Mapping of nucleocapsid residues important for HIV-1 genomic RNA dimerization and packaging

HIV-1 RNA genome packaging: it's G-rated

A member of the Retroviridae, human immunodeficiency virus type 1 (HIV-1), uses the RNA genome packaged into nascent virions to transfer genetic information to its progeny. The genome packaging step is a highly regulated and extremely efficient process as a vast majority of virus particles contain two copies of full-length unspliced HIV-1 RNA that form a dimer. Thus, during virus assembly HIV-1 can identify and selectively encapsidate HIV-1 unspliced RNA from an abundant pool of cellular RNAs and various spliced HIV-1 RNAs. Several "G" features facilitate the packaging of a dimeric RNA genome. The viral polyprotein Gag orchestrates virus assembly and mediates RNA genome packaging. During this process, Gag preferentially binds unpaired guanosines within the highly structured 5' untranslated region (UTR) of HIV-1 RNA. In addition, the HIV-1 unspliced RNA provides a scaffold that promotes Gag:Gag interactions and virus assembly, thereby ensuring its packaging. Intriguingly, recent studies have shown that the use of different guanosines at the junction of U3 and R as transcription start sites results in HIV-1 unspliced RNA species with 99.9% identical sequences but dramatically distinct 5' UTR conformations. Consequently, one species of unspliced RNA is preferentially packaged over other nearly identical RNAs. These studies reveal how conformations affect the functions of HIV-1 RNA elements and the complex regulation of HIV-1 replication. In this review, we summarize cis- and trans-acting elements critical for HIV-1 RNA packaging, locations of Gag:RNA interactions that mediate genome encapsidation, and the effects of transcription start sites on the structure and packaging of HIV-1 RNA.

The human cellular protein NoL12 is a specific partner of the HIV-1 nucleocapsid protein NCp7

The human immunodeficiency virus-1 (HIV-1) nucleocapsid protein (NCp7) is a nucleic acid chaperone protein with two highly conserved zinc fingers. To exert its key roles in the viral cycle, NCp7 interacts with several host proteins. Among them, the human NoL12 protein (hNoL12) was previously identified in genome wide screens as a potential partner of NCp7. hNoL12 is a highly conserved 25 kDa nucleolar RNA-binding protein implicated in the 5'end processing of ribosomal RNA in the nucleolus and thus in the assembly and maturation of ribosomes. In this work, we confirmed the NCp7/hNoL12 interaction in cells by Förster resonance energy transfer visualized by Fluorescence Lifetime Imaging Microscopy and co-immunoprecipitation. The interaction between NCp7 and hNoL12 was found to strongly depend on their both binding to RNA, as shown by the loss of interaction when the cell lysates were pretreated with RNase. Deletion mutants of hNoL12 were tested for their co-immunoprecipitation with NCp7, leading to the identification of the exonuclease domain of hNoL12 as the binding domain for NCp7. Finally, the interaction with hNoL12 was found to be specific of the mature NCp7 and to require NCp7 basic residues. IMPORTANCE HIV-1 mature nucleocapsid (NCp7) results from the maturation of the Gag precursor in the viral particle and is thus mostly abundant in the first phase of the infection which ends with the genomic viral DNA integration in the cell genome. Most if not all the nucleocapsid partners identified so far are not specific of the mature form. We described here the specific interaction in the nucleolus between NCp7 and the human nucleolar protein 12, a protein implicated in ribosomal RNA maturation and DNA damage response. This interaction takes place in the cell nucleolus, a subcellular compartment where NCp7 accumulates. The absence of binding between hNoL12 and Gag makes hNoL12 one of the few known specific cellular partners of NCp7.

Influence of HIV-1 Genomic RNA on the Formation of Gag Biomolecular Condensates

Biomolecular condensates (BMCs) play an important role in the replication of a growing number of viruses, but many important mechanistic details remain to be elucidated. Previously, we demonstrated that the pan-retroviral nucleocapsid (NC) and HIV-1 pr55Gag (Gag) proteins phase separate into condensates, and that HIV-1 protease (PR)-mediated maturation of Gag and Gag-Pol precursor proteins yields self-assembling BMCs that have HIV-1 core architecture. Using biochemical and imaging techniques, we aimed to further characterize the phase separation of HIV-1 Gag by determining which of its intrinsically disordered regions (IDRs) influence the formation of BMCs, and how the HIV-1 viral genomic RNA (gRNA) could influence BMC abundance and size. We found that mutations in the Gag matrix (MA) domain or the NC zinc finger motifs altered condensate number and size in a salt-dependent manner. Gag BMCs were also bimodally influenced by the gRNA, with a condensate-promoting regime at lower protein concentrations and a gel dissolution at higher protein concentrations. Interestingly, incubation of Gag with CD4+ T cell nuclear lysates led to the formation of larger BMCs compared to much smaller ones observed in the presence of cytoplasmic lysates. These findings suggest that the composition and properties of Gag-containing BMCs may be altered by differential association of host factors in nuclear and cytosolic compartments during virus assembly. This study significantly advances our understanding of HIV-1 Gag BMC formation and provides a foundation for future therapeutic targeting of virion assembly.

Influence of HIV-1 genomic RNA on the formation of Gag biomolecular condensates

Biomolecular condensates (BMCs) play an important role in the replication of a growing number of viruses, but many important mechanistic details remain to be elucidated. Previously, we demonstrated that pan-retroviral nucleocapsid (NC) and the HIV-1 pr55 ^Gag (Gag) proteins phase separate into condensates, and that HIV-1 protease (PR)-mediated maturation of Gag and Gag-Pol precursor proteins yield self-assembling BMCs having HIV-1 core architecture. Using biochemical and imaging techniques, we aimed to further characterize the phase separation of HIV-1 Gag by determining which of its intrinsically disordered regions (IDRs) influence the formation of BMCs and how the HIV-1 viral genomic RNA (gRNA) could influence BMC abundance and size. We found that mutations in the Gag matrix (MA) domain or the NC zinc finger motifs altered condensate number and size in a salt-dependent manner. Gag BMCs were also bimodally influenced by the gRNA, with a condensate-promoting regime at lower protein concentrations and a gel dissolution at higher protein concentrations. Interestingly, incubation of Gag with CD4 ⁺ T cell nuclear lysates led to the formation of larger BMCs as compared to much smaller ones observed in the presence of cytoplasmic lysates. These findings suggests that the composition and properties of Gag-containing BMCs may be altered by differential association of host factors in nuclear and cytosolic compartments during virus assembly. This study significantly advances our understanding of HIV-1 Gag BMC formation and provides a foundation for future therapeutic targeting of virion assembly.

Human Immunodeficiency Virus Type 2 Capsid Protein Mutagenesis Reveals Amino Acid Residues Important for Virus Particle Assembly

Human immunodeficiency virus (HIV) Gag drives virus particle assembly. The capsid (CA) domain is critical for Gag multimerization mediated by protein-protein interactions. The Gag protein interaction network defines critical aspects of the retroviral lifecycle at steps such as particle assembly and maturation. Previous studies have demonstrated that the immature particle morphology of HIV-2 is intriguingly distinct relative to that of HIV-1. Based upon this observation, we sought to determine the amino acid residues important for virus assembly that might help explain the differences between HIV-1 and HIV-2. To do this, we conducted site-directed mutagenesis of targeted locations in the HIV-2 CA domain of Gag and analyzed various aspects of virus particle assembly. A panel of 31 site-directed mutants of residues that reside at the HIV-2 CA inter-hexamer interface, intra-hexamer interface and CA inter-domain linker were created and analyzed for their effects on the efficiency of particle production, particle morphology, particle infectivity, Gag subcellular distribution and in vitro protein assembly. Seven conserved residues between HIV-1 and HIV-2 (L19, A41, I152, K153, K157, N194, D196) and two non-conserved residues (G38, N127) were found to significantly impact Gag multimerization and particle assembly. Taken together, these observations complement structural analyses of immature HIV-2 particle morphology and Gag lattice organization as well as provide important comparative insights into the key amino acid residues that can help explain the observed differences between HIV immature particle morphology and its association with virus replication and particle infectivity.

The Zinc Content of HIV-1 NCp7 Affects Its Selectivity for Packaging Signal and Affinity for Stem-Loop 3

The nucleocapsid (NC) protein of human immunodeficiency (HIV) is a small, highly basic protein containing two CCHC zinc-finger motifs, which is cleaved from the NC domain of the Gag polyprotein during virus maturation. We previously reported that recombinant HIV-1 Gag and NCp7 overexpressed in an E. coli host contains two and one zinc ions, respectively, and Gag exhibited much higher selectivity for packaging signal (Psi) and affinity for the stem-loop (SL)-3 of Psi than NCp7. In this study, we prepared NCp7 containing 0 (⁰NCp7), 1 (NCp7) or 2 (²NCp7) zinc ions, and compared their secondary structure, Psi-selectivity and SL3-affinity. Along with the decrease of the zinc content, less ordered conformations were detected. Compared to NCp7, ²NCp7 exhibited a much higher Psi-selectivity and SL3-affinity, similar to Gag, whereas ⁰NCp7 exhibited a lower Psi-selectivity and SL3-affinity, similar to the H23&H44K double mutant of NCp7, indicating that the different RNA-binding property of Gag NC domain and the mature NCp7 may be resulted, at least partially, from their different zinc content. This study will be helpful to elucidate the critical roles that zinc played in the viral life cycle, and benefit further investigations of the functional switch from the NC domain of Gag to the mature NCp7.

Specificity of the HIV-1 Protease on Substrates Representing the Cleavage Site in the Proximal Zinc-Finger of HIV-1 Nucleocapsid Protein

To explore the sequence context-dependent nature of the human immunodeficiency virus type 1 (HIV-1) protease’s specificity and to provide a rationale for viral mutagenesis to study the potential role of the nucleocapsid (NC) processing in HIV-1 replication, synthetic oligopeptide substrates representing the wild-type and modified versions of the proximal cleavage site of HIV-1 NC were assayed as substrates of the HIV-1 protease (PR). The S1′ substrate binding site of HIV-1 PR was studied by an in vitro assay using KIVKCF↓NCGK decapeptides having amino acid substitutions of N17 residue of the cleavage site of the first zinc-finger domain, and in silico calculations were also performed to investigate amino acid preferences of S1′ site. Second site substitutions have also been designed to produce “revertant” substrates and convert a non-hydrolysable sequence (having glycine in place of N17) to a substrate. The specificity constants obtained for peptides containing non-charged P1′ substitutions correlated well with the residue volume, while the correlation with the calculated interaction energies showed the importance of hydrophobicity: interaction energies with polar residues were related to substantially lower specificity constants. Cleavable “revertants” showed one residue shift of cleavage position due to an alternative productive binding mode, and surprisingly, a double cleavage of a substrate was also observed. The results revealed the importance of alternative binding possibilities of substrates into the HIV-1 PR. The introduction of the “revertant” mutations into infectious virus clones may provide further insights into the potential role of NC processing in the early phase of the viral life-cycle.

Overview of the Nucleic-Acid Binding Properties of the HIV-1 Nucleocapsid Protein in Its Different Maturation States

HIV-1 Gag polyprotein orchestrates the assembly of viral particles. Its C-terminus consists of the nucleocapsid (NC) domain that interacts with nucleic acids, and p1 and p6, two unstructured regions, p6 containing the motifs to bind ALIX, the cellular ESCRT factor TSG101 and the viral protein Vpr. The processing of Gag by the viral protease subsequently liberates NCp15 (NC-p1-p6), NCp9 (NC-p1) and NCp7, NCp7 displaying the optimal chaperone activity of nucleic acids. This review focuses on the nucleic acid binding properties of the NC domain in the different maturation states during the HIV-1 viral cycle.

Comparison of HIV-1 Gag and NCp7 in their selectivity for package signal, affinity for stem-loop 3, and Zn2+ content

The human immunodeficiency virus type 1 (HIV-1) Gag recognizes viral packaging signal (Psi) specifically via its nucleocapsid (NC) domain, resulting in the encapsidation of two copies of genomic RNA (gRNA) into the viral particle. The NCp7, which is cleaved from Gag during viral maturation, is a nucleic acid chaperone, coating and protecting the gRNA. In this study, an RT-qPCR-based approach was developed to quantitatively compare the Psi-selectivity of Gag and NCp7 in the presence of bacterial or 293T total RNAs. The binding affinity of Gag and NCp7 to the stem-loop (SL) 3 of Psi was also compared using surface plasmon resonance. We found that Gag selected more Psi-RNA than NCp7 from both E. coli BL21 (DE3) and in vitro binding reactions, and Gag bound to SL3-RNA with a higher affinity than NCp7. Moreover, Gag contained two Zn²⁺ whereas NCp7 contained one. The N-terminal zinc-finger motif of NCp7 lost most of its Zn²⁺-binding activity. Deletion of N-terminal amino acids 1-11 of NCp7 resulted in increased Psi-selectivity, SL3-affinity and Zn²⁺ content. These results indicated that Zn²⁺ coordination of Gag is critical for Psi-binding and selection. Removal of Zn²⁺ from the first zinc-finger motif during or after Gag cleavage to generate mature NCp7 might serve as a switch to regulate the functions of Gag NC domain and mature NCp7. Our study will be helpful to elucidate the important roles that Zn²⁺ plays in the viral life cycle, and may benefit further investigations of the function of HIV-1 Gag and NCp7.

Investigation of HIV-1 Gag binding with RNAs and lipids using Atomic Force Microscopy

Atomic Force Microscopy was utilized to study the morphology of Gag, ΨRNA, and their binding complexes with lipids in a solution environment with 0.1Å vertical and 1nm lateral resolution. TARpolyA RNA was used as a RNA control. The lipid used was phospha-tidylinositol-(4,5)-bisphosphate (PI(4,5)P2). The morphology of specific complexes Gag-ΨRNA, Gag-TARpolyA RNA, Gag-PI(4,5)P2 and PI(4,5)P2-ΨRNA-Gag were studied. They were imaged on either positively or negatively charged mica substrates depending on the net charges carried. Gag and its complexes consist of monomers, dimers and tetramers, which was confirmed by gel electrophoresis. The addition of specific ΨRNA to Gag is found to increase Gag multimerization. Non-specific TARpolyA RNA was found not to lead to an increase in Gag multimerization. The addition PI(4,5)P2 to Gag increases Gag multimerization, but to a lesser extent than ΨRNA. When both ΨRNA and PI(4,5)P2 are present Gag undergoes comformational changes and an even higher degree of multimerization.

ReAsH/tetracystein-based correlative light-electron microscopy for HIV-1 imaging during the early stages of infection

Visualization of viruses in the host cell during the course of infection by correlative light-electron microscopy (CLEM) requires a specific labelling of the viral structures in order to recognize the nanometric viral cores in the intracellular environment. For Human immunodeficiency virus type 1 (HIV-1), the labelling approaches developed for fluorescence microscopy are generally not suited for transmission electron microscopy (TEM), so that imaging of HIV-1 particles in infected cells by CLEM is not straightforward. Herein, we adapt the labeling approach with a tetracystein tag (TC) and a biarsenical resorufin-based label (ReAsH) for monitoring the HIV-1 particles during the early stages of HIV-1 infection by CLEM. In this approach, the ReAsH fluorophore triggers the photo-conversion of 3,3-diaminobenzidine tetrahydrochloride (DAB), generating a precipitate sensitive to osmium tetroxide staining that can be visualized by transmission electron microscopy. The TC tag is fused to the nucleocapsid protein NCp7, a nucleic acid chaperone that binds to the viral genome. HeLa cells, infected by ReAsH-labeled pseudoviruses containg NCp7-TC proteins exhibit strong fluorescent cytoplasmic spots that overlap with dark precipitates in the TEM sections. The DAB precipitates corresponding to single viral cores are observed all over the cytoplasm, and notably near microtubules and nuclear pores. This work describes for the first time a specific contrast given by HIV-1 viral proteins in TEM images and opens new perspectives for the use of CLEM to monitor the intracellular traffic of viral complexes.

Conserved cysteines in Mason-Pfizer monkey virus capsid protein are essential for infectious mature particle formation

Retrovirus assembly is driven mostly by Gag polyprotein oligomerization, which is mediated by inter and intra protein-protein interactions among its capsid (CA) domains. Mason-Pfizer monkey virus (M-PMV) CA contains three cysteines (C82, C193 and C213), where the latter two are highly conserved among most retroviruses. To determine the importance of these cysteines, we introduced mutations of these residues in both bacterial and proviral vectors and studied their impact on the M-PMV life cycle. These studies revealed that the presence of both conserved cysteines of M-PMV CA is necessary for both proper assembly and virus infectivity. Our findings suggest a crucial role of these cysteines in the formation of infectious mature particles.

Disparate Contributions of Human Retrovirus Capsid Subdomains to Gag-Gag Oligomerization, Virus Morphology, and Particle Biogenesis

The capsid domain (CA) of the retroviral Gag protein is a primary determinant of Gag oligomerization, which is a critical step for immature Gag lattice formation and virus particle budding. Although the human immunodeficiency virus type 1 (HIV-1) CA carboxy-terminal domain (CTD) is essential for CA-CA interactions, the CA CTD has been suggested to be largely dispensable for human T-cell leukemia virus type 1 (HTLV-1) particle biogenesis. To more clearly define the roles of the HTLV-1 CA amino-terminal domain (NTD) and CA CTD in particle biogenesis, we generated and analyzed a panel of Gag proteins with chimeric HIV-1/HTLV-1 CA domains. Subcellular distribution and protein expression levels indicated that Gag proteins with a chimeric HIV-1 CA NTD/HTLV-1 CA CTD did not result in Gag oligomerization regardless of the parent Gag background. Furthermore, chimeric Gag proteins with the HTLV-1 CA NTD produced particles phenotypically similar to HTLV-1 immature particles, highlighting the importance of the HTLV-1 CA NTD in HTLV-1 immature particle morphology. Taken together, these observations support the conclusion that the HTLV-1 CA NTD can functionally replace the HIV-1 CA CTD, but the HIV-1 CA NTD cannot replace the HTLV-1 CA CTD, indicating that the HTLV-1 CA subdomains provide distinct contributions to Gag-Gag oligomerization, particle morphology, and biogenesis. Furthermore, we have shown for the first time that HIV-1 and HTLV-1 Gag domains outside the CA (e.g., matrix and nucleocapsid) impact Gag oligomerization as well as immature particle size and morphology.IMPORTANCE A key aspect in virus replication is virus particle assembly, which is a poorly understood process for most viruses. For retroviruses, the Gag structural protein is the primary driver of virus particle biogenesis, and the CA CTD is the primary determinant of Gag-Gag interactions for HIV-1. In this study, the HTLV-1 capsid amino-terminal domain was found to provide distinct contributions to Gag-Gag oligomerization, particle morphology, and biogenesis. This study provides information that will aid efforts for discovery of therapeutic targets for intervention.

HIV-1 Pr55Gag binds genomic and spliced RNAs with different affinity and stoichiometry

The HIV-1 Pr55^Gag precursor specifically selects genomic RNA (gRNA) from a large variety of cellular and spliced viral RNAs (svRNAs), however the molecular mechanisms of this selective recognition remains poorly understood. To gain better understanding of this process, we analyzed the interactions between Pr55^Gag and a large panel of viral RNA (vRNA) fragments encompassing the main packaging signal (Psi) and its flanking regions by fluorescence spectroscopy. We showed that the gRNA harbors a high affinity binding site which is absent from svRNA species, suggesting that this site might be crucial for selecting the HIV-1 genome. Our stoichiometry analysis of protein/RNA complexes revealed that few copies of Pr55^Gag specifically associate with the 5' region of the gRNA. Besides, we found that gRNA dimerization significantly impacts Pr55^Gag binding, and we confirmed that the internal loop of stem-loop 1 (SL1) in Psi is crucial for specific interaction with Pr55^Gag. Our analysis of gRNA fragments of different length supports the existence of a long-range tertiary interaction involving sequences upstream and downstream of the Psi region. This long-range interaction might promote optimal exposure of SL1 for efficient Pr55^Gag recognition. Altogether, our results shed light on the molecular mechanisms allowing the specific selection of gRNA by Pr55^Gag among a variety of svRNAs, all harboring SL1 in their first common exon.

Imaging HIV-1 RNA dimerization in cells by multicolor super-resolution and fluctuation microscopies

Dimerization is a unique and vital characteristic of retroviral genomes. It is commonly accepted that genomic RNA (gRNA) must be dimeric at the plasma membrane of the infected cells to be packaged during virus assembly. However, where, when and how HIV-1 gRNA find each other and dimerize in the cell are long-standing questions that cannot be answered using conventional approaches. Here, we combine two state-of-the-art, multicolor RNA labeling strategies with two single-molecule microscopy technologies to address these questions. We used 3D-super-resolution structured illumination microscopy to analyze and quantify the spatial gRNA association throughout the cell and monitored the dynamics of RNA-RNA complexes in living-cells by cross-correlation fluctuation analysis. These sensitive and complementary approaches, combined with trans-complementation experiments, reveal for the first time the presence of interacting gRNA in the cytosol, a challenging observation due to the low frequency of these events and their dilution among the bulk of other RNAs, and allow the determination of the subcellular orchestration of the HIV-1 dimerization process.

Requirements for nucleocapsid-mediated regulation of reverse transcription during the late steps of HIV-1 assembly

HIV-1 is a retrovirus replicating within cells by reverse transcribing its genomic RNA (gRNA) into DNA. Within cells, virus assembly requires the structural Gag proteins with few accessory proteins, notably the viral infectivity factor (Vif) and two copies of gRNA as well as cellular factors to converge to the plasma membrane. In this process, the nucleocapsid (NC) domain of Gag binds to the packaging signal of gRNA which consists of a series of stem-loops (SL1-SL3) ensuring gRNA selection and packaging into virions. Interestingly, mutating NC activates a late-occurring reverse transcription (RT) step in producer cells, leading to the release of DNA-containing HIV-1 particles. In order to decipher the molecular mechanism regulating this late RT, we explored the role of several key partners of NC, such as Vif, gRNA and the cellular cytidine deaminase APOBEC3G that restricts HIV-1 infection by targeting the RT. By studying combinations of deletions of these putative players, we revealed that NC, SL1-SL3 and in lesser extent Vif, but not APOBEC3G, interplay regulates the late RT.

HIV-1 nucleocapsid protein localizes efficiently to the nucleus and nucleolus

The HIV-1 nucleocapsid (NC) is an essential viral protein containing two highly conserved retroviral-type zinc finger (ZF) motifs, which functions in multiple stages of the HIV-1 life cycle. Although a number of functions for NC either in its mature form or as a domain of Gag have been revealed, little is known about the intracellular localization of NC and, moreover, its role in Gag protein trafficking. Here, we have investigated various forms of HIV-1 NC protein for its cellular localization and found that the NC has a strong nuclear and nucleolar localization activity. The linker region, composed of a stretch of basic amino acids between the two ZF motifs, was necessary and sufficient for the activity.

Investigating the cellular distribution and interactions of HIV-1 nucleocapsid protein by quantitative fluorescence microscopy

The nucleocapsid protein (NCp7) of the Human immunodeficiency virus type 1 (HIV-1) is a small basic protein containing two zinc fingers. About 2000 NCp7 molecules coat the genomic RNA in the HIV-1 virion. After infection of a target cell, the viral core enters into the cytoplasm, where NCp7 chaperones the reverse transcription of the genomic RNA into the proviral DNA. As a consequence of their much lower affinity for double-stranded DNA as compared to single-stranded RNAs, NCp7 molecules are thought to be released in the cytoplasm and the nucleus of infected cells in the late steps of reverse transcription. Yet, little is known on the cellular distribution of the released NCp7 molecules and on their possible interactions with cell components. Hence, the aim of this study was to identify potential cellular partners of NCp7 and to monitor its intracellular distribution and dynamics by means of confocal fluorescence microscopy, fluorescence lifetime imaging microscopy, fluorescence recovery after photobleaching, fluorescence correlation and cross-correlation spectroscopy, and raster imaging correlation spectroscopy. HeLa cells transfected with eGFP-labeled NCp7 were used as a model system. We found that NCp7-eGFP localizes mainly in the cytoplasm and the nucleoli, where it binds to cellular RNAs, and notably to ribosomal RNAs which are the most abundant. The binding of NCp7 to ribosomes was further substantiated by the intracellular co-diffusion of NCp7 with the ribosomal protein 26, a component of the large ribosomal subunit. Finally, gradient centrifugation experiments demonstrate a direct association of NCp7 with purified 80S ribosomes. Thus, our data suggest that NCp7 molecules released in newly infected cells may primarily bind to ribosomes, where they may exert a new potential role in HIV-1 infection.

Single aromatic residue location alters nucleic acid binding and chaperone function of FIV nucleocapsid protein

Feline immunodeficiency virus (FIV) is a retrovirus that infects domestic cats, and is an excellent animal model for human immunodeficiency virus type 1 (HIV-1) pathogenesis. The nucleocapsid (NC) protein is critical for replication in both retroviruses. FIV NC has several structural features that differ from HIV-1 NC. While both NC proteins have a single conserved aromatic residue in each of the two zinc fingers, the aromatic residue on the second finger of FIV NC is located on the opposite C-terminal side relative to its location in HIV-1 NC. In addition, whereas HIV-1 NC has a highly charged cationic N-terminal tail and a relatively short C-terminal extension, the opposite is true for FIV NC. To probe the impact of these differences on the nucleic acid (NA) binding and chaperone properties of FIV NC, we carried out ensemble and single-molecule assays with wild-type (WT) and mutant proteins. The ensemble studies show that FIV NC binding to DNA is strongly electrostatic, with a higher effective charge than that observed for HIV-1 NC. The C-terminal basic domain contributes significantly to the NA binding capability of FIV NC. In addition, the non-electrostatic component of DNA binding is much weaker for FIV NC than for HIV-1 NC. Mutation of both aromatic residues in the zinc fingers to Ala (F12A/W44A) further increases the effective charge of FIV NC and reduces its non-electrostatic binding affinity. Interestingly, switching the location of the C-terminal aromatic residue to mimic the HIV-1 NC sequence (N31W/W44A) reduces the effective charge of FIV NC and increases its non-electrostatic binding affinity to values similar to HIV-1 NC. Consistent with the results of these ensemble studies, single-molecule DNA stretching studies show that while WT FIV NC has reduced stacking capability relative to HIV-1 NC, the aromatic switch mutant recovers the ability to intercalate between the DNA bases. Our results demonstrate that altering the position of a single aromatic residue switches the binding mode of FIV NC from primarily electrostatic binding to more non-electrostatic binding, conferring upon it NA interaction properties comparable to that of HIV-1 NC.

The HIV-1 nucleocapsid protein recruits negatively charged lipids to ensure its optimal binding to lipid membranes

The HIV-1 Gag polyprotein precursor composed of the matrix (MA), capsid (CA), nucleocapsid (NC), and p6 domains orchestrates virus assembly via interactions between MA and the cell plasma membrane (PM) on one hand and NC and the genomic RNA on the other hand. As the Gag precursor can adopt a bent conformation, a potential interaction of the NC domain with the PM cannot be excluded during Gag assembly at the PM. To investigate the possible interaction of NC with lipid membranes in the absence of any interference from the other domains of Gag, we quantitatively characterized by fluorescence spectroscopy the binding of the mature NC protein to large unilamellar vesicles (LUVs) used as membrane models. We found that NC, either in its free form or bound to an oligonucleotide, was binding with high affinity (∼ 10(7) M(-1)) to negatively charged LUVs. The number of NC binding sites, but not the binding constant, was observed to decrease with the percentage of negatively charged lipids in the LUV composition, suggesting that NC and NC/oligonucleotide complexes were able to recruit negatively charged lipids to ensure optimal binding. However, in contrast to MA, NC did not exhibit a preference for phosphatidylinositol-(4,5)-bisphosphate. These results lead us to propose a modified Gag assembly model where the NC domain contributes to the initial binding of the bent form of Gag to the PM.

IMPORTANCE

The NC protein is a highly conserved nucleic acid binding protein that plays numerous key roles in HIV-1 replication. While accumulating evidence shows that NC either as a mature protein or as a domain of the Gag precursor also interacts with host proteins, only a few data are available on the possible interaction of NC with lipid membranes. Interestingly, during HIV-1 assembly, the Gag precursor is thought to adopt a bent conformation where the NC domain may interact with the plasma membrane. In this context, we quantitatively characterized the binding of NC, as a free protein or as a complex with nucleic acids, to lipid membranes and showed that the latter constitute a binding platform for NC. Taken together, our data suggest that the NC domain may play a role in the initial binding events of Gag to the plasma membrane during HIV-1 assembly.

Dynamics of linker residues modulate the nucleic acid binding properties of the HIV-1 nucleocapsid protein zinc fingers

The HIV-1 nucleocapsid protein (NC) is a small basic protein containing two zinc fingers (ZF) separated by a short linker. It is involved in several steps of the replication cycle and acts as a nucleic acid chaperone protein in facilitating nucleic acid strand transfers occurring during reverse transcription. Recent analysis of three-dimensional structures of NC-nucleic acids complexes established a new property: the unpaired guanines targeted by NC are more often inserted in the C-terminal zinc finger (ZF2) than in the N-terminal zinc finger (ZF1). Although previous NMR dynamic studies were performed with NC, the dynamic behavior of the linker residues connecting the two ZF domains remains unclear. This prompted us to investigate the dynamic behavior of the linker residues. Here, we collected ¹⁵N NMR relaxation data and used for the first time data at several fields to probe the protein dynamics. The analysis at two fields allows us to detect a slow motion occurring between the two domains around a hinge located in the linker at the G35 position. However, the amplitude of motion appears limited in our conditions. In addition, we showed that the neighboring linker residues R29, A30, P31, R32, K33 displayed restricted motion and numerous contacts with residues of ZF1. Our results are fully consistent with a model in which the ZF1-linker contacts prevent the ZF1 domain to interact with unpaired guanines, whereas the ZF2 domain is more accessible and competent to interact with unpaired guanines. In contrast, ZF1 with its large hydrophobic plateau is able to destabilize the double-stranded regions adjacent to the guanines bound by ZF2. The linker residues and the internal dynamics of NC regulate therefore the different functions of the two zinc fingers that are required for an optimal chaperone activity.

Human APOBEC3F incorporation into human immunodeficiency virus type 1 particles

APOBEC3 proteins are a family of cytidine deaminases that exhibit broad antiretroviral activity. Among APOBEC3 proteins, APOBEC3G (hA3G) and APOBEC3F (hA3F) exhibit the most potent anti-HIV-1 activities. Although the incorporation of hA3F into virions is a prerequisite for exerting its antiviral function, the detail mechanism underlying remains incompletely understood. In this work, we present data showing that the nucleocapsid (NC) domain of HIV-1 Gag and a linker sequence between the two cytidine deaminase domains within hA3F, i.e., 104-156 amino acids, are required for viral packaging of hA3F. A detailed mapping study reveals that the cluster of basic residues surrounding the N-terminal zinc finger (ZF) and the linker region between the ZFs of HIV-1 NC play an important role in A3F incorporation, in addition, at least one of two ZFs is required. A hA3F fragment is able to compete with both hA3G and hA3F for viral incorporation, suggesting a common mechanism underlying virion encapsidation of hA3G and hA3F. Taken together, these results shed a light on the detail mechanism underlying viral incorporation of hA3F.

Distinct nucleic acid interaction properties of HIV-1 nucleocapsid protein precursor NCp15 explain reduced viral infectivity

During human immunodeficiency virus type 1 (HIV-1) maturation, three different forms of nucleocapsid (NC) protein—NCp15 (p9 + p6), NCp9 (p7 + SP2) and NCp7—appear successively. A mutant virus expressing NCp15 shows greatly reduced infectivity. Mature NCp7 is a chaperone protein that facilitates remodeling of nucleic acids (NAs) during reverse transcription. To understand the strict requirement for NCp15 processing, we compared the chaperone function of the three forms of NC. NCp15 anneals tRNA to the primer-binding site at a similar rate as NCp7, whereas NCp9 is the most efficient annealing protein. Assays to measure NA destabilization show a similar trend. Dynamic light scattering studies reveal that NCp15 forms much smaller aggregates relative to those formed by NCp7 and NCp9. Nuclear magnetic resonance studies suggest that the acidic p6 domain of HIV-1 NCp15 folds back and interacts with the basic zinc fingers. Neutralizing the acidic residues in p6 improves the annealing and aggregation activity of NCp15 to the level of NCp9 and increases the protein–NA aggregate size. Slower NCp15 dissociation kinetics is observed by single-molecule DNA stretching, consistent with the formation of electrostatic inter-protein contacts, which likely contribute to the distinct aggregate morphology, irregular HIV-1 core formation and non-infectious virus.

Retrovirus-specific differences in matrix and nucleocapsid protein-nucleic acid interactions: implications for genomic RNA packaging

Retroviral RNA encapsidation involves a recognition event between genomic RNA (gRNA) and one or more domains in Gag. In HIV-1, the nucleocapsid (NC) domain is involved in gRNA packaging and displays robust nucleic acid (NA) binding and chaperone functions. In comparison, NC of human T-cell leukemia virus type 1 (HTLV-1), a deltaretrovirus, displays weaker NA binding and chaperone activity. Mutation of conserved charged residues in the deltaretrovirus bovine leukemia virus (BLV) matrix (MA) and NC domains affects virus replication and gRNA packaging efficiency. Based on these observations, we hypothesized that the MA domain may generally contribute to NA binding and genome encapsidation in deltaretroviruses. Here, we examined the interaction between HTLV-2 and HIV-1 MA proteins and various NAs in vitro. HTLV-2 MA displays higher NA binding affinity and better chaperone activity than HIV-1 MA. HTLV-2 MA also binds NAs with higher affinity than HTLV-2 NC and displays more robust chaperone function. Mutation of two basic residues in HTLV-2 MA α-helix II, previously implicated in BLV gRNA packaging, reduces NA binding affinity. HTLV-2 MA binds with high affinity and specificity to RNA derived from the putative packaging signal of HTLV-2 relative to nonspecific NA. Furthermore, an HIV-1 MA triple mutant designed to mimic the basic character of HTLV-2 MA α-helix II dramatically improves binding affinity and chaperone activity of HIV-1 MA in vitro and restores RNA packaging to a ΔNC HIV-1 variant in cell-based assays. Taken together, these results are consistent with a role for deltaretrovirus MA proteins in viral RNA packaging.

Distinct binding interactions of HIV-1 Gag to Psi and non-Psi RNAs: implications for viral genomic RNA packaging

Despite the vast excess of cellular RNAs, precisely two copies of viral genomic RNA (gRNA) are selectively packaged into new human immunodeficiency type 1 (HIV-1) particles via specific interactions between the HIV-1 Gag and the gRNA psi (ψ) packaging signal. Gag consists of the matrix (MA), capsid, nucleocapsid (NC), and p6 domains. Binding of the Gag NC domain to ψ is necessary for gRNA packaging, but the mechanism by which Gag selectively interacts with ψ is unclear. Here, we investigate the binding of NC and Gag variants to an RNA derived from ψ (Psi RNA), as well as to a non-ψ region (TARPolyA). Binding was measured as a function of salt to obtain the effective charge (Zeff) and nonelectrostatic (i.e., specific) component of binding, Kd(1M). Gag binds to Psi RNA with a dramatically reduced Kd(1M) and lower Zeff relative to TARPolyA. NC, GagΔMA, and a dimerization mutant of Gag bind TARPolyA with reduced Zeff relative to WT Gag. Mutations involving the NC zinc finger motifs of Gag or changes to the G-rich NC-binding regions of Psi RNA significantly reduce the nonelectrostatic component of binding, leading to an increase in Zeff. These results show that Gag interacts with gRNA using different binding modes; both the NC and MA domains are bound to RNA in the case of TARPolyA, whereas binding to Psi RNA involves only the NC domain. Taken together, these results suggest a novel mechanism for selective gRNA encapsidation.

HIV-1 assembly, budding, and maturation

A defining property of retroviruses is their ability to assemble into particles that can leave producer cells and spread infection to susceptible cells and hosts. Virion morphogenesis can be divided into three stages: assembly, wherein the virion is created and essential components are packaged; budding, wherein the virion crosses the plasma membrane and obtains its lipid envelope; and maturation, wherein the virion changes structure and becomes infectious. All of these stages are coordinated by the Gag polyprotein and its proteolytic maturation products, which function as the major structural proteins of the virus. Here, we review our current understanding of the mechanisms of HIV-1 assembly, budding, and maturation, starting with a general overview and then providing detailed descriptions of each of the different stages of virion morphogenesis.

Roles played by capsid-dependent induction of membrane curvature and Gag-ESCRT interactions in tetherin recruitment to HIV-1 assembly sites

Tetherin/BST-2 (here called tetherin) is an antiviral protein that restricts release of diverse enveloped viruses from infected cells through physically tethering virus envelope and host plasma membrane. For HIV-1, specific recruitment of tetherin to assembly sites has been observed as its colocalization with the viral structural protein Gag or its accumulation in virus particles. Because of its broad range of targets, we hypothesized that tetherin is recruited through conserved features shared among various enveloped viruses, such as lipid raft association, membrane curvature, or ESCRT dependence. We observed that reduction of cellular cholesterol does not block tetherin anti-HIV-1 function, excluding an essential role for lipid rafts. In contrast, mutations in the capsid domain of Gag, which inhibit induction of membrane curvature, prevented tetherin-Gag colocalization detectable by confocal microscopy. Disruption of Gag-ESCRT interactions also inhibited tetherin-Gag colocalization when disruption was accomplished via amino acid substitutions in late domain motifs, expression of a dominant-negative Tsg101 derivative, or small interfering RNA (siRNA)-mediated depletion of Tsg101 or Alix. However, further analyses of these conditions by quantitative superresolution localization microscopy revealed that Gag-tetherin coclustering is significantly reduced but persists at intermediate levels. Notably, this residual tetherin recruitment was still sufficient for the full restriction of HIV-1 release. Unlike the late domain mutants, the capsid mutants defective in inducing membrane curvature showed little or no coclustering with tetherin in superresolution analyses. These results support a model in which both Gag-induced membrane curvature and Gag-ESCRT interactions promote tetherin recruitment, but the recruitment level achieved by the former is sufficient for full restriction.

Site-selective probing of cTAR destabilization highlights the necessary plasticity of the HIV-1 nucleocapsid protein to chaperone the first strand transfer

The HIV-1 nucleocapsid protein (NCp7) is a nucleic acid chaperone required during reverse transcription. During the first strand transfer, NCp7 is thought to destabilize cTAR, the (−)DNA copy of the TAR RNA hairpin, and subsequently direct the TAR/cTAR annealing through the zipping of their destabilized stem ends. To further characterize the destabilizing activity of NCp7, we locally probe the structure and dynamics of cTAR by steady-state and time resolved fluorescence spectroscopy. NC(11–55), a truncated NCp7 version corresponding to its zinc-finger domain, was found to bind all over the sequence and to preferentially destabilize the penultimate double-stranded segment in the lower part of the cTAR stem. This destabilization is achieved through zinc-finger–dependent binding of NC to the G₁₀ and G₅₀ residues. Sequence comparison further revealed that C•A mismatches close to the two G residues were critical for fine tuning the stability of the lower part of the cTAR stem and conferring to G₁₀ and G₅₀ the appropriate mobility and accessibility for specific recognition by NC. Our data also highlight the necessary plasticity of NCp7 to adapt to the sequence and structure variability of cTAR to chaperone its annealing with TAR through a specific pathway.

Aromatic residue mutations reveal direct correlation between HIV-1 nucleocapsid protein's nucleic acid chaperone activity and retroviral replication

The human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) protein plays an essential role in several stages of HIV-1 replication. One important function of HIV-1 NC is to act as a nucleic acid chaperone, in which the protein facilitates nucleic acid rearrangements important for reverse transcription and recombination. NC contains only 55 amino acids, with 15 basic residues and two zinc fingers, each having a single aromatic residue (Phe16 and Trp37). Despite its simple structure, HIV-1 NC appears to have optimal chaperone activity, including the ability to strongly aggregate nucleic acids, destabilize nucleic acid secondary structure, and facilitate rapid nucleic acid annealing. Here we combine single molecule DNA stretching experiments with ensemble solution studies of protein-nucleic acid binding affinity, oligonucleotide annealing, and nucleic acid aggregation to measure the characteristics of wild-type (WT) and aromatic residue mutants of HIV-1 NC that are important for nucleic acid chaperone activity. These in vitro results are compared to in vivo HIV-1 replication for viruses containing the same mutations. This work allows us to directly relate HIV-1 NC structure with its function as a nucleic acid chaperone in vitro and in vivo. We show that replacement of either aromatic residue with another aromatic residue results in a protein that strongly resembles WT NC. In contrast, single amino acid substitutions of either Phe16Ala or Trp37Ala significantly slow down NC's DNA interaction kinetics, while retaining some helix-destabilization capability. A double Phe16Ala/Trp37Ala substitution further reduces the latter activity. Surprisingly, the ensemble nucleic acid binding, annealing, and aggregation properties are not significantly altered for any mutant except the double aromatic substitution with Ala. Thus, elimination of a single aromatic residue from either zinc finger strongly reduces NC's chaperone activity as determined by single molecule DNA stretching experiments without significantly altering its ensemble-averaged biochemical properties. Importantly, the substitution of aromatic residues with Ala progressively decreases NC's nucleic acid chaperone activity while also progressively inhibiting viral replication. Taken together, these data support the critical role of HIV-1 NC's aromatic residues, and establish a direct and statistically significant correlation between nucleic acid chaperone activity and viral replication.

Molecular determinants of HIV-1 NCp7 chaperone activity in maturation of the HIV-1 dimerization initiation site

Human immunodeficiency virus genome dimerization is initiated through an RNA-RNA kissing interaction formed via the dimerization initiation site (DIS) loop sequence, which has been proposed to be converted to a more thermodynamically stable linkage by the viral p7 form of the nucleocapsid protein (NC). Here, we systematically probed the role of specific amino acids of NCp7 in its chaperone activity in the DIS conversion using 2-aminopurine (2-AP) fluorescence and nuclear magnetic resonance spectroscopy. Through comparative analysis of NCp7 mutants, the presence of positively charged residues in the N-terminus was found to be essential for both helix destabilization and strand transfer functions. It was also observed that the presence and type of the Zn finger is important for NCp7 chaperone activity, but not the order of the Zn fingers. Swapping single aromatic residues between Zn fingers had a significant effect on NCp7 activity; however, these mutants did not exhibit the same activity as mutants in which the order of the Zn fingers was changed, indicating a functional role for other flanking residues. RNA chaperone activity is further correlated with NCp7 structure and interaction with RNA through comparative analysis of nuclear magnetic resonance spectra of NCp7 variants, and complexes of these proteins with the DIS dimer.

Dominant role of the 5' TAR bulge in dimerization of HIV-1 genomic RNA, but no evidence of TAR-TAR kissing during in vivo virus assembly

The 5' untranslated region of HIV-1 genomic RNA (gRNA) contains two stem-loop structures that appear to be equally important for gRNA dimerization: the 57-nucleotide 5' TAR, at the very 5' end, and the 35-nucleotide SL1 (nucleotides 243-277). SL1 is well-known for containing the dimerization initiation site (DIS) in its apical loop. The DIS is a six-nucleotide palindrome. Here, we investigated the mechanism of TAR-directed gRNA dimerization. We found that the trinucleotide bulge (UCU24) of the 5' TAR has dominant impacts on both formation of HIV-1 RNA dimers and maturation of the formed dimers. The ΔUCU trinucleotide deletion strongly inhibited the first process and blocked the other, thus impairing gRNA dimerization as severely as deletion of the entire 5' TAR, and more severely than deletion of the DIS, inactivation of the viral protease, or most severe mutations in the nucleocapsid protein. The apical loop of TAR contains a 10-nucleotide palindrome that has been postulated to stimulate gRNA dimerization by a TAR-TAR kissing mechanism analogous to the one used by SL1 to stimulate dimerization. Using mutations that strongly destabilize formation of the TAR palindrome duplex, as well as compensatory mutations that restore duplex formation to a wild-type-like level, we found no evidence of TAR-TAR kissing, even though mutations nullifying the kissing potential of the TAR palindrome could impair dimerization by a mechanism other than hindering of SL1. However, nullifying the kissing potential of TAR had much less severe effects than ΔUCU. By not uncovering a dimerization mechanism intrinsic to TAR, our data suggest that TAR mutations exert their effect 3' of TAR, yet not on SL1, because TAR and SL1 mutations have synergistic effects on gRNA dimerization.

Role of HIV-1 RNA and protein determinants for the selective packaging of spliced and unspliced viral RNA and host U6 and 7SL RNA in virus particles

HIV-1 particles contain RNA species other than the unspliced viral RNA genome. For instance, viral spliced RNAs and host 7SL and U6 RNAs are natural components that are non-randomly incorporated. To understand the mechanism of packaging selectivity, we analyzed the content of a large panel of HIV-1 variants mutated either in the 5'UTR structures of the viral RNA or in the Gag-nucleocapsid protein (GagNC). In parallel, we determined whether the selection of host 7SL and U6 RNAs is dependent or not on viral RNA and/or GagNC. Our results reveal that the polyA hairpin in the 5'UTR is a major packaging determinant for both spliced and unspliced viral RNAs. In contrast, 5'UTR RNA structures have little influence on the U6 and 7SL RNAs, indicating that packaging of these host RNAs is independent of viral RNA packaging. Experiments with GagNC mutants indicated that the two zinc-fingers and N-terminal basic residues restrict the incorporation of the spliced RNAs, while favoring unspliced RNA packaging. GagNC through the zinc-finger motifs also restricts the packaging of 7SL and U6 RNAs. Thus, GagNC is a major contributor to the packaging selectivity. Altogether our results provide new molecular insight on how HIV selects distinct RNA species for incorporation into particles.

Diverse interactions of retroviral Gag proteins with RNAs

Retrovirus particles are constructed from a single virus-encoded protein, termed Gag. Given that assembly is an essential step in the viral replication cycle, it is a potential target for antiviral therapy. However, such an approach has not yet been exploited because of the lack of fundamental knowledge concerning the structures and interactions responsible for assembly. Assembling an infectious particle entails a remarkably diverse array of interactions, both specific and nonspecific, between Gag proteins and RNAs. These interactions are essential for the construction of the particle, for packaging of the viral RNA into the particle, and for placement of the primer for viral DNA synthesis. Recent results have provided some new insights into each of these interactions. In the case of HIV-1 Gag, it is clear that more than one domain of the protein contributes to Gag-RNA interaction.

Cell biology of retroviral RNA packaging

Generation of infectious retroviral particles rely on the targeting of all structural components to the correct cellular sites at the correct time. Gag, the main structural protein, orchestrates the assembly process and the mechanisms that trigger its targeting to assembly sites are well described. Gag is also responsible for the packaging of the viral genome and the molecular details of the Gag/RNA interaction are well characterized. Until recently, much less was understood about the cell biology of retrovirus RNA packaging. However, novel biochemical and live-cell microscopic approaches have identified where in the cell the initial events of genome recognition by Gag occur. These recent developments have shed light on the role played by the viral genome during virion assembly. Other central issues of the cell biology of RNA packaging, such as how the Gag-RNA complex traffics through the cytoplasm towards assembly sites, await characterization.

SHAPE analysis of the FIV Leader RNA reveals a structural switch potentially controlling viral packaging and genome dimerization

Feline immunodeficiency virus (FIV) infects many species of cat, and is related to HIV, causing a similar pathology. High-throughput selective 2′ hydroxyl acylation analysed by primer extension (SHAPE), a technique that allows structural interrogation at each nucleotide, was used to map the secondary structure of the FIV packaging signal RNA. Previous studies of this RNA showed four conserved stem–loops, extensive long-range interactions (LRIs) and a small, palindromic stem–loop (SL5) within the gag open reading frame (ORF) that may act as a dimerization initiation site (DIS), enabling the virus to package two copies of its genome. Our analyses of wild-type (wt) and mutant RNAs suggest that although the four conserved stem–loops are static structures, the 5′ and 3′ regions previously shown to form LRI also adopt an alternative, yet similarly conserved conformation, in which the putative DIS is occluded, and which may thus favour translational and splicing functions over encapsidation. SHAPE and in vitro dimerization assays were used to examine SL5 mutants. Dimerization contacts appear to be made between palindromic loop sequences in SL5. As this stem–loop is located within the gag ORF, recognition of a dimeric RNA provides a possible mechanism for the specific packaging of genomic over spliced viral RNAs.

RIG-I-mediated antiviral signaling is inhibited in HIV-1 infection by a protease-mediated sequestration of RIG-I

The rapid induction of type I interferon (IFN) is essential for establishing innate antiviral responses. During infection, cytoplasmic viral RNA is sensed by two DExD/H box RNA helicases, RIG-I and MDA5, ultimately driving IFN production. Here, we demonstrate that purified genomic RNA from HIV-1 induces a RIG-I-dependent type I IFN response. Both the dimeric and monomeric forms of HIV-1 were sensed by RIG-I, but not MDA5, with monomeric RNA, usually found in defective HIV-1 particles, acting as a better inducer of IFN than dimeric RNA. However, despite the presence of HIV-1 RNA in the de novo infection of monocyte-derived macrophages, HIV-1 replication did not lead to a substantial induction of IFN signaling. We demonstrate the existence of an evasion mechanism based on the inhibition of the RIG-I sensor through the action of the HIV-1 protease (PR). Indeed, the ectopic expression of PR resulted in the inhibition of IFN regulatory factor 3 (IRF-3) phosphorylation and decreased expression of IFN and interferon-stimulated genes. A downregulation of cytoplasmic RIG-I levels occurred in cells undergoing a single-cycle infection with wild-type provirus BH10 but not in cells transfected with a protease-deficient provirus, BH10-PR(-). Cellular fractionation and confocal microscopy studies revealed that RIG-I translocated from the cytosol to an insoluble fraction during the de novo HIV-1 infection of monocyte-derived macrophages, in the presence of PR. The loss of cytoplasmic RIG-I was prevented by the lysosomal inhibitor E64, suggesting that PR targets RIG-I to the lysosomes. This study reveals a novel PR-dependent mechanism employed by HIV-1 to counteract the early IFN response to viral RNA in infected cells.

Matrix domain modulates HIV-1 Gag's nucleic acid chaperone activity via inositol phosphate binding

Retroviruses replicate by reverse transcribing their single-stranded RNA genomes into double-stranded DNA using specific cellular tRNAs to prime cDNA synthesis. In HIV-1, human tRNA(3)(Lys) serves as the primer and is packaged into virions during assembly. The viral Gag protein is believed to chaperone tRNA(3)(Lys) placement onto the genomic RNA primer binding site; however, the timing and possible regulation of this event are currently unknown. Composed of the matrix (MA), capsid (CA), nucleocapsid (NC), and p6 domains, the multifunctional HIV-1 Gag polyprotein orchestrates the highly coordinated process of virion assembly, but the contribution of these domains to tRNA(3)(Lys) annealing is unclear. Here, we show that NC is absolutely essential for annealing and that the MA domain inhibits Gag's tRNA annealing capability. During assembly, MA specifically interacts with inositol phosphate (IP)-containing lipids in the plasma membrane (PM). Surprisingly, we find that IPs stimulate Gag-facilitated tRNA annealing but do not stimulate annealing in Gag variants lacking the MA domain or containing point mutations involved in PM binding. Moreover, we find that IPs prevent MA from binding to nucleic acids but have little effect on NC or Gag. We propose that Gag binds to RNA either with both NC and MA domains or with NC alone and that MA-IP interactions alter Gag's binding mode. We propose that MA's interactions with the PM trigger the switch between these two binding modes and stimulate Gag's chaperone function, which may be important for the regulation of events such as tRNA primer annealing.

Role of HIV-1 nucleocapsid protein in HIV-1 reverse transcription

The HIV-1 nucleocapsid protein (NC) is a nucleic acid chaperone, which remodels nucleic acid structures so that the most thermodynamically stable conformations are formed. This activity is essential for virus replication and has a critical role in mediating highly specific and efficient reverse transcription. NC's function in this process depends upon three properties: (1) ability to aggregate nucleic acids; (2) moderate duplex destabilization activity; and (3) rapid on-off binding kinetics. Here, we present a detailed molecular analysis of the individual events that occur during viral DNA synthesis and show how NC's properties are important for almost every step in the pathway. Finally, we also review biological aspects of reverse transcription during infection and the interplay between NC, reverse transcriptase, and human APOBEC3G, an HIV-1 restriction factor that inhibits reverse transcription and virus replication in the absence of the HIV-1 Vif protein.

Features, processing states, and heterologous protein interactions in the modulation of the retroviral nucleocapsid protein function

Retroviral nucleocapsid (NC) is central to viral replication. Nucleic acid chaperoning is a key function for NC through the action of its conserved basic amino acids and zinc-finger structures. NC manipulates genomic RNA from its packaging in the producer cell to reverse transcription into the infected host cell. This chaperone function, in conjunction with NC's aggregating properties, is up-modulated by successive NC processing events, from the Gag precursor to the fully mature protein, resulting in the condensation of the nucleocapsid within the capsid shell. Reverse transcription also depends on NC processing, whereas this process provokes NC dissociation from double-stranded DNA, leading to a preintegration complex (PIC), competent for host chromosomal integration. In addition NC interacts with cellular proteins, some of which are involved in viral budding, and also with several viral proteins. All of these properties are reviewed here, focusing on HIV-1 as a paradigmatic reference and highlighting the plasticity of the nucleocapsid architecture.

Formation of immature and mature genomic RNA dimers in wild-type and protease-inactive HIV-1: differential roles of the Gag polyprotein, nucleocapsid proteins NCp15, NCp9, NCp7, and the dimerization initiation site

Formation of immature genomic RNA (gRNA) dimers is exquisitely nucleocapsid (NC)-dependent in protease-inactive (PR-in) HIV-1. This establishes that Pr55gag/Pr160gag-pol has NC-dependent chaperone activity within intact HIV-1. Mutations in the proximal zinc finger and the linker of the NC sequence of Pr55gag/Pr160gag-pol abolish gRNA dimerization in PR-in HIV-1. In wild type, where the NC of Pr55gag is processed into progressively smaller proteins termed NCp15 (NCp7-p1-p6), NCp9 (NCp7-p1) and NCp7, formation of immature dimers is much swifter than in PR-in HIV-1. NCp7 and NCp15 direct this rapid accumulation. NCp9 is sluggish in this process, but it stimulates the transition from immature to mature gRNA dimer as well as NCp7 and much better than NCp15. The amino-terminus, proximal zinc finger, linker, and distal zinc finger of NCp7 contribute to this maturation event in intact HIV-1. The DIS is a dimerization initiation site for all immature gRNA dimers, irrespective of their mechanism of formation.

C-terminal domain modulates the nucleic acid chaperone activity of human T-cell leukemia virus type 1 nucleocapsid protein via an electrostatic mechanism

Retroviral nucleocapsid (NC) proteins are molecular chaperones that facilitate nucleic acid (NA) remodeling events critical in viral replication processes such as reverse transcription. Surprisingly, the NC protein from human T-cell leukemia virus type 1 (HTLV-1) is an extremely poor NA chaperone. Using bulk and single molecule methods, we find that removal of the anionic C-terminal domain (CTD) of HTLV-1 NC results in a protein with chaperone properties comparable with that of other retroviral NCs. Increasing the ionic strength of the solution also improves the chaperone activity of full-length HTLV-1 NC. To determine how the CTD negatively modulates the chaperone activity of HTLV-1 NC, we quantified the thermodynamics and kinetics of wild-type and mutant HTLV-1 NC/NA interactions. The wild-type protein exhibits very slow dissociation kinetics, and removal of the CTD or mutations that eliminate acidic residues dramatically increase the protein/DNA interaction kinetics. Taken together, these results suggest that the anionic CTD interacts with the cationic N-terminal domain intramolecularly when HTLV-1 NC is not bound to nucleic acids, and similar interactions occur between neighboring molecules when NC is NA-bound. The intramolecular N-terminal domain-CTD attraction slows down the association of the HTLV-1 NC with NA, whereas the intermolecular interaction leads to multimerization of HTLV-1 NC on the NA. The latter inhibits both NA/NC aggregation and rapid protein dissociation from single-stranded DNA. These features make HTLV-1 NC a poor NA chaperone, despite its robust duplex destabilizing capability.

Intracellular transport of human immunodeficiency virus type 1 genomic RNA and viral production are dependent on dynein motor function and late endosome positioning

Our earlier work indicated that the human immunodeficiency virus type 1 (HIV-1) genomic RNA (vRNA) is trafficked to the microtubule-organizing center (MTOC) when heterogeneous nuclear ribonucleoprotein A2/B1 is depleted from cells. Also, Rab7-interacting lysosomal protein promoted dynein motor complex, late endosome and vRNA clustering at the MTOC suggesting that the dynein motor and late endosomes were involved in vRNA trafficking. To investigate the role of the dynein motor in vRNA trafficking, dynein motor function was disrupted by small interference RNA-mediated depletion of the dynein heavy chain or by p50/dynamitin overexpression. These treatments led to a marked relocalization of vRNA and viral structural protein Gag to the cell periphery with late endosomes and a severalfold increase in HIV-1 production. In contrast, rerouting vRNA to the MTOC reduced virus production. vRNA localization depended on Gag membrane association as shown using both myristoylation and Gag nucleocapsid domain proviral mutants. Furthermore, the cytoplasmic localization of vRNA and Gag was not attributable to intracellular or internalized endocytosed virus particles. Our results demonstrate that dynein motor function is important for regulating Gag and vRNA egress on endosomal membranes in the cytoplasm to directly impact on viral production.

The conserved N-terminal basic residues and zinc-finger motifs of HIV-1 nucleocapsid restrict the viral cDNA synthesis during virus formation and maturation

Reverse transcription of the genomic RNA by reverse transcriptase occurs soon after HIV-1 infection of target cells. The viral nucleocapsid (NC) protein chaperones this process via its nucleic acid annealing activities and its interactions with the reverse transcriptase enzyme. To function, NC needs its two conserved zinc fingers and flanking basic residues. We recently reported a new role for NC, whereby it negatively controls reverse transcription in the course of virus formation. Indeed, deleting its zinc fingers causes reverse transcription activation in virus producer cells. To investigate this new NC function, we used viruses with subtle mutations in the conserved zinc fingers and its flanking domains. We monitored by quantitative PCR the HIV-1 DNA content in producer cells and in produced virions. Results showed that the two intact zinc-finger structures are required for the temporal control of reverse transcription by NC throughout the virus replication cycle. The N-terminal basic residues also contributed to this new role of NC, while Pro-31 residue between the zinc fingers and Lys-59 in the C-terminal region did not. These findings further highlight the importance of NC as a major target for anti-HIV-1 drugs.