut) and VI405KK (SIM2mut) completely lost their ability to block HIV-1 infection. Interestingly, these mutants also lost the recently described property of TRIM5α(rh) to shuttle into the nucleus. Analysis of these variants revealed that they are unable to interact with the HIV-1 core, which might explain the reason that these variants are not active against HIV-1. Furthermore, NMR titration experiments to assay the binding between the PRYSPRY domain of TRIM5α(rh) and the small ubiquitin-like modifier 1(SUMO-1) revealed no interaction. In addition, we examined the role of SUMOylation in restriction, and find out that inhibition of SUMOylation by the adenoviral protein Gam1 did not alter the retroviral restriction ability of TRIM5α. Overall, our results do not support a role for SIMs or SUMOylation in the antiviral properties of TRIM5α.

Moloney murine leukemia virus integrase and reverse transcriptase interact with PML proteins

Pull-down assay and co-immunoprecipitation of cell extracts in which the integrase or reverse transcriptase of Moloney murine leukemia virus was transiently expressed showed that both enzymes interacted with PML proteins. In infected cells, interaction between the integrase and PML was also observed. Transient expression of PIASy and SUMO proteins facilitated SUMOylation of the integrase but had no apparent effects on the interaction with PML. A FLAG-tagged integrase co-localized with PML protein possibly in the PML body. Knockdown of PML by small interfering RNA resulted in reduced viral cDNA levels and integration efficiency. This suggested that PML proteins activated reverse transcription.

Sumoylation at the host-pathogen interface

Many viral proteins have been shown to be sumoylated with corresponding regulatory effects on their protein function, indicating that this host cell modification process is widely exploited by viral pathogens to control viral activity. In addition to using sumoylation to regulate their own proteins, several viral pathogens have been shown to modulate overall host sumoylation levels. Given the large number of cellular targets for SUMO addition and the breadth of critical cellular processes that are regulated via sumoylation, viral modulation of overall sumoylation presumably alters the cellular environment to ensure that it is favorable for viral reproduction and/or persistence. Like some viruses, certain bacterial plant pathogens also target the sumoylation system, usually decreasing sumoylation to disrupt host anti-pathogen responses. The recent demonstration that Listeria monocytogenes also disrupts host sumoylation, and that this is required for efficient infection, extends the plant pathogen observations to a human pathogen and suggests that pathogen modulation of host sumoylation may be more widespread than previously appreciated. This review will focus on recent aspects of how pathogens modulate the host sumoylation system and how this benefits the pathogen.

Emerging roles for ubiquitin in adenovirus cell entry

Adenovirus relies on numerous interactions between viral and host cell proteins to efficiently enter cells. Undoubtedly, post-translational modifications of host and cellular proteins can impact the efficiency of this cell entry process. Ubiquitylation, once simply thought of as a modification targeting proteins for proteasomal degradation, is now known to regulate protein trafficking within cells, protein-protein interactions and cell signalling pathways. Accumulating evidence suggests that protein ubiquitylation can influence all stages of the life cycle of other viruses such as cell entry, replication and egress. Until recently, the influence of ubiquitylation has only been documented during adenovirus replication. This review highlights the most recent evidence demonstrating direct engagement of host ubiquitylation and SUMOylation machinery by adenovirus during cell entry. Additionally, potential roles for host protein ubiquitylation and the potential for adenovirus regulation of host ubiquitylation machinery during cell entry are discussed.

Human pathogens and the host cell SUMOylation system

Since posttranslational modification (PTM) by the small ubiquitin-related modifiers (SUMOs) was discovered over a decade ago, a huge number of cellular proteins have been found to be reversibly modified, resulting in alteration of differential cellular pathways. Although the molecular consequences of SUMO attachment are difficult to predict, the underlying principle of SUMOylation is altering inter- and/or intramolecular interactions of the modified substrate, changing localization, stability, and/or activity. Unsurprisingly, many different pathogens have evolved to exploit the cellular SUMO modification system due to its functional flexibility and far-reaching functional downstream consequences. Although the extensive knowledge gained so far is impressive, a definitive conclusion about the role of SUMO modification during virus infection in general remains elusive and is still restricted to a few, yet promising concepts. Based on the available data, this review aims, first, to provide a detailed overview of the current state of knowledge and, second, to evaluate the currently known common principles/molecular mechanisms of how human pathogenic microbes, especially viruses and their regulatory proteins, exploit the host cell SUMO modification system.

Sumoylation of the P protein at K254 plays an important role in growth of parainfluenza virus 5

The P protein of parainfluenza virus 5 (PIV5) is an essential cofactor of the viral RNA-dependent RNA polymerase. Phosphorylation of the P protein can positively or negatively regulate viral gene expression, depending on the precise phosphorylation sites. Sumoylation, a process of adding small ubiquitin-like modifier (SUMO) to proteins posttranslationally, plays an important role in regulating protein function. In this study, we have found that the P protein of PIV5 was sumoylated with SUMO1 in both transfected and infected cells. The K254 residue of the P protein is within a consensus sumoylation motif. Mutation of the P protein at K254 to arginine (P-K254R) reduced PIV5 minigenome activity, as well as the sumoylation level of the P protein. Incorporation of K254R into a recombinant PIV5 (rPIV5-P-K254R) resulted in a virus that grew to a lower titer and had lower levels of viral RNA synthesis and protein expression than wild-type PIV5, suggesting that sumoylation of the P protein at K254 is important for PIV5 growth. Biochemical studies did not reveal any defect of P-K254R in its interactions with viral proteins NP and L or formation of homotetramers. We propose that sumoylation of the P protein at K254 regulates PIV5 gene expression through a host protein.

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.

The SUMOylation of matrix protein M1 modulates the assembly and morphogenesis of influenza A virus

SUMOylation is an important posttranslational modification for regulation of cellular functions and viral replication. Here, we report that protein SUMOylation regulates the replication of influenza A virus at the steps of viral maturation and assembly. Knocking down the SUMO-conjugating enzyme Ubc9 resulted in the reduction of virus production. Dissection of the virus life cycle revealed that SUMOylation is involved in the processes of virus maturation and assembly. The viral matrix protein M1 is SUMOylated at K242. A virus carrying the SUMO-defective M1 produced a lower titer of virus, while its viral proteins and viral RNA (vRNA) accumulated in the cells. Furthermore, the mechanistic studies showed that the SUMOylation of M1 is required for the interaction between M1 and viral RNP (vRNP) to form the M1-vRNP complex. The lack of M1 SUMOylation prevented the nuclear export of vRNP and subsequent viral morphogenesis. Taken together, our findings elucidate that the maturation and assembly of influenza A virus is controlled by the SUMO modification of M1 protein. Therefore, we suggest that M1 can serve as a target for developing a new generation of drugs for flu therapy.

SUMO-interacting motifs of human TRIM5α are important for antiviral activity

Human TRIM5α potently restricts particular strains of murine leukemia viruses (the so-called N-tropic strains) but not others (the B- or NB-tropic strains) during early stages of infection. We show that overexpression of SUMO-1 in human 293T cells, but not in mouse MDTF cells, profoundly blocks N-MLV infection. This block is dependent on the tropism of the incoming virus, as neither B-, NB-, nor the mutant R110E of N-MLV CA (a B-tropic switch) are affected by SUMO-1 overexpression. The block occurred prior to reverse transcription and could be abrogated by large amounts of restricted virus. Knockdown of TRIM5α in 293T SUMO-1-overexpressing cells resulted in ablation of the SUMO-1 antiviral effects, and this loss of restriction could be restored by expression of a human TRIM5α shRNA-resistant plasmid. Amino acid sequence analysis of human TRIM5α revealed a consensus SUMO conjugation site at the N-terminus and three putative SUMO interacting motifs (SIMs) in the B30.2 domain. Mutations of the TRIM5α consensus SUMO conjugation site did not affect the antiviral activity of TRIM5α in any of the cell types tested. Mutation of the SIM consensus sequences, however, abolished TRIM5α antiviral activity against N-MLV. Mutation of lysines at a potential site of SUMOylation in the CA region of the Gag gene reduced the SUMO-1 block and the TRIM5α restriction of N-MLV. Our data suggest a novel aspect of TRIM5α-mediated restriction, in which the presence of intact SIMs in TRIM5α, and also the SUMO conjugation of CA, are required for restriction. We propose that at least a portion of the antiviral activity of TRIM5α is mediated through the binding of its SIMs to SUMO-conjugated CA.

Deciphering the code for retroviral integration target site selection

Upon cell invasion, retroviruses generate a DNA copy of their RNA genome and integrate retroviral cDNA within host chromosomal DNA. Integration occurs throughout the host cell genome, but target site selection is not random. Each subgroup of retrovirus is distinguished from the others by attraction to particular features on chromosomes. Despite extensive efforts to identify host factors that interact with retrovirion components or chromosome features predictive of integration, little is known about how integration sites are selected. We attempted to identify markers predictive of retroviral integration by exploiting Precision-Recall methods for extracting information from highly skewed datasets to derive robust and discriminating measures of association. ChIPSeq datasets for more than 60 factors were compared with 14 retroviral integration datasets. When compared with MLV, PERV or XMRV integration sites, strong association was observed with STAT1, acetylation of H3 and H4 at several positions, and methylation of H2AZ, H3K4, and K9. By combining peaks from ChIPSeq datasets, a supermarker was identified that localized within 2 kB of 75% of MLV proviruses and detected differences in integration preferences among different cell types. The supermarker predicted the likelihood of integration within specific chromosomal regions in a cell-type specific manner, yielding probabilities for integration into proto-oncogene LMO2 identical to experimentally determined values. The supermarker thus identifies chromosomal features highly favored for retroviral integration, provides clues to the mechanism by which retrovirus integration sites are selected, and offers a tool for predicting cell-type specific proto-oncogene activation by retroviruses.

When HIV-1, murine leukemia virus (MLV), or other retroviruses infect a cell, the virus generates a DNA copy of the viral RNA genome and ligates the cDNA within host chromosomal DNA. This integration reaction occurs at sites throughout the host cell genome, but little is known about how integration sites are selected. We attempted to identify markers predictive of retroviral integration by comparing the genome-wide binding sites for more than 60 factors with 14 retroviral integration datasets. We borrowed Precision-Recall methods from the Information Retrieval field for extracting information from highly skewed datasets such as these. For MLV and other gammaretroviruses, strong association was observed with STAT1, acetylation of H3 and H4 at several positions, and methylation of H2AZ, H3K4, and K9. We generated a supermarker by combining high scoring markers. The supermarker localized within 2 kB of 75% of MLV proviruses and predicted the likelihood of integration within specific chromosomal regions in a cell-type specific manner. This study identified chromosomal features highly favored for retroviral integration. It also provides clues to the mechanism by which retrovirus integration sites are selected, and offers a tool for predicting cell-type specific proto-oncogene activation by retroviruses.

Effects of the interactions of classical swine fever virus Core protein with proteins of the SUMOylation pathway on virulence in swine

Here we have identified host cell proteins involved with the cellular SUMOylation pathway, SUMO-1 (small ubiquitin-like modifier) and UBC9, a SUMO-1 conjugating enzyme that interact with classical swine fever virus (CSFV) Core protein. Five highly conserved lysine residues (K179, K180, K220, K221, and K246) within the CSFV Core were identified as putative SUMOylation sites. Analysis of these interactions showed that K179A, K180A, and K221A substitutions disrupt Core-SUMO-1 binding, while K220A substitution precludes Core-UBC9 binding. In vivo, Core mutant viruses (K179A, K180A, K220A, K221A) and (K220A, K221A) harboring those substitutions were attenuated in swine. These data shows a clear correlation between the disruption of Core protein binding to SUMO-1 and UBC9 and CSFV attenuation. Overall, these data suggest that the interaction of Core with the cellular SUMOylation pathway plays a significant role in the CSFV growth cycle in vivo.

Human Ubc9 contributes to production of fully infectious human immunodeficiency virus type 1 virions

Ubc9 was identified as a cellular protein that interacts with the Gag protein of Mason-Pfizer monkey virus. We show here that Ubc9 also interacts with the human immunodeficiency virus type 1 (HIV-1) Gag protein and that their interaction is important for virus replication. Gag was found to colocalize with Ubc9 predominantly at perinuclear puncta. While cells in which Ubc9 expression was suppressed with RNA interference produced normal numbers of virions, these particles were 8- to 10-fold less infectious than those produced in the presence of Ubc9. The nature of this defect was assayed for dependence on Ubc9 during viral assembly, trafficking, and Env incorporation. The Gag-mediated assembly of virus particles and protease-mediated processing of Gag and Gag-Pol were unchanged in the absence of Ubc9. However, the stability of the cell-associated Env glycoprotein was decreased and Env incorporation into released virions was altered. Interestingly, overexpression of the Ubc9 trans-dominant-negative mutant C93A, which is a defective E2-SUMO-1 conjugase, suggests that this activity may not be required for interaction with Gag, virion assembly, or infectivity. This finding demonstrates that Ubc9 plays an important role in the production of infectious HIV-1 virions.

The host cell sulfonation pathway contributes to retroviral infection at a step coincident with provirus establishment

The early steps of retrovirus replication leading up to provirus establishment are highly dependent on cellular processes and represent a time when the virus is particularly vulnerable to antivirals and host defense mechanisms. However, the roles played by cellular factors are only partially understood. To identify cellular processes that participate in these critical steps, we employed a high volume screening of insertionally mutagenized somatic cells using a murine leukemia virus (MLV) vector. This approach identified a role for 3′-phosphoadenosine 5′-phosphosulfate synthase 1 (PAPSS1), one of two enzymes that synthesize PAPS, the high energy sulfate donor used in all sulfonation reactions catalyzed by cellular sulfotransferases. The role of the cellular sulfonation pathway was confirmed using chemical inhibitors of PAPS synthases and cellular sulfotransferases. The requirement for sulfonation was mapped to a stage during or shortly after MLV provirus establishment and influenced subsequent gene expression from the viral long terminal repeat (LTR) promoter. Infection of cells by an HIV vector was also shown to be highly dependent on the cellular sulfonation pathway. These studies have uncovered a heretofore unknown regulatory step of retroviral replication, have defined a new biological function for sulfonation in nuclear gene expression, and provide a potentially valuable new target for HIV/AIDS therapy.

A genetic screen was used to identify host cell functions important for the replication of retroviruses, including human immunodeficiency viruses. These studies have uncovered a heretofore unexpected role for the cellular sulfonation pathway in an intracellular step of retroviral replication. Through the addition of sulfate groups, this pathway is responsible for modifying and regulating different types of cellular factors including proteins, lipids, carbohydrates and hormones. The role of this pathway was further confirmed by using specific chemical inhibitors. The sulfonation requirement was mapped to a step during viral DNA integration into the host genome that has a subsequent effect upon the level of expression of viral genes. These studies have uncovered a new regulatory mechanism of retroviral replication and suggest that components of the host cell sulfonation pathway might represent attractive targets for antiviral development.

Global analysis of host-pathogen interactions that regulate early-stage HIV-1 replication

Human Immunodeficiency Viruses (HIV-1 and HIV-2) rely upon host-encoded proteins to facilitate their replication. Here, we combined genome-wide siRNA analyses with interrogation of human interactome databases to assemble a host-pathogen biochemical network containing 213 confirmed host cellular factors and 11 HIV-1-encoded proteins. Protein complexes that regulate ubiquitin conjugation, proteolysis, DNA-damage response, and RNA splicing were identified as important modulators of early-stage HIV-1 infection. Additionally, over 40 new factors were shown to specifically influence the initiation and/or kinetics of HIV-1 DNA synthesis, including cytoskeletal regulatory proteins, modulators of posttranslational modification, and nucleic acid-binding proteins. Finally, 15 proteins with diverse functional roles, including nuclear transport, prostaglandin synthesis, ubiquitination, and transcription, were found to influence nuclear import or viral DNA integration. Taken together, the multiscale approach described here has uncovered multiprotein virus-host interactions that likely act in concert to facilitate the early steps of HIV-1 infection.

Kinesin KIF4 regulates intracellular trafficking and stability of the human immunodeficiency virus type 1 Gag polyprotein

Retroviral Gag proteins are synthesized as soluble, myristoylated precursors that traffic to the plasma membrane and promote viral particle production. The intracellular transport of human immunodeficiency virus type 1 (HIV-1) Gag to the plasma membrane remains poorly understood, and cellular motor proteins responsible for Gag movement are not known. Here we show that disrupting the function of KIF4, a kinesin family member, slowed temporal progression of Gag through its trafficking intermediates and inhibited virus-like particle production. Knockdown of KIF4 also led to increased Gag degradation, resulting in reduced intracellular Gag protein levels; this phenotype was rescued by reintroduction of KIF4. When KIF4 function was blocked, Gag transiently accumulated in discrete, perinuclear, nonendocytic clusters that colocalized with endogenous KIF4, with Ubc9, an E2 SUMO-1 conjugating enzyme, and with SUMO. These studies identify a novel transit station through which Gag traffics en route to particle assembly and highlight the importance of KIF4 in regulating HIV-1 Gag trafficking and stability.

An improved SUMO fusion protein system for effective production of native proteins

Expression of recombinant proteins as fusions with SUMO (small ubiquitin-related modifier) protein has significantly increased the yield of difficult-to-express proteins in Escherichia coli. The benefit of this technique is further enhanced by the availability of naturally occurring SUMO proteases, which remove SUMO from the fusion protein. Here we have improved the exiting SUMO fusion protein approach for effective production of native proteins. First, a sticky-end PCR strategy was applied to design a new SUMO fusion protein vector that allows directional cloning of any target gene using two universal cloning sites (Sfo1 at the 5'-end and XhoI at the 3'-end). No restriction digestion is required for the target gene PCR product, even the insert target gene contains a SfoI or XhoI restriction site. This vector produces a fusion protein (denoted as His(6)-Smt3-X) in which the protein of interest (X) is fused to a hexahistidine (His(6))-tagged Smt3. Smt3 is the yeast SUMO protein. His(6)-Smt3-X was purified by Ni(2+) resin. Removal of His(6)-Smt3 was performed on the Ni(2+) resin by an engineered SUMO protease, His(6)-Ulp1(403-621)-His(6). Because of its dual His(6) tags, His(6)-Ulp1(403-621)-His(6) exhibits a high affinity for Ni(2) resin and associates with Ni(2+) resin after cleavage reaction. One can carry out both fusion protein purification and SUMO protease cleavage using one Ni(2+)-resin column. The eluant contains only the native target protein. Such a one-column protocol is useful in developing a better high-throughput platform. Finally, this new system was shown to be effective for cloning, expression, and rapid purification of several difficult-to-produce authentic proteins.

Host factors exploited by retroviruses

Retroviruses make a long and complex journey from outside the cell to the nucleus in the early stages of infection, and then an equally long journey back out again in the late stages of infection. Ongoing efforts are identifying an enormous array of cellular proteins that are used by the viruses in the course of their travels. These host factors are potential new targets for therapeutic intervention.

Genetic Studies of the beta-hairpin loop of Rous sarcoma virus capsid protein

The first few residues of the Rous sarcoma virus (RSV) CA protein comprise a structurally dynamic region that forms part of a Gag-Gag interface in immature virus particles. Dissociation of this interaction during maturation allows refolding and formation of a beta-hairpin structure important for assembly of CA monomers into the mature capsid shell. A consensus binding site for the cellular Ubc9 protein was previously identified within this region, suggesting that binding of Ubc9 and subsequent small ubiquitin-like modifier protein 1 (SUMO-1) modification of CA may play a role either in regulating the assembly activity of CA in immature particles or mature cores or in controlling postentry function(s) during the establishment of infection. In the present study, mutations designed to eliminate the consensus binding site were used to dissect the potentially overlapping functions of these residues. The resulting replication defects could not be traced to a failure to form particles of normal composition but, rather, to a deficit in genome replication. Genetic suppressors of two detrimental beta-hairpin mutations improved infectivity without restoring the consensus site or creating a novel one elsewhere. Optimal restoration of infectivity to a Lys-to-Arg mutant required a combination of secondary changes, one on the surface of each domain of CA. Rather than arguing for a critical role of Ubc9 and SUMO in RSV replication, these findings provide strong support for a structural role of the N-terminal residues and a particularly striking example of long-range interactions between regions of CA in achieving a functional core competent for genome replication.

HIV infection of non-dividing cells: a divisive problem

Understanding how lentiviruses can infect terminally differentiated, non-dividing cells has proven a very complex and controversial problem. It is, however, a problem worth investigating, for it is central to HIV-1 transmission and AIDS pathogenesis. Here I shall attempt to summarise what is our current understanding for HIV-1 infection of non-dividing cells. In some cases I shall also attempt to make sense of controversies in the field and advance one or two modest proposals.

Viruses and sumoylation: recent highlights

Since its discovery in 1997, SUMO (small ubiquitin-like modifier) has been implicated in a range of activities, indicating that this protein is as important in the cell as ubiquitin is. Although it can function throughout the cell, it appears to be involved more in nuclear functions. The growing list of substrates that are covalently modified by SUMO includes many viral proteins; SUMO appears to facilitate viral infection of cells, making it a possible target for antiviral therapies. It therefore is important to understand how viruses manipulate the cellular sumoylation system and how sumoylation affects viral functions.

From cell surface to the nucleus, a short but critical journey for retroviruses

From the cell surface to the nucleus, retroviruses will face multiple obstacles, crossing physical barriers such as the plasma membrane, but also finding their pathway through a viscous cytoplasm. At the same time, retroviruses have to overcome cellular defenses, interfering with the early steps of the virus life cycle. Although the general outcomes of this journey have been known for several decades, the stepwise interactions taking place between cellular and viral factors, which will transform the incoming viral RNA genome into a double-stranded DNA competent for integration, remain largely unknown. In this sense, the uncoating process and the molecular basis of intracellular trafficking of preintegration complexes are still poorly defined. Additionally, other key stages, which have been the focus of many reports, still require some clarification, as is the case for the precise implication of viral and cellular determinants involved in nuclear import of preintegration complexes.