Efficiency and selectivity of RNA packaging by Rous sarcoma virus Gag deletion mutants

HIV-1 Hijacking of Host ATPases and GTPases That Control Protein Trafficking

The human immunodeficiency virus (HIV-1) modifies the host cell environment to ensure efficient and sustained viral replication. Key to these processes is the capacity of the virus to hijack ATPases, GTPases and the associated proteins that control intracellular protein trafficking. The functions of these energy-harnessing enzymes can be seized by HIV-1 to allow the intracellular transport of viral components within the host cell or to change the subcellular distribution of antiviral factors, leading to immune evasion. Here, we summarize how energy-related proteins deviate from their normal functions in host protein trafficking to aid the virus in different phases of its replicative cycle. Recent discoveries regarding the interplay among HIV-1 and host ATPases and GTPases may shed light on potential targets for pharmacological intervention.

The roles of lipids and nucleic acids in HIV-1 assembly

During HIV-1 assembly, precursor Gag (PrGag) proteins are delivered to plasma membrane (PM) assembly sites, where they are triggered to oligomerize and bud from cells as immature virus particles. The delivery and triggering processes are coordinated by the PrGag matrix (MA) and nucleocapsid (NC) domains. Targeting of PrGag proteins to membranes enriched in cholesterol and phosphatidylinositol-4,5-bisphosphate (PI[4,5]P2) is mediated by the MA domain, which also has been shown to bind both RNA and DNA. Evidence suggests that the nucleic-acid-binding function of MA serves to inhibit PrGag binding to inappropriate intracellular membranes, prior to delivery to the PM. At the PM, MA domains putatively trade RNA ligands for PI(4,5)P2 ligands, fostering high-affinity membrane binding. Triggering of oligomerization, budding, and virus particle release results when NC domains on adjacent PrGag proteins bind to viral RNA, leading to capsid (CA) domain oligomerization. This process leads to the assembly of immature virus shells in which hexamers of membrane-bound MA trimers appear to organize above interlinked CA hexamers. Here, we review the functions of retroviral MA proteins, with an emphasis on the nucleic-acid-binding capability of the HIV-1 MA protein, and its effects on membrane binding.

Mechanistic differences between nucleic acid chaperone activities of the Gag proteins of Rous sarcoma virus and human immunodeficiency virus type 1 are attributed to the MA domain

Host cell tRNAs are recruited for use as primers to initiate reverse transcription in retroviruses. Human immunodeficiency virus type 1 (HIV-1) uses tRNA(Lys3) as the replication primer, whereas Rous sarcoma virus (RSV) uses tRNA(Trp). The nucleic acid (NA) chaperone function of the nucleocapsid (NC) domain of HIV-1 Gag is responsible for annealing tRNA(Lys3) to the genomic RNA (gRNA) primer binding site (PBS). Compared to HIV-1, little is known about the chaperone activity of RSV Gag. In this work, using purified RSV Gag containing an N-terminal His tag and a deletion of the majority of the protease domain (H6.Gag.3h), gel shift assays were used to monitor the annealing of tRNA(Trp) to a PBS-containing RSV RNA. Here, we show that similar to HIV-1 Gag lacking the p6 domain (GagΔp6), RSV H6.Gag.3h is a more efficient chaperone on a molar basis than NC; however, in contrast to the HIV-1 system, both RSV H6.Gag.3h and NC have comparable annealing rates at protein saturation. The NC domain of RSV H6.Gag.3h is required for annealing, whereas deletion of the matrix (MA) domain, which stimulates the rate of HIV-1 GagΔp6 annealing, has little effect on RSV H6.Gag.3h chaperone function. Competition assays confirmed that RSV MA binds inositol phosphates (IPs), but in contrast to HIV-1 GagΔp6, IPs do not stimulate RSV H6.Gag.3h chaperone activity unless the MA domain is replaced with HIV-1 MA. We conclude that differences in the MA domains are primarily responsible for mechanistic differences in RSV and HIV-1 Gag NA chaperone function. Importance: Mounting evidence suggests that the Gag polyprotein is responsible for annealing primer tRNAs to the PBS to initiate reverse transcription in retroviruses, but only HIV-1 Gag chaperone activity has been demonstrated in vitro. Understanding RSV Gag's NA chaperone function will allow us to determine whether there is a common mechanism among retroviruses. This report shows for the first time that full-length RSV Gag lacking the protease domain is a highly efficient NA chaperone in vitro, and NC is required for this activity. In contrast to results obtained for HIV-1 Gag, due to the weak nucleic acid binding affinity of the RSV MA domain, inositol phosphates do not regulate RSV Gag-facilitated tRNA annealing despite the fact that they bind to MA. These studies provide insight into the viral regulation of tRNA primer annealing, which is a potential target for antiretroviral therapy.

Dimeric RNA recognition regulates HIV-1 genome packaging

How retroviruses regulate the amount of RNA genome packaged into each virion has remained a long-standing question. Our previous study showed that most HIV-1 particles contain two copies of viral RNA, indicating that the number of genomes packaged is tightly regulated. In this report, we examine the mechanism that controls the number of RNA genomes encapsidated into HIV-1 particles. We hypothesize that HIV-1 regulates genome packaging by either the mass or copy number of the viral RNA. These two distinct mechanisms predict different outcomes when the genome size deviates significantly from that of wild type. Regulation by RNA mass would result in multiple copies of a small genome or one copy of a large genome being packaged, whereas regulation by copy number would result in two copies of a genome being packaged independent of size. To distinguish between these two hypotheses, we examined the packaging of viral RNA that was larger (≈17 kb) or smaller (≈3 kb) than that of wild-type HIV-1 (≈9 kb) and found that most particles packaged two copies of the viral genome regardless of whether they were 17 kb or 3 kb. Therefore, HIV-1 regulates RNA genome encapsidation not by the mass of RNA but by packaging two copies of RNA. To further explore the mechanism that governs this regulation, we examined the packaging of viral RNAs containing two packaging signals that can form intermolecular dimers or intramolecular dimers (self-dimers) and found that one self-dimer is packaged. Therefore, HIV-1 recognizes one dimeric RNA instead of two copies of RNA. Our findings reveal that dimeric RNA recognition is the key mechanism that regulates HIV-1 genome encapsidation and provide insights into a critical step in the generation of infectious viruses.

Beyond plasma membrane targeting: role of the MA domain of Gag in retroviral genome encapsidation

The MA (matrix) domain of the retroviral Gag polyprotein plays several critical roles during virus assembly. Although best known for targeting the Gag polyprotein to the inner leaflet of the plasma membrane for virus budding, recent studies have revealed that MA also contributes to selective packaging of the genomic RNA (gRNA) into virions. In this Review, we summarize recent progress in understanding how MA participates in genome incorporation. We compare the mechanisms by which the MA domains of different retroviral Gag proteins influence gRNA packaging, highlighting variations and similarities in how MA directs the subcellular trafficking of Gag, interacts with host factors and binds to nucleic acids. A deeper understanding of how MA participates in these diverse functions at different stages in the virus assembly pathway will require more detailed information about the structure of the MA domain within the full-length Gag polyprotein. In particular, it will be necessary to understand the structural basis of the interaction of MA with gRNA, host transport factors and membrane phospholipids. A better appreciation of the multiple roles MA plays in genome packaging and Gag localization might guide the development of novel antiviral strategies in the future.

The remarkable frequency of human immunodeficiency virus type 1 genetic recombination

The genetic diversity of human immunodeficiency virus type 1 (HIV-1) results from a combination of point mutations and genetic recombination, and rates of both processes are unusually high. This review focuses on the mechanisms and outcomes of HIV-1 genetic recombination and on the parameters that make recombination so remarkably frequent. Experimental work has demonstrated that the process that leads to recombination--a copy choice mechanism involving the migration of reverse transcriptase between viral RNA templates--occurs several times on average during every round of HIV-1 DNA synthesis. Key biological factors that lead to high recombination rates for all retroviruses are the recombination-prone nature of their reverse transcription machinery and their pseudodiploid RNA genomes. However, HIV-1 genes recombine even more frequently than do those of many other retroviruses. This reflects the way in which HIV-1 selects genomic RNAs for coencapsidation as well as cell-to-cell transmission properties that lead to unusually frequent associations between distinct viral genotypes. HIV-1 faces strong and changeable selective conditions during replication within patients. The mode of HIV-1 persistence as integrated proviruses and strong selection for defective proviruses in vivo provide conditions for archiving alleles, which can be resuscitated years after initial provirus establishment. Recombination can facilitate drug resistance and may allow superinfecting HIV-1 strains to evade preexisting immune responses, thus adding to challenges in vaccine development. These properties converge to provide HIV-1 with the means, motive, and opportunity to recombine its genetic material at an unprecedented high rate and to allow genetic recombination to serve as one of the highest barriers to HIV-1 eradication.

Basic residues of the retroviral nucleocapsid play different roles in gag-gag and Gag-Psi RNA interactions

The Orthoretrovirus Gag interaction (I) domain maps to the nucleocapsid (NC) domain in the Gag polyprotein. We used the yeast two-hybrid system to analyze the role of Alpharetrovirus NC in Gag-Gag interactions and also examined the efficiency of viral assembly and release in vivo. We could delete either or both of the two Cys-His (CH) boxes without abrogating Gag-Gag interactions. We found that as few as eight clustered basic residues, attached to the C terminus of the spacer peptide separating the capsid (CA) and NC domains in the absence of NC, was sufficient for Gag-Gag interactions. Our results support the idea that a sufficient number of basic residues, rather than the CH boxes, play the important role in Gag multimerization. We also examined the requirement for basic residues in Gag for packaging of specific packaging signal (Psi)-containing RNA. Using a yeast three-hybrid RNA-protein interaction assay, second-site suppressors of a packaging-defective Gag mutant were isolated, which restored Psi RNA binding. These suppressors mapped to the p10 or CA domains in Gag and resulted in either introduction of a positively charged residue or elimination of a negatively charged one. These results imply that the structural interactions of NC with other domains of Gag are necessary for Psi RNA binding. Taken together, our results show that while Gag assembly only requires a certain number of positively charged amino acids, Gag binding to genomic RNA for packaging requires more complex interactions inherent in the protein tertiary structure.

Link between genome packaging and rate of budding for Rous sarcoma virus

The subcellular location at which genomic RNA is packaged by Gag proteins during retrovirus assembly remains unknown. Since the membrane-binding (M) domain is most critical for targeting Gag to the plasma membrane, changes to this determinant might alter the path taken through the cell and reduce the efficiency of genome packaging. In this report, a Rous sarcoma virus (RSV) mutant having two acidic-to-basic substitutions in the M domain is described. This mutant, designated Super M, produced particles much faster than the wild type, but the mutant virions were noninfectious and contained only 1/10 the amount of genomic RNA found in wild-type particles. To identify the cause(s) of these defects, we considered data that suggest that RSV Gag traffics through the nucleus to package the viral genome. Although inhibition of the CRM-1 pathway of nuclear export caused the accumulation of wild-type Gag in the nucleus, nuclear accumulation did not occur with Super M. The importance of the nucleocapsid (NC) domain in membrane targeting was also determined, and, importantly, deletion of the NC sequence prevented plasma membrane localization by wild-type Gag but not by Super M Gag. Based on these results, we reasoned that the enhanced membrane-targeting properties of Super M inhibit genome packaging. Consistent with this interpretation, substitutions that reestablished the wild-type number of basic and acidic residues in the Super M Gag M domain reduced the budding efficiency and restored genome packaging and infectivity. Therefore, these data suggest that Gag targeting and genome packaging are normally linked to ensure that RSV particles contain viral RNA.

Mapping the encapsidation determinants of feline immunodeficiency virus

Encapsidation of retroviral RNA involves specific interactions between viral proteins and cis-acting genomic RNA sequences. Human immunodeficiency virus type 1 (HIV-1) RNA encapsidation determinants appear to be more complex and dispersed than those of murine retroviruses. Feline lentiviral (feline immunodeficiency virus [FIV]) encapsidation has not been studied. To gain comparative insight into lentiviral encapsidation and to optimize FIV-based vectors, we used RNase protection assays of cellular and virion RNAs to determine packaging efficiencies of FIV deletion mutants, and we studied replicative phenotypes of mutant viruses. Unlike the case for other mammalian retroviruses, the sequences between the major splice donor (MSD) and the start codon of gag contribute negligibly to FIV encapsidation. Moreover, molecular clones having deletions in this region were replication competent. In contrast, sequences upstream of the MSD were important for encapsidation, and deletion of the U5 element markedly reduced genomic RNA packaging. The contribution of gag sequences to packaging was systematically investigated with subgenomic FIV vectors containing variable portions of the gag open reading frame, with all virion proteins supplied in trans. When no gag sequence was present, packaging was abolished and marker gene transduction was absent. Inclusion of the first 144 nucleotides (nt) of gag increased vector encapsidation to detectable levels, while inclusion of the first 311 nt increased it to nearly wild-type levels and resulted in high-titer FIV vectors. However, the identified proximal gag sequence is necessary but not sufficient, since viral mRNAs that contain all coding regions, with or without as much as 119 nt of adjacent upstream 5' leader, were excluded from encapsidation. The results identify a mechanism whereby FIV can encapsidate its genomic mRNA in preference to subgenomic mRNAs.

cis-Acting elements important for retroviral RNA packaging specificity

Spleen necrosis virus (SNV) proteins can package RNA from distantly related murine leukemia virus (MLV), whereas MLV proteins cannot package SNV RNA efficiently. We used this nonreciprocal recognition to investigate regions of packaging signals that influence viral RNA encapsidation specificity. Although the MLV and SNV packaging signals (Psi and E, respectively) do not contain significant sequence homology, they both contain a pair of hairpins. This hairpin pair was previously proposed to be the core element in MLV Psi. In the present study, MLV-based vectors were generated to contain chimeric SNV/MLV packaging signals in which the hairpins were replaced with the heterologous counterpart. The interactions between these chimeras and MLV or SNV proteins were examined by virus replication and RNA analyses. SNV proteins recognized all of the chimeras, indicating that these chimeras were functional. We found that replacing the hairpin pair did not drastically alter the ability of MLV proteins to package these chimeras. These results indicate that, despite the important role of the hairpin pair in RNA packaging, it is not the major motif responsible for the ability of MLV proteins to discriminate between the MLV and SNV packaging signals. To determine the role of sequences flanking the hairpins in RNA packaging specificity, vectors with swapped flanking regions were generated and evaluated. SNV proteins packaged all of these chimeras efficiently. In contrast, MLV proteins strongly favored chimeras with the MLV 5'-flanking regions. These data indicated that MLV Gag recognizes multiple elements in the viral packaging signal, including the hairpin structure and flanking regions.

Role of RNA in facilitating Gag/Gag-Pol interaction

We have examined the influence of RNA upon the interaction of Gag-Pol with Gag during human immunodeficiency virus type 1 (HIV-1) assembly. COS7 cells were transfected with protease-negative HIV-1 proviral DNA, and Gag/Gag-Pol complexes were detected by coimmunoprecipitation with anti-integrase. In COS7 cells, Gag/Gag-Pol is found almost entirely in pelletable, membrane-bound complexes. Exposure of cells to 1% Triton X-100 releases Gag/Gag-Pol from bulk membrane, but the complexes remain pelletable. The role of RNA in facilitating the interaction between Gag and Gag-Pol was examined in these bulk membrane-free, pelletable complexes. The specific presence of viral genomic RNA is not required to maintain the Gag/Gag-Pol interaction, but some type of RNA is, since exposure to RNase destabilized the Gag/Gag-Pol complex. When present only in Gag, the nucleocapsid mutation R7R10K11S, which inhibits Gag binding to RNA, inhibits the formation of both Gag and Gag/Gag-Pol complexes. When present only in Gag-Pol, this mutation has no effect upon complex formation. This result indicates that Gag-Pol may not interact directly with RNA but rather requires RNA-facilitated Gag multimerization for its interaction with Gag.

Yeast three-hybrid screening of rous sarcoma virus mutants with randomly mutagenized minimal packaging signals reveals regions important for gag interactions

We previously showed that the yeast three-hybrid system provides a genetic assay of both RNA and protein components for avian retroviral RNA encapsidation. In the current study, we used this assay to precisely define cis-acting determinants involved in avian leukosis sarcoma virus packaging RNA binding to Gag protein. In vivo screening of Rous sarcoma virus mutants was performed with randomly mutated minimal packaging sequences (MPsi) made using PCR amplification after cotransformation with GagDeltaPR protein into yeast cells. Colonies with low beta-galactosidase activity were analyzed to locate mutations in MPsi sequences affecting binding to Gag proteins. This genetic assay delineated secondary structural elements that are important for efficient RNA binding, including a single-stranded small bulge containing the initiation codon for uORF3, as well as adjacent stem structures. This implies a possible tertiary structure favoring the high-affinity binding sites for Gag. In most cases, results from the three-hybrid assay were well correlated with those from the viral RNA packaging assays. The results from random mutagenesis using the rapid three-hybrid binding assay are consistent with those from site-directed mutagenesis using in vivo packaging assays.

NMR structure of the HIV-1 nucleocapsid protein bound to stem-loop SL2 of the psi-RNA packaging signal. Implications for genome recognition

The RNA genome of the human immunodeficiency virus type-1 (HIV-1) contains a approximately 120 nucleotide Psi-packaging signal that is recognized by the nucleocapsid (NC) domain of the Gag polyprotein during virus assembly. The Psi-site contains four stem-loops (SL1-SL4) that possess overlapping and possibly redundant functions. The present studies demonstrate that the 19 residue SL2 stem-loop binds NC with affinity (K(d)=110(+/-50) nM) similar to that observed for NC binding to SL3 (K(d)=170(+/-65) nM) and tighter than expected on the basis of earlier work, suggesting that NC-SL2 interactions probably play a direct role in the specific recognition and packaging of the full-length, unspliced genome. The structure of the NC-SL2 complex was determined by heteronuclear NMR methods using (15)N,(13)C-isotopically labeled NC protein and SL2 RNA. The N and C-terminal "zinc knuckles" (Cys-X(2)-Cys-X(4)-His-X(4)-Cys; X=variable amino acid) of HIV-1 NC bind to exposed guanosine bases G9 and G11, respectively, of the G8-G9-U10-G11 tetraloop, and residues Lys3-Lys11 of the N-terminal tail forms a 3(10) helix that packs against the proximal zinc knuckle and interacts with the RNA stem. These structural features are similar to those observed previously in the NMR structure of NC bound to SL3. Other features of the complex are substantially different. In particular, the N-terminal zinc knuckle interacts with an A-U-A base triple platform in the minor groove of the SL2 RNA stem, but binds to the major groove of SL3. In addition, the relative orientations of the N and C-terminal zinc knuckles differ in the NC-SL2 and NC-SL3 complexes, and the side-chain of Phe6 makes minor groove hydrophobic contacts with G11 in the NC-SL2 complex but does not interact with RNA in the NC-SL3 complex. Finally, the N-terminal helix of NC interacts with the phosphodiester backbone of the SL2 RNA stem mainly via electrostatic interactions, but does not bind in the major groove or make specific H-bonding contacts as observed in the NC-SL3 structure. These findings demonstrate that NC binds in an adaptive manner to SL2 and SL3 via different subsets of inter and intra-molecular interactions, and support a genome recognition/packaging mechanism that involves interactions of two or more NC domains of assembling HIV-1 Gag molecules with multiple Psi-site stem-loop packaging elements during the early stages of retrovirus assembly.

The double-stranded RNA-binding protein Staufen is incorporated in human immunodeficiency virus type 1: evidence for a role in genomic RNA encapsidation

Human Staufen (hStau), a double-stranded RNA (dsRNA)-binding protein that is involved in mRNA transport, is incorporated in human immunodeficiency virus type 1 (HIV-1) and in other retroviruses, including HIV-2 and Moloney murine leukemia virus. Sucrose and Optiprep gradient analyses reveal cosedimentation of hStau with purified HIV-1, while subtilisin assays demonstrate that it is internalized. hStau incorporation in HIV-1 is selective, is dependent on an intact functional dsRNA-binding domain, and quantitatively correlates with levels of encapsidated HIV-1 genomic RNA. By coimmunoprecipitation and reverse transcription-PCR analyses, we demonstrate that hStau is associated with HIV-1 genomic RNA in HIV-1-expressing cells and purified virus. Overexpression of hStau enhances virion incorporation levels, and a corresponding, threefold increase in HIV-1 genomic RNA encapsidation levels. This coordinated increase in hStau and genomic RNA packaging had a significant negative effect on viral infectivity. This study is the first to describe hStau within HIV-1 particles and provides evidence that hStau binds HIV-1 genomic RNA, indicating that it may be implicated in retroviral genome selection and packaging into assembling virions.

Replication of lengthened Moloney murine leukemia virus genomes is impaired at multiple stages

It has been assumed that RNA packaging constraints limit the size of retroviral genomes. This notion of a retroviral "headful" was tested by examining the ability of Moloney murine leukemia virus genomes lengthened by 4, 8, or 11 kb to participate in a single replication cycle. Overall, replication of these lengthened genomes was 5- to 10-fold less efficient than that of native-length genomes. When RNA expression and virion formation, RNA packaging, and early stages of replication were compared, long genomes were found to complete each step less efficiently than did normal-length genomes. To test whether short RNAs might facilitate the packaging of lengthy RNAs by heterodimerization, some experiments involved coexpression of a short packageable RNA. However, enhancement of neither long vector RNA packaging nor long vector DNA synthesis was observed in the presence of the short RNA. Most of the proviruses templated by 12 and 16 kb vectors appeared to be full length. Most products of a 19. 2-kb vector contained deletions, but some integrated proviruses were around twice the native genome length. These results demonstrate that lengthy retroviral genomes can be packaged and that genome length is not strictly limited at any individual replication step. These observations also suggest that the lengthy read-through RNAs postulated to be intermediates in retroviral transduction can be packaged directly without further processing.

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.

The nucleocapsid domain is responsible for the ability of spleen necrosis virus (SNV) Gag polyprotein to package both SNV and murine leukemia virus RNA

Murine leukemia virus (MLV)-based vector RNA can be packaged and propagated by the proteins of spleen necrosis virus (SNV). We recently demonstrated that MLV proteins cannot support the replication of an SNV-based vector; RNA analysis revealed that MLV proteins cannot efficiently package SNV-based vector RNA. The domain in Gag responsible for the specificity of RNA packaging was identified using chimeric gag-pol expression constructs. A competitive packaging system was established by generating a cell line that expresses one viral vector RNA containing the MLV packaging signal (Psi) and another viral vector RNA containing the SNV packaging signal (E). The chimeric gag-pol expression constructs were introduced into the cells, and vector titers as well as the efficiency of RNA packaging were examined. Our data confirm that Gag is solely responsible for the selection of viral RNAs. Furthermore, the nucleocapsid (NC) domain in the SNV Gag is responsible for its ability to interact with both SNV E and MLV Psi. Replacement of the SNV NC with the MLV NC generated a chimeric Gag that could not package SNV RNA but retained its ability to package MLV RNA. A construct expressing SNV gag-MLV pol supported the replication of both MLV and SNV vectors, indicating that the gag and pol gene products from two different viruses can functionally cooperate to perform one cycle of retroviral replication. Viral titer data indicated that SNV cis-acting elements are not ideal substrates for MLV pol gene products since infectious viruses were generated at a lower efficiency. These results indicate that the nonreciprocal recognition between SNV and MLV extends beyond the Gag-RNA interaction and also includes interactions between Pol and other cis-acting elements.

The gag domains required for avian retroviral RNA encapsidation determined by using two independent assays

The Rous sarcoma virus (RSV) Gag precursor polyprotein is the only viral protein which is necessary for specific packaging of genomic RNA. To map domains within Gag which are important for packaging, we constructed a series of Gag mutations in conjunction with a protease (PR) active-site point mutation in a full-length viral construct. We found that deletion of either the matrix (MA), the capsid (CA), or the protease (PR) domain did not abrogate packaging, although the MA domain is likely to be required for proper assembly. A previously characterized deletion of both Cys-His motifs in RSV nucleocapsid protein (NC) reduced both the efficiency of particle release and specific RNA packaging by 6- to 10-fold, consistent with previous observations that the NC Cys-His motifs played a role in assembly and RNA packaging. Most strikingly, when amino acid changes at Arg 549 and 551 immediately downstream of the distal NC Cys-His box were made, RNA packaging was reduced by more than 25-fold with no defect in particle release, demonstrating the importance of this basic amino acid region in packaging. We also used the yeast three-hybrid system to study avian retroviral RNA-Gag interactions. Using this assay, we found that the interactions of the minimal packaging region (Mpsi) with Gag are of high affinity and specificity. Using a number of Mpsi and Gag mutants, we have found a clear correlation between a reporter gene activation in a yeast three-hybrid binding system and an in vivo packaging assay. Our results showed that the binding assay provides a rapid genetic assay of both RNA and protein components for specific encapsidation.

Late domain function identified in the vesicular stomatitis virus M protein by use of rhabdovirus-retrovirus chimeras

Little is known about the mechanisms used by enveloped viruses to separate themselves from the cell surface at the final step of budding. However, small sequences in the Gag proteins of several retroviruses (L domains) have been implicated in this process. A sequence has been identified in the M proteins of rhabdoviruses that closely resembles the PPPPY motif in the L domain of Rous sarcoma virus (RSV), an avian retrovirus. To evaluate whether the PPPY sequence in vesicular stomatitis virus (VSV) M protein has an activity analogous to that of the retroviral sequence, M-Gag chimeras were characterized. The N-terminal 74 amino acids of the VSV (Indiana) M protein, including the PPPY motif, was able to replace the L domain of RSV Gag and allow the assembly and release of virus-like particles. Alanine substitutions in the VSV PPPY motif severely compromised the budding activity of this hybrid protein but not that of another chimera which also contained the RSV PPPPY sequence. We conclude that this VSV sequence is functionally homologous to the RSV L domain in promoting virus particle release, making this the first example of such an activity in a virus other than a retrovirus. Both the RSV and VSV motifs have been shown to interact in vitro with certain cellular proteins that contain a WW interaction module, suggesting that the L domains are sites of interaction with unknown host machinery involved in virus release.

Virus maturation by budding

Enveloped viruses mature by budding at cellular membranes. It has been generally thought that this process is driven by interactions between the viral transmembrane proteins and the internal virion components (core, capsid, or nucleocapsid). This model was particularly applicable to alphaviruses, which require both spike proteins and a nucleocapsid for budding. However, genetic studies have clearly shown that the retrovirus core protein, i.e., the Gag protein, is able to form enveloped particles by itself. Also, budding of negative-strand RNA viruses (rhabdoviruses, orthomyxoviruses, and paramyxoviruses) seems to be accomplished mainly by internal components, most probably the matrix protein, since the spike proteins are not absolutely required for budding of these viruses either. In contrast, budding of coronavirus particles can occur in the absence of the nucleocapsid and appears to require two membrane proteins only. Biochemical and structural data suggest that the proteins, which play a key role in budding, drive this process by forming a three-dimensional (cage-like) protein lattice at the surface of or within the membrane. Similarly, recent electron microscopic studies revealed that the alphavirus spike proteins are also engaged in extensive lateral interactions, forming a dense protein shell at the outer surface of the viral envelope. On the basis of these data, we propose that the budding of enveloped viruses in general is governed by lateral interactions between peripheral or integral membrane proteins. This new concept also provides answers to the question of how viral and cellular membrane proteins are sorted during budding. In addition, it has implications for the mechanism by which the virion is uncoated during virus entry.

The role of nucleocapsid of HIV-1 in virus assembly

The role of the nucleocapsid protein of HIV-1 Gag in virus assembly was investigated using Gag truncation mutants, a nucleocapsid deletion mutant, and point mutations in the nucleocapsid region of Gag, in transfected COS cells, and in stable T-cell lines. Consistent with previous investigations, a truncation containing only the matrix and capsid regions of Gag was unable to assemble efficiently into particles; also, the pelletable material released was lighter than the density of wild-type HIV-1. A deletion mutant lacking p7 nucleocapsid but containing the C-terminal p6 protein was also inefficient in particle release and released lighter particles, while a truncation containing only the first zinc finger of p7 could assemble more efficiently into virions. These results clearly show that p7 is indispensable for virus assembly and release. Some point mutations in the N-terminal basic domain and in the basic linker region between the two zinc fingers, which had been previously shown to have reduced RNA binding in vitro [Schmalzbauer, E., Strack, B., Dannull, J., Guehmann, S., and Moelling, K. (1996). J. Virol. 70: 771-777], were shown to reduce virus assembly dramatically when expressed in full-length viral clones. A fusion protein consisting of matrix and capsid fused to a heterologous viral protein known to have nonspecific RNA binding activity [Ribas, J. C., Fujimura, T., and Wickner, R. B. (1994) J. Biol. Chem. 269: 28420-28428] released pelletable material slightly more efficiently than matrix and capsid alone, and these particles had density higher than matrix and capsid alone. These results demonstrate the essential role of HIV-1 nucleocapsid in the virus assembly process and show that the positively charged N terminus of p7 is critical for this role.

Molecular events in the assembly of retrovirus particles

Retrovirus assembly results from the ability of a single gene product, the gag polyprotein precursor, to coalesce into a spherical particle capable of release from the cell. In conjunction with this primary process of capsid formation additional viral gene products such as the replicative enzymes and envelope glycoproteins as well as the genomic RNA are incorporated to form an infectious virus.

Nonreciprocal pseudotyping: murine leukemia virus proteins cannot efficiently package spleen necrosis virus-based vector RNA

It has been documented that spleen necrosis virus (SNV) can package murine leukemia virus (MLV) RNA efficiently and propagate MLV vectors to the same titers as it propagates SNV-based vectors. Although the SNV packaging signal (E) and MLV packaging signal (Psi) have little sequence homology, similar double-hairpin RNA structures were predicted and supported by experimental evidence. To test whether SNV RNA can be packaged by MLV proteins, we modified an SNV vector to be expressed in an MLV-based murine helper cell line. Surprisingly, we found that MLV proteins could not support the replication of SNV vectors. The decrease in titer was approximately 2,000- to 20,000-fold in one round of retroviral replication. RNA analysis revealed that SNV RNA was not efficiently packaged by MLV proteins. RNA hybridization of the cellular and viral RNAs indicated that SNV RNA was packaged at least 25-fold less efficiently than MLV RNA, which was the sensitivity limit of the hybridization assay. The contrast between the MLV and SNV packaging specificity is striking. SNV proteins can recognize both SNV E and MLV Psi, but MLV can recognize only MLV Psi. This is the first demonstration of two retroviruses with nonreciprocal packaging specificities.

Distinct functions and requirements for the Cys-His boxes of the human immunodeficiency virus type 1 nucleocapsid protein during RNA encapsidation and replication

The process of retroviral RNA encapsidation involves interaction between trans-acting viral proteins and cis-acting RNA elements. The encapsidation signal on human immunodeficiency virus type 1 (HIV-1) RNA is a multipartite structure composed of functional stem-loop structures. The nucleocapsid (NC) domain of the Gag polyprotein precursor contains two copies of a Cys-His box motif that have been demonstrated to be important in RNA encapsidation. To further characterize the role of the Cys-His boxes of the HIV-1 NC protein in RNA encapsidation, the relative efficiency of RNA encapsidation for virus particles that contained mutations within the Cys-His boxes was measured. Mutations that disrupted the first Cys-His box of the NC protein resulted in virus particles that encapsidated genomic RNA less efficiently and subgenomic RNA more efficiently than did wild-type virus. Mutations within the second Cys-His box did not significantly affect RNA encapsidation. In addition, a full complement of wild-type NC protein in virus particles is not required for efficient RNA encapsidation or virus replication. Finally, both Cys-His boxes of the NC protein play additional roles in virus replication.

In vitro assembly of virus-like particles with Rous sarcoma virus Gag deletion mutants: identification of the p10 domain as a morphological determinant in the formation of spherical particles

Retroviruses are unusual in that expression of a single protein, Gag, leads to budding of virus-like particles into the extracellular space. We have developed conditions under which virus-like particles are formed spontaneously in vitro from fragments of Rous sarcoma virus (RSV) Gag protein purified after expression in Escherichia coli. The CA-NC fragment of Gag was shown previously to assemble into hollow cylinders (S. Campbell and V. M. Vogt, J. Virol. 69:6487-6497, 1995). We have now extended these studies to larger Gag proteins. In every case examined, assembly into regular structures required RNA. A nearly full-length Gag missing only the C-terminal PR domain, as well as similar proteins missing in addition the N-terminal half of MA, the C-terminal half of MA, the entire MA sequence, or the entire p2 sequence, all assembled into spherical particles resembling RSV in size. By contrast, proteins missing p10 assembled into cylindrical particles like those formed by CA-NC alone. Thin section electron microscopy showed that each of these Gag proteins formed in the expressing E. coli cells particles similar in shape to those seen in vitro. We conclude from these results that neither the sequences required for membrane binding in vivo, near the N terminus of Gag, nor the sequences required for a late step in budding, in the p2 portion of Gag, are essential for formation of virus-like particles in this system. Furthermore, we postulate the existence of a shape-determining sequence in p10, which provides or facilitates interactions required for the growing particle to be constrained to a spherical shape.

Analysis of cleavage site mutations between the NC and PR Gag domains of Rous sarcoma virus

In retroviruses, the viral protease (PR) is released as a mature protein by cleavage of Gag, Gag-Pro, or Gag-Pro-Pol precursor polypeptides. In avian sarcoma and leukemia viruses (ASLV), PR forms the C-terminal domain of Gag. Based on the properties of a mutation (cs22) in the cleavage site between the upstream NC domain and the PR domain, the proteolytic liberation of PR previously was inferred to be essential for processing of Gag and Pol proteins. To study this process in more detail, we have analyzed the effects that several mutations at the NC-PR cleavage site have on proteolytic processing in virus-like particles expressed in COS and quail cells. Mutant Gag proteins carrying the same mutations also were synthesized in vitro and tested for processing with purified PR. In both types of studies, N-terminal sequencing of the liberated PR domain was carried out to exactly identify the site of cleavage. Finally, synthetic peptides corresponding to the mutant proteins were assessed for the ability to act as substrates for PR. The results were all consistent and led to the following conclusions. (i) In vivo, if normal processing between NC and PR is prevented by mutations, limited cleavage occurs at a previously unrecognized alternative site three amino acids downstream, i.e., in PR. This N-terminally truncated PR is inactive as an enzyme, as inferred from the global processing defect in cs22 and a similar mutant. (ii) In Gag proteins translated in vitro, purified PR cleaves this alternative site as rapidly as it does the wild-type site. (iii) Contrary to previously accepted rules describing retroviral cleavage sites, an isoleucine residue placed at the P1 position of the NC-PR cleavage site does not hinder normal processing. (iv) A proline residue placed at the P2 position in this cleavage site blocks normal processing.

Autogenous regulation of RNA translation and packaging by Rous sarcoma virus Pr76gag

Unspliced cytoplasmic retroviral RNA in chronically infected cells either is encapsidated by Gag proteins in the manufacture of virus or is used to direct synthesis of Gag proteins. Several models have been suggested to explain the sorting of viral RNA for these two purposes. Here we present evidence supporting a simple biochemical mechanism that accounts for the routing of retroviral RNA. Our results indicate that ribosomes compete with the Gag proteins to determine the fate of nascent retroviral RNA. Although the integrity of the entire Rous sarcoma virus leader sequence is important for retroviral packaging and translation, the RNA structure around the third small open reading frame, which neighbors the psi site required for packaging of the RNA, is particularly critical for maintenance of the balance between translation and packaging. These results support the hypothesis that Gag proteins autogenously regulate their synthesis and encapsidation of retroviral RNA and that an equilibrium exists between RNA destined for translation and packaging that is based on the intracellular levels of Gag proteins and ribosomes. To test the model, mRNAs with natural or mutated 5' leader sequences from Rous sarcoma virus were expressed in avian cells in the presence and absence of Pr76gag. We demonstrate that Pr76gag acts as a translational repressor of these mRNAs in a dose-dependent manner, supporting the hypothesis that Pr76gag can sort retroviral RNA for translation and encapsidation.

Autogenous regulation of RNA translation and packaging by Rous sarcoma virus Pr76gag

Unspliced cytoplasmic retroviral RNA in chronically infected cells either is encapsidated by Gag proteins in the manufacture of virus or is used to direct synthesis of Gag proteins. Several models have been suggested to explain the sorting of viral RNA for these two purposes. Here we present evidence supporting a simple biochemical mechanism that accounts for the routing of retroviral RNA. Our results indicate that ribosomes compete with the Gag proteins to determine the fate of nascent retroviral RNA. Although the integrity of the entire Rous sarcoma virus leader sequence is important for retroviral packaging and translation, the RNA structure around the third small open reading frame, which neighbors the psi site required for packaging of the RNA, is particularly critical for maintenance of the balance between translation and packaging. These results support the hypothesis that Gag proteins autogenously regulate their synthesis and encapsidation of retroviral RNA and that an equilibrium exists between RNA destined for translation and packaging that is based on the intracellular levels of Gag proteins and ribosomes. To test the model, mRNAs with natural or mutated 5' leader sequences from Rous sarcoma virus were expressed in avian cells in the presence and absence of Pr76gag. We demonstrate that Pr76gag acts as a translational repressor of these mRNAs in a dose-dependent manner, supporting the hypothesis that Pr76gag can sort retroviral RNA for translation and encapsidation.

Synthesis and assembly of retrovirus Gag precursors into immature capsids in vitro

The assembly of retroviral particles is mediated by the product of the gag gene; no other retroviral gene products are necessary for this process. While most retroviruses assemble their capsids at the plasma membrane, viruses of the type D class preassemble immature capsids within the cytoplasm of infected cells. This has allowed us to determine whether immature capsids of the prototypical type D retrovirus, Mason-Pfizer monkey virus (M-PMV), can assemble in a cell-free protein synthesis system. We report here that assembly of M-PMV Gag precursor proteins can occur in this in vitro system. Synthesized particles sediment in isopycnic gradients to the appropriate density and in thin-section electron micrographs have a size and appearance consistent with those of immature retrovirus capsids. The in vitro system described in this report appears to faithfully mimic the process of assembly which occurs in the host cell cytoplasm, since M-PMV gag mutants defective in in vivo assembly also fail to assemble in vitro. Likewise, the Gag precursor proteins of retroviruses that undergo type C morphogenesis, Rous sarcoma virus and human immunodeficiency virus, which do not preassemble capsids in vivo, fail to assemble particles in this system. Additionally, we demonstrate, with the use of anti-Gag antibodies, that this cell-free system can be utilized for analysis in vitro of potential inhibitors of retrovirus assembly.

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.

Genetic analysis of the zinc finger in the Moloney murine leukemia virus nucleocapsid domain: replacement of zinc-coordinating residues with other zinc-coordinating residues yields noninfectious particles containing genomic RNA

The effect of changing zinc (Zn2+)-coordinating residues in the nucleocapsid protein of Moloney murine leukemia virus was investigated by introducing a His-34-to-Cys or Cys-39-to-His mutation into the putative Zn2+ finger. Mutant virions contained normal levels of properly processed Gag and Env proteins and wild-type levels of full-length viral RNA. However, the specific infectivity of the mutants was approximately 4 x 10(-4) that of wild-type particles. They were probably noninfectious because of the inability of the particles to synthesize cDNA transcripts, since full-length viral DNA could not be detected in Hirt supernatants of NIH 3T3 cells infected with the CCCC or CCHH virus. These mutants will provide an extremely valuable tool for analysis of the role of retroviral Zn2+ fingers in infection processes, independent of viral RNA recognition and packaging.

Evidence for a second function of the MA sequence in the Rous sarcoma virus Gag protein

During retrovirus assembly, Gag proteins bind to the inner leaflet of the plasma membrane to initiate the budding process. The molecular basis of this protein-lipid interaction is poorly understood. For the human, immunodeficiency virus type 1 Gag protein, we recently reported that the membrane-binding domain resides within the N-terminal 31 amino acids and consists of two components: myristate and a cluster of basic residues, which together promote membrane binding in vitro and budding in vivo (W. Zhou, L. J. Parent, J. W. Wills, and M. D. Resh, J. Virol. 68:2556-2569, 1994). The positively charged residues associate electrostatically with acidic phospholipids to stabilize membrane binding, while myristate provides membrane-binding energy via hydrophobic interactions. Here we demonstrate that the human immunodeficiency virus type 1 Gag membrane-binding domain can fully replace the membrane-targeting function of the N-terminal 100 residues of the non-myristylated Rous sarcoma virus (RSV) Gag protein. To further explore the importance of myristate and basic residues in membrane binding, we developed a gain-of-function assay whereby budding was restored to defective mutants of RSV Gag. Detailed mutational analysis revealed that the position, number, and context of charged residues are crucial to budding. Myristate provides additional membrane-binding energy, which is critical when a Gag protein is near the threshold of stable membrane association. Finally, viruses with altered matrix (MA) proteins that are noninfectious, even though they produce particles with high efficiency, were identified. Thus, we present the first evidence that the RSV MA sequence plays two distinct roles, membrane binding during particle assembly and a second, as yet undefined function required for viral infectivity.

The packaging phenotype of the SE21Q1b provirus is related to high proviral expression and not trans-acting factors

The avian packaging cell line SE21Q1b produces particles which encapsidate cellular RNAs. Such RNAs can be reverse transcribed by endogenous polymerase and integrated into the genomes of newly infected cells (M. Linial, Cell 49:93-102, 1987). Genomic RNA is not packaged because the packaging (psi) region of the provirus is deleted. The provirus also lacks the negative-strand primer binding site, which prevents efficient reverse transcription of randomly packaged genomic RNA. Previous work from our laboratory suggested that the trans-acting defect which allows packaging of cellular mRNA mapped to the provirus but did not map to the nucleocapsid region of the gag gene (D.J. Anderson, P. Lee, K. L. Levine, J. Sang, S. A. Shah, O. O. Yang, P. R. Shank, and M. L. Linial, J. Virol. 66:204-216, 1992). We have found, using proviral recombinants between SE21Q1b and wild-type Rous sarcoma virus, that packaging of cellular RNAs does not map to the gag gene. Rather, the propensity of SE21Q1b particles to package cellular mRNA is a function of the high level of particle production in these cells and not of any specific viral structural proteins.

Self-assembly in vitro of purified CA-NC proteins from Rous sarcoma virus and human immunodeficiency virus type 1

The internal structural proteins of retroviruses are proteolytically processed from the Gag polyprotein, which alone is able to assemble into virus-like particles when expressed in cells. All Gag proteins contain domains corresponding to the three structural proteins MA, CA, and NC. We have expressed the CA and NC domains together as a unit in Escherichia coli, both for Rous sarcoma virus (RSV) and for human immunodeficiency virus type 1 (HIV-1). We also expressed a similar HIV-1 protein carrying the C-terminal p6 domain. RSV CA-NC, HIV-1 CA-NC, and HIV-1 CA-NC-p6 were purified in native form by classic methods. After adjustment of the pH and salt concentration, each of these proteins was found to assemble at a low level of efficiency into structures that resembled circular sheets and roughly spherical particles. The presence of RNA dramatically increased the efficiency of assembly, and in this case all three proteins formed hollow, cylindrical particles whose lengths were determined by the size of the RNA. The optimal pH at which assembly occurred was 5.5 for the RSV protein and 8.0 for the HIV-1 proteins. The treatment of the RSV CA-NC cylindrical particles with nonionic detergent, with ribonuclease, or with viral protease caused disassembly. These results suggest that RNA plays an important structural role in the virion and that it may initiate and organize the assembly process. The in vitro system described should facilitate the dissection of assembly pathways in retroviruses.

Retroviral nucleocapsid domains mediate the specific recognition of genomic viral RNAs by chimeric Gag polyproteins during RNA packaging in vivo

The retroviral nucleocapsid (NC) protein is necessary for the specific encapsidation of the viral genomic RNA by the assembling virion. However, it is unclear whether NC contains the determinants for the specific recognition of the viral RNA or instead contributes nonspecific RNA contacts to strengthen a specific contact made elsewhere in the Gag polyprotein. To discriminate between these two possibilities, we have swapped the NC domains of the human immunodeficiency virus type 1 (HIV-1) and Moloney murine leukemia virus (M-MuLV), generating an HIV-1 mutant containing the M-MuLV NC domain and an M-MuLV mutant containing the HIV-1 NC domain. These mutants, as well as several others, were characterized for their abilities to encapsidate HIV-1, M-MuLV, and nonviral RNAs and to preferentially package genomic viral RNAs over spliced viral RNAs. We found that the M-MuLV NC domain mediates the specific packaging of RNAs containing the M-MuLV psi packaging element, while the HIV-1 NC domain confers an ability to package the unspliced HIV-1 RNA over spliced HIV-1 RNAs. In addition, we found that the HIV-1 mutant containing the M-MuLV NC domain exhibited a 20-fold greater ability than wild-type HIV-1 to package a nonviral RNA. These results help confirm the notion that the NC domain specifically recognizes the retroviral genomic RNA during RNA encapsidation.

Nucleocapsid protein effects on the specificity of retrovirus RNA encapsidation

We have analyzed the roles of Gag protein nucleocapsid (NC) domains in the packaging or encapsidation of retroviral RNAs into virus particles. We found that mutation of both zinc finger motifs of the human immunodeficiency virus (HIV) NC domain reduced but did not eliminate encapsidation of the HIV viral RNA. However, the NC mutations also resulted in a three- to fourfold reduction in the specificity of RNA encapsidation, as determined by comparison of virus-associated genomic and spliced RNA levels. As a complementary approach, we replaced the NC domain of Moloney murine leukemia virus (M-MuLV) with that of HIV. Chimeric virus particles assembled efficiently, were of wild-type M-MuLV density, and cross-linked at NC cysteines. In encapsidation studies, wild-type M-MuLV precursor Gag (PrGag) proteins packaged M-MuLV transcripts more efficiently than HIV RNAs. In contrast, chimeric PrGag proteins possessing the HIV-1 NC domain in the context of the M-MuLV MA (matrix), p12, and CA (capsid) domains encapsidated HIV transcripts to a greater extent than M-MuLV transcripts. Our results support the notion that retroviral NC domains contribute toward both the efficiency and specificity of viral genomic RNA packaging.

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.