Conditional infectivity of a human immunodeficiency virus matrix domain deletion mutant

Assembly of retrovirus capsid-nucleocapsid proteins in the presence of membranes or RNA

Retrovirus Gag precursor (PrGag) proteins direct the assembly of roughly spherical immature virus particles, while after proteolytic processing events, the Gag capsid (CA) and nucleocapsid (NC) domains condense on viral RNAs to form mature retrovirus core structures. To investigate the process of retroviral morphogenesis, we examined the properties of histidine-tagged (His-tagged) Moloney murine leukemia (M-MuLV) capsid plus nucleocapsid (CANC) (His-MoCANC) proteins in vitro. The His-MoCANC proteins bound RNA, possessed nucleic acid-annealing activities, and assembled into strand, circle (or sphere), and tube forms in the presence of RNA. Image analysis of electron micrographs revealed that tubes were formed by cage-like lattices of CANC proteins surrounding at least two different types of protein-free cage holes. By virtue of a His tag association with nickel-chelating lipids, His-MoCANC proteins also assembled into planar sheets on lipid monolayers, mimicking the membrane-associated immature PrGag protein forms. Membrane-bound His-MoCANC proteins organized into two-dimensional (2D) cage-like lattices that were closely related to the tube forms, and in the presence of both nickel-chelating lipids and RNAs, 2D lattice forms appeared similar to lattices assembled in the absence of RNA. Our observations are consistent with a M-MuLV morphogenesis model in which proteolytic processing of membrane-bound Gag proteins permits CA and NC domains to rearrange from an immature spherical structure to a condensed mature form while maintaining local protein-protein contacts.

Efficient particle production by minimal Gag constructs which retain the carboxy-terminal domain of human immunodeficiency virus type 1 capsid-p2 and a late assembly domain

The human immunodeficiency virus type 1 (HIV-1) Gag precursor Pr55(gag) by itself is capable of assembling into retrovirus-like particles (VLP). In the present study, we attempted to identify the minimal Gag sequences required for the formation of VLP. Our results show that about 80% of Pr55(gag) can be either deleted or replaced by heterologous sequences without significantly compromising VLP production. The smallest chimeric molecule still able to efficiently form VLP was only about 16 kDa. This minimal Gag construct contained the leucine zipper domain of the yeast transcription factor GCN4 to substitute for the assembly function of nucleocapsid (NC), followed by a P-P-P-P-Y motif to provide late budding (L) domain function, and retained only the myristylation signal and the C-terminal capsid-p2 domain of Pr55(gag). We also show that the L domain function of HIV-1 p6(gag) is not dependent on the presence of an active viral protease and that the NC domain of Pr55(gag) is dispensable for the incorporation of Vpr into VLP.

Relationship between human immunodeficiency virus type 1 Gag multimerization and membrane binding

The human immunodeficiency virus type 1 (HIV-1) Gag precursor, Pr55(Gag), is necessary and sufficient for the assembly and release of viruslike particles. Binding of Gag to membrane and Gag multimerization are both essential steps in virus assembly, yet the domains responsible for these events have not been fully defined. In addition, the relationship between membrane binding and Gag-Gag interaction remains to be elucidated. To investigate these issues, we analyzed, in vivo, the membrane-binding and assembly properties of a series of C-terminally truncated Gag mutants. Pr55(Gag) was truncated at the C terminus of matrix (MAstop), between the N- and C-terminal domains of capsid (CA146stop), at the C terminus of capsid (p41stop), at the C terminus of p2 (p43stop), and after the N-terminal 35 amino acids of nucleocapsid (NC35stop). The ability of these truncated Gag molecules to assemble and release viruslike particles and their capacity to copackage into particles when coexpressed with full-length Gag were determined. We demonstrate that the amount of truncated Gag incorporated into particles is incrementally increased by extension from CA146 to NC35, suggesting that multiple sites in this region are involved in Gag multimerization. Using membrane flotation centrifugation, we observe that MA shows significantly reduced membrane binding relative to full-length Gag but that CA146 displays steady-state membrane-binding properties comparable to those of Pr55(Gag). The finding that the CA146 mutant, which contains only matrix and the N-terminal domain of capsid, exhibits levels of steady-state membrane binding equivalent to those of full-length Gag indicates that strong Gag-Gag interaction domains are not required for the efficient binding of HIV-1 Gag to membrane.

Role of the Gag matrix domain in targeting human immunodeficiency virus type 1 assembly

Human immunodeficiency virus type 1 (HIV-1) particle formation and the subsequent initiation of protease-mediated maturation occur predominantly on the plasma membrane. However, the mechanism by which HIV-1 assembly is targeted specifically to the plasma membrane versus intracellular membranes is largely unknown. Previously, we observed that mutations between residues 84 and 88 of the matrix (MA) domain of HIV-1 Gag cause a retargeting of virus particle formation to an intracellular site. In this study, we demonstrate that the mutant virus assembly occurs in the Golgi or in post-Golgi vesicles. These particles undergo core condensation in a protease-dependent manner, indicating that virus maturation can occur not only on the plasma membrane but also in the Golgi or post-Golgi vesicles. The intracellular assembly of mutant particles is dependent on Gag myristylation but is not influenced by p6(Gag) or envelope glycoprotein expression. Previous characterization of viral revertants suggested a functional relationship between the highly basic domain of MA (amino acids 17 to 31) and residues 84 to 88. We now demonstrate that mutations in the highly basic domain also retarget virus particle formation to the Golgi or post-Golgi vesicles. Although the basic domain has been implicated in Gag membrane binding, no correlation was observed between the impact of mutations on membrane binding and Gag targeting, indicating that these two functions of MA are genetically separable. Plasma membrane targeting of Gag proteins with mutations in either the basic domain or between residues 84 and 88 was rescued by coexpression with wild-type Gag; however, the two groups of MA mutants could not rescue each other. We propose that the highly basic domain of MA contains a major determinant of HIV-1 Gag plasma membrane targeting and that mutations between residues 84 and 88 disrupt plasma membrane targeting through an effect on the basic domain.

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

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

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

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

Binding of human immunodeficiency virus type 1 Gag to membrane: role of the matrix amino terminus

Binding of the human immunodeficiency virus type 1 (HIV-1) Gag protein precursor, Pr55(Gag), to membrane is an indispensable step in virus assembly. Previously, we reported that a matrix (MA) residue 6 substitution (6VR) imposed a virus assembly defect similar to that observed with myristylation-defective mutants, suggesting that the 6VR change impaired membrane binding. Intriguingly, the 6VR mutation had no effect on Gag myristylation. The defective phenotype imposed by 6VR was reversed by changes at other positions in MA, including residue 97. In this study, we use several biochemical methods to demonstrate that the residue 6 mutation, as well as additional substitutions in MA amino acids 7 and 8, reduce membrane binding without affecting N-terminal myristylation. This effect is observed in the context of Pr55(Gag), a truncated Gag containing only MA and CA, and in MA itself. The membrane binding defect imposed by the 6VR mutation is reversed by second-site changes in MA residues 20 and 97, both of which, when present alone, increase membrane binding to levels greater than those for the wild type. Both reduced and enhanced membrane binding imposed by the MA substitutions depend upon the presence of the N-terminal myristate. The results support the myristyl switch model recently proposed for the regulation of Gag membrane binding, according to which membrane binding is determined by the degree of exposure or sequestration of the N-terminal myristate moiety. Alternatively, insertion of the myristate into the lipid bilayer might be a prerequisite event for the function of other distinct MA-encoded membrane binding domains.

Opposing effects of human immunodeficiency virus type 1 matrix mutations support a myristyl switch model of gag membrane targeting

Targeting of the human immunodeficiency virus type 1 (HIV-1) Gag precursor Pr55(gag) to the plasma membrane, the site of virus assembly, is primarily mediated by the N-terminal matrix (MA) domain. N-myristylation of MA is essential for the stable association of Pr55(gag) with membranes and for virus assembly. We now show that single amino acid substitutions near the N terminus of MA can dramatically impair assembly without compromising myristylation. Subcellular fractionation demonstrated that Gag membrane binding was compromised to a similar extent as in the absence of the myristyl acceptor site, indicating that the myristyl group was not available for membrane insertion. Remarkably, the effects of the N-terminal modifications could be completely suppressed by second-site mutations in the globular core of MA. The compensatory mutations enhanced Gag membrane binding and increased viral particle yields above wild-type levels, consistent with an increase in the exposure of the myristyl group. Our results support a model in which the compact globular core of MA sequesters the myristyl group to prevent aberrant binding to intracellular membranes, while the N terminus is critical to allow the controlled exposure of the myristyl group for insertion into the plasma membrane.

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

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

Structural biology of HIV

The human immunodeficiency virus (HIV) genome encodes a total of three structural proteins, two envelope proteins, three enzymes, and six accessory proteins. Studies over the past ten years have provided high-resolution three-dimensional structural information for all of the viral enzymes, structural proteins and envelope proteins, as well as for three of the accessory proteins. In some cases it has been possible to solve the structures of the intact, native proteins, but in most cases structural data were obtained for isolated protein domains, peptidic fragments, or mutants. Peptide complexes with two regulatory RNA fragments and a protein complex with an RNA recognition/encapsidation element have also been structurally characterized. This article summarizes the high-resolution structural information that is currently available for HIV proteins and reviews current structure-function and structure-biological relationships.

Gag protein from human immunodeficiency virus type 1 assembles in the absence of cyclophilin A

Human immunodeficiency virus type 1 (HIV-1) replication requires coordinated activities of host and viral factors. We reported previously that interactions of the host factor cyclophilin A with HIV-1 Gag polyproteins affected Gag processing and maturation of virus particles (Streblow et al., 1998. Virology 245, 197-202). We now use in vitro translation and physical analysis of Gag structures to refine our understanding of how cyclophilin A affects HIV-1 replication. Gag assembled into oligomeric structures in vitro in the presence or absence of cyclophilin A, and proteins synthesized under the two conditions were equally susceptible to cleavage by exogenous HIV-1 protease. These and previous data show that Cyclophilin A is required at a step between Gag assembly and Gag processing/virion morphogenesis. Cyclophilin A may be required for Gag conformational changes subsequent to assembly, that are required for efficient dimerization and activation of the viral protease.

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

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

HIV-1 gag proteins: diverse functions in the virus life cycle

The Gag proteins of HIV-1, like those of other retroviruses, are necessary and sufficient for the assembly of virus-like particles. The roles played by HIV-1 Gag proteins during the life cycle are numerous and complex, involving not only assembly but also virion maturation after particle release and early postentry steps in virus replication. As the individual Gag domains carry out their diverse functions, they must engage in interactions with themselves, other Gag proteins, other viral proteins, lipid, nucleic acid (DNA and RNA), and host cell proteins. This review briefly summarizes our current understanding of how HIV-1 Gag proteins function in the virus life cycle.

The C-terminal half of the human immunodeficiency virus type 1 Gag precursor is sufficient for efficient particle assembly

Human immunodeficiency virus type 1 particle assembly is directed by the Gag polyprotein Pr55(gag), the precursor for the matrix (MA), capsid (CA), and nucleocapsid proteins of the mature virion. We now show that CA sequences N terminal to the major homology region (MHR), which form a distinct domain, are dispensable for particle formation. However, slightly larger deletions which extend into the MHR severely impair particle production. Remarkably, a deletion which removed essentially all MA and CA sequences between the N-terminal myristyl anchor and the MHR reduced the yield of extracellular particles only moderately. Particle formation even exceeded wild-type levels when additional MA sequences, either from the N or the C terminus of the domain, were retained. We conclude that no distinct region between the myristyl anchor and the MHR is required for efficient particle assembly or release.

Analysis of minimal human immunodeficiency virus type 1 gag coding sequences capable of virus-like particle assembly and release

We have constructed a series of human immunodeficiency virus (HIV) gag mutants by progressive truncation of the gag coding sequence from the C terminus and have combined these mutants with an assembly-competent matrix domain deletion mutation (DeltaMA). By using several methods, the particle-producing capabilities of each mutant were examined. Our analysis indicated that truncated Gag precursors lacking most of C-terminal gag gene products assembled and were released from 293T cells. Additionally, a mutant with a combined deletion of the MA (DeltaMA) and p6 domains even produced particles at levels comparable to that of the wild-type (wt) virus. However, most mutants derived from combination of the DeltaMA and the C-terminal truncation mutations did not release particles as well as the wt. Our smallest HIV gag gene product capable of virus-like particle formation was a 28-kDa protein which consists of a few MA amino acids and the CA-p2 domain. Sucrose density gradient fractionation analysis indicated that most mutants exhibited a wt retrovirus particle density. Exceptions to this rule were mutants with an intact MA domain but deleted downstream of the p2 domains. These C-terminal truncation mutants possessed particle densities of 1.13 to 1.15 g/ml, lower than that of the wt. The N-terminal portions of the CA domain, which have been shown to be dispensable for core assembly, became critical when most of the MA domain was deleted, suggesting a requirement for an intact CA domain to assemble and release particles.

Detection of a trimeric human immunodeficiency virus type 1 Gag intermediate is dependent on sequences in the matrix protein, p17

Previous studies have shown that single amino acid changes in the amino-terminal matrix (MA) domain, p17, of the human immunodeficiency virus type 1 Gag precursor Pr55, can abrogate virion particle assembly. In the three-dimensional structure of MA such mutations lie in a single helix spanning residues 54 to 68, suggesting a key role for this helix in the assembly process. The fundamental nature of this involvement, however, remains poorly understood. In the present study, the essential features of the MA helix required for virus assembly have been investigated through the analysis of a further 15 site-directed mutants. With previous mutants that failed to assemble, residues mapped as critical for assembly were all located on the hydrophobic face of the helix and had a key role in stabilizing the trimeric interface. This implies a role for the MA trimer in virus assembly. We support this interpretation by showing that purified MA is trimeric in solution and that mutations that prevent virus assembly also prevent trimerization. Trimerization in solution was also a property of a larger MA-capsid (CA) Gag molecule, while under the same conditions CA only was a monomer. These data suggest that Gag trimerization driven by the MA domain is an intermediate stage in normal virion assembly and that it relies, in turn, on an MA conformation dependent on the hydrophobic core of the molecule.

The matrix protein of HIV-1 is not sufficient for assembly and release of virus-like particles

The matrix (MA) proteins of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) are known to be important for the targeting and assembly of lentiviral proteins. The objective of the present study was to determine whether the MA protein of HIV-1 was sufficient for particle assembly and release. Eukaryotic expression of wild-type HIV-1 Gag-Pol, HIV-1 MA alone, or SIV MA alone was analyzed with radio-immunoprecipitation, density centrifugation, and a protease protection assay. Cells that expressed HIV-1 Gag-Pol or SIV MA alone released virus-like particles (VLPs) with sucrose gradient densities of 1.15 or 1.12 g/ml, respectively. The MA and/or capsid proteins in these particles were protected from protease degradation, indicating the presence of a protective outer membrane. Expression of HIV-1 MA protein alone resulted in release of MA which pelleted through a 20% sucrose cushion but failed to enter a 20-60% sucrose gradient and was not protected from protease degradation. The MA protein of SIV was previously reported to be sufficient for production of VLPs (S. A. Gonzalez, H, K, Affrachino, H. R. Gelderblom, and A. Burney. Virology 194, 548-556, 1993; V. Liska, D. Spehner, M. Mehtali, D. Schmitt, A. Kirn, and A. M. Aubertin. J. Gen. Virol. 75, 2955-2962, 1994). Our study confirmed that result, but indicated that the MA protein of HIV-1 was not sufficient to assemble and release VLPs.

N-Terminal extension of human immunodeficiency virus capsid protein converts the in vitro assembly phenotype from tubular to spherical particles

Expression of retroviral Gag polyproteins is sufficient for morphogenesis of virus-like particles with a spherical immature protein shell. Proteolytic cleavage of Gag into the matrix (MA), capsid (CA), nucleocapsid (NC), and p6 domains (in the case of human immunodeficiency virus [HIV]) leads to condensation to the mature cone-shaped core. We have analyzed the formation of spherical or cylindrical particles on in vitro assembly of purified HIV proteins or inside Escherichia coli cells. CA protein alone yielded cylindrical particles, while all N-terminal extensions of CA abolished cylinder formation. Spherical particles with heterogeneous diameters or amorphous protein aggregates were observed instead. Extending CA by 5 amino acids was sufficient to convert the assembly phenotype to spherical particles. Sequences C-terminal of CA were not required for sphere formation. Proteolytic cleavage of N-terminally extended CA proteins prior to in vitro assembly led to the formation of cylindrical particles, while proteolysis of in vitro assembly products caused disruption of spheres but not formation of cylinders. In vitro assembly of CA and extended CA proteins in the presence of cyclophilin A (CypA) at a CA-to-CypA molar ratio of 10:1 yielded significantly longer cylinders and heterogeneous spheres, while higher concentrations of CypA completely disrupted particle formation. We conclude that the spherical shape of immature HIV particles is determined by the presence of an N-terminal extension on the CA domain and that core condensation during virion maturation requires the liberation of the N terminus of CA.

Efficient HIV-1 replication can occur in the absence of the viral matrix protein

Matrix (MA), a major structural protein of retroviruses, is thought to play a critical role in several steps of the HIV-1 replication cycle, including the plasma membrane targeting of Gag, the incorporation of envelope (Env) glycoproteins into nascent particles, and the nuclear import of the viral genome in non-dividing cells. We now show that the entire MA protein is dispensable for the incorporation of HIV-1 Env glycoproteins with a shortened cytoplasmic domain. Furthermore, efficient HIV-1 replication in the absence of up to 90% of MA was observed in a cell line in which the cytoplasmic domain of Env is not required. Additional compensatory changes in Gag permitted efficient virus replication even if all of MA was replaced by a heterologous membrane targeting signal. Viruses which lacked the globular domain of MA but retained its N-terminal myristyl anchor exhibited an increased ability to form both extracellular and intracellular virus particles, consistent with a myristyl switch model of Gag membrane targeting. Pseudotyped HIV-1 particles that lacked the structurally conserved globular head of MA efficiently infected macrophages, indicating that MA is dispensable for nuclear import in terminally differentiated cells.

The molecular basis of HIV capsid assembly

In common with many aspects of the HIV life cycle, the assembly of the virus particle has been the subject of intense investigation over recent years. Study of the subject is facilitated by the fact that only a single gene product, the Pr55 Gag protein, is required for virus assembly. A combination of site directed mutagenesis, biochemical characterisation and structural studies have led to a picture of the overall architecture of the particle, the partial structure of Pr55, and the subdomains involved in oligomerisation. Copyright 1998 John Wiley & Sons, Ltd.

Analysis of the assembly function of the human immunodeficiency virus type 1 gag protein nucleocapsid domain

Previous studies have shown that in addition to its function in specific RNA encapsidation, the human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) is required for efficient virus particle assembly. However, the mechanism by which NC facilitates the assembly process is not clearly established. Formally, NC could act by constraining the Pr559gag polyprotein into an assembly-competent conformation or by masking residues which block the assembly process. Alternatively, the capacity of NC to bind RNA or make interprotein contacts might affect particle assembly. To examine its role in the assembly process, we replaced the NC domain in Pr55gag with polypeptide domains of known function, and the chimeric proteins were analyzed for their abilities to direct the release of virus-like particles. Our results indicate that NC does not mask inhibitory domains and does not act passively, by simply providing a stable folded monomeric structure. However, replacement of NC by polypeptides which form interprotein contacts permitted efficient virus particle assembly and release, even when RNA was not detected in the particles. These results suggest that formation of interprotein contacts by NC is essential to the normal HIV-1 assembly process.

A role for two hairpin structures as a core RNA encapsidation signal in murine leukemia virus virions

Four putative hairpin structures (hairpins A to D) are involved in the specific encapsidation of Moloney murine leukemia virus (M-MuLV) RNA into M-MuLV virus particles. The C and D elements, encompassing M-MuLV viral nucleotides 310 to 374, facilitate encapsidation of heterologous RNA into virions. Thus, these two elements appear to act as a core RNA encapsidation signal. The loop sequences of the putative C and D hairpins are identical (GACG). However, when GACG loops were introduced into RNAs on heterologous stem sequences, they increased encapsidation levels only three- to fourfold. These results suggest that C and D stem-and-loop sequences contribute to the M-MuLV cis-acting site for encapsidation.

Effects of nucleocapsid mutations on human immunodeficiency virus assembly and RNA encapsidation

The human immunodeficiency virus (HIV) Pr55Gag precursor proteins direct virus particle assembly. While Gag-Gag protein interactions which affect HIV assembly occur in the capsid (CA) domain of Pr55Gag, the nucleocapsid (NC) domain, which functions in viral RNA encapsidation, also appears to participate in virus assembly. In order to dissect the roles of the NC domain and the p6 domain, the C-terminal Gag protein domain, we examined the effects of NC and p6 mutations on virus assembly and RNA encapsidation. In our experimental system, the p6 domain did not appear to affect virus release efficiency but p6 deletions and truncations reduced the specificity of genomic HIV-1 RNA encapsidation. Mutations in the nucleocapsid region reduced particle release, especially when the p2 interdomain peptide or the amino-terminal portion of the NC region was mutated, and NC mutations also reduced both the specificity and the efficiency of HIV-1 RNA encapsidation. These results implicated a linkage between RNA encapsidation and virus particle assembly or release. However, we found that the mutant ApoMTRB, in which the nucleocapsid and p6 domains of HIV-1 Pr55Gag were replaced with the Bacillus subtilis MtrB protein domain, released particles efficiently but packaged no detectable RNA. These results suggest that, for the purposes of virus-like particle assembly and release, NC can be replaced by a protein that does not appear to encapsidate RNA.

Truncation of the human immunodeficiency virus type 1 envelope glycoprotein allows efficient pseudotyping of Moloney murine leukemia virus particles and gene transfer into CD4+ cells

Human immunodeficiency virus type 1 (HIV-1) can readily accept envelope (Env) glycoproteins from distantly related retroviruses. However, we previously showed that the HIV-1 Env glycoprotein complex is excluded even from particles formed by the Gag proteins of another lentivirus, visna virus, unless the matrix domain of the visna virus Gag polyprotein is replaced by that of HIV-1. We also showed that the integrity of the HIV-1 matrix domain is critical for the incorporation of wild-type HIV-1 Env protein but not for the incorporation of a truncated form which lacks the 144 C-terminal amino acids of the cytoplasmic domain of the transmembrane glycoprotein. We report here that the C-terminal truncation of the transmembrane glycoprotein also allows the efficient incorporation of HIV-1 Env proteins into viral particles formed by the Gag proteins of the widely divergent Moloney murine leukemia virus (Mo-MLV). Additionally, pseudotyping of a Mo-MLV-based vector with the truncated rather than the full-length HIV-1 Env allowed efficient transduction of human CD4+ cells. These results establish that Mo-MLV-based vectors can be used to target cells susceptible to infection by HIV-1.

Pleiotropic mutations in the HIV-1 matrix protein that affect diverse steps in replication

The matrix domain of the Gag precursor protein, and the mature matrix protein, which is derived from processing of the Gag precursor, functions in several steps of the human immunodeficiency virus type-1 (HIV-1) life cycle. We made numerous mutations throughout the matrix protein and identified three mutants in the N-terminal portion of the matrix that drastically diminish the ability of the virus to replicate. Each of these replication-defective mutants was unable to acquire efficiently the envelope glycoprotein of HIV-1. To determine whether these same mutations affect other steps in viral replication we pseudotyped mutant particles with the envelope glycoprotein from an amphotropic murine leukemia virus. Each of these mutants was also hampered in other steps in virus replication. Two mutants were defective in entry or uncoating, and the third was hampered in a step following reverse transcription. Since viral replication was analyzed under conditions in which the nuclear localization function of the matrix protein is not required, the matrix protein may be required for an additional replication step following reverse transcription.

Three-dimensional structure of the HTLV-II matrix protein and comparative analysis of matrix proteins from the different classes of pathogenic human retroviruses

The matrix protein performs similar roles in all retroviruses, initially directing membrane localization of the assembling viral particle and subsequently forming a stable structural shell associated with the inner surface of the mature viral membrane. Although conserved structural elements are likely to perform these functions in all retroviral matrix proteins, invariant motifs are not evident at the primary sequence level and three-dimensional structures have been available for only the primate lentiviral matrix proteins. We have therefore used NMR spectroscopy to determine the structure of the matrix protein from human T-cell leukemia virus type II (HTLV-II), a member of the human oncovirus subclass of retroviruses. A total of 577 distance restraints were used to build 20 refined models that superimpose with an rmsd of 0.71 A for the backbone atoms of the structured regions. The globular HTLV-II matrix structure is composed of four alpha-helices and a 3(10) helix. Exposed basic residues near the C terminus of helix II form a putative membrane binding surface which could act in concert with the N-terminal myristoyl group to anchor the protein on the viral membrane surface. Clear structural similarities between the HTLV-II and HIV-1 matrix proteins suggest that the topology and exposed cationic membrane binding surface are likely to be conserved features of retroviral matrix proteins.

Comparison of the NMR and X-ray structures of the HIV-1 matrix protein: evidence for conformational changes during viral assembly

The three-dimensional solution- and solid-state structures of the human immunodeficiency virus type-1 (HIV-1) matrix protein have been determined recently in our laboratories by NMR and X-ray crystallographic methods (Massiah et al. 1994. J Mol Biol 244:198-223; Hill et al. 1996. Proc Natl Acad Sci USA 93:3099-3104). The matrix protein exists as a monomer in solution at low millimolar protein concentrations, but forms trimers in three different crystal lattices. Although the NMR and X-ray structures are similar, detailed comparisons have revealed an approximately 6 A displacement of a short 3(10) helix (Pro 66-Gly 71) located at the trimer interface. High quality electron density and nuclear Overhauser effect (NOE) data support the integrity of the X-ray and NMR models, respectively. Because matrix apparently associates with the viral membrane as a trimer, displacement of the 3(10) helix may reflect a physiologically relevant conformational change that occurs during virion assembly and disassembly. These findings further suggest that Pro 66 and Gly 71, which bracket the 3(10) helix, serve as "hinges" that allow the 3(10) helix to undergo this structural reorientation.

cis-active structural motifs involved in specific encapsidation of Moloney murine leukemia virus RNA

We have analyzed the roles of RNA structural motifs located in the 5' part of the Moloney murine leukemia virus (M-MuLV) encapsidation domain (Psi region) with regard to their effects on viral replication. Four putative stem-loop structures between the 5' splice donor site and the gag initiation codon have been examined: stem structure A, corresponding to M-MuLV viral nucleotides 211 to 224; stem-loop B, nucleotides 278 to 303; stem-loop C, nucleotides 310 to 352; and stem-loop D, nucleotides 355 to 374. By measuring infectivities, encapsidation and splicing efficiencies, and endogenous reverse transcription levels of motif A, B, C, and D deletion mutants, we identified mutations which affect replication at the encapsidation step. In particular, deletion of all four motifs in a single mutant eliminated encapsidation of viral RNA, while deletion of individual elements moderately reduced the encapsidation efficiencies. Through analysis of different deletion combinations, we found that deletion of the first two motifs (A plus B) reduced both encapsidation and reverse transcription efficiencies, while deletion of the 3' motifs (C plus D) eliminated encapsidation. Interestingly, the C and D motifs both contain a GACG loop sequence and are highly conserved among murine type C retroviruses. Our results indicate that M-MuLV motifs C and D are necessary for efficient encapsidation, and the presence of at least one of these two stem-loops is crucial to encapsidation and virus replication.

Structural analysis of human immunodeficiency virus type 1 Gag protein interactions, using cysteine-specific reagents

We have examined structural interactions of Gag proteins in human immunodeficiency virus type 1 (HIV-1) particles by utilizing cysteine mutagenesis and cysteine-specific modifying reagents. In immature protease-minus but otherwise wild-type (wt) particles, precursor Pr55Gag proteins did not form intermolecular cystines naturally but could be cross-linked at cysteines, and cross-linking appeared to occur across nucleocapsid (NC) domains. Capsid (CA) proteins in wt mature viruses possess cysteines near their carboxy termini at gag codons 330 and 350, but these residues are not involved in natural covalent intermolecular bonds, nor can they be intermolecularly cross-linked by using the membrane-permeable cross-linker bis-maleimido hexane. The cysteine at gag codon 350 (C-350) is highly reactive to thiol-specific modifying reagents, while the one at codon 330 (C-330) appears considerably less reactive, even in the presence of ionic detergent. These results suggest that the HIV-1 CA C terminus forms an unusually stable conformation. Mutagenesis of C-350 to a serine residue in the mutant C350S (C-350 changed to serine) virtually eliminated particle assembly, attesting to the importance of this region. We also examined a C330S mutant, as well as mutants in which cysteines were created midway through the capsid domain or in the C-terminal section of the major homology region. All such mutants appeared wt on the basis of biochemical assays but showed greatly reduced infectivities, indicative of a postassembly, postprocessing replicative block. Interestingly, capsid proteins of mature major homology region mutant particles could be cysteine cross-linked, implying either that these mutations permit cross-linking of the native C-terminal CA cysteines or that major homology regions on neighbor capsid proteins are in close proximity in mature virions.

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

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

Human immunodeficiency virus type 2 glycoprotein enhancement of particle budding: role of the cytoplasmic domain

Previous studies have shown that the glycoprotein cytoplasmic domains of human immunodeficiency virus type 2 (HIV-2) or simian immunodeficiency virus of macaques modulate biological activities of the viral glycoprotein complex, including syncytium formation, exterior glycoprotein conformation, and glycoprotein incorporation into budding virus particles. We have now utilized a recombinant expression system to study interactions of full-length or truncated HIV-2 glycoproteins with coexpressed HIV-2 Gag proteins which self-assemble and bud as virus-like particles. Interestingly, budding of HIV-2 virus-like particles from cells was enhanced 5- to 24-fold when Gag was coexpressed with the full-length HIV-2 glycoprotein, compared with Gag expressed either alone or with a truncated HIV-2 glycoprotein. The results obtained in this model system indicate that an additional effect of the lengthy cytoplasmic domain of the glycoprotein of HIV-2 is enhancement of particle budding. We speculate that the cytoplasmic domain of the viral glycoprotein of HIV-2 enhances budding by (i) potentiation of Gag structure or function or (ii) membrane modulation.

Spectroscopic study of an HIV-1 capsid protein major homology region peptide analog

The capsid (CA) domain of retroviral Gag proteins possesses one subdomain, the major homology region (MHR), which is conserved among nearly all avian and mammalian retroviruses. While it is known that the mutagenesis of residues in the MHR will impair virus infectivity, the precise structure and function of the MHR is not known. In order to obtain further information on the MHR, we have examined the structure of a synthetic peptide encompassing the MHR of human immunodeficiency virus type I (HIV-1) CA protein. Multiple sequence alignment and secondary structure prediction indicate that the peptide could form 50% alpha-helix and 10% beta-sheet. In addition, circular dichroism studies indicate that, in the presence of 50% trifluoroethanol (TFE), the peptide adopts an alpha-helical structure over half of its length. Further analysis by proton nuclear magnetic resonance spectroscopy suggests that the C-terminal portion of the MHR forms a helix in aqueous solution. Upon the addition of TFE, the position of the helix remains nearly constant, but the magnitude of the changes in H alpha chemical shifts of the residues indicate a more stable helix. These results suggest that a helical C-terminus of retroviral MHRs may be integral to the function of this region.

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.

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.

A single amino acid change in the cytoplasmic domain of the simian immunodeficiency virus transmembrane molecule increases envelope glycoprotein expression on infected cells

We have described a virus termed CP-MAC, derived from the BK28 molecular clone of simian immunodeficiency virus, that was remarkable for its ability to infect Sup-T1 cells with rapid kinetics, cell fusion, and CD4 down-modulation (C. C. LaBranche, M. M. Sauter, B. S. Haggarty, P. J. Vance, J. Romano, T. K. Hart, P. J. Bugelski, and J. A. Hoxie, J. Virol. 68:5509-5522, 1994 [Erratum 68:7665-7667]). Compared with BK28, CP-MAC exhibited a number of changes in its envelope glycoproteins, including a highly stable association between the external (SU) and transmembrane (TM) molecules, a more rapid electrophoretic mobility of TM, and, of particular interest, a marked increase in the level of envelope protein expression on the surface of infected cells. These changes were shown to be associated with 11 coding mutations in the env gene (5 in SU and 6 in TM). In this report, we demonstrate that a single amino acid mutation of a Tyr to a Cys at position 723 (Y723C) in the TM cytoplasmic domain of CP-MAC is the principal determinant for the increased expression of envelope glycoproteins on the cell surface. When introduced into the env gene of BK28, the Y723C mutation produced up to a 25-fold increase in the levels of SU and TM on chronically infected cells, as determined by fluorescence-activated cell sorter analysis with monoclonal and polyclonal antibodies. A similar effect was observed when a Tyr-to-Cys change was introduced at the analogous position (amino acid 721) in the SIVmac239 molecular clone, which, unlike BK28 does not contain a premature stop codon in its TM cytoplasmic tail. Substituting other amino acids, including Ala, Ile, and Ser, at this position produced increases in surface envelope glycoproteins that were similar to that observed for the Cys substitution, while a Tyr-to-Phe mutation produced a smaller increase. These results could not be accounted for by differences in the kinetics or efficiency of envelope glycoprotein processing or by shedding of SU from infected cells. However, immunoelectron microscopy demonstrated that the Y723C mutation in BK28 produced a striking redistribution of cell surface envelope molecules from localized patches to a diffuse pattern that covered the entire plasma membrane. This finding suggests that mutation of a Tyr residue in the simian immunodeficiency virus TM cytoplasmic domain may disrupt a structural element that can modulate envelope glycoprotein expression on the surface of infected cells.

Rescue of human immunodeficiency virus type 1 matrix protein mutants by envelope glycoproteins with short cytoplasmic domains

The matrix (MA) protein of human immunodeficiency virus type 1 (HIV-1) forms the outer protein shell directly underneath the lipid envelope of the virion. The MA protein has a key role in different aspects of virus assembly, including the incorporation of the HIV-1 Env protein complex, which contains a transmembrane glycoprotein with an unusually long cytoplasmic tail. In this study, we compared the abilities of HIV-1 MA mutants to incorporate Env protein complexes with long and short cytoplasmic tails. While the mutant particles failed to incorporate the authentic HIV-1 Env protein complex, they retained the ability to efficiently and functionally incorporate the amphotropic murine leukemia virus Env protein complex, which has a short cytoplasmic tail. Moreover, incorporation of the autologous Env protein complex could be restored by a second-site mutation that resulted in the truncation of the cytoplasmic tail of the HIV-1 transmembrane glycoprotein. Remarkably, the second-site mutation also restored the ability of MA mutants to replicate in MT-4 cells. These results imply that the long cytoplasmic tail of the transmembrane glycoprotein is responsible for the exclusion of the HIV-1 Env protein complex from MA mutant particles.

Virion incorporation of envelope glycoproteins with long but not short cytoplasmic tails is blocked by specific, single amino acid substitutions in the human immunodeficiency virus type 1 matrix

Incorporation of envelope glycoproteins into a budding retrovirus is an essential step in the formation of an infectious virus particle. By using site-directed mutagenesis, we identified specific amino acid residues in the matrix domain of the human immunodeficiency virus type 1 (HIV-1) Gag protein that are critical to the incorporation of HIV-1 envelope glycoproteins into virus particles. Pseudotyping analyses were used to demonstrate that two heterologous envelope glycoproteins with short cytoplasmic tails (the envelope of the amphotropic murine leukemia virus and a naturally truncated HIV-2 envelope) are efficiently incorporated into HIV-1 particles bearing the matrix mutations. Furthermore, deletion of the cytoplasmic tail of HIV-1 transmembrane envelope glycoprotein gp41 from 150 to 7 or 47 residues reversed the incorporation block imposed by the matrix mutations. These results suggest the existence of a specific functional interaction between the HIV-1 matrix and the gp41 cytoplasmic tail.

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

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

Efficient particle formation can occur if the matrix domain of human immunodeficiency virus type 1 Gag is substituted by a myristylation signal

Lentiviruses, such as human immunodeficiency virus type 1 (HIV-1), assemble at and bud through the cytoplasmic membrane. Both the matrix (MA) domain of Gag and its amino-terminal myristylation have been implicated in these processes. We have created HIV-1 proviruses lacking the entire matrix domain of gag which either lack or contain an amino-terminal myristate addition sequence at the beginning of the capsid domain. Myristate- and matrix-deficient [myr(-)MA(-)] viruses produced after transient transfection are still able to assemble into particles, although the majority do not form at the plasma membrane or bud efficiently. Myristylation of the amino terminus of the truncated Gag precursor permits a much more efficient release of the mutant virions. While myr(-)MA(-) particles were inefficient in proteolytic processing of the Gag precursor, myristylation enabled efficient proteolysis of the mutant Gag. All matrix-deficient viruses are noninfectious. Particles produced by matrix-deficient mutants contain low levels of glycoproteins, indicating the importance of matrix in either incorporation or stable retention of Env. Since matrix-deficient viruses contain a normal complement of viral genomic RNA, a role for MA in genomic incorporation can be excluded. Contrary to previous reports, the HIV-1 genome does not require sequences between the 5' splice donor site and the gag start codon for efficient packaging.

Specificity and sequence requirements for interactions between various retroviral Gag proteins

We previously established a genetic assay for retroviral Gag polyprotein multimerization (J. Luban, K. B. Alin, K. L. Bossolt, T. Humaran, and S. P. Goff, J. Virol. 66:5157-5160, 1992). Here we use this assay to demonstrate homomeric interactions between Gag polyproteins encoded by six different retroviruses. Of the Gag polyproteins tested, only those encoded by closely related retroviruses formed heteromultimers. To determine the primary sequence requirements for human immunodeficiency virus type 1 Gag polyprotein multimerization, we studied the effects on multimerization of deletion and linker insertion mutations. Sequences necessary for this process were located between the C-terminal one-third of the capsid domain and the C terminus of the nucleocapsid domain.

Single amino acid changes in the human immunodeficiency virus type 1 matrix protein block virus particle production

The matrix protein of human immunodeficiency virus type 1 is encoded by the amino-terminal portion of the Gag precursor and is postulated to be involved in a variety of functions in the virus life cycle. To define domains and specific amino acid residues of the matrix protein that are involved in virus particle assembly, we introduced 35 amino acid substitution mutations in the human immunodeficiency virus type 1 matrix protein. Using reverse transcriptase and radioimmunoprecipitation analyses and transmission electron microscopy, we assessed the mutants for their ability to form virus particles and to function in the infection process. This study has identified several domains of the matrix protein in which single amino acid substitutions dramatically reduce the efficiency of virus particle production. These domains include the six amino-terminal residues of matrix, the region of matrix between amino acids 55 and 59, and the region between amino acids 84 and 95. Single amino acid substitutions in one of these domains (between matrix amino acids 84 and 88) result in a redirection of the majority of virus particle formation to sites within cytoplasmic vacuoles.

Functional exchange of an oncoretrovirus and a lentivirus matrix protein

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