Polypeptides of Mason-Pfizer monkey virus. I. Synthesis and processing of the gag-gene products

Expression, purification, and characterization of biologically active full-length Mason-Pfizer monkey virus (MPMV) Pr78Gag

MPMV precursor polypeptide Pr78^Gag orchestrates assembly and packaging of genomic RNA (gRNA) into virus particles. Therefore, we have expressed recombinant full-length Pr78^Gag either with or without His₆-tag in bacterial as well as eukaryotic cultures and purified the recombinant protein from soluble fractions of the bacterial cultures. The recombinant Pr78^Gag protein has the intrinsic ability to assemble in vitro to form virus like particles (VLPs). Consistent with this observation, the recombinant protein could form VLPs in both prokaryotes and eukaryotes. VLPs formed in eukaryotic cells by recombinant Pr78^Gag with or without His₆-tag can encapsidate MPMV transfer vector RNA, suggesting that the inclusion of the His₆-tag to the full-length Pr78^Gag did not interfere with its expression or biological function. This study demonstrates the expression and purification of a biologically active, recombinant Pr78^Gag, which should pave the way to study RNA-protein interactions involved in the MPMV gRNA packaging process.

Molecular aspects of the interaction between Mason-Pfizer monkey virus matrix protein and artificial phospholipid membrane

The Mason-Pfizer monkey virus is a type D retrovirus, which assembles its immature particles in the cytoplasm prior to their transport to the host cell membrane. The association with the membrane is mediated by the N-terminally myristoylated matrix protein. To reveal the role of particular residues which are involved in the capsid-membrane interaction, covalent labelling of arginine, lysine and tyrosine residues of the Mason-Pfizer monkey virus matrix protein bound to artificial liposomes containing 95% of phosphatidylcholine and 5% phosphatidylinositol-(4,5)-bisphosphate (PI(4,5)P₂ ) was performed. The experimental results were interpreted by multiscale molecular dynamics simulations. The application of these two complementary approaches helped us to reveal that matrix protein specifically recognizes the PI(4,5)P₂ molecule by the residues K20, K25, K27, K74, and Y28, while the residues K92 and K93 stabilizes the matrix protein orientation on the membrane by the interaction with another PI(4,5)P₂ molecule. Residues K33, K39, K54, Y66, Y67, and K87 appear to be involved in the matrix protein oligomerization. All arginine residues remained accessible during the interaction with liposomes which indicates that they neither contribute to the interaction with membrane nor are involved in protein oligomerization. Proteins 2016; 84:1717-1727. © 2016 Wiley Periodicals, Inc.

An siRNA screen of membrane trafficking genes highlights pathways common to HIV-1 and M-PMV virus assembly and release

The assembly and release of retroviruses from the host cells requires a coordinated series of interactions between viral structural proteins and cellular trafficking pathways. Although a number of cellular factors involved in retrovirus assembly have been identified, it is likely that retroviruses utilize additional trafficking factors to expedite their assembly and budding that have not yet been defined. We performed a screen using an siRNA library targeting host membrane trafficking genes in order to identify new host factors that contribute to retrovirus assembly or release. We utilized two retroviruses that follow very distinct assembly pathways, HIV-1 and Mason-Pfizer monkey virus (M-PMV) in order to identify host pathways that are generally applicable in retrovirus assembly versus those that are unique to HIV or M-PMV. Here we report the identification of 24 host proteins identified in the screen and subsequently validated in follow-up experiments as contributors to the assembly or release of both viruses. In addition to identifying a number of previously unsuspected individual trafficking factors, we noted multiple hits among proteins involved in modulation of the actin cytoskeleton, clathrin-mediated transport pathways, and phosphoinositide metabolism. Our study shows that distant genera of retroviruses share a number of common interaction strategies with host cell trafficking machinery, and identifies new cellular factors involved in the late stages of retroviral replication.

ROCK1 and LIM kinase modulate retrovirus particle release and cell-cell transmission events

The assembly and release of retroviruses from the host cells require dynamic interactions between viral structural proteins and a variety of cellular factors. It has been long speculated that the actin cytoskeleton is involved in retrovirus production, and actin and actin-related proteins are enriched in HIV-1 virions. However, the specific role of actin in retrovirus assembly and release remains unknown. Here we identified LIM kinase 1 (LIMK1) as a cellular factor regulating HIV-1 and Mason-Pfizer monkey virus (M-PMV) particle release. Depletion of LIMK1 reduced not only particle output but also virus cell-cell transmission and was rescued by LIMK1 replenishment. Depletion of the upstream LIMK1 regulator ROCK1 inhibited particle release, as did a competitive peptide inhibitor of LIMK1 activity that prevented cofilin phosphorylation. Disruption of either ROCK1 or LIMK1 led to enhanced particle accumulation on the plasma membrane as revealed by total internal reflection fluorescence microscopy (TIRFM). Electron microscopy demonstrated a block to particle release, with clusters of fully mature particles on the surface of the cells. Our studies support a model in which ROCK1- and LIMK1-regulated phosphorylation of cofilin and subsequent local disruption of dynamic actin turnover play a role in retrovirus release from host cells and in cell-cell transmission events.

IMPORTANCE

Viruses often interact with the cellular cytoskeletal machinery in order to deliver their components to the site of assembly and budding. This study indicates that a key regulator of actin dynamics at the plasma membrane, LIM kinase, is important for the release of viral particles for HIV as well as for particle release by a distantly related retrovirus, Mason-Pfizer monkey virus. Moreover, disruption of LIM kinase greatly diminished the spread of HIV from cell to cell. These findings suggest that LIM kinase and its dynamic modulation of the actin cytoskeleton in the cell may be an important host factor for the production, release, and transmission of retroviruses.

A Mason-Pfizer Monkey virus Gag-GFP fusion vector allows visualization of capsid transport in live cells and demonstrates a role for microtubules

Immature capsids of the Betaretrovirus, Mason-Pfizer Monkey virus (M-PMV), are assembled in the pericentriolar region of the cell, and are then transported to the plasma membrane for budding. Although several studies, utilizing mutagenesis, biochemistry, and immunofluorescence, have defined the role of some viral and host cells factors involved in these processes, they have the disadvantage of population analysis, rather than analyzing individual capsid movement in real time. In this study, we created an M-PMV vector in which the enhanced green fluorescent protein, eGFP, was fused to the carboxyl-terminus of the M-PMV Gag polyprotein, to create a Gag-GFP fusion that could be visualized in live cells. In order to express this fusion protein in the context of an M-PMV proviral backbone, it was necessary to codon-optimize gag, optimize the Kozak sequence preceding the initiating methionine, and mutate an internal methionine codon to one for alanine (M100A) to prevent internal initiation of translation. Co-expression of this pSARM-Gag-GFP-M100A vector with a WT M-PMV provirus resulted in efficient assembly and release of capsids. Results from fixed-cell immunofluorescence and pulse-chase analyses of wild type and mutant Gag-GFP constructs demonstrated comparable intracellular localization and release of capsids to untagged counterparts. Real-time, live-cell visualization and analysis of the GFP-tagged capsids provided strong evidence for a role for microtubules in the intracellular transport of M-PMV capsids. Thus, this M-PMV Gag-GFP vector is a useful tool for identifying novel virus-cell interactions involved in intracellular M-PMV capsid transport in a dynamic, real-time system.

An updated review of simian betaretrovirus (SRV) in macaque hosts

This review is an updated summary of nearly 30 years of SRV history and provides new and critical findings of original research accomplished in the last 5 years including, but not limited to, the pathogenetic mechanisms underlying the origin of hematopoietic abnormalities observed in infected hosts and proposed new SRV serotypes. Despite major advances in the understanding and control of SRV disease, much more remains to be learned and SRV continues to be an exciting and attractive primate model for comparative studies of the mechanisms of retroviral immunosuppression.

The complete genome and genetic characteristics of SRV-4 isolated from cynomolgus monkeys (Macaca fascicularis)

At least 5 serotypes of exogenous simian retrovirus type D (SRV/D) have been found in nonhuman primates, but only SRV-1, 2 and 3 have been completely sequenced. SRV-4 was recovered once from cynomolgus macaques in California in 1984, but its genome sequences are unknown. Here we report the second identification of SRV-4 and its complete genome from infected cynomolgus macaques with Indochinese and Indonesian/Indochinese mixed ancestry. Phylogenetic analysis demonstrated that SRV-4 was distantly related to SRV-1, 2, 3, 5, 6 and 7. SRV/D-T, a new SRV/D recovered in 2005 from cynomolgus monkeys at Tsukuba Primate Center in Japan, clustered with the SRV-4 isolates from California and Texas and was shown to be another occurrence of SRV-4 infection. The repeated occurrence of SRV-4 in cynomolgus monkeys in different areas of the world and across 25years suggests that this species is the natural host of SRV-4.

Conformational changes of the N-terminal part of Mason-Pfizer monkey virus p12 protein during multimerization

The Mason-Pfizer monkey virus is a prototype Betaretrovirus with the defining characteristic that it assembles spherical immature particles from Gag-related polyprotein precursors within the cytoplasm of the infected cell. It was shown previously that the N-terminal part of the Gag p12 domain (wt-Np12) is required for efficient assembly. However, the precise role for p12 in mediating Gag-Gag interaction is still poorly understood. In this study we employed detailed circular dichroism spectroscopy, electron microscopy and ultracentrifugation analyses of recombinant wt-Np12 prepared by in vitro transcription and translation. The wt-Np12 domain fragment forms fibrillar structures in a concentration-dependent manner. Assembly into fibers is linked to a conformational transition from unfolded or another non-periodical state to alpha-helix during multimerization.

The impact of altered polyprotein ratios on the assembly and infectivity of Mason-Pfizer monkey virus

Most retroviruses employ a frameshift mechanism during polyprotein synthesis to balance appropriate ratios of structural proteins and enzymes. To investigate the requirements for individual precursors in retrovirus assembly, we modified the polyprotein repertoire of Mason-Pfizer monkey virus (M-PMV) by mutating the frameshift sites to imitate the polyprotein organization of Rous sarcoma virus (Gag-Pro and Gag-Pro-Pol) or Human immunodeficiency virus (Gag and Gag-Pro-Pol). For the "Rous-like" virus, assembly was impaired with no incorporation of Gag-Pro-Pol into particles and for the "HIV-like" virus an altered morphogenesis was observed. A mutant expressing Gag and Gag-Pro polyproteins and lacking Gag-Pro-Pol assembled intracellular particles at a level similar to the wild-type. Gag-Pro-Pol polyprotein alone neither formed immature particles nor processed the precursor. All the mutants were non-infectious except the "HIV-like", which retained fractional infectivity.

Basic residues in the Mason-Pfizer monkey virus gag matrix domain regulate intracellular trafficking and capsid-membrane interactions

Mason-Pfizer monkey virus (M-PMV) capsids that have assembled in the cytoplasm must be transported to and associate with the plasma membrane prior to being enveloped by a lipid bilayer during viral release. Structural studies have identified a positive-charge density on the membrane-proximal surface of the matrix (MA) protein component of the Gag polyprotein. To investigate if basic amino acids in MA play a role in intracellular transport and capsid-membrane interactions, mutants were constructed in which lysine and arginine residues (R10, K16, K20, R22, K25, K27, K33, and K39) potentially exposed on the capsid surface were replaced singly and in pairs by alanine. A majority of the charge substitution mutants were released less efficiently than the wild type. Electron microscopy of mutant Gag-expressing cells revealed four distinct phenotypes: K16A and K20A immature capsids accumulated on and budded into intracellular vesicles; R10A, K27A, and R22A capsid transport was arrested at the cellular cortical actin network, while K25A immature capsids were dispersed throughout the cytoplasm and appeared to be defective at an earlier stage of intracellular transport; and the remaining mutant (K33A and K39A) capsids accumulated at the inner surface of the plasma membrane. All mutants that released virions exhibited near-wild-type infectivity in a single-round assay. Thus, basic amino acids in the M-PMV MA define both cellular location and efficiency of virus release.

Phosphorylation of the HTLV-1 matrix L-domain-containing protein by virus-associated ERK-2 kinase

L-domain-containing proteins from animal retroviruses play a critical role in the recruitment of the host cell endocytic machinery that is required for retroviruses budding. We recently demonstrated that phosphorylation of the p6(gag) protein containing the L-domain of the human immunodeficiency virus type 1 regulates viral assembly and budding. Here, we investigated whether or not the L-domain-containing protein from another human retrovirus, namely the matrix protein of the human T-cell leukemia virus type 1, that contains the canonical PTAP and PPPY L-domain motifs, shares similar functional properties. We found that MA is phosphorylated at several sites. We identified one phosphorylated amino acid in the HTLV-1 MA protein as being S105, located in the close vicinity to the L-domain sequence. S105 phosphorylation was found to be mediated by the cellular kinase ERK-2 that is incorporated within HTLV-1 virus particles in an active form. Mutation of the ERK-2 target S105 residue into an alanine was found to decrease viral release and budding efficiency of the HTLV-1(ACH) molecular clone from transfected cells. Our data thus support the postulate that phosphorylation of retroviral L-domain proteins is a common feature to retroviruses that participates in the regulation of viral budding.

Localization of self-interacting domains within betaretrovirus Gag polyproteins

The Betaretrovirus genus is characterized by the ability to preassemble immature capsids within the cytoplasm. For Mason-Pfizer monkey virus (M-PMV) this ability depends in part upon the unique Internal Scaffold Domain (ISD) within the p12 region of Gag. In this study, we have further characterized the ability of M-PMV p12 to promote Gag-Gag interaction and have examined the Gag polyprotein of the related mouse mammary tumor virus (MMTV) to potentially identify a region with equivalent function. Using the yeast two-hybrid system, we confirmed that both Gag polyproteins strongly interact, primarily through the CA-NC regions, but also through additional domains N-terminal to CA. For M-PMV, this auxiliary interaction domain was p12. For MMTV, no single strongly self-interacting protein was identified. Instead, MMTV Gag appears to utilize the weak contributions of several protein domains to support the main interaction of its CA-NC. Our findings suggest that, in addition to the canonical NC "I-domain" interaction, MMTV Gag self-association results from the concerted action of multiple regions of the polyprotein while M-PMV Gag relies mainly on its p12 domain.

Retrovirus budding

The release of retrovirus particles from the infected cell is greatly stimulated by short motifs, known as "late" or "L" domains, present within the Gag precursor protein. Three distinct classes of L domains have been identified; these bear the core sequence: Pro-Thr/Ser-Ala-Pro [P(T/S)AP], Pro-Pro-x-Tyr (PPxY), or Tyr-Pro-x-Leu (YPxL). A number of recent studies have demonstrated that L domains function by interacting with components of the machinery responsible for sorting cellular proteins into the multivesicular body (MVB) pathway. This review traces the history of L domain discovery and characterization, and highlights the relationship between L domain activity, retrovirus release, and the host endosomal sorting machinery.

Isolation and characterization of a new simian retrovirus type D subtype from monkeys at the Tsukuba Primate Center, Japan

Exogenous type D simian retroviruses (SRV/D) are prevalent in captive and feral populations of various macaque monkeys. Thus far, five subtypes of SRV/Ds have been reported, three of which (SRV-1, -2 and -3) have been molecularly characterized. Two SRV/D strains (N27 and T150) were isolated from seropositive cynomolgus macaques at the Tsukuba Primate Center (TPC) in Japan, showing clinical signs of SRV/D infection, including anemia and persistent unresponsive diarrhea. Electron microscopy demonstrated that both SRV/D isolates have a virion morphology typical of type D retrovirus. The SRV/D N27 and T150 isolates were essentially the same based on sequence analysis. From homology analysis of the entire gag sequence, the N27 isolate is closely related to the other known SRV/Ds but is distinct from the three molecularly characterized SRV/Ds. Thus, we have tentatively designated the N27 and T150 viruses isolated from TPC cynomolgus macaques as SRV/D-Tsukuba (SRV/D-T).

Isolation and characterization of the Mason–Pfizer monkey virus p12 protein

The Mason-Pfizer monkey virus (M-PMV) Gag protein, precursor to the structural proteins of the infectious virion, assembles into immature capsid-like particles when expressed at high levels in bacterial cells. Similar capsid-like particles can be obtained by in vitro assembly using a high concentration of isolated Gag. M-PMV Gag contains a p12 protein that has no corresponding analogues in most other retroviruses and has been suggested to contain an internal scaffold domain (ISD). We have expressed and purified p12 and the N- and C-terminal halves (Np12 and Cp12) that are predicted to be structurally independent domains. The behavior of these proteins was analyzed using chemical cross-linking, CD spectroscopy, and electron microscopy. The N-terminal half of p12 is largely alpha-helical although the C-terminal portion lacks any apparent ordered structure. Both p12 and Np12 form high-order oligomers in vitro and when expressed in E. coli produce organized structures that are visible by electron microscopy. Interestingly, Cp12, as well as the whole protein, can form dimers in the presence of SDS. The data show that both domains of p12 contribute to its ability to multimerize with much of this potential residing in its N-terminal part, most probably within the leucine zipper-like (LZL) sequence.

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.

The host cell MAP kinase ERK-2 regulates viral assembly and release by phosphorylating the p6gag protein of HIV-1

The host cell MAP kinase ERK-2 incorporated within human immunodeficiency virus type 1 particles plays a critical role in virus infectivity by phosphorylating viral proteins. Recently, a fraction of the virus incorporated late (L) domain-containing p6(gag) protein, which has an essential function in the release of viral particles from the cell surface, was reported to be phosphorylated by an unknown virus-associated cellular protein kinase (Muller, B., Patschinsky, T., and Krausslich, H. G. (2002) J. Virol. 76, 1015-1024). The present study demonstrates the contribution of the MAP kinase ERK-2 in p6(gag) phosphorylation. According to mutational analysis, a single ERK-2-phosphorylated threonine residue, belonging to a highly conserved phosphorylation MAP kinase consensus site, was identified at position 23 within p6(gag). Substitution by an alanine of the Thr(23) phosphorylable residue within the pNL4.3 molecular clone was found to decrease viral release from various cell types. As observed from electron microscopy experiments, most virions produced from this molecular clone remained incompletely separated from the host cell membrane with an immature morphology and displayed a reduced infectivity in single round infection experiments. Analysis of protein processing by Western blotting experiments revealed an incomplete Pr55(gag) maturation and a reduction in the virion-associated reverse transcriptase proteins was observed that was not related to differences in intracellular viral protein expression. Altogether, these data suggest that phosphorylation of p6(gag) protein by virus-associated ERK-2 is involved in the budding stage of HIV-1 life cycle.

The endogenous langur type D retrovirus PO-1-Lu and its exogenous counterparts in macaque and langur monkeys

PO-1-Lu, the endogenous type D retrovirus of langurs (Trachypithecus obscurus) has previously been considered a progenitor to the prototype type D retrovirus, Mason Pfizer monkey virus (M-PMV/SRV-3), that became established in macaque monkeys (Macaca spp.) following a zoonosis. This study reevaluates this hypothesis to include other exogenous SRVs. New sequence information from the gp70(SU)-encoding region of PO-1-Lu shows striking similarity to the newly identified exogenous langur retrovirus, SRV-6, recently isolated from the Hanuman Langur (Semnopithecus entellus). An unrooted, bootstrapped neighbor-joining tree derived from env gene nucleotide sequences shows PO-1-Lu and SRV-6 appear more closely related genetically to SRV-2 than SRV-1 or SRV-3 (M-PMV). This is also reflected in our observations that the M-PMV envelope glycoprotein precursor gPr86(Env) and gp70(SU) were antigenically distinct from PO-1-Lu, although the gp22(TM) glycoproteins were antigenically cross-reactive. The potential that SRV-6 represents an exogenous form of PO-1-Lu that has arisen following a recent zoonosis is discussed.

A tyrosine motif in the cytoplasmic domain of mason-pfizer monkey virus is essential for the incorporation of glycoprotein into virions

Mason-Pfizer monkey virus (M-PMV) encodes a transmembrane (TM) glycoprotein with a 38-amino-acid-long cytoplasmic domain. After the release of the immature virus, a viral protease-mediated cleavage occurs within the cytoplasmic domain, resulting in the loss of 17 amino acids from the carboxy terminus. This maturational cleavage occurs between a histidine at position 21 and a tyrosine at position 22 in the cytoplasmic domain of the TM protein. We have demonstrated previously that a truncated TM glycoprotein with a 21-amino-acid-long cytoplasmic tail showed enhanced fusogenicity but could not be incorporated into virions. These results suggest that postassembly cleavage of the cytoplasmic domain removes a necessary incorporation signal and activates fusion activity. To investigate the contribution of tyrosine residues to the function of the glycoprotein complex and virus replication, we have introduced amino acid substitutions into two tyrosine residues found in the cytoplasmic domain. The effects of these mutations on glycoprotein biosynthesis and function, as well as on virus infectivity, have been examined. Mutation of tyrosine 34 to alanine had little effect on glycoprotein function. In contrast, substitutions at tyrosine 22 modulated fusion activity in either a positive or negative manner, depending on the substituting amino acid. Moreover, any nonaromatic substitution at this position blocked glycoprotein incorporation into virions and abolished infectivity. These results demonstrate that M-PMV employs a tyrosine signal for the selective incorporation of glycoprotein into budding virions. Antibody uptake studies show that tyrosine 22 is part of an efficient internalization signal in the cytoplasmic domain of the M-PMV glycoprotein that can also be positively and negatively influenced by changes at this site.

Molecular biology of jaagsiekte sheep retrovirus

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA), a contagious lung cancer of sheep. Until recently, research on JSRV/OPA was hampered by the lack of a tissue culture system for the propagation of the virus. Historically, pathological samples (lung fluid) collected from sheep affected by OPA were the only source of infectious JSRV. Thus studies on the JSRV/OPA system were conducted only where field isolates of OPA cases were readily available. In the past 10 years, the deduction of the JSRV sequence (York et al. 1991; York 1992), the isolation of an infectious and oncogenic JSRV molecular clone (JSRV21) (Palmarini et al. 1999a) and the establishment of a rapid method to produce infectious virus in vitro (Palmarini et al. 1999a) sparked many studies at the molecular level that strengthened past observations and revealed new properties of this unique virus. Here, we will review the data accumulated so far on the molecular biology of JSRV using the infectious and oncogenic JSRV21 molecular clone as virus of reference.

The late-domain-containing protein p6 is the predominant phosphoprotein of human immunodeficiency virus type 1 particles

The Gag-derived protein p6 of human immunodeficiency virus type 1 (HIV-1) plays a crucial role in the release of virions from the membranes of infected cells. It is presumed that p6 and functionally related proteins from other viruses act as adapters, recruiting cellular factors to the budding site. This interaction is mediated by so-called late domains within the viral proteins. Previous studies had suggested that virus release from the plasma membrane shares elements with the cellular endocytosis machinery. Since protein phosphorylation is known to be a regulatory mechanism in these processes, we have investigated the phosphorylation of HIV-1 structural proteins. Here we show that p6 is the major phosphoprotein of HIV-1 particles. After metabolic labeling of infected cells with [ortho-32P]phosphate, we found that phosphorylated p6 from infected cells and from virus particles consisted of several forms, suggesting differential phosphorylation at multiple sites. Apparently, phosphorylation occurred shortly before or after the release of p6 from Gag and involved only a minor fraction of the total virion-associated p6 molecules. Phosphoamino acid analysis indicated phosphorylation at Ser and Thr, as well as a trace of Tyr phosphorylation, supporting the conclusion that multiple phosphorylation events do occur. In vitro experiments using purified virus revealed that endogenous or exogenously added p6 was efficiently phosphorylated by virion-associated cellular kinase(s). Inhibition experiments suggested that a cyclin-dependent kinase or a related kinase, most likely ERK2, was involved in p6 phosphorylation by virion-associated enzymes.

Activation of the Mason-Pfizer monkey virus protease within immature capsids in vitro

For all retroviruses, the completion of the viral budding process correlates with the activation of the viral protease by an unknown mechanism, and, as the structural (Gag) polyproteins are cleaved by the viral protease, maturation of the immature virus-like particle into an infectious virion. Unlike most retroviruses, the Mason-Pfizer monkey virus Gag polyproteins assemble into immature capsids within the cytoplasm of the cell before the viral budding event. The results reported here describe a unique experimental system in which Mason-Pfizer monkey virus immature capsids are removed from the cell, and the protease is activated in vitro by the addition of a reducing agent. The cleavage of the protease from the precursor form is a primary event, which proceeds with a half time of 14 min, and is followed by authentic processing of the Gag polyproteins. Activity of the viral protease in vitro depends on pH, with an increase in catalytic rates at acidic and neutral pH. The initiation of protease activity within immature capsids in vitro demonstrates that viral protease activity is sensitive to oxidation-reduction conditions, and that the viral protease can be activated in the absence of viral budding.

Sequences in the 5' leader of Mason-Pfizer monkey virus which affect viral particle production and genomic RNA packaging: development of MPMV packaging cell lines

We used a series of deletion mutations in the 5' untranslated region of the prototype D type retrovirus, Mason-Pfizer Monkey Virus (MPMV), to analyse RNA encapsidation. A region was identified upstream of the major splice donor which reduced particle production but had a proportionally greater effect on RNA packaging. A small deletion downstream of the splice donor had little effect on RNA production and caused no significant packaging defect. A large deletion encompassing the end of the primer binding site down to the splice donor had a dramatic effect, disrupting viral protein synthesis. Stable cell lines were produced containing packaging-defective virus. These first-generation packaging cell lines were used to package and transfer an MPMV-based vector.

Type D retrovirus Gag polyprotein interacts with the cytosolic chaperonin TRiC

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

Role of matrix protein in the type D retrovirus replication cycle: importance of the arginine residue at position 55

We previously reported that a mutant of Mason-Pfizer monkey virus (M-PMV), which has an amino acid substitution in the matrix (MA) protein at position 55, MA-R55W, showed altered viral morphogenesis, reduced glycoprotein incorporation, and loss of infectivity. In this report, we show that two additional amino acid substitutions at this site in MA, R55F and R55Y, also result in similar altered morphogenesis, Env incorporation, and infectivity, demonstrating that these changes are not specific for the substitution of tryptophan in place of arginine 55. Attempts to isolate second site infectious revertants from cells transfected with the R55W mutant genome resulted only in the recovery of infectious viruses in which the codon at position 55 had reverted to one encoding arginine. In contrast, no revertants were obtained from the phenylalanine and tyrosine mutants in which three nucleotide changes had been engineered into the arginine codon. These results confirm that the arginine residue at position 55 is critical for intracellular targeting of M-PMV Gag molecules and support the concept that as part of a cytoplasmic transport retention signal R55 interacts with cellular trafficking components rather than other regions of Gag.

A cell-line-specific defect in the intracellular transport and release of assembled retroviral capsids

Retrovirus assembly involves a complex series of events in which a large number of proteins must be targeted to a point on the plasma membrane where immature viruses bud from the cell. Gag polyproteins of most retroviruses assemble an immature capsid on the cytoplasmic side of the plasma membrane during the budding process (C-type assembly), but a few assemble immature capsids deep in the cytoplasm and are then transported to the plasma membrane (B- or D-type assembly), where they are enveloped. With both assembly phenotypes, Gag polyproteins must be transported to the site of viral budding in either a relatively unassembled form (C type) or a completely assembled form (B and D types). The molecular nature of this transport process and the host cell factors that are involved have remained obscure. During the development of a recombinant baculovirus/insect cell system for the expression of both C-type and D-type Gag polyproteins, we discovered an insect cell line (High Five) with two distinct defects that resulted in the reduced release of virus-like particles. The first of these was a pronounced defect in the transport of D-type but not C-type Gag polyproteins to the plasma membrane. High Five cells expressing wild-type Mason-Pfizer monkey virus (M-PMV) Gag precursors accumulate assembled immature capsids in large cytoplasmic aggregates similar to a transport-defective mutant (MA-A18V). In contrast, a larger fraction of the Gag molecules encoded by the M-PMV C-type morphogenesis mutant (MA-R55W) and those of human immunodeficiency virus were transported to the plasma membrane for assembly and budding of virions. When pulse-labeled Gag precursors from High Five cells were fractionated on velocity gradients, they sedimented more rapidly, indicating that they are sequestered in a higher-molecular-mass complex. Compared to Sf9 insect cells, the High Five cells also demonstrate a defect in the release of C-type virus particles. These findings support the hypothesis that host cell factors are important in the process of Gag transport and in the release of enveloped viral particles.

In vitro infection of ovine cell lines by Jaagsiekte sheep retrovirus

Sheep pulmonary adenomatosis (SPA), also known as jaagsiekte or ovine pulmonary carcinoma, is a contagious lung cancer of sheep, originating from type II pneumocytes and Clara cells. Previous studies have implicated a type D retrovirus (jaagsiekte sheep retrovirus [JSRV]) as the causative agent of SPA. We recently isolated a proviral clone of JSRV from an animal with a spontaneous case of SPA (JSRV(21)) and showed that it harbors an infectious and oncogenic virus. This demonstrated that JSRV is necessary and sufficient to induce SPA. A major impediment in research on JSRV has been the lack of an in vitro tissue culture system for the virus. The experiments reported here show the first successful in vitro infection with this virus, using the JSRV(21) clone. JSRV(21) virus was obtained by transiently transfecting human 293T cells with a plasmid containing the JSRV(21) provirus driven by the human cytomegalovirus immediate-early promoter. Virus produced in this manner exhibited reverse transcriptase (RT) activity that banded at 1.15 g/ml in sucrose density gradients. Infection of concentrated JSRV(21) into ovine choroid plexus (CP), testes (OAT-T3), turbinate (FLT), and intestinal carcinoma (ST6) cell lines resulted in establishment of infection as measured by PCR amplification. Evidence that this reflected genuine infection included the fact that heat inactivation of the virus eliminated it, the levels of viral DNA increased with passage of the infected cells, and the infected cells released active RT as measured by the sensitive product enhancement RT assay. The RT activity released from the infected cells banded at 1.15 g/ml, and JSRV(21) provirus was transmitted from infected cells to uninfected ones by cocultivation. However, the amount of virus released from infected cells was low. These results suggest that the JSRV receptor is present on many ovine cell types and that the observed restriction of JSRV expression in vivo to tumor cells might be controlled by factors other than the viral receptor. Finally we tagged the U3 of pJSRV(21) with the bacterial supF gene, an amber suppressor tRNA gene. The resulting clone, termed pJSRV(supF), is infectious in vitro. It may be a useful tool for future studies on viral DNA integration, since the normal sheep genome contains 15 to 20 copies of highly JSRV-related endogenous sequences that cross-react with many JSRV hybridization probes.

Molecular cloning and cell-specific growth characterization of polymorphic variants of type D serogroup 2 simian retroviruses

Simian retroviruses (SRVs), the etiological agent of a spontaneous Simian acquired immunodeficiency syndrome, endemically infects large percentages of Asian macaques housed in biomedical research colonies and severely compromises the effective use of these species as a viable research animal. We recently described the molecular cloning of a serogroup 2 SRV, D2/RHE/OR, which causes mild immunosuppression in rhesus macaques. A restriction site variant, D2/RHE/OR/V1, has also been recovered from severely ill animals endemically infected with D2/RHE/OR. We now report the complete nucleotide sequences of D2/RHE/OR and D2/RHE/OR/V1. Both infectious molecular clones retain the genetic structure typical of type D SRVs (5' LTR-gag-prt-pol-env-3'LTR) and encode identically sized 8105-bp proviruses. D2/RHE/OR and D2/RHE/OR/V1 are 99.3% similar at the amino acid level, exhibiting only 17 residue differences, of which 10 are located in the envelope glycoproteins. The molecular clones and reciprocal chimeric viruses were used to assess the contribution of different genetic domains to virus infectivity in a T cell infection assay. These experiments indicate that D2/RHE/OR has a reduced ability to infect specific T cell lines, especially Hut-78 and MT-4 cells, and that the envelope gene is not the sole determinant of in vitro tropism.

Characterization of human MMTV-like (HML) elements similar to a sequence that was highly expressed in a human breast cancer: further definition of the HML-6 group

Previously, we found a retroviral sequence, HML-6.2BC1, to be expressed at high levels in a multifocal ductal breast cancer from a 41-year-old woman who also developed ovarian carcinoma. The sequence of a human genomic clone (HML-6.28) selected by high-stringency hybridization with HML-6.2BC1 is reported here. It was 99% identical to HML-6.2BC1 and gave the same restriction fragments as total DNA. HML-6.28 is a 4.7-kb provirus with a 5'LTR, truncated in RT. Data from two similar genomic clones and sequences found in GenBank are also reported. Overlaps between them gave a rather complete picture of the HML-6.2BC1-like human endogenous retroviral elements. Work with somatic cell hybrids and FISH localized HML-6.28 to chromosome 6, band p21, close to the MHC region. The causal role of HML-6.28 in breast cancer remains unclear. Nevertheless, the ca. 20 Myr old HML-6 sequences enabled the definition of common and unique features of type A, B, and D (ABD) retroviruses. In Gag, HML-6 has no intervening sequences between matrix and capsid proteins, unlike extant exogenous ABD viruses, possibly an ancestral feature. Alignment of the dUTPase showed it to be present in all ABD viruses, but gave a phylogenetic tree different from trees made from other ABD genes, indicating a distinct phylogeny of dUTPase. A conserved 24-mer sequence in the amino terminus of some ABD envelope genes suggested a conserved function.

Three active forms of aspartic proteinase from Mason-Pfizer monkey virus

Mason-Pfizer monkey virus (M-PMV) proteinase, released by the autocatalytic cleavage of Gag-Pro and Gag-Pro-Pol polypeptide precursors, catalyzes the processing of viral precursors to yield the structural proteins and enzymes of the virion. In retroviruses, usually only one proteolytically active form of proteinase exists. Here, we describe an unusual feature of M-PMV, the existence of three active forms of a retroviral proteinase with molecular masses of 17, 13, and 12 kDa as determined by mass spectroscopy. These forms arise in vitro by self-processing of a 26-kDa proteinase precursor. We have developed a process for isolation of each truncated product and demonstrate that all three forms display proteolytic activity. Amino acid analyses, as well as the determination of N- and C-terminal sequences, revealed that the N-termini of all three forms are identical, confirming that in vitro autoprocessing of the 17-kDa form occurs at the C-terminus to yield the truncated forms. The 17-kDa form and the newly described 13-kDa form of proteinase were identified in virions collected from the rhesus monkey CMMT cell line chronically infected with M-PMV, confirming that multiple forms exist in vivo.

A proline-rich motif (PPPY) in the Gag polyprotein of Mason-Pfizer monkey virus plays a maturation-independent role in virion release

Virus assembly represents one of the last steps in the retrovirus life cycle. During this process, Gag polyproteins assemble at specific sites within the cell to form viral capsids and induce membrane extrusion (viral budding) either as assembly progresses (type C virus) or following formation of a complete capsid (type B and type D viruses). Finally, the membrane must undergo a fusion event to pinch off the particle in order to release a complete enveloped virion. Structural elements within the MA region of the Gag polyprotein define the route taken to the plasma membrane and direct the process of virus budding. Results presented here suggest that a distinct region of Gag is necessary for virus release. The pp24 and pp16 proteins of the type D retrovirus Mason-Pfizer monkey virus (M-PMV) are phosphoproteins that are encoded in the gag gene of the virus. The pp16 protein is a C-terminally located cleavage product of pp24 and contains a proline-rich motif (PPPY) that is conserved among the Gag proteins of a wide variety of retroviruses. By performing a functional analysis of this coding region with deletion mutants, we have shown that the pp16 protein is dispensable for capsid assembly but essential for virion release. Moreover, additional experiments indicated that the virus release function of pp16 was abolished by the deletion of only the PPPY motif and could be restored when this motif alone was reinserted into a Gag polyprotein lacking the entire pp16 domain. Single-amino-acid substitutions for any of the residues within this motif confer a similar virion release-defective phenotype. It is unlikely that the function of the proline-rich motif is simply to inhibit premature activation of protease, since the PPPY deletion blocked virion release in the context of a protease-defective provirus. These results demonstrate that in type D retroviruses a PPPY motif plays a key role in a late stage of virus budding that is independent of and occurs prior to virion maturation.

The three-dimensional solution structure of the matrix protein from the type D retrovirus, the Mason-Pfizer monkey virus, and implications for the morphology of retroviral assembly

The Mason-Pfizer monkey virus (M-PMV) is the prototype of the type D retroviruses. In type B and D retroviruses, the Gag protein pre-assembles before association with the membrane, whereas in type C retroviruses (lentiviruses, BLV/HTLV group) Gag is targeted efficiently to the plasma membrane, where the particle formation occurs. The N-terminal domain of Gag, the matrix protein (MA), plays a critical role in determining this morphogenic difference. We have determined the three-dimensional solution structure of the M-PMV MA by heteronuclear nuclear magnetic resonance. The protein contains four alpha-helices that are structurally similar to the known type C MA structures. This similarity implies possible common assembly units and membrane-binding mechanisms for type C and B/D retroviruses. In addition to this, the interpretation of mutagenesis data has enabled us to identify, for the first time, the structural basis of a putative intracellular targeting motif.

Type D retrovirus markers in healthy Africans from Guinea

Sixteen matching sera and DNA samples from healthy African blood donors living in rural areas of Guinea were analysed for the presence of type D retrovirus markers. Screening for the antibodies against structural proteins of Mason-Pfizer monkey virus (M-PMV) was carried out by Western blot with a purified M-PMV as an antigen. Eight out of 16 sera samples were found to contain antibodies against at least two gag gene-coded proteins, and three of these were weakly positive against env gene-coded protein. Using PCR amplification and Southern hybridization, we detected M-PMV-like gag sequences in 11 out of 16 samples and env-related sequences in 8 out of 16 samples. Six DNAs were found to contain both M-PMV gag- and env-related sequences. Restriction endonuclease analysis of the PCR-amplified gag sequences from two individuals and direct DNA sequencing analysis of the amplimers confirmed their M-PMV-like origin. Detection of antibodies and M-PMV-related sequences in blood donors from Guinea, but not in French or Algerian blood donors, indicated exogenous SRV infection in humans from certain geographic areas of Western Africa.

Linker insertion mutations in the human immunodeficiency virus type 1 gag gene: effects on virion particle assembly, release, and infectivity

The phenotypes of a series of mutant human immunodeficiency virus type 1 proviruses with linker insertion and deletion mutations within the gag coding region were characterized. These mutants were tested for their ability to make and release viral particles in COS7 cells and for their viability in vivo. Of the 12 mutant proviruses, 4 did not make extracellular virion particles when transfected into COS7 cells. All four of these mutants had mutations in the C-terminal domain of CA. These mutants appeared to have defects both in the ability to accumulate high-molecular-weight intracellular structures containing Gag and Pol products and in the ability to release virion particles. Seven of the mutant proviruses retained the ability to make, release, and process virion particles from COS7 cells. These particles contained the Env glycoprotein, viral genomic RNA, and the mature products of the Gag and Gag-Pol polyproteins, yet they were noninfectious or poorly infectious. The defect in these mutants appears to be in one of the early steps of the viral life cycle. Thus, multiple regions throughout Gag appear to be important in mediating the early steps of the viral life cycle.

Importance of p12 protein in Mason-Pfizer monkey virus assembly and infectivity

Mason-Pfizer monkey virus (M-PMV) represents the prototype type D retrovirus, characterized by the assembly of intracytoplasmic A-type particles within the infected-cell cytoplasm. These immature particles migrate to the plasma membrane, where they are released by budding. The gag gene of M-PMV encodes a novel protein, p12, just 5' of the major capsid protein (CA) p27 on the polyprotein precursor. The function of p12 is not known, but an equivalent protein is found in mouse mammary tumor virus and is absent from the type C retroviruses. In order to determine whether the p12 protein plays a role in the intracytoplasmic assembly of capsids, a series of in-frame deletion mutations were constructed in the p12 coding domain. The mutant gag genes were expressed by a recombinant vaccinia virus-T7 polymerase-based system in CV-1 cells or in the context of the viral genome in COS-1 cells. In both of these high-level expression systems, mutant Gag precursors were competent to assemble but were not infectious. In contrast, when stable transfectant HeLa cell lines were established, assembly of the mutant precursors into capsids was drastically reduced. Instead, the polyprotein precursors remained predominantly soluble in the cytoplasm. These results show that while p12 is not required for the intracytoplasmic assembly of M-PMV capsids, under the conditions of low-level protein biosynthesis seen in virus-infected cells, it may assist in the stable association of polyprotein precursors for capsid assembly. Moreover, the presence of the p12 coding domain is absolutely required for the infectivity of M-PMV virions.

Mutational analysis of the major homology region of Mason-Pfizer monkey virus by use of saturation mutagenesis

The major capsid (CA) protein of retroviruses possesses a stretch of 20 amino acids, called the major homology region (MHR), which is evolutionarily conserved and invariant in location within the primary sequence of the protein. The function of this region was investigated by examining the effect of random single-amino-acid substitutions within the central 13 positions of the MHR on the life cycle of Mason-Pfizer monkey virus (M-PMV), an immunosuppressive D-type retrovirus. When these mutants were subcloned into an M-PMV proviral vector and expressed in COS cells, one of two major phenotypes was observed. The first group, containing three mutants bearing drastic amino acid substitutions, was unable to assemble capsids in the cytoplasm of the host cell. The second and more common group of mutants was able to assemble and release virions, but these either displayed greatly reduced levels of infectivity or were completely noninfectious. Included within this second group were two mutants with unusual phenotypes; mutant D158Y exhibited a novel cleavage site for the viral protease that resulted in cleavage of the major capsid protein, p27 (CA), within the MHR, whereas mutant F156L appeared to have lost a major site for antibody recognition within the mature CA protein. The results of this mutagenic analysis suggest that changes in the MHR sequence can interfere with the assembly of viral capsids and block an early stage of the infection cycle of M-PMV.

Effect of retroviral proteinase inhibitors on Mason-Pfizer monkey virus maturation and transmembrane glycoprotein cleavage

Mason-Pfizer monkey virus (M-PMV) is the prototype type D retrovirus which preassembles immature intracytoplasmic type A particles within the infected cell cytoplasm. Intracytoplasmic type A particles are composed of uncleaved polyprotein precursors which upon release are cleaved by the viral proteinase to their constituent mature proteins. This results in a morphological change in the virion described as maturation. We have investigated the role of the viral proteinase in virus maturation and infectivity by inhibiting the function of the enzyme through mutagenesis of the proteinase gene and by using peptide inhibitors originally designed to block human immunodeficiency virus type 1 proteinase activity. Mutation of the active-site aspartic acid, Asp-26, to asparagine abrogated the activity of the M-PMV proteinase but did not affect the assembly of noninfectious, immature virus particles. In mutant virions, the transmembrane glycoprotein (TM) of M-PMV, initially synthesized as a cell-associated gp22, is not cleaved to gp20, as is observed with wild-type virions. This demonstrates that the viral proteinase is responsible for this cleavage event. Hydroxyethylene isostere human immunodeficiency virus type 1 proteinase inhibitors were shown to block M-PMV proteinase cleavage of the TM glycoprotein and Gag-containing precursors in a dose-dependent manner. The TM cleavage event was more sensitive than cleavage of the Gag precursors to inhibition. The infectivity of treated particles was reduced significantly, but experiments showed that inhibition of precursor and TM cleavage may be at least partially reversible. These results demonstrate that the M-PMV aspartyl proteinase is activated in released virions and that the hydroxyethylene isostere proteinase inhibitors used in this study exhibit a broad spectrum of antiretroviral activity.

A viral protease-mediated cleavage of the transmembrane glycoprotein of Mason-Pfizer monkey virus can be suppressed by mutations within the matrix protein

The envelope glycoprotein precursor of retroviruses undergoes proteolytic cleavage in the Golgi complex to yield the mature surface and transmembrane (TM) glycoproteins of the virus. We report here that the TM glycoprotein of Mason-Pfizer monkey virus undergoes a second proteolytic processing event during a late maturation step that can follow virus release and Gag polyprotein cleavage. Cleavage results in the conversion of the cell-associated TM glycoprotein (gp22) to a virus-associated gp20. Processing continues after virus release and yields virions that contain predominantly gp20. A mutation within the active site of the Mason-Pfizer monkey virus aspartyl protease was shown to block both TM glycoprotein cleavage and the processing of the Gag polyprotein precursor. The role of the viral protease in cleavage of the TM glycoprotein localizes the cleavage site to the cytoplasmic domain of this protein. Surprisingly, point mutations within the matrix (MA) coding region of the gag gene can affect the extent to which gp22 is processed to gp20 and in one case [p10(MA)-A79V] results in greater than 90% inhibition of gp22 cleavage. The data provide genetic evidence of a specific interaction between the capsid proteins and the cytoplasmic domain of the TM glycoprotein of a retrovirus. This interaction is required for cytoplasmic domain cleavage to occur and may play a critical role in virus assembly and viral infectivity.

A single amino acid substitution within the matrix protein of a type D retrovirus converts its morphogenesis to that of a type C retrovirus

Two different morphogenic processes of retroviral capsid assembly have been observed: the capsid is either assembled at the plasma membrane during the budding process (type C), or preassembled within the cytoplasm (types B and D). We describe here a gag mutant of Mason-Pfizer monkey virus, a type D retrovirus, in which a tryptophan substituted for an arginine in the matrix protein results in efficient assembly of capsids at the plasma membrane through a morphogenic process similar to that of type C retroviruses. We conclude that a type D retrovirus Gag polyprotein contains an additional, dominant signal that prevents immediate transport of precursors from the site of biosynthesis to the plasma membrane. Instead, they are directed to and retained at a cytoplasmic site where a concentration sufficient for self-assembly into capsids occurs. Thus, capsid assembly processes for different retroviruses appear to differ only in the intracellular site to which capsid precursors are directed.

Study of the gag-coded polyproteins of type D retroviruses. Block of glycosylation inhibits the processing of major structural protein

The study of the gag-coded glycosylated polyprotein (gPr78gag) from type D retrovirus-producing cells is reported. It is shown that glycosylation inhibitors tunicamycin and 2-deoxy-D-glucose are blocking the synthesis of gPr78gag. This demonstrates that gPr78gag is a N-glycan. The protein part of gPr78gag is supposedly identified (MW = 65 k). The inhibitors used partially stopped the processing of gag-coded polyprotein Pr78gag into core protein p27 as shown in pulse-chase experiments.

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.

Preassembled capsids of type D retroviruses contain a signal sufficient for targeting specifically to the plasma membrane

The capsids of Mason-Pfizer monkey virus (M-PMV), an immunosuppressive type D retrovirus, are preassembled in the infected cell cytoplasm and are then transported to the plasma membrane, where they are enveloped in a virus glycoprotein-containing lipid bilayer. The role of viral glycoprotein in intracellular transport of M-PMV capsids was investigated with a spontaneous mutant (5A) of M-PMV, which we show here to be defective in envelope glycoprotein biosynthesis. DNA sequence analysis of the env gene of mutant 5A reveals a single nucleotide deletion in the middle of the gene, which results in the synthesis of a truncated form of the envelope glycoprotein. Evidence is presented showing that the mutant glycoprotein is not expressed at the cell surface but is retained in the endoplasmic reticulum. Normal levels of gag-pro-pol precursor polyproteins are made and processed in mutant genome-transfected cells, and high levels of noninfectious particles lacking viral glycoprotein are released with normal kinetics into the culture medium. No intracisternal budding of capsids is observed. We conclude that viral glycoprotein is required neither for targeting preassembled capsids of M-PMV to the plasma membrane for final maturation nor for the budding process. Since the presence or absence of M-PMV glycoprotein at the site of budding does not affect the efficiency or kinetics of the targeting process, the preassembled capsid of M-PMV, in contrast to those of intracisternal type A particles, appears to have an intrinsic signal for intracellular transport to the plasma membrane.

Expression of avian reticuloendotheliosis virus envelope confers host resistance

We constructed two reticuloendotheliosis virus (REV) envelope gene expression plasmids, one containing the REV-A envelope gene, the other the spleen necrosis virus (SNV) envelope gene. Cell lines were generated by transfecting each of the REV envelope plasmids into D17 cells, a canine cell line. The levels of REV envelope glycoprotein in the cell lines were assayed by immunoprecipitating the envelope glycoproteins from lysates of cells that were labeled with [35S]methionine. Virological challenge assays determined the degree of resistance of each of the cell lines to REV-A or SNV infection. The expression of either envelope gene protected the cells from infection by either REV-A or SNV virus. Several cell lines were significantly more resistant to REV infection than the parental D17 cells, and two lines were 25,000-fold more resistant, approaching the resistance of REV-infected D17 cells to reinfection. The resistant cell lines were not able to confer resistance to susceptible cells by cocultivation. The level of resistance was correlated with the uniformity of expression of the REV envelope glycoproteins by the individual cells in a cell line and not with the absolute level of expression by the population of cells.

Molecular cloning of a type D retrovirus from human cells (PMFV) and its homology to simian acquired immunodeficiency type D retroviruses

Unintegrated circular proviral DNA of a type D retrovirus (PMFV) isolated from a permanent human cell line was molecularly cloned in the bacteriophage vector L47.1 and subcloned in the plasmid vector pGEM-2. A restriction endonuclease map of PMFV DNA was established using 10 different enzymes for single and multiple digestions of closed circular and cloned DNA molecules. By restriction endonuclease analysis cloned PMFV DNA represented full-length viral DNA with one long terminal repeat (LTR). The comparison of the physical map of cloned PMFV to those of other cloned type D retroviruses revealed closest homology to the map of retrovirus D/New England (pD398) and SAIDS retrovirus type 1 (SRV-1). The relatedness of PMFV to further type D retroviruses (Mason-Pfizer monkey virus, MPMV; SAIDS retrovirus type 2, SRV-2) was also demonstrated by cross-hybridization of cloned DNAs under different stringencies (i) using full-length genomic probes of PMFV, MPMV, and SRV-2 and (ii) by DNA sequence analysis of regions of the group specific antigen (gag) protease (prt), polymerase (pol), and envelope (env) genes.

Simian retrovirus-D serotype 1 (SRV-1) envelope glycoproteins gp70 and gp20: expression in yeast cells and identification of specific antibodies in sera from monkeys that recovered from SRV-1 infection

The gp70 and transmembrane gp20 envelope proteins of simian retrovirus-D serotype 1 (SRV-1) were expressed in Saccharomyces cerevisiae as fusion proteins with human superoxide dismutase (SOD). Expression of the SOD-gp70 and SOD-gp20 sequences yielded fusion proteins of 52 and 29 kilodaltons, respectively. The yeast-expressed SRV-1 envelope proteins were used in an enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies in the sera of rhesus macaques that recovered from SRV-1. Sera from 47 of 49 such monkeys tested positive for antibodies to the SOD-gp70 fusion protein, while 45 of 49 reacted positively to SOD-gp20. None of 26 SRV-1-nonexposed monkeys tested positive in either ELISA. Monkeys immunized with the recombinant SRV-1 gp20 and gp70 proteins made good ELISA and Western blot (immunoblot) antibodies to whole SRV-1. This antibody was not neutralizing in vitro, however.

Biochemical and immunological characterization of structural proteins from retrovirus-D/New England and comparison to Mason-Pfizer monkey virus and permanent human fibroblast virus

Biochemical and immunological properties of retrovirus-D/New England (here referred as R-D/NE) recently isolated at the New England Regional Primate Research Center, Southborough, Ma, from a rhesus monkey with acquired immune deficiency syndrome were investigated and compared to the prototype type D retroviruses Mason-Pfizer monkey virus (MPMV) and permanent human fibroblast virus (PMFV) isolated from a breast carcinoma of a rhesus monkey and a continuous human cell line, respectively. The polypeptide composition of R-D/NE propagated in a human lymphoid B cell line (Raji cells) has been investigated using SDS-polyacrylamide gel electrophoresis. Staining with Coomassie blue and labelling with 14C amino acids revealed seven viral polypeptides with molecular weights of 4,000, 10,000, 12,000, 15,000, 18,000, 27,000, and 80,000 Da which were also shared by MPMV and PMFV. The 80,000 Da protein was shown to be a glycoprotein by incorporation of 3H glucosamine. The 18,000 Da protein was identified as a phosphoprotein of R-D/NE. p18 structural proteins of MPMV and PMFV represent phosphoproteins of their respective viruses as well. All three phosphorylated proteins contain O-phosphoserine as major phosphoamino acid. The comparison of tryptic peptide maps of the major internal structural proteins of R-D/NE, MPMV, and PMFV revealed a striking similarity among p 10/p 12 and p 15. proteins. A minor difference was detected among the tryptic peptide digests of p 4 and p 27 proteins. Antiserum against p 15 of MPMV showed a significantly weaker binding to R-D/NE than to MPMV and PMFV at high dilutions.

Two efficient ribosomal frameshifting events are required for synthesis of mouse mammary tumor virus gag-related polyproteins

The primary translation products of retroviral pol genes are polyproteins initiated in an upstream gene (gag). To investigate the manner in which the gag-initiated polyproteins of the mouse mammary tumor virus are produced, we determined the nucleotide sequence of a 1.8-kilobase DNA fragment that spans the region between gag and pol in the C3H strain of mouse mammary tumor virus. The sequence reveals three overlapping open reading frames: the first encodes products of gag (p27gag and p14gag); the second encodes a protein domain of unknown function (termed X) that is highly related to a similarly positioned sequence in simian type D retroviruses and the viral protease (pro); and the third encodes the reverse transcriptase. The reading frames are organized to permit uninterrupted readthrough from gag to pol if ribosomal frameshifts occur in the -1 direction within each of the two overlapping regions, one of which is 16 nucleotides in length and the other 13 nucleotides. Cell-free translation of RNA containing these overlap regions shows that fusion of the reading frames by ribosomal frameshifting occurs efficiently: about one-fourth of the ribosomes traversing the gag-X/pro overlap and one-tenth traversing the X/pro-pol overlap shift frames, generating gag-related polyproteins in ratios similar to those observed in vivo. Synthetic oligonucleotides containing either of the overlap regions inserted into novel contexts do not induce frameshifting; hence the overlapping portions of the reading frames are not sufficient to induce a frameshift event, and a larger sequence context or secondary structure may be implicated.