Amino acids encoded downstream of gag are not required by Rous sarcoma virus protease during gag-mediated assembly

Non-canonical nematode endogenous retroviruses resulting from RNA virus glycoprotein gene capture by a metavirus

Reverse-transcribing retroviruses exist as horizontally transmitted infectious agents or vertically transmitted endogenous retroviruses (ERVs) resident in eukaryotic genomes, and they are phylogenetically related to the long terminal repeat (LTR) class of retrotransposons. ERVs and retrotransposons are often distinguished only by the presence or absence of a gene encoding the envelope glycoprotein (env). Endogenous elements of the virus family Metaviridae include the insect-restricted Errantivirus genus of ERVs, for which some members possess env, and the pan-eukaryotic Metavirus genus that lacks an envelope glycoprotein gene. Here we report a novel Nematoda endogenous retrovirus (NERV) clade with core retroviral genes arranged uniquely as a continuous gag-env-pro-pol ORF. Reverse transcriptase sequences were phylogenetically related to metaviruses, but envelope glycoprotein sequences resembled those of the Nyamiviridae and Chrysoviridae RNA virus families, suggesting env gene capture during host cell infection by an RNA virus. NERVs were monophyletic, restricted to the nematode subclass Chromadoria, and included additional ORFs for a small hypothetical protein or a large Upf1-like RNA-dependent AAA-ATPase/helicase indicative of viral transduction of a host gene. Provirus LTR identity, low copy number, ORF integrity and segregation of three loci in Meloidogyne incognita, taken together with detection of NERV transcriptional activity, support potential infectivity of NERVs, along with their recent emergence and integration. Altogether, NERVs constitute a new and distinct Metaviridae lineage demonstrating retroviral evolution through sequential heterologous gene capture events.

Functional Equivalence of Retroviral MA Domains in Facilitating Psi RNA Binding Specificity by Gag

Retroviruses specifically package full-length, dimeric genomic RNA (gRNA) even in the presence of a vast excess of cellular RNA. The “psi” (Ψ) element within the 5′-untranslated region (5′UTR) of gRNA is critical for packaging through interaction with the nucleocapsid (NC) domain of Gag. However, in vitro Gag binding affinity for Ψ versus non-Ψ RNAs is not significantly different. Previous salt-titration binding assays revealed that human immunodeficiency virus type 1 (HIV-1) Gag bound to Ψ RNA with high specificity and relatively few charge interactions, whereas binding to non-Ψ RNA was less specific and involved more electrostatic interactions. The NC domain was critical for specific Ψ binding, but surprisingly, a Gag mutant lacking the matrix (MA) domain was less effective at discriminating Ψ from non-Ψ RNA. We now find that Rous sarcoma virus (RSV) Gag also effectively discriminates RSV Ψ from non-Ψ RNA in a MA-dependent manner. Interestingly, Gag chimeras, wherein the HIV-1 and RSV MA domains were swapped, maintained high binding specificity to cognate Ψ RNAs. Using Ψ RNA mutant constructs, determinants responsible for promoting high Gag binding specificity were identified in both systems. Taken together, these studies reveal the functional equivalence of HIV-1 and RSV MA domains in facilitating Ψ RNA selectivity by Gag, as well as Ψ elements that promote this selectivity.

The Structure of Immature Virus-Like Rous Sarcoma Virus Gag Particles Reveals a Structural Role for the p10 Domain in Assembly

The polyprotein Gag is the primary structural component of retroviruses. Gag consists of independently folded domains connected by flexible linkers. Interactions between the conserved capsid (CA) domains of Gag mediate formation of hexameric protein lattices that drive assembly of immature virus particles. Proteolytic cleavage of Gag by the viral protease (PR) is required for maturation of retroviruses from an immature form into an infectious form. Within the assembled Gag lattices of HIV-1 and Mason-Pfizer monkey virus (M-PMV), the C-terminal domain of CA adopts similar quaternary arrangements, while the N-terminal domain of CA is packed in very different manners. Here, we have used cryo-electron tomography and subtomogram averaging to study in vitro-assembled, immature virus-like Rous sarcoma virus (RSV) Gag particles and have determined the structure of CA and the surrounding regions to a resolution of ∼8 Å. We found that the C-terminal domain of RSV CA is arranged similarly to HIV-1 and M-PMV, whereas the N-terminal domain of CA adopts a novel arrangement in which the upstream p10 domain folds back into the CA lattice. In this position the cleavage site between CA and p10 appears to be inaccessible to PR. Below CA, an extended density is consistent with the presence of a six-helix bundle formed by the spacer-peptide region. We have also assessed the affect of lattice assembly on proteolytic processing by exogenous PR. The cleavage between p10 and CA is indeed inhibited in the assembled lattice, a finding consistent with structural regulation of proteolytic maturation.

IMPORTANCE

Retroviruses first assemble into immature virus particles, requiring interactions between Gag proteins that form a protein layer under the viral membrane. Subsequently, Gag is cleaved by the viral protease enzyme into separate domains, leading to rearrangement of the virus into its infectious form. It is important to understand how Gag is arranged within immature retroviruses, in order to understand how virus assembly occurs, and how maturation takes place. We used the techniques cryo-electron tomography and subtomogram averaging to obtain a detailed structural picture of the CA domains in immature assembled Rous sarcoma virus Gag particles. We found that part of Gag next to CA, called p10, folds back and interacts with CA when Gag assembles. This arrangement is different from that seen in HIV-1 and Mason-Pfizer monkey virus, illustrating further structural diversity of retroviral structures. The structure provides new information on how the virus assembles and undergoes maturation.

Potential role for CA-SP in nucleating retroviral capsid maturation

During virion maturation, the Rous sarcoma virus (RSV) capsid protein is cleaved from the Gag protein as the proteolytic intermediate CA-SP. Further trimming at two C-terminal sites removes the spacer peptide (SP), producing the mature capsid proteins CA and CA-S. Abundant genetic and structural evidence shows that the SP plays a critical role in stabilizing hexameric Gag interactions that form immature particles. Freeing of CA-SP from Gag breaks immature interfaces and initiates the formation of mature capsids. The transient persistence of CA-SP in maturing virions and the identification of second-site mutations in SP that restore infectivity to maturation-defective mutant viruses led us to hypothesize that SP may play an important role in promoting the assembly of mature capsids. This study presents a biophysical and biochemical characterization of CA-SP and its assembly behavior. Our results confirm cryo-electron microscopy (cryo-EM) structures reported previously by Keller et al. (J. Virol. 87:13655-13664, 2013, doi:10.1128/JVI.01408-13) showing that monomeric CA-SP is fully capable of assembling into capsid-like structures identical to those formed by CA. Furthermore, SP confers aggressive assembly kinetics, which is suggestive of higher-affinity CA-SP interactions than observed with either of the mature capsid proteins. This aggressive assembly is largely independent of the SP amino acid sequence, but the formation of well-ordered particles is sensitive to the presence of the N-terminal β-hairpin. Additionally, CA-SP can nucleate the assembly of CA and CA-S. These results suggest a model in which CA-SP, once separated from the Gag lattice, can actively promote the interactions that form mature capsids and provide a nucleation point for mature capsid assembly.

IMPORTANCE

The spacer peptide is a documented target for antiretroviral therapy. This study examines the biochemical and biophysical properties of CA-SP, an intermediate form of the retrovirus capsid protein. The results demonstrate a previously unrecognized activity of SP in promoting capsid assembly during maturation.

A large extension to HIV-1 Gag, like Pol, has negative impacts on virion assembly

The GagPol protein of HIV-1 harbors viral enzymes, such as protease (PR), reverse transcriptase, and integrase, that are all crucial for virion infectivity. Previous studies have suggested that expression of GagPol alone does not produce viral particles and that the budding defect is caused by the presence of the Pol region. However, it has remained unknown why GagPol fails to produce viral particles. We show here that HIV-1 GagPol is incapable of membrane binding and subsequent particle assembly. Our confocal data indicated that, despite full N-myristoylation, GagPol protein failed to target plasma membrane with diffuse distribution in the cytoplasm. Membrane flotation analysis confirmed these findings. Progressive C-terminal truncation of GagPol to give GagPR allowed for plasma membrane targeting but still not for particle production. Conversely, the C-terminal addition of a noncognate protein, such as ß-galactosidase or 4 tandem GFP, to Gag impaired the membrane affinity, indicating that the Pol region, a large extension to Gag, inhibits membrane binding in the context of GagPol. The addition of the 10 N-terminal amino acids of Fyn kinase [Fyn(10)], a tight membrane-binding signal, conferred plasma membrane targeting on GagPol, but the Fyn(10)GagPol did not produce viral particles. The defect in particle budding was not rescued by the introduction of the PTAP motif, which is responsible for a late stage of viral particle budding. Rather, electron microscopy suggested that the budding defect of GagPR occurred at an early stage of particle morphogenesis. Our data, which were consistent with previous observations, demonstrate the defects of GagPol in membrane binding and particle assembly.

Orthoretroviral-like prototype foamy virus Gag-Pol expression is compatible with viral replication

Background

Foamy viruses (FVs) unlike orthoretroviruses express Pol as a separate precursor protein and not as a Gag-Pol fusion protein. A unique packaging strategy, involving recognition of briding viral RNA by both Pol precursor and Gag as well as potential Gag-Pol protein interactions, ensures Pol particle encapsidation.

Results

Several Prototype FV (PFV) Gag-Pol fusion protein constructs were generated to examine whether PFV replication is compatible with an orthoretroviral-like Pol expression. During their analysis, non-particle-associated secreted Pol precursor protein was discovered in extracellular wild type PFV particle preparations of different origin, copurifying in simple virion enrichment protocols. Different analysis methods suggest that extracellular wild type PFV particles contain predominantly mature p85^PR-RT and p40^IN Pol subunits. Characterization of various PFV Gag-Pol fusion constructs revealed that PFV Pol expression in an orthoretroviral manner is compatible with PFV replication as long as a proteolytic processing between Gag and Pol proteins is possible. PFV Gag-Pol translation by a HIV-1 like ribosomal frameshift signal resulted in production of replication-competent virions, although cell- and particle-associated Pol levels were reduced in comparison to wild type. In-frame fusion of PFV Gag and Pol ORFs led to increased cellular Pol levels, but particle incorporation was only marginally elevated. Unlike that reported for similar orthoretroviral constructs, a full-length in-frame PFV Gag-Pol fusion construct showed wildtype-like particle release and infectivity characteristics. In contrast, in-frame PFV Gag-Pol fusion with C-terminal Gag ORF truncations or non-removable Gag peptide addition to Pol displayed wildtype particle release, but reduced particle infectivity. PFV Gag-Pol precursor fusion proteins with inactivated protease were highly deficient in regular particle release, although coexpression of p71^Gag resulted in a significant copackaging of these proteins.

Conclusions

Non-particle associated PFV Pol appears to be naturally released from infected cells by a yet unknown mechanism. The absence of particle-associated Pol precursor suggests its rapid processing upon particle incorporation. Analysis of different PFV Gag-Pol fusion constructs demonstrates that orthoretroviral-like Pol expression is compatible with FV replication in principal as long as fusion protein processing is possible. Furthermore, unlike orthoretroviruses, PFV particle release and infectivity tolerate larger differences in relative cellular Gag/Pol levels.

Functional complementation of nucleocapsid and late domain PTAP mutants of human immunodeficiency virus type 1 during replication

During human immunodeficiency virus type 1 (HIV-1) assembly, the nucleocapsid (NC) and the PTAP motif in p6 of Gag play important roles in RNA encapsidation and virus release, respectively. We have previously demonstrated that functional complementation occurs between an NC mutant and a PTAP mutant to rescue viral replication. In this report, we examined the amounts of functional NC and PTAP motif that are required during virus replication. When NC and PTAP mutants were coexpressed at 5:1, 5:5, and 1:5 ratios, virus titers were rescued at 5%, 51%, and 86% of the wild-type level, respectively. These results indicate that HIV-1 requires a small amount of functional PTAP motif but far more functional NC to complete efficient replication. Further analyses reveal that RNA packaging can be significantly rescued in viruses containing a small amount of functional NC. However, most of the NC proteins must be functional to generate the wild-type level of R-U5 DNA product. Once the R-U5 product is generated, viruses containing half of the functional NC can complete reverse transcription and DNA integration at near-wild-type efficiency. These results define the quantitative requirements of NC and p6 during HIV-1 replication and provide insights into the requirement for the development of anti-HIV strategies using NC and p6 as targets.

Irregular and Semi-Regular Polyhedral Models for Rous Sarcoma Virus Cores

Whereas many viruses have capsids of uniquely defined sizes that observe icosahedral symmetry, retrovirus capsids are highly polymorphic. Nevertheless, they may also be described as polyhedral foldings of a fullerene lattice on which the capsid protein (CA) is arrayed. Lacking the high order of symmetry that facilitates the reconstruction of icosahedral capsids from cryo-electron micrographs, the three-dimensional structures of individual retrovirus capsids may be determined by cryo-electron tomography, albeit at lower resolution. Here we describe computational and graphical methods to construct polyhedral models that match in size and shape, capsids of Rous sarcoma virus (RSV) observed within intact virions [8]. The capsids fall into several shape classes, including tubes, "lozenges", and "coffins". The extent to which a capsid departs from icosahedral symmetry reflects the irregularity of the distribution of pentamers, which are always 12 in number for a closed polyhedral capsid. The number of geometrically distinct polyhedra grows rapidly with increasing quotas of hexamers, and ranks in the millions for RSV capsids, which typically have 150 - 300 hexamers. Unlike the capsid proteins of icosahedral viruses that assume a minimal number of quasi-equivalent conformations equal to the triangulation number (T), retroviral CAs exhibit a near-continuum of quasi-equivalent conformations - a property that may be attributed to the flexible hinge linking the N- and C-terminal domains.

Overlapping roles of the Rous sarcoma virus Gag p10 domain in nuclear export and virion core morphology

Nucleocytoplasmic shuttling of the Rous sarcoma virus (RSV) Gag polyprotein is an integral step in virus particle assembly. A nuclear export signal (NES) was previously identified within the p10 domain of RSV Gag. Gag mutants containing deletions of the p10 NES or mutations of critical hydrophobic residues at positions 219, 222, 225, or 229 become trapped within the nucleus and exhibit defects in the efficiency of virus particle release. To investigate other potential roles for Gag nuclear trafficking in RSV replication, we created viruses bearing NES mutant Gag proteins. Viruses carrying p10 mutations produced low levels of particles, as anticipated, and those particles that were released were noninfectious. The p10 mutant viruses contained approximately normal amounts of Gag, Gag-Pol, and Env proteins and genomic viral RNA (vRNA), but several major structural defects were found. Thin-section transmission electron microscopy revealed that the mature particles appeared misshapen, while the viral cores were cylindrical, horseshoe-shaped, or fragmented, with some particles containing multiple small, electron-dense aggregates. Immature virus-like particles produced by the expression of Gag proteins bearing p10 mutations were also aberrant, with both spherical and tubular filamentous particles produced. Interestingly, the secondary structure of the encapsidated vRNA was altered; although dimeric vRNA was predominant, there was an additional high-molecular-weight fraction. Together, these results indicate that the p10 NES domain of Gag is critical for virus replication and that it plays overlapping roles required for the nuclear shuttling of Gag and for the maintenance of proper virion core morphology.

Mutation of YMYL in the Nipah virus matrix protein abrogates budding and alters subcellular localization

Matrix (M) proteins reportedly direct the budding of paramyxoviruses from infected cells. In order to begin to characterize the assembly process for the highly lethal, emerging paramyxovirus Nipah virus (NiV), we have examined the budding of NiV M. We demonstrated that expression of the NiV M protein is sufficient to produce budding virus-like particles (VLPs) that are physically and morphologically similar to NiV. We identified in NiV M a sequence, YMYL, with similarity to the YPDL late domain found in the equine infectious anemia virus Gag protein. When the YMYL within NiV M was mutated, VLP release was abolished and M was relocalized to the nucleus, but the mutant M proteins retained oligomerization activity. When YMYL was fused to a late-domain mutant of the Ebola virus VP40 matrix protein, VP40 budding was restored. These results suggest that the YMYL sequence may act as a trafficking signal and a late domain for NiV M.

Phosphorylation and proteolytic cleavage of gag proteins in budded simian immunodeficiency virus

The lentiviral Gag polyprotein (Pr55(Gag)) is cleaved by the viral protease during the late stages of the virus life cycle. Proteolytic cleavage of Pr55(Gag) is necessary for virion maturation, a structural rearrangement required for infectivity that occurs in budded virions. In this study, we investigate the relationship between phosphorylation of capsid (CA) domains in Pr55(Gag) and its cleavage intermediates and their cleavage by the viral protease in simian immunodeficiency virus (SIV). First, we demonstrate that phosphorylated forms of Pr55(Gag), several CA-containing cleavage intermediates of Pr55(Gag), and the free CA protein are detectable in SIV virions but not in virus-producing cells, indicating that phosphorylation of these CA-containing Gag proteins may require an environment that is unique to the virion. Second, we show that the CA domain of Pr55(Gag) can be phosphorylated in budded virus and that this phosphorylation does not require the presence of an active viral protease. Further, we provide evidence that CA domains (i.e., incompletely cleaved CA) are phosphorylated to a greater extent than free (completely cleaved) CA and that CA-containing Gag proteins can be cleaved by the viral protease in SIV virions. Finally, we demonstrate that Pr55(Gag) and several of its intermediates, but not free CA, are actively phosphorylated in budded virus. Taken together, these data indicate that, in SIV virions, phosphorylation of CA domains in Pr55(Gag) and several of its cleavage intermediates likely precedes the cleavage of these domains by the viral protease.

Insertion of capsid proteins from nonenveloped viruses into the retroviral budding pathway

Retroviral Gag proteins direct the assembly and release of virus particles from the plasma membrane. The budding machinery consists of three small domains, the M (membrane-binding), I (interaction), and L (late or "pinching-off") domains. In addition, Gag proteins contain sequences that control particle size. For Rous sarcoma virus (RSV), the size determinant maps to the capsid (CA)-spacer peptide (SP) sequence, but it functions only when I domains are present to enable particles of normal density to be produced. Small deletions throughout the CA-SP sequence result in the release of particles that are very large and heterogeneous, even when I domains are present. In this report, we show that particles of relatively uniform size and normal density are released by budding when the size determinant and I domains in RSV Gag are replaced with capsid proteins from two unrelated, nonenveloped viruses: simian virus 40 and satellite tobacco mosaic virus. These results indicate that capsid proteins of nonenveloped viruses can interact among themselves within the context of Gag and be inserted into the retroviral budding pathway merely by attaching the M and L domains to their amino termini. Thus, the differences in the assembly pathways of enveloped and nonenveloped viruses may be far simpler than previously thought.

Type D retrovirus Gag polyprotein interacts with the cytosolic chaperonin TRiC

The carboxy terminus-encoding portion of the gag gene of Mason-Pfizer monkey virus (M-PMV), the prototype immunosuppressive primate type D retrovirus, encodes a 36-amino-acid, proline-rich protein domain that, in the mature virion, becomes the p4 capsid protein. The p4 domain has no known role in M-PMV replication. We found that two mutants with premature termination codons that remove half or all of the p4 domain produced lower levels of stable Gag protein and of self-assembled capsids. Interestingly, yeast two-hybrid screening revealed that p4 specifically interacted with TCP-1gamma, a subunit of the chaperonin TRiC (TCP-1 ring complex). TRiC is a cytosolic chaperonin that is known to be involved in both folding and subunit assembly of a variety of cellular proteins. TCP-1gamma also associated with high specificity with the M-PMV pp24/16-p12 domain and human immunodeficiency virus p6. Moreover, in cells, Gag polyprotein associated with the TRiC chaperonin complex and this association depended on ATP hydrolysis. In the p4 truncation mutants, the Gag-TRiC association was significantly reduced. These results strongly suggest that cytosolic chaperonin TRiC is involved in Gag folding and/or capsid assembly. We propose that TRiC associates transiently with nascent M-PMV Gag molecules to assist in their folding. Consequently, properly folded Gag molecules carry out the intermolecular interactions involved in self-assembly of the immature capsid.

Membrane targeting properties of a herpesvirus tegument protein-retrovirus Gag chimera

The retroviral Gag protein is capable of directing the production and release of virus-like particles in the absence of all other viral components. Budding normally occurs after Gag is transported to the plasma membrane by its membrane-targeting and -binding (M) domain. In the Rous sarcoma virus (RSV) Gag protein, the M domain is contained within the first 86 amino acids. When M is deleted, membrane association and budding fail to occur. Budding is restored when M is replaced with foreign membrane-binding sequences, such as that of the Src oncoprotein. Moreover, the RSV M domain is capable of targeting heterologous proteins to the plasma membrane. Although the solution structure of the RSV M domain has been determined, the mechanism by which M specifically targets Gag to the plasma membrane rather than to one or more of the large number of internal membrane surfaces (e.g., the Golgi apparatus, endoplasmic reticulum, and nuclear, mitochondrial, or lysosomal membranes) is unknown. To further investigate the requirements for targeting proteins to discrete cellular locations, we have replaced the M domain of RSV with the product of the unique long region 11 (U(L)11) gene of herpes simplex virus type 1. This 96-amino-acid myristylated protein is thought to be involved in virion transport and envelopment at internal membrane sites. When the first 100 amino acids of RSV Gag (including the M domain) were replaced by the entire UL11 sequence, the chimeric protein localized at and budded into the Golgi apparatus rather than being targeted to the plasma membrane. Myristate was found to be required for this specific targeting, as were the first 49 amino acids of UL11, which contain an acidic cluster motif. In addition to shedding new light on UL11, these experiments demonstrate that RSV Gag can be directed to internal cellular membranes and suggest that regions outside of the M domain do not contain a dominant plasma membrane-targeting motif.

Systematic mutational analysis of the active-site threonine of HIV-1 proteinase: rethinking the "fireman's grip" hypothesis

Aspartic proteinases share a conserved network of hydrogen bonds (termed "fireman's grip"), which involves the hydroxyl groups of two threonine residues in the active site Asp-Thr-Gly triplets (Thr26 in the case of human immunodeficiency virus type 1 (HIV-1) PR). In the case of retroviral proteinases (PRs), which are active as symmetrical homodimers, these interactions occur at the dimer interface. For a systematic analysis of the "fireman's grip," Thr26 of HIV-1 PR was changed to either Ser, Cys, or Ala. The variant enzymes were tested for cleavage of HIV-1 derived peptide and polyprotein substrates. PR(T26S) and PR(T26C) showed similar or slightly reduced activity compared to wild-type HIV-1 PR, indicating that the sulfhydryl group of cysteine can substitute for the hydroxyl of the conserved threonine in this position. PR(T26A), which lacks the "fireman's grip" interaction, was virtually inactive and was monomeric in solution at conditions where wild-type PR exhibited a monomer-dimer equilibrium. All three mutations had little effect when introduced into only one chain of a linked dimer of HIV-1 PR. In this case, even changing both Thr residues to Ala yielded residual activity suggesting that the "fireman's grip" is not essential for activity but contributes significantly to dimer formation. Taken together, these results indicate that the "fireman's grip" is crucial for stabilization of the retroviral PR dimer and for overall stability of the enzyme.

A proline-rich motif within the matrix protein of vesicular stomatitis virus and rabies virus interacts with WW domains of cellular proteins: implications for viral budding

The matrix (M) protein of rhabdoviruses has been shown to play a key role in virus assembly and budding; however, the precise mechanism by which M mediates these processes remains unclear. We have associated a highly conserved, proline-rich motif (PPxY or PY motif, where P denotes proline, Y represents tyrosine, and x denotes any amino acid) of rhabdoviral M proteins with a possible role in budding mediated by the M protein. Point mutations that disrupt the PY motif of the M protein of vesicular stomatitis virus (VSV) have no obvious effect on membrane localization of M but instead lead to a decrease in the amount of M protein released from cells in a functional budding assay. Interestingly, the PPxY sequence within rhabdoviral M proteins is identical to that of the ligand which interacts with WW domains of cellular proteins. Indeed, results from two in vitro binding assays demonstrate that amino acids 17 through 33 and 29 through 44, which contain the PY motifs of VSV and rabies virus M proteins, respectively, mediate interactions with WW domains of specific cellular proteins. Point mutations that disrupt the consensus PY motif of VSV or rabies virus M protein result in a significant decrease in their ability to interact with the WW domains. These properties of the PY motif of rhabdovirus M proteins are strikingly analogous to those of the late (L) budding domain identified in the gag-specific protein p2b of Rous sarcoma virus. Thus, it is possible that rhabdoviruses may usurp host proteins to facilitate the budding process and that late stages in the budding process of rhabdoviruses and retroviruses may have features in common.

The major site of phosphorylation within the Rous sarcoma virus MA protein is not required for replication

About one-third of the MA protein in Rous sarcoma virus (RSV) is phosphorylated. Previous analyses of this fraction have suggested that serine residues 68 and 106 are the major sites of phosphorylation. As a follow-up to that study, we have characterized mutants which have these putative phosphorylation sites changed to alanine, either separately or together. None of the substitutions (S68A, S106A, or S68/106A) had an effect on the budding efficiency or infectivity of the virus. Upon examination of the 32P-labeled viral proteins, we found that the S68A substitution did not affect phosphorylation in vivo at all. In contrast, the S106A substitution prevented all detectable phosphorylation of MA, suggesting that there is only one major site of phosphorylation in MA. We also found that the RSV MA protein is phosphorylated on tyrosine, but the amount was low and detectable only with large numbers of virions and an antibody specific for phosphotyrosine.

Genetic determinants of Rous sarcoma virus particle size

The Gag proteins of retroviruses are the only viral products required for the release of membrane-enclosed particles by budding from the host cell. Particles released when these proteins are expressed alone are identical to authentic virions in their rates of budding, proteolytic processing, and core morphology, as well as density and size. We have previously mapped three very small, modular regions of the Rous sarcoma virus (RSV) Gag protein that are necessary for budding. These assembly domains constitute only 20% of RSV Gag, and alterations within them block or severely impair particle formation. Regions outside of these domains can be deleted without any effect on the density of the particles that are released. However, since density and size are independent parameters for retroviral particles, we employed rate-zonal gradients and electron microscopy in an exhaustive study of mutants lacking the various dispensable segments of Gag to determine which regions would be required to constrain or define the particle dimensions. The only sequence found to be absolutely critical for determining particle size was that of the initial capsid cleavage product, CA-SP, which contains all of the CA sequence plus the spacer peptides located between CA and NC. Some regions of CA-SP appear to be more important than others. In particular, the major homology region does not contribute to defining particle size. Further evidence for interactions among CA-SP domains was obtained from genetic complementation experiments using mutant deltaNC, which lacks the RNA interaction domains in the NC sequence but retains a complete CA-SP sequence. This mutant produces low-density particles heterogeneous in size. It was rescued into particles of normal size and density, but only when the complementing Gag molecules contained the complete CA-SP sequence. We conclude that CA-SP functions during budding in a manner that is independent of the other assembly domains.

Conditional human immunodeficiency virus type 1 protease mutants show no role for the viral protease early in virus replication

The human immunodeficiency virus type 1 protease plays a critical role in the proteolytic processing of precursor polyproteins during virion maturation. Contradictory evidence has been obtained for a possible role for the protease early after infection, i.e., during DNA synthesis and/or integration. We have reexamined this question by using conditional mutants of the protease. In one set of experiments, protease mutants that confer a temperature-sensitive phenotype for processing were used to assess the need for protease activity early after infection. No significant difference from results with wild-type virus was seen when infections were carried out at either 35 or 40 degrees C. In a separate set of experiments, infections were carried out in the presence of a protease inhibitor. In this case, both wild-type virus and a drug-resistant variant were used, the latter as a control to ensure a specific effect of the inhibitor. Infection with either virus was not inhibited at drug concentrations that were up to 10-fold higher than those needed to inhibit intracellular processing by the viral protease. The results obtained by both of these experimental protocols provide evidence that the human immunodeficiency virus type 1 protease does not play a role early after infection.

Fine mapping and characterization of the Rous sarcoma virus Pr76gag late assembly domain

The p2 region of the Rous sarcoma virus (RSV) Gag polyprotein contains an assembly domain, which is required late in replication for efficient budding of virus-like particles from cells (J. W. Wills, C. E. Cameron, C. B. Wilson, Y. Xiang, R. P. Bennett, and J. Leis, J. Virol. 68:6605-6618, 1994). This domain, referred to as the L domain, was previously mapped to the 11 amino acids of p2b. Through the analysis of a series of deletion and substitution mutations, the L domain has now been fine mapped to a highly conserved amino acid sequence, PPPPYV of p2b. Sequences flanking PPPPYV motif can be deleted without any effect on budding. Defects caused by L-domain deletions can be rescued by placing a wild-type copy of the sequence at several other positions in RSV Gag. A proline-rich P(S/T)APP motif is found in many retroviral Gag polyproteins; the motif found in the p6 region of human immunodeficiency virus type 1 has been implicated in late functions of the virus. Substitution of the RSV L domain with this motif in a 10-amino-acid sequence derived from visna leukemia virus results in wild-type release of virus particles from cells. In contrast, the slightly different sequences from Gibbon ape leukemia virus, Moloney leukemia virus, PSAPP alone, or a proline-rich SH3 binding sequence do not efficiently rescue RSV L-domain mutations.

Analysis of Rous sarcoma virus Gag protein by mass spectrometry indicates trimming by host exopeptidase

We have used electrospray ionization-mass spectrometry to investigate Gag protein structure and processing in Rous sarcoma virus, the prototype of the avian sarcoma and leukemia viruses. Molecular masses determined for the mature virion proteins MA, CA, NC, and PR agree closely with those predicted by currently accepted models for their structures. However, the data for p10 imply that only about 10% of the product has the predicted mass while the remainder is missing the C-terminal methionine residue. Molecular masses also were obtained for products generated by PR cleavage in vitro of a Gag precursor polyprotein expressed in Escherichia coli. The data confirm the predicted Gag cleavage sites for PR. Thus, carboxypeptidase activity appears to be responsible for generating the des-Met form of p10. The same activity may account for the small amount of the mature des-Met CA, as previously reported. Analysis of cleavage products generated in vitro also serves to define the PR processing site separating the p2a and p2b peptides, Asn-164-Cys-165. In conjunction with published characterizations of these two peptides processed from the segment of Gag between MA and p10, these data suggest trimming of p2b by an aminopeptidase. Finally, the molecular masses determined for the MA-related species p19f, p23, and p35 now accurately define the structures of these proteins.

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

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

Transport and processing of the Rous sarcoma virus Gag protein in the endoplasmic reticulum

The Gag proteins of replication-competent retroviruses direct budding at the plasma membrane and are cleaved by the viral protease (PR) just before or very soon after particle release. In contrast, defective retroviruses that bud into the endoplasmic reticulum (ER) have been found, and morphologically these appear to contain uncleaved Gag proteins. From this, it has been proposed that activation of PR may depend upon a host factor found only at the plasma membrane. However, if Gag proteins were cleaved by PR before the particle could pinch off the ER membrane, then the only particles that would remain visible are those that packaged smaller-than-normal amounts of PR, and these would have an immature morphology. To distinguish between these two hypotheses, we made use of the Rous sarcoma virus (RSV) Gag protein, the PR of RSV IS included on each Gag molecule. To target Gag to the ER, a signal peptide was installed at its amino terminus in place of the plasma membrane-binding domain. An intervening, hydrophobic, transmembrane anchor was included to keep Gag extended into the cytoplasm. We found that PR-mediated processing occurred, although the cleavage products were rapidly degraded. When the anchor was removed, allowing the entire protein to be inserted into the lumen of the ER, Gag processing occurred with a high level of efficiency, and the cleavage products were quite stable. Thus, PR activation does not require targeting of Gag molecules to the plasma membrane. Unexpectedly, molecules lacking the transmembrane anchor were rapidly secreted from the cell in a nonmembrane-enclosed form and in a manner that was very sensitive to brefeldin A and monensin. In contrast, the wild-type RSV and Moloney murine leukemia virus Gag proteins were completely insensitive to these inhibitors, suggesting that the normal mechanism of transport to the plasma membrane does not require interactions with the secretory pathway.

Differential proteolytic processing leads to multiple forms of the CA protein in avian sarcoma and leukemia viruses

The CA (capsid) protein of avian sarcoma and leukemia viruses occurs in multiple species. Only one form has been previously characterized biochemically. We have now determined that the mature CA protein of avian sarcoma and leukemia viruses exists as three species with different C termini, ending in amino acid residues A-476, A-478, and M-479 of the Gag precursor, respectively. These structures were deduced from a combination of cyanogen bromide peptide mapping, sequence analysis of tryptic peptides, and electrospray mass spectrometry. The three forms of CA were detected in the same ratios in Rous sarcoma virus and avian myeloblastosis virus and therefore are likely to represent a common feature of members of this genus of avian retroviruses. The only previously reported CA species, CAM-479, accounts for only about 36% of the total CA protein, while CAA-476 and CAA-478 account for 55 and 9%, respectively. From the analysis of peptides cleaved in vitro by PR, the viral protease, we infer that the cleavage site between A-476 and A-477 not only is recognized by PR but is the preferred site. We were unable to determine if A-478/A-479 is a cleavage site for PR or alternatively if CAA-478 results from further processing of CAM-479 by a carboxypeptidase. To study the biological significance of residues A-477 to M-479, we constructed genetically altered viruses in which deletions removed either residues 477 to 479 or 477 to 488. The resulting virus particles appeared to assembly with normal efficiencies, but the latter mutant showed slowed proteolytic processing. Neither of the mutants was infectious.

Positionally independent and exchangeable late budding functions of the Rous sarcoma virus and human immunodeficiency virus Gag proteins

The Gag proteins of Rous sarcoma virus and human immunodeficiency virus (HIV) each contain a function involved in a late step in budding, defects in which result in the accumulation of these molecules at the plasma membrane. In the Rous sarcoma virus Gag protein (Pr76gag), this assembly domain is associated with a PPPY motif, which is located at an internal position between the MA and CA sequences. This motif is not contained anywhere within the HIV Gag protein (Pr55gag), and the MA sequence is linked directly to CA. Instead, a late assembly function of HIV has been associated with the p6 sequence situated at the C terminus of Gag. Here we demonstrate the remarkable finding that the late assembly domains from these two unrelated Gag proteins are exchangeable between retroviruses and can function in a positionally independent manner.

Genetic analysis of the major homology region of the Rous sarcoma virus Gag protein

The mature cores of all retroviruses contain a major structural protein known as the CA (capsid) protein. Although it appears to form a shell around the ribonucleoprotein complex that contains the viral RNA, its function in viral replication is largely unknown. Little sequence similarity exists between the CA proteins of different retroviruses, except for a region of about 20 amino acids termed the major homology region (MHR). To examine the role of the CA protein in particle assembly and release, mutants of Rous sarcoma virus were created in which segments of CA were deleted or single conserved residues in the MHR were altered. The ability of the deletion mutants to release particles at rates similar to the wild-type protein demonstrated that the CA domain of Gag is not an essential component of the minimal budding machinery. Certain point mutations in the MHR region did block assembly and release in certain cell types, presumably by perturbing the global structure of the Gag precursor. Another group of MHR substitutions produced noninfectious or poorly infectious particles that were normal in their content of gag and pol gene products and viral RNA. The mutants were capable of initiating reverse transcription in vitro; however, the association of CA protein with the core was compromised, as indicated by its sensitivity to extraction with nonionic detergent. Prominent blebs on the virion envelope also indicated a disturbance at the membrane. Finally, an anti-peptide serum directed against MHR was found to react with the uncleaved Gag protein but not with mature CA, suggesting that MHR undergoes a dynamic rearrangement upon liberation from the polyprotein. We conclude that the MHR is involved in the very late steps in maturation of the virion (i.e., ones that occur after budding is initiated) and is essential for proper function of the core upon entry into a new host cell.

An assembly domain of the Rous sarcoma virus Gag protein required late in budding

The Gag protein of Rous sarcoma virus has the ability to direct particle assembly at the plasma membrane in the absence of all the other virus-encoded components. An extensive deletion analysis has revealed that very large regions of this protein can be deleted without impairing budding and has suggested that the essential functions map to three discrete regions. In the studies reported here, we establish the location of assembly domain 2 (AD2) within the proline-rich p2b sequence of this Gag protein. AD2 mutants lacking the p2b sequence were completely defective for particle release even though their Gag proteins were tightly associated with the membrane fraction and exhibited high levels of protease activity. Mutations that inactivate the viral protease did not restore budding to wild-type levels for these mutants, indicating that the defect is not due simply to a loss of protease regulation. AD2 mutants could be rescued into dense particles in genetic complementation assays, indicating that their defect is not due to a gross alteration of the overall conformation of the protein and that the assembly function is not needed on every Gag molecule in the population. Several mutants with amino acid substitutions in the p2b sequence were found to have an intermediate capacity for budding. Inactivation of the protease of these mutants stabilized the Gag polyprotein within the cells and allowed an increase in particle release; however, the rate of budding remained slow. We favor the idea that AD2 is a dynamic region of movement, perhaps serving as a molecular hinge to allow the particle to emerge from the surface of the cell during budding.

Identification of a membrane-binding domain within the amino-terminal region of human immunodeficiency virus type 1 Gag protein which interacts with acidic phospholipids

Retroviral Gag proteins are targeted to the plasma membrane, where they play the central role in virion formation. Several studies have suggested that the membrane-binding signal is contained within the amino-terminal matrix sequence; however, the precise location has never been determined for the Gag protein of any retrovirus. In this report, we show that the first 31 residues of human immunodeficiency virus type 1 Gag protein can function independently as a membrane-targeting domain when fused to heterologous proteins. A bipartite membrane-targeting motif was identified, consisting of the myristylated N-terminal 14 amino acids and a highly basic region that binds acidic phospholipids. Replacement of the N-terminal membrane-targeting domain of pp60v-src with that of human immunodeficiency virus type 1 Gag elicits efficient membrane binding and a transforming phenotype. Removal of myristate or the basic region results in decreased membrane binding of Gag-Src chimeras in vitro and impaired virion formation by Pr55gag in vivo. We propose that the N-terminal Gag sequence functions as a targeting signal to direct interaction with acidic phospholipids on the cytoplasmic leaflet of the plasma membrane.

Functional chimeras of the Rous sarcoma virus and human immunodeficiency virus gag proteins

The Gag protein encoded by Rous sarcoma virus (RSV) is the only viral product required for the process of budding whereby virus particles are formed at the plasma membrane. Deletion analysis of this Gag molecule has revealed several regions (assembly domains) that are important for budding. One of these domains is located at the amino terminus and is needed for membrane binding. Another is located within the carboxy-terminal third of the protein. Though there is little sequence homology among the Gag proteins of unrelated retroviruses, it seemed possible that their assembly domains might be functionally conserved, and to explore this idea, numerous Gag chimeras were made. The results indicate that the first 10 amino acids of the human immunodeficiency virus (HIV) Gag protein can suppress the block to budding caused by deletions in the RSV MA sequence, much as described previously for the first 10 residues from the Src oncoprotein (J.W. Wills, R.C. Craven, R. A. Weldon, Jr., T. D. Nelle, and C.R. Erdie, J. Virol. 65:3804-3812, 1991). In addition, the carboxy-terminal half of the HIV Gag protein was fused to a truncated RSV Gag molecule, mutant Bg-Bs, which is unable to direct core assembly. This chimera was able to produce particles at a rate identical to that of RSV and of a density similar to that of authentic virions. Deletion analysis of the carboxy-terminal chimera revealed two small regions within the HIV NC protein that were sufficient for endowing mutant Bg-Bs with these properties. Chimeras lacking both regions produced particles of a low density, suggesting that these sequences may be involved in the tight packing of Gag molecules during assembly. In a related set of experiments, replacement of the RSV protease with that of HIV resulted in premature processing within the RSV sequence and a block to budding. Particle assembly was restored when the HIV PR activity was inactivated by mutagenesis. Collectively, the data presented here illustrate the functional similarities of Gag proteins from unrelated retroviruses.

Necessity of the spacer peptide between CA and NC in the Rous sarcoma virus gag protein

A mutant of Rous sarcoma virus was constructed in which the nine amino acids that separate the CA and NC sequences in the Gag protein were deleted. The spacer peptide deletion mutant produced particles containing the normal complement of viral RNA and all of the viral proteins, including reverse transcriptase. Though electron microscopy revealed particles of normal morphology, the particles were noninfectious. The normally slow maturation of the CA protein, which involves cleavage of the spacer peptide from the carboxy terminus, was bypassed in this mutant, and the association between CA and the internal components of the core appears to have been disrupted. The results suggest that the spacer peptide has an essential role in directing folding and/or oligomerization of the CA subunits within the capsid structure.

The 3;21 translocation in myelodysplasia results in a fusion transcript between the AML1 gene and the gene for EAP, a highly conserved protein associated with the Epstein-Barr virus small RNA EBER 1

In the 8;21 translocation, the AML1 gene, located at chromosome band 21q22, is translocated to chromosome 8 (q22), where it is fused to the ETO gene and transcribed as a chimeric gene. AML1 is the human homolog of the recently cloned mouse gene pebp2 alpha B, homologous to the DNA binding alpha subunit of the polyoma enhancer factor pebp2. AML1 is also involved in a translocation with chromosome 3 that is seen in patients with therapy-related acute myeloid leukemia and myelodysplastic syndrome and in chronic myelogenous leukemia in blast crisis. We have isolated a fusion cDNA clone from a t(3;21) library derived from a patient with therapy-related myelodysplastic syndrome; this clone contains sequences from AML1 and from EAP, which we have now localized to band 3q26. EAP has previously been characterized as a highly expressed small nuclear protein of 128 residues (EBER 1) associated with Epstein-Barr virus small RNA. The fusion clone contains the DNA binding 5' part of AML1 that is fused to ETO in the t(8;21) and, in addition, at least one other exon. The translocation replaces the last nine codons of AML1 with the last 96 codons of EAP. The fusion does not maintain the correct reading frame of EAP and may not lead to a functional chimeric protein.

The nonmyristylated Pr160gag-pol polyprotein of human immunodeficiency virus type 1 interacts with Pr55gag and is incorporated into viruslike particles

The expression of the pol gene of human immunodeficiency virus type 1 occurs via a ribosomal frameshift between the gag and pol genes. The resulting protein, a Gag-Pol polyprotein, is produced at a level 5 to 10% of that of the Gag protein. The Gag-Pol polyprotein is incorporated into virions and provides viral protease, reverse transcriptase, and integrase, which are essential for infectivity. It is generally believed that the Gag-Pol polyprotein is incorporated into virions via interaction with the Gag protein, although the details of the mechanism are unknown. To further study this problem, we have constructed a human immunodeficiency virus type 1 proviral genome which overexpresses the Gag-Pol polyprotein (Pr160gag-pol). Transfection of this proviral genome (pGPpr-) into COS-1 cells resulted in the expression of full-length Pr160gag-pol polyprotein. Although the majority of the Pr160gag-pol was confined to the cells, low levels of reverse transcriptase activity were detectable in the cell supernatants. The cotransfection of pGPpr- with a second plasmid which expresses only the Pr55gag precursor (pGAG) resulted in a significantly higher level of Pr160gag-pol in the medium of transfected cells. Sedimentation analysis using sucrose density gradients demonstrated that most Pr160gag-pol was found in fractions corresponding to the density of virion particles, indicating that the Pr160gag-pol polyprotein was released in association with a Pr55gag viruslike particle. To further characterize the requirements for the release, a mutation was constructed to express an unmyristylated Pr160gag-pol polyprotein. Coexpression with Pr55gag demonstrated that the unmyristylated Pr160gag-pol was also incorporated into virion particles. Subcellular fractionation experiments revealed that the distributions of the Pr160gag-polmyr- and Pr160gag-pol in the membrane and cytosol were similar under low- or high-ionic-strength conditions. Taken together, these results suggest that myristylation of the Pr160gag-pol polyprotein is not required for the interaction with the Pr55gag necessary for packaging into a viruslike particle.

Transposition of a Ty3 GAG3-POL3 fusion mutant is limited by availability of capsid protein

Ty3 encodes structural proteins in its upstream open reading frame (GAG3) and catalytic proteins in an overlapping open reading frame (POL3). As is the case for retroviruses, high levels of structural protein versus catalytic proteins are synthesized and we show here that catalytic proteins are derived from a GAG3-POL3 fusion polyprotein. To evaluate the relative contributions of structural and catalytic components of the Ty3 particle, we perturbed the balance of these proteins by fusing the GAG3 and POL3 frames. This fusion Ty3 was capable of complementing low levels of transposition of a donor Ty3 which contained only cis-acting sequences required for transposition. Examination of extracts of cells expressing the GAG3-POL3 fusion mutant showed that particle formation differed qualitatively and quantitatively from viruslike particle formation by wild-type Ty3. Suprisingly, expression of 238 codons of GAG3, encoding only capsid protein, complemented transposition and particle formation defects of the fusion mutant, showing that the limiting deficiency was in capsid, and not in nucleocapsid, function. In addition, protein containing the capsid domain expressed alone accumulated in the same particulate fraction as viruslike particles, showing that it was sufficient for particle formation. The activity of the Ty3 fusion mutant contrasts with the inviability of mutant retroviruses in which gag and pol frames were fused and argues that retrotransposons tolerate considerable variation in the nucleoprotein complexes that permit replication and integration.

Complementation studies with Rous sarcoma virus gag and gag-pol polyprotein mutants

Avian retroviruses (with the notable exception of spleen necrosis virus) express their protease (PR) both in their gag and their gag-pol polyprotein precursors, in contrast to other retroviruses, notably, the mammalian retroviruses, in which PR is encoded in the gag-pol polyprotein or in a separate reading frame as a gag-pro product. The consequence is that the avian PR is expressed in stoichiometric rather than catalytic amounts. To investigate the significance of the particular genome organization of the avian retrovirus prototype Rous sarcoma virus, we developed an assay that measures complementation between the gag and the gag-pol polyproteins by expressing them from two different plasmids in transfected cells. By using this assay, we showed that the protease PR from the gag-pol polyprotein is capable of autocatalytic self-cleavage and -activation when coexpressed with a protease-deficient gag protein and that the PR domain has a role in viral particle assembly. Furthermore, this complementation assay can be used to investigate the role of the gag domain in the gag-pol polyprotein by determining whether it can rescue a defect in the gag polyprotein. We report here the results of such an experiment, which studied a mutation in the N terminus of the gag gene.

Processing of avian retroviral gag polyprotein precursors is blocked by a mutation at the NC-PR cleavage site

The avian sarcoma and leukosis viruses (ASLV) encode a protease (PR) at the C terminus of gag which in vivo catalyzes the processing of both gag and gag-pol precursors. The studies reported here were undertaken to determine whether PR is able to cleave these polyproteins while it is still part of the gag precursor or whether the release of its N terminus to form free PR is necessary for full proteolytic activity. To address this question, we created a mutation that disrupts the PR cleavage site between the NC and PR coding regions of the gag gene. This mutation was introduced into a eukaryotic vector that expresses only the gag precursor and into an otherwise infectious clone of ASLV that carries the neo gene as a selectable marker. These constructs were expressed in monkey COS cells or in quail QT35 cells, respectively. Processing was impaired in both systems. Mutant particles were formed, but they contained no mature processed gag proteins. We observed only the uncleaved gag precursor polypeptide Pr76 in one case or Pr76 and a cleaved product of about 60 kDa in the other. Processing of the mutant gag precursor could be complemented in trans by from a wild-type construct, suggesting that the mutation did not induce gross structural alterations in its precursor. Our results suggest that the PR first must be released from its precursor before it can attack other sites in the gag and gag-pol polyproteins and that cleavage at the NC-PR boundary is a prerequisite for the initiation of the PR-directed processing.

Assembly and processing of avian retroviral gag polyproteins containing linked protease dimers

Assembly and maturation of retroviral particles requires the aggregation and controlled proteolytic cleavage of polyprotein core precursors by a precursor-encoded protease (PR). Active, mature retroviral PR is a dimer, and the accumulation of precursors at sites of assembly may facilitate subunit interaction and subsequent activation of this enzyme. In addition, it has been suggested that cellular cytoplasmic components act as inhibitors of PR activity, so that processing is delayed until the nascent virions leave this compartment and separate from the surface of host cells. To investigate the mechanisms that control PR activity during virus assembly, we studied the in vivo processing of retroviral gag precursors that contain tandemly linked PR subunits in which dimerization is concentration independent. Sequences encoding four different linked protease dimers were independently joined to the end of the Rous sarcoma virus (RSV) gag gene in a simian virus 40-based plasmid vector which expresses a myristoylated gag precursor upon transfection of COS-1 cells. Three of these plasmids produced gag precursors that were incorporated into viruslike particles and proteolytically cleaved by the dimers to mature core proteins that were indistinguishable from the processed products of wild-type gag. The amount of viral gag protein that was assembled and packaged in these transfections was inversely related to the relative proteolytic activities of the linked PR dimers. The fourth gag precursor, which contained the most active linked PR dimer, underwent rapid intracellular processing and did not form viruslike particles. In the absence of the plasma membrane targeting signal, processing of all four linked PR dimer-containing gag precursors was completed entirely within the cell. From these results, we conclude that the delay in polyprotein core precursor processing that occurs during normal virion assembly does not depend on a cytoplasmic inhibitor of PR activity. We suggest that dimer formation is not only necessary but may be sufficient for the initiation of PR-directed maturation of gag and gag-pol precursors.

trans-acting viral protease is necessary and sufficient for activation of avian leukosis virus reverse transcriptase

The structural and enzymatic components of retroviral cores are formed by proteolytic cleavage of precursor polypeptides, mediated by the viral protease (PR). We described previously the construction of PR-defective avian leukosis viruses. These mutant viruses are noninfectious, and their major internal components are the uncleaved gag and gag-pol polyproteins (Pr76gag and Pr180gag-pol). The reverse transcriptase (RT) activity associated with the PR-defective virions is approximately 500-fold reduced relative to that of wild-type virions, suggesting that specific cleavages activate RT activity. To gain a better understanding of the role that PR plays in the processing and activation of RT, we performed complementation experiments wherein wild-type or PR mutant gag precursors were separately coexpressed with frame-corrected wild-type or PR mutant gag-pol precursors. The results demonstrate that, as in other retrovirus systems, gag-pol precursors can be assembled into virions only when they are rescued by a gag precursor. If the gag precursor is wild type, then the rescued Pr180gag-pol is completely and properly matured, irrespective of whether its embedded PR domain is wild type or mutant. In both cases, the virions produced are fully and equally infectious. This indicates that an active-site mutation in the PR domain of the gag-pol precursor has no effect on avian leukosis virus infectivity when particles are assembled from wild-type gag precursors. In contrast, if the gag precursor has an active-site mutation in PR or is deleted for PR, then the virions are noninfectious and the gag and gag-pol precursors remain unprocessed, even if the embedded PR domain of Pr180gag-pol is wild type. Thus, in this system, virion-associated Pr180gag-pol displays no detectable cis- or trans-acting PR activity. As assayed with an exogenous template, virions with processed gag-pol polyprotein display high levels of RT activity while those with unprocessed Pr180gag-pol display greatly reduced RT activity. These results demonstrate that during virion assembly, the PR supplied by a gag precursor is both necessary and sufficient for trans-activation of RT through proteolytic maturation of copackaged gag-pol polyprotein.

Role of the avian retroviral protease in the activation of reverse transcriptase during virion assembly

The retroviruses of the avian sarcoma-leukosis virus group synthesize their viral protease (PR) in two precursor forms--as a carboxy-terminal domain of the Gag precursor and as an embedded domain within the Gag-Pol precursor. We have shown previously that the Gag-derived PR is fully capable of processing the Gag precursor in the absence of the embedded PR (R.P. Bennett, S. Rhee, R.C. Craven, E. Hunter, and J.W. Wills, J. Virol. 65:272-280, 1991). In this study, we examined the question of whether or not the PR domain of Gag-Pol has an essential role in the maturation of the Pol proteins. The Gag-Pol precursor was expressed in the absence of Gag by use of a simian virus 40-based vector in which the gag and pol reading frames were fused. The fusion protein accumulated to high levels in transfected cells without being released into the medium but could be rescued into particles by coexpression of the Gag protein from a second vector. The resulting particles contained mature Gag and Pol proteins and active reverse transcriptase (RT). Using this complementation system, the effects of PR defects in the Gag and/or Gag-Pol proteins on the activation of RT were examined. The results showed that the presence of a functional PR on the Gag precursor, but not on Gag-Pol, was required for full activation of RT. The embedded PR of Gag-Pol was unable to carry out any detectable processing of the Gag precursor and was able to activate RT to only a low level in the absence of a functional Gag PR domain. Finally, some point mutations in the Gag-Pol PR domain inhibited activation of RT in trans by a wild-type PR, suggesting that the correct conformation of the PR domain in Gag-Pol is prerequisite for activation of RT.

Overexpression of the gag-pol precursor from human immunodeficiency virus type 1 proviral genomes results in efficient proteolytic processing in the absence of virion production

The expression of the gag-pol polyprotein of human immunodeficiency virus type 1 (HIV-1) occurs via ribosomal frameshifting between the gag and pol genes. Because low levels of the gag-pol precursor are naturally produced in HIV-1-infected cells, a limited amount of information is available on the biology of this molecule. To further study this polyprotein, two mutant HIV-1 proviral genomes were created to position the gag and pol genes in the same translational reading frame. The mutations inserted a single thymidine nucleotide at the site of ribosomal frameshifting (nucleotide 1635), which results in the addition of a phenylalanine residue (frameshift 1 [FS1]), or a single adenine nucleotide, which results in the addition of a leucine residue (frameshift 2 [FS2]). Transfection of the mutant proviral genomes into COS-1 cells resulted in the expression of the p160gag-pol polyprotein precursor as well as the proteolytically processed gag and pol gene products. Metabolic labeling of the transfected cells with [3H]myristic acid revealed that the p160gag-pol and p17gag proteins expressed from the mutant genomes were myristylated. While the supernatants from COS-1 cells transfected with wild-type or mutant proviral genomes contained similar amounts of p24 antigen, the levels of reverse transcriptase were, on the average, 10 times greater in the supernatants from cells transfected with the FS1 and FS2 proviral genomes. The cells transfected with the wild-type proviral genome released infectious viral particles, while the mutant proviral genomes released p24 and reverse transcriptase in the absence of detectable particle formation. The mutant proviral genomes were completely noninfectious as determined by coculture of the transfected COS-1 cells with SupT1 cells. These results demonstrate that the gag-pol polyprotein of HIV-1 contains the appropriate signals for proteolytic processing and association with intracytoplasmic membranes in the absence of virion formation.

Suppression of retroviral MA deletions by the amino-terminal membrane-binding domain of p60src

The molecular mechanism by which retroviral Gag proteins are directed to the plasma membrane for the formation of particles (budding) is unknown, but it is widely believed that the MA domain, located at the amino terminus, plays a critical role. Consistent with this idea, we found that small deletions in this segment of the Rous sarcoma virus Gag protein completely blocked particle formation. The mutant proteins appear to have suffered only localized structural damage since they could be rescued (i.e., packaged into particles) when coexpressed with Gag proteins that are competent for particle formation. To our surprise, the effects of the MA deletions could be completely suppressed by fusing as few as seven residues of the myristylated amino terminus of the oncoprotein p60src to the beginning of the mutant Gag proteins. Particles produced by the chimeras were of the same density as the wild type. Two myristylated peptides having sequences distinct from that of p60src were entirely unable to suppress MA deletions, indicating that myristate alone is not a sufficient membrane targeting signal. We hypothesize that the amino terminus of p60src suppresses the effects of MA deletions by diverting the Rous sarcoma virus Gag protein from its normal site of assembly to the Src receptor for particle formation.