In vivo modification of retroviral gag gene-encoded polyproteins by myristic acid

Stephan Oroszlan and the Proteolytic Processing of Retroviral Proteins: Following A Pro

Steve Oroszlan determined the sequences at the ends of virion proteins for a number of different retroviruses. This work led to the insight that the amino-terminal amino acid of the mature viral CA protein is always proline. In this remembrance, we review Steve’s work that led to this insight and show how that insight was a necessary precursor to the work we have done in the subsequent years exploring the cleavage rate determinants of viral protease processing sites and the multiple roles the amino-terminal proline of CA plays after protease cleavage liberates it from its position in a protease processing site.

Across the Hall from Pioneers

I was fortunate to be associated with the lab of Stephen Oroszlan at the US National Cancer Institute from ~1982 until his conversion to Emeritus status in 1995. His lab made groundbreaking discoveries on retroviral proteins during that time, including many features that could not have been inferred or anticipated from straightforward sequence information. Building on the Oroszlan lab results, my colleagues and I demonstrated that the zinc fingers in nucleocapsid proteins play a crucial role in genomic RNA encapsidation; that the N-terminal myristylation of the Gag proteins of many retroviruses is important for their association with the plasma membrane before particle assembly is completed; and that gammaretroviruses initially synthesize their Env protein as an inactive precursor and then truncate the cytoplasmic tail of the transmembrane protein, activating Env fusogenicity, during virus maturation. We also elucidated several aspects of the mechanism of translational suppression in pol gene expression in gammaretroviruses; amazingly, this is a fundamentally different mechanism of suppression from that in most other retroviral genera.

AKT capture by feline leukemia virus

Oncogene-containing retroviruses are generated by recombination events between viral and cellular sequences, a phenomenon called "oncogene capture". The captured cellular genes, referred to as "v-onc" genes, then acquire new oncogenic properties. We report a novel feline leukemia virus (FeLV), designated "FeLV-AKT", that has captured feline c-AKT1 in feline lymphoma. FeLV-AKT contains a gag-AKT fusion gene that encodes the myristoylated Gag matrix protein and the kinase domain of feline c-AKT1, but not its pleckstrin homology domain. Therefore, it differs structurally from the v-Akt gene of murine retrovirus AKT8. AKT may be involved in the mechanisms underlying malignant diseases in cats.

Syntheses and Biological Evaluation of 5′-O-Myristoyl Derivatives of Thymidine against Human Immunodeficiency Virus

A series of 5′- Oacyl derivatives of thymidine (dThd) were prepared by direct acylation of thymidine using the Mitsunobu reaction. Further reaction of the bromo analogues with sodium azide gave azido ester analogues. Anti-human immunodeficiency virus type 1 (HIV-1) activities were determined against HIV-infected T4 lymphocytes. 5′- O-(12-Azidododecanoyl)thymidine exhibited moderate activity (EC50 4.6 μM) against HIV-infected T4 lymphocytes. 5- O-(2-Bromotetradecanoyl)-thymidine was found to be the most stable ester (t1/2 15.3 min) to hydrolysis by porcine liver esterase in vitro. Partition coefficients (P) in n-octanol-phosphate buffer were determined (log10 P range 4.15–6.72) and compared with the theoretical values calculated (log10 P 3.96–6.53) using the PALLAS program. Anti-HIV structure-activity data suggest that the experimental partition coefficient should be in the log10 P 4.6–4.8 range for optimum anti-HIV activity. The structures of these thymidine analogues were optimized using molecular mechanics (MM+ force field) and semi-empirical quantum mechanics PM3 calculations. The moderately active compounds adopted a similar C-2′ endo sugar conformation and exhibited similar energies for the lowest energy conformer. A quantitative structure-activity relationship (QSAR) regression equation was developed, based on the optimized structures and anti-HIV data using the SciQSAR program, which showed that log P was a determinant of anti-HIV activity.

Tracking interspecies transmission and long-term evolution of an ancient retrovirus using the genomes of modern mammals

Mammalian genomes typically contain hundreds of thousands of endogenous retroviruses (ERVs), derived from ancient retroviral infections. Using this molecular 'fossil' record, we reconstructed the natural history of a specific retrovirus lineage (ERV-Fc) that disseminated widely between ~33 and ~15 million years ago, corresponding to the Oligocene and early Miocene epochs. Intercontinental viral spread, numerous instances of interspecies transmission and emergence in hosts representing at least 11 mammalian orders, and a significant role for recombination in diversification of this viral lineage were also revealed. By reconstructing the canonical retroviral genes, we identified patterns of adaptation consistent with selection to maintain essential viral protein functions. Our results demonstrate the unique potential of the ERV fossil record for studying the processes of viral spread and emergence as they play out across macro-evolutionary timescales, such that looking back in time may prove insightful for predicting the long-term consequences of newly emerging viral infections.

DOI:http://dx.doi.org/10.7554/eLife.12704.001

Viruses have been with us for billions of years, and exist everywhere in nature that life is found. Viruses therefore have had a significant impact on the evolution of all organisms, from bacteria to humans. Unfortunately, viruses do not leave fossils, and so we know very little about how viruses originate and evolve over time. Fortunately, over the course of millions of years, genetic sequences from the viruses accumulate in the DNA genomes of living organisms (including humans). These sequences can serve as molecular “fossils” for exploring the natural history of viruses and their hosts.

Diehl et al. have now searched the genomes of 50 modern mammals for “fossil” viral remnants of an ancient group of viruses known as ERV-Fc. This revealed that ERV-Fc viruses infected the ancestors of at least 28 of these mammal species between 15 million and 30 million years ago. The viruses affected a diverse range of hosts, including carnivores, rodents and primates. The distribution of ERV-Fc among different mammals indicates that the viruses spread to every continent except Antarctica and Australia, and that they jumped between species more than 20 times.

Diehl et al. also pinpointed patterns of evolutionary change in the genes of the ERV-Fc viruses that reflect how the viruses adapted to different host mammals. As part of this process, the viruses often exchanged genes with each other and with other types of viruses. Such genetic recombination is likely to have played a significant role in the evolutionary success of the ERV-Fc viruses.

Mammalian genomes contain hundreds of thousands of ancient viral fossils similar to ERV-Fc. Future work could study these to improve our understanding of when and why new viruses emerge and how long-term contact with viruses affects the evolution of their host organisms.

DOI:http://dx.doi.org/10.7554/eLife.12704.002

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.

Myristic acid increases dihydroceramide Δ4-desaturase 1 (DES1) activity in cultured rat hepatocytes

Dihydroceramide Δ4-desaturase 1 (DES1) catalyzes the last step of the de novo ceramide biosynthesis, which consists of the introduction of a trans Δ4-double bond in the carbon chain of the dihydroceramide. It was previously observed that myristic acid binds DES1 through N-myristoylation. This N-terminal modification significantly increased the activity of the recombinant DES1 in COS-7 cells and targeted part of the enzyme initially present in the endoplasmic reticulum to the mitochondrial outer membrane, leading to an increase in ceramide levels. Since these results were obtained in a recombinant COS-7 cell model with high expression of rat DES1, the purpose of the present study was to investigate if the native DES1 enzyme was really upregulated by its N-myristoylation in cultured rat hepatocytes. We first showed that DES1 was the main dihydroceramide desaturase isoform expressed in rat hepatocytes. In this model, the wild-type myristoylable recombinant form of rat DES1 was found in both the endoplasmic reticulum and the mitochondria whereas the mutated non-myristoylable recombinant form (N-terminal glycine replaced by an alanine) was almost exclusively localized in the endoplasmic reticulum, which evidenced the importance of the myristoylation. Then, we showed that compared to other fatty acids, myristic acid was the only one to increase native DES1 activity, in both total cell lysates and mitochondrial fractions. The myristic acid-associated increase in DES1 activity was not linked to elevated mRNA or protein expression but more likely to its N-terminal myristoylation. Finally, the myristic acid-associated increase in DES1 activity slightly enhanced the number of apoptotic cells.

Retroviral matrix and lipids, the intimate interaction

Retroviruses are enveloped viruses that assemble on the inner leaflet of cellular membranes. Improving biophysical techniques has recently unveiled many molecular aspects of the interaction between the retroviral structural protein Gag and the cellular membrane lipids. This interaction is driven by the N-terminal matrix domain of the protein, which probably undergoes important structural modifications during this process, and could induce membrane lipid distribution changes as well. This review aims at describing the molecular events occurring during MA-membrane interaction, and pointing out their consequences in terms of viral assembly. The striking conservation of the matrix membrane binding mode among retroviruses indicates that this particular step is most probably a relevant target for antiviral research.

Post-translational myristoylation: Fat matters in cellular life and death

Myristoylation corresponds to the irreversible covalent linkage of the 14-carbon saturated fatty acid, myristic acid, to the N-terminal glycine of many eukaryotic and viral proteins. It is catalyzed by N-myristoyltransferase. Typically, the myristate moiety participates in protein subcellular localization by facilitating protein-membrane interactions as well as protein-protein interactions. Myristoylated proteins are crucial components of a wide variety of functions, which include many signalling pathways, oncogenesis or viral replication. Initially, myristoylation was described as a co-translational reaction that occurs after the removal of the initiator methionine residue. However, it is now well established that myristoylation can also occur post-translationally in apoptotic cells. Indeed, during apoptosis hundreds of proteins are cleaved by caspases and in many cases this cleavage exposes an N-terminal glycine within a cryptic myristoylation consensus sequence, which can be myristoylated. The principal objective of this review is to provide an overview on the implication of myristoylation in health and disease with a special emphasis on post-translational myristoylation. In addition, new advancements in the detection and identification of myristoylated proteins are also briefly reviewed.

[New regulatory and signal functions for myristic acid]

Myristic acid is a 14 carbon saturated fatty acid, which is mostly found in milk fat. In industrialized countries, its excessive consumption is correlated with an increase in plasma cholesterol and mortality due to cardiovascular diseases. Nevertheless, one feature of this fatty acid is its ability to acylate proteins, a reaction which is called N-terminal myristoylation. This article describes various examples of important cellular regulations where the intervention of myristic acid is proven. Modulations of the cellular concentration of this fatty acid and its associated myristoylation function might be used as regulators of these metabolic pathways.

An early stage of Mason-Pfizer monkey virus budding is regulated by the hydrophobicity of the Gag matrix domain core

Intracellular capsid transport and release of Mason-Pfizer monkey virus are dependent on myristylation of the Gag matrix domain (MA). A myristylated MA mutant, in which Thr41 and Thr78 are replaced with isoleucines, assembles capsids that are transported to the plasma membrane but are blocked in an early budding step. Since the nuclear magnetic resonance structure of MA showed that these Thr residues point into the hydrophobic core of the protein, it was hypothesized that the T41I/T78I mutant was defective in release of myristic acid from the more hydrophobic core. In order to further investigate whether an increase in the hydrophobicity of the MA core modulates capsid-membrane interactions and viral budding, three tyrosine residues (11, 28, and 67), oriented toward the MA core, were replaced individually or in a pair-wise combination with the more hydrophobic phenylalanine residue(s). As a control, Tyr82, oriented toward the outer surface of MA, was also replaced with phenylalanine. These Tyr-to-Phe substitutions did not alter capsid assembly compared to wild type in a capsid assembly assay. Pulse-chase, immunofluorescence, and electron microscopy studies demonstrated that single substitutions of Tyr11, Tyr28, and Tyr67 recapitulated the T41I/T78I mutant phenotype of decreased budding kinetics and accumulation of capsids at the plasma membrane. MA double mutants with a combination of these Tyr substitutions exhibited a phenotype that was even more defective in budding. In contrast, MA mutants with Tyr82 replaced by Phe resulted in a transport-defective phenotype. These results strongly support the hypothesis that myristic acid is sequestered inside MA prior to capsid-membrane interactions.

Murine leukemia virus particle assembly quantitated by fluorescence microscopy: role of Gag-Gag interactions and membrane association

In order to track the assembly of murine leukemia virus (MLV), we used fluorescence microscopy to visualize particles containing Gag molecules fused to fluorescent proteins (FPs). Gag-FP chimeras budded from cells to produce fluorescent spots, which passed through the same pore-size filters and sedimented at the same velocity as authentic MLV. N-terminal myristylation of Gag-FPs was necessary for particle formation unless wild-type Gag was coexpressed. By labeling nonmyristylated Gag with yellow FP and wild-type Gag with cyan FP, we could quantitate the coincorporation of two proteins into single particles. This experiment showed that nonmyristylated Gag was incorporated into mixed particles at approximately 50% the efficiency of wild-type Gag. Mutations that inhibit Gag-Gag interactions (K. Alin and S. P. Goff, Virology 216:418-424, 1996; K. Alin and S. P. Goff, Virology 222:339-351, 1996) were then introduced into the capsid (CA) region of Gag-FPs. The mutations P150L and R119C/P133L inhibited fluorescent particle formation by these Gag-FPs, but Gag-FPs containing these mutations could be efficiently incorporated into particles when coexpressed with wild-type Gag. When these mutations were introduced into nonmyristylated Gag-FPs, no incorporation into particles in the presence of wild-type Gag was detected. These data suggest that two independent mechanisms, CA interactions and membrane association following myristylation, cooperate in MLV Gag assembly and budding.

Analysis of bovine leukemia virus gag membrane targeting and late domain function

Assembly of retrovirus-like particles only requires the expression of the Gag polyprotein precursor. We have exploited this in the development of a model system for studying the virus particle assembly pathway for bovine leukemia virus (BLV). BLV is closely related to the human T-cell leukemia viruses (HTLVs), and all are members of the Deltaretrovirus genus of the Retroviridae family. Overexpression of a BLV Gag polyprotein containing a carboxy-terminal influenza virus hemagglutinin (HA) epitope tag in mammalian cells led to the robust production of virus-like particles (VLPs). Site-directed mutations were introduced into HA-tagged Gag to test the usefulness of this model system for studying certain aspects of the virus assembly pathway. First, mutations that disrupted the amino-terminal glycine residue that is important for Gag myristylation led to a drastic reduction in VLP production. Predictably, the nature of the VLP production defect was correlated to Gag membrane localization. Second, mutation of the PPPY motif (located in the MA domain) greatly reduced VLP production in the absence of the viral protease. This reduction in VLP production was more severe in the presence of an active viral protease. Examination of particles by electron microscopy revealed an abundance of particles that began to pinch off from the plasma membrane but were not completely released from the cell surface, indicating that the PPPY motif functions as a late domain (L domain).

Caprine arthritis encephalitis virus: evidence for a B/D-type assembly pathway in a C-type lentivirus replication

Lentiviruses, among which is caprine arthritis encephalitis virus (CAEV), are known to concomitantly assemble and bud at the plasma membrane of infected cells, in a C-type defined pathway. Electron microscopy analysis of CAEV-infected cells demonstrated viral particles budding at the plasma membrane and into intracellular membrane-surrounded vesicles. Furthermore, nonenveloped immature virus-like particles, resembling intracytoplasmic type-A particles (ICAPs), accumulated within the cytoplasm of those cells. Fractionation on sucrose density gradients of cytoplasmic lysates from CAEV-infected cells revealed that enveloped immature or mature viral particles had a density of 1.16--1.17 g/ml, whereas ICAPs sedimented at a density of 1.2--1.27 g/ml. Endogenous reverse transcriptase activity was only associated with the 1.16--1.17 g/ml density particles despite the presence of viral RNA in both populations. The intracellular enveloped particles were found to be infectious. The CAEV Gag precursor by itself was shown to direct assembly, budding, and release of immature virus-like particles when expressed in goat primary synovial membrane cells using the same pathways of assembly and budding as observed in CAEV-infected cells. These data suggest that CAEV assembly, driven by the Gag precursor, could unusually proceed via two simultaneous pathways characteristic of type-C and type-B/D retroviruses.

Protein kinase B regulates T lymphocyte survival, nuclear factor kappaB activation, and Bcl-X(L) levels in vivo

The serine/threonine kinase protein kinase B (PKB)/Akt mediates cell survival in a variety of systems. We have generated transgenic mice expressing a constitutively active form of PKB (gag-PKB) to examine the effects of PKB activity on T lymphocyte survival. Thymocytes and mature T cells overexpressing gag-PKB displayed increased active PKB, enhanced viability in culture, and resistance to a variety of apoptotic stimuli. PKB activity prolonged the survival of CD4(+)CD8(+) double positive (DP) thymocytes in fetal thymic organ culture, but was unable to prevent antigen-induced clonal deletion of thymocytes expressing the major histocompatibility complex class I-restricted P14 T cell receptor (TCR). In mature T lymphocytes, PKB can be activated in response to TCR stimulation, and peptide-antigen-specific proliferation is enhanced in T cells expressing the gag-PKB transgene. Both thymocytes and T cells overexpressing gag-PKB displayed elevated levels of the antiapoptotic molecule Bcl-X(L). In addition, the activation of peripheral T cells led to enhanced nuclear factor (NF)-kappaB activation via accelerated degradation of the NF-kappaB inhibitory protein IkappaBalpha. Our data highlight a physiological role for PKB in promoting survival of DP thymocytes and mature T cells, and provide evidence for the direct association of three major survival molecules (PKB, Bcl-X(L), and NF-kappaB) in vivo in T lymphocytes.

The catalytic subunit of phosphoinositide 3-kinase: requirements for oncogenicity

The retroviral oncogene p3k (v-p3k) of avian sarcoma virus 16 (ASV16) codes for the catalytic subunit of phosphoinositide (PI) 3-kinase, p110alpha. The v-P3k protein is oncogenic in vivo and in vitro; its cellular counterpart, c-P3k, lacks oncogenicity. Fusion of viral Gag sequences to the amino terminus of c-P3k activates the transforming potential. Activation can also be achieved by the addition of a myristylation signal to the amino terminus or of a farnesylation signal to the carboxyl terminus of c-P3k. A mutated myristylation signal was equally effective; it also caused a strong increase in the kinase activity of P3k. Mutations that inactivate lipid kinase activity abolish oncogenicity. The transforming activity of P3k is correlated with the ability to induce activating phosphorylation in Akt. Point mutations and amino-terminal deletions recorded in v-P3k were shown to be irrelevant to the activation of oncogenic potential. Interactions of P3k with the regulatory subunit of PI 3-kinase, p85, or with Ras are not required for transformation. These results support the conclusion that the oncogenicity of P3k depends on constitutive lipid kinase activity. Akt is an important and probably essential downstream component of the oncogenic signal from P3k.

Cloning, expression and N-terminal myristoylation of CpCPK1, a calcium-dependent protein kinase from zucchini (Cucurbita pepo L.)

We have isolated a full-length cDNA clone (CpCDPK1) encoding a calcium-dependent protein kinase (CDPK) gene from zucchini (Cucurbita pepo L.). The predicted amino acid sequence of the cDNA shows a remarkably high degree of similarity to members of the CDPK gene family from Arabidopsis thaliana, especially AtCPK1 and AtCPK2. Northern analysis of steady-state mRNA levels for CpCPK1 in etiolated and light-grown zucchini seedlings shows that the transcript is most abundant in etiolated hypocotyls and overall expression is suppressed by light. As described for other members of the CDPK gene family from different species, the CpCPK1 clone has a putative N-terminal myristoylation sequence. In this study, site-directed mutagenesis and an in vitro coupled transcription/translation system were used to demonstrate that the protein encoded by this cDNA is specifically myristoylated by a plant N-myristoyl transferase. This is the first demonstration of myristoylation of a CDPK protein which may contribute to the mechanism by which this protein is localized to the plasma membrane.

Synthesis, in vitro anti-HIV activity, and biological stability of 5'-O-myristoyl analogue derivatives of 3'-fluoro-2',3'-dideoxythymidine (FLT) as potential bifunctional prodrugs of FLT

A group of 5'-O-myristoyl analogue derivatives of FLT (2) were evaluated as potential anti-HIV agents that were designed to serve as prodrugs to FLT. 3'-Fluoro-2',3'-dideoxy-5'-O-(12-methoxydodecanoyl)thymidine (4) (EC50 = 3.8 nM) and 3'-fluoro-2',3'-dideoxy-5'-O-(12-azidododecanoyl)thymidine (8) (EC50 = 2.8 nM) were the most effective anti-HIV-1 agents. There was a linear correlation between Log P and HPLC Log retention time for the 5'-O-FLT esters. The in vitro enzymatic hydrolysis half-life (t1/2), among the group of esters (3-8) in porcine liver esterase, rat plasma and rat brain homogenate was longer for 3'-fluoro-2',3'-dideoxy-5'-O-(myristoyl)thymidine (7), with t1/2 values of 20.3, 4.6 and 17.5 min, respectively.

Blockage of N-myristoylation of HIV-1 gag induces the production of impotent progeny virus

The role of the N-myristoylation of the human immunodeficiency virus type 1 (HIV-1) gag protein in ACH-2 cells was studied. The infectivity of HIV-1 from the cells stimulated with phorbol 12-myristate 13-acetate (PMA) was suppressed by pretreatment with N-myristoyl glycinal diethylacetal (N-Myr-GOA), a potent N-myristoylation inhibitor, and the blockage of myristoylation resulted in accumulation of immature gag precursors. The viral particles which budded from the non-N-Myr-GOA-treated ACH-2 cells stimulated with PMA exhibited a typical viral phenotype, whereas those which budded from the N-Myr-GOA-treated ACH-2 cells stimulated with PMA were twisted, as observed electron microscopically. In electron microscopic analyses with gold-labeled monoclonal antibodies to gag and env, gag and env were detected adjacent to each other in the PMA-stimulated ACH-2, but no env was detected in the cells treated with N-Myr-GOA. Taken together, the results suggest that the myristoylation of HIV-1 gag seems to be responsible for both maturation of gag and acquisition of HIV-1 infectivity.

Myristoylation

N-myristoylation is an acylation process absolutely specific to the N-terminal amino acid glycine in proteins. This maturation process concerns about a hundred proteins in lower and higher eukaryotes involved in oncogenesis, in secondary cellular signalling, in infectivity of retroviruses and, marginally, of other virus types. Thy cytosolic enzyme responsible for this activity, N-myristoyltransferase (NMT), studied since 1987, has been purified from different sources. However, the studies of the specificities of the various NMTs have not progressed in detail except for those relating to the yeast cytosolic enzyme. Still to be explained are differences in species specificity and between various putative isoenzymes, also whether the data obtained from the yeast enzyme can be transposed to other NMTs. The present review discusses data on the various addressing processes subsequent to myristoylation, a patchwork of pathways that suggests myristoylation is only the first step of the mechanisms by which a protein associates with the membrane. Concerning the enzyme itself, there are evidences that NMT is also present in the endoplasmic reticulum and that its substrate specificity is different from that of the cytosolic enzyme(s). These differences have major implications for their differential inhibition and for their respective roles in several pathologies. For instance, the NMTs from mammalians are clearly different from those found in several microorganisms, which raises the question whether the NMT may be a new targets for fungicides. Finally, since myristoylation has a central role in virus maturation and oncogenesis, specific NMT inhibitors might lead to potent antivirus and anticancer agents.

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.

Complete inhibition of human immunodeficiency virus Gag myristoylation is necessary for inhibition of particle budding

Myristoylation of human immunodeficiency virus (HIV) Gag protein is essential for virus particle budding. Two reactions are involved; activation of free myristate to myristoyl-CoA and transfer of the myristoyl residue to the Gag N-terminal glycine. We have investigated the effects of triacsin C, an inhibitor of long chain acyl-CoA synthetase, on release of HIV Gag virus-like particle (VLP) produced using the recombinant baculovirus system. First, inhibition of acyl-CoA formation by triacsin C was confirmed using the membrane fractions of insect Sf9 cells as an enzyme source. Second, when HIV Gag protein was expressed in the presence of triacsin C (0-48 microM), Gag myristoylation was inhibited in a dose-dependent manner. Budding of Gag VLP, however, did not follow similar inhibition kinetics but appeared unaffected up to 24 microM, yet was completely abolished at 48 microM when the myristoylation of Gag protein was also completely inhibited. The "all-or-none" inhibition of Gag VLP budding suggests that although inhibition of acyl-CoA synthetase blocks the production of myristoylated Gag protein, only complete inhibition of Gag myristoylation prevents VLP budding. Thus, relatively few myristoylated Gag molecules are sufficient for plasma membrane targeting and VLP budding.

Production of avian leukosis virus particles in mammalian cells can be mediated by the interaction of the human immunodeficiency virus protein Rev and the Rev-responsive element

In human immunodeficiency virus type 1-infected cells, the efficient expression of viral proteins from unspliced and singly spliced RNAs is dependent on two factors: the presence in the cell of the viral protein Rev and the presence in the viral RNA of the Rev-responsive element (RRE). We show here that the HIV-1 Rev/RRE system can increase the expression of avian leukosis virus (ALV) structural proteins in mammalian cells (D-17 canine osteosarcoma) and promote the release of mature ALV virions from these cells. In this system, the Rev/RRE interaction appears to facilitate the export of full-length unspliced ALV RNA from the nucleus to the cytoplasm, allowing increased production of the ALV structural proteins. Gag protein is produced in the cytoplasm of the ALV-transfected cells even in the absence of a Rev/RRE interaction. However, a functional Rev/RRE interaction increases the amount of Gag present intracellularly and, more strikingly, results in the release of mature ALV particles into the supernatant. RCAS virus containing an RRE is replication-competent in chicken embryo fibroblasts; however, we have been unable to determine whether the particles produced in D-17 cells are as infectious as the particles produced in chicken embryo fibroblasts.

Localization of the palmitoylation site in the transmembrane protein p12E of Friend murine leukaemia virus

Friend murine leukaemia virus complex was propagated on murine cells in the presence of [9,10-3H]palmitic acid. Virus particles were harvested from the culture supernatant and lysed with detergents. The viral transmembrane protein, p12E, was isolated from the lysates by size-exclusion chromatography and purified by narrowbore reverse-phase HPLC. Analysis of the purified product by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) revealed that the protein is palmitoylated carrying one fatty acid residue. The radiolabelled fatty acid was released by hydroxylamine treatment at pH 7, indicating that acylation occurred via a thioester linkage. For allocation of the acylation site, p12E was digested with trypsin. The resulting peptides were either directly subjected to MALDI-TOF-MS or fractionated by microbore reverse-phase HPLC prior to mass spectrometry. The results revealed that p12E of Friend murine leukaemia virus is acylated at a cysteine residue situated at the C-terminal side of the putative transmembrane anchor of the polypeptide. Fatty acid analysis of the purified acylpeptide demonstrated that p12E carries almost exclusively palmitic acid.

Acylation of myelin basic protein peptide 1-21 with alkyl carboxylates 2-10 carbons long affects secondary structure and posttranslational modification

A peptide consisting of the first 21 residues of human myelin basic protein (MBP) was synthesized. The N-terminal alanine of portions was blocked in separate experiments with alkyl carboxylates varying in size from 2 to 10 carbon atoms. The effects of these different alkyl carboxylates at the N-terminus on the secondary structure was studied by circular dichroism (250-190 nm). In water, the spectra of the unblocked peptide suggested unordered structure with large negative ellipticities at 198 nm. Addition of an acetyl group altered the magnitude of [theta]198 from -21856 +/- 2319 to -11095 +/- 1000 deg cm2 dmol-1, suggesting a significant increase in ordered structure. When peptides with longer alkyl carboxylates, acylated at the N-termini, were studied, the magnitude of theta 198 approached that of the unblocked peptide but greater negative ellipticities were observed for the C8 and C10 alkyl carboxylates. The theta 222 values were generally low (-1803 +/- 463) but increased with increasing length of the alkyl carboxylate to about -3200 deg cm2 dmol-1, suggesting that little alpha-helical structure was present. The spectra were also taken in lipid-mimetic solvents, including 2-propanol, methanol, and lysophosphatidylglycerol (lysoPG). In general the theta 198 and theta 222 values were suggestive of increased structure in these environments compared to water. In 90% 2-propanol the theta 198 of the unblocked peptide did not change when an acetyl group was added to the N-terminus (9088 compared to 8477 deg cm2 dmol-1). Addition of longer alkyl carboxylates correlated with larger, negative ellipticities.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of two active HeT-A retroposons of Drosophila melanogaster

HeT-A elements are Drosophila melanogaster LINE-like retroposons that transpose to broken chromosome ends by attaching themselves with an oligo(A) tail. Since this family of elements is believed to be involved in the vital function of telomere elongation in Drosophila, it is important to understand their transposition mechanism and the molecular aspects of activity. By comparison of several elements we have defined here the unit length of HeT-A elements to be approximately 6 kb. Also, we studied an active HeT-A element that had transposed very recently to the end of a terminally deleted X chromosome. The 12 kb of newly transposed DNA consisted of a tandem array of three different HeT-A elements joined by oligo(A) tails to each other and to the chromosome end broken in the yellow gene. Such an array may have transposed as a single unit or resulted from rapid successive transpositions of individual HeT-A elements. By sequence comparison with another recently transposed HeT-A element, conserved domains in the single open reading frame (ORF), encoding a gag-like polypeptide, of these elements were defined. We conclude that for transposition an intact ORF is required in cis, while the reverse transcriptase is not encoded on the HeT-A element but is provided in trans. This would make HeT-A elements dependent on an external reverse transcriptase for transposition and establish control of the genome over the activity of HeT-A elements. This distinguishes the Drosophila HeT-A element, which has been implicated in Drosophila telomere elongation, from the other, 'selfish' LINE-like elements.

Role of the matrix protein in the virion association of the human immunodeficiency virus type 1 envelope glycoprotein

The matrix (MA) protein of human immunodeficiency virus type 1 (HIV-1) forms an inner coat directly underneath the lipid envelope of the virion. The outer surface of the lipid envelope surrounding the capsid is coated by the viral Env glycoproteins. We report here that the HIV-1 capsid-Env glycoprotein association is very sensitive to minor alterations in the MA protein. The results indicate that most of the MA domain of the Gag precursor, except for its carboxy terminus, is essential for this association. Viral particles produced by proviruses with small missense or deletion mutations in the region coding for the amino-terminal 100 amino acids of the MA protein lacked both the surface glycoprotein gp120 and the transmembrane glycoprotein gp41, indicating a defect at the level of Env glycoprotein incorporation. Alterations at the carboxy terminus of the MA domain had no significant effect on the levels of particle-associated Env glycoprotein or on virus replication. The presence of HIV-1 MA protein sequences was sufficient for the stable association of HIV-1 Env glycoprotein with hybrid particles that contain the capsid (CA) and nucleocapsid (NC) proteins of visna virus. The association of HIV-1 Env glycoprotein with the hybrid particles was dependent upon the presence of the HIV-1 MA protein domain, as HIV-1 Env glycoprotein was not efficiently recruited into virus particles when coexpressed with authentic visna virus Gag proteins.

Synthesis of myristoyl CoA analogues and myristoyl peptides as inhibitors of myristoyl CoA:protein N-myristoyltransferase

To develop inhibitors of myristoyl CoA:protein N-myristoyltransferase (NMT), a series of myristoyl coenzyme A analogues and myristoyl peptides were synthesized, including S-(2-oxopentadecyl)-CoA (1), S-(2-hydroxypentadecyl)-CoA (2), S-(2-oxopentadecyl)-pantetheine (3), Myr-N-Gly-(L)-Phe (4), Myr-N-Gly-(L)-Tyr (5), and Myr-N-Gly-(L)-Asn-Ala- Ala-Ser-Ala-Arg-(NH2) (6). Biological evaluation of these compounds in an in vitro NMT enzyme assay revealed that the nonhydrolyzable acyl CoA analogue 1 was the most potent inhibitor [inhibitor dissociation constant (Ki) = 24 nM]. A preliminary structure-activity relationship study showed that the adenosine moiety and the 2-keto group in this nonhydrolyzable analogue were necessary for inhibitory activity. A possible mechanism for the inhibition of NMT by 1 was proposed, in which 1 might block the reaction at the stage of an acyl-CoA-NMT-peptide complex. Product analogues such as the myristoylated peptides 4-6 were poor inhibitors of NMT.

Expression of virus-encoded proteinases: functional and structural similarities with cellular enzymes

Many viruses express their genome, or part of their genome, initially as a polyprotein precursor that undergoes proteolytic processing. Molecular genetic analyses of viral gene expression have revealed that many of these processing events are mediated by virus-encoded proteinases. Biochemical activity studies and structural analyses of these viral enzymes reveal that they have remarkable similarities to cellular proteinases. However, the viral proteinases have evolved unique features that permit them to function in a cellular environment. In this article, the current status of plant and animal virus proteinases is described along with their role in the viral replication cycle. The reactions catalyzed by viral proteinases are not simple enzyme-substrate interactions; rather, the processing steps are highly regulated, are coordinated with other viral processes, and frequently involve the participation of other factors.

A subset of Pr65gag is nucleus associated in murine leukemia virus-infected cells

Nuclei of cells infected with Moloney murine leukemia virus (MoMuLV) were examined for the presence of gag proteins. This analysis was performed in conjunction with other studies suggesting a possible role for gag proteins in regulating nuclear events relating to processing and/or transport of viral genomic RNA. We detected Pr65gag and a p30-related protein in a nuclear fraction of infected cells. We also found evidence that a highly conserved amino acid sequence, which is shared by p30 and U1 small nuclear ribonucleoprotein 70-kDa protein, is a component of the nuclear targeting sequence for Pr65gag. Immunoelectron microscopy studies with a monoclonal anti-p12 antibody established that approximately 18% of gag-containing proteins of MoMuLV are located in the nucleus. Such gag-containing proteins from a mutant MoMuLV that lacks N-terminal myristic acid had greater affinity for the nucleus, suggesting that fatty acid acylation of Pr65gag plays a role in overcoming the proposed nuclear transport signal. The possible roles that nuclear gag proteins may play in retroviral replication are discussed.

Purification of immature cores of mouse mammary tumor virus and immunolocalization of protein domains

The immature capsids of the mouse mammary tumor virus (MMTV), known as intracytoplasmic A particles, have been isolated from murine L1210 leukemia cells. The diameter of the isolated particles was 80 nm as determined by negative staining. Two polypeptides of 77 and 110 kDa were found to be their major polypeptide components, in agreement with the expected sizes of the Gag and Gag-Pro precursor polypeptides of the mature MMTV proteins. Both polypeptides were recognized by antibodies directed toward the matrix (p10) and capsid (p27) proteins of MMTV. Immunogold labeling of p10 on isolated A particles, visualized by negative staining, showed that this protein is located at the surface of the immature capsids, whereas p27 can be detected only in broken or disrupted particles, suggesting that it has an internal location. These observations were confirmed by immunolabeling of both proteins on thin sections of A particle-producing cells. In addition, the viral protease had a more internal position than p27. Since the sequential order of the viral proteins in the Gag precursor is p10-pp21-p27-p14 and that in Gag-Pro is p10-pp21-p27-p30-protease, our results demonstrate the radial organization of the polypeptide precursors forming the intracytoplasmic A particles.

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

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

Studies on processing, particle formation, and immunogenicity of the HIV-1 gag gene product: a possible component of a HIV vaccine

Antigens in a particulate conformation were shown to be highly immunogenic in mammals. For this reason, the particle forming capacity of derivatives of the HIV-1 group specific core antigen p55 gag was assayed and compared dependent on various expression systems: recombinant bacteria, vaccinia- and baculoviruses were established encoding the entire core protein p55 either in its authentic sequence or lacking the myristylation consensus signal. Moreover, p55 gag was expressed in combination with the protease (p55-PR) or with the entire polymerase (p55-pol), respectively. Budding of 100-160 nm p55 core particles, resembling immature HIV-virions, was observed in the eucaryotic expression systems only. In comparison to the vaccinia virus driven expression of p55 in mammalian cells, considerably higher yields of particulate core antigen were obtained by infection of Spodoptera frugiperda (Sf9) insect cells with the recombinant Autographa californica nuclear polyhedrosis (AcMNPV) baculovirus. Mutation of the NH2-terminal myristylation signal sequence prevented budding of the immature core particles. Expression of the HIV p55-PR gene construct by recombinant baculovirus resulted in complete processing of the p55 gag precursor molecule in this system. The introduction of an artificial frameshift near the natural frameshift site resulted in constitutive expression of the viral protease and complete processing of p55, both in Escherichia coli and in vaccinia virus infected cells. Interestingly, significant processing of p55 resembling that of HIV infected H9 cells could also be achieved in the vaccinia system by fusing the entire pol gene to the gag gene. Moreover, processing was not found to be dependent on amino-terminal myristylation of the gag procursor molecule, which is in contrast to observations with type C and type D retrovirus. However, complete processing of p55 into p24, p17, p9 and p6 abolished particle formation. Purified immature HIV-virus like particles were highly immunogenic in rabbits, leading to a strong humoral immune response after immunization. Empty immature p55 gag particles represent a noninfectious and attractive candidate for a basic vaccine component.

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

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

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

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

Analyses of the requirements for the synthesis of virus-like particles by feline immunodeficiency virus gag using baculovirus vectors

Feline immunodeficiency virus (FIV) gag gene was expressed in baculovirus vectors to investigate its potential for the assembly of virus-like particles. The unprocessed 50-kDa FIV gag precursor made in infected insect cells by recombinant AcFIVGAG-1 was myristoylated, assembled at the cell surface into virus-like particles (with diameters of approximately 100 nm), and efficiently released into the culture supernatant fluids. The presence of the complete viral-coded protease component of the FIV pol gene engineered into a second expression vector (AcFIVGAG-P5) resulted in the efficient processing of the gag precursor to its component proteins and abolished particle formation and secretion. Insertion of a stop codon in this vector upstream of the putative gag-pol frameshift site (GGGAAAC) resulted in the derivation of an expression vector (AcFIVGAG-R) that made a truncated, unprocessed 46-kDa FIV gag precursor lacking some 34 amino acids in the p10 carboxy-proximal coding region of gag. This vector synthesized tubular structures in the cytoplasm of infected cells and released them into the cell supernatant. The results demonstrate that the FIV gag precursor can spontaneously assemble into virus-like particles without any other virus proteins and that the carboxy-terminal part of the precursor gag protein is essential for such assembly.

A superantigen encoded in the open reading frame of the 3' long terminal repeat of mouse mammary tumour virus

Mice express a collection of superantigens, which bind to class II major histocompatibility proteins and interact with T cells bearing particular V beta chains as part of their alpha beta receptors. These superantigens have been suggested to be encoded by exogenous or endogenous mouse mammary tumour viruses. One such superantigen is now shown to be encoded in the open reading frame of the long terminal repeat of a mammary tumour virus, a gene of previously unknown function.

Synthesis and characterization of inhibitors of myristoyl-CoA:protein N-myristoyltransferase

Several substrate and product analogs were synthesized and tested as in vitro inhibitors of bovine brain N-myristoyl-CoA:protein N-myristoyltransferase (NMT; EC 2.3.1.97). At 40 microM, the acyl CoA analog, S-(2-ketopentadecyl)-CoA, completely inhibited NMT in the presence of 80 microM myristoyl CoA. Decreasing but marked inhibition was also observed with the acyl CoA analogs, S-(2-bromo-tetradecanoyl)-CoA and S-(3-(epoxymethylene)dodecanoyl)-CoA, and the multisubstrate derivative N-(2-S-CoA-tetradecanoyl)glycinamide in the presence of 40 microM myristoyl CoA. Inhibition was also observed with the non-coenzyme A myristoyl analog, 1-bromo-2-pentadecanone. All of the above compounds exhibited reversible competitive inhibition kinetics with respect to myristoyl CoA with Ki values of 0.11 to 24 microM. Two additional acyl CoA analogs, S-(cis-3-tetradecenoyl)-CoA and S-(3-tetradecynoyl)-CoA, functioned as alternative substrates for NMT.

Characterization of the gag/fusion protein encoded by the defective Duplan retrovirus inducing murine acquired immunodeficiency syndrome

Murine acquired immunodeficiency syndrome is induced by a defective retrovirus. Sequencing of this defective viral genome revealed a long open reading frame which encodes a putative gag/fusion protein, N-MA-p12-CA-NC-COOH, (D. C. Aziz, Z. Hanna, and P. Jolicoeur, Nature (London) 338:505-508, 1989). We raised a specific antibody to the unique p12 domain of this gag fusion precursor, Pr60gag. We found that Pr60gag was indeed encoded by the defective viral genome both in cell-free translation reticulocyte extracts and in infected mouse fibroblasts. Pr60gag was found to be myristylated, phosphorylated, and attached to the cell membrane, like other helper murine leukemia virus (MuLV) gag precursors. Pr60gag was not substantially cleaved within the nonproducer cells and was not released from these cells. However, in the presence of helper MuLV proteins, it formed phenotypically mixed particles. In these particles, Pr60gag was only partially cleaved. In helper MuLV-producing cells harboring the defective virus, a gag-related p40 intermediate was generated both intracellularly and extracellularly. In these cells, Pr60gag appeared to behave as a dominant negative mutant, interfering with proper cleavage of helper Pr65gag. Our data indicate that Pr60gag is a major (and possibly the only) gene product of the defective murine acquired immunodeficiency syndrome virus and is likely to harbor some determinants of pathogenicity of this virus.

Myristylation of Rous sarcoma virus Gag protein does not prevent replication in avian cells

Rous sarcoma virus is an example of a replication-competent retrovirus whose Gag protein is not modified with myristic acid. The purpose of the experiments described in this report was to determine whether the addition of this 14-carbon fatty acid would interfere with the replication of Rous sarcoma virus. We found that myristylated derivatives of the Rous sarcoma virus Gag protein are fully functional for particle formation in avian cells and that the addition of myristic acid has very little effect on infectivity.

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

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

N myristoylation of the spleen necrosis virus matrix protein is required for correct association of the Gag polyprotein with intracellular membranes and for particle formation

To determine whether myristoylation is required for spleen necrosis virus replication, we constructed a substitution mutation in the gag gene that alters the putative myristate acceptor glycine residue. This single amino acid change was lethal for virus replication, resulted in aberrant proteolytic processing, and interrupted virion assembly and the release of virus from cells. Immunofluorescence analysis indicated that the amount of Gag polyprotein at the cell periphery and in Golgi-associated vesicles is severely reduced in the myristoylation mutant, indicating that correct intracellular targeting is affected by a lack of myristoylation. Coexpression of wild-type Gag polyprotein did not complement and rescue the replication-defective phenotype of the myristoylation mutant. Thus, it appears that the nonmyristoylated polyproteins are incapable of interacting with their myristoylated counterparts to form biologically active particles.

Mechanism of chemical activation of expression of the endogenous ecotropic murine leukemia provirus Emv-3

DBA/2 mice carry a single endogenous ecotropic murine leukemia provirus, Emv-3. This provirus is defective; it is very poorly expressed in young DBA/2 mice. The defect in Emv-3 is caused by a single base substitution in codon 3 of p15gag. The resulting amino acid substitution inhibits myristylation of the gag precursor and subsequent virus assembly. Despite this defect, percutaneous treatment of DBA/2 mice with the carcinogen and mutagen 7,12-dimethylbenz[a]anthracene (DMBA) induces ecotropic murine leukemia virus replication in virtually all treated mice. We hypothesized that this induction is the result of a DMBA-induced reverse mutation in codon 3 of p15gag which allows for efficient myristylation. We tested this hypothesis by isolating ecotropic viruses from DMBA-treated mice and determining the DNA sequences of selected regions of p15gag, including codon 3. In support of the above-described model, all of the viruses examined contained single nucleotide substitutions in codon 3. In addition, most of the replication-competent viruses that were sequenced appeared to result from simple mutation of Emv-3 rather than recombination with other endogenous murine leukemia viruses. These studies may provide a basis for development of a sensitive assay for the mutagenic activity of a variety of chemical carcinogens in vivo.

RNA-binding properties of the matrix protein (p19gag) of avian sarcoma and leukemia viruses

We have reinvestigated the ability of the matrix protein (MA) (p19gag) of avian sarcoma and leukemia viruses to interact with RNA. Previous reports claimed on the one hand that MA can bind tightly and with a high degree of specificity to avian sarcoma and leukemia virus RNA in vitro and on the other that it cannot bind to RNA at all. We found that MA purified by any of several methods does bind to RNA, as measured by its ability to cause retention of radioactive RNA on nitrocellulose membranes in a filtration assay. However, this interaction is weak and lacks specificity. The interaction of MA with RNA was barely detectable by classical sedimentation analysis, and from this observation we estimate that the intrinsic MA-RNA association constant is ca. 10(3) M-1, at least 3 orders of magnitude smaller than the constant describing the interaction of the viral nucleocapsid protein (NC) (p12gag) with RNA, ca. 10(6) M-1. Separately purified phosphorylated and nonphosphorylated MA species bound RNA equally. We also found that MA can bind to DNA with an affinity similar to that for RNA. The large quantitative discrepancy between our results and earlier published reports can be traced in part to methods of data analysis.