The C terminus of human immunodeficiency virus type 1 matrix protein is involved in early steps of the virus life cycle

HIV-1 envelope facilitates the development of protease inhibitor resistance through acquiring mutations associated with viral entry and immune escape

Introduction

There is increasing evidence supporting a role for HIV-1 envelope in the development of Protease Inhibitor drug resistance, and a recent report from our group suggested that Env mutations co-evolve with Gag-Protease mutations in the pathway to Lopinavir resistance. In this study, we investigated the effect of co-evolving Env mutations on virus function and structure.

Methods

Co-receptor usage and n-linked glycosylation were investigated using Geno2Pheno as well as tools available at the Los Alamos sequence database. Molecular dynamics simulations were performed using Amber 18 and analyzed using Cpptraj, and molecular interactions were calculated using the Ring server.

Results

The results showed that under Protease Inhibitor drug selection pressure, the envelope gene modulates viral entry by protecting the virus from antibody recognition through the increased length and number of N-glycosylation sites observed in V1/V2 and to some extent V5. Furthermore, gp120 mutations appear to modulate viral entry through a switch to the CXCR4 coreceptor, induced by higher charge in the V3 region and specific mutations at the coreceptor binding sites. In gp41, S534A formed a hydrogen bond with L602 found in the disulfide loop region between the Heptad Repeat 1 and Heptad Repeat 2 domains and could negatively affect the association of gp120-gp41 during viral entry. Lastly, P724Q/S formed both intermolecular and intramolecular interactions with residues within the Kennedy loop, a known epitope.

Discussion

In conclusion, the results suggest that mutations in envelope during Protease Inhibitor treatment failure are related to immune escape and that S534A mutants could preferentially use the cell-to-cell route of infection.

Association of VH4-59 Antibody Variable Gene Usage with Recognition of an Immunodominant Epitope on the HIV-1 Gag Protein

The human antibody response against HIV-1 infection recognizes diverse antigenic subunits of the virion, and includes a high level of antibodies to the Gag protein. We report here the isolation and characterization of a subset of Gag-specific human monoclonal antibodies (mAbs) that were prevalent in the antibody repertoire of an HIV-infected individual. Several lineages of Gag-specifc mAbs were encoded by a single antibody heavy chain variable region, V_H4-59, and a representative antibody from this group designated mAb 3E4 recognized a linear epitope on the globular head of the p17 subunit of Gag. We found no evidence that mAb 3E4 exhibited any function in laboratory studies aimed at elucidating the immunologic activity, including assays for neutralization, Ab-dependent cell-mediated virus inhibition, or enhanced T cell reactivity caused by Gag-3E4 complexes. The findings suggest this immunodominant epitope in Gag protein, which is associated with V_H4-59 germline gene usage, may induce a high level of B cells that encode binding but non-functional antibodies that occupy significant repertoire space following HIV infection. The studies define an additional specific molecular mechanism in the immune distraction activity of the HIV virion.

Biophysical characterization and crystal structure of the Feline Immunodeficiency Virus p15 matrix protein

BACKGROUND

Feline Immunodeficiency Virus (FIV) is a viral pathogen that infects domestic cats and wild felids. During the viral replication cycle, the FIV p15 matrix protein oligomerizes to form a closed matrix that underlies the lipidic envelope of the virion. Because of its crucial role in the early and late stages of viral morphogenesis, especially in viral assembly, FIV p15 is an interesting target in the development of potential new therapeutic strategies.

RESULTS

Our biochemical study of FIV p15 revealed that it forms a stable dimer in solution under acidic conditions and at high concentration, unlike other retroviral matrix proteins. We determined the crystal structure of full-length FIV p15 to 2 Å resolution and observed a helical organization of the protein, typical for retroviral matrix proteins. A hydrophobic pocket that could accommodate a myristoyl group was identified, and the C-terminal end of FIV p15, which is mainly unstructured, was visible in electron density maps. As FIV p15 crystallizes in acidic conditions but with one monomer in the asymmetric unit, we searched for the presence of a biological dimer in the crystal. No biological assembly was detected by the PISA server, but the three most buried crystallographic interfaces have interesting features: the first one displays a highly conserved tryptophan acting as a binding platform, the second one is located along a 2-fold symmetry axis and the third one resembles the dimeric interface of EIAV p15. Because the C-terminal end of p15 is involved in two of these three interfaces, we investigated the structure and assembly of a C-terminal-truncated form of p15 lacking 14 residues. The truncated FIV p15 dimerizes in solution at a lower concentration and crystallizes with two molecules in the asymmetric unit. The EIAV-like dimeric interface is the only one to be retained in the new crystal form.

CONCLUSION

The dimeric form of FIV p15 in solution and its extended C-terminal end are characteristic among lentiviral matrix proteins. Crystallographic interfaces revealed several interactions that might be involved in FIV replication. Further studies are needed to better understand their biological relevance in the function of FIV Gag during viral replication.

Viral and cellular factors that regulate HIV-1 uncoating

PURPOSE OF REVIEW

The immediate events in HIV-1 infection following fusion of HIV-1 particles with the target cells are poorly defined and difficult to study. It is generally thought that the viral capsid undergoes a disassembly process that has broadly been referred to as uncoating. The recent identification of species-specific host restriction factors that target the viral capsid has sparked new interest in retroviral uncoating.

RECENT FINDINGS

Recent studies have used purified HIV-1 cores to study HIV-1 uncoating in vitro. This review summarizes the recent literature relevant to HIV-1 uncoating with a specific emphasis on viral and cellular factors that may regulate capsid stability.

SUMMARY

Uncoating of the viral core is a key step in the infection of HIV-1 that is highly sensitive to alterations in capsid stability. The uncoating step of HIV-1 infection may thus represent an attractive target for the development of novel antiretroviral therapies.

A highly conserved residue in the C-terminal helix of HIV-1 matrix is required for envelope incorporation into virus particles

The incorporation of viral envelope (Env) glycoproteins into nascent particles is an essential step in the production of infectious human immunodeficiency virus type 1 (HIV-1). This process has been shown to require interactions between Env and the matrix (MA) domain of the Gag polyprotein. Previous studies indicate that several residues in the N-terminal region of MA are required for Env incorporation. However, the precise mechanism by which Env proteins are acquired during virus assembly has yet to be fully defined. Here, we examine whether a highly conserved glutamate at position 99 in the C-terminal helix is required for MA function and HIV-1 replication. We analyze a panel of mutant viruses that contain different amino acid substitutions at this position using viral infectivity studies, virus-cell fusion assays, and immunoblotting. We find that E99V mutant viruses are defective for fusion with cell membranes and thus are noninfectious. We show that E99V mutant particles of HIV-1 strains LAI and NL4.3 lack wild-type levels of Env proteins. We identify a compensatory substitution in MA residue 84 and show that it can reverse the E99V-associated defects. Taken together, these results indicate that the C-terminal hydrophobic pocket of MA, which encompasses both residues 84 and 99, has a previously unsuspected and key role in HIV-1 Env incorporation.

Assembly and replication of HIV-1 in T cells with low levels of phosphatidylinositol-(4,5)-bisphosphate

HIV-1 Gag assembles into virus particles predominantly at the plasma membrane (PM). Previously, we observed that phosphatidylinositol-(4,5)-bisphosphate [PI(4,5)P(2)] is essential for Gag binding to the plasma membrane and virus release in HeLa cells. In the current study, we found that PI(4,5)P(2) also facilitates Gag binding to the PM and efficient virus release in T cells. Notably, serial passage of HIV-1 in an A3.01 clone that expresses polyphosphoinositide 5-phosphatase IV (5ptaseIV), which depletes cellular PI(4,5)P(2), yielded an adapted mutant with a Leu-to-Arg change at matrix residue 74 (74LR). Virus replication in T cells expressing 5ptaseIV was accelerated by the 74LR mutation relative to replication of wild type HIV-1 (WT). This accelerated replication of the 74LR mutant was not due to improved virus release. In control T cells, the 74LR mutant releases virus less efficiently than does the WT, whereas in cells expressing 5ptaseIV, the WT and the 74LR mutant are similarly inefficient in virus release. Unexpectedly, we found that the 74LR mutation increased virus infectivity and compensated for the inefficient virus release. Altogether, these results indicate that PI(4,5)P(2) is essential for Gag-membrane binding, targeting of Gag to the PM, and efficient virus release in T cells, which in turn likely promotes efficient virus spread in T cell cultures. In T cells with low PI(4,5)P(2) levels, however, the reduced virus particle production can be compensated for by a mutation that enhances virus infectivity.

Mutations at human immunodeficiency virus type 1 reverse transcriptase tryptophan repeat motif attenuate the inhibitory effect of efavirenz on virus production

HIV-1 virus particle processing is mediated by protease (PR), with enzymatic activation triggered by Gag-Pol/Gag-Pol interaction. We previously reported that truncation mutations at the reverse transcriptase (RT) connection subdomain markedly impair virus particle processing, suggesting an important role for the RT subdomain in PR-mediated virus processing. A highly conserved tryptophan (Trp) repeat motif of the HIV-1 RT connection subdomain is involved in RT dimerization. Our goal in this study was to determine whether mutations at the Trp repeat motif have any effect on PR-mediated virus processing. Our results indicate that even though alanine substitutions at W401 (W401A) or at both W401 and W402 (W401A/W402A) have no major effect on steady-state virus processing, the combined W401A/W402A mutations partially negate and the W401A mutation almost completely negates an efavirenz (EFV)-imposed barrier to virus production. The combination of RT instability and poor enzymatic activity reflects a RT dimerization defect incurred by the mutations. We also found that an artificial p66RT carrying the W401A or W401A/W402A mutations was packaged into virions more efficiently than wild-type p66RT, and that the viral incorporation of p66RT is significantly reduced by EFV, implying a novel effect of EFV on RT-Gag interaction. Our results suggest that the Trp repeat motif may play a role in the Gag-Pol/Gag-Pol interaction that contributes to subsequent PR activation.

Impact of natural sheep-goat transmission on detection and control of small ruminant lentivirus group C infections

Dissemination of small ruminant lentivirus (SRLV) infections in Norway is affected by the different control strategies used for maedi-visna virus (MVV) infections in sheep and caprine arthritis-encephalitis virus (CAEV) infections in goats. Here we investigated SRLV phylogenetic group variants in sheep. CAEV-like isolates, belonging to phylogenetic group C, were found among both seropositive sheep and goats in mixed flocks, in which sheep and goats are kept together. Intra-herd clustering confirmed that mixed flock animals were infected by the same virus variant, suggesting ongoing interspecies transmission. Few sheep flocks were found to be infected with the MVV-like phylogenetic group A. The apparent absence of SRLV group A type in goats is probably due to the MVV control programme and animal management practices. SRLV group C targets lungs and mammary glands in sheep, and induces typical SRLV pathological lesions. SRLV group C isolated from the sheep mammary glands suggested a productive infection and potential for transmission to offspring. SRLV group C was most prevalent among goats. A lower PCR sensitivity in seropositive sheep suggested a lower load of SRLV group C provirus in sheep than in goats. Higher genetic divergence of group C than in other SRLV groups and extensive heterogeneity among group C isolates in the matrix C-terminal region demonstrate the need for identifying conserved target regions when developing PCR protocols for SRLV detection. As sheep and goats may serve as reservoirs for all SRLV genogroup types, successful control programmes require inclusion of both species.

Molecular epidemiology demonstrated three emerging clusters of human immunodeficiency virus type 1 subtype B infection in Hong Kong

We conducted a molecular epidemiological study on newly diagnosed human immunodeficiency virus type 1 (HIV-1)-infected patients in Hong Kong to identify the epidemiological linkage of HIV-1 infection in the locality. Reverse transcription polymerase chain reaction (RT-PCR) for HIV-1 was performed on newly diagnosed HIV-1-positive sera collected from January 2002 to December 2006. PCR products correspond to the env C2V3V4 region and gag p17/p24 junction of the HIV-1 genome were nucleotide sequenced. Phylogenetic analyses performed on the acquired nucleotide sequences revealed that CRF01_AE and subtype B were the two dominant HIV-1 subtypes. Analyses also demonstrated the presence of three emerging HIV-1 clusters among the subtype B sequences in Hong Kong. Individual cluster possesses a unique cluster-specific amino acid signature for identification. Data show that one of the clusters (Cluster I) is rapidly expanding. In addition to the unique cluster-specific amino acid signature, the majority of sequences in Cluster I harbor a 6-amino acid insertion at the gag p17/p24 junction in a region that is thought to be closely associated with HIV-1 infectivity.

Real-time visualization of HIV-1 GAG trafficking in infected macrophages

HIV-1 particle production is driven by the Gag precursor protein Pr55(Gag). Despite significant progress in defining both the viral and cellular determinants of HIV-1 assembly and release, the trafficking pathway used by Gag to reach its site of assembly in the infected cell remains to be elucidated. The Gag trafficking itinerary in primary monocyte-derived macrophages is especially poorly understood. To define the site of assembly and characterize the Gag trafficking pathway in this physiologically relevant cell type, we have made use of the biarsenical-tetracysteine system. A small tetracysteine tag was introduced near the C-terminus of the matrix domain of Gag. The insertion of the tag at this position did not interfere with Gag trafficking, virus assembly or release, particle infectivity, or the kinetics of virus replication. By using this in vivo detection system to visualize Gag trafficking in living macrophages, Gag was observed to accumulate both at the plasma membrane and in an apparently internal compartment that bears markers characteristic of late endosomes or multivesicular bodies. Significantly, the internal Gag rapidly translocated to the junction between the infected macrophages and uninfected T cells following macrophage/T-cell synapse formation. These data indicate that a population of Gag in infected macrophages remains sequestered internally and is presented to uninfected target cells at a virological synapse.

Characterization of replication defects induced by mutations in the basic domain and C-terminus of HIV-1 matrix

Extensive mutagenesis has defined distinct functional domains in the HIV-1 matrix domain (MA). In an attempt to more clearly define functions of regions of MA which affect viral entry, we analyzed mutations in the N-terminal basic and the C-terminal helical domains. Deletions of 8-10 amino acid residues of the C-terminal fifth helix of MA resulted in viruses that were only mildly defective in infectivity and fusion. The defect exhibited by these mutations could largely be attributed to a reduction in levels of viral envelope incorporated into mature virions. Truncation of the gp41 cytoplasmic tail (gp41CT) could rescue the phenotype of one of these mutants. In contrast, mutations of multiple basic residues in the N-terminus of MA were severely defective in both infectivity and fusion. While these mutations induce severe envelope incorporation defects, they also result in virus crippled at a post-entry step, since truncation of the gp41CT could not rescue the infectivity defect.

Incorporation of human immunodeficiency virus type 1 reverse transcriptase into virus-like particles

We demonstrate that a genetically engineered human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) composed mainly of p66 or p51 subunits can be incorporated into virus-like particles (VLPs) when coexpressed with HIV-1 Pr55(gag). VLP-associated RT exhibited a detergent-resistant association with immature cores during sucrose gradient equilibrium centrifugation, suggesting that RT is incorporated into VLPs. However, RT that retains downstream integrase (IN) is severely inhibited in terms of incorporation into VLPs. Results from immunofluorescence tests reveal that RT-IN is primarily localized at the perinuclear area and exhibits poor colocalization with Gag. IN removal leads to a redistribution of RT throughout the cytoplasm and improved RT incorporation into VLPs. Similar results were observed for RT-IN in which alanine was substituted for 186-Lys-Arg-Lys-188 residues of the IN putative nuclear localization signal, suggesting that IN karyophilic properties may partly account for the inhibitory effect of IN on RT incorporation. Although the membrane-binding capacity of RT was markedly reduced compared to that of wild-type Gag or Gag-Pol, the correlation of membrane-binding ability with particle incorporation efficiency was incomplete. Furthermore, we observed that membrane-binding-defective myristylation-minus RT can be packaged into VLPs at the same level as its normal myristylated counterpart. This suggests that the incorporation of RT into VLPs is independent of membrane affinity but very dependent on RT-Gag interaction. Results from a genetic analysis suggest that the Gag-interacting regions of RT mainly reside in the thumb subdomain and that the RT-binding domains of Gag are located in the matrix (MA) and p6 regions.

A mutation in the human immunodeficiency virus type 1 Gag protein destabilizes the interaction of the envelope protein subunits gp120 and gp41

The Gag protein of human immunodeficiency virus type 1 (HIV-1) associates with the envelope protein complex during virus assembly. The available evidence indicates that this interaction involves recognition of the gp41 cytoplasmic tail (CT) by the matrix protein (MA) region of Pr55(Gag). Here we show that substitution of Asp for Leu at position 49 (L49D) in MA results in a specific reduction in particle-associated gp120 without affecting the levels of gp41. Mutant virions were markedly reduced in single-cycle infectivity despite a relatively modest defect in fusion with target cells. Studies with HIV-1 particles containing decreased levels of envelope proteins suggested that the L49D mutation also inhibits a postentry step in infection. Truncation of the gp41 tail, or pseudotyping by vesicular stomatitis virus glycoprotein, restored both the fusion and infectivity of L49D mutant virions to wild-type levels. Truncation of gp41 also resulted in equivalent levels of gp120 on particles with and without the MA mutation and enhanced the replication of the L49D mutant virus in T cells. The impaired fusion and infectivity of L49D mutant particles were also complemented by a single point mutation in the gp41 CT that disrupted the tyrosine-containing endocytic motif. Our results suggest that an altered interaction between the MA domain of Gag and the gp41 cytoplasmic tail leads to dissociation of gp120 from gp41 during HIV-1 particle assembly, thus resulting in impaired fusion and infectivity.

Emergence of a three codon deletion in gag p17 in HIV type 1 subtype C long-term survivors, and general population spread

In a population-based study in northern Malawi we investigated HIV-1 subtype C gag and env gene sequences associated with long-term survival. DNA samples were available from 31 individuals surviving between population surveys carried out in the 1980s and 1990s. Most survivors with paired sequences dating from the 1980s and the 1990s had a three codon deletion in the gag p17 region of the sequence retrieved from the sample collected in the 1990s that was not present in the sequence from the same individual dating from the 1980s. This deletion was also not present in any other 1980s sequences from Malawi, but was common in samples collected in Malawi in the 1990s. The deletion is equivalent to the loss of three amino acids in the D helix region of the gag protein, and may be associated with longer survival and onward transmission.

Genetic control of retrovirus susceptibility in mammalian cells

Host cellular genes can have profound effects on retrovirus replication. Many of these genes encode restriction factors that block virus infection; others encode positive factors that are exploited by the viruses. Recently, a number of such genes have been cloned and characterized, bringing into sharper focus the mechanisms and pathways exploited by these viruses. The major host factors involved in the early phase of the viral life cycle are discussed.

Construction and characterization of a fluorescently labeled infectious human immunodeficiency virus type 1 derivative

The introduction of a label which can be detected in living cells opens new possibilities for the direct analysis of dynamic processes in virus replication, such as the transport and assembly of structural proteins. Our aim was to generate a tool for the analysis of the trafficking of the main structural protein of human immunodeficiency virus type 1 (HIV-1), Gag, as well as for the analysis of virus-host cell interactions in an authentic setting. We describe here the construction and characterization of infectious HIV derivatives carrying a label within the Gag polyprotein. Based on our initial finding that a short epitope tag could be inserted near the C terminus of the matrix domain of Gag without affecting viral replication, we constructed HIV derivatives carrying the egfp gene at the analogous position, resulting in the expression of a Gag-EGFP fusion protein in the authentic viral context. Particles displaying normal viral protein compositions were released from transfected cells, and Gag-EGFP was efficiently processed by the viral protease, yielding the expected products. Furthermore, particles with mature morphology were observed by thin-section electron microscopy. The modified virus was even found to be infectious, albeit with reduced relative infectivity. By preparing mixed particles containing equimolar amounts of Gag-EGFP and Gag, we were able to obtain highly fluorescently labeled virion preparations which displayed normal morphology and full wild-type infectivity, demonstrating that the process of HIV particle assembly displays a remarkable flexibility. The fluorescent virus derivative is a useful tool for investigating the interaction of HIV with live cells.

Role of human immunodeficiency virus type 1 matrix phosphorylation in an early postentry step of virus replication

The matrix domain (MA) is important for targeting of human immunodeficiency virus type 1 Gag assembly to the plasma membrane, envelope incorporation into virions, preintegration complex import into the nucleus, and nuclear export of viral RNA. Myristylation and phosphorylation are key regulatory events for MA function. Previous studies have indicated that MA phosphorylation at serine (Ser) residues is important for viral replication. This study defines the molecular mechanisms of virus particle assembly and infectivity through a detailed study of the role of MA serine phosphorylation. We show that the combined mutation of Ser residues at positions 9, 67, 72, and 77 impairs viral infectivity in dividing and nondividing cells, although the assembly of these Ser mutant viruses is comparable to that of wild-type virus. This defect can be rescued by pseudotyping these mutant viruses with vesicular stomatitis virus G protein, suggesting that these serine residues are critical in an early postentry step of viral infection. The phosphorylation level of MA in defective mutant viruses was severely reduced compared to that of the wild type, suggesting that phosphorylation of Ser-9, -67, -72, and -77 is important for an early postentry step during virus infection.

Mason-Pfizer monkey virus Gag proteins interact with the human sumo conjugating enzyme, hUbc9

Retroviral Gag proteins function during early and late stages of the viral life cycle. To gain additional insight into the cellular requirements for viral replication, a two-hybrid screen was used to identify cellular proteins that interact with the Mason-Pfizer monkey virus Gag protein. One of the cellular proteins found was identified as hUbc9, a nuclear pore-associated E2 SUMO conjugating enzyme. In vitro protein interaction assays verified the association and mapped the interaction domain to the CA protein. In vivo, hUbc9 and Gag colocalized in the cytoplasm as discrete foci near the nuclear membrane. In addition, overexpression of hUbc9 in cells caused a fraction of Gag to colocalize with hUbc9 in the nucleus. These experiments demonstrate that hUbc9 and Gag interact in cells, strengthen the hypothesis that Gag proteins transiently associate with the nuclear compartment during viral replication, and suggest that hUbc9 plays a role in this process.

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.

Characterization of producer cell-dependent restriction of murine leukemia virus replication

We previously reported that the human bronchocarcinoma cell line A549 produces poorly infectious gibbon ape leukemia virus-pseudotyped Moloney murine leukemia virus (MLV). In contrast, similar amounts of virions recovered from human fibrosarcoma HT1080 cells result in 10-fold-higher transduction rates (G. Duisit, A. Salvetti, P. Moullier, and F. Cosset, Hum. Gene Ther. 10:189-200, 1999). We have now extended this initial observation to other type-C envelope (Env) pseudotypes and analyzed the mechanism involved. Structural and morphological analysis showed that viral particles recovered from A549 (A549-MLV) and HT1080 (HT1080-MLV) cells were normal and indistinguishable from each other. They expressed equivalent levels of mature Env proteins and bound similarly to the target cells. Furthermore, incoming particles reached the cytosol and directed the synthesis of linear viral DNA equally efficiently. However, almost no detectable circular DNAs could be detected in A549-MLV-infected cells, indicating that the block of infection resulted from defective nuclear translocation of the preintegration complex. Interestingly, pseudotyping of A549-MLV with vesicular stomatitis virus glycoprotein G restored the amount of circular DNA forms as well as the transduction rates to HT1080-MLV levels, suggesting that the postentry blockage could be overcome by endocytic delivery of the core particles downstream of the restriction point. Thus, in contrast to the previously described target cell-dependent Fv-1 (or Fv1-like) restriction in mammalian cells (P. Pryciak and H. E. Varmus, J. Virol. 66:5959-5966, 1992; G. Towers, M. Bock, S. Martin, Y. Takeuchi, J. P. Stoye, and O. Danos, Proc. Natl. Acad. Sci. USA 97:12295-12299, 2000), we report here a new restriction of MLV replication that relies only on the producer cell type.

Characterization of a virtually full-length human immunodeficiency virus type 1 genome of a prevalent intersubtype (C/B') recombinant strain in China

A molecular epidemiology study was conducted among more than 100 human immunodeficiency virus type 1 (HIV-1) subtype C seropositive intravenous drug users (IDUs) from China. Genotyping based on the envelope C2V3 coding region revealed the highest homology of the most prevalent virus strains circulating throughout China to subtype C sequences of Indian origin. Based on these results, a virtually full-length genome representing the most prevalent class of clade C strains circulating throughout China was directly amplified from peripheral blood mononuclear cells of a selected HIV-infected IDU and subcloned. Sequence analysis identified a mosaic structure, suggesting extensive intersubtype recombination events between genomes of the prevalent clade C and (B')-subtype Thai virus strains of that geographic region. Recombinant Identification Program analysis and phylogenetic bootstrapping suggested that there were 10 breakpoints (i) in the gag-pol coding region, (ii) in vpr and at the 3' end of the vpu gene, and (iii) in the nef open reading frame. (B')-sequences therefore include (i) several insertions in the gag-pol coding region; (ii) 3'-vpr, the complete vpu gene, and the first exons of tat and rev; and (iii) the 5' half of the nef gene. Breakpoints located in the vpr/vpu coding region as well as in the nef gene of 97cn54 were found at almost identical positions of all subtype C strains isolated from IDUs living in different areas of China, suggesting a common ancestor for the C/B' recombinant strains. More than 50% of well-defined subtype B-derived cytotoxic T-lymphocyte epitopes within Gag and Pol and 10% of the known epitopes in Env were found to exactly match sequences within in this clade C/B' chimeric reference strain. These results may substantially facilitate a biological comparison of clade C-derived reference strains as well as the generation of useful reagents supporting vaccine-related efforts in China.

The carboxy-terminal fragment of nucleolin interacts with the nucleocapsid domain of retroviral gag proteins and inhibits virion assembly

A yeast two-hybrid screen for cellular proteins that interact with the murine leukemia virus (MuLV) Gag protein resulted in the identification of nucleolin, a host protein known to function in ribosome assembly. The interacting fusions contained the carboxy-terminal 212 amino acids of nucleolin [Nuc(212)]. The nucleocapsid (NC) portion of Gag was necessary and sufficient to mediate the binding to Nuc(212). The interaction of Gag with Nuc(212) could be demonstrated in vitro and was manifested in vivo by the NC-dependent incorporation of Nuc(212) inside MuLV virions. Overexpression of Nuc(212), but not full-length nucleolin, potently and specifically blocked MuLV virion assembly and/or release. A mutant of MuLV, selected to specifically disrupt the binding to Nuc(212), was found to be severely defective for virion assembly. This mutant harbors a single point mutation in capsid (CA) adjacent to the CA-NC junction, suggesting a role for this region in Moloney MuLV assembly. These experiments demonstrate that selection for proteins that bind assembly domain(s) can yield potent inhibitors of virion assembly. These experiments also raise the possibility that a nucleolin-Gag interaction may be involved in virion assembly.

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

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

Translation elongation factor 1-alpha interacts specifically with the human immunodeficiency virus type 1 Gag polyprotein

Human immunodeficiency virus type 1 (HIV-1) gag-encoded proteins play key functions at almost all stages of the viral life cycle. Since these functions may require association with cellular factors, the HIV-1 matrix protein (MA) was used as bait in a yeast two-hybrid screen to identify MA-interacting proteins. MA was found to interact with elongation factor 1-alpha (EF1alpha), an essential component of the translation machinery that delivers aminoacyl-tRNA to ribosomes. EF1alpha was then shown to bind the entire HIV-1 Gag polyprotein. This interaction is mediated not only by MA, but also by the nucleocapsid domain, which provides a second, independent EF1alpha-binding site on the Gag polyprotein. EF1alpha is incorporated within HIV-1 virion membranes, where it is cleaved by the viral protease and protected from digestion by exogenously added subtilisin. The specificity of the interaction is demonstrated by the fact that EF1alpha does not bind to nonlentiviral MAs and does not associate with Moloney murine leukemia virus virions. The Gag-EF1alpha interaction appears to be mediated by RNA, in that basic residues in MA and NC are required for binding to EF1alpha, RNase disrupts the interaction, and a Gag mutant with undetectable EF1alpha-binding activity is impaired in its ability to associate with tRNA in cells. Finally, the interaction between MA and EF1alpha impairs translation in vitro, a result consistent with a previously proposed model in which inhibition of translation by the accumulation of Gag serves to release viral RNA from polysomes, permitting the RNA to be packaged into nascent virions.

Somatic cell mutants resistant to retrovirus replication: intracellular blocks during the early stages of infection

To identify cellular functions involved in the early phase of the retroviral life cycle, somatic cell mutants were isolated after selection for resistance to infection. Rat2 fibroblasts were treated with chemical mutagens, and individual virus-resistant clones were recovered after selection for resistance to infection. Two clones were characterized in detail. Both mutant lines were resistant to infection by both ecotropic and amphotropic murine viruses, as well as by human immunodeficiency virus type 1 pseudotypes. One clone showed a strong block to reverse transcription of the retroviral RNA, including formation of the earliest DNA products. The second clone showed normal levels of viral DNA synthesis but did not allow formation of the circular DNAs normally found in the nucleus. Cell fractionation showed that the viral preintegration complex was present in a form that could not be extracted under conditions that readily extracted the complex from wild-type cells. The results suggest that the DNA was trapped in a nonproductive state and excluded from the nucleus of the infected cell. The properties of these two mutant lines suggest that host gene products play important roles both before and after reverse transcription.

Reversion of a human immunodeficiency virus type 1 matrix mutation affecting Gag membrane binding, endogenous reverse transcriptase activity, and virus infectivity

We previously characterized mutations in the human immunodeficiency virus type 1 matrix (MA) protein that displayed reduced infectivity in single-round assays, defects in the stable synthesis of viral DNA in infected cells, and impaired endogenous reverse transcriptase activity. The mutants, which contained substitutions in a highly conserved Leu at MA amino acid 20, also increased binding of Gag to membrane. To elucidate further the role of MA in the virus replication cycle, we have characterized a viral revertant of an amino acid 20 mutant (20LK). The revertant virus, which replicates with essentially wild-type kinetics in H9 cells, contains second-site compensatory changes at MA amino acids 73 (E-->K) and 82 (A-->T), while retaining the original 20LK mutation. Single-cycle infectivity assays, performed with luciferase-expressing viruses, show that the 20LK/73EK/82AT triple mutant displays markedly improved infectivity relative to the original 20LK mutant. The stable synthesis of viral DNA in infected cells is also significantly increased compared with that of 20LK DNA. Furthermore, activity of revertant virions in endogenous reverse transcriptase assays is restored to near-wild-type-levels. Interestingly, although 20LK/73EK/82AT reverses the defects in replication kinetics, postentry events, and endogenous reverse transcriptase activity induced by the 20LK mutation, the reversion does not affect the 20LK-imposed increase in Gag membrane binding. Mutants containing single and double amino acid substitutions were constructed, and their growth kinetics were examined. Only virus containing all three changes (20LK/73EK/82AT) grew with significantly accelerated kinetics; 73EK, 73EK/82AT, and 20LK/82AT mutants displayed pronounced defects in virus particle production. Viral core-like complexes were isolated by sucrose density gradient centrifugation of detergent-treated virions. Intriguingly, the protein composition of wild-type and mutant detergent-resistant complexes differed markedly. In wild-type and 20LK complexes, MA was removed following detergent solubilization of the viral membrane. In contrast, in revertant preparations, the majority of MA cosedimented with the detergent-resistant complex. These results suggest that the 20LK/73EK/82AT mutations induced a significant alteration in MA-MA or MA-core interactions.

Human immunodeficiency virus type 1 protease triggers a myristoyl switch that modulates membrane binding of Pr55(gag) and p17MA

The human immunodeficiency virus type 1 (HIV-1) Pr55(gag) gene product directs the assembly of virions at the inner surface of the cell plasma membrane. The specificity of plasma membrane binding by Pr55(gag) is conferred by a combination of an N-terminal myristoyl moiety and a basic residue-rich domain. Although the myristate plus basic domain is also present in the p17MA proteolytic product formed upon Pr55(gag) maturation, the ability of p17MA to bind to membranes is significantly reduced. It was previously reported that the reduced membrane binding of p17MA was due to sequestration of the myristate moiety by a myristoyl switch (W. Zhou and M. D. Resh, J. Virol. 70:8540-8548, 1996). Here we demonstrate directly that treatment of membrane-bound Pr55(gag) in situ with HIV-1 protease generates p17MA, which is then released from the membrane. Pr55(gag) was synthesized in reticulocyte lysates, bound to membranes, and incubated with purified HIV-1 protease. The p17MA product in the membrane-bound and soluble fractions was analyzed following proteolysis. Newly generated p17MA initially was membrane bound but then displayed a slow, time-dependent dissociation resulting in 65% solubilization. Residual p17MA could be extracted from the membranes with either high pH or high salt. Treatment of membranes from transfected COS-1 cells with protease revealed that Pr55(gag) was present within sealed membrane vesicles and that the release of p17MA occurred only when detergent and salt were added. We present a model proposing that the HIV-1 protease is the "trigger" for a myristoyl switch mechanism that modulates the membrane associations of Pr55(gag) and p17MA in virions and membranes.

Multiple functions for the basic amino acids of the human T-cell leukemia virus type 1 matrix protein in viral transmission

We studied the involvement of the human T-cell leukemia virus type 1 (HTLV-1) Gag matrix protein in the cell-to-cell transmission of the virus using missense mutations of the basic amino acids. These basic amino acids are clustered at the N terminus of the protein in other retroviruses and are responsible for targeting the Gag proteins to the plasma membrane. In the HTLV-bovine leukemia virus genus of retroviruses, the basic amino acids are distributed throughout the matrix protein sequence. The HTLV-1 matrix protein contains 11 such residues. A wild-type phenotype was obtained only for mutant viruses with mutations at one of two positions in the matrix protein. The phenotypes of the other nine mutant viruses showed that the basic amino acids are involved at various steps of the replication cycle, including some after membrane targeting. Most of these nine mutations allowed normal synthesis, transport, and cleavage of the Gag precursor, but particle release was greatly affected for seven of them. In addition, four mutated proteins with correct particle release and envelope glycoprotein incorporation did not however permit cell-to-cell transmission of HTLV-1. Thus, particle release, although required, is not sufficient for the cell-to-cell transmission of HTLV-1, and the basic residues of the matrix protein are involved in steps that occur after viral particle release.

Cleavage of the murine leukemia virus transmembrane env protein by human immunodeficiency virus type 1 protease: transdominant inhibition by matrix mutations

We have identified mutations in the human immunodeficiency virus type 1 (HIV-1) matrix protein (MA) which block infectivity of virions pseudotyped with murine leukemia virus (MuLV) envelope (Env) glycoproteins without affecting infectivity conferred by HIV-1 Env or vesicular stomatitis virus G glycoproteins. This inhibition is very potent and displays a strong transdominant effect; infectivity is reduced more than 100-fold when wild-type and mutant molecular clones are cotransfected at a 1:1 ratio. This phenomenon is observed with both ecotropic and amphotropic MuLV Env. The MA mutations do not affect the incorporation of MuLV Env into virions. We demonstrate that in HIV-1 virions pseudotyped with MuLV Env, the HIV-1 protease (PR) efficiently catalyzes the cleavage of the p15(E) transmembrane (TM) protein to p12(E). Immunoprecipitation analysis of pseudotyped virions reveals that the mutant MA blocks this HIV-1 PR-mediated cleavage of MuLV TM. Furthermore, the transdominant inhibition exerted by the mutant MA on wild-type infectivity correlates with the relative level of p15(E) cleavage. Consistent with the hypothesis that abrogation of infectivity imposed by the mutant MA is due to inhibition of p15(E) cleavage, mutant virions are significantly more infectious when pseudotyped with a truncated p12(E) form of MuLV Env. These results indicate that HIV-1 Gag sequences can influence the viral PR-mediated processing of the MuLV TM Env protein p15(E). These findings have implications for the development of HIV-1-based retroviral vectors pseudotyped with MuLV Env, since p15(E) cleavage is essential for activating membrane fusion and virus infectivity.

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.

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.

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.

Mutations of the human immunodeficiency virus type 1 p6Gag domain result in reduced retention of Pol proteins during virus assembly

One of the crucial steps in the assembly of the human immunodeficiency virus type 1 (HIV-1) and other retroviruses is the incorporation and retention of all the key viral enzymes in released virions. The viral enzymes protease, reverse transcriptase, and integrase of HIV-1 are initially synthesized as Gag-Pol fusion polyproteins. It has been shown that the incorporation of Gag-Pol polyproteins during virus assembly requires the Gag domains that are shared by the Gag and Gag-Pol precursors. We now report that truncation of the C-terminal p6 domain of HIV-1 Gag, which is present in the Gag precursor but not in the Gag-Pol precursor, drastically reduced the amount of Pol proteins in the mutant virions. Mutations in the lentivirus conserved motif P(T/S)APP in p6 also drastically reduced the amount of Pol proteins in mutant virions. The steady-state levels of Gag-Pol precursors and cleaved Pol proteins in the transfected cells were not affected by mutations in p6. The incorporation of unprocessed Gag-Pol precursors into p6 mutant virions was detected when the viral protease was mutated, suggesting that the interactions among mutant Gag molecules and Gag-Pol precursors were not significantly affected. These results suggest that the p6 domain of HIV-1 Gag may play an important role in recruiting or retaining cleaved Pol proteins during virus assembly.

The I domain is required for efficient plasma membrane binding of human immunodeficiency virus type 1 Pr55Gag

The interaction of the human immunodeficiency virus type 1 (HIV-1) Pr55Gag molecule with the plasma membrane of an infected cell is an essential step of the viral life cycle. Myristic acid and positively charged residues within the N-terminal portion of MA constitute the membrane-binding domain of Pr55Gag. A separate assembly domain, termed the interaction (I) domain, is located nearer the C-terminal end of the molecule. The I domain is required for production of dense retroviral particles, but has not previously been described to influence the efficiency of membrane binding or the subcellular distribution of Gag. This study used a series of Gag-green fluorescent protein fusion constructs to define a region outside of MA which determines efficient plasma membrane interaction. This function was mapped to the nucleocapsid (NC) region of Gag. The minimal region in a series of C-terminally truncated Gag proteins conferring plasma membrane fluorescence was identified as the N-terminal 14 amino acids of NC. This same region was sufficient to create a density shift in released retrovirus-like particles from 1.13 to 1.17 g/ml. The functional assembly domain previously termed the I domain is thus required for the efficient plasma membrane binding of Gag, in addition to its role in determining the density of released particles. We propose a model in which the I domain facilitates the interaction of the N-terminal membrane-binding domain of Pr55Gag with the plasma membrane.

HIV p17 enhances lymphocyte proliferation and HIV-1 replication after binding to a human serum factor

OBJECTIVE

To analyse the role of recombinant HIV-1 protein p17 in the modulation of cell activity.

METHODS

Peripheral blood mononuclear cells (PBMC) obtained from healthy donors were cultured in the presence or absence of p17 with mitogens such as phytohaemagglutinin or interleukin-2 and their response assayed by cell proliferation. Cross-linking experiments were employed to investigate the presence of a binding between p17 and factor(s) present in human serum. An immunoenzymatic assay for p24 antigen detection was used to analyse the effect of the addition of exogenous p17 to cultures of PBMC infected with HIV-1 in vitro.

RESULTS

Purified recombinant p17 protein at a concentration of 0.25 microg/ml significantly increased the proliferation of preactivated PBMC obtained from healthy donors. This effect was obtained by binding p17 to factor(s) present in human serum and observed on both CD4+ and CD8+ T cells. Recombinant p17 also induced an increased rate of HIV-1 replication, probably due to enhanced T-cell proliferation. The activity of p17 protein was inhibited by anti-p17 antibodies generated by injecting recombinant p17 in rabbits, but not by human antibodies generated during the natural course of HIV infection.

CONCLUSION

Characterization of the human factor(s) and identification of the interacting p17 epitope(s) will improve our understanding of the mechanisms used by HIV to efficiently replicate in our organisms.

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.

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.

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.

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.

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.

The role of Gag in human immunodeficiency virus type 1 virion morphogenesis and early steps of the viral life cycle

The phenotypes of a series of mutant human immunodeficiency virus type 1 proviruses with linker insertion and deletion mutations within the gag coding region were characterized. These mutants, with mutations in the matrix, capsid, and p2 coding regions, produced replication-defective virion particles with defects in the early steps of the viral life cycle. To investigate this phenotype further, the abilities of mutant virion particles to enter T cells, initiate and complete reverse transcription, and transport the newly transcribed proviral DNA were investigated. Only 4 of 10 of the mutants appeared to make wild-type levels of viral DNA. Biochemical analyses of the mutants revealed the middle region of CA as being important in determining virion particle density and sedimentation in velocity gradients. This region also appears to be critical in determining the morphology of mature virion particles by electron microscopy. Particles with aberrant morphology were uninfectious, and only those mutants which displayed cone-shaped cores were capable of carrying out the early steps of the viral life cycle. Thus, the normal morphology of human immunodeficiency virus type 1 appears to be critical to infectivity.

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.