Linker insertion mutations in the human immunodeficiency virus type 1 gag gene: effects on virion particle assembly, release, and infectivity

Identification of novel interactions in HIV-1 capsid protein assembly by high-resolution mass spectrometry

The pleomorphic nature of the immature and mature HIV-1 virions has made it difficult to characterize intersubunit interactions using traditional approaches. While the structures of isolated domains are known, the challenge is to identify intersubunit interactions and thereby pack these domains into supramolecular structures. Using high-resolution mass spectrometry, we have measured the amide hydrogen exchange protection factors for the soluble capsid protein (CA) and CA assembled in vitro. Comparison of the protection factors as well as chemical crosslinking experiments has led to a map of the subunit/subunit interfaces in the assembled tubes. This analysis provides direct biochemical evidence for the homotypic N domain and C domain interactions proposed from cryo-electron microscopy image reconstruction of CA tubes. Most significantly, we have identified a previously unrecognized intersubunit N domain-C domain interaction. The detection of this interaction reconciles previously discrepant biophysical and genetic data.

Tripeptide interference with human immunodeficiency virus type 1 morphogenesis

Capsid assembly during virus replication is a potential target for antiviral therapy. The Gag polyprotein is the main structural component of retroviral particles, and in human immunodeficiency virus type 1 (HIV-1), it contains the sequences for the matrix, capsid, nucleocapsid, and several small polypeptides. Here, we report that at a concentration of 100 micro M, 7 of 83 tripeptide amides from the carboxyl-terminal sequence of the HIV-1 capsid protein p24 suppressed HIV-1 replication (>80%). The three most potent tripeptides, glycyl-prolyl-glycine-amide (GPG-NH(2)), alanyl-leucyl-glycine-amide (ALG-NH(2)), and arginyl-glutaminyl-glycine-amide (RQG-NH(2)), were found to interact with p24. With electron microscopy, disarranged core structures of HIV-1 progeny were extensively observed when the cells were treated with GPG-NH(2) and ALG-NH(2). Furthermore, nodular structures of approximately the same size as the broad end of HIV-1 conical capsids were observed at the plasma membranes of treated cells only, possibly indicating an arrest of the budding process. Corresponding tripeptides with nonamidated carboxyl termini were not biologically active and did not interact with p24.

Characterization of a putative alpha-helix across the capsid-SP1 boundary that is critical for the multimerization of human immunodeficiency virus type 1 gag

A 14-amino-acid spacer peptide termed SP1 that separates the capsid (CA) and nucleocapsid (NC) sequences plays an active role in the assembly of human immunodeficiency virus type 1. This activity of SP1 involves its amino-terminal residues that, together with adjacent CA residues, constitute a putative alpha-helical structure spanning Gag residues from positions 359 to 371. In this study, we have determined that the virus assembly determinants within this putative alpha-helix were residues H359, K360, A361, L364, A367, and M368, of which K360 and A367 contribute to virus production to lesser extents. Notably, changes of the two basic amino acids H359 and K360 to arginine (R) impaired virus production, whereas mutations L364I and M368I, in contrast to L364A and M368A, generated near-wild-type levels of virus particles. This suggests that within Gag complexes, amino acids H359 and K360 are involved in stricter steric interactions than L364 and M368. Since L364 and M368 are separated by four residues and thus presumably located on the same side of the helical surface, they may initiate synergistic hydrophobic interactions to stabilize Gag association. Further analysis in the context of the protease-negative mutation D185H confirmed the key roles of amino acids H359, A361, L364, and M368 in virus assembly. Importantly, when transfected cells were subjected to Dounce homogenization and the cell lysates were treated by ultracentrifugation at 100,000 x g, Gag molecules containing each of the H359A, A361V, L364A, and M368A mutations were found mainly in the supernatant fraction (S100), whereas approximately 80% of wild-type Gag proteins were found in the pellet. Therefore, these four mutations must have prevented Gag from generating large complexes.

Kinetic analysis of the role of intersubunit interactions in human immunodeficiency virus type 1 capsid protein assembly in vitro

The human immunodeficiency virus type 1 (HIV-1) capsid protein (CA) plays a crucial role in both assembly and maturation of the virion. Numerous recent studies have focused on either the soluble form of CA or the polymer end product of in vitro CA assembly. The CA polymer, in particular, has been used to study CA-CA interactions because it is a good model for the CA interactions within the virion core. However, analysis of the process of in vitro CA assembly can yield valuable insights into CA-CA interactions and the mechanism of core assembly. We describe here a method for the analysis of CA assembly kinetics wherein the progress of assembly is monitored by using turbidity. At pH 7.0 the addition of either of the isolated CA domains (i.e., the N or the C domain) to an assembly reaction caused a decrease in the assembly rate by competing for binding to the full-length CA protein. At pH 8.0 the addition of the isolated C domain had a similar inhibitory affect on CA assembly. However, at pH 8.0 the isolated N domain had no affect on the rate of CA assembly but, when mixed with the C domain, it alleviated the C-domain inhibition. These data provide biochemical evidence for a pH-sensitive homotypic N-domain interaction, as well as for an N- and C-domain interaction.

Intracellular trafficking of retroviral genomes during the early phase of infection: viral exploitation of cellular pathways

Retroviruses enter cells through specific cell-surface receptors and then embark on a journey that ultimately leads to the establishment of the integrated proviral DNA. The steps of the journey include the reverse transcription of the viral RNA into DNA, the trafficking of the viral protein-DNA complex through the cytoplasm, the entry of the complex into the nucleus, and the insertion of the linear viral DNA into the host genome. All these steps are likely to involve specific interactions of viral proteins with host machinery. Our knowledge of the details of these interactions is very limited but is rapidly expanding, and should provide a deeper understanding of the pathways and components used by the different classes of retroviruses. This knowledge in turn should enable the development of better and more efficient retroviral vectors for use in gene therapy protocols in vivo.

Human immunodeficiency virus type 1 N-terminal capsid mutants that exhibit aberrant core morphology and are blocked in initiation of reverse transcription in infected cells

A group of conserved hydrophobic residues faces the interior of the coiled-coil-like structure within the N-terminal domain of the human immunodeficiency virus type 1 (HIV-1) capsid protein (CA). It has been suggested that these residues are important for maintaining stable structure and functional activity. To investigate this possibility, we constructed two HIV-1 clones, in which Trp23 or Phe40 was changed to Ala. We also constructed a third mutant, D51A, which has a mutation that destroys a salt bridge between Pro1 and Asp51. All three mutants are replication defective but produce virus particles. Mutant virions contain all of the viral proteins, although the amount and stability of CA are decreased and levels of virion-associated integrase are reduced. The mutations do not affect endogenous reverse transcriptase activity; however, the mutants are blocked in their ability to initiate reverse transcription in infected cells and no minus-strand strong-stop DNA is detected. The defect in reverse transcription is associated with striking defects in the morphology of mutant virus cores, as determined by transmission electron microscopy. Our data indicate that the mutations made in this study disrupt CA structure and prevent proper maturation of virus cores. We propose that this results in a defect in core stability or in an early postentry event preceding reverse transcription.

In vitro assembly of feline immunodeficiency virus capsid protein: biological role of conserved cysteines

Core assembly, a key step in the retroviral life cycle, is poorly understood. Previous studies have shown that the entire gag region is needed to form the assembled particles. In this report, we have shown that the assembly process is driven by recombinant capsid protein (p26) of feline immunodeficiency virus itself. Proteins are expressed in a bacterial system and soluble forms of wild-type and modified proteins are purified from bacterial extracts and are examined on gel-filtration chromatography fitted to an HPLC system. It has also been shown that changing residue Cys190 (one of the two conserved cysteines of feline immunodeficiency virus which are also conserved for all the immunodeficiency viruses including HIV) to serine by site-directed mutagenesis disrupts the assembly process. In addition, this modification causes considerable thermal instability of the protein while substitutions at nonconserved cysteines do not significantly affect the thermal stability and assembly of the protein. These findings indicate that conserved cysteine residues play a vital role in the capsid protein assembly and, therefore, are critical for virus infectivity.

The NH2-terminal domain of the human T-cell leukemia virus type 1 capsid protein is involved in particle formation

The human immunodeficiency virus type 1 (HIV-1) and human T-cell leukemia virus type 1 (HTLV-1) capsid proteins (CA) display similar structures formed by two independently folded N-terminal (NTD) and C-terminal (CTD) domains. To characterize the functions harbored by the HTLV-1 CA domains in particle formation, 12 sites scattered throughout the protein were mutated. The effects of the mutations on Gag membrane binding, proteolytic processing, and virus-like particle secretion were analyzed. It appears that the NTD is the major partner of indirect or direct Gag-Gag interactions. In particular, most of the NTD mutations impaired virion morphogenesis, and no mutation located in the NTD could be fully rescued by coexpression of wild-type Gag. In contrast, the CTD seems not to be involved in Gag-Gag interactions. Nevertheless, an unknown function required for particle formation is located in the CTD. Thus, despite an overall structural similarity between the HIV-1 and HTLV-1 CA proteins, their NTDs and CTDs exhibit different functions.

Membrane relocation but not tight binding of human immunodeficiency virus type 1 Gag particles myristoylated in Escherichia coli

Expression of human immunodeficiency virus Gag protein and the N-terminal matrix (MA) domain in Escherichia coli yielded spherical structures in the cytoplasm. When human N-myristoyltransferase was coexpressed, both Gag and MA were fully myristoylated and spherical structures were relocated in close proximity to the cytoplasmic membrane. However, neither myristoylated Gag nor MA exhibited tight binding to E. coli membrane, suggesting that myristoylation in E. coli did not confer membrane affinity on Gag despite the relocation. Our data also suggest that the morphogenetic pathway of Gag particles in prokaryotic cells differs from that in eukaryotic cells despite biochemical similarities of in the form of Gag expressed.

Urea equilibrium unfolding of the major core protein of the retrovirus feline immunodeficiency virus and its tryptophan mutants

Circular dichroism and fluorescence spectroscopy have been employed to study the urea unfolding mechanism of a recombinant form of the major core protein of feline immunodeficiency virus (FIV-rp24) and its native tryptophan mutants. The equilibrium denaturation curves indicate the existence of two transitions. The first unfolding transition most likely reflects the denaturation of the carboxy-terminal region of FIV-rp24. Consequently, the second transition, where the changes in fluorescence are produced, should reflect the denaturation of the amino-terminal region. If the intermediate observed upon urea denaturation is an on-pathway species, the data described herein can reflect the sequential and independent loss of structure of the two domains that this type of proteins possesses.

Structural analysis of the N-terminal domain of the human T-cell leukemia virus capsid protein

The N-terminal domain of the retroviral capsid (CA) protein is one of the least conserved regions encoded in the genome. Surprisingly, the three-dimensional structures of the CA from different genera exhibit alpha-helical structural features that are highly conserved. The N-terminal residues of the human immunodeficiency virus type 1 (HIV-1) and Rous sarcoma virus (RSV) capsid proteins form a beta-hairpin. To determine if this feature is conserved in the retroviral family, we cloned, expressed, purified, and solved the structure of a N-terminal 134 amino acid fragment (CA(134)) from the human T-cell leukemia virus type 1 (HTLV-I) using high resolution nuclear magnetic resonance (NMR) spectroscopy. The CA(134) fragment contains an N-terminal beta-hairpin and a central coiled-coil-like structure composed of six alpha-helices. The N-terminal Pro1 residue contacts Asp54 in the helical cluster through a salt bridge. Thus, the beta-hairpin is conserved and the helical cluster is structurally similar to other retroviral CA domains. However, although the same Asp residue defines the orientation of the hairpin in both the HTLV-1 and HIV-1 CA proteins, the HTLV-I hairpin is oriented away, rather than towards, the helical core. Significant differences were also detected in the spatial orientation and helical content of the long centrally located loop connecting the helices in the core. It has been proposed that the salt bridge allows the formation of a CA-CA interface that is important for the assembly of the conical cores that are characteristic of HIV-1. As HTLV-I forms spherical cores, the salt-bridge feature is apparently not conserved for this function although its role in determining the orientation of the beta-hairpin may be critical, along with the central loop. Comparison of three-dimensional structures is expected to elucidate the relationships between the retroviral capsid protein structure and its function.

RNA trafficking signals in human immunodeficiency virus type 1

Intracellular trafficking of retroviral RNAs is a potential mechanism to target viral gene expression to specific regions of infected cells. Here we show that the human immunodeficiency virus type 1 (HIV-1) genome contains two sequences similar to the hnRNP A2 response element (A2RE), a cis-acting RNA trafficking sequence that binds to the trans-acting trafficking factor, hnRNP A2, and mediates a specific RNA trafficking pathway characterized extensively in oligodendrocytes. The two HIV-1 sequences, designated A2RE-1, within the major homology region of the gag gene, and A2RE-2, in a region of overlap between the vpr and tat genes, both bind to hnRNP A2 in vitro and are necessary and sufficient for RNA transport in oligodendrocytes in vivo. A single base change (A8G) in either sequence reduces hnRNP A2 binding and, in the case of A2RE-2, inhibits RNA transport. A2RE-mediated RNA transport is microtubule and hnRNP A2 dependent. Differentially labelled gag and vpr RNAs, containing A2RE-1 and A2RE-2, respectively, coassemble into the same RNA trafficking granules and are cotransported to the periphery of the cell. tat RNA, although it contains A2RE-2, is not transported as efficiently as vpr RNA. An A2RE/hnRNP A2-mediated trafficking pathway for HIV RNA is proposed, and the role of RNA trafficking in targeting HIV gene expression is discussed.

Assembly and release of human immunodeficiency virus type 1 Gag proteins containing tandem repeats of the matrix protein coding sequences in the matrix domain

We have constructed human immunodeficiency virus (HIV) gag mutants by increasing the matrix protein (MA) sequences via tandemly repeated duplication of the central 107-MA codons. Instead of a total of 132 amino acid residues for the wild-type MA, the resultant mutants designated as MA2, MA3, and MA4 contained a total of 242, 352, and 462 codons in the MA domains, respectively. Analysis indicated that the addition of 110 or 220 amino acid residues to the MA did not significantly affect the assembly, release, and processing of particles; however, particle production was markedly reduced when another copy of 110 residues was added to the MA. Subcellular fractionation analysis suggested that the MA tandem repeat mutations enhanced the Gag membrane affinity, in a manner which correlated with the copy number of MA sequences. The effects of enhanced membrane affinity were substantially reduced when sequences downstream of the capsid (CA) domain were deleted. Sucrose density gradient fractionation analysis showed that particles produced by the large insertion mutants possessed wild-type (wt) HIV particle density. Truncation of sequences downstream of the nucleocapsid (NC) domains of the mutants did not influence the budding of particles. In contrast, particle budding was severely impaired when sequences downstream of the CA domain were truncated. Particle densities for the large Gag proteins, which were truncated at the C-terminus of CA, were about 1.12-1.14 g/ml lower than that for wt. Our results suggest that the HIV MA domain could adopt insertions of large protein sequences, and strongly support the proposal that the NC and p2 domains play a crucial role in the process of correct Gag protein packing.

Construction of infectious SIV/HIV-2 chimeras

OBJECTIVE

To construct SIV/HIV-2 chimeras (SHIV) that replicate in vivo. These would be valuable tools to elucidate the mechanism by which HIV-2 can bypass protection conferred by live attenuated SIV vaccines.

METHOD

Novel SHIV were constructed to express either the vpx, vpr, tat, rev and env genes (SHIV-2isy env) or the gag and pol genes (SHIV-2isy gag/pol) of the infectious molecular clone HIV-2isy in an SIVmac backbone. The replication of SHIV-2isy env and SHIV-2isy gag/pol were evaluated on selected cell lines and peripheral blood mononuclear cells (PBMC) in vitro. In addition, their infectivity was assessed in vivo.

RESULT

Virus stocks of SHIV-2isy env and SHIV-2isy gag/pol were prepared in vitro. For SHIV-2isy gag/pol both the 5' and 3' boundaries of the chimeric construct were critical for infectivity in vitro. The growth of each chimera on T cell lines in vitro mirrors that of the parental viruses donating the envelope gene. On PBMCs SHIV-2isy env replicated well on human and simian PBMC whereas SHIV-2isy gag/pol replicated to detectable levels on human PBMC only. In vivo, SHIV-2isy env virus was isolated from one of two cynomolgus macaques challenged intravenously, SHIV-2isy gag/pol was isolated from one of two cynomolgus macaques and both rhesus macaques challenged intravenously.

CONCLUSION

This is the first report of SIV/HIV-2 chimeras that are infectious in macaques. Moreover, this is the first report of an infectious chimera in which both SIV gag and pol have been replaced with the equivalent regions of an HIV isolate.

Cell-dependent gag mutants of HIV-1 are crucially defective at the stage of uncoating/reverse transcription in non-permissive cells

We have previously shown that some of the human immunodeficiency virus type 1 (HIV-1) gag matrix (MA), capsid (CA), and nucleocapsid (NC) mutants display host-cell-dependent replication potential, and that they are defective at the early phase of the virus replication cycle in non-permissive cells. To determine the defective replication stage of the cell-dependent mutants precisely, the processes of virus entry into cells and virus DNA synthesis were monitored by the highly sensitive enzyme-linked immunosorbent assay and polymerase chain reaction amplification analysis. The results obtained indicated that all the cell-dependent MA, CA and NC mutants are defective at the stage of uncoating/reverse transcription, and that a cellular factor(s) is involved in this process.

Three-dimensional organization of retroviral capsid proteins on a lipid monolayer

We have used a method for the two-dimensional crystallization of retroviral structural proteins to obtain a three-dimensional structure of negatively stained, membrane-bound, histidine-tagged Moloney murine leukemia virus (M-MuLV) capsid protein (his-MoCA) arrays. Tilted and untilted micrographs from crystals formed by purified his-MoCA proteins incubated beneath lipid monolayers containing nickel-chelating lipids were used in 3D reconstructions. The 2D crystals had unit cell dimensions of a=72.6 A, b=72.5 A and gamma=119.5 degrees, but appeared to have no intrinsic symmetry (p1) in 3D, in contrast to the trigonal or hexagonal appearance of their 2D projections. Membrane-bound his-MoCA proteins showed a strand-like organization, apparently with dimer building blocks. Membrane-proximal regions, or putative N-terminal domains (NTDs), dimerized with different partners than the membrane-distal putative C-terminal domains (CTDs). Evidence also suggests that CTDs can adopt alternate orientations relative to their NTDs, forming interstrand connections. Our results are consistent with helical-spiral models for retrovirus particle assembly, but are not easily reconcilable with icosahedral models.

Analysis of a human immunodeficiency virus type 1 gag mutant with an engineered 110-amino-acid insertion in the matrix protein domain

A human immunodeficiency virus (HIV) matrix (MA) protein mutant was constructed by duplication of 107 codons of the HIV-1 MA domain. This MA protein duplication mutant (MAII) still could assemble and process particles, had a wild-type (wt) HIV particle density, and possessed reverse transcriptase activity of about 80% of the wild type virus level. The incorporation of HIV Env and viral RNA genome was not greatly affected. The MAII was noninfectious or poorly infectious, however, when pseudotyped with an amphotropic murine leukemia virus envelope protein or with the HIV envelope protein. Although the MAII mutant displayed an immunofluorescence staining pattern similar to that of the wild type virus, subcellular fractionation studies indicated that the membrane association of MAII Gag precursors was unstable under high-salt conditions. Electron microscopic studies showed that the mutant had a decreased density of particle cores compared with that of the wild type virus, suggesting an altered arrangement of the packed proteins. As this insertion in the MA gene caused no major effects on virus assembly implies that the HIV-1 gag has the potential to adapt large insertions of extra coding sequences without loss of the ability to direct particle assembly and release.

Efficient particle production by minimal Gag constructs which retain the carboxy-terminal domain of human immunodeficiency virus type 1 capsid-p2 and a late assembly domain

The human immunodeficiency virus type 1 (HIV-1) Gag precursor Pr55(gag) by itself is capable of assembling into retrovirus-like particles (VLP). In the present study, we attempted to identify the minimal Gag sequences required for the formation of VLP. Our results show that about 80% of Pr55(gag) can be either deleted or replaced by heterologous sequences without significantly compromising VLP production. The smallest chimeric molecule still able to efficiently form VLP was only about 16 kDa. This minimal Gag construct contained the leucine zipper domain of the yeast transcription factor GCN4 to substitute for the assembly function of nucleocapsid (NC), followed by a P-P-P-P-Y motif to provide late budding (L) domain function, and retained only the myristylation signal and the C-terminal capsid-p2 domain of Pr55(gag). We also show that the L domain function of HIV-1 p6(gag) is not dependent on the presence of an active viral protease and that the NC domain of Pr55(gag) is dispensable for the incorporation of Vpr into VLP.

Relationship between human immunodeficiency virus type 1 Gag multimerization and membrane binding

The human immunodeficiency virus type 1 (HIV-1) Gag precursor, Pr55(Gag), is necessary and sufficient for the assembly and release of viruslike particles. Binding of Gag to membrane and Gag multimerization are both essential steps in virus assembly, yet the domains responsible for these events have not been fully defined. In addition, the relationship between membrane binding and Gag-Gag interaction remains to be elucidated. To investigate these issues, we analyzed, in vivo, the membrane-binding and assembly properties of a series of C-terminally truncated Gag mutants. Pr55(Gag) was truncated at the C terminus of matrix (MAstop), between the N- and C-terminal domains of capsid (CA146stop), at the C terminus of capsid (p41stop), at the C terminus of p2 (p43stop), and after the N-terminal 35 amino acids of nucleocapsid (NC35stop). The ability of these truncated Gag molecules to assemble and release viruslike particles and their capacity to copackage into particles when coexpressed with full-length Gag were determined. We demonstrate that the amount of truncated Gag incorporated into particles is incrementally increased by extension from CA146 to NC35, suggesting that multiple sites in this region are involved in Gag multimerization. Using membrane flotation centrifugation, we observe that MA shows significantly reduced membrane binding relative to full-length Gag but that CA146 displays steady-state membrane-binding properties comparable to those of Pr55(Gag). The finding that the CA146 mutant, which contains only matrix and the N-terminal domain of capsid, exhibits levels of steady-state membrane binding equivalent to those of full-length Gag indicates that strong Gag-Gag interaction domains are not required for the efficient binding of HIV-1 Gag to membrane.

Crosslink analysis of N-terminal, C-terminal, and N/B determining regions of the Moloney murine leukemia virus capsid protein

To analyze contacts made by Moloney murine leukemia virus (M-MuLV) capsid (CA) proteins in immature and mature virus particles, we have employed a cysteine-specific crosslinking approach that permits the identification of retroviral Gag protein interactions at particular residues. For analysis, single cysteine creation mutations were made in the context of protease-deficient or protease-competent parental constructs. Cysteine creation mutations were chosen near the N- and C-termini of CA and at a site adjacent to the M-MuLV Glu-Ala Fv1 N/B host range determination sequence. Analysis of immature virions showed that PrGag proteins were crosslinked at C-terminal CA residues to form dimers while crosslinking of particle-associated N-terminal and N/B region mutant proteins did not yield dimers, but showed evidence of linking to an unknown 140- to 160-kDa partner. Analysis of mature virions demonstrated that both N- and C-terminal CA residues participated in dimer formation, suggesting that processed CA N- and C-termini are free to establish interprotein associations. Interestingly, N/B region mutant residues in mature virus particles did not crosslink to form dimers, but showed a novel crosslinked band, consistent with an interaction between the N/B tropism determining region and a cellular protein of 45-55 kDa.

Enhancement of the basal-level activity of HIV-1 long terminal repeat by HIV-1 nucleocapsid protein

Two HIV-1 proteins, Tat and NCp7 (NC), have zinc finger-like structures. NC is a virion protein and has been shown to accumulate in the nucleus 8 h postinfection. Since transcription factors with zinc fingers assist the transcriptional activity of both RNA polymerases II and III, we examined the effect of NC on HIV-1 LTR-directed gene expression. The HIV-1 NC binds to the HIV-1 LTR and results in a mobility shift in polyacrylamide gel electrophoresis. Competition assays with cold probes revealed that the binding of NC and formation of a DNA-protein complex could be prevented by the addition of excess unlabeled LTR self-probe, but not the HIV-1 V3 envelope gene. The DNase I footprint analysis showed that NC binds to six regions within HIV-1 LTR, four of which are near the transcription start site. The NC alone enhances LTR basal-level activity in RNA runoff experiments. When the general transcription factors (GTFs) were added in the assay, NC enhances NF-kappaB, Sp1, and TFIIB-induced HIV-1 LTR-directed RNA transcription. RNA transcription directed by the adenovirus major late promoter, however, is not significantly affected by NC in the cell-free system. Transient transfection of human T lymphocytes with the plasmids containing HIV-1 nc or gag showed enhancement of LTR-CAT activity. Moreover, transfection of HIV-1 provirus containing mutations in NC zinc-finger domains dramatically decreases the enhancement activity in human T cells, in which HIV-1 LTR is stably integrated into the cellular genome. These observations show that NC binds to HIV-1 LTR and cooperatively enhances GTFs and NF-kappaB induced HIV-1 LTR basal-level activity. NC may play the role of a nucleation protein, which binds to LTR and enhances basal-level transcription by recruiting cellular transcription factors to the HIV-1 promoter in competition with cellular promoters.

Characterization of a discontinuous epitope of the human immunodeficiency virus (HIV) core protein p24 by epitope excision and differential chemical modification followed by mass spectrometric peptide mapping analysis

A combination of epitope excision, epitope extraction, and differential chemical modification followed by mass spectrometric peptide mapping was used for the characterization of a discontinuous epitope that is recognized by the mouse anti-HIV-p24 monoclonal antibody 5E2.A3. In epitope excision, the protein is first bound to an immobilized antibody and then digested with proteolytic enzymes. In epitope extraction, the protein is first digested and subsequently allowed to react with the antibody. After epitope excision of the p24-5E2.A3 complex with endoproteinase Lys-C, a large fragment remained affinity bound corresponding to amino acids 1-158 of HIV-p24 (fragment 1-158). Further digestion, however, resulted in loss of affinity. Moreover, no affinity-bound fragments were observed after an epitope extraction experiment. These data from the epitope excision and extraction experiments suggest that the epitope is discontinuous. For the further characterization of the epitope, amino groups in the epitope-containing fragment were acetylated in both the affinity bound and free states followed by mass spectrometric analysis. Two successive acetylation reactions were performed: (1) the first used a low molar excess of acetic anhydride, and (2) the second, after separation from the antibody, a high molar excess of its hexadeuteroderivative. This isotopic labeling procedure, in combination with high resolution mass spectrometry, allowed the precise determination of relative reactivities of amino groups. In this study, no differences were observed in the ranking of the relative reactivities of five lysine residues. However, the N-terminal amino group was found to be part of the discontinuous epitope.

Functional characterization of the human immunodeficiency virus type 1 genome by genetic footprinting

We present a detailed and quantitative analysis of the functional characteristics of the 1,000-nucleotide segment at the 5' end of the human immunodeficiency virus type 1 (HIV-1) RNA genome. This segment of the viral genome contains several important cis-acting sequences, including the TAR, polyadenylation, viral att site, minus-strand primer-binding site, and 5' splice donor sequences, as well as coding sequences for the matrix protein and the N-terminal half of the capsid protein. The genetic footprinting technique was used to determine quantitatively the abilities of 134 independent insertion mutations to (i) make stable viral RNA, (ii) assemble and release viral RNA-containing viral particles, and (iii) enter host cells, complete reverse transcription, enter the nuclei of host cells, and generate proviruses in the host genome by integration. All of the mutants were constructed and analyzed en masse, greatly decreasing the labor typically involved in mutagenesis studies. The results confirmed the presence of several previously known functional features in this region of the HIV genome and provided evidence for several novel features, including newly identified cis-acting sequences that appeared to contribute to (i) the formation of stable viral transcripts, (ii) viral RNA packaging, and (iii) an early step in viral replication. The results also pointed to an unanticipated trans-acting role for the N-terminal portion of matrix in the formation of stable viral RNA transcripts. Finally, in contrast to previous reports, the results of this study suggested that detrimental mutations in the matrix and capsid proteins principally interfered with viral assembly.

Roles of matrix, p2, and N-terminal myristoylation in human immunodeficiency virus type 1 Gag assembly

Human immunodeficiency virus type 1 Gag protein is cotranslationally myristoylated at the N terminus and targeted to the plasma membrane, where virus particle assembly occurs. Particle assembly requires the ordered multimerization of Gag proteins, yet there is little direct evidence of intermediates of the reaction or of the domains that lead to each stage of the oligomerization process. In this study, following the expression in insect cells of C-terminally truncated Gag proteins and their purification, both the multimeric nature of each Gag protein and the ability to form Gag virus-like particles (VLP) were analyzed. Our results show that (i) the matrix (MA) domain forms a trimer and contributes to a similar level of oligomerization of the assembly-competent Gag; (ii) the p2 domain, located at the capsid/nucleocapsid junction, is essential for a higher order of multimerization (>1,000 kDa); (iii) the latter multimerization is accompanied by a change in Gag assembly morphology from tubes to spheres and results in VLP production; and (iv) N-terminal myristoylation is not required for either of the multimerization stages but plays a key role in conversion of these multimers to Gag VLP. We suggest that the Gag trimer and the > 1,000-kDa multimer are intermediates in the assembly reaction and form before Gag targeting to the plasma membrane. Our data identify a minimum of three stages for VLP development and suggest that each stage involves a separate domain, MA, p2, or N-terminal myristoylation, each of which contributes to HIV particle assembly.

Circular dichroism and fluorescence spectroscopic properties of the major core protein of feline immunodeficiency virus and its tryptophan mutants. Assignment of the individual contribution of the aromatic sidechains

The gene coding for the major capsid protein of feline immunodeficiency virus (FIV) has been cloned into the expression vector pQE60, which allows protein purification by affinity chromatography on a nitrilotriacetic acid/Ni/agarose column. The gene was expressed in Escherichia coli and the resultant soluble protein (FIV-rp24) purified to electrophoretic homogeneity. The amino-acid composition of the recombinant protein is almost identical to that predicted from the DNA sequence. This protein has two tryptophan residues at positions 40 and 126 that have been replaced by phenylalanine by site-directed mutagenesis to obtain two single mutants and a double mutant. Circular dichroism and fluorescence spectroscopy were employed to study the structural features of FIV-rp24 protein and its tryptophan mutants. The analysis of the CD spectra indicated that alpha-helix is the major secondary structural element (48-52%) and that the overall three-dimensional structure is not modified by the mutations. The fluorescence emission spectra showed that both tryptophan residues occupy a highly hydrophobic environment. Moreover, the different tyrosine fluorescence intensities of wild-type and mutant proteins are indicative of the existence of resonance energy transfer processes to nearby tryptophan. The individual contributions of each tryptophan residue to the spectroscopic properties of the wild-type protein were obtained from the spectra of all these proteins. Thermal denaturation studies indicate that the two tryptophan residues do not contribute equally to the stabilization of the three-dimensional structure.

Human immunodeficiency virus type 1 Gag polyprotein multimerization requires the nucleocapsid domain and RNA and is promoted by the capsid-dimer interface and the basic region of matrix protein

The human immunodeficiency virus type 1 (HIV-1) Gag polyprotein directs the formation of virions from productively infected cells. Many gag mutations disrupt virion assembly, but little is known about the biochemical effects of many of these mutations. Protein-protein interactions among Gag monomers are believed to be necessary for virion assembly, and data suggest that RNA may modify protein-protein interactions or even serve as a bridge linking Gag polyprotein monomers. To evaluate the primary sequence requirements for HIV-1 Gag homomeric interactions, a panel of HIV-1 Gag deletion mutants was expressed in bacteria and evaluated for the ability to associate with full-length Gag in vitro. The nucleocapsid protein, the major RNA-binding domain of Gag, exhibited activity comparable to that of the complete polyprotein. In the absence of the nucleocapsid protein, relatively weak activity was observed that was dependent upon both the capsid-dimer interface and basic residues within the matrix domain. The relevance of the in vitro findings was confirmed with an assay in which nonmyristylated mutant Gags were assessed for the ability to be incorporated into virions produced by wild-type Gag expressed in trans. Evidence of the importance of RNA for Gag-Gag interaction was provided by the demonstration that RNase impairs the Gag-Gag interaction and that HIV-1 Gag interacts efficiently with Gags encoded by distantly related retroviruses and with structurally unrelated RNA-binding proteins. These results are consistent with models in which Gag multimerization involves indirect contacts via an RNA bridge as well as direct protein-protein interactions.

In vitro assembly of human immunodeficiency virus type 1 Gag protein

Retroviral Gag protein is sufficient to produce Gag virus-like particles when expressed in higher eukaryotic cells. Here we describe the in vitro assembly reaction of human immunodeficiency virus Gag protein, which consists of two sequential steps showing the optimal conditions for each reaction. Following expression and purification, Gag protein lacking only the C-terminal p6 domain was present as a monomer (50 kDa) by velocity sedimentation analysis. Initial assembly of the Gag protein to 60 S intermediates occurred by dialysis at 4 degrees C in low salt at neutral to alkaline pH. However, higher order of assembly required incubation at 37 degrees C and was facilitated by the addition of Mg(2+). Prolonged incubation under these conditions produced complete assembly (600 S), equivalent to Gag virus-like particles obtained from Gag-expressing cells. Neither form disassembled by treatment with nonionic detergent, suggesting that correct assembly might occur in vitro. Electron microscopic observation confirmed that the 600 S assembly products were spherical particles similar to authentic immature human immunodeficiency virus particles. The latter assembly stage but not the former was accelerated by the addition of RNA although not inhibited by RNaseA treatment. These results suggest that Gag protein alone assembles in vitro, but that additional RNA facilitates the assembly reaction.

Inhibition of HIV/SIV replication by dominant negative Gag mutants

There are several major strategies against HIV/AIDS. Of these, the gene therapy is a novel, challenging, and promising one. The target genes, which have been extensively studied for the potential gene therapy of HIV/AIDS, include those of cellular and viral origins. Especially, trans-dominant negative Tat, Rev, Env, Pol, and Gag mutants of HIV have currently attracted considerable attention. In this brief review, we summarize the nature of the HIV/SIV mutants of this category and discuss their future use for gene therapy with special reference to the dominant negative Gag mutants of HIV-1.

Solution structure of the capsid protein from the human T-cell leukemia virus type-I

The solution structure of the capsid protein (CA) from the human T-cell leukemia virus type one (HTLV-I), a retrovirus that causes T-cell leukemia and HTLV-I-associated myelopathy in humans, has been determined by NMR methods. The protein consists of independent N and C-terminal domains connected by a flexible linker. The domains are structurally similar to the N-terminal "core" and C-terminal "dimerization" domains, respectively, of the human immunodeficiency virus type one (HIV-1) and equine infectious anemia virus (EIAV) capsid proteins, although several important differences exist. In particular, hydrophobic residues near the major homology region are partially buried in HTLV-I CA, which is monomeric in solution, whereas analogous residues in HIV-1 and EIAV CA project from the C-terminal domain and promote dimerization. These differences in the structure and oligomerization state of the proteins appear to be related to, and possibly controlled by, the oxidation state of conserved cysteine residues, which are reduced in HTLV-I CA but form a disulfide bond in the HIV-1 and EIAV CA crystal structures. The results are consistent with an oxidative capsid assembly mechanism, in which CA oligomerization or maturation is triggered by disulfide bo nd formation as the budding virus enters the oxidizing environment of the bloodstream.

Isolation and characterization of an oligomerization-negative mutant of HIV-1 integrase

The yeast two-hybrid method was used to screen mutagenized DNAs to isolate a variant of the human immunodeficiency virus type 1 integrase (IN) that does not interact with the wild-type IN. The responsible mutation, leading to a single amino acid change (V260E) in the C-terminal domain of IN, blocks IN-IN multimerization but has only small effect on binding to a host interacting protein, INI1 (hSNF5). Binding studies in vitro confirmed the defect in multimerization of the mutant IN. Biochemical analyses of the mutant IN enzyme expressed in bacteria detected only subtle changes in its properties, suggesting that the yeast system is a sensitive reporter of correct IN conformation. Mutant virus carrying the V260E substitution was blocked in replication at the time of DNA integration, consistent with IN multimerization being important for its activity in vivo.

Cyclophilin A incorporation is not required for human immunodeficiency virus type 1 particle maturation and does not destabilize the mature capsid

The cellular protein cyclophilin A (CypA) is packaged into human immunodeficiency virus type 1 (HIV-1) virions through a specific interaction with the capsid (CA) domain of the Gag polyprotein. CypA is important for infectivity, but its role in viral replication is currently unknown. Previous reports suggested that CypA promotes uncoating or enhances maturation. We analyzed the morphology and capsid stability of HIV-1 variants defective in CypA binding and of virus grown in the presence of cyclosporin. Both cyclosporin treatment and alteration of Gly89 or Pro90 in the CypA-binding site of CA caused a 5- to 20-fold decrease in CypA incorporation. Virus produced from cyclosporin-treated cells and variants G89V and G89A were 10- to 100-fold less infectious but exhibited normal virion morphologies with regular cone-shaped capsids. Irregular capsid morphologies and lower infectivities were observed for some other variants in the CypA-binding region. Decreased CypA incorporation did not reduce the kinetics of intracellular polyprotein processing or of virus release. No increase in immature particles was observed. These results suggest that CypA does not promote virion maturation. Furthermore, detergent stripping of virus particles with various CypA contents revealed no difference in capsid stability. Based on these results and those reported in the accompanying paper, it appears likely that CypA also is not an uncoating factor. Alternative models for CypA function are discussed.

Vif and the p55(Gag) polyprotein of human immunodeficiency virus type 1 are present in colocalizing membrane-free cytoplasmic complexes

The Vif protein of human immunodeficiency virus type 1 (HIV-1) is a potent regulator of viral infectivity. Current data posit that Vif functions late in replication to modulate assembly, budding, and/or maturation. Consistent with this model, earlier indirect immunofluorescence analyses of HIV-1-infected cells demonstrated that Vif and Gag colocalize to a substantial degree (J. H. M. Simon, R. A. M. Fouchier, T. E. Southerling, C. B. Guerra, C. K. Grant, and M. H. Malim, J. Virol. 71:5259-5267, 1997). Here, we describe a series of subcellular fractionation studies which indicate that Vif and the p55(Gag) polyprotein are present in membrane-free cytoplasmic complexes that copurify in sucrose density gradients and are stable in nonionic detergents. Both Vif and Gag are targeted to these complexes independent of each other, and their association with them appears to be mediated by protein-protein interactions. We propose that these complexes may represent viral assembly intermediates and that Vif is appropriately localized to influence the final stages of the viral life cycle and, therefore, the infectivity of progeny virions.

Head-to-tail dimers and interdomain flexibility revealed by the crystal structure of HIV-1 capsid protein (p24) complexed with a monoclonal antibody Fab

The crystal structure of an intact molecule of HIV-1 capsid protein (p24) in complex with a monoclonal antibody fragment recognizing an epitope on the C-terminal domain has been determined at 3 A resolution. The helical N- and C-terminal domains of p24 are linked by an extended peptide forming a flexibly linked dumb-bell-shaped molecule 75 A in overall length. The p24 construct used is a variant with an N-terminal extension that mimics to some extent the Gag context of p24. We observed a novel head-to-tail dimer of p24 molecules which occurs through the formation of a substantial intermolecular interface between the N- and C-terminal domains. Comparison with previously observed p24 dimers shows that the same residues and secondary structural elements can partake in different interfaces revealing a remarkable stickiness and plasticity of the p24 molecule, properties which, combined with the inter-domain flexibility, are presumably important in the assembly and maturation of viral particles. Previous mutagenesis studies designed to test specific N-N and C-C homodimer interfaces do not discriminate fully against the possibility of the observed N-C interface.

Model for lentivirus capsid core assembly based on crystal dimers of EIAV p26

Two crystal forms of recombinant p26 capsid protein (CA) from the equine infectious anemia virus (EIAV) have in common an antiparallel four-helix bundle dimer interface between N-terminal domains (NTDs). The dimer interface provides a lenient scaffold to accommodate the wide sequence variation in these helices within lentivirus CA. Pairs of dimers weakly associate to form exact or approximate D2 symmetry tetramers. In one of the two crystal forms, the tetramers are linked via dimerization of C-terminal domains (CTDs). We propose that the observed NTD and CTD homodimer interactions are involved in the assembly of the lentivirus capsid. The NTD homodimer shape readily suggests a model for the mature capsid core, based on hexagonal packing with dimensions and surface topology resembling described EIAV capsid cores. Combining available data for human immunodeficiency virus and EIAV CA, we also propose an assembly pathway for maturation of the lentivirus capsid core following proteolytic cleavage of the gag polyprotein precursor.

Structural biology of HIV

The human immunodeficiency virus (HIV) genome encodes a total of three structural proteins, two envelope proteins, three enzymes, and six accessory proteins. Studies over the past ten years have provided high-resolution three-dimensional structural information for all of the viral enzymes, structural proteins and envelope proteins, as well as for three of the accessory proteins. In some cases it has been possible to solve the structures of the intact, native proteins, but in most cases structural data were obtained for isolated protein domains, peptidic fragments, or mutants. Peptide complexes with two regulatory RNA fragments and a protein complex with an RNA recognition/encapsidation element have also been structurally characterized. This article summarizes the high-resolution structural information that is currently available for HIV proteins and reviews current structure-function and structure-biological relationships.

Mutations in nonconserved domains of Ty3 integrase affect multiple stages of the Ty3 life cycle

Ty3, a retroviruslike element of Saccharomyces cerevisiae, transposes into positions immediately upstream of RNA polymerase III-transcribed genes. The Ty3 integrase (IN) protein is required for integration of the replicated, extrachromosomal Ty3 DNA. In retroviral IN, a conserved core region is sufficient for strand transfer activity. In this study, charged-to-alanine scanning mutagenesis was used to investigate the roles of the nonconserved amino- and carboxyl-terminal regions of Ty3 IN. Each of the 20 IN mutants was defective for transposition, but no mutant was grossly defective for capsid maturation. All mutations affecting steady-state levels of mature IN protein resulted in reduced levels of replicated DNA, even when polymerase activity was not grossly defective as measured by exogenous reverse transcriptase activity assay. Thus, IN could contribute to nonpolymerase functions required for DNA production in vivo or to the stability of the DNA product. Several mutations in the carboxyl-terminal domain resulted in relatively low levels of processed 3' ends of the replicated DNA, suggesting that this domain may be important for binding of IN to the long terminal repeat. Another class of mutants produced wild-type amounts of DNA with correctly processed 3' ends. This class could include mutants affected in nuclear entry and target association. Collectively, these mutations demonstrate that in vivo, within the preintegration complex, IN performs a central role in coordinating multiple late stages of the retrotransposition life cycle.

HIV-1 gag proteins: diverse functions in the virus life cycle

The Gag proteins of HIV-1, like those of other retroviruses, are necessary and sufficient for the assembly of virus-like particles. The roles played by HIV-1 Gag proteins during the life cycle are numerous and complex, involving not only assembly but also virion maturation after particle release and early postentry steps in virus replication. As the individual Gag domains carry out their diverse functions, they must engage in interactions with themselves, other Gag proteins, other viral proteins, lipid, nucleic acid (DNA and RNA), and host cell proteins. This review briefly summarizes our current understanding of how HIV-1 Gag proteins function in the virus life cycle.

The C-terminal half of the human immunodeficiency virus type 1 Gag precursor is sufficient for efficient particle assembly

Human immunodeficiency virus type 1 particle assembly is directed by the Gag polyprotein Pr55(gag), the precursor for the matrix (MA), capsid (CA), and nucleocapsid proteins of the mature virion. We now show that CA sequences N terminal to the major homology region (MHR), which form a distinct domain, are dispensable for particle formation. However, slightly larger deletions which extend into the MHR severely impair particle production. Remarkably, a deletion which removed essentially all MA and CA sequences between the N-terminal myristyl anchor and the MHR reduced the yield of extracellular particles only moderately. Particle formation even exceeded wild-type levels when additional MA sequences, either from the N or the C terminus of the domain, were retained. We conclude that no distinct region between the myristyl anchor and the MHR is required for efficient particle assembly or release.

Analysis of minimal human immunodeficiency virus type 1 gag coding sequences capable of virus-like particle assembly and release

We have constructed a series of human immunodeficiency virus (HIV) gag mutants by progressive truncation of the gag coding sequence from the C terminus and have combined these mutants with an assembly-competent matrix domain deletion mutation (DeltaMA). By using several methods, the particle-producing capabilities of each mutant were examined. Our analysis indicated that truncated Gag precursors lacking most of C-terminal gag gene products assembled and were released from 293T cells. Additionally, a mutant with a combined deletion of the MA (DeltaMA) and p6 domains even produced particles at levels comparable to that of the wild-type (wt) virus. However, most mutants derived from combination of the DeltaMA and the C-terminal truncation mutations did not release particles as well as the wt. Our smallest HIV gag gene product capable of virus-like particle formation was a 28-kDa protein which consists of a few MA amino acids and the CA-p2 domain. Sucrose density gradient fractionation analysis indicated that most mutants exhibited a wt retrovirus particle density. Exceptions to this rule were mutants with an intact MA domain but deleted downstream of the p2 domains. These C-terminal truncation mutants possessed particle densities of 1.13 to 1.15 g/ml, lower than that of the wt. The N-terminal portions of the CA domain, which have been shown to be dispensable for core assembly, became critical when most of the MA domain was deleted, suggesting a requirement for an intact CA domain to assemble and release particles.

Early function of HIV-1 Gag proteins is cell-dependent

Various gag gene mutants of human immunodeficiency virus type 1 (HIV-1) were monitored for their replication potentials and defective replication sites in various CD4-positive T-cell lines. Some matrix, capsid, and nucleocapsid mutants displayed a replication defect in a cell-dependent manner. The single-round replication assays demonstrated that these mutants were defective at an early infection phase also in a cell-dependent way. These results indicated that interaction of a cell factor(s) and Gag proteins is involved in an early process of HIV-1 replication.

Particle size determinants in the human immunodeficiency virus type 1 Gag protein

The retroviral Gag protein plays the central role in the assembly process and can form membrane-enclosed, virus-like particles in the absence of any other viral products. These particles are similar to authentic virions in density and size. Three small domains of the human immunodeficiency virus type 1 (HIV-1) Gag protein have been previously identified as being important for budding. Regions that lie outside these domains can be deleted without any effect on particle release or density. However, the regions of Gag that control the size of HIV-1 particles are less well understood. In the case of Rous sarcoma virus (RSV), the size determinant maps to the CA (capsid) and adjacent spacer sequences within Gag, but systematic mapping of the HIV Gag protein has not been reported. To locate the size determinants of HIV-1, we analyzed a large collection of Gag mutants. To our surprise, all mutants with defects in the MA (matrix), CA, and the N-terminal part of NC (nucleocapsid) sequences produced dense particles of normal size, suggesting that oncoviruses (RSV) and lentiviruses (HIV-1) have different size-controlling elements. The most important region found to be critical for determining HIV-1 particle size is the p6 sequence. Particles lacking all or small parts of p6 were uniform in size distribution but very large as measured by rate zonal gradients. Further evidence for this novel function of p6 was obtained by placing this sequence at the C terminus of RSV CA mutants that produce heterogeneously sized particles. We found that the RSV-p6 chimeras produced normally sized particles. Thus, we present evidence that the entire p6 sequence plays a role in determining the size of a retroviral particle.

Cryoelectron microscopic examination of human immunodeficiency virus type 1 virions with mutations in the cyclophilin A binding loop

The human immunodeficiency virus type 1 capsid protein contains a conserved P217X4PX2PX5P231 motif. Mutation at Pro-222 decreases virion incorporation of cyclophilin A, while mutation at Pro-231 abolishes infectivity. Although viral RNA incorporation and protease cleavage of the Gag precursor were not affected by these mutations, cryoelectron microscopy revealed a loss of virion maturation in P231A particles.

The molecular basis of HIV capsid assembly

In common with many aspects of the HIV life cycle, the assembly of the virus particle has been the subject of intense investigation over recent years. Study of the subject is facilitated by the fact that only a single gene product, the Pr55 Gag protein, is required for virus assembly. A combination of site directed mutagenesis, biochemical characterisation and structural studies have led to a picture of the overall architecture of the particle, the partial structure of Pr55, and the subdomains involved in oligomerisation. Copyright 1998 John Wiley & Sons, Ltd.

Sequential steps in human immunodeficiency virus particle maturation revealed by alterations of individual Gag polyprotein cleavage sites

Retroviruses are produced as immature particles containing structural polyproteins, which are subsequently cleaved by the viral proteinase (PR). Extracellular maturation leads to condensation of the spherical core to a capsid shell formed by the capsid (CA) protein, which encases the genomic RNA complexed with nucleocapsid (NC) proteins. CA and NC are separated by a short spacer peptide (spacer peptide 1 [SP1]) on the human immunodeficiency virus type 1 (HIV-1) Gag polyprotein and released by sequential PR-mediated cleavages. To assess the role of individual cleavages in maturation, we constructed point mutations abolishing cleavage at these sites, either alone or in combination. When all three sites between CA and NC were mutated, immature particles containing stable CA-NC were observed, with no apparent effect on other cleavages. Delayed maturation with irregular morphology of the ribonucleoprotein core was observed when cleavage of SP1 from NC was prevented. Blocking the release of SP1 from CA, on the other hand, yielded normal condensation of the ribonucleoprotein core but prevented capsid condensation. A thin, electron-dense layer near the viral membrane was observed in this case, and mutant capsids were significantly less stable against detergent treatment than wild-type HIV-1. We suggest that HIV maturation is a sequential process controlled by the rate of cleavage at individual sites. Initial rapid cleavage at the C terminus of SP1 releases the RNA-binding NC protein and leads to condensation of the ribonucleoprotein core. Subsequently, CA is separated from the membrane by cleavage between the matrix protein and CA, and late release of SP1 from CA is required for capsid condensation.

Proteolytic refolding of the HIV-1 capsid protein amino-terminus facilitates viral core assembly

After budding, the human immunodeficiency virus (HIV) must 'mature' into an infectious viral particle. Viral maturation requires proteolytic processing of the Gag polyprotein at the matrix-capsid junction, which liberates the capsid (CA) domain to condense from the spherical protein coat of the immature virus into the conical core of the mature virus. We propose that upon proteolysis, the amino-terminal end of the capsid refolds into a beta-hairpin/helix structure that is stabilized by formation of a salt bridge between the processed amino-terminus (Pro1) and a highly conserved aspartate residue (Asp51). The refolded amino-terminus then creates a new CA-CA interface that is essential for assembling the condensed conical core. Consistent with this model, we found that recombinant capsid proteins with as few as four matrix residues fused to their amino-termini formed spheres in vitro, but that removing these residues refolded the capsid amino-terminus and redirected protein assembly from spheres to cylinders. Moreover, point mutations throughout the putative CA-CA interface blocked capsid assembly in vitro, core assembly in vivo and viral infectivity. Disruption of the conserved amino-terminal capsid salt bridge also abolished the infectivity of Moloney murine leukemia viral particles, suggesting that lenti- and oncoviruses mature via analogous pathways.

A putative alpha-helical structure which overlaps the capsid-p2 boundary in the human immunodeficiency virus type 1 Gag precursor is crucial for viral particle assembly

The capsid (CA) and nucleocapsid domains of the human immunodeficiency virus type 1 Gag polyprotein are separated by the p2 spacer peptide, which is essential for virus replication. Previous studies have revealed that p2 has an important role in virus morphogenesis. In this paper, we show that a crucial assembly determinant maps to the highly conserved N terminus of p2, which is predicted to form part of an alpha-helix that begins in CA. A mutational analysis indicates that the ability of the N terminus of p2 to adopt an alpha-helical structure is essential for its function during virus assembly. To prevent CA-p2 processing, it was necessary to mutate both the CA-p2 cleavage site and an internal cleavage site within p2. Virions produced by the double mutant lacked a conical core shell and instead contained a thin electron-dense shell about 10 nm underneath the virion membrane. These results suggest that p2 is transiently required for proper assembly, but needs to be removed from the C terminus of CA to weaken CA-CA interactions and allow the rearrangement of the virion core shell during virus maturation.

Cells with high cyclophilin A content support replication of human immunodeficiency virus type 1 Gag mutants with decreased ability to incorporate cyclophilin A

Gag polyprotein-mediated incorporation of cellular cyclophilin A (CyPA) into virions is essential for the formation of infectious human immunodeficiency virus type 1 (HIV-1) virions. Either a point mutation in Gag (P222A) or drugs which bind CyPA decrease virion incorporation of CyPA and interfere with HIV-1 replication. We have found that lymphoid cells varied greatly in their CyPA content and that cells with high CyPA content supported the replication of P222A HIV-1 Gag mutants. These experiments demonstrated that a higher cellular CyPA content of some cells was able to compensate for the decreased binding affinity of P222A mutant Gag for CyPA, allowing virus replication to occur.

Cryo-electron microscopy structure of yeast Ty retrotransposon virus-like particles

The virus-like particles (VLPs) produced by the yeast retrotransposon Ty1 are functionally related to retroviral cores. These particles are unusual in that they have variable radif. A paired mass-radius analysis of VLPs by scanning transmission electron microscopy showed that many of these particles form an icosahedral T-number series. Three-dimensional reconstruction to 38-A resolution from cryo-electron micrographs of T = 3 and T = 4 shells revealed that the single structural protein encoded by the TYA gene assembles into spiky shells from trimeric units.