Nuclear import of human immunodeficiency virus type-1 preintegration complexes

Determinants of Retroviral Integration and Implications for Gene Therapeutic MLV-Based Vectors and for a Cure for HIV-1 Infection

To complete their replication cycle, retroviruses need to integrate a DNA copy of their RNA genome into a host chromosome. Integration site selection is not random and is driven by multiple viral and cellular host factors specific to different classes of retroviruses. Today, overwhelming evidence from cell culture, animal experiments and clinical data suggests that integration sites are important for retroviral replication, oncogenesis and/or latency. In this review, we will summarize the increasing knowledge of the mechanisms underlying the integration site selection of the gammaretrovirus MLV and the lentivirus HIV-1. We will discuss how host factors of the integration site selection of retroviruses may steer the development of safer viral vectors for gene therapy. Next, we will discuss how altering the integration site preference of HIV-1 using small molecules could lead to a cure for HIV-1 infection.

Foamy virus assembly with emphasis on pol encapsidation

Foamy viruses (FVs) differ from all other genera of retroviruses (orthoretroviruses) in many aspects of viral replication. In this review, we discuss FV assembly, with special emphasis on Pol incorporation. FV assembly takes place intracellularly, near the pericentriolar region, at a site similar to that used by betaretroviruses. The regions of Gag, Pol and genomic RNA required for viral assembly are described. In contrast to orthoretroviral Pol, which is synthesized as a Gag-Pol fusion protein and packaged through Gag-Gag interactions, FV Pol is synthesized from a spliced mRNA lacking all Gag sequences. Thus, encapsidation of FV Pol requires a different mechanism. We detail how WT Pol lacking Gag sequences is incorporated into virus particles. In addition, a mutant in which Pol is expressed as an orthoretroviral-like Gag-Pol fusion protein is discussed. We also discuss temporal regulation of the protease, reverse transcriptase and integrase activities of WT FV Pol.

Prototype foamy virus gag nuclear localization: a novel pathway among retroviruses

Gag nuclear localization has long been recognized as a hallmark of foamy virus (FV) infection. Two required motifs, a chromatin-binding site (CBS) and a nuclear localization signal (NLS), both located in glycine-arginine-rich box II (GRII), have been described. However, the underlying mechanisms of Gag nuclear translocation are largely unknown. We analyzed prototype FV (PFV) Gag nuclear localization using a novel live-cell fluorescence microscopy assay. Furthermore, we characterized the nuclear localization route of Gag mutants tagged with the simian vacuolating virus 40-NLS (SV40-NLS) and also dissected the respective contributions of the CBS and the NLS. We found that PFV Gag does not translocate to the nucleus of interphase cells by NLS-mediated nuclear import and does not possess a functional NLS. PFV Gag nuclear localization occurred only by tethering to chromatin during mitosis. This mechanism was found for endogenously expressed Gag as well as for Gag delivered by infecting viral particles. Thereby, the CBS was absolutely essential, while the NLS was dispensable. Gag CBS-dependent nuclear localization was neither essential for infectivity nor necessary for Pol encapsidation. Interestingly, Gag localization was independent of the presence of Pol, Env, and viral RNA. The addition of a heterologous SV40-NLS resulted in the nuclear import of PFV Gag in interphase cells, rescued the nuclear localization deficiency but not the infectivity defect of a PFV Gag ΔGRII mutant, and did not enhance FV's ability to infect G(1)/S-phase-arrested cells. Thus, PFV Gag nuclear localization follows a novel pathway among orthoretroviral Gag proteins.

Retroviral vectors for gene therapy

Since their first clinical trial 20 years ago, retroviral (gretroviral and lentiviral) vectors have now been used in more than 350 gene-therapy studies. Retroviral vectors are particularly suited for gene-correction of cells due to long-term and stable expression of the transferred transgene(s), and also because little effort is required for their cloning and production. Several monogenic inherited diseases, mostly immunodeficiencies, can now be successfully treated. The occurrence of insertional mutagenesis in some studies allowed extensive analysis of integration profiles of retroviral vectors, as well as the design of lentiviral vectors with increased safety properties. These new-generation vectors will enable us to continue the successful story of gene therapy, and treat more patients and even more complex diseases.

Perturbation of host nuclear membrane component RanBP2 impairs the nuclear import of human immunodeficiency virus -1 preintegration complex (DNA)

HIV-1 is a RNA virus that requires an intermediate DNA phase via reverse transcription (RT) step in order to establish productive infection in the host cell. The nascent viral DNA synthesized via RT step and the preformed viral proteins are assembled into pre-integration complex (PIC) in the cell cytoplasm. To integrate the viral DNA into the host genome, the PIC must cross cell nuclear membrane through the nuclear pore complex (NPC). RanBP2, also known as Nup358, is a major component of the cytoplasmic filaments that emanates from the nuclear pore complex and has been implicated in various nucleo-cytoplasmic transport pathways including those for HIV Rev-protein. We sought to investigate the role of RanBP2 in HIV-1 replication. In our investigations, we found that RanBP2 depletion via RNAi resulted in profound inhibition of HIV-1 infection and played a pivotal role in the nuclear entry of HIV DNA. More precisely, there was a profound decline in 2-LTR DNA copies (marker for nuclear entry of HIV DNA) and an unchanged level of viral reverse transcription in RanBP2-ablated HIV-infected cells compared to RanBP3-depleted or non-specific siRNA controls. We further demonstrated that the function of Rev was unaffected in RanBP2-depleted latently HIV infected cells (reactivated). We also serendipitously found that RanBP2 depletion inhibited the global ectopic gene expression. In conclusion, RanBP2 is a host factor that is involved in the nuclear import of HIV-1 PIC (DNA), but is not critical to the nuclear export of the viral mRNAs or nucleo-cytoplasmic shuttling of Rev. RanBP2 could be a potential target for efficient inhibition of HIV.

Retroviral reverse transcriptases

Reverse transcription is a critical step in the life cycle of all retroviruses and related retrotransposons. This complex process is performed exclusively by the retroviral reverse transcriptase (RT) enzyme that converts the viral single-stranded RNA into integration-competent double-stranded DNA. Although all RTs have similar catalytic activities, they significantly differ in several aspects of their catalytic properties, their structures and subunit composition. The RT of human immunodeficiency virus type-1 (HIV-1), the virus causing acquired immunodeficiency syndrome (AIDS), is a prime target for the development of antiretroviral drug therapy of HIV-1/AIDS carriers. Therefore, despite the fundamental contributions of other RTs to the understanding of RTs and retrovirology, most recent RT studies are related to HIV-1 RT. In this review we summarize the basic properties of different RTs. These include, among other topics, their structures, enzymatic activities, interactions with both viral and host proteins, RT inhibition and resistance to antiretroviral drugs.

Analysis of the viral elements required in the nuclear import of HIV-1 DNA

HIV-1 possesses an exquisite ability to infect cells independently from their cycling status by undergoing an active phase of nuclear import through the nuclear pore. This property has been ascribed to the presence of karyophilic elements present in viral nucleoprotein complexes, such as the matrix protein (MA); Vpr; the integrase (IN); and a cis-acting structure present in the newly synthesized DNA, the DNA flap. However, their role in nuclear import remains controversial at best. In the present study, we carried out a comprehensive analysis of the role of these elements in nuclear import in a comparison between several primary cell types, including stimulated lymphocytes, macrophages, and dendritic cells. We show that despite the fact that none of these elements is absolutely required for nuclear import, disruption of the central polypurine tract-central termination sequence (cPPT-CTS) clearly affects the kinetics of viral DNA entry into the nucleus. This effect is independent of the cell cycle status of the target cells and is observed in cycling as well as in nondividing primary cells, suggesting that nuclear import of viral DNA may occur similarly under both conditions. Nonetheless, this study indicates that other components are utilized along with the cPPT-CTS for an efficient entry of viral DNA into the nucleus.

[How proviral DNA is integrated into the host cell DNA and how this process can be inhibited]

The HIV replication cycle passes through a stage of integrating proviral DNA into the cell's DNA. In this process, the viral enzyme, integrase, catalyses two reactions. The first reaction, which seems to occur in the cytoplasm, involves 3'-end processing, in which two nucleotides are removed from the 3' ends of the viral DNA by integrase. The second reaction, which occurs in the nucleus, involves the strand transfer reaction, catalyzed by integrase, in which the recessed 3' ends of the viral DNA are joined to the protruding 5' ends in the target DNA. Although this activity has not yet been completely defined and the structure of the active form of integrase, probably a tetramer, has not been resolved, drugs of the diketoacid (DKA) family have been found. These drugs are highly potent inhibitors of the second phase, the strand transfer reaction. Through a series of optimizations, a highly effective molecule for clinical use, raltegravir, has been achieved. The present article provides a summary of basic knowledge on integrase, as well as the activity and the modes of inhibition of this enzyme. Also discussed is the reduced, but nevertheless real, development of resistance to raltegravir, requiring second-generation integrase inhibitors to be designed.

Human immunodeficiency virus type 1 Vpr modulates cellular expression of UNG2 via a negative transcriptional effect

It was recently reported that human immunodeficiency virus type 1 (HIV-1) Vpr induced the proteasomal degradation of the nuclear UNG2 enzyme for efficient virus replication. We confirm here that HIV-1 infection and Vpr expression reduce the level of endogenous UNG2, but this effect is not reverted by treatment with the proteasome inhibitor MG132. Moreover, this reduction is not mediated by Vpr binding to UNG2 and is independent of the Vpr-induced G(2) arrest. Finally, we show that Vpr influences the UNG2 promoter without affecting UNG1 gene expression. These data indicate that the Vpr-induced decrease of UNG2 level is mainly related to a transcriptional effect.

Nuclear import of Avian Sarcoma Virus integrase is facilitated by host cell factors

BACKGROUND

Integration of retroviral DNA into the host cell genome is an obligatory step in the virus life cycle. In previous reports we identified a sequence (amino acids 201-236) in the linker region between the catalytic core and C-terminal domains of the avian sarcoma virus (ASV) integrase protein that functions as a transferable nuclear localization signal (NLS) in mammalian cells. The sequence is distinct from all known NLSs but, like many, contains basic residues that are essential for activity.

RESULTS

Our present studies with digitonin-permeabilized HeLa cells show that nuclear import mediated by the NLS of ASV integrase is an active, saturable, and ATP-dependent process. As expected for transport through nuclear pore complexes, import is blocked by treatment of cells with wheat germ agglutinin. We also show that import of ASV integrase requires soluble cellular factors but does not depend on binding the classical adapter Importin-alpha. Results from competition studies indicate that ASV integrase relies on one or more of the soluble components that mediate transport of the linker histone H1.

CONCLUSION

These results are consistent with a role for ASV integrase and cytoplasmic cellular factors in the nuclear import of its viral DNA substrate, and lay the foundation for identification of host cell components that mediate this reaction.

Reconstitution of the myeloid and lymphoid compartments after the transplantation of autologous and genetically modified CD34+ bone marrow cells, following gamma irradiation in cynomolgus macaques

Background

Prolonged, altered hematopoietic reconstitution is commonly observed in patients undergoing myeloablative conditioning and bone marrow and/or mobilized peripheral blood-derived stem cell transplantation. We studied the reconstitution of myeloid and lymphoid compartments after the transplantation of autologous CD34⁺ bone marrow cells following gamma irradiation in cynomolgus macaques.

Results

The bone marrow cells were first transduced ex vivo with a lentiviral vector encoding eGFP, with a mean efficiency of 72% ± 4%. The vector used was derived from the simian immunodeficiency lentivirus SIVmac251, VSV-g pseudotyped and encoded eGFP under the control of the phosphoglycerate kinase promoter. After myeloid differentiation, GFP was detected in colony-forming cells (37% ± 10%). A previous study showed that transduction rates did not differ significantly between colony-forming cells and immature cells capable of initiating long-term cultures, indicating that progenitor cells and highly immature hematopoietic cells were transduced with similar efficiency. Blood cells producingeGFP were detected as early as three days after transplantation, and eGFP-producing granulocyte and mononuclear cells persisted for more than one year in the periphery.

Conclusion

The transplantation of CD34⁺ bone marrow cells had beneficial effects for the ex vivo proliferation and differentiation of hematopoietic progenitors, favoring reconstitution of the T- and B-lymphocyte, thrombocyte and red blood cell compartments.

Quantitative analysis of the interactions between HIV-1 integrase and retroviral reverse transcriptases

The RT (reverse transcriptase) of HIV-1 interacts with HIV-1 IN (integrase) and inhibits its enzymatic activities. However, the molecular mechanisms underling these interactions are not well understood. In order to study these mechanisms, we have analysed the interactions of HIV-1 IN with HIV-1 RT and with two other related RTs: those of HIV-2 and MLV (murine-leukaemia virus). All three RTs inhibited HIV-1 IN, albeit to a different extent, suggesting a common site of binding that could be slightly modified for each one of the studied RTs. Using surface plasmon resonance technology, which monitors direct protein–protein interactions, we performed kinetic analyses of the binding of HIV-1 IN to these three RTs and observed interesting binding patterns. The interaction of HIV-1 RT with HIV-1 IN was unique and followed a two-state reaction model. According to this model, the initial IN–RT complex formation was followed by a conformational change in the complex that led to an elevation of the total affinity between these two proteins. In contrast, HIV-2 and MLV RTs interacted with IN in a simple bi-molecular manner, without any apparent secondary conformational changes. Interestingly, HIV-1 and HIV-2 RTs were the most efficient inhibitors of HIV-1 IN activity, whereas HIV-1 and MLV RTs showed the highest affinity towards HIV-1 IN. These modes of direct protein interactions, along with the apparent rate constants calculated and the correlations of the interaction kinetics with the capacity of the RTs to inhibit IN activities, are all discussed.

Human gene therapy vectors derived from feline lentiviruses

Lentiviral vectors are useful for gene transfer to dividing and nondividing cells. Feline immunodeficiency virus (FIV) vectors transduce most human cell types with good efficiency and may have advantages for clinical gene therapy applications. This article reviews significant progress in the development and refinement of FIV vector systems.

FIV as a Model for HIV: An Overview

Animal models for human immunodeficiency virus (HIV) infection play a key role in understanding the pathogenesis of AIDS and the development of therapeutic agents and vaccines. As the only lentivirus that causes an immunodeficiency resembling that of HIV infection, in its natural host, feline immunodeficiency virus (FIV) has been a unique and powerful model for AIDS research. FIV was first described in 1987 by Niels Pedersen and co-workers as the causative agent for a fatal immunodeficiency syndrome observed in cats housed in a cattery in Petaluma, California. Since this landmark observation, multiple studies have shown that natural and experimental infection of cats with biological isolates of FIV produces an AIDS syndrome very similar in pathogenesis to that observed for human AIDS. FIV infection induces an acute viremia associated with Tcell alterations including depressed CD4 :CD8 T-cell ratios and CD4 T-cell depletion, peripheral lymphadenopathy, and neutropenia. In later stages of FIV infection, the host suffers from chronic persistent infections that are typically self-limiting in an immunocompetent host, as well as opportunistic infections, chronic diarrhea and wasting, blood dyscracias, significant CD4 T-cell depletion, neurologic disorders, and B-cell lymphomas. Importantly, chronic FIV infection induces a progressive lymphoid and CD4 T-cell depletion in the infected cat. The primary mode of natural FIV transmission appears to be blood-borne facilitated by fighting and biting. However, experimental infection through transmucosal routes (rectal and vaginal mucosa and perinatal) have been well documented for specific FIV isolates. Accordingly, FIV disease pathogenesis exhibits striking similarities to that described for HIV-1 infection.

Inhibition of the activities of reverse transcriptase and integrase of human immunodeficiency virus type-1 by peptides derived from the homologous viral protein R (Vpr)

Shortly after infection by human immunodeficiency virus (HIV), two complexes are formed in a stepwise manner in the cytoplasm of infected cells: the reverse transcription complex that later becomes the preintegration complex. Both complexes include, in addition to cellular proteins, viral RNA or DNA and several proteins, such as reverse transcriptase (RT), integrase (IN), and viral protein R (Vpr). These proteins are positioned in close spatial proximity within these complexes, enabling mutual interactions between the proteins. Physical in vitro interactions between RT and IN that affect their enzymatic activities were already reported. Moreover, we found recently that HIV-1 RT-derived peptides bind and inhibit HIV-1 IN and that an IN-derived peptide binds and inhibits HIV-1 RT. Additionally, HIV-1 Vpr and its C-terminal domain affected in vitro the integration activity of HIV-1 IN. Here, we describe the associations of Vpr-derived peptides with RT and IN. Of a peptide library that spans the 96-residue-long Vpr protein, three partially overlapping peptides, derived from the C-terminal domain, bind both enzymes. Two of these peptides inhibit both RT and IN. Another peptide, derived from the Vpr N-terminal domain, binds IN and inhibits its activities, without binding and affecting RT. Interestingly, two sequential C-terminal peptides (derived from residues 57-71 and 61-75 of full-length Vpr) are the most effective inhibitors of both enzymes. The data and the molecular modeling presented suggest that RT and IN are inhibited as a result of steric hindrance or conformational changes of their active sites, whereas a second mechanism of blocking its dimerization state could be also attributed to the inhibition of IN.

The road to chromatin - nuclear entry of retroviruses

Human immunodeficiency virus 1 (HIV-1) and other retroviruses synthesize a DNA copy of their genome after entry into the host cell. Integration of this DNA into the host cell's genome is an essential step in the viral replication cycle. The viral DNA is synthesized in the cytoplasm and is associated with viral and cellular proteins in a large nucleoprotein complex. Before integration into the host genome can occur, this complex must be transported to the nucleus and must cross the nuclear envelope. This Review summarizes our current knowledge of how this journey is accomplished.

Inhibition of human immunodeficiency virus type-1 reverse transcriptase by a novel peptide derived from the viral integrase

Previous studies show that the reverse transcriptase (RT) of human immunodeficiency virus type-1 (HIV-1) and RT-derived peptides interact with and inhibit the viral integrase (IN). In the present study, we have performed the complementary study by screening a complete library of HIV-1 IN-derived peptides for their effects on the RT. We have identified a 20-residues long peptide, derived from the IN (residues 46-65) that binds the RT and inhibits its DNA-polymerase activities (without affecting the ribonuclease-H activity). The full 20-residues sequence is required for maximal inhibition. This inhibition is non-competitive and probably results from obstructing the formation of RT-DNA complexes by the peptide. The data and the molecular docking model presented suggest that this inhibition is probably caused by a steric hindrance or conformational changes of the RT. These results can facilitate the development of novel and specific peptide-based HIV-1 RT inhibitors that might help in the fight against AIDS.

Vpr and HIV-1 disease progression: R77Q mutation is associated with long-term control of HIV-1 infection in different groups of patients

BACKGROUND

Vpr (viral protein R) is a 96 amino acids soluble protein that is expressed late during viral replication. Recent studies have focused on the role of a mutation at position 77 that might be associated with the condition of long-term non-progression, but data are still controversial.

PATIENTS AND METHODS

Fifteen long-term non-progressors (LTNP), 19 therapy-naive HIV-1-infected patients with progressive disease (Pr), 23 HIV-1-infected patients receiving sub-optimal therapy with dual nucleoside [nucleoside reverse transcriptase inhibitor (NRTI)] therapy but efficiently controlling viral replication (STP) and 19 antiretroviral therapy multi-experienced patients with actively replicating virus (MEP) were analysed. HIV-RNA was extracted from plasma samples, the Vpr region was amplified, cloned and sequenced. The Pol gene was amplified, directly sequenced and analysed using Sequence Navigator software.

RESULTS

A significantly higher prevalence of the R77Q mutation was evidenced both in LTNP (86.7%) and STP (73.9%) in comparison with Pr (42.1%) and MEP (42.1%), (P = 0.007). Comparing groups of patients with progressive disease (Pr + MEP) and groups with non-progressive disease (LTNP + STP) the probability of harbouring the R77Q mutation was significantly higher in non-progressors (odds ratio, 5.16; P = 0.001).

CONCLUSIONS

Our results support the hypothesis of the association of R77Q mutation in the Vpr gene with delayed progression of HIV-1 disease. R77Q does not seem to be linked to a particular viral strain but might be associated to immunologic selection. The R77Q mutation might reduce CD4+ T-cell depletion possibly affecting T-cell survival in vivo by altering the pro-apoptotic activity of Vpr.

Analysis of HIV-2 Vpx by modeling and insertional mutagenesis

Vpx facilitates HIV-2 nuclear localization by a poorly understood mechanism. We have compared Vpx to an NMR structure HIV-1 Vpr in a central helical domain and probed regions of Vpx by insertional mutagenesis. A predicted loop between helices two and three appears to be unique, overlapping with a known novel nuclear localization signal. Overall, Vpx was found to be surprisingly flexible, tolerating a series of large insertions. We found that insertion within the polyproline-containing C-terminus destabilizes nuclear localization, whereas mutating a second helix in the central domain disrupts viral packaging. Other insertional mutants in the predicted loop and in a linker region between the central domain and the C-terminus may be useful as sites of intramolecular tags as they could be packaged adequately and retained preintegration complex associated integration activity in a serum starvation assay. An unexpected result was found within a previously defined nuclear localization motif near aa 71. This mutant retained robust nuclear localization in a GFP fusion assay and was competent for preintegration complex associated nuclear import. In summary, we have modeled helical content in Vpx and assessed potential sites of intramolecular tags which may prove useful for protein-protein interactions studies.

Lentiviral vectors

We review the use of lentiviral vectors in current human gene therapy applications that involve genetic modification of nondividing tissues with integrated transgenes. Safety issues, including insertional mutagenesis and replication-competent retroviruses, are discussed. Innate cellular defenses against retroviruses and their implications for human gene therapy with different lentiviral vectors are also addressed.

Roles of HIV-1 auxiliary proteins in viral pathogenesis and host-pathogen interactions

Active host-pathogen interactions take place during infection of human immunodeficiency virus type 1 (HIV-1). Outcomes of these interactions determine the efficiency of viral infection and subsequent disease progression. HIV-infected cells respond to viral invasion with various defensive strategies such as innate, cellular and humoral immune antiviral mechanisms. On the other hand, the virus has also developed various offensive tactics to suppress these host cellular responses. Among many of the viral offensive strategies, HIV-1 viral auxiliary proteins (Tat, Rev, Nef, Vif, Vpr and Vpu) play important roles in the host-pathogen interaction and thus have significant impacts on the outcome of HIV infection. One of the best examples is the interaction of Vif with a host cytidine deaminase APOBEC3G. Although specific roles of other auxiliary proteins are not as well described as Vif-APOBEC3G interaction, it is the goal of this brief review to summarize some of the preliminary findings with the hope to stimulate further discussion and investigation in this exhilarating area of research.

The C-terminal domain of the HIV-1 regulatory protein Vpr adopts an antiparallel dimeric structure in solution via its leucine-zipper-like domain

HIV-1 Vpr is a highly conserved accessory protein that is involved in many functions of the virus life cycle. Vpr facilitates the entry of the HIV pre-integration complex through the nuclear pore, induces G2 cell cycle arrest, regulates cell apoptosis, increases transcription from the long terminal repeat and enhances viral replication. Vpr contains a Leu/Ile-rich domain (amino acids 60-81) in its C-terminal part, which is critical for dimerization. The sequence comprising residues 52-96 is implicated in properties of the protein such as DNA interaction and apoptosis via interaction with the adenine nucleotide translocator. To understand the specific interactions of Vpr-(52-96), the ability of this peptide to dimerize via a leucine-zipper mechanism has been investigated, by NMR and fluorescence spectroscopy. In contrast with results from a study performed in the presence of trifluoroethanol, our results, obtained in 30% (v/v) [2H]acetonitrile, show that Vpr-(52-96) in solution still forms an a-helix spanning residues 53-75, but dimerizes in an antiparallel orientation, through hydrophobic interactions between leucine and isoleucine residues and stacking between His71 and Trp54. Moreover, to demonstrate the physiological relevance of the dimer structure, fluorescence spectroscopy experiments have been performed in a Mes buffer, which confirmed the formation of the dimer in aqueous solution and highlighted the spatial proximity between Trp54 and His71. Surprisingly, the leucine-zipper structure shown in the present work for Vpr-(52-96) mimics the structure of full-length Vpr-(1-96), and this could explain why some of the properties of Vpr-(52-96) and Vpr-(1-96) are identical, while some are even enhanced for Vpr-(52-96), particularly in the case of DNA transfection experiments.

The long terminal repeat-containing retrotransposon Tf1 possesses amino acids in gag that regulate nuclear localization and particle formation

Tf1 is a long terminal repeat-containing retrotransposon of Schizosaccharomyces pombe that is studied to further our understanding of retrovirus propagation. One important application is to examine Tf1 as a model for how human immunodeficiency virus type 1 proteins enter the nucleus. The accumulation of Tf1 Gag in the nucleus requires an N-terminal nuclear localization signal (NLS) and the nuclear pore factor Nup124p. Here, we report that NLS activity is regulated by adjacent residues. Five mutant transposons were made, each with sequential tracts of four amino acids in Gag replaced by alanines. All five versions of Tf1 transposed with frequencies that were significantly lower than that of the wild type. Although all five made normal amounts of Gag, two of the mutations did not make cDNA, indicating that Gag contributed to reverse transcription. The localization of the Gag in the nucleus was significantly reduced by mutations A1, A2, and A3. These results identified residues in Gag that contribute to the function of the NLS. The Gags of A4 and A5 localized within the nucleus but exhibited severe defects in the formation of virus-like particles. Of particular interest was that the mutations in Gag-A4 and Gag-A5 caused their nuclear localization to become independent of Nup124p. These results suggested that Nup124p was only required for import of Tf1 Gag because of its extensive multimerization.

Importin-alpha promotes passage through the nuclear pore complex of human immunodeficiency virus type 1 Vpr

Viral protein R (Vpr) of human immunodeficiency virus type 1 has potent karyophilic properties, but details of the mechanism by which it enters the nucleus remain to be clarified. We reported previously that two regions, located between residues 17 and 34 (alphaH1) and between residues 46 and 74 (alphaH2), are indispensable for the nuclear localization of Vpr. Here, we reveal that a chimeric protein composed of the nuclear localization signal of Vpr, glutathione S-transferase, and green fluorescent protein was localized at the nuclear envelope and then entered the nucleus upon addition of importin-alpha. An in vitro transport assay using a series of derivatives of importin-alpha demonstrated that the carboxyl terminus was required for this nuclear import process. We also showed that Vpr interacts with importin-alpha through alphaH1 and alphaH2; only the interaction via alphaH1 is indispensable for the nuclear entry of Vpr. These observations indicate that importin-alpha functions as a mediator for the nuclear entry of Vpr.

Peptides derived from the reverse transcriptase of human immunodeficiency virus type 1 as novel inhibitors of the viral integrase

Recent studies have shown that the integrase (IN) of HIV-1 is inhibited in vitro by HIV-1 reverse transcriptase (RT). We further investigated the specific protein sequences of RT that were involved in this inhibition by screening a complete library of RT-derived peptides for their inhibition of IN activities. Two 20-residue peptides, peptide 4286, derived from the RT DNA polymerase domain, and the one designated 4321, from the RT ribonuclease H domain, inhibit the enzymatic activities of IN in vitro. The former peptide inhibits all three IN-associated activities (3'-end processing, strand transfer, and disintegration), whereas the latter one inhibits primarily the first two functions. We showed the importance of the sequences and peptide length for the effective inhibition of IN activities. Binding assays of the peptides to IN (with no DNA substrate present) indicated that the two inhibitory peptides (as well as several non-inhibitory peptides) interact directly with IN. Moreover, the isolated catalytic core domain of IN also interacted directly with the two inhibitory peptides. Nevertheless, only peptide 4286 can inhibit the disintegration activity associated with the IN core domain, because this activity is the only one exhibited by this domain. This result was expected from the lack of inhibition of disintegration of full-length IN by peptide 4321. The data and the three-dimensional models presented suggested that the inhibition resulted from steric hindrance of the catalytic domain of IN. This information can substantially facilitate the development of novel drugs against HIV INs and thus contribute to the fight against AIDS.

Genetic control of retrovirus susceptibility in mammalian cells

Host cellular genes can have profound effects on retrovirus replication. Many of these genes encode restriction factors that block virus infection; others encode positive factors that are exploited by the viruses. Recently, a number of such genes have been cloned and characterized, bringing into sharper focus the mechanisms and pathways exploited by these viruses. The major host factors involved in the early phase of the viral life cycle are discussed.

The Vpr protein from HIV-1: distinct roles along the viral life cycle

The genomes of human and simian immunodeficiency viruses (HIV and SIV) encode the gag, pol and env genes and contain at least six supplementary open reading frames termed tat, rev, nef, vif, vpr, vpx and vpu. While the tat and rev genes encode regulatory proteins absolutely required for virus replication, nef, vif, vpr, vpx and vpu encode for small proteins referred to "auxiliary" (or "accessory"), since their expression is usually dispensable for virus growth in many in vitro systems. However, these auxiliary proteins are essential for viral replication and pathogenesis in vivo. The two vpr- and vpx-related genes are found only in members of the HIV-2/SIVsm/SIVmac group, whereas primate lentiviruses from other lineages (HIV-1, SIVcpz, SIVagm, SIVmnd and SIVsyk) contain a single vpr gene. In this review, we will mainly focus on vpr from HIV-1 and discuss the most recent developments in our understanding of Vpr functions and its role during the virus replication cycle.

Macrophages and HIV-1: dangerous liaisons

HIV-1, like the other lentiviruses, has evolved the ability to infect nondividing cells including macrophages. HIV-1 replication in monocytes/macrophages entails peculiar features and differs in many respects from that in CD4 T lymphocytes. HIV-1 exhibits different tropism for CD4 T cells and macrophages. The virus can enter macrophages via several routes. Mitosis is not required for nuclear import of viral DNA or for its integration into the host cell genome. Specific cellular factors are required for HIV-1 transcription in macrophages. The assembly and budding of viral particles in macrophages take place in late endosomal compartments. Viral particles can use the exosome pathway to exit cells. Given their functions in host defence against pathogens and the regulation of the immune response plus their permissivity to HIV-1 infection, monocytes/macrophages exert a dual role in HIV infection. They contribute to the establishment and persistence of HIV-1 infection, and may activate surrounding T cells favouring their infection. Furthermore, monocytes/macrophages act as a Trojan horse to transmit HIV-1 to the central nervous system. They also exhibit antiviral activity and express many molecules that inhibit HIV-1 replication. Activated microglia and macrophages may also exert a neurotrophic and neuroprotective effect on infected brain regulating glutamate metabolism or by secretion of neurotrophins. This review will discuss specific aspects of viral replication in monocytes/macrophages and the role of their interactions with the cellular environment in HIV-1 infection swinging between protection and pathogenesis.

The functionally conserved nucleoporins Nup124p from fission yeast and the human Nup153 mediate nuclear import and activity of the Tf1 retrotransposon and HIV-1 Vpr

We report that the fission yeast nucleoporin Nup124p is required for the nuclear import of both, retrotransposon Tf1-Gag as well as the retroviral HIV-1 Vpr. Failure to import Tf1-Gag into the nucleus in a nup124 null mutant resulted in complete loss of Tf1 transposition. Similarly, nuclear import of HIV-1 Vpr was impaired in nup124 null mutant strains and cells became resistant to Vpr's cell-killing activity. On the basis of protein domain similarity, the human nucleoporin Nup153 was identified as a putative homolog of Nup124p. We demonstrate that in vitro-translated Nup124p and Nup153 coimmunoprecipitate Tf1-Gag or HIV-1 Vpr. Though full-length Nup153 was unable to complement the Tf1 transposition defect in a nup124 null mutant, we provide evidence that both nucleoporins share a unique N-terminal domain, Nup124p(AA264-454) and Nup153(AA448-634) that is absolutely essential for Tf1 transposition. Epigenetic overexpression of this domain in a wild-type (nup124(+)) background blocked Tf1 activity implying that sequences from Nup124p and the human Nup153 challenged the same pathway affecting Tf1 transposition. Our results establish a unique relationship between two analogous nucleoporins Nup124p and Nup153 wherein the function of a common domain in retrotransposition is conserved.

Vpx proteins of SIVmac239 and HIV-2ROD interact with the cytoskeletal protein alpha-actinin 1

vpx genes of human immunodeficiency virus type 2 (HIV-2) and immunodeficiency viruses from macaques (SIVmac), sooty mangabeys (SIVsm) and red-capped mangabeys (SIVrcm) encode a 112 aa protein that is packed into virion particles via interaction with the p6 domain of p55(gag). Vpx localizes to the nucleus when expressed in the absence of other viral proteins. Moreover, Vpx is necessary for efficient nuclear import of the pre-integration complex (PIC) and critical for virus replication in quiescent cells, such as terminally differentiated macrophages and memory T cells. Vpx does not contain sequence elements that are homologous to previously characterized nuclear localization signals (NLSs). Therefore, it is likely that Vpx-dependent import of the PIC is mediated by interaction of Vpx with cellular proteins that do not belong to the classical import pathways. By using a yeast two-hybrid screen, alpha-actinin 1, a cytoskeletal protein, was identified to interact with SIVmac239 Vpx. Interestingly, deletion of the proline-rich C-terminal domain (aa 101-112) of Vpx, which is important for nuclear localization, resulted in loss of interaction with alpha-actinin 1. These findings suggest that the interaction with alpha-actinin 1 may play an important role in the transport of Vpx to the nucleus and in Vpx-mediated nuclear import of the PIC.

Cell cycle regulation of human immunodeficiency virus type 1 integration in T cells: antagonistic effects of nuclear envelope breakdown and chromatin condensation

We examined the influence of mitosis on the kinetics of human immunodeficiency virus type 1 integration in T cells. Single-round infection of cells arrested in G1b or allowed to synchronously proceed through division showed that mitosis delays virus integration until 18-24 h postinfection, whereas integration reaches maximum levels by 15 h in G1b-arrested cells. Subcellular fractionation of metaphase-arrested cells indicated that, while nuclear envelope disassembly facilitates docking of viral DNA to chromatin, chromosome condensation directly antagonizes and therefore delays integration. As a result of the balance between the two effects, virus integration efficiency is eventually up to threefold greater in dividing cells. At the single-cell level, using a green fluorescent protein-expressing reporter virus, we found that passage through mitosis leads to prominent asymmetric segregation of the viral genome in daughter cells without interfering with provirus expression.

LAP2alpha and BAF collaborate to organize the Moloney murine leukemia virus preintegration complex

Integration of viral DNA into the host genome is an essential step in retroviral replication. The viral DNA made by reverse transcription is a component of the preintegration complex (PIC) that also contains the viral integrase protein, the enzyme that integrates the viral DNA. Several other viral and cellular proteins are present in the PIC, but their functional roles are less well established. Barrier-to-autointegration factor (BAF) is a cellular protein component of the PIC that blocks autointegration of the viral DNA and stimulates intermolecular integration. In uninfected cells, BAF interacts with members of the LEM family of inner nuclear membrane and nucleoplasmic proteins. Here, we demonstrate that one of the LEM proteins, lamina-associated polypeptide 2alpha (LAP2alpha), is a component of the PIC. LAP2alpha stabilizes the association of BAF with the PIC to stimulate intermolecular integration and suppress autointegration. To further understand the role of LAP2alpha, we established LAP2alpha-knockdown cell lines. Depletion of LAP2alpha significantly inhibited viral replication. Our results demonstrate a critical contribution of LAP2alpha to the nucleoprotein organization of the PIC and to viral replication.

Nuclear import of HIV-1 integrase is inhibited in vitro by styrylquinoline derivatives

Nuclear import of HIV-1 preintegration complexes (PICs) allows the virus to infect nondividing cells. Integrase (IN), the PIC-associated viral enzyme responsible for the integration of the viral genome into the host cell DNA, displays karyophilic properties and has been proposed to participate to the nuclear import of the PIC. Styrylquinolines (SQs) have been shown to block viral replication at nontoxic concentrations and to inhibit IN 3'-processing activity in vitro by competing with the DNA substrate binding. However, several lines of evidence suggested that SQs could have a postentry, preintegrative antiviral effect in infected cells. To gain new insights on the mechanism of their antiviral activity, SQs were assayed for their ability to affect nuclear import of HIV-1 IN and compared with the effect of a specific strand transfer inhibitor. Using an in vitro transport assay, we have previously shown that IN import is a saturable mechanism, thus showing that a limiting cellular factor is involved in this process. We now demonstrate that SQs specifically and efficiently inhibit in vitro nuclear import of IN without affecting other import pathways, whereas a specific strand transfer inhibitor does not affect IN import. These data suggest that SQs not only inhibit IN-DNA interaction but would also inhibit the interaction between IN and the cellular factor required for its nuclear import.

Effect of HIV-1 Vpr on cell cycle regulators

Cell cycle is one of the most complex processes in the life of a dividing cell. It involves numerous regulatory proteins, which direct the cell through a specific sequence of events for the production of two daughter cells. Cyclin-dependent kinases (cdks), which complex with the cyclin proteins, are the main players in the cell cycle. They can regulate the progression of the cells through different stages regulated by several proteins including p53, p21(WAF1), p19, p16, and cdc25. Downstream targets of cyclin-cdk complexes include pRB and E2F. A cell cycle can be altered to the advantage of many viral agents, most notably polyomaviruses, papillomaviruses, adenoviruses, and retroviruses. In addition, viral protein R (Vpr) is a protein encoded by the human immunodeficiency virus type 1 (HIV-1). HIV-1, the causative agent of acquired immunodeficiency syndrome (AIDS), is a member of the lentivirus class of retroviruses. This accessory protein plays an important role in the regulation of the cell cycle by causing G(2) arrest and affecting cell cycle regulators. Vpr prevents infected cells from proliferating, and collaborates with the matrix protein (MA) to enable HIV-1 to enter the nucleus of nondividing cells. Studies from different labs including ours showed that Vpr affects the functions of cell cycle proteins, including p53 and p21(WAF1). Thus, the replication of HIV-1, and ultimately its pathogenesis, are intrinsically tied to cell-cycle control.

Capsid is a dominant determinant of retrovirus infectivity in nondividing cells

A major difference between lentiviruses such as human immunodeficiency virus (HIV) and most other retroviruses is their ability to productively infect nondividing cells. We present here genetic evidence for involvement of the capsid protein (CA) in the infectious phenotype in nondividing cells. A chimeric HIV type 1 (HIV-1) in which the MA and CA of HIV-1 are replaced with the MA, p12, and CA encoding sequences from murine leukemia virus (MLV) loses the ability to efficiently infect nondividing cells. Analysis of the accumulation of two-long-terminal-repeat circles implies that the impairment of nuclear transport of preintegration complexes is responsible for the restricted infection of this chimeric virus in nondividing cells. Incorporation of MLV MA and MLV p12 into HIV virions alone does not exert any adverse effects on viral infection in interphase cells. These results suggest that CA is the dominant determinant for the difference between HIV and MLV in the ability to transduce nondividing cells.

Human immunodeficiency virus type 1 DNA nuclear import and integration are mitosis independent in cycling cells

An essential step in human immunodeficiency virus type 1 (HIV-1) replication is the movement of the viral preintegration complex from the cytoplasm into the nucleus. The pathway(s) and timing for HIV-1 DNA nuclear entry in cycling cells have not been established. Here, we show that if cycling cells are infected before S phase, viral DNA can be integrated prior to passage of the host DNA replication fork through the integration site, as indicated by stable inheritance in both daughter cells. We conclude that efficient nuclear entry can occur independently of mitotic nuclear disassembly in cycling cells.

Intracellular trafficking of HIV-1 cores: journey to the center of the cell

After entry into the cytoplasm, many diverse viruses, including both RNA and DNA viruses, require import into the nucleus and access to the cellular nuclear machinery for productive replication to proceed. Because diffusion through the crowded cytoplasmic environment is greatly restricted, most (if not all) of these viruses must first be actively transported from the site of cytoplasmic entry to the nuclear periphery (Luby-Phelps 2000; Lukacs et al. 2000; Sodeik 2000). Having reached the nucleus, viruses have evolved assorted methods to overcome the formidable physical barrier that is presented by the nuclear envelope. This review examines how these issues relate to human immunodeficiency virus type-1 (HIV-1) infection. Specifically, HIV-1 uncoating, cytoplasmic transport, and nuclear entry are addressed.

Quantitative analysis of human immunodeficiency virus type 1 DNA dynamics by real-time PCR: integration efficiency in stimulated and unstimulated peripheral blood mononuclear cells

We established a set of real-time PCR assay to accurately quantify human immunodeficiency virus type 1 (HIV-1) DNA in infected cells. Using this assay we were able to measure the strong-stop, full-length/ 1-LTR circle, 2-LTR circle, and integrated forms of viral DNA, and the data provided was quite consistent with the characteristics of mutant viruses in early phase of infection. Since our assay is particularly applicable to quantify the integrated DNA in small scale of samples, we measured the level of integrated DNA in wild-type virus (WT)- or Vpr-defective virus (deltaVpr)-infected peripheral blood mononuclear cells (PBMC), and examined whether quiescent condition of the PBMC influences integration step of HIV-1. Under stimulating condition approximately 25% of total viral DNA was in integrated form in either WT- or DeltaVpr-infected cells. In contrast, under unstimulated condition the level of integration efficiency was not significantly reduced in WT-infected cells, while this efficiency was severely impaired in the absence of vpr gene. This result clearly demonstrated a crucial role of the Vpr for nuclear localization and subsequent integration of viral DNA in nondividing cells. Therefore, our assay is useful for analyzing the events in early phase of HIV-1 infection under various conditions.

Intracellular transport of human immunodeficiency virus type 1 integrase

The human immunodeficiency virus type 1 integrase protein has karyophilic properties; that is, it localizes to the cell nucleus according to a range of assays. As an essential component of the preintegration complex, it has been suggested that the karyophilic properties of integrase might facilitate transport of the preintegration complex through the nuclear pore complexes of nondividing cells. However, no experiments have satisfactorily identified a nuclear localization signal within integrase. In this work, we investigated the karyophilic properties of integrase in intact cells with hopes of identifying a genuine transferable nuclear localization signal. Our results confirm that integrase tightly binds chromosomal DNA in vivo. However, our analysis determined that large integrase fusion proteins are unable to access the nucleus, indicating that integrase might lack a transferable nuclear localization signal. In addition, we present several lines of evidence to indicate that DNA binding might facilitate integrase nuclear accumulation. Furthermore, our data indicate integrase is degraded in the cytoplasm by a proteasome-dependent process, an event that probably contributes to the apparent nuclear accumulation of integrase. These results provide new insight into human immunodeficiency virus type 1 integrase intracellular dynamics.

LEDGF/p75 is essential for nuclear and chromosomal targeting of HIV-1 integrase in human cells

We have reported that human immunodeficiency virus type 1 (HIV-1) integrase (IN) forms a specific nuclear complex with human lens epithelium-derived growth factor/transcription co-activator p75 (LEDGF/p75) protein. We now studied the IN-LEDGF/p75 interaction and nuclear import of IN in living cells using fusions of IN and LEDGF/p75 with enhanced green fluorescent protein and far-red fluorescent protein HcRed1. We show that both the N-terminal zinc binding domain and the central core domains of IN are involved in the interaction with LEDGF/p75. Both domains are essential for nuclear localization of IN as well as for the association of IN with condensed chromosomes during mitosis. However, upon overexpression of LEDGF/p75, the core domain fragment of IN was recruited to the nuclei and mitotic chromosomes with a distribution pattern characteristic of the full-length protein, indicating that it harbors the main determinant for interaction with LEDGF/p75. Although the C-terminal domain of IN was dispensable for nuclear/chromosomal localization, a fusion of the C-terminal IN fragment with enhanced green fluorescent protein was found exclusively in the nucleus, with a diffuse nuclear/nucleolar distribution, suggesting that the C-terminal domain may also play a role in the nuclear import of IN. In contrast to LEDGF/p75, its alternative splice variant, p52, did not interact with HIV-1 IN in vitro and in living cells. Finally, RNA interference-mediated knock-down of endogenous LEDGF/p75 expression abolished nuclear/chromosomal localization of IN. We conclude, therefore, that the interaction with LEDGF/p75 accounts for the karyophilic properties and chromosomal targeting of HIV-1 IN.

Identification of the nuclear localization signal of human immunodeficiency virus type 2 Vpx

The Vpx protein of human immunodeficiency virus type 2 (HIV-2) is a viral accessory protein related to, but distinct from, the Vpr protein of HIV-1. Vpx is packaged into virions and, as a component of the viral preintegration complex (PIC), Vpx is required for efficient virus replication in nondividing cells. Therefore, the localization of Vpx in cells is dynamic and dependent upon discrete domains of the protein. Expressed in the absence of other viral proteins, Vpx localizes to the nucleus of cells. However, if expressed with the Gag protein of HIV-2, Vpx localizes to the plasma membrane of cells. To further understand the regulation of Vpx localization, we fused regions of Vpx to beta-galactosidase to identify regions of the protein sufficient to mediate nuclear localization. The minimal transferable region of Vpx that conferred nuclear localization in these assays was aa 65 to 72. Alanine substitution of K(68) and R(70) in a GFP-Vpx construct abolished nuclear localization, suggesting that the basic residues in this region are important for nuclear import. Analysis of the membrane transport of several GFP-Vpx alanine mutants demonstrated that while separable, the domains of Vpx required for nuclear localization are not distinct from the domains required for membrane transport. The results of heterokaryon shuttling assays indicated that Vpx is not a shuttling protein; however, HIV-2 Vpr did shuttle similar to HIV-1 Vpr.

Regulation of T cell activation by HIV-1 accessory proteins: Vpr acts via distinct mechanisms to cooperate with Nef in NFAT-directed gene expression and to promote transactivation by CREB

Nef and Vpr are lentiviral accessory proteins that have been implicated in regulation of cellular gene expression. We noticed that Vpr can potentiate Nef-induced activation of nuclear factor of activated T cells (NFAT)-dependent transcription. Unlike Nef, which stimulated calcium signaling to activate NFAT, Vpr functioned farther downstream. Similar to the positive effects of Vpr on most of the transcriptional test systems that we used, potentiation of NFAT-directed gene expression was relatively modest in magnitude (two- to threefold) and depended on the cell cycle-arresting capacity of Vpr. By contrast, we found that Vpr could cause more than fivefold upregulation of cyclic AMP response element (CRE)-directed transcription via a mechanism that did not require Vpr-induced G2/M arrest. This effect, however, was only evident under suboptimal conditions known to lead to serine phosphorylation of the CRE binding factor (CREB) but not to CREB-dependent gene expression. This suggested that Vpr may act by stabilizing interactions with CREB and its transcriptional cofactor CREB binding protein (CBP). Indeed, this effect could be blocked by cotransfection of the adenoviral CBP inhibitor E1A. These results provide additional evidence for cell cycle-independent regulation of gene expression by Vpr and implicate CREB as a potentially important target for Vpr action in HIV-infected host cells.

Subcellular localization of feline immunodeficiency virus integrase and mapping of its karyophilic determinant

Feline immunodeficiency virus (FIV), like other members of the lentivirus subfamily, such as human immunodeficiency virus type 1 (HIV-1), can infect nondividing and terminally differentiated cells. The transport of the preintegration complex into the nucleus is cell cycle-independent, but the mechanism is not well understood. Integrase is a key component of the complex and has been suggested to play a role in nuclear import during HIV-1 replication. To determine its karyophilic property, FIV integrase fused with glutathione S-transferase and enhanced green fluorescent protein was expressed in various feline and human cells and the subcellular localization was visualized by fluorescence microscopy. Wild-type FIV integrase was karyophilic in all cell lines tested and capable of targeting the fusion protein to the nuclei of transfected cells. Analysis of deletion and point mutation variants of FIV integrase failed to reveal any canonical nuclear localization signal, and the karyophilic determinant was mapped to the highly conserved N-terminal zinc-binding HHCC motif. A region near the C-terminal domain enriched with basic amino acid residues also affected the nuclear import of integrase. However, the role of this region is only modulatory in comparison to that of the zinc-binding domain. The N-terminal zinc-binding domain does not bind DNA and instead is essential in integrase multimerization. We therefore postulate that the karyophilic property of FIV integrase requires subunit multimerization promoted by the HHCC motif. Alternatively, the HHCC motif may directly promote interaction between FIV integrase and cellular proteins involved in nuclear import.

NMR structure of the HIV-1 regulatory protein VPR

The human immunodeficiency virus type 1 (HIV-1) genome encodes a highly conserved regulatory gene product, Vpr (96 residues, 14kDa), which is incorporated into virions. In the infected cells, Vpr, expressed late in the virus cycle, is believed to function in the early phases of HIV-1 replication, such as nuclear migration of pre-integration complex, transcription of the proviral genome, viral multiplication by blocking cells in G2 phase and regulation of apoptosis phenomenon. Vpr has a critical role in long term AIDS disease by inducing infection in non-dividing cells such as monocytes and macrophages. To gain insight into the structure-function relationships of Vpr, the (1-96)Vpr protein was synthesized with 22 labeled amino acids. Its 3D structure was analyzed in the presence of CD(3)CN and in pure water at low pH and refined by restrained simulated annealing. The structure of the protein is characterized by three well-defined alpha-helices: 17-33, 38-50 and 56-77 surrounded by flexible N and C-terminal domains. In contrast to the structure obtained in the presence of TFE, the three alpha-helices are folded around a hydrophobic core constituted of Leu, Ile, Val and aromatic residues as illustrated by numerous long range NOEs. This structure accounts for the interaction of Vpr with different targets.

Nuclear import of viral DNA genomes

The genomes of many viruses traffic into the nucleus, where they are either integrated into host chromosomes or maintained as episomal DNA and then transcriptionally activated or silenced. Here, we discuss the existing evidence on how the lentiviruses, adenoviruses, herpesviruses, hepadnaviruses and autonomous parvoviruses enter the nucleus. Depending on the size of the capsid enclosing the genome, three principles of viral nucleic acids import are discussed. The first principle is that the capsid disassembles in the cytosol or in a docked state at the nuclear pore complex and a subviral genomic complex is trafficked through the pore. Second, the genome is injected from a capsid that is docked to the pore complex, and third, import factors are recruited to cytosolic capsids to increase capsid affinity to the pore complex, mediate translocation and allow disassembly in the nucleoplasm.