Blocking of human immunodeficiency virus type-1 virion autolysis by autologous p2(gag) peptide

Our previous study suggested that the p2(gag) peptide, AEAMSQVTNTATIM, inhibits human immunodeficiency virus type 1 (HIV-1) protease (PR) activity in vitro. In this study, Ala substitutions (Met4Ala and Thr8Ala) and deletion of amino acid Asn9 within the nona p2(gag) peptide (AEAMSQVTN) were found to decrease the inhibitory effect on HIV-1 PR activity. Furthermore, treatment of PMA-activated latently infected T lymphocytes, ACH-2 cells, with the p2(gag) peptide (100 and 250 micro M) resulted in a decrease in the amount of p24(gag )in the resultant viral lysates derived from the cell-free supernatant. In addition, the HIV-1-Tat-p2(gag) fusion peptide was synthesized to effectively deliver the p2(gag) peptide into the cells. The fusion peptide was incorporated into chronically infected T lymphocytes, CEM/LAV-1 cells, as detected on indirect immunofluorescence analysis using anti-p2(gag) peptide monoclonal antibodies, which recognize the nona peptide (AEAMSQVTN) derived from the N-terminus of the p2(gag) peptide, and cleaved by HIV-1 PR in vitro. Treatment of CEM/LAV-1 cells with the fusion peptide also resulted in a decrease in the amount of p24(gag )in the resultant viral lysate derived from the cell-free supernatant. Taken together, these data suggest that the p2(gag) peptide consequently blocks the autolysis of HIV-1 virions for the conservation of viral species.     

Localization of the Vpx packaging signal within the C terminus of the human immunodeficiency virus type 2 Gag precursor protein.

Viral protein X (Vpx) is a human immunodeficiency virus type 2 (HIV-2) and simian immunodeficiency virus accessory protein that is packaged into virions in molar amounts equivalent to Gag proteins. To delineate the processes of virus assembly that mediate Vpx packaging, we used a recombinant vaccinia virus-T7 RNA polymerase system to facilitate Gag protein expression, particle assembly, and extracellular release. HIV genes were placed under control of the bacteriophage T7 promoter and transfected into HeLa cells expressing T7 RNA polymerase. Western immunoblot analysis detected p55gag and its cleavage products p39 and p27 in purified particles derived by expression of gag and gag-pol, respectively. In trans expression of vpx with either HIV-2 gag or gag-pol gave rise to virus-like particles that contained Vpx in amounts similar to that detected in HIV-2 virus produced from productively infected T cells. Using C-terminal deletion and truncation mutants of HIV-2 Gag, we mapped the p15 coding sequence for determinants of Vpx packaging. This analysis revealed a region (residues 439 to 497) downstream of the nucleocapsid protein (NC) required for incorporation of Vpx into virions. HIV-1/HIV-2 gag chimeras were constructed to further characterize the requirements for incorporation of Vpx into virions. Chimeric HIV-1/HIV-2 Gag particles consisting of HIV-1 p17 and p24 fused in frame at the C terminus with HIV-2 p15 effectively incorporate Vpx, while chimeric HIV-2/HIV-1 Gag particles consisting of HIV-2 p17 and p27 fused in frame at the C terminus with HIV-1 p15 do not. Expression of a 68-amino-acid sequence of HIV-2 containing residues 439 to 497 fused to the coding regions of HIV-1 p17 and p24 also produced virus-like particles capable of packaging Vpx in amounts similar to that of full-length HIV-2 Gag. Sucrose gradient analysis confirmed particle association of Vpx and Gag proteins. These results demonstrate that the HIV-2 Gag precursor (p55) regulates incorporation of Vpx into virions and indicates that the packaging signal is located within residues 439 to 497.     

Formation of virus-like particles by HIV-1 Gag proteins, expressed by a recombinant vaccinia virus

Monkey kidney cells CV-1 were infected with recombinant vaccinia virus carrying HIV-1 gag gene with a deletion of 230 nucleotide pairs from the 3'-terminus. The main gene product detected in the lysates of infected cells was the gag precursor rp50. The protein was accumulated on the cell membranes suggesting that it had a myristylated N-terminus, and was cleaved by a recombinant virus specific protease with the formation of two proteins, p17 and p24 corresponding in molecular masses to mature gag proteins. Virus-like particles similar to immature HIV virions were budding from the surface of infected cells. They look like the ring of optically dense material covered with a lipid bilayer, of the same size (100-120 nm) and of the same density in a sucrose gradient (1.16-1.18 g/ml) as HIV-1 virions. The particles contained rp50 and cellular heterogeneous RNA. Thus, the unprocessed gag precursor with deleted 77 amino acid residues from the C-terminus is able to form virus-like particles in the absence of env proteins and virus-specific RNA, and these particles are budding from the cell surface. The question about the use of extracellular Gag-particles for AIDS diagnostic work and construction of vaccines is discussed.     

Development of HIV/AIDS vaccine using chimeric gag-env virus-like particles

We attempted to develop a candidate HIV/AIDS vaccine, by using unprocessed HIV-2 gag pr45 precursor protein. We found that a 45 kDa unprocessed HIV-2 gag precursor protein (pr45), with a deletion of a portion of the viral protease, assembles as virus-like particles (VLP). We mapped the functional domain of HIV-2 gag VLP formation in order to find the minimum length of gag protein to form VLP. A series of deletion mutants was constructed by sequentially removing the C-terminal region of HIV-2 gag precursor protein and expressed truncated genes in Spodoptera frugiperda (SF) cells by infecting recombinant baculoviruses. We found that deletion of up to 143 amino acids at the C-terminus of HIV-2 gag, leaving 376 amino acids at the N-terminus of the protein, did not affect VLP formation. There is a proline-rich region at the amino acid positions 373 to 377 of HIV-2 gag, and replacement of these proline residues by site-directed mutagenesis completely abolished VLP assembly. Our data demonstrate that the C-terminal p12 region of HIV-2 gag precursor protein, and zinc finger domains, are dispensable for gag VLP assembly, but the presence of at least one of the three prolines at amino acid positions 373, 375 or 377 of HIV-2NIH-Z is required for VLP formation. Animals immunized with these gag particles produced high titer antibodies and Western blot analyses showed that anti-gag pr45 rabbit sera react with p17, p24 and p55 gag proteins of HIV-1. We then constructed chimeric gag genes, which carry the hypervariable V3 region of HIV-1 gp120, because the V3 loop is known to interact with chemokine receptor as a coreceptor, and known to induce the major neutralizing antibodies and stimulate the cytoxic T lymphocyte responses in humans and mice. We expressed chimeric fusion protein of HIV-2 gag with 3 tandem copies of consensus V3 domain that were derived from 245 different isolates of HIV-1. In addition, we also constructed and expressed chimeric fusion protein that contains HIV-2 gag with V3 domains of HIV-1IIIB, HIV-1MN, HIV-1SF2 and HIV-1RF. The chimeric gag-env particles had a spherical morphology, and the size was slightly larger than that of a gag particle. Immunoprecipitation and Western blot analyses show that these chimeric proteins were recognized by HIV-1 positive human sera and antisera raised against V3 peptides, as well as by rabbit anti-gp120 serum. We obtained virus neutralizing antibodies in rabbits by immunizing these gag-env VLPs. In addition, we found that gag-env chimeric VLPs induce a strong CTL activity against V3 peptide-treated target cells. Our results indicate that V3 peptides from all major clades of HIV-1 carried by HIV-2 gag can be used as a potential HIV/AIDS vaccine.     

The p6gag domain of human immunodeficiency virus type 1 is sufficient for the incorporation of Vpr into heterologous viral particles.

The vpr gene of human immunodeficiency virus type 1 (HIV-1) encodes a virion-associated regulatory protein. Mutagenesis has shown that the virion association of Vpr requires sequences near the C terminus of the HIV-1 Gag polyprotein Pr55gag. To investigate whether Vpr incorporation is mediated by a specific domain of Pr55gag, we examined the ability of chimeric HIV-1/Moloney murine leukemia virus (MLV) Gag polyproteins to direct the incorporation of Vpr. Vpr expressed in trans did not associate with particles formed by the authentic MLV Gag polyprotein or with particles formed by chimeric Gag polyproteins that had the matrix (MA) or capsid (CA) domain of MLV precisely replaced by the corresponding domain of HIV-1HXB2. By contrast, Vpr was efficiently incorporated upon replacement of the C-terminal nucleocapsid (NC) domain of the MLV Gag polyprotein with HIV-1 p15 sequences. Vpr was also efficiently incorporated into particles formed by a MLV Gag polyprotein that had the HIV-1 p6 domain fused to its C terminus. Furthermore, a deletion analysis revealed that a conserved region near the C terminus of the p6 domain is essential for Vpr incorporation, whereas sequences downstream of the conserved region are dispensable. These results show that a virion association motif for Vpr is located within residues 1 to 46 of p6.     

Active cAMP-dependent protein kinase incorporated within highly purified HIV-1 particles is required for viral infectivity and interacts with viral capsid protein

Host cell components, including protein kinases such as ERK-2/mitogen-activated protein kinase, incorporated within human immunodeficiency virus type 1 (HIV-1) virions play a pivotal role in the ability of HIV to infect and replicate in permissive cells. The present work provides evidence that the catalytic subunit of cAMP-dependent protein kinase (C-PKA) is packaged within HIV-1 virions as demonstrated using purified subtilisin-digested viral particles. Virus-associated C-PKA was shown to be enzymatically active and able to phosphorylate synthetic substrate in vitro. Suppression of virion-associated C-PKA activity by specific synthetic inhibitor had no apparent effect on viral precursor maturation and virus assembly. However, virus-associated C-PKA activity was demonstrated to regulate HIV-1 infectivity as assessed by single round infection assays performed by using viruses produced from cells expressing an inactive form of C-PKA. In addition, virus-associated C-PKA was found to co-precipitate with and to phosphorylate the CAp24gag protein. Altogether our results indicate that virus-associated C-PKA regulates HIV-1 infectivity, possibly by catalyzing phosphorylation of the viral CAp24gag protein.     

The p2 domain of human immunodeficiency virus type 1 Gag regulates sequential proteolytic processing and is required to produce fully infectious virions.

The proteolytic processing sites of the human immunodeficiency virus type 1 (HIV-1) Gag precursor are cleaved in a sequential manner by the viral protease. We investigated the factors that regulate sequential processing. When full-length Gag protein was digested with recombinant HIV-1 protease in vitro, four of the five major processing sites in Gag were cleaved at rates that differ by as much as 400-fold. Three of these four processing sites were cleaved independently of the others. The CA/p2 site, however, was cleaved approximately 20-fold faster when the adjacent downstream p2/NC site was blocked from cleavage or when the p2 domain of Gag was deleted. These results suggest that the presence of a C-terminal p2 tail on processing intermediates slows cleavage at the upstream CA/p2 site. We also found that lower pH selectively accelerated cleavage of the CA/p2 processing site in the full-length precursor and as a peptide primarily by a sequence-based mechanism rather than by a change in protein conformation. Deletion of the p2 domain of Gag results in released virions that are less infectious despite the presence of the processed final products of Gag. These findings suggest that the p2 domain of HIV-1 Gag regulates the rate of cleavage at the CA/p2 processing site during sequential processing in vitro and in infected cells and that p2 may function in the proper assembly of virions.     

Cyclophilin A-independent replication of a human immunodeficiency virus type 1 isolate carrying a small portion of the simian immunodeficiency virus SIV(MAC) gag capsid region

Hybrid viruses between human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus strain mac (SIV(MAC)) are invaluable to various fields of HIV-1 research. To date, however, no replication-competent HIV-1 strain containing the gag capsid (CA) region of SIV(MAC) has been reported. To obtain the viable gag gene chimeric virus in an HIV-1 background, seven HIV-1 strains carrying a part of SIV(MAC) CA or a small deletion in the CA region were constructed and examined for their biological and biochemical characteristics. While all the recombinants and mutants were found to express Gag and to produce progeny virions on transfection, only one chimeric virus, which has 18 bp of SIV gag CA sequence in place of the region encoding the HIV-1 CA cyclophilin A (CyPA)-binding loop, was infectious for human cell lines. Although this chimeric virus was unable to grow in monkey lymphocytic cells like wild-type (wt) HIV-1 did, it grew much better than wt virus in the presence of cyclosporin A in a human cell line which supports HIV-1 replication in a CyPA-dependent manner. These results indicate that the transfer of a small portion of the SIV(MAC) CA region to HIV-1 could confer the CyPA-independent replication potential of SIV(MAC) on the virus.     

Variability at human immunodeficiency virus type 1 subtype C protease cleavage sites: an indication of viral fitness?

Naturally occurring polymorphisms in the protease of human immunodeficiency virus type 1 (HIV-1) subtype C would be expected to lead to adaptive (compensatory) changes in protease cleavage sites. To test this hypothesis, we examined the prevalences and patterns of cleavage site polymorphisms in the Gag, Gag-Pol, and Nef cleavage sites of C compared to those in non-C subtypes. Codon-based maximum-likelihood methods were used to assess the natural selection and evolutionary history of individual cleavage sites. Seven cleavage sites (p17/p24, p24/p2, NC/p1, NC/TFP, PR/RT, RT/p66, and p66/IN) were well conserved over time and in all HIV-1 subtypes. One site (p1/p6(gag)) exhibited moderate variation, and four sites (p2/NC, TFP/p6(pol), p6(pol)/PR, and Nef) were highly variable, both within and between subtypes. Three of the variable sites are known to be major determinants of polyprotein processing and virion production. P2/NC controls the rate and order of cleavage, p6(gag) is an important phosphoprotein required for virion release, and TFP/p6(pol), a novel cleavage site in the transframe domain, influences the specificity of Gag-Pol processing and the activation of protease. Overall, 58.3% of the 12 HIV-1 cleavage sites were significantly more diverse in C than in B viruses. When analyzed as a single concatenated fragment of 360 bp, 96.0% of group M cleavage site sequences fell into subtype-specific phylogenetic clusters, suggesting that they coevolved with the virus. Natural variation at C cleavage sites may play an important role, not only in regulation of the viral cycle but also in disease progression and response to therapy.     

Inhibitors of human immunodeficiency virus type 1 zinc fingers prevent normal processing of gag precursors and result in the release of noninfectious virus particles.

The Cys-Xaa2-Cys-Xaa4-His-Xaa4-Cys zinc fingers of retroviral nucleocapsid proteins are prime antiviral targets because of conservation of the Cys and His chelating residues and the absolute requirement of these fingers in both early and late phases of retroviral replication. We previously reported that certain disulfide-substituted benzamides (DIBAs) chemically modify the Cys residues of the fingers, resulting in inhibition of human immunodeficiency virus type 1 (HIV-1) replication (W. G. Rice, J. G. Supko, L. Malspeis, R. W. Buckheit, Jr., D. Clanton, M. Bu, L. Graham, C. A. Schaeffer, J. A. Turpin, J. Domagala, R. Gogliotti, J. P. Bader, S. M. Halliday, L. Coren, R. C. Sowder II, L. O. Arthur, and L. E. Henderson, Science 270:1194-1197, 1995). We now examine the consequences of the interaction of DIBAs with the zinc fingers of the HIV-1 p7 nucleocapsid protein and its Pr55gag precursor. In HIV-1-infected U1 cells, DIBAs inhibited the release of infectious virions, and even under conditions in which virion particles were produced, the particles were noninfectious. DIBAs caused abnormal processing of Gag precursors, and the inhibitory effect on processing was not due to inhibition of the HIV-1 protease enzyme or Pr55gag myristoylation. Rather, the defect in processing was due to the formation of intermolecular cross-linkages among the zinc fingers of adjacent Gag molecules, rendering the precursors no longer recognizable by HIV-1 protease. Likewise, DIBAs caused intermolecular cross-linkage among recombinant Pr55gag packaged into pseudovirions, thereby generating modified precursors that were resistant to the action of protease. Thus, DIBAs chemically modified the mutationally intolerant retroviral zinc fingers in infected cells, interrupting protease-mediated maturation of virions and leading ultimately to the production of compromised virions.