The matrix protein of human immunodeficiency virus type 1 is required for incorporation of viral envelope protein into mature virions

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.

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.

Further evidence for hexagonal organization of HIV gag protein in prebudding assemblies and immature virus-like particles

The fullerene-like model for the organization of HIV gag encoded precursor pr55gag was based on the study of prebudding assemblies at the plasma membrane of cells infected with a recombinant baculovirus expressing HIV-1 gag protein. The objective of the present study was to support the model by image processing of virus-like particles (VLP). In this work we used VLP purified by density gradient centrifugation, which caused partial or occasionally complete loss of the lipid bilayer in some VLP without the use of detergent. In addition more plasma membrane-associated pr55gag protein assemblies were processed. Image processing of negatively stained specimens revealed the presence of threefold symmetry and a hexagonal network of rings with a resolution of 29 A in VLP and better than 25 A in membrane associated assemblies. The center-to-center spacing of the rings was 67 A in VLP and 70 A in membrane assemblies. Patches of gag protein oligomers at the plasma membrane were usually round and varying in size, but some of them were triangular. Indication of triangular-shaped gag protein assemblies was also seen in partly dissociated VLP. Since the hexagonal network is formed by the uncleaved gag polyprotein, we conclude that the threefold symmetry applies to all domains including p24gag. The presence of threefold symmetry and the hexagonal network in VLP are consistent with the hypothesis that immature HIV particles possess icosahedral symmetry.

cDNA Encoding a Single-Chain Antibody to HIV p17 with Cytoplasmic or Nuclear Retention Signals Inhibits HIV-1 Replication

HIV-1 gag p17 protein is an attractive target for molecular intervention, because it is involved in the viral replication cycle at both the pre- and postintegration levels. In the present experiments, we targeted p17 by intracellularly expressing a cDNA encoding an Ab to p17. cDNA from a hybridoma-secreting Ab to p17 was cloned, sequenced, reconstructed as a single-chain Ab fragment (scFv), and expressed in the cytoplasm or nucleus with appropriate retention signals. The expressed scFvs had no effect on T cell growth or CD4 expression and bound specifically to HIV-1 p17. Human CD4+ Jurkat T cells that expressed scFvs and were infected with HIV-1 showed a marked reduction in virus replication compared with cells expressing vector alone. The inhibition of virus replication was more pronounced when scFvs were expressed in the cytoplasm rather than the nucleus. From these studies, we conclude that the intracellular expression of a single-chain Ab to p17 inhibits HIV replication; in addition, the degree of inhibition is related to the intracellular targeting site.

Detection of a trimeric human immunodeficiency virus type 1 Gag intermediate is dependent on sequences in the matrix protein, p17

Previous studies have shown that single amino acid changes in the amino-terminal matrix (MA) domain, p17, of the human immunodeficiency virus type 1 Gag precursor Pr55, can abrogate virion particle assembly. In the three-dimensional structure of MA such mutations lie in a single helix spanning residues 54 to 68, suggesting a key role for this helix in the assembly process. The fundamental nature of this involvement, however, remains poorly understood. In the present study, the essential features of the MA helix required for virus assembly have been investigated through the analysis of a further 15 site-directed mutants. With previous mutants that failed to assemble, residues mapped as critical for assembly were all located on the hydrophobic face of the helix and had a key role in stabilizing the trimeric interface. This implies a role for the MA trimer in virus assembly. We support this interpretation by showing that purified MA is trimeric in solution and that mutations that prevent virus assembly also prevent trimerization. Trimerization in solution was also a property of a larger MA-capsid (CA) Gag molecule, while under the same conditions CA only was a monomer. These data suggest that Gag trimerization driven by the MA domain is an intermediate stage in normal virion assembly and that it relies, in turn, on an MA conformation dependent on the hydrophobic core of the molecule.

Retention of the human immunodeficiency virus type 1 envelope glycoprotein in the endoplasmic reticulum does not redirect virus assembly from the plasma membrane

The envelope glycoprotein (Env) of human immunodeficiency virus type 1 (HIV-1) has been shown to redirect the site of virus assembly in polarized epithelial cells. To test whether localization of the glycoprotein exclusively to the endoplasmic reticulum (ER) could redirect virus assembly to that organelle in nonpolarized cells, an ER -retrieval signal was engineered into an epitope-tagged variant of Env. The epitope tag, attached to the C terminus of Env, did not affect the normal maturation and transport of the glycoprotein or the incorporation of Env into virions. The epitope-tagged Env was also capable of mediating syncytium formation and virus entry with a similar efficiency to that of wild-type Env. When the epitope was modified to contain a consensus K(X)KXX ER retrieval signal, however, the glycoprotein was no longer proteolytically processed into its surface and transmembrane subunits and Env could not be detected at the cell surface by biotinylation. Endoglycosidase H analysis revealed that the modified Env was not transported to the Golgi apparatus. Immunofluorescent staining patterns were also consistent with the exclusion of Env from the Golgi. As expected, cells expressing the modified Env failed to form syncytia with CD4(+) permissive cells. Despite this tight localization of Env to the ER, when the modified Env was expressed in the context of virus, virions continued to be produced efficiently from the plasma membrane of transfected cells. However, these virions contained no detectable glycoprotein and were noninfectious. Electron microscopy revealed virus budding from the plasma membrane of these cells, but no virus was seen assembling at the ER membrane and no assembled virions were found within the cell. These results suggest that the accumulation of Env in an intracellular compartment is not sufficient to redirect the assembly of HIV Gag in nonpolarized cells.

The matrix protein of HIV-1 is not sufficient for assembly and release of virus-like particles

The matrix (MA) proteins of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) are known to be important for the targeting and assembly of lentiviral proteins. The objective of the present study was to determine whether the MA protein of HIV-1 was sufficient for particle assembly and release. Eukaryotic expression of wild-type HIV-1 Gag-Pol, HIV-1 MA alone, or SIV MA alone was analyzed with radio-immunoprecipitation, density centrifugation, and a protease protection assay. Cells that expressed HIV-1 Gag-Pol or SIV MA alone released virus-like particles (VLPs) with sucrose gradient densities of 1.15 or 1.12 g/ml, respectively. The MA and/or capsid proteins in these particles were protected from protease degradation, indicating the presence of a protective outer membrane. Expression of HIV-1 MA protein alone resulted in release of MA which pelleted through a 20% sucrose cushion but failed to enter a 20-60% sucrose gradient and was not protected from protease degradation. The MA protein of SIV was previously reported to be sufficient for production of VLPs (S. A. Gonzalez, H, K, Affrachino, H. R. Gelderblom, and A. Burney. Virology 194, 548-556, 1993; V. Liska, D. Spehner, M. Mehtali, D. Schmitt, A. Kirn, and A. M. Aubertin. J. Gen. Virol. 75, 2955-2962, 1994). Our study confirmed that result, but indicated that the MA protein of HIV-1 was not sufficient to assemble and release VLPs.

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.

Efficient HIV-1 replication can occur in the absence of the viral matrix protein

Matrix (MA), a major structural protein of retroviruses, is thought to play a critical role in several steps of the HIV-1 replication cycle, including the plasma membrane targeting of Gag, the incorporation of envelope (Env) glycoproteins into nascent particles, and the nuclear import of the viral genome in non-dividing cells. We now show that the entire MA protein is dispensable for the incorporation of HIV-1 Env glycoproteins with a shortened cytoplasmic domain. Furthermore, efficient HIV-1 replication in the absence of up to 90% of MA was observed in a cell line in which the cytoplasmic domain of Env is not required. Additional compensatory changes in Gag permitted efficient virus replication even if all of MA was replaced by a heterologous membrane targeting signal. Viruses which lacked the globular domain of MA but retained its N-terminal myristyl anchor exhibited an increased ability to form both extracellular and intracellular virus particles, consistent with a myristyl switch model of Gag membrane targeting. Pseudotyped HIV-1 particles that lacked the structurally conserved globular head of MA efficiently infected macrophages, indicating that MA is dispensable for nuclear import in terminally differentiated cells.

Role of matrix in an early postentry step in the human immunodeficiency virus type 1 life cycle

The matrix protein of human immunodeficiency virus type 1 (HIV-1) has been reported to play a crucial role in the targeting of the Gag polyprotein precursor to the plasma membrane and in the incorporation of viral envelope glycoproteins into budding virions. In this report, we present evidence that mutation of a highly conserved Leu at matrix amino acid 20 blocks or markedly delays virus replication in a range of cell types, including T-cell lines, primary human peripheral blood mononuclear cells, and monocyte-derived macrophages. These mutations do not impair virus assembly and release, RNA encapsidation, or envelope glycoprotein incorporation into virions but rather cause significant defects in an early step in the virus life cycle, as measured by single-cycle infectivity assays and the analysis of viral DNA synthesis early postinfection. This infectivity defect is independent of the type of envelope glycoprotein carried on mutant virions; similar results are obtained in pseudotyping experiments using wild-type or truncated HIV-1 envelope glycoproteins, the amphotropic murine leukemia virus envelope, or the vesicular stomatitis G protein. Intriguingly, matrix residue 20 mutations also increase the apparent binding of Gag to membrane, accelerate the kinetics of Gag processing, and induce defects in endogenous reverse transcriptase activity without affecting virion density or morphology. These results help elucidate the function of matrix in HIV-1 replication.

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.

Human immunodeficiency virus type 1 matrix protein interacts with cellular protein HO3

The matrix (MA) protein of human immunodeficiency virus type 1 (HIV-1) plays a critical role in virion morphogenesis and fulfills important functions during the early steps of infection. In an effort to identify cellular partners of MA, a Saccharomyces cerevisiae two-hybrid screen was utilized. A specific interaction between MA and HO3, a putative histidyl-tRNA synthetase, was demonstrated in this system. HO3-specific mRNA was detected in several tissues relevant for HIV infection, such as spleen, thymus, and peripheral blood lymphocytes, as well as in a number of T-lymphoid-cell lines. The binding of MA to HO3 was confirmed in transfected cells by coimmunoprecipitation. This interaction was abrogated by replacing two lysine residues at positions 26 and 27 of MA by threonine (MA(KK27TT)). HO3 localized both to the cytoplasm and to the nucleus of acutely transfected 293T cells. When overexpressed in HIV-1-producing cells, HO3 was incorporated into wild-type virions but not in ones containing the dilysine-mutated variant of MA. Correspondingly, overexpression of HO3 in virus producer cells enhanced the infectivity of wild-type but not MA(KK27AA) HIV-1 particles. The stimulating effect of HO3 was independent from the presence of Envelope, Vpr, or Vpu. Taken together, these results suggest that HO3, through its recognition of MA, plays a role in the life cycle of HIV-1.

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.

Gene therapy for infectious diseases

Gene therapy is being investigated as an alternative treatment for a wide range of infectious diseases that are not amenable to standard clinical management. Approaches to gene therapy for infectious diseases can be divided into three broad categories: (i) gene therapies based on nucleic acid moieties, including antisense DNA or RNA, RNA decoys, and catalytic RNA moieties (ribozymes); (ii) protein approaches such as transdominant negative proteins and single-chain antibodies; and (iii) immunotherapeutic approaches involving genetic vaccines or pathogen-specific lymphocytes. It is further possible that combinations of the aforementioned approaches will be used simultaneously to inhibit multiple stages of the life cycle of the infectious agent.

The HIV-1 matrix domain of Gag is required for Vpu responsiveness during particle release

HIV-1 viral protein U (Vpu) facilitates virus particle release. To determine whether Gag is sufficient for generation of a target for Vpu-mediated particle release, we expressed HIV-1 Gag protein in the absence of the other viral genes. The resulting particles were still Vpu responsive. Mutational analysis of Gag indicated that the matrix domain (MA) is required for Vpu responsiveness. However, additional mutations in other domains of Gag, which affect the formation of stable virus particles, also abrogate Vpu responsiveness on total Gag release. Coexpression of the wild-type gag gene and a gag mutant lacking the MA domain renders the MA- mutant Vpu responsive. This indicates that Gag molecules lacking MA are still incorporated into particles through association with wild-type Gag molecules and that the resulting composite particles are sufficient for Vpu-mediated exit.

The entire SU subunit is required for the incorporation of the HIV-1 envelope glycoprotein complex into virions

A modified envelope glycoprotein of the human immunodeficiency virus type 1 (HIV-1) containing an intact TM subunit, but lacking most of the gp120/SU subunit was transported and expressed on the membrane of COS-1 cells. However, this deleted glycoprotein, failed to be incorporated into the budding viral particles. This suggested that a particular domain(s) of the gp120/SU glycoprotein subunit could be required for envelope incorporation. To explore this possibilty, we constructed envelope genes containing specific domains of the SU protein in-frame with the TM subunit. Transient expression studies indicated that any envelope primary translation product containing one or more of the gp 20/SU variable domains and the entire gp41/TM protein was transported and stably expressed on the cell surface. However, efficient proteolytic processing of these Env precursors into gp41, was not observed. The addition of more than 90% of the SU sequences into the deleted Env product, including the five variable domains, were insufficient to promote incorporation of this glycoprotein precursor into virions. These results suggest that the native conformation of the SU subunit is an essential requirement for the efficient incorporation of the Env complex into virons. The C1 domain of the SU glycoprotein subunit constitutes an important determinant that makes the envelope complex assembly-competent, but, by itself, it is not sufficient to drive this process.

Human immunodeficiency virus matrix tyrosine phosphorylation: characterization of the kinase and its substrate requirements

During virus assembly, a subset of human immunodeficiency virus (HIV) matrix (MA) molecules is phosphorylated on C-terminal tyrosine. This modification facilitates infection of nondividing cells by allowing for the recruitment of the karyophilic MA into the viral core and preintegration complex. MA tyrosine phosphorylation is accomplished by a cellular protein kinase which is incorporated into virions. In this study, we have investigated the nature of this enzyme as well as the determinants of MA necessary for its phosphorylation. Employing an in vitro kinase assay, we found that the MA tyrosine kinase activity is present in various cultured cell lines including CEM and SupT1 T-lymphoid cells, Namalwa B cells, 293 and CV-1 kidney fibroblasts, and P4 HeLa cells. In addition, it could be detected in platelets, macrophages, and activated peripheral blood lymphocytes (PBLs) but not in erythrocytes and resting PBLs isolated from human blood. Subcellular localization of the kinase activity by cell fractionation demonstrated that it is enriched in cellular membranes. In HIV type 2 (HIV-2) particles, the MA tyrosine kinase is associated with the inner leaflet of the viral membrane, while the tyrosine-phosphorylated MA is localized to the core. Individual mutations of each of the last eight residues immediately upstream of the C-terminal tyrosine (Y132) of HIV-1 MA did not prevent Y132 phosphorylation, suggesting that the kinase does not require a highly specific sequence adjacent to the C-terminal tyrosine. Confirming this, we found that the MA of murine leukemia virus, the sequence of which is only moderately homologous to that of HIV-1 and HIV-2 MA, is also C-terminally tyrosine phosphorylated.

Membrane binding of human immunodeficiency virus type 1 matrix protein in vivo supports a conformational myristyl switch mechanism

The interaction of the human immunodeficiency virus (HIV) Gag protein with the plasma membrane of a cell is a critical event in the assembly of HIV particles. The matrix protein region (MA) of HIV type 1 (HIV-1) Pr55Gag has previously been demonstrated to confer membrane-binding properties on the precursor polyprotein. Both the myristic acid moiety and additional determinants within MA are essential for plasma membrane binding and subsequent particle formation. In this study, we demonstrated the myristylation-dependent membrane interaction of MA in an in vivo membrane-binding assay. When expressed within mammalian cells, MA was found both in association with cellular membranes and in a membrane-free form. In contrast, the intact precursor Pr55Gag molecule analyzed in an identical manner was found almost exclusively bound to membranes. Both membrane-bound and membrane-free forms of MA were myristylated and phosphorylated. Differential membrane binding was not due to the formation of multimers, as dimeric and trimeric forms of MA were also found in both membrane-bound and membrane-free fractions. To define the requirements for membrane binding of MA, we analyzed the membrane binding of a series of MA deletion mutants. Surprisingly, deletions within alpha-helical regions forming the globular head of MA led to a dramatic increase in overall membrane binding. The stability of the MA-membrane interaction was not affected by these deletions, and no deletion eliminated membrane binding of the molecule. These results establish that myristic acid is a primary determinant of the stability of the Gag protein-membrane interaction and provide support for the hypothesis that a significant proportion of HIV-1 MA molecules may adopt a conformation in which myristic acid is hidden and unavailable for membrane interaction.

Blockage of N-myristoylation of HIV-1 gag induces the production of impotent progeny virus

The role of the N-myristoylation of the human immunodeficiency virus type 1 (HIV-1) gag protein in ACH-2 cells was studied. The infectivity of HIV-1 from the cells stimulated with phorbol 12-myristate 13-acetate (PMA) was suppressed by pretreatment with N-myristoyl glycinal diethylacetal (N-Myr-GOA), a potent N-myristoylation inhibitor, and the blockage of myristoylation resulted in accumulation of immature gag precursors. The viral particles which budded from the non-N-Myr-GOA-treated ACH-2 cells stimulated with PMA exhibited a typical viral phenotype, whereas those which budded from the N-Myr-GOA-treated ACH-2 cells stimulated with PMA were twisted, as observed electron microscopically. In electron microscopic analyses with gold-labeled monoclonal antibodies to gag and env, gag and env were detected adjacent to each other in the PMA-stimulated ACH-2, but no env was detected in the cells treated with N-Myr-GOA. Taken together, the results suggest that the myristoylation of HIV-1 gag seems to be responsible for both maturation of gag and acquisition of HIV-1 infectivity.

Increased incorporation of chimeric human immunodeficiency virus type 1 gp120 proteins into Pr55gag virus-like particles by an Epstein-Barr virus gp220/350-derived transmembrane domain

Noninfectious Pr55gag virus-like particles containing high quantities of oligomeric human immunodeficiency virus type 1 (HIV-1) envelope (Env) proteins represent potential candidate immunogens for a vaccine against HIV-1 infection. Thus, chimeric env genes were constructed encoding the HIV-1 exterior glycoprotein gp120 which was covalently linked at different C-terminal positions to a transmembrane domain (TM) from the Epstein-Barr virus (EBV) major Env glycoprotein gp220/ 350. All chimeric Env-TM polypeptides as well as the wild-type HIV Env proteins were equally produced and incorporated at the outer surface of insect cells using the baculovirus expression system. In the presence of coexpressed HIV Pr55gag polyproteins significantly decreased amounts of wild-type Env proteins were presented at the cell surface, whereas the membrane incorporation of the Env-TM chimeras was not affected. Biochemical and immunoelectron microscopical analysis of particles that were efficiently released from these cells displayed the incorporation of both wild-type Env and chimeric Env-TM proteins on the surface of VLPs. However, the quantities of particle-associated chimeric Env-TM proteins exceeded those of incorporated wild-type Env proteins by a factor of 5-10. Chemical cross-linking and subsequent polyacrylamide gel electrophoresis of VLP-entrapped Env proteins revealed that the chimeric Env-TM proteins form homodimers and a higher-order oligomer, similar to that observed for wild-type Env proteins. Thus, the results of this study clearly demonstrate that the replacement of the gp41 transmembrane protein of gp160 by a heterologous, EBV gp220/350-derived membrane anchor provides an effective strategy to incorporate high quantities of oligomeric HIV gp120 proteins on the surface of Pr55gag virus-like particles.

Mutagenesis analysis of the murine leukemia virus matrix protein: identification of regions important for membrane localization and intracellular transport

We have created two sets of substitution mutations in the Moloney murine leukemia virus (Mo-MuLV) matrix protein in order to identify domains involved in association with the plasma membrane and in incorporation of the viral envelope glycoproteins into virus particles. The first set of mutations was targeted at putative membrane-associating regions similar to those of the human immunodeficiency virus type 1 matrix protein, which include a polybasic region at the N terminus of the Mo-MuLV matrix protein and two regions predicted to form beta strands. The second set of mutations was created within hydrophobic residues to test for the production of virus particles lacking envelope proteins, with the speculation of an involvement of the membrane-spanning region of the envelope protein in incorporation into virus particles. We have found that mutation of the N-terminal polybasic region redirected virus assembly to the cytoplasm, and we show that tryptophan residues may also play a significant role in the intracellular transport of the matrix protein. In total, 21 mutants of the Mo-MuLV matrix protein were produced, but we did not observe any mutant virus particles lacking the envelope glycoproteins, suggesting that a direct interaction between the Mo-MuLV matrix protein and envelope proteins either may not exist or may occur through multiple redundant interactions.

The Nef protein of human immunodeficiency virus type 1 enhances serine phosphorylation of the viral matrix

The human immunodeficiency virus type 1 matrix (MA) protein is phosphorylated during virion maturation on its C-terminal tyrosine and on several serine residues. Whereas MA tyrosine phosphorylation facilitates viral nuclear import, the significance of MA serine phosphorylation remains unclear. Here, we report that MA serine but not tyrosine phosphorylation is strongly enhanced by Nef. Mutations that abrogated the membrane association of Nef and its ability to bind a cellular serine/threonine kinase greatly diminished the extent of virion MA serine phosphorylation. Correspondingly, a protein kinase coimmunoprecipitated with Nef could phosphorylate MA on serine in vitro, producing a phosphopeptide pattern reminiscent of that of virion MA. Recombinant p21-activated kinase hPAK65, a recently proposed relative of the Nef-associated kinase, achieved a comparable result. Taken together, these data suggest that MA is a target of the Nef-associated serine kinase.

Characterization of human immunodeficiency virus type 1 matrix revertants: effects on virus assembly, Gag processing, and Env incorporation into virions

The matrix protein of human immunodeficiency virus type 1 (HIV-1) has been postulated to serve a variety of functions in the virus life cycle. Previously, we introduced a large number of mutations into the HIV-1 matrix and determined the effects on virus replication. These studies identified domains involved in virus assembly and release and envelope glycoprotein incorporation into virions. Here we describe the identification and characterization of viral revertants containing second-site changes in the matrix which compensate for the effects of four of the original mutations on matrix function. Specifically, mutations at matrix residues 4 and 6 severely impaired virus assembly and release; substitutions at residues 4 and 6 reversed the phenotype of the amino acid 4 change while second-site mutations at matrix positions 10, 69, and 97 partially or fully reversed the phenotype of the amino acid 6 substitution. A mutation at matrix residue 62 reversed the effect of a position 34 change which blocks envelope glycoprotein incorporation into virions, and substitutions at residues 27 and 51 reversed the phenotype of a position 86 mutation which redirects virus assembly to the cytoplasm. In addition to determining the effects of the compensatory changes in the context of the original mutations, we also introduced and analyzed the second-site changes alone in the context of the wild-type molecular clone. The data presented here define potential intermolecular and intramolecular interactions which occur in the matrix during the virus life cycle and have implications for our understanding of the relationship between matrix structure and function.

Contribution of virion ICAM-1 to human immunodeficiency virus infectivity and sensitivity to neutralization

Incorporation of the intercellular adhesion molecule ICAM-1 into human immunodeficiency virus type 1 (HIV-1) particles increased virus infectivity on peripheral blood mononuclear cells (PBMCs) by two- to sevenfold. The degree of ICAM-1-mediated enhancement was greater for viruses bearing envelope glycoproteins derived from primary HIV-1 isolates than for those bearing envelope glycoproteins from laboratory-adapted strains. Treatment of target PBMCs with an antibody against LFA-1, a principal ICAM-1 receptor, was able to nullify the ICAM-1-mediated enhancement. The incorporation of ICAM-1 rendered HIV-1 virions less susceptible to neutralization by a monoclonal antibody directed against the viral envelope glycoproteins. Surprisingly, an antibody against ICAM-1 completely neutralized infection by ICAM-1-containing viruses, reducing the efficiency of virus entry by almost 100-fold. Thus, HIV-1 neutralization by an ICAM-1-directed antibody involves more than an inhibition of the contribution of ICAM-1 to virus entry.

Structure-function studies of the human immunodeficiency virus type 1 matrix protein, p17

The human immunodeficiency virus type 1 (HIV-1) matrix protein, p17, plays important roles in both the early and late stages of the viral life cycle. Using our previously determined solution structure of p17, we have undertaken a rational mutagenesis program aimed at mapping structure-function relationships within the molecule. Amino acids hypothesized to be important for p17 function were mutated and examined for effect in an infectious proviral clone of HIV-1. In parallel, we analyzed by nuclear magnetic resonance spectroscopy the structure of recombinant p17 protein containing such substitutions. These analyses identified three classes of mutants that were defective in viral replication: (i) proteins containing substitutions at internal residues that grossly distorted the structure of recombinant p17 and prevented viral particle formation, (ii) mutations at putative p17 trimer interfaces that allowed correct folding of recombinant protein but produced virus that was defective in particle assembly, and (iii) substitution of basic residues in helix A that caused some relocation of virus assembly to intracellular locations and produced normally budded virions that were completely noninfectious.

Truncation of the human immunodeficiency virus type 1 envelope glycoprotein allows efficient pseudotyping of Moloney murine leukemia virus particles and gene transfer into CD4+ cells

Human immunodeficiency virus type 1 (HIV-1) can readily accept envelope (Env) glycoproteins from distantly related retroviruses. However, we previously showed that the HIV-1 Env glycoprotein complex is excluded even from particles formed by the Gag proteins of another lentivirus, visna virus, unless the matrix domain of the visna virus Gag polyprotein is replaced by that of HIV-1. We also showed that the integrity of the HIV-1 matrix domain is critical for the incorporation of wild-type HIV-1 Env protein but not for the incorporation of a truncated form which lacks the 144 C-terminal amino acids of the cytoplasmic domain of the transmembrane glycoprotein. We report here that the C-terminal truncation of the transmembrane glycoprotein also allows the efficient incorporation of HIV-1 Env proteins into viral particles formed by the Gag proteins of the widely divergent Moloney murine leukemia virus (Mo-MLV). Additionally, pseudotyping of a Mo-MLV-based vector with the truncated rather than the full-length HIV-1 Env allowed efficient transduction of human CD4+ cells. These results establish that Mo-MLV-based vectors can be used to target cells susceptible to infection by HIV-1.

Microvesicles are a source of contaminating cellular proteins found in purified HIV-1 preparations

Identification and quantitation of cellular proteins associated with HIV-1 particles are complicated by the presence of nonvirion-associated cellular proteins that copurify with virions. Many cellular proteins are associated with nonviral particles that bud from the surface of cells called microvesicles. Microvesicles band in sucrose gradients in a range of densities that includes the same density as retroviruses. To characterize these microvesicles, HIV-1-infected and uninfected human T-cell lines were propagated and virus and microvesicles were purified from clarified cell culture supernatants by sucrose density gradient centrifugation or centrifugation through 20% sucrose pads. Microvesicles were found to contain various proteins, including HLA DR and beta 2-M, and a substantial amount of RNA and DNA. The concentrations of HIV-1 p24CA, HLA DR and beta 2-microglobulin (beta 2-M) were determined by radioimmunoassay. The ratios of HIV-1 p24CA to HLA DR and beta 2-M were found to vary with respect to the HIV-1 isolate, host cell, and other factors. Electron microscopic analysis of microvesicles revealed that they consisted of particles of various sizes and morphologies. Although HIV-1 particles are known to contain some cellular proteins, microvesicles from HIV-1 infected H9 cells appeared to contain little or no HIV-1 gp120SU.

Pleiotropic mutations in the HIV-1 matrix protein that affect diverse steps in replication

The matrix domain of the Gag precursor protein, and the mature matrix protein, which is derived from processing of the Gag precursor, functions in several steps of the human immunodeficiency virus type-1 (HIV-1) life cycle. We made numerous mutations throughout the matrix protein and identified three mutants in the N-terminal portion of the matrix that drastically diminish the ability of the virus to replicate. Each of these replication-defective mutants was unable to acquire efficiently the envelope glycoprotein of HIV-1. To determine whether these same mutations affect other steps in viral replication we pseudotyped mutant particles with the envelope glycoprotein from an amphotropic murine leukemia virus. Each of these mutants was also hampered in other steps in virus replication. Two mutants were defective in entry or uncoating, and the third was hampered in a step following reverse transcription. Since viral replication was analyzed under conditions in which the nuclear localization function of the matrix protein is not required, the matrix protein may be required for an additional replication step following reverse transcription.

Inhibition of early and late events of the HIV-1 replication cycle by cytoplasmic Fab intrabodies against the matrix protein, p17

BACKGROUND

The HIV-1 matrix (MA) protein, p17, contains two subcellular localization signals that facilitate both nuclear import of the viral preintegration complex early during infection and virus particle assembly late in infection. The dual role of MA in both the afferent and efferent arms of the HIV-1 life cycle makes it an important target for intracellular immunization-based gene therapy strategies.

MATERIALS AND METHODS

Here we report, using a new bicistronic vector, that an intracellular Fab antibody, or Fab intrabody, directed against a carboxy-terminal epitope of MA from the Clade B HIV-1 genotype, can inhibit HIV-1 infection when expressed in the cytoplasm of actively dividing CD4+ T cells.

RESULTS

Marked inhibition of proviral gene expression occurred when single-round HIV-1 CAT virus was used for infections. In challenge experiments using both laboratory strains and syncytium-inducing primary isolates of HIV-1, a substantial reduction in the infectivity of virions released from the cells was also observed.

CONCLUSIONS

This novel strategy of simultaneously blocking early and late events of the HIV-1 life cycle may prove useful in clinical gene therapy approaches for the treatment of HIV-1 infection and AIDS, particularly when combined with genetic or pharmacologic-based strategies that inhibit other HIV-1 target molecules simultaneously.

Mutations in the matrix protein of human immunodeficiency virus type 1 inhibit surface expression and virion incorporation of viral envelope glycoproteins in CD4+ T lymphocytes

Highly conserved amino acids in the second helix structure of the human immunodeficiency virus type 1 (HIV-1) MA protein were identified to be critical for the incorporation of viral Env proteins into HIV-1 virions from transfected COS-7 cells. The effects of these MA mutations on viral replication in the HIV-1 natural target cells, CD4+ T lymphocytes, were evaluated by using a newly developed system. In CD4+ T lymphocytes, mutations in the MA domain of HIV-1 Gag also inhibited the incorporation of viral Env proteins into mature HIV-1 virions. Furthermore, mutations in the MA domain of HIV-1 Gag reduced surface expression of viral Env proteins in CD4+ T lymphocytes. The synthesis of gp160 and cleavage of gp160 to gp120 were not significantly affected by MA mutations. On the other hand, the stability of gp120 in MA mutant-infected cells was significantly reduced compared to that in the parental wild-type virus-infected cells. These results suggest that functional interaction between HIV-1 Gag and Env proteins is not only critical for efficient incorporation of Env proteins into mature virions but also important for proper intracellular transport and stable surface expression of viral Env proteins in infected CD4+ T lymphocytes. A single amino acid substitution in MA abolished virus infectivity in dividing CD4+ T lymphocytes without significantly affecting virus assembly, virus release, or incorporation of Gag-Pol and Env proteins, suggesting that in addition to its functional role in virus assembly, the MA protein of HIV-1 also plays an important role in other steps of virus replication.

Three-dimensional structure of the HTLV-II matrix protein and comparative analysis of matrix proteins from the different classes of pathogenic human retroviruses

The matrix protein performs similar roles in all retroviruses, initially directing membrane localization of the assembling viral particle and subsequently forming a stable structural shell associated with the inner surface of the mature viral membrane. Although conserved structural elements are likely to perform these functions in all retroviral matrix proteins, invariant motifs are not evident at the primary sequence level and three-dimensional structures have been available for only the primate lentiviral matrix proteins. We have therefore used NMR spectroscopy to determine the structure of the matrix protein from human T-cell leukemia virus type II (HTLV-II), a member of the human oncovirus subclass of retroviruses. A total of 577 distance restraints were used to build 20 refined models that superimpose with an rmsd of 0.71 A for the backbone atoms of the structured regions. The globular HTLV-II matrix structure is composed of four alpha-helices and a 3(10) helix. Exposed basic residues near the C terminus of helix II form a putative membrane binding surface which could act in concert with the N-terminal myristoyl group to anchor the protein on the viral membrane surface. Clear structural similarities between the HTLV-II and HIV-1 matrix proteins suggest that the topology and exposed cationic membrane binding surface are likely to be conserved features of retroviral matrix proteins.

Comparison of the NMR and X-ray structures of the HIV-1 matrix protein: evidence for conformational changes during viral assembly

The three-dimensional solution- and solid-state structures of the human immunodeficiency virus type-1 (HIV-1) matrix protein have been determined recently in our laboratories by NMR and X-ray crystallographic methods (Massiah et al. 1994. J Mol Biol 244:198-223; Hill et al. 1996. Proc Natl Acad Sci USA 93:3099-3104). The matrix protein exists as a monomer in solution at low millimolar protein concentrations, but forms trimers in three different crystal lattices. Although the NMR and X-ray structures are similar, detailed comparisons have revealed an approximately 6 A displacement of a short 3(10) helix (Pro 66-Gly 71) located at the trimer interface. High quality electron density and nuclear Overhauser effect (NOE) data support the integrity of the X-ray and NMR models, respectively. Because matrix apparently associates with the viral membrane as a trimer, displacement of the 3(10) helix may reflect a physiologically relevant conformational change that occurs during virion assembly and disassembly. These findings further suggest that Pro 66 and Gly 71, which bracket the 3(10) helix, serve as "hinges" that allow the 3(10) helix to undergo this structural reorientation.

The carboxyl terminus of the human foamy virus Gag protein contains separable nucleic acid binding and nuclear transport domains

The Gag protein of human foamy virus (HFV) lacks Cys-His boxes present in the nucleocapsid (NC) domains of other retroviruses; instead it contains three glycine-arginine-rich motifs (GR boxes). We have expressed the carboxyl end of HFV Gag containing the GR boxes (the NC domain equivalent) and analyzed its nucleic acid binding properties. Our results show that the NC domain of HFV Gag binds with high affinity to both RNA and DNA, in a sequence-independent manner, as determined by filter binding assays. Analysis of a mutant containing a heterologous sequence in place of GR box I indicates that this motif is required for nucleic acid binding and for viral replication. A mutant in GR box II still binds to RNA and DNA in vitro, but virus containing this mutation does not replicate and no nuclear staining of the Gag protein is found in transfected cells. Surprisingly, a revertant from this mutant that completely lacks GR box II and exhibits very little nuclear transport of Gag can readily replicate in tissue culture. This finding thus provides a direct evidence that although the sequences in GR box II can serve as a nuclear transport signal, they are not required for HFV replication and it is unlikely that nuclear localization of Gag protein plays any critical role during viral infection. Taken together, our results suggest that the Gag protein of HFV may be more analogous to the core protein of the hepatitis B virus family than to conventional retroviral Gag protein.

Human immunodeficiency virus type 1 envelope protein endocytosis mediated by a highly conserved intrinsic internalization signal in the cytoplasmic domain of gp41 is suppressed in the presence of the Pr55gag precursor protein

The mechanisms involved in the incorporation of viral glycoproteins into virions are incompletely understood. For retroviruses, incorporation may involve interactions between the Gag proteins of these viruses and the cytoplasmic domains of the relevant envelope (Env) glycoproteins. Recent studies have identified within the cytoplasmic tail of the human immunodeficiency virus type 1 (HIV-1) Env protein a tyrosine-containing internalization motif similar to those found in the cytoplasmic domains of certain cell surface proteins that undergo rapid constitutive endocytosis in clathrin-coated pits. Given that surface expression of the HIV-1 Env protein is essential for the production of infectious virus, the presence of this internalization motif is surprising. We show here that in contrast to the rapid rate of Env protein internalization observed in cells expressing the Env protein in the absence of other HIV-1 proteins, the rate of internalization of Env protein from the surfaces of HIV-1-infected cells is extremely slow. The presence of the Pr55gag precursor protein is necessary and sufficient for inhibition of Env protein internalization, while a mutant Pr55-gag that is incapable of mediating Env incorporation into virions is also unable to inhibit endocytosis of the Env protein. The failure of the Env protein to undergo endocytosis from the surface of an HIV-1-infected cell may reflect the fact that the proposed interaction of the matrix domain of the Gag protein with Env during assembly prevents the interaction of Env with host adaptin molecules that recruit plasma membrane molecules such as the transferrin receptor into clathrin-coated pits. When the normal ratio of Gag and Env proteins in the infected cells is altered by overexpression of Env protein, this mechanism allows removal of excess Env protein from the cell surface. Taken together, these results suggest that a highly conserved system to reduce surface levels of the Env protein functions to remove Env protein that is not associated with Gag and that is therefore not destined for incorporation into virions. This mechanism for the regulation of surface levels of Env protein may protect infected cells from Env-dependent cytopathic effects or Env-specific immune responses.

Identification of domains in the simian immunodeficiency virus matrix protein essential for assembly and envelope glycoprotein incorporation

The matrix domain (MA) of the simian immunodeficiency virus (SIV) is encoded by the amino-terminal region of the Gag polyprotein precursor and is the component of the viral capsid that lines the inner surface of the virus envelope. To define domains of the SIV MA protein that are involved in virus morphogenesis, deletion and substitution mutations were introduced in this protein in the context of a gag-protease construct and expressed in the vaccinia virus vector system. The MA mutants were characterized with respect to synthesis and processing of the Gag precursor, assembly and release of virus-like particles, and incorporation of the envelope (Env) glycoprotein into particles. We have identified two regions of the SIV MA which are critical for particle formation. Both domains are located in a central hydrophobic alpha-helix of the SIV MA, according to data on the structure of this protein. In addition, we have characterized a domain whose mutation impairs the incorporation of SIV Env glycoproteins with long transmembrane cytoplasmic tails into particles. Interestingly, these mutant particles retained the ability to associate with SIV Env proteins with short cytoplasmic tails.

Molecular biology of human immunodeficiency virus type-1

Tremendous progress has been made in our understanding of the multiplication and pathogenesis of the human immunodeficiency virus, the causative agent of acquired immunodeficiency syndrome (AIDS). To block virus multiplication several targets in the life cycle of the virus have already been identified for which antiviral drugs can be developed and gene therapy can be envisaged as a possible treatment or cure of AIDS. The combination of several therapies might be needed for effective treatment. Prevention of HIV infections through effective vaccines still awaits novel, unconventional strategies.

Cyclosporine A-resistant human immunodeficiency virus type 1 mutants demonstrate that Gag encodes the functional target of cyclophilin A

The cellular peptidyl-prolyl isomerase cyclophilin A is incorporated into human immunodeficiency virus type 1 virions via contacts with the proline-rich domain of the Gag polyprotein. Cyclosporine A and nonimmunosuppressive analogs bind with high affinity to cyclophilin A, compete with Gag for binding to cyclophilin A, and prevent incorporation of cyclophilin A into virions; in parallel with the disruption of cyclophilin A incorporation into virions, there is a linear reduction in the initiation of reverse transcription after infection of a T cell. Passage of human immunodeficiency virus type 1 in the presence of the drug selects one of two mutations, either of which alters the proline-rich domain of Gag and is sufficient to confer drug resistance on the cloned wild-type provirus. Neither mutation alters Gag's cyclophilin A-binding properties in vitro, and cyclophilin A incorporation into drug-resistant virions is effectively disrupted by cyclosporine A, indicating that the drug-resistant mutants do not require virion-associated cyclophilin A to initiate infection. That Gag's functional dependence on cyclophilin A can be differentiated genetically from its ability to bind cyclophilin A is further demonstrated by the rescue of a mutation precluding cyclophilin A packaging by a mutation conferring cyclosporine A resistance. These experiments demonstrate that, in addition to its ability to package cyclophilin A into virions, gag encodes the functional target of cyclophilin A.

Structural analysis of human immunodeficiency virus type 1 Gag protein interactions, using cysteine-specific reagents

We have examined structural interactions of Gag proteins in human immunodeficiency virus type 1 (HIV-1) particles by utilizing cysteine mutagenesis and cysteine-specific modifying reagents. In immature protease-minus but otherwise wild-type (wt) particles, precursor Pr55Gag proteins did not form intermolecular cystines naturally but could be cross-linked at cysteines, and cross-linking appeared to occur across nucleocapsid (NC) domains. Capsid (CA) proteins in wt mature viruses possess cysteines near their carboxy termini at gag codons 330 and 350, but these residues are not involved in natural covalent intermolecular bonds, nor can they be intermolecularly cross-linked by using the membrane-permeable cross-linker bis-maleimido hexane. The cysteine at gag codon 350 (C-350) is highly reactive to thiol-specific modifying reagents, while the one at codon 330 (C-330) appears considerably less reactive, even in the presence of ionic detergent. These results suggest that the HIV-1 CA C terminus forms an unusually stable conformation. Mutagenesis of C-350 to a serine residue in the mutant C350S (C-350 changed to serine) virtually eliminated particle assembly, attesting to the importance of this region. We also examined a C330S mutant, as well as mutants in which cysteines were created midway through the capsid domain or in the C-terminal section of the major homology region. All such mutants appeared wt on the basis of biochemical assays but showed greatly reduced infectivities, indicative of a postassembly, postprocessing replicative block. Interestingly, capsid proteins of mature major homology region mutant particles could be cysteine cross-linked, implying either that these mutations permit cross-linking of the native C-terminal CA cysteines or that major homology regions on neighbor capsid proteins are in close proximity in mature virions.

Characterization of human immunodeficiency virus type 1 p17 matrix protein motifs associated with mother-to-child transmission

In order to determine if viral selection occurs during mother-to-child transmission of human immunodeficiency virus type 1 (HIV-1), we used a direct solid-phase sequencing method to sequence the p17 matrix protein-encoding regions of viral isolates from 12 HIV-1-infected mother-and-child pairs, 4 infected infants, 4 transmitting mothers, and 22 nontransmitting mothers and compared the sequences. The blood samples were collected during the delivery period for the mothers and during the first month of life for most of the children. The p17 nucleic sequences were distributed among several clades corresponding to the HIV-1 A, B, and G subtypes. At the amino acid level, no significant differences within the known p17 functional regions were observed among the subtypes. Statistical analyses could be performed with the B subtype. Within the major p17 antibody binding site, a constant KIEEEQN motif (amino acids 103 to 109) was found in all mother-and-child isolates from the B subtype. On the other hand, 9 of 17 nontransmitting mother isolates were variable in this 103 to 109 region. Thus, this motif was significantly associated with the transmitting status (chi square, P = 0.0034). A valine residue at position 104 was significantly associated with the nontransmitting phenotype (chi square, P = 0.014), suggesting that it has a protective role during vertical transmission. The C-terminal end of p17 was globally conserved among nontransmitting mother isolates (chi square, P = 0.0037). These results might improve the understanding of the pathogenesis of HIV-1 vertical transmission and might allow the screening of seropositive mothers by a rapid molecular or peptide test.

TM domain swapping of murine leukemia virus and human T-cell leukemia virus envelopes confers different infectious abilities despite similar incorporation into virions

We investigated the influence of transmembrane protein (TM) domains on incorporation of retroviral envelopes into virions and on infectivity. We introduced complete, truncated, or chimeric Friend murine leukemia virus (F-MuLV) and human T-cell leukemia virus type 1 (HTLV-1) envelopes into an MuLV particle-producing complementation cell line. As shown previously for HTLV-1 envelopes containing extracellular domains of F-MuLV TM (C. Denesvre, P. Sonigo, A. Corbin, H. Ellerbrok, and M. Sitbon, J. Virol. 69:4149-4157, 1995), reverse chimeric F-MuLV envelopes containing the extracellular domain of HTLV-1 TM were not processed. In contrast, a chimeric MuLV envelope containing the entire HTLV membrane-spanning and cytoplasmic domains (FHTMi) was efficiently processed, fusogenic as tested in a cell-to-cell assay, and efficiently incorporated into MuLV particles. However, these MuLV particles bearing FHTMi envelope proteins could not infect mouse or rat cells which are susceptible to wild-type F-MuLV. Therefore, envelopes which are readily fusogenic in cell-to-cell assays and also efficiently incorporated into virions may not necessarily confer virus-to-cell fusogenicity. HTLV envelopes, whether parental, chimeric (containing the MuLV cytoplasmic tail) or with a truncated cytoplasmic domain, were incorporated into MuLV particles with equal efficiencies, indicating that the cytoplasmic tails of these envelopes did not determine their incorporation into virions. In contrast to FHTMi envelope, HTLV-1 envelopes with F-MuLV membrane-spanning and cytoplasmic domains, as well as wild-type HTLV-1 envelopes, conferred virion infectivity. These results help to define requirements for envelope incorporation into retroviral particles and their cell-free infectivity.

Binding of the human immunodeficiency virus type 1 Gag polyprotein to cyclophilin A is mediated by the central region of capsid and requires Gag dimerization

The cellular peptidyl-prolyl isomerase cyclophilin A (CyPA) is incorporated into human immunodeficiency virus type 1 (HIV-1) virions via direct contacts with the HIV-1 Gag polyprotein. Disruption of the Gag-CyPA interaction leads to the production of HIV-1 particles lacking CyPA; these virions are noninfectious, indicating that contacts between CyPA and Gag are necessary for HIV-1 replication. Here, we have used the yeast two-hybrid system in conjunction with an in vitro binding assay to identify the minimal domain of Gag required for binding to CyPA. Analysis of a panel of gag deletion mutants in the two-hybrid system indicated that a region spanning the central portion of the capsid (CA) domain was sufficient for interactions with CyPA, but discrepancies between results obtained in different fusion protein contexts suggested that multimerization of Gag might also be necessary for binding to CyPA. Consistent with a requirement for multimerization, the binding of Gag to CyPA in vitro required a region within the nucleocapsid (NC) domain shown previously to be important for Gag self-association. Substitution of a heterologous dimerization motif for the region from NC also promoted specific binding to CyPA, confirming that interactions with CyPA are dependent on Gag multimerization. Fusion of the heterologous dimerization motif to a 100-amino-acid domain from CA was sufficient for binding to CyPA in vitro. These results define the minimal CyPA-binding domain within Gag and provide insight into the mechanism by which CyPA is incorporated into HIV-1 virions.

Cyclophilin A is required for an early step in the life cycle of human immunodeficiency virus type 1 before the initiation of reverse transcription

Cyclophilin A (CyPA) is incorporated into human immunodeficiency virus type 1 (HIV-1) virions via contact with the Gag polyprotein. Genetic or pharmacologic disruption of CyPA incorporation causes a quantitative reduction in virion infectivity with no discernible effects on virion assembly or on endogenous reverse transcriptase activity. Instead, the reduction of virion-associated CyPA is accompanied by a parallel, quantitative decrease in the initiation of viral DNA synthesis after infection of T cells. The infectivity of CyPA-deficient virions is not restored by pseudotyping with Env of amphotropic murine leukemia virus, demonstrating that CyPA is not required for the HIV-1-Env-CD4 interaction. These results indicate that CyPA is required for an early step in the HIV-1 life cycle following receptor binding and membrane fusion but preceding reverse transcription. CyPA is the first cellular protein other than the cell surface receptor shown to be required for an early event in the life cycle of a retrovirus.

Crystal structures of the trimeric human immunodeficiency virus type 1 matrix protein: implications for membrane association and assembly

The human immunodeficiency virus type 1 (HIV-1) matrix protein forms a structural shell associated with the inner viral membrane and performs other essential functions throughout the viral life cycle. The crystal structure of the HIV-1 matrix protein, determined at 2.3 angstrom resolution, reveals that individual matrix molecules are composed of five major helices capped by a three-stranded mixed beta-sheet. Unexpectedly, the protein assembles into a trimer in three different crystal lattices, burying 1880 angstrom2 of accessible surface area at the trimer interfaces. Trimerization appears to create a large, bipartite membrane binding surface in which exposed basic residues could cooperate with the N-terminal myristoyl groups to anchor the protein on the acidic inner membrane of the virus.

A large region within the Rous sarcoma virus matrix protein is dispensable for budding and infectivity

All retroviruses have a layer of matrix protein (MA) situated directly beneath the lipid of their envelope. This protein is initially expressed as the amino-terminal sequence of the Gag polyprotein, where it plays an important role in binding Gag to the plasma membrane during the early steps of the budding process. Others have suggested that MA may provide additional functions during virion assembly, including the selective incorporation of viral glycoproteins and the RNA genome into the emerging virion. To further study the role of the Rous sarcoma virus MA sequence in the viral replication cycle, we have pursued an extensive deletion analysis. Surprisingly, the entire second half of MA (residues 87 to 155) and part of the neighboring p2 sequence were found to be dispensable not only for budding but also for infectivity in avian cells. Thus, all of the functions associated with the Rous sarcoma virus MA sequence must be contained within its first half.

Human immunodeficiency virus type 2 glycoprotein enhancement of particle budding: role of the cytoplasmic domain

Previous studies have shown that the glycoprotein cytoplasmic domains of human immunodeficiency virus type 2 (HIV-2) or simian immunodeficiency virus of macaques modulate biological activities of the viral glycoprotein complex, including syncytium formation, exterior glycoprotein conformation, and glycoprotein incorporation into budding virus particles. We have now utilized a recombinant expression system to study interactions of full-length or truncated HIV-2 glycoproteins with coexpressed HIV-2 Gag proteins which self-assemble and bud as virus-like particles. Interestingly, budding of HIV-2 virus-like particles from cells was enhanced 5- to 24-fold when Gag was coexpressed with the full-length HIV-2 glycoprotein, compared with Gag expressed either alone or with a truncated HIV-2 glycoprotein. The results obtained in this model system indicate that an additional effect of the lengthy cytoplasmic domain of the glycoprotein of HIV-2 is enhancement of particle budding. We speculate that the cytoplasmic domain of the viral glycoprotein of HIV-2 enhances budding by (i) potentiation of Gag structure or function or (ii) membrane modulation.

The membrane-binding domain of the Rous sarcoma virus Gag protein

The Gag protein of Rous sarcoma virus (RSV) can direct particle assembly and budding at the plasma membrane independently of the other virus-encoded products. A previous deletion analysis has suggested that the first 86 amino acids of RSV Gag constitute a large membrane-binding domain that is absolutely required for these processes. To test this hypothesis, we inserted these residues in place of the N-terminal membrane-binding domain of the pp60v-src, a transforming protein whose biological activity requires plasma membrane localization. The ability of the Src chimera to induce cellular transformation suggests that the RSV sequence indeed contains an independent, functional domain.

Mutations affecting the cytoplasmic domain of the Moloney murine leukemia virus envelope protein: rapid reversion during replication

Five premature termination mutations and five missense mutations were introduced into the portion of cloned Moloney murine leukemia virus (M-MuLV) DNA encoding the Env cytoplasmic domain. All of the mutant DNAs gave rise to replication-competent virus after transfection of NIH/3T3 cells, but several of the mutant DNAs scored as replication-defective when introduced into Rat2 cells. Cell lines stably expressing the mutant DNAs all released virion particles, and in all but one case infectious virus were generated. These viable mutants were all found to have reverted to the wild-type sequence. To generate fully mutant virus stocks, the mutant DNAs were introduced transiently into COS cells, which are resistant to infection with MuLV, thus prohibiting reversion by error-prone mechanisms involving reverse transcription. Virions harvested from the COS cells were confirmed as mutant by analyzing both virion proteins and the viral DNA they generated, and were then tested for infectivity in NIH/3T3 cells. The mutant viruses were infectious, but still rapidly gave rise to revertants. We conclude that the mutations within the cytoplasmic domain do not provide an absolute block to virus replication, but that the mutants replicate more slowly than the wild-type and quickly give rise to revertants with selective advantage for replication.

An internalization signal in the simian immunodeficiency virus transmembrane protein cytoplasmic domain modulates expression of envelope glycoproteins on the cell surface

A Tyr to Cys mutation at amino acid position 723 in the cytoplasmic domain of the simian immunodeficiency virus (SIV) transmembrane (TM) molecule has been shown to increase expression of envelope glycoproteins on the surface of infected cells. Here we show that Tyr-723 contributes to a sorting signal that directs the rapid endocytosis of viral glycoproteins from the plasma membrane via coated pits. On cells infected by SIVs with a Tyr at position 723, envelope glycoproteins were transiently expressed on the cell surface and then rapidly endocytosed. Similar findings were noted for envelope molecules expressed in the absence of other viral proteins. Immunoelectron microscopy demonstrated that these molecules were localized in patches on the cell surface and were frequently associated with coated pits. In contrast, envelope glycoproteins containing a Y723C mutation were diffusely distributed over the entire plasma membrane. To determine if an internalization signal was present in the SIV TM, chimeric molecules were constructed that contained the CD4 external and membrane spanning domains and a SIV TM cytoplasmic tail with a Tyr or other amino acids at SIV position 723. In Hela cells stably expressing these molecules, chimeras with a Tyr-723 were rapidly endocytosed, while chimeras containing other amino acids at position 723, including a Phe, were internalized at rates only slightly faster than a CD4 molecule that lacked a cytoplasmic domain. In addition, the biological effects of the internalization signal were evaluated in infectious viruses. A mutation that disrupted the signal and as a result, increased the level of viral envelope glycoprotein on infected cells, was associated with accelerated infection kinetics and increased cell fusion during viral replication. These results demonstrate that a Tyr-dependent motif in the SIV TM cytoplasmic domain can function as an internalization signal that can modulate expression of the viral envelope molecules on the cell surface and affect the biological properties of infectious viruses. The conservation of an analogous Tyr in all human and simian immunodeficiency viruses suggests that this signal may be present in other primate lentiviruses and could be important in the pathogenesis of these viruses in vivo.