Mutations in gag proteins P12 and P15 of Moloney murine leukemia virus block early stages of infection

BET-Independent Murine Leukemia Virus Integration Is Retargeted In Vivo and Selects Distinct Genomic Elements for Lymphomagenesis

Moloney murine leukemia virus (MLV) infects BALB/c mice and induces T-cell lymphoma in mice. Retroviral integration is mediated by the interaction of the MLV integrase (IN) with members of the bromodomain and extraterminal motif (BET) protein family (BRD2, BRD3, and BRD4). The introduction of the W390A mutation into MLV IN abolishes the BET interaction. Here, we compared the replication of W390A MLV to that of wild-type (WT) MLV in adult BALB/c mice to study the role of BET proteins in replication, integration, and tumorigenesis in vivo. Comparing WT and W390A MLV infections revealed similar viral loads in the blood, thymus, and spleen cells. Interestingly, W390A MLV integration was retargeted away from GC-enriched genomic regions. However, both WT MLV- and W390A MLV-infected mice developed T-cell lymphoma after similar latencies represented by an enlarged thymus and spleen and multiorgan tumor infiltration. Integration site sequencing from splenic tumor cells revealed clonal expansion in all WT MLV- and W390A MLV-infected mice. However, the integration profiles of W390A MLV and WT MLV differed significantly. Integrations were enriched in enhancers and promoters, but compared to the WT, W390A MLV integrated less frequently into enhancers and more frequently into oncogene bodies such as Notch1 and Ppp1r16b. We conclude that host factors direct MLV in vivo integration site selection. Although BET proteins target WT MLV integration preferentially toward enhancers and promoters, insertional lymphomagenesis can occur independently from BET, likely due to the intrinsically strong enhancer/promoter of the MLV long terminal repeat (LTR).

IMPORTANCE In this study, we have shown that the in vivo replication of murine leukemia virus happens independently of BET proteins, which are key host determinants involved in retroviral integration site selection. This finding opens a new research line in the discovery of alternative viral or host factors that may complement the dominant host factor. In addition, our results show that BET-independent murine leukemia virus uncouples insertional mutagenesis from gene enhancers, although lymphomagenesis still occurs despite the lack of an interaction with BET proteins. Our findings also have implications for the engineering of BET-independent MLV-based vectors for gene therapy, which may not be a safe alternative.

Covalent conjugation of the equine infectious anemia virus Gag with SUMO

The conjugation of small ubiquitin-like modifier (SUMO) to the target protein, namely, SUMOylation, is involved in the regulation of many important biological events including host-pathogen interaction. Some viruses have evolved to exploit the host SUMOylation machinery to modify their own protein. Retroviral Gag protein plays critical roles in the viral life cycle. The HIV-1 p6 and the Moloney murine leukemia virus CA have been reported to be conjugated with SUMO. In this study, we report for the first time, to our knowledge, the covalent conjugation of equine infectious anemia virus (EIAV) Gag with SUMO. The C-terminal p9 domain of Gag is a main target for SUMOylation and SUMO is attached to multiple sites of p9, including K30 whose mutation abolished p9 SUMOylation completely. The SUMOylation of p9, but not the p9-K30 mutant, was also detected in equine fibroblastic cells ATCC^® CCL-57™. Ubc9 and its C93 residue are indispensable for the SUMOylation of p9. Using confocal microscopy, it is found that EIAV Gag localizes primarily, if not exclusively, in the cytoplasm of the cell and the co-localization of EIAV Gag with Ubc9 was observed. Our findings that EIAV Gag is SUMOylated at p9-K30, together with previous findings on the defects of p9-K30 mutant in viral DNA translocation from cytoplasm to the nucleus, suggests that SUMOylation of Gag may be involved in such functions.

p12 tethers the murine leukemia virus pre-integration complex to mitotic chromosomes

The p12 protein of the murine leukemia virus (MLV) is a constituent of the pre-integration complex (PIC) but its function in this complex remains unknown. We developed an imaging system to monitor MLV PIC trafficking in live cells. This allowed the visualization of PIC docking to mitotic chromosomes and its release upon exit from mitosis. Docking occurred concomitantly with nuclear envelope breakdown and was impaired for PICs of viruses with lethal p12 mutations. Insertion of a heterologous chromatin binding module into p12 of one of these mutants restored PICs attachment to the chromosomes and partially rescued virus replication. Capsid dissociated from wild type PICs in mitotic cells but remained associated with PICs harboring tethering-negative p12 mutants. Altogether, these results explain, in part, MLV restriction to dividing cells and reveal a role for p12 as a factor that tethers MLV PIC to mitotic chromosomes.

Retroviruses, including the murine leukemia virus (MLV), reverse transcribe their RNA genome to a DNA copy, which travels from the cytoplasm to the nucleus as part of a ‘pre-integration complex’ (PIC), to integrate into cellular chromosomes. The viral p12 protein is a constituent of the MLV PIC, but its function in this complex has remained unknown. We developed a real-time imaging system to detect p12 and MLV PICs in live cells. This revealed that p12 tethers the MLV PIC to mitotic chromosomes. Accordingly, PICs derived from viruses with specific lethal mutations in p12 failed to attach to the chromosomes, and insertion of a heterologous chromatin binding module into p12 restored PICs attachment to the chromosomes and rescued virus replication. In addition, docking of wild type PICs to chromosomes coincided with nuclear envelope breakdown during mitosis, and detachment occurred upon exit from mitosis. Capsid - another viral component of the PIC - dissociated from wild type PICs in mitotic cells but remained associated with PICs harboring tethering-negative p12 mutants, suggesting interplay between these two proteins in regulating targeting of mitotic chromosomes by the PIC. These results highlight steps contributing to the high tropism of MLV to dividing cells.

The gammaretroviral p12 protein has multiple domains that function during the early stages of replication

Background

The Moloney murine leukaemia virus (Mo-MLV) gag gene encodes three main structural proteins, matrix, capsid and nucleocapsid and a protein called p12. In addition to its role during the late stages of infection, p12 has an essential, but undefined, function during early post-entry events. As these stages of retroviral infection remain poorly understood, we set out to investigate the function of p12.

Results

Examination of the infectivity of Mo-MLV virus-like particles containing a mixture of wild type and mutant p12 revealed that the N- and C-terminal regions of p12 are sequentially acting domains, both required for p12 function, and that the N-terminal activity precedes the C-terminal activity in the viral life cycle. By creating a panel of p12 mutants in other gammaretroviruses, we showed that these domains are conserved in this retroviral genus. We also undertook a detailed mutational analysis of each domain, identifying residues essential for function. These data show that different regions of the N-terminal domain are necessary for infectivity in different gammaretroviruses, in stark contrast to the C-terminal domain where the same region is essential for all viruses. Moreover, chimeras between the p12 proteins of Mo-MLV and gibbon ape leukaemia virus revealed that the C-terminal domains are interchangeable whereas the N-terminal domains are not. Finally, we identified potential functions for each domain. We observed that particles with defects in the N-terminus of p12 were unable to abrogate restriction factors, implying that their cores were impaired. We further showed that defects in the C-terminal domain of p12 could be overcome by introducing a chromatin binding motif into the protein.

Conclusions

Based on these data, we propose a model for p12 function where the N-terminus of p12 interacts with, and stabilizes, the viral core, allowing the C-terminus of p12 to tether the preintegration complex to host chromatin during mitosis, facilitating integration.

The Gag cleavage product, p12, is a functional constituent of the murine leukemia virus pre-integration complex

The p12 protein is a cleavage product of the Gag precursor of the murine leukemia virus (MLV). Specific mutations in p12 have been described that affect early stages of infection, rendering the virus replication-defective. Such mutants showed normal generation of genomic DNA but no formation of circular forms, which are markers of nuclear entry by the viral DNA. This suggested that p12 may function in early stages of infection but the precise mechanism of p12 action is not known. To address the function and follow the intracellular localization of the wt p12 protein, we generated tagged p12 proteins in the context of a replication-competent virus, which allowed for the detection of p12 at early stages of infection by immunofluorescence. p12 was found to be distributed to discrete puncta, indicative of macromolecular complexes. These complexes were localized to the cytoplasm early after infection, and thereafter accumulated adjacent to mitotic chromosomes. This chromosomal accumulation was impaired for p12 proteins with a mutation that rendered the virus integration-defective. Immunofluorescence demonstrated that intracellular p12 complexes co-localized with capsid, a known constituent of the MLV pre-integration complex (PIC), and immunofluorescence combined with fluorescent in situ hybridization (FISH) revealed co-localization of the p12 proteins with the incoming reverse transcribed viral DNA. Interactions of p12 with the capsid and with the viral DNA were also demonstrated by co-immunoprecipitation. These results imply that p12 proteins are components of the MLV PIC. Furthermore, a large excess of wt PICs did not rescue the defect in integration of PICs derived from mutant p12 particles, demonstrating that p12 exerts its function as part of this complex. Altogether, these results imply that p12 proteins are constituent of the MLV PIC and function in directing the PIC from the cytoplasm towards integration.

All retroviruses reverse transcribe their RNA genome to a DNA copy in the cytoplasm of the infected cell. To be expressed, the viral genomic DNA has to travel to the cell nucleus and to integrate into the cellular chromosomes. This trafficking is governed by cellular and viral proteins that associate with the viral genome to form a ‘pre-integration complex’ (PIC), yet the full composition of this complex is unknown. Former studies showed that for the murine leukemia virus (MLV), mutations in a viral protein named p12 abrogate MLV infection, after reverse transcription and prior to the integration step, suggesting a role for this protein in early stages of infection. However, the precise mechanism of p12 action is not known. We combined microscopic, genetic and biochemical techniques to provide evidence that the p12 protein is part of the MLV PIC and that it exerts its function from within this complex. These analyses also suggest a role for p12 in the trafficking of the PIC from the cytoplasm to the chromosomes of the infected cell. Altogether, these findings highlight an important ‘building block’ of a complex that is essential for MLV infection.

PDZD8 is a novel Gag-interacting factor that promotes retroviral infection

In a yeast two-hybrid screen for cellular factors that could interact with human immunodeficiency virus type 1 (HIV-1) Gag protein, we identified PDZD8 and confirmed the interaction by coimmunoprecipitation (co-IP). PDZD8 overexpression promoted the initiation of reverse transcription and increased infection by pseudotyped retroviruses independent of the route of viral entry, while transient knockdown of endogenous levels decreased HIV-1 infection. A mutant of PDZD8 lacking a predicted coiled-coil domain in its Gag-interacting region failed to bind Gag and promote HIV-1 infection, identifying the domain of PDZD8 required for mediating these effects. As such, we identify PDZD8 as a novel positive mediator of retroviral infection.

Random screening for dominant-negative mutants of the cytomegalovirus nuclear egress protein M50

Inactivation of gene products by dominant-negative (DN) mutants is a powerful tool to assign functions to proteins. Here, we present a two-step procedure to establish a random screen for DN alleles, using the essential murine cytomegalovirus gene M50 as an example. First, loss-of-function mutants from a linker-scanning library were tested for inhibition of virus reconstitution with the help of FLP-mediated ectopic insertion of the mutants into the viral genome. Second, DN candidates were confirmed by conditional expression of the inhibitory proteins in the virus context. This allowed the quantification of the inhibitory effect, the identification of the morphogenesis block, and the construction of DN mutants with improved activity. Based on these observations a DN mutant of the homologous gene (UL50) in human cytomegalovirus was predicted and constructed. Our data suggest that a proline-rich sequence motif in the variable region of M50/UL50 represents a new functional site which is essential for nuclear egress of cytomegalovirus capsids.

Interaction of Moloney murine leukemia virus matrix protein with IQGAP

The matrix protein (MA) of the Moloney murine leukemia virus (M-MuLV) was found to interact with IQGAP1, a prominent regulator of the cytoskeleton. Mutational studies defined residues of MA critical for the interaction, and tests of viruses carrying MA mutations revealed a near-perfect correlation between binding and virus replication. The replication-defective mutants showed defects in both early and late stages of the life cycle. Four viable second-site revertant viruses were isolated from three different replication-defective parental mutants, and in all cases the interaction with IQGAP1 was restored by the suppressor mutations. The interaction of MA and IQGAP1 was readily detected in vitro and in vivo. Virus replication was potently inhibited by a C-terminal fragment of IQGAP1, and impaired by RNAi knockdown of IQGAP1 and 2. We suggest that the IQGAPs link the virus to the cytoskeleton for trafficking both into and out of the cell.

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.

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.

Phosphorylated serine residues and an arginine-rich domain of the moloney murine leukemia virus p12 protein are required for early events of viral infection

Mutational analyses of the p12 Gag phosphoprotein of Moloney murine leukemia virus have demonstrated its participation in both virus assembly and the early stages of infection. The molecular mechanisms by which p12 functions in these events are still poorly understood. We performed studies to examine the significance of p12 phosphorylation in the viral life cycle. Alanine substitutions were introduced at the potential phosphorylation sites in p12, and the resulting mutants were tested for replication. Mutant viruses with changes at S61 and S78 were severely impaired, whereas the other mutant viruses were viable. S61 was shown to be required for normal levels of phosphorylation of p12 in vivo. These defective mutant viruses showed no apparent alteration to Gag protein processing or reduction in the yield of virions after transient transfection, but the mutants failed to form circular viral DNAs in acutely infected cells. Sequence analysis of revertant clones derived from S(61,65)A mutant virus revealed two classes: one group with a single mutation at a residue adjacent to S61 and another group with mutations introducing new positive charges surrounding S61. In vivo [32P]orthophosphate labeling indicated that the rescue of the S(61,65)A mutant virus did not result in a significant increase in the phosphorylation level of p12. Alanine substitutions of an arginine-rich stretch near S61 (at R-66, -68, -70, and -71) resulted in the same phenotype as the S(61,65)A mutant virus. The restored function of S(61,65)A mutant virus by second or third site mutations may result from a structural change or the addition of positively charged residues in the arginine-rich region.

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.

Identification of residues of the Moloney murine leukemia virus nucleocapsid critical for viral DNA synthesis in vivo

The nucleocapsid (NC) protein of retroviruses is a small nucleic acid-binding protein important in virion assembly and in the encapsidation of the viral RNA genome into the virion particle. Multiple single-amino-acid substitutions were introduced into the NC of Moloney murine leukemia virus to examine further its role in viral replication. Two residues were shown to play important roles in the early events of replication. Unlike viruses with previously characterized NC mutations, these viruses showed no impairment in the late events of replication. Viruses containing the substitutions L21A and K30A expressed the normal complement of properly processed viral Gag proteins. Analysis of the RNA content of mutant virions revealed normal levels of unspliced and spliced viral RNA, and the tRNA(Pro) primer was properly annealed to the primer binding site on the viral genome. The virions demonstrated no defect in initiation of reverse transcription using the endogenous tRNA primer or in the synthesis of long viral DNA products in vitro. Nonetheless, viruses possessing these NC mutations demonstrated significant defects in the synthesis and accumulation of viral DNA products in vivo.

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.

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

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

RNA dimerization defect in a Rous sarcoma virus matrix mutant

The retrovirus matrix (MA) sequence of the Gag polyprotein has been shown to contain functions required for membrane targeting and binding during particle assembly and budding. Additional functions for MA have been proposed based on the existence of MA mutants in Rous sarcoma virus (RSV), murine leukemia virus, human immunodeficiency virus type 1, and human T-cell leukemia virus type 1 that lack infectivity even though they release particles of normal composition. Here we describe an RSV MA mutant with a surprising and previously unreported phenotype. In the mutant known as Myr1E, the small membrane-binding domain of the Src oncoprotein has been added as an N-terminal extension of Gag. While Myr1E is not infectious, full infectivity can be reestablished by a single amino acid substitution in the Src sequence (G2E), which eliminates the addition of myristic acid and the membrane-binding capacity of this foreign sequence. The presence of myristic acid at the N terminus of the Myr1E Gag protein does not explain its replication defect, because other myristylated derivatives of RSV Gag are fully infectious (e.g., Myr2 [C. R. Erdie and J. W. Wills, J. Virol. 64:5204-5208, 1990]). Biochemical analyses of Myr1E particles reveal that they contain wild-type levels of the Gag cleavage products, Env glycoproteins, and reverse transcriptase activity when measured on an exogenous template. Genomic RNA incorporation appears to be mildly reduced compared to the wild-type level. Unexpectedly, RNA isolated from Myr1E particles is monomeric when analyzed on nondenaturing Northern blots. Importantly, the insertional mutation does not lie within previously identified dimer linkage sites. In spite of the dimerization defect, the genomic RNA from Myr1E particles serves efficiently as a template for reverse transcription as measured by an endogenous reverse transcriptase assay. In marked contrast, after infection of avian cells, the products of reverse transcription are nearly undetectable. These findings might be explained either by the loss of a normal function of MA needed in the formation or stabilization of RNA dimers or by the interference in such events by the mutant MA molecules. It is possible that Myr1E viruses package a single copy of viral RNA.

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.

HEED, the product of the human homolog of the murine eed gene, binds to the matrix protein of HIV-1

heed, the human homolog of mouse eed and Drosophila esc, two members of the trithorax (trx) and Polycomb group (Pc-G) of genes, was isolated by screening an activated lymphocyte cDNA library versus the immunodeficiency virus type 1 (HIV-1) MA protein used as a bait in a two-hybrid system in yeast. The human EED protein (HEED) had 99. 5% identity with the mouse EED protein and contained seven WD repeats. Two heed gene transcripts were identified, with a putative 407-nucleotide-long intron, giving rise to two HEED protein isoforms of 535 and 494 residues in length, respectively. The shorter HEED isoform, originated from the unspliced message, lacked the seventh WD repeat. HEED was found to bind to MA protein in vitro, as efficiently as in vivo in yeast cells. Site-directed mutagenesis and phage biopanning suggested that the interaction between HEED and MA involved the N-terminal region of the MA protein, including the first polybasic signal, in a MA conformation-dependent manner. In the HEED protein, however, two discrete linear MA-binding motifs were identified within residues 388-403, overlapping the origin of the fifth WD repeat. Deletion of the C-terminal 41 residues of HEED, spanning the seventh WD repeat, as in the 494-residue HEED protein, was detrimental to HEED-MA interaction in vivo, suggesting the existence of another C-terminal binding site and/or a conformational role of the HEED C-terminal domain in the MA-HEED interaction. MA and HEED proteins co-localized within the nucleus of co-transfected human cells and of recombinant baculovirus co-infected insect cells. This and the failure of HEED to bind to uncleaved GAG precursor suggested a role of HEED at the early stages of virus infection, rather than late in the virus life cycle.

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.

Genetic analysis of interactions between Gag proteins of Rous sarcoma virus

The yeast two-hybrid system was used to characterize homomeric interactions between the Gag proteins of Rous sarcoma virus (RSV). The RSV Gag precursor was found to interact strongly with itself and not with various control proteins. The RSV Gag did not interact significantly with Gag proteins of a variety of other retroviruses, including murine leukemia viruses and primate lentiviruses. Deletion analysis suggested that two nonoverlapping regions are independently sufficient to mediate RSV Gag-Gag dimerization. One such region lies near the N terminus and contains p2, p10, and a large N-terminal part of the capsid (CA) domain; the other is localized in the C terminus and includes a small C-terminal portion of CA and the nucleocapsid protein. These interaction domains may play roles in viral assembly.

Structural analysis of membrane-bound retrovirus capsid proteins

We have developed a system for analysis of histidine-tagged (His-tagged) retrovirus core (Gag) proteins, assembled in vitro on lipid monolayers consisting of egg phosphatidylcholine (PC) plus the novel lipid DHGN. DHGN was shown to chelate nickel by atomic absorption spectrometry, and DHGN-containing monolayers specifically bound gold conjugates of His-tagged proteins. Using PC + DHGN monolayers, we examined membrane-bound arrays of an N-terminal His-tagged Moloney murine leukemia virus (M-MuLV) capsid (CA) protein, His-MoCA, and in vivo studies suggest that in vitro-derived His-MoCA arrays reflect some of the Gag protein interactions which occur in assembling virus particles. The His-MoCA proteins formed extensive two-dimensional (2D) protein crystals, with reflections out to 9.5 A resolution. The image-analyzed 2D projection of His-MoCA arrays revealed a distinct cage-like network. The asymmetry of the individual building blocks of the network led to the formation of two types of hexamer rings, surrounding protein-free cage holes. These results predict that Gag hexamers constitute a retrovirus core substructure, and that cage hole sizes define an exclusion limit for entry of retrovirus envelope proteins, or other plasma membrane proteins, into virus particles. We believe that the 2D crystallization method will permit the detailed analysis of retroviral Gag proteins and other His-tagged proteins.

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

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

Structural interactions between retroviral Gag proteins examined by cysteine cross-linking

We have examined structural interactions between Gag proteins within Moloney murine leukemia virus (M-MuLV) particles by making use of the cysteine-specific cross-linking agents iodine and bis-maleimido hexane. Virion-associated wild-type M-MuLV Pr65Gag proteins in immature particles were intermolecularly cross-linked at cysteines to form Pr65Gag oligomers, from dimers to pentamers or hexamers. Following a systematic approach of cysteine-to-serine mutagenesis, we have shown that cross-linking of Pr65Gag occurred at cysteines of the nucleocapsid (NC) Cys-His motif, suggesting that the Cys-His motifs within virus particles are packed in close proximity. The M-MuLV Pr65Gag protein did not cross-link to the human immunodeficiency virus Pr55Gag protein when the two molecules were coexpressed, indicating either that they did not coassemble or that heterologous Gag proteins were not in close enough proximity to be cross-linked. Using an assembly-competent, protease-minus, cysteine-minus Pr65Gag protein as a template, novel cysteine residues were generated in the M-MuLV capsid domain major homology region (MHR). Cross-linking of proteins containing MHR cysteines showed above-background levels of Gag-Gag dimers but also identified a novel cellular factor, present in virions, that cross-linked to MHR residues. Although the NC cysteine mutation was compatible with M-MuLV particle assembly, deletions of the NC domain were not tolerated. These results suggest that the Cys-His motif is held in close proximity within immature M-MuLV particles by interactions between CA domains and/or non-Cys-His motif domains of the NC.

Human immunodeficiency virus type 1 MA deletion mutants expressed in baculovirus-infected cells: cis and trans effects on the Gag precursor assembly pathway

The role of the matrix protein (MA) of human immunodeficiency virus type 1 in intracellular transport, assembly, and extracellular release of Gag polyprotein precursor (Pr55gag) was investigated by deletion mutagenesis of the MA domain of recombinant Gag precursor expressed in baculovirus-infected cells. In addition, three carboxy-terminally truncated forms of the Gag precursor, representing mainly the MA, were constructed. One corresponded to an MA with a deletion of its last 12 residues (amb120), while the others corresponded to the entire MA with an additional sequence from the N-terminal portion of the CA (amb143 and och180). Deletions within the MA central region (residues 41 to 78) appeared to be detrimental to Gag particle assembly and budding from the plasma membrane. A slightly narrower domain, between amino acids 41 and 68, was found to be critical for soluble Gag secretion. Mutations which totally or partially deleted one or the other of the two polybasic signals altered the transport of N-myristylated Gag precursor to the plasma membrane. In coexpression with wild-type Gag precursor, a discrete trans-dominant negative effect on wild-type Gag particle assembly and release was observed with deletion mutants located in the central MA region (residues 41 to 78). A more significant negative effect was obtained with the two recombinant proteins of amb120 and och180, which redirected the Gag particle assembly pathway from the plasma membrane compartment to intracellular vesicles (amb120) and to the nuclear compartment (och180).

Functional exchange of an oncoretrovirus and a lentivirus matrix protein

To map functional domains in the retroviral Gag protein we have constructed chimeric viruses where regions of the murine leukemia virus (MuLV) Gag protein have been replaced with analogous sequences from human immunodeficiency virus type 1 (HIV-1). Here we describe the chimeric virus MuLV(MAHIV) which contains the HIV-1 matrix (MA) protein in place of the MuLV MA. MuLV(MAHIV) is infectious but grows at a reduced rate compared with wild-type MuLV. We found that the partial defect in replication of the chimeric virus is at a late stage in the viral life cycle. The MuLV(MAHIV) Gag proteins are distributed aberrantly within cells and are not associated with cellular membranes. Unlike MuLV, HIV-1 is able to integrate into growth-arrested cells. Incorporation of the HIV-1 MA, which is known to play a role in infection of nondividing cells, does not enable MuLV(MAHIV) to be expressed in growth-arrested cells. While it possesses no amino acid homology, we found that the HIV-1 MA can efficiently replace the MuLV matrix protein in infection.

Conditional infectivity of a human immunodeficiency virus matrix domain deletion mutant

We constructed a human immunodeficiency virus (HIV) matrix (MA) deletion mutant by deletion of about 80% of the HIV type 1 Gag MA domain but retaining myristylation and proteolytic processing signals. The effects of this deletion matrix (dl.MA) mutant on HIV particle assembly, processing, and infectivity were analyzed. Surprisingly, the dl.MA mutant still could assemble and process virus particles, had a wild-type (wt) retrovirus particle density, and possessed wt reverse transcriptase activity. RNase protection experiments showed that dl.MA mutant particles preferentially packaged viral genomic RNA. When both mutant and wt particles were pseudotyped with an amphotropic murine leukemia virus envelope protein, mutant infectivity was about 10% of wt level. In contrast, infectivity of the dl.MA mutant was 1,000-fold less than that of wild-type when mutant and wt particles were pseudotyped with the HIV envelope protein. Protein analyses of pseudotyped virions indicated that there were no major differences between mutant and wt viruses in the efficiency of amphotropic murine leukemia virus envelope protein incorporation. In contrast, there was a reduction in the amount of mutant particle-associated HIV envelope protein gp120. Our results suggest that an intact HIV matrix domain is not absolutely required for reverse transcription, nuclear localization, or integration but is necessary for appropriate HIV envelope protein function.

Incorporation of human immunodeficiency virus type 1 Gag proteins into murine leukemia virus virions

The retroviral Gag polyprotein is necessary and sufficient for assembly and budding of viral particles. However, the exact inter- and intramolecular interactions of the Gag polyproteins during this process are not known. To locate functional domains within Gag, we generated chimeric proviruses between human immunodeficiency virus type 1 (HIV-1) and murine leukemia virus (MuLV). In these chimeric proviruses, the matrix or capsid proteins of MuLV were precisely replaced with the matrix or capsid proteins of HIV-1. Although the chimeric proviruses were unable to efficiently assemble into mature viral particles by themselves, coexpression of wild-type MuLV Gag rescued the HIV proteins into virions. The specificity of the rescue of HIV proteins into MuLV virions shows that specific interactions involving homologous matrix or capsid regions of Gag are necessary for retroviral particle formation.

Viral determinants that control the neuropathogenicity of PVC-211 murine leukemia virus in vivo determine brain capillary endothelial cell tropism of the virus in vitro

PVC-211 murine leukemia virus (MuLV) is a neuropathogenic, weakly leukemogenic variant of the nonneuropathogenic, highly leukemogenic Friend MuLV (F-MuLV). Chimeric viruses constructed from PVC-211 MuLV clone 3d and F-MuLV clone 57 indicate that the env gene of PVC-211 MuLV contains the determinant(s) responsible for pathological changes in the central nervous system. However, sequences within the 5' one-third (AatII-EcoRI region) of the PVC-211 MuLV genome, which include the 5' leader sequence, the gag gene, and the 5' quarter of the pol gene, are also needed in conjunction with the env gene determinant(s) to cause clinically evident neurological disease in the majority of virus-infected animals after a short latency. In the presence of the AatII-EcoRI region of the PVC-211 MuLV genome, the PVC-211 MuLV env gene sequences encoding the amino-terminal half of the SU protein, which contains the receptor-binding region of the protein, were sufficient to cause rapidly progressive neurological disease. When PVC-211 MuLV, F-MuLV, and various chimeric viruses were tested for their ability to replicate in cultured brain capillary endothelial cells (BCEC), the primary site of PVC-211 MuLV replication within the central nervous system, there was a direct correlation between the replication efficiency of a virus in BCEC in vitro and its ability to cause neurological disease in vivo. This observation indicates that the sequences in PVC-211 MuLV that render it neuropathogenic affect its replication in BCEC and suggests that rapid and efficient replication of the virus in BCEC is crucial for the pathological changes in the central nervous system that result in development of neurological disease.

Analysis of deletions and thermosensitive mutations in Rous sarcoma virus gag protein p10

Rous sarcoma virus protein p10 is a gag component of the virion present in stoichiometric amount but of unknown function. To characterize this protein, a series of mutants of p10 with linker insertions or deletions was generated by site-directed mutagenesis of a cloned proviral DNA. The deletions and two of the linkers insertions, which disrupted proline pairs, reduced the yield of virus particles upon transfection. These two linker insertion mutants were moreover thermosensitive for this phenotype, producing fewer virus particles at 41 degrees C than at 36 degrees C. Examination of the intracellular viral proteins demonstrated that for all mutants, the amount of gag precursor was similar to the wild-type level. Moreover, the amount of mature gag CA that could be detected by this analysis was similar between each of the mutants and the wild type. This finding suggests that the transport of gag to the membrane and the initial stages of maturation were not affected by the mutations. The virus particles contained normal amounts of active reverse transcriptase, showing that the gag-pol polyprotein was incorporated and cleaved properly. Viral RNA was quantitatively and qualitatively similar in mutant and wild-type virions. However, the infectivity of the mutants virions differed; one of the thermosensitive linker insertions that had no effect on particle production at 36 degrees C was nevertheless noninfectious at that temperature. Together, these data suggest that the p10 protein is involved in a late steps of virus maturation, possibly budding, and perhaps also in an early event of viral infection.

Assembly, processing, and infectivity of human immunodeficiency virus type 1 gag mutants

We studied the effects of gag mutations on human immunodeficiency virus type 1 (HIV-1) assembly, processing, and infectivity by using a replication-defective HIV expression system. HIV mutants were screened for infectivity by transduction of a selectable marker and were examined for assembly by monitoring particle release from transfected cells. Gag protein processing and reverse transcriptase activities of mutant particles were also assayed. Surprisingly, most Gag protein mutants were assembled and processed. The two exceptions to this rule were a myristylation-minus mutant, and one gag matrix domain mutant which expressed proteins that were trapped intracellularly. Interestingly, a mutant with a 56-amino-acid deletion within the HIV gag capsid domain still could assemble and process virus particles, exhibited a wild-type retrovirus particle density, and had wild-type reverse transcriptase activity. Indeed, although most HIV-1 gag mutants were noninfectious or poorly infectious, they produced apparently normal particles which possessed significant reverse transcriptase activities. These results strongly support the notion that the HIV-1 Gag proteins are functionally involved in post-assembly, postprocessing stages of virus infectivity.

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

Deletion mutations at the C terminus of the matrix (MA) protein of human immunodeficiency virus type 1 (HIV-1) were generated by site-directed mutagenesis. The resultant mutant viruses had a severe defect in virus infectivity. This defect did not involve late steps of the virus life cycle, as the synthesis and processing of the Gag polyprotein and the assembly and release of mutant virions were not greatly affected. The incorporation of viral proteins and the viral RNA genome was similar for mutant and wild-type virions. In contrast, the early steps of the virus life cycle were severely affected, as the synthesis of viral DNA postinfection was dramatically reduced in mutant-virus-infected cells. One stretch of amino acids that was deleted in one of the mutants has significant homology with a region in VP1 of the picornavirus family. This region of VP1 is presumably involved in poliovirus penetration into cells. These results suggest that in addition to its functional role in virus assembly, the MA protein of HIV-1, and possibly of other retroviruses, plays an important role in virus entry.

Matrix protein of Akv murine leukemia virus: genetic mapping of regions essential for particle formation

Type C retroviruses assemble at the plasma membrane of the infected cell. Attachment of myristic acid to the N terminus of the Gag precursor polyprotein has been shown to be essential for membrane localization and virus morphogenesis. Here, we report that the matrix (MA) protein contains regions that in conjunction with myristylation are important for Gag protein stability and the assembly of murine leukemia viruses. We identified these domains by generating a series of Akv murine leukemia virus mutants carrying small in-frame deletions within the coding region of the MA protein encompassing 129 amino acids. Studies show that mutants with deletions within the segment encoding the first 102 amino acids were all replication defective, whereas the C-terminal residues 103 to 124 seem not to have any critical function in virus maturation. Cells expressing the replication-defective genomes did not release any detectable Gag proteins. In one mutant, deletion of 3 amino acids in the N terminus resulted in an inefficiently myristylated, stable Gag polyprotein. The remaining defect genomes encoded unstable Gag proteins, although they were modified with myristic acid. The results suggest that the matrix domain plays an important role in stabilizing the Gag polyprotein.

Bovine leukemia virus matrix-associated protein MA(p15): further processing and formation of a specific complex with the dimer of the 5'-terminal genomic RNA fragment

The retrovirus precursor protein has an arrangement of several characteristic domains with which it achieves selective and efficient packaging of the genome RNA during particle assembly. In this study, we analyzed the composition of the bovine leukemia virus (BLV) gag proteins and examined their RNA-binding properties in gel mobility shift assays, using various genomic RNA probes synthesized in vitro. Results obtained in amino acid sequence and composition analyses indicate that the matrix-associated protein MA(p15) is further processed by the BLV protease (PR) to generate MA(p10), a short peptide of seven amino acid residues, and p4. The gag precursor is now mapped as NH2-MA(p10)-p4-CA(p24)-NC(p12)-COOH. MA(p15) formed a specific complex with the dimer RNA of the U5-5' gag region presumed to contain the BLV packaging signal but not with other RNAs. The NH2-terminal cleavage product, MA(p10), bound all RNA fragments tested, while the COOH-terminal peptides with a sequence common to mammalian type C retroviruses had little affinity for RNA. The nucleocapsid protein NC(p12) bound to RNAs nonspecifically and randomly in the presence or absence of zinc ions. These results suggest a possible interaction of the NH2 terminus of the gag precursor with the 5' terminus of the genomic RNA in an early phase of particle assembly, when the conserved structure between the MA and CA domains might be involved.

Rearrangements in unintegrated retroviral DNA are complex and are the result of multiple genetic determinants

We used a replication-competent retrovirus shuttle vector based on a DNA clone of the Schmidt-Ruppin A strain of Rous sarcoma virus to characterize rearrangements in circular viral DNA. In this system, circular molecules of viral DNA present after acute infection of cultured cells were cloned as plasmids directly into bacteria. The use of a replication-competent shuttle vector permitted convenient isolation of a large number of viral DNA clones; in this study, over 1,000 clones were analyzed. The circular DNA molecules could be placed into a limited number of categories. Approximately one-third of the rescued molecules had deletions in which one boundary was very near the edge of a long terminal repeat (LTR) unit. Subtle differences in the patterns of deletions in circular DNAs with one versus two copies of the LTR sequence were observed, and differences between deletions emanating from the right and left boundaries of the LTR were seen. A virus with a missense mutation in the region of the pol gene responsible for integration and exhibiting a temperature sensitivity phenotype for replication had a marked decrease in the number of rescued molecules with LTR-associated deletions when infection was performed at the nonpermissive temperature. This result suggests that determinants in the pol gene, possibly in the integration protein, play a role in the generation of LTR-associated deletions. Sequences in a second region of the genome, probably within the viral gag gene, were also found to affect the types of circular viral DNA molecules present after infection. Sequences in this region from different strains of avian sarcoma-leukosis viruses influenced the fraction of circular molecules with LTR-associated deletions, as well as the relative proportion of circular molecules with either one or two copies of the LTR. Thus, the profile of rearrangements in unintegrated viral DNA is complex and dependent upon the nature of sequences in the gag and pol regions.

Transport and assembly of gag proteins into Moloney murine leukemia virus

We have studied the process of Moloney murine leukemia virus (M-MuLV) assembly by characterization of core (gag) protein mutants and analysis of wild-type (wt) gag proteins produced by cells in the presence of the ionophore monensin. Our genetic studies involved examination of linker insertion mutants of a Gag-beta-galactosidase (Gag-beta-gal) fusion protein, GBG2051, which is incorporated into virus particles when expressed in the presence of wt viral proteins. Analysis indicated that the amino-terminal two-thirds of the gag matrix domain is essential for targeting of proteins to the plasma membrane; mutant proteins localized to the cytoplasm or were trapped on intracellular membranes. Mutations through most of the coding region of the gag capsid domain generated proteins which were released from cells in membrane vesicles but not in virions. In contrast, linker insertions into p12gag or carboxy-terminal portions of the matrix or capsid coding regions did not affect assembly of fusion proteins into virus particles. Monensin, which blocks vesicular transport, inhibited gag protein intracellular transport and release from cells. Our results suggest that a significant proportion of M-MuLV myristylated gag proteins travel via vesicles to the cell surface. Specific matrix protein polypeptide regions and myristic acid modification are both necessary for appropriate gag protein transport, while capsid protein interactions appear to mediate the final phase of virion formation.

Structural role of the matrix protein of type D retroviruses in gag polyprotein stability and capsid assembly

To obtain a better understanding of the role of the gag gene-encoded matrix (MA) protein in the assembly and maturation of type D retroviruses, we have made five mutants with specific in-frame deletions within the p10-coding region by the use of oligonucleotide-directed mutagenesis. The changes in the Gag polyprotein made by these mutations resulted in almost identical phenotypes. In cells expressing mutant genomes, the mutant Gag polyproteins were synthesized and modified with myristic acid in a normal manner. However, they were so unstable that the bulk of the newly synthesized polyproteins was degraded within 1 h without being processed into mature structural polypeptides. In contrast, wild-type polyproteins have a processing half-life of 3.0 to 3.5 h. The mutant Gag polyproteins were assembled with very low efficiency into capsids in the cytoplasm of the mutant-infected cells. Moreover, the few capsids that formed were neither released from nor accumulated in the cells. These results suggest that the matrix protein plays an important role in guiding the correct folding of the Gag polyprotein, which is presumably crucial for both stabilizing the molecule and facilitating the intermolecular interactions that occur during assembly of immature capsids.

Assembly of gag-beta-galactosidase proteins into retrovirus particles

We studied the expression of beta-galactosidase (beta-gal) and 15 gag-beta-gal fusion proteins in the presence of Moloney murine leukemia virus wild-type core (gag) proteins. Analysis indicated that proteins retaining the amino-terminal portion of gag through the capsid protein-coding region were incorporated into retrovirus particles. Proteins which deleted portions of the capsid protein were assembled into virions at low efficiency, indicating the importance of capsid protein interactions in retrovirus assembly. Fusion proteins which retained the amino-terminal matrix protein of the gag polyprotein but which lacked the capsid protein were released efficiently from cells in a nonviral form. The nonviral form was characterized by a high sedimentation coefficient and a low density, suggestive of membrane vesicles. While beta-gal was present in the cytoplasm of expressing cells, all fusion constructs were associated with cellular membranes. gag-beta-gal proteins which were capable of release from cells demonstrated a two-component immunofluorescence staining pattern consisting of a circle of fluorescence around the nucleus and a punctate pattern of staining throughout the remainder of the cell. Interestingly, fusions within the matrix protein were trapped intracellularly and yielded distinct perinuclear staining patterns, possibly localizing to the rough endoplasmic reticulum and/or Golgi. This observation suggests that Moloney murine leukemia virus gag proteins travel to the plasma membrane by vesicular transport associated with the cytoplasmic face of intracellular vesicles.

Rapid reversion of a deletion mutation in Moloney murine leukemia virus by recombination with a closely related endogenous provirus

During abortive infection of mouse cells, defective retroviruses carrying deletions in essential functions can recombine with endogenous retroviral sequences to form viable, replication-competent viruses. We have examined the reversion of a mutant Moloney murine leukemia virus with a deletion in the protease domain of the pol gene after infection of NIH/3T3 cells. In this system revertants arise quickly, only 2 weeks after infection. Analysis of DNA clones of the revertant viral genomes showed that they were derived by recombination with a long sequence of gag and pol exhibiting 95% sequence identity to Moloney virus. One such cloned recombinant was fully infectious, indicating that the repertoire of viral sequences in the NIH/3T3 genome must include substantial stretches of functional viral genes. Examination of the viral DNAs very early in the infection revealed the presence of defective genomes, formed by nonhomologous crossovers between the two parental sequences. We suggest that these may serve as intermediates in the eventual formation of the viable revertant genomes.

HIV-1 Gag mutants can dominantly interfere with the replication of the wild-type virus

The products of the human immunodeficiency virus (HIV) gag gene exist in a highly multimerized state in the mature virion. For that reason, they may represent a particularly suitable target for the generation of dominant negative mutants. A number of HIV site-directed Gag mutants did show interference with the production of infectious viral particles from cells in which they were cotransfected with a wild-type proviral DNA. Furthermore, cells constitutively expressing such HIV Gag mutants had an impaired ability to support HIV replication when infected with wild-type virus. The block was localized to the late stages of the virus life cycle. Such Gag variants could constitute prototypes for the development of anti-HIV intracellular immunization.

An insertion mutation in the pol gene of Moloney murine leukemia virus results in temperature-sensitive pol maturation and viral replication

An insertion mutation in the pol gene of Moloney murine leukemia virus (M-MuLV) was found to render the virus temperature-sensitive for replication. A provirus containing a 12-bp insertion at the boundary between the reverse transcriptase (RT) and integrase (IN) domains induced the formation of mutant virions containing a partially processed RT-IN fusion protein. Some proteolytic processing to form mature RT and IN was observed at 32 degrees, but only aberrantly processed proteins were detected at 39 degrees. The uncleaved precursor was found to exhibit DNA polymerase activity, even though it could not support replication of the virus in vivo at 39 degrees.

Characterization of Moloney murine leukemia virus mutants with single-amino-acid substitutions in the Cys-His box of the nucleocapsid protein

To study the function of the retroviral nucleocapsid protein (NC), we have constructed point mutations in the gag gene of Moloney murine leukemia virus (MuLV) that affect a conserved cysteine-histidine motif of NC. The mutants were characterized biologically and biochemically. Cell lines producing the mutant virions were constructed in NIH 3T3 and rat2 cells, and the viral particles released by these cells were characterized for protein and RNA content. The results indicated that most mutations block replication and specifically inhibit the packaging of the MuLV genomic RNA. In some of the mutants, the packaging of the endogenous rat VL30 RNA was not affected as profoundly as was MuLV RNA. NC also seems to have another function distinct from dimer formation and packaging: one mutation reduced viral RNA packaging by only fivefold but completely abolished viral cDNA synthesis, suggesting a defect in reverse transcription.

Expression of the gag-pol fusion protein of Moloney murine leukemia virus without gag protein does not induce virion formation or proteolytic processing

A mutant of Moloney murine leukemia virus was generated in which the UAG termination codon at the 3' end of the gag gene was changed to a CAG codon encoding glutamine. Cells carrying the mutant provirus constitutively express the gag-pol fusion protein and no detectable gag protein. The precursor is stable, is not processed by the protease domain within the precursor, and does not induce assembly and release of virion particles.

Mutants of murine leukemia viruses and retroviral replication

The analysis of retroviral mutants has played a critical role in the development of our understanding of the complex viral life cycle. The most fundamental result of that analysis has been the definition of the replication functions encoded by the viruses. From a biochemical examination of a particular step in the life cycle it is difficult to determine, for example, whether that step is catalyzed by a viral or a host enzyme; but the isolation of a viral mutant defective in that step can firmly establish that a viral function is involved. In this way many facts about the viruses have been established. We know that reverse transcriptase is encoded by the virus; that RNAase H and DNA polymerase activities reside on the same gene product; that processing of many precursor proteins is mediated by a viral proteinase; and that establishment of the integrated provirus requires a viral protein. The list of functions mediated by viral enzymes has largely been defined by the mutants isolated and studied in various laboratories. The second significant result of the studies of viral mutants has been the assignation of the replication functions to particular viral genes, and then more specifically to particular domains of these genes. Mutants and viral variants have been essential in the determination, for example, that the gag protein is the critical gene product for the assembly of a virion particle; that the env protein is the determinant of species specificity of infection; or that the LTR is a major determinant of tissue tropism and leukemogenicity. The subdivisions of functions within a given gene have similarly hinged on mutants. Genetic mapping was needed to establish that P30 is the most important region for assembly; that the proteinase and integrase functions reside, respectively, in the 5' and 3' portions of the pol gene; and that the glycosylated gag protein is dispensable for replication. A third important area of knowledge has depended heavily on viral mutants: the determination of host functions and proteins that interact with viral proteins. Variant viruses with altered or restricted host ranges serve to define differences between pairs of different host cells, and the mapping of the viral mutations serves to define the viral protein important in that interaction with the host. These studies are only in their infancy, but it is clear that substantial efforts will be made to further analyze these host functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Rous sarcoma virus nucleic acid-binding protein p12 is necessary for viral 70S RNA dimer formation and packaging

To study the function(s) of the Rous sarcoma virus nucleic acid-binding protein p12, we constructed mutants by using two restriction sites in the p12 proviral coding sequence of the Prague C strain to insert KpnI synthetic linkers. The two restriction sites are in the same reading frame, which allowed us to construct a deletion mutant lacking the two conserved Cys-His regions and a duplication mutant containing three intact Cys-His boxes. These mutant DNAs were transfected into chicken embryo fibroblasts, and the viral particles produced in a transient assay were characterized biochemically and for infectivity. Our results indicate that the Rous sarcoma virus nucleic acid-binding protein p12 is necessary for genomic RNA packaging but not for particle assembly and is implicated in the formation of a stable 70S dimeric RNA. Moreover, the fact that one mutant was apparently able to package normal 70S RNA but was not infectious suggests a role for p12 during the infection process.

Evidence for H-2-linked control of retrovirus production in Friend virus-induced tumor cell lines

In Friend leukemia virus-induced tumor cell lines derived from mice congenic with respect to the H-2 complex, most cell lines expressing the H-2k haplotype continuously produced infectious exogenous virus in culture, whereas most cell lines expressing the H-2b or H-2d haplotype stopped producing virus during in vitro passage. This apparent H-2-linked control of virus production did not appear to be the result of alteration of the provirus or resistance to superinfection. The implications of this finding with respect to virus-induced leukemogenesis are discussed.

Point mutations in the P30 domain of the gag gene of Moloney murine leukemia virus

A series of point mutations in the P30 domain of the Moloney murine leukemia virus gag gene was generated by bisulfite treatment of heteroduplex DNAs containing a single-stranded region in the gag gene. One virus bearing such a mutation exhibited a coordinate defect in gag and pol function, and was similar to previously described deletion mutants with alterations in this gene. One mutant virus displayed a different phenotype: it could assemble virion particles and provide pol function, but the particles were defective in the early stages of infection. The continued concordance of the mutants' failure or ability to both assemble virions and provide pol lends further support to the proposal that similar parts of the gag gene are required for these two processes.