A molecular determinant of human immunodeficiency virus particle assembly located in matrix antigen p17

Low-molecular-weight anti-HIV-1 agents targeting HIV-1 capsid proteins

The HIV-1 capsid is a shell that encapsulates viral RNA, and forms a conical structure by assembling oligomers of capsid (CA) proteins. Since the CA proteins are highly conserved among many strains of HIV-1, the inhibition of the CA function could be an appropriate goal for suppression of HIV-1 replication, but to date, no drug targeting CA has been developed. Hydrophobic interactions between two CA molecules through Trp184 and Met185 in the protein are known to be indispensable for conformational stabilization of the CA multimer. In our previous study, a small molecule designed by in silico screening as a dipeptide mimic of Trp184 and Met185 in the interaction site was synthesized and found to have significant anti-HIV-1 activity. In the present study, molecules with different scaffolds based on a dipeptide mimic of Trp184 and Met185 have been designed and synthesized. Their significant anti-HIV activity and their advantages compared to the previous compounds were examined. The present results should be useful in the design of novel CA-targeting anti-HIV agents.

Small-Molecule Anti-HIV-1 Agents Based on HIV-1 Capsid Proteins

The capsid of human immunodeficiency virus type 1 (HIV-1) is a shell that encloses viral RNA and is highly conserved among many strains of the virus. It forms a conical structure by assembling oligomers of capsid (CA) proteins. CA dysfunction is expected to be an important target of suppression of HIV-1 replication, and it is important to understand a new mechanism that could lead to the CA dysfunction. A drug targeting CA however, has not been developed to date. Hydrophobic interactions between two CA molecules via Trp184/Met185 in CA were recently reported to be important for stabilization of the multimeric structure of CA. In the present study, a small molecule designed by in silico screening as a dipeptide mimic of Trp184 and Met185 in the interaction site, was synthesized and its significant anti-HIV-1 activity was confirmed. Structure activity relationship (SAR) studies of its derivatives were performed and provided results that are expected to be useful in the future design and development of novel anti-HIV agents targeting CA.

Potent leads based on CA-19L, an anti-HIV active HIV-1 capsid fragment

Several anti-HIV-1 peptides have previously been found among overlapping fragment peptide libraries that contain an octa-arginyl moiety and cover the whole sequence of an HIV-1 capsid (CA) protein. Several derivatives based on a potent CA fragment peptide CA-19L have been synthesized. CA-19L overlaps with the Helix 9 region of the CA protein, which could be important for oligomerization of the CA proteins. Derivatives of CA-19L in which several amino acid residues were added to the N- and C-termini according to the natural CA sequence, were synthesized and their anti-HIV activity was evaluated. Some potent compounds were found, and these potential new anti-HIV agents are expected to be useful as new tools for elucidation of CA functions.

Exploratory studies on CA-15L, an anti-HIV active HIV-1 capsid fragment

Utilizing overlapping fragment peptide libraries covering the whole sequence of an HIV-1 capsid (CA) protein with the addition of an octa-arginyl moiety, we had previously found several peptides with anti-HIV-1 activity. Herein, among these potent CA fragment peptides, CA-15L was examined because this peptide sequence overlaps with Helix 7, a helix region of the CA protein, which may be important for oligomerization of the CA proteins. A CA-15L surrogate with hydrophilic residues, and its derivatives, in which amino acid sequences are shifted toward the C-terminus by one or more residues, were synthesized and their anti-HIV activity was evaluated. In addition, its derivatives with substitution for the Ser149 residue were synthesized and their anti-HIV activity was evaluated because Ser149 might be phosphorylated in the step of degradation of CA protein oligomers. Several active compounds were found and might become new anti-HIV agents and new tools for elucidation of CA functions.

Development of anti-HIV peptides based on a viral capsid protein

Peptide inhibitors with cell permeability targeting an HIV-1 capsid (CA) protein might make therapeutic by regulating HIV-1 replication. Overlapping fragment peptide libraries covering the whole sequence of an HIV-1 CA protein have been synthesized with the addition of an octa-arginyl moiety to increase their cell permeability. Amongst these peptides, several compounds which inhibit the HIV-1 replication cycle have been found. Conjugation of cell-penetrating functions such as an octa-arginyl group to individual peptides in combination with the addition of chloroquine in cell-based anti-HIV assays was previously proven to be a useful assay method with which to search for active peptides. Anti-HIV assays have been performed in the presence or absence of chloroquine and found that most of compounds have higher anti-HIV activity in the presence, rather than in the absence of chloroquine. Some potent seeds as anti-HIV agents might naturally lie hidden in CA proteins, and could become useful leads to HIV inhibitors.

Anti-HIV screening for cell-penetrating peptides using chloroquine and identification of anti-HIV peptides derived from matrix proteins

Previously, compounds which inhibit the HIV-1 replication cycle were found in overlapping peptide libraries covering the whole sequence of an HIV-1 matrix (MA) protein constructed with the addition of an octa-arginyl group. The two top lead compounds are sequential fragments MA-8L and MA-9L. In the present study, the addition of chloroquine in cell-based anti-HIV assays was proven to be an efficient method with which to find anti-HIV compounds among several peptides conjugated by cell-penetrating signals such as an octa-arginyl group: the conjugation of an octa-arginyl group to individual peptides contained in whole proteins in combination with the addition of chloroquine in cells is a useful assay method to search active peptides. To find more potent fragment peptides, individual peptides between MA-8L and MA-9L, having the same peptide chain length but with sequences shifted by one amino acid residue, were synthesized in this paper and their anti-HIV activity was evaluated with an anti-HIV assay using chloroquine. As a result, the peptides in the C-terminal side of the series, which are relatively close to MA-9L, showed more potent inhibitory activity against both X4-HIV-1 and R5-HIV-1 than the peptides in the N-terminal side.

Conjugation of cell-penetrating peptides leads to identification of anti-HIV peptides from matrix proteins

Compounds which inhibit the HIV-1 replication cycle have been found amongst fragment peptides derived from an HIV-1 matrix (MA) protein. Overlapping peptide libraries covering the whole sequence of MA were designed and constructed with the addition of an octa-arginyl group to increase their cell membrane permeability. Imaging experiments with fluorescent-labeled peptides demonstrated these peptides with an octa-arginyl group can penetrate cell membranes. The fusion of an octa-arginyl group was proven to be an efficient way to find active peptides in cells such as HIV-inhibitory peptides.

Covalent bonded Gag multimers in human immunodeficiency virus type-1 particles

The oligomerization of HIV-1 Gag and Gag-Pol proteins, which are assembled at the plasma membrane, leads to viral budding. The budding generally places the viral components under non-reducing conditions. Here the effects of non-reducing conditions on Gag structures and viral RNA protection were examined. Using different reducing conditions and SDS-PAGE, it was shown that oligomerized Gag possesses intermolecular covalent bonds under non-reducing conditions. In addition, it was demonstrated that the mature viral core contains a large amount of covalent bonded Gag multimers, as does the immature core. Viral genomic RNA becomes sensitive to ribonuclease in reducing conditions. These results suggest that, under non-reducing conditions, covalent bonded Gag multimers are formed within the viral particles and play a role in protection of the viral genome.

SOCS1 is an inducible host factor during HIV-1 infection and regulates the intracellular trafficking and stability of HIV-1 Gag

Human immunodeficiency virus type 1 (HIV-1) utilizes the macromolecular machinery of the infected host cell to produce progeny virus. The discovery of cellular factors that participate in HIV-1 replication pathways has provided further insight into the molecular basis of virus-host cell interactions. Here, we report that the suppressor of cytokine signaling 1 (SOCS1) is an inducible host factor during HIV-1 infection and regulates the late stages of the HIV-1 replication pathway. SOCS1 can directly bind to the matrix and nucleocapsid regions of the HIV-1 p55 Gag polyprotein and enhance its stability and trafficking, resulting in the efficient production of HIV-1 particles via an IFN signaling-independent mechanism. The depletion of SOCS1 by siRNA reduces both the targeted trafficking and assembly of HIV-1 Gag, resulting in its accumulation as perinuclear solid aggregates that are eventually subjected to lysosomal degradation. These results together indicate that SOCS1 is a crucial host factor that regulates the intracellular dynamism of HIV-1 Gag and could therefore be a potential new therapeutic target for AIDS and its related disorders.

Evolution of feline immunodeficiency virus Gag proteins

We evaluated the predicted biochemical properties of Gag proteins from a diverse group of feline immunodeficiency viruses (FIV) to determine how different evolutionary histories of virus and host have changed or constrained these important structural proteins. Our data are based on FIV sequences derived from domestic cat (FIVfca), cougar (FIVpco), and lions (FIVple). Analyses consisted of determining the selective forces acting at each position in the protein and the comparing predictions for secondary structure, charge, hydrophobicity and flexibility for matrix, capsid and nucleocapsid, and the C-terminal peptide, which comprise the Gag proteins. We demonstrate that differences among the FIV Gag proteins have largely arisen by neutral evolution, although many neutrally evolving regions have maintained biochemical features. Regions with predicted differences in biochemical features appear to involve intramolecular interactions and structural elements that undergo conformational changes during particle maturation. In contrast, the majority of sites involved in intermolecular contacts on the protein surface are constrained by purifying selection. There is also conservation of sites that interact with host proteins associated with cellular trafficking and particle budding. NC is the only protein with evidence of positive selection, two of which occur in the N-terminal region responsible for RNA binding and interaction with host proteins.

In vivo homodimerisation of HTLV-1 Gag and MA gives clues to the retroviral capsid and TM envelope protein arrangement

During retroviral particle formation, the capsid precursors (Gag) associate with the cell membrane via their matrix (MA) domain to form viral assembling particles. After budding, Gag and its proteolytically matured MA, form a shell in the released immature and mature particles, respectively. Although the arrangement of Gag domains in vitro and their radial organisation in retroviral particles have been extensively studied, little is known concerning Gag inter-subunit interactions in authentic retroviruses. We report that human T-cell leukemia virus type 1 Gag homodimerises in the cell via a disulphide bonding at cysteine 61 in the MA domain. Most Gags are homodimeric after budding and MAs are also dimeric in mature authentic virions. Molecular modelling of the MA domain indicates that non-covalent interactions at the MA dimer interface may also be important for Gag (and MA) dimerisation. In addition, all amino acids previously reported to be involved in MA-transmembrane (TM) interactions are located on the MA face opposite to the dimer interface. The model reveals that homodimerisation is compatible with a hexameric network of Gag and MA dimers that look like the hexameric networks observed for other retroviruses. These data, together with previous studies, lead us to propose a supra-molecular arrangement model in which the transmembrane glycoproteins of the virion envelope are anchored in a hexameric cage hole formed by the MA.

The molecular basis of HIV capsid assembly--five years of progress

The assembly of HIV is relatively poorly investigated when compared with the process of virus entry. Yet a detailed understanding of the mechanism of assembly is fundamental to our knowledge of the complete life cycle of this virus and also has the potential to inform the development of new antiviral strategies. The repeated multiple interaction of the basic structural unit, Gag, might first appear to be little more than concentration dependent self-assembly but the precise mechanisms emerging for HIV are far from simple. Gag interacts not only with itself but also with host cell lipids and proteins in an ordered and stepwise manner. It binds both the genomic RNA and the virus envelope protein and must do this at an appropriate time and place within the infected cell. The assembled virus particle must successfully release from the cell surface and, whilst being robust enough for transmission between hosts, must nonetheless be primed for rapid disassembly when infection occurs. Our current understanding of these processes and the domains of Gag involved at each stage is the subject of this review.

The central domain of the matrix protein of HIV-1: influence on protein structure and virus infectivity

The central region of the matrix protein p17 of HIV-1 is known to be essential during virus assembly. We substituted alanines for amino acid triplets in this region of p17 (amino acid residues 47 to 55: NPG LLE TSE). Introduction of the respective mutations into the gag-coding sequence of HI-proviruses and subsequent transfection into Cos-7 cells led to particle production and release. Exchange of LLE resulted in the production of non-infectious particles. These residues may be important for correct folding and assembly of the processed matrix protein and the production of infectious HIV. In vitro studies of wild-type and mutated matrix proteins using spectroscopic methods (NMR, fluorescence, CD) yielded detailed data about structure and stability. Two-dimensional NMR spectroscopy showed that wild-type and mutant proteins (p17-NPG and p17-TSE) are well folded. Besides structural changes at the mutated site, chemical shift changes indicate small but significant long range structural rearrangements. The stability against chemically and thermally induced unfolding of the mutants p17-NPG and p17-TSE was slightly decreased, while that of p17-LLE was drastically diminished. The alterations have only a local effect on protein folding for the mutants p17-NPG and p17-TSE, and the globular tertiary structure remains nearly unchanged. For p17-LLE, however, the substitutions seem to trigger significant changes in structural elements.

High-density lipoprotein concentrations relate to the clinical course of HIV viral load in patients undergoing antiretroviral therapy

OBJECTIVE

To determine whether levels of HDL are associated with viral load response in HIV-treated patients, and to seek an explanation based on amino acid sequence similarity between the key apolipoprotein A1 and HIV proteins concerned in viral replication.

DESIGN

The major HDL lipoprotein is apolipoprotein A1, which is able to inhibit HIV-induced syncytium formation. This retrospective clinical study assessed the relationship between the response to antiretroviral treatment (time of undetectable viral load/duration of viral suppression below the limit of detection) and HDL-cholesterol levels on commencing antiretroviral treatment.

PATIENTS AND METHODS

HIV-treated patients with undetectable HIV viral loads were followed every 3 months for 36 months. We measured total cholesterol, HDL-cholesterol, triglycerides, previous responses to antiretroviral treatment, opportunistic infections, sex and age. These variables were assessed in relation to the time of undetectable viral load until viral rebound. Amino acid sequence alignment was performed with HIV proteins and apolipoprotein A1 to detect shared similarity.

RESULTS

The Cox proportional hazards model showed a significant association between HDL-cholesterol and the time of undetectable viral load. The other variables studied were not associated. There was 30% sequence similarity in an area of 50 amino acids shared between apolipoprotein A1 and p17 Gag-HIV protein.

CONCLUSION

High levels of HDL-cholesterol are associated with a better viral response in treated HIV patients. This association could be related to the sequence similarity and structure homology between apolipoprotein A1 and p17 Gag-HIV protein, which raises the intriguing clinical possibility that inducing an increase in HDL could assist HIV therapy.

Atomic resolution structure of Moloney murine leukemia virus matrix protein and its relationship to other retroviral matrix proteins

Matrix proteins associated with the viral membrane are important in the formation of the viral particle and in virus maturation. The 1.0 A crystal structure of the ecotropic Gammaretrovirus Moloney murine leukemia virus (M-MuLV) matrix protein reveals the conserved topology of other retroviral matrix proteins, despite undetectable sequence similarity. The N terminus (normally myristylated) is exposed and adjacent to a basic surface patch, features likely to contribute to membrane binding. The four proteins in the asymmetric unit make varied contacts. The M-MuLV matrix structure is intermediate, between those of the lentiviruses and other retroviruses. The protein fold appears to be maintained, in part, by the conservation of side chain packing, which may provide a useful tool for searching for weak distant similarities in proteins.

Comparative analysis of the roles of simian immunodeficiency and bovine leukemia virus matrix proteins in Gag assembly in insect cells

The role of the matrix (MA) domain of simian immunodeficiency virus (SIV) and bovine leukaemia virus (BLV) Gag in the assembly of virus-like particles (VLP) in insect cells has been investigated. Wild-type SIV and BLV Gag assembled to form discrete VLP structures typical of many retroviruses analysed by similar systems. When amino acids predicated by the three-dimensional structure to be at the interface of SIV MA monomers were deleted, VLP assembly was abolished consistent with a role for MA multimerization in assembly. When amino acids predicted to be in the analogous positions in BLV MA were mutated, however, VLP assembly was not affected. These data indicate that the models of assembly derived from one model retrovirus may not necessarily apply to more distantly related viruses despite the structural similarity present in equivalent Gag domains.

Cleavage of vesicular stomatitis virus matrix protein prevents self-association and leads to crystallization

The matrix protein (M) of vesicular stomatitis virus is responsible for the budding of newly formed virions out of host cells. In vitro, it has been shown to self-associate, a property that may be related to the role of M in virus assembly but also prevents crystallization. Using limited proteolysis by thermolysin, we have isolated and characterized two soluble fragments of the protein that remain noncovalently associated. The digestion product does not self-associate nor is it recruited in aggregates formed by intact M molecules. These results identify a peptide, located at the surface of the protein and disorganized by thermolysin cleavage, responsible for M self-association. The thermolysin-resistant core of M has been crystallized and the crystals diffract to 2-A resolution.

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.

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.

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.

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.

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.

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.

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.

Assembly of the reovirus outer capsid requires mu 1/sigma 3 interactions which are prevented by misfolded sigma 3 protein in temperature-sensitive mutant tsG453

A temperature-sensitive reovirus mutant, tsG453, whose defect was mapped to major outer capsid protein sigma 3, makes core particles but fails to assemble the outer capsid around the core at non-permissive temperature. Previous studies that made use of electron cryo-microscopy and image reconstructions showed that mu 1, the other major outer capsid protein, but not sigma 3, interact extensively with the core capsid. Although wild-type sigma 3 and mu 1 interact with each other, immunocoprecipitation studies showed that mutant sigma 3 protein was incapable of interacting with mu 1 at the non-permissive temperature. In addition, restrictively-grown mutant sigma 3 protein could not be precipitated by some sigma 3-specific monoclonal antibodies. These observations suggest that in a wild-type infection, specific sigma 3 and mu 1 interactions result in changes in mu 1 conformation which are required to allow mu 1/sigma 3 complexes to condense onto the core capsid shell during outer capsid assembly, and that sigma 3 in non-permissive tsG453 infections is misfolded such that it cannot interact with mu 1.

Human immunodeficiency virus type 1 capsid formation in reticulocyte lysates

The Gag polyprotein of human immunodeficiency virus (HIV) (Pr55Gag) contains sufficient information to direct particle assembly events when expressed within tissue culture cells. HIV Gag proteins normally form particles at a plasma membrane assembly site, in a manner analogous to that of the type C avian and mammalian leukemia/sarcoma viruses. It has not previously been demonstrated that immature HIV capsids can form without budding through an intact cellular membrane. In this study, a rabbit reticulocyte lysate translation reaction was used to recreate HIV capsid formation in vitro. Production of HIV-1 Pr55Gag and of a matrix-deleted Gag construct resulted in the formation of a subset of Gag protein structures with an equilibrium density of 1.15 g/ml. Gel filtration chromatography revealed these Gag protein structures to be larger than 2 x 10(6) Da, consistent with the formation of large multimers or capsids. These Gag protein structures were protease sensitive in the absence of detergent, indicating that they did not contain a complete lipid envelope. Spherical structures were detected by electron microscopy within the reticulocyte lysate reaction mixtures and appeared essentially identical to immature HIV capsids or retrovirus-like particles. These results demonstrate that the HIV Gag protein is capable of producing immature capsids in a cell-free reaction and that such capsids lack a complete lipid envelope.

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.

Mutations in the Ty3 major homology region affect multiple steps in Ty3 retrotransposition

The Saccharomyces cerevisiae retroviruslike element Ty3 encodes the major structural proteins capsid (CA) and nucleocapsid in the GAG3 open reading frame. The Ty3 CA protein contains a sequence (QGX2EX5FX3LX3H, where H is a hydrophobic residue) which has not been observed in other retrotransposons but which is similar to the major homology region (MHR) described for retrovirus CA. In this study the effects of mutations in the Ty3 MHR on particle formation, processing, DNA synthesis, and transposition were examined. Each of the mutations tested resulted in severe defects in transposition, with disruption occurring prior to or at particle formation, subsequent to particle formation and prior to completion of DNA synthesis, and subsequent to DNA synthesis. Changing the Q in the motif to R had relatively little effect on particle formation but decreased transposition to about 13% of that of a wild-type element. Changing G to A or V almost completely eliminated the formation of intracellular particles, possibly by disruption of CA-CA interactions. Changes introduced at the position of E resulted in blocked processing, blocked DNA synthesis, or a block at some post-reverse transcription step, depending on the nature of the mutation introduced. These results showed that the integrity of the Ty3 MHR is required for multiple aspects of Ty3 replication involving CA. These functions are independent of extracellular budding and of infection, aspects of the retroviral life cycle which are not recapitulated in replication of the Ty3 retrotransposon.

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.

Assembly of macromolecular complexes in bacterial and baculovirus expression systems

Many proteins exist normally as oligomers or complexes with other proteins. Recent advances in vector design have allowed this aspect of protein function to be mimicked in recombinant expression systems. Examples of the ordered oligomerization of a single protein through to the assembly of eight different proteins have been documented in recombinant Escherichia coli and recombinant baculovirus systems.

Assembly and immunogenicity of chimeric Gag-Env proteins derived from the human immunodeficiency virus type 1

We evaluated the potential of the precursor Gag protein (Pr55) of the human immunodeficiency virus type 1 (HIV-1) as a carrier for the presentation of envelope epitopes. Recombinant chimeric core-envelope protein-expressing constructs were derived by deletion of regions within the gag gene, especially of regions encoding p24 capsid epitopes. Sequences encoding either the principal neutralization determinant (PND) and/or the CD4-binding domains (CD4BS) were then inserted. Deletion of residues 196-226 within the p24 capsid protein did not prevent self-assembly into virus-like particles (VLPs) whereas deletion of residues 299-328 completely abolished VLP formation. Thus the major homology region (MHR) and proximal sequences are required for capsid assembly. An immunization study in mice showed that assembled chimeric proteins elicited strong anti-Gag, weak anti-envelope, and no neutralizing humoral responses. Nonassembled chimeric proteins were poor immunogens. Mapping of Pr55 antigenic sites using sera from immunized mice and peptides overlapping the entire Gag precursor showed that p24 capsid and p17 matrix epitopes presented to the immune system differed from the mature form (p24 or p17) and the multimeric immature form (Pr55).

Crystal structure of SIV matrix antigen and implications for virus assembly

Simian immunodeficiency virus (SIV) is closely related to human immunodeficiency virus (HIV), their matrix antigens (MAs) sharing some 50% sequence identity. MA is a component of Pr55Gag, the sole protein required for assembly of the virion shell. MA targets Pr55 to the plasma membrane, and facilitates incorporation of the virus envelope protein and assembly of the Pr55Gag shell. Cleavage of Pr55 by the viral protease produces the mature protein of relative molecular mass 17-18K, which underlies the host-derived membrane and is important in both virus entry and nuclear localization of the virion core. Here we report the crystal structure of SIV MA. The molecule forms a trimer consistent with oligomerization in vitro, the observed virion architecture, and various biological properties of MA.