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

N-terminal Gag domain required for foamy virus particle assembly and export

Among the Retroviridae, foamy viruses (FVs) exhibit an unusual way of particle assembly and a highly specific incorporation of envelope protein into progeny virions. We have analyzed deletions and point mutants of the prototypic FV gag gene for capsid assembly and egress, envelope protein incorporation, infectivity, and ultrastructure. Deletions introduced at the 3' end of gag revealed the first 297 amino acids (aa) to be sufficient for specific Env incorporation and export of particulate material. Deletions introduced at the 5' end showed the region between aa 6 and 200 to be dispensable for virus capsid assembly but required for the incorporation of Env and particle egress. Point mutations were introduced in the 5' region of gag to target residues conserved among FVs from different species. Alanine substitutions of residues in a region between aa 40 and 60 resulted in severe alterations in particle morphology. Furthermore, at position 50, this region harbors the conserved arginine that is presumably at the center of a signal sequence directing FV Gag proteins to a cytoplasmic assembly site.

Atomic resolution structure of Moloney murine leukemia virus matrix protein and its relationship to other retroviral matrix proteins

Matrix proteins associated with the viral membrane are important in the formation of the viral particle and in virus maturation. The 1.0 A crystal structure of the ecotropic Gammaretrovirus Moloney murine leukemia virus (M-MuLV) matrix protein reveals the conserved topology of other retroviral matrix proteins, despite undetectable sequence similarity. The N terminus (normally myristylated) is exposed and adjacent to a basic surface patch, features likely to contribute to membrane binding. The four proteins in the asymmetric unit make varied contacts. The M-MuLV matrix structure is intermediate, between those of the lentiviruses and other retroviruses. The protein fold appears to be maintained, in part, by the conservation of side chain packing, which may provide a useful tool for searching for weak distant similarities in proteins.

Conditions for copackaging rous sarcoma virus and murine leukemia virus Gag proteins during retroviral budding

Rous sarcoma virus (RSV) and murine leukemia virus (MLV) are examples of distantly related retroviruses that normally do not encounter one another in nature. Their Gag proteins direct particle assembly at the plasma membrane but possess very little sequence similarity. As expected, coexpression of these two Gag proteins did not result in particles that contain both. However, when the N-terminal membrane-binding domain of each molecule was replaced with that of the Src oncoprotein, which is also targeted to the cytoplasmic face of the plasma membrane, efficient copackaging was observed in genetic complementation and coimmunoprecipitation assays. We hypothesize that the RSV and MLV Gag proteins normally use distinct locations on the plasma membrane for particle assembly but otherwise have assembly domains that are sufficiently similar in function (but not sequence) to allow heterologous interactions when these proteins are redirected to a common membrane location.

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

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

Mutagenesis analysis of the murine leukemia virus matrix protein: identification of regions important for membrane localization and intracellular transport

We have created two sets of substitution mutations in the Moloney murine leukemia virus (Mo-MuLV) matrix protein in order to identify domains involved in association with the plasma membrane and in incorporation of the viral envelope glycoproteins into virus particles. The first set of mutations was targeted at putative membrane-associating regions similar to those of the human immunodeficiency virus type 1 matrix protein, which include a polybasic region at the N terminus of the Mo-MuLV matrix protein and two regions predicted to form beta strands. The second set of mutations was created within hydrophobic residues to test for the production of virus particles lacking envelope proteins, with the speculation of an involvement of the membrane-spanning region of the envelope protein in incorporation into virus particles. We have found that mutation of the N-terminal polybasic region redirected virus assembly to the cytoplasm, and we show that tryptophan residues may also play a significant role in the intracellular transport of the matrix protein. In total, 21 mutants of the Mo-MuLV matrix protein were produced, but we did not observe any mutant virus particles lacking the envelope glycoproteins, suggesting that a direct interaction between the Mo-MuLV matrix protein and envelope proteins either may not exist or may occur through multiple redundant interactions.

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.

Therapeutic effect of Gag-nuclease fusion protein on retrovirus-infected cell cultures

Capsid-targeted viral inactivation is a novel protein-based strategy for the treatment of viral infections. Virus particles are inactivated by targeting toxic fusion proteins to virions, where they destroy viral components from within. We have fused Staphylococcus nuclease (SN) to the C-terminal end of Moloney murine leukemia virus Gag and demonstrated that expression of this fusion protein in chronically infected chicken embryo fibroblasts resulted in its incorporation into virions and subsequent inactivation of the virus particles by degradation of viral RNA. Release of particles incorporating Gag-SN fusion proteins into the extracellular milieu activates the nuclease and results in destruction of the virion from within. By comparing the effects of incorporated SN and SN*, an enzymatically inactive missense mutant form of SN, on the infectivity of virus particles, we have clearly demonstrated that nucleolytic activity is the antiviral mechanism. Expression of Gag-SN fusion proteins as a therapeutic agent causes a stable reduction of infectious titers by 20- to 60-fold. The antiviral effect of capsid-targeted viral inactivation in our model system, using both prophylactic and therapeutic approaches, suggests that a similar anti-human immunodeficiency virus strategy might be successful.

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.

The membrane-binding domain of the Rous sarcoma virus Gag protein

The Gag protein of Rous sarcoma virus (RSV) can direct particle assembly and budding at the plasma membrane independently of the other virus-encoded products. A previous deletion analysis has suggested that the first 86 amino acids of RSV Gag constitute a large membrane-binding domain that is absolutely required for these processes. To test this hypothesis, we inserted these residues in place of the N-terminal membrane-binding domain of the pp60v-src, a transforming protein whose biological activity requires plasma membrane localization. The ability of the Src chimera to induce cellular transformation suggests that the RSV sequence indeed contains an independent, functional domain.

p6Gag is required for particle production from full-length human immunodeficiency virus type 1 molecular clones expressing protease

The human immunodeficiency virus type 1 (HIV-1) Gag protein precursor, Pr55Gag, contains at its C-terminal end a proline-rich, 6-kDa domain designated p6. Two functions have been proposed for p6: incorporation of the HIV-1 accessory protein Vpr into virus particles and virus particle production. To characterize the role of p6 in the HIV-1 life cycle and to map functional domains within p6, we introduced a number of nonsense and single and multiple amino acid substitution mutations into p6. Following the introduction of the mutations into the full-length HIV-1 molecular clone pNL4-3, the effects on Gag protein expression and processing, virus particle production, and virus infectivity were analyzed. The production of mutant virus particles was also examined by transmission electron microscopy. The results indicate that (i) p6 is required for efficient virus particle production from a full-length HIV-1 molecular clone; (ii) a Pro-Thr-Ala-Pro sequence, located between residues 7 and 10 of p6, is critical for virus particle production; (iii) mutations outside the Pro-Thr-Ala-Pro motif have little or no effect on virus assembly and release; (iv) the p6 defect is manifested at a late stage in the budding process; and (v) mutations in p6 that severely reduce virion production in HeLa cells also block or significantly delay the establishment of a productive infection in the CEM (12D-7) T-cell line. We further demonstrate that mutational inactivation of the viral protease reverses the p6 defect, suggesting a functional linkage between p6 and the proteolytic processing of the Gag precursor protein during the budding of progeny virions.

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.

Identification of human immunodeficiency virus type 1 Gag protein domains essential to membrane binding and particle assembly

Assembly of human immunodeficiency virus type 1 (HIV-1) particles occurs at the plasma membrane of infected cells. Myristylation of HIV-1 Gag precursor polyprotein Pr55Gag is required for stable membrane binding and for assembly of viral particles. We expressed a series of proteins representing major regions of the HIV-1 Gag protein both with and without an intact myristyl acceptor glycine and performed subcellular fractionation studies to identify additional regions critical for membrane binding. Myristylation-dependent binding of Pr55Gag was demonstrated by using the vaccinia virus/T7 hybrid system for protein expression. Domains within the matrix protein (MA) region downstream of the initial 15 amino acids were required for membrane binding which was resistant to a high salt concentration (1 M NaCl). A myristylated construct lacking most of the matrix protein did not associate with the plasma membrane but formed intracellular retrovirus-like particles. A nonmyristylated construct lacking most of the MA region also was demonstrated by electron microscopy to form intracellular particles. Retrovirus-like extracellular particles were produced with a Gag protein construct lacking all of p6 and most of the nucleocapsid region. These studies suggest that a domain within the MA region downstream from the myristylation site is required for transport of Gag polyprotein to the plasma membrane and that stable plasma membrane binding requires both myristic acid and a downstream MA domain. The carboxyl-terminal p6 region and most of the nucleocapsid region are not required for retrovirus-like particle formation.

Mutated primer binding sites interacting with different tRNAs allow efficient murine leukemia virus replication

Two Akv murine leukemia virus-based retroviral vectors with primer binding sites matching tRNA(Gln-1) and tRNA(Lys-3) were constructed. The transduction efficiency of these mutated vectors was found to be comparable to that of a vector carrying the wild-type primer binding site matching tRNA(Pro). Polymerase chain reaction amplification and sequence analysis of transduced proviruses confirmed the transfer of vectors with mutated primer binding sites and further showed that tRNA(Gln-2) may act efficiently in conjunction with the tRNA(Gln-1) primer binding site. We conclude that murine leukemia virus can replicate by using various tRNA molecules as primers and propose primer binding site-tRNA primer interactions to be of major importance for tRNA primer selection. However, efficient primer selection does not require perfect Watson-Crick base pairing at all 18 positions of the primer binding site.

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

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

A large deletion in the matrix domain of the human immunodeficiency virus gag gene redirects virus particle assembly from the plasma membrane to the endoplasmic reticulum

Morphogenesis of retroviruses involves assembly of the structural Gag and Gag-Pol polyproteins with subsequent budding of the virus particle from the plasma membrane and proteolytic cleavage by the viral proteinase. The matrix (MA) domain, representing the N-terminal segment of Gag, plays a critical role in this process. We constructed an in-frame deletion in the MA coding region (lacking codons 16 to 99) of the human immunodeficiency virus (HIV) type 1 gag gene. Following transient transfection of the complete proviral DNA carrying the deletion, the mutant polyprotein was synthesized and proteolytically processed like the wild-type polyprotein. However, release of virus particles was reduced approximately 10-fold. The extracellular particles that were released did not contain viral glycoproteins and were noninfectious. Electron micrographs revealed budding of virus particles into the endoplasmic reticulum (ER) of transfected cells and large numbers of particles within the ER. These particles were all immature and morphologically indistinguishable from intracisternal A-type particles, a class of murine endogenous retrovirus elements. Budding structures at the plasma membrane were rarely seen and only a few extracellular particles were observed, but in contrast to those in the ER, these particles had the morphology of mature particles, similar to that of wild-type HIV, except for the lack of surface projections.