Monoclonal antibody to the amino-terminal L sequence of murine leukemia virus glycosylated gag polyproteins demonstrates their unusual orientation in the cell membrane

Murine Leukemia Virus Glycosylated Gag Reduces Murine SERINC5 Protein Expression at Steady-State Levels via the Endosome/Lysosome Pathway to Counteract SERINC5 Antiretroviral Activity

Glycosylated Gag (glycoGag) is an accessory protein expressed by most gammaretroviruses, including murine leukemia virus (MLV). MLV glycoGag not only enhances MLV replication and disease progression but also increases human immunodeficiency virus type 1 (HIV-1) infectivity as Nef does. Recently, SERINC5 (Ser5) was identified as the target for Nef, and the glycoGag Nef-like activity has been attributed to the Ser5 antagonism. Here, we investigated how glycoGag antagonizes Ser5 using MLV glycoMA and murine Ser5 proteins. We confirm previous observations that glycoMA relocalizes Ser5 from plasma membrane to perinuclear punctated compartments and the important role of its Y₃₆XXL₃₉ motif in this process. We find that glycoMA decreases Ser5 expression at steady-state levels and identify two other glycoGag crucial residues, P31 and R63, for the Ser5 downregulation. The glycoMA and Ser5 interaction is detected in live cells using a bimolecular fluorescence complementation assay. Ser5 is internalized via receptor-mediated endocytosis and relocalized to Rab5⁺ early, Rab7⁺ late, and Rab11⁺ recycling endosomes by glycoMA. Although glycoMA is not polyubiquitinated, the Ser5 downregulation requires Ser5 polyubiquitination via the K48- and K63-linkage, resulting in Ser5 destruction in lysosomes. Although P31, Y36, L39, and R63 are not required for glycoMA interaction with Ser5, they are required for Ser5 relocalization to lysosomes for destruction. In addition, although murine Ser1, Ser2, and Ser3 exhibit very poor antiviral activity, they are also targeted by glycoMA for lysosomal destruction. We conclude that glycoGag has a broad activity to downregulate SERINC proteins via the cellular endosome/lysosome pathway, which promotes viral replication.IMPORTANCE MLV glycoGag not only enhances MLV replication but also increases HIV-1 infectivity similarly as Nef. Recent studies have discovered that both glycoGag and Nef antagonize a novel host restriction factor Ser5 and promote viral replication. Compared to Nef, the glycoGag antagonism of Ser5 is still poorly understood. MLV glycoGag is a transmembrane version of the structural Gag protein with an extra 88-amino-acid leader region that determines its activity. We now show that glycoGag interacts with Ser5 in live cells and internalizes Ser5 via receptor-mediated endocytosis. Ser5 is polyubiquitinated and relocalized to endosomes and lysosomes for massive destruction. In addition to the previously identified tyrosine-based sorting signal, we find two more important residues for Ser5 relocalization and downregulation. We also find that the Ser5 sensitivity to glycoGag is conserved in the SERINC family. Together, our findings highlight the important role of endosome/lysosome pathway in the enhancement of viral replication by viral proteins.

Selective Isolation of Retroviruses from Extracellular Vesicles by Intact Virion Immunoprecipitation

There exists a wide variety of techniques to isolate and purify viral particles from cell culture supernatants. However, these techniques vary greatly in ease of use, purity, yield and impact on viral structural integrity. Most importantly, it is becoming evident that secreted extracellular vesicles (EVs) co-purify with retroviruses using nearly all purification methods due to nearly indistinguishable biophysical characteristics such as size, buoyant density and nucleic acid content. Recently, our group has illustrated a means of isolating intact and highly enriched retroviral virions from EV-containing cell supernatants using an immunoprecipitation approach targeting the viral envelope glycoprotein of the Moloney Murine Leukemia Virus ( Renner et al., 2018 ). This technique, that we call intact virion immunoprecipitation (IVIP), enabled us to characterize the accessibility of epitopes on the surface of these retroviruses and assess the orientation of the virus-encoded integral membrane protein Glycogag (gPr80) in the viral envelope. Proper implementation of this protocol enables fast, simple and reproducible preparations of intact and highly purified retroviral particles devoid of detectable EV contaminants.

Deaminase-Dead Mouse APOBEC3 Is an In Vivo Retroviral Restriction Factor

The apolipoprotein B editing complex 3 (APOBEC3) proteins are potent retroviral restriction factors that are under strong positive selection, both in terms of gene copy number and sequence diversity. A common feature of all the members of the APOBEC3 family is the presence of one or two cytidine deamination domains, essential for cytidine deamination of retroviral reverse transcripts as well as packaging into virions. Several studies have indicated that human and mouse APOBEC3 proteins restrict retrovirus infection via cytidine deaminase (CD)-dependent and -independent means. To understand the relative contribution of CD-independent restriction in vivo, we created strains of transgenic mice on an APOBEC3 knockout background that express a deaminase-dead mouse APOBEC3 due to point mutations in both CD domains (E73Q/E253Q). Here, we show that the CD-dead APOBEC3 can restrict murine retroviruses in vivo Moreover, unlike the wild-type protein, the mutant APOBEC3 is not packaged into virions but acts only as a cell-intrinsic restriction factor that blocks reverse transcription by incoming viruses. Finally, we show that wild-type and CD-dead mouse APOBEC3 can bind to murine leukemia virus (MLV) reverse transcriptase. Our findings suggest that the mouse APOBEC3 cytidine deaminase activity is not required for retrovirus restriction.IMPORTANCE APOBEC3 proteins are important host cellular restriction factors essential for restricting retrovirus infection by causing mutations in the virus genome and by blocking reverse transcription. While both methods of restriction function in vitro, little is known about their role during in vivo infection. By developing transgenic mice with mutations in the cytidine deamination domains needed for enzymatic activity and interaction with viral RNA, we show that APOBEC3 proteins can still restrict in vivo infection by interacting with reverse transcriptase and blocking its activity. These studies demonstrate that APOBEC3 proteins have evolved multiple means for blocking retrovirus infection and that all of these means function in vivo.

Full-Length Glycosylated Gag of Murine Leukemia Virus Can Associate with the Viral Envelope as a Type I Integral Membrane Protein

The glycosylated Gag protein (gPr80) of murine leukemia viruses (MLVs) has been shown to exhibit multiple roles in facilitating retrovirus release, infection, and resistance to host-encoded retroviral restriction factors, such as APOBEC3, SERINC3, and SERINC5. One way in which gPr80 helps MLVs to escape host innate immune restriction is by increasing capsid stability, a feature that protects viral replication intermediates from being detected by cytosolic DNA sensors. gPr80 also increases the resistance of MLVs to deamination and restriction by mouse APOBEC3 (mA3). How the gPr80 accessory protein, with its three N-linked glycosylation sites, contributes to these resistance mechanisms is still not fully understood. Here we further characterized the function of gPr80 and, more specifically, revealed that the asparagines targeted for glycosylation in gPr80 also contribute to capsid stability through their parallel involvement in the Pr65 Gag structural polyprotein. In fact, we demonstrate that sensitivity to deamination by the mA3 and human A3 proteins is directly linked to capsid stability. We also show that full-length gPr80 is detected in purified viruses. However, our results suggest that gPr80 is inserted in the NexoCcyto orientation of a type I integral membrane protein. Additionally, our experiments have revealed the existence of a large population of Env-deficient virus-like particles (VLPs) harboring gPr80 inserted in the opposite (NcytoCexo) polarity, which is typical of type II integral membrane proteins. Overall this study provides new insight into the complex nature of the MLV gPr80 accessory protein.IMPORTANCE Viruses have evolved numerous strategies to infect, spread in, and persist in their hosts. Here we analyze the details of how the MLV-encoded glycosylated Gag (gPr80) protein protects the virus from being restricted by host innate immune defenses. gPr80 is a variant of the structural Pr65 Gag protein with an 88-amino-acid extended leader sequence that directs the protein for translation and glycosylation in the endoplasmic reticulum. This study dissects the specific contributions of gPr80 glycans and capsid stability in helping the virus to infect cells, spread, and counteract the effects of the host intrinsic restriction factor APOBEC3. Overall this study provides further insight into the elusive role of the gPr80 protein.

Novel host restriction factors implicated in HIV-1 replication

Human immunodeficiency virus-1 (HIV-1) is known to interact with multiple host cellular proteins during its replication in the target cell. While many of these host cellular proteins facilitate viral replication, a number of them are reported to inhibit HIV-1 replication at various stages of its life cycle. These host cellular proteins, which are known as restriction factors, constitute an integral part of the host's first line of defence against the viral pathogen. Since the discovery of apolipoprotein B mRNA-editing enzyme 3G (APOBEC3G) as an HIV-1 restriction factor, several human proteins have been identified that exhibit anti-HIV-1 restriction. While each restriction factor employs a distinct mechanism of inhibition, the HIV-1 virus has equally evolved complex counter strategies to neutralize their inhibitory effect. APOBEC3G, tetherin, sterile alpha motif and histidine-aspartate domain 1 (SAMHD1), and trim-5α are some of the best known HIV-1 restriction factors that have been studied in great detail. Recently, six novel restriction factors were discovered that exhibit significant antiviral activity: endoplasmic reticulum α1,2-mannosidase I (ERManI), translocator protein (TSPO), guanylate-binding protein 5 (GBP5), serine incorporator (SERINC3/5) and zinc-finger antiviral protein (ZAP). The focus of this review is to discuss the antiviral mechanism of action of these six restriction factors and provide insights into the probable counter-evasion strategies employed by the HIV-1 virus. The recent discovery of new restriction factors substantiates the complex host-pathogen interactions occurring during HIV-1 pathogenesis and makes it imperative that further investigations are conducted to elucidate the molecular basis of HIV-1 replication.

SERINC3 and SERINC5 restrict HIV-1 infectivity and are counteracted by Nef

HIV-1 Nef and the unrelated murine leukemia virus glycoGag strongly enhance the infectivity of HIV-1 virions produced in certain cell types in a clathrin-dependent manner. Here we show that Nef and glycoGag prevent the incorporation of the multipass transmembrane proteins SERINC3 and SERINC5 into HIV-1 virions to an extent that correlates with infectivity enhancement. Silencing of SERINC3 together with SERINC5 precisely phenocopied the effects of Nef and glycoGag on HIV-1 infectivities. The infectivity of nef-deficient virions increased more than 100-fold when produced in double-knockout human CD4+ T cells that lack both SERINC3 and SERINC5, and re-expression experiments confirmed that the absence of SERINC3 and SERINC5 accounted for the infectivity enhancement. Furthermore, SERINC3 and SERINC5 together restricted HIV-1 replication, and this restriction was evaded by Nef. SERINC3 and SERINC5 are highly expressed in primary human HIV-1 target cells, and inhibiting their downregulation by Nef is a potential strategy to combat HIV/AIDS.

The Nef-like effect of murine leukemia virus glycosylated gag on HIV-1 infectivity is mediated by its cytoplasmic domain and depends on the AP-2 adaptor complex

Human immunodeficiency virus type 1 (HIV-1) Nef enhances the infectivity of progeny virions. However, Nef is dispensable for the production of HIV-1 virions of optimal infectivity if the producer cells are superinfected with certain gammaretroviruses. In the case of the ecotropic Moloney murine leukemia virus (M-MLV), the Nef-like effect is mediated by the glycosylated Gag (glycoGag) protein. We now show that the N-terminal intracellular domain of the type II transmembrane protein glycoGag is responsible for its effect on HIV-1 infectivity. In the context of a fully active minimal M-MLV glycoGag construct, truncations of the cytoplasmic domain led to a near total loss of activity. Furthermore, the cytoplasmic domain of M-MLV glycoGag was fully sufficient to transfer the activity to an unrelated type II transmembrane protein. Although the intracellular region of glycoGag is relatively poorly conserved even among ecotropic and xenotropic MLVs, it was also fully sufficient for the rescue of nef-deficient HIV-1 when derived from a xenotropic virus. A mutagenic analysis showed that only a core region of the intracellular domain that exhibits at least some conservation between murine and feline leukemia viruses is crucial for activity. In particular, a conserved YXXL motif in the center of this core region was critical. In addition, expression of the μ2 subunit of the AP-2 adaptor complex in virus producer cells was essential for activity. We conclude that the ability to enhance HIV-1 infectivity is a conserved property of the MLV glycoGag cytoplasmic domain and involves AP-2-mediated endocytosis.

IMPORTANCE

The Nef protein of HIV-1 and the entirely unrelated glycosylated Gag (glycoGag) protein of a murine leukemia virus (MLV) similarly enhance the infectiousness of HIV-1 particles by an unknown mechanism. MLV glycoGag is an alternative version of the structural viral Gag protein with an extra upstream region that provides a cytosolic domain and a plasma membrane anchor. We now show for the first time that the cytosolic domain of MLV glycoGag contains all the information needed to enhance HIV-1 infectivity and that this function of the cytosolic domain is conserved despite limited sequence conservation. Within the cytosolic domain, a motif that resembles a cellular sorting signal is critical for activity. Furthermore, the enhancement of HIV-1 infectivity depends on an endocytic cellular protein that is known to interact with such sorting signals. Together, our findings implicate the endocytic machinery in the enhancement of HIV-1 infectivity by MLV glycoGag.

HIV-1 Nef responsiveness is determined by Env variable regions involved in trimer association and correlates with neutralization sensitivity

HIV-1 Nef and the unrelated murine leukemia virus glycoGag similarly enhance the infectivity of HIV-1 virions. We now show that the effects of Nef and glycoGag are similarly determined by variable regions of HIV-1 gp120 that control Env trimer association and neutralization sensitivity. Whereas neutralization-sensitive X4-tropic Env proteins conferred high responsiveness to Nef and glycoGag, particles bearing neutralization-resistant R5-tropic Envs were considerably less affected. The profoundly different Nef/glycoGag responsiveness of a neutralization-resistant and a neutralization-sensitive R5-tropic Env could be switched by exchanging their gp120 V1/V2 regions, which also switches their neutralization sensitivity. Within V1/V2, the same determinants governed Nef/glycoGag responsiveness and neutralization sensitivity, indicating that these phenotypes are mechanistically linked. The V1/V2 and V3 regions, which form an apical trimer-association domain, together determined the Nef and glycoGag responsiveness of an X4-tropic Env. Our results suggest that Nef and glycoGag counteract the inactivation of Env spikes with relatively unstable apical trimer-association domains.

Murine leukemia virus glycosylated Gag blocks apolipoprotein B editing complex 3 and cytosolic sensor access to the reverse transcription complex

Pathogenic retroviruses have evolved multiple means for evading host restriction factors such as apolipoprotein B editing complex (APOBEC3) proteins. Here, we show that murine leukemia virus (MLV) has a unique means of counteracting APOBEC3 and other cytosolic sensors of viral nucleic acid. Using virus isolated from infected WT and APOBEC3 KO mice, we demonstrate that the MLV glycosylated Gag protein (glyco-Gag) enhances viral core stability. Moreover, in vitro endogenous reverse transcription reactions of the glyco-Gag mutant virus were substantially inhibited compared with WT virus, but only in the presence of APOBEC3. Thus, glyco-Gag rendered the reverse transcription complex in the viral core resistant to APOBEC3. Glyco-Gag in the virion also rendered MLV resistant to other cytosolic sensors of viral reverse transcription products in newly infected cells. Strikingly, glyco-Gag mutant virus reverted to glyco-Gag-containing virus only in WT and not APOBEC3 KO mice, indicating that counteracting APOBEC3 is the major function of glyco-Gag. Thus, in contrast to the HIV viral infectivity factor protein, which prevents APOBEC3 packaging in the virion, the MLV glyco-Gag protein uses a unique mechanism to counteract the antiviral action of APOBEC3 in vivo--namely, protecting the reverse transcription complex in viral cores from APOBEC3. These data suggest that capsid integrity may play a critical role in virus resistance to intrinsic cellular antiviral resistance factors that act at the early stages of infection.

The cellular protein La functions in enhancement of virus release through lipid rafts facilitated by murine leukemia virus glycosylated Gag

Murine leukemia viruses (MuLVs) encode two forms of Gag polyprotein: the precursor for the viral core proteins (Pr65^gag for Moloney MuLV [M-MuLV]) and a longer glycosylated form (glyco-gag, or gPr80^gag). gPr80^gag is translated from the same unspliced viral RNA as Pr65^gag, from an upstream in-frame CUG initiation codon. As a result, gPr80^gag contains 88 unique N-terminal amino acids that include a signal peptide that conducts gPr80^gag into the rough endoplasmic reticulum, where it is glycosylated, exported to the cell surface, and cleaved into two proteins of 55 and 40 kDa. The amino-terminal 55-kDa protein remains cell associated with the 88 unique amino acids exposed to the cytosol. We previously showed that gPr80^gag facilitates efficient M-MuLV release through lipid rafts. In this report, we found that the unique N-terminal domain of gPr80^gag is sufficient to facilitate enhanced M-MuLV particle release from transfected 293T cells. A search for cellular proteins involved in gPr80^gag function led to cellular La protein. Overexpression of mouse or human La enhanced M-MuLV particle release in the absence of glyco-gag, and the released virus had a reduced buoyant density characteristic of increased cholesterol content. Moreover, small interfering RNA (siRNA) knockdown of human La abolished glyco-gag enhancement of M-MuLV release. These results implicate La as a cellular protein involved in M-MuLV glyco-gag function. We also found that overexpression of mouse or human La could enhance HIV-1 release in the absence of gPr80^gag. Therefore, M-MuLV and HIV-1 may share a pathway for release through lipid rafts involving La.

Retroviruses cause diseases such as leukemia and AIDS. An important aspect of viral replication is how viruses are released from infected cells. We previously found that a unique protein encoded by murine leukemia viruses (MuLVs), glyco-gag (or gPr80^gag), enhances efficient virus release through cholesterol-rich membrane subdomains called lipid rafts. In this study, we found that the N-terminal domain of gPr80^gag is sufficient to enhance viral release. A search for cellular proteins that participate in gPr80^gag function led to cellular La protein. Overexpression of La phenocopied glyco-gag in enhancing M-MuLV release, and knockdown of La abolished glyco-gag function. M-MuLV glyco-gag also enhanced release of HIV-1, as did overexpression La in the absence of glyco-gag. Thus, M-MuLV and HIV-1 may share a cellular pathway for release through lipid rafts involving La. These results may also be relevant for other viruses that are released through lipid rafts.

The glycosylated Gag protein of a murine leukemia virus inhibits the antiretroviral function of APOBEC3

APOBEC proteins have evolved as innate defenses against retroviral infections. Human immunodeficiency virus (HIV) encodes the Vif protein to evade human APOBEC3G; however, mouse retroviruses do not encode a Vif homologue, and it has not been understood how they evade mouse APOBEC3. We report here a murine leukemia virus (MuLV) that utilizes its glycosylated Gag protein (gGag) to evade APOBEC3. gGag is critical for infection of in vitro cell lines in the presence of APOBEC3. Furthermore, a gGag-deficient virus restricted for replication in wild-type mice replicates efficiently in APOBEC3 knockout mice, implying a novel role of gGag in circumventing the action of APOBEC3 in vivo.

Mutation of capsid protein phosphorylation sites abolishes cauliflower mosaic virus infectivity

The cauliflower mosaic virus (CaMV) capsid protein is derived by bidirectional processing of the precapsid protein (CP56). We expressed several derivatives of CP56 in Escherichia coli and used them as substrates for virus-associated kinase and casein kinase II purified from plant cells. Three serine residues located at the N terminus of the mature viral protein CP44 were identified as phosphorylation targets. A mutation of one of them in the viral context had little or no effect on viral infectivity, but a mutation of all three serines abolished infectivity. The mapping of phosphorylation sites in CP44, but not CP39 or CP37, and immunodetection of the Zn finger motif in CP44 and CP39, but not CP37, support the model that CP39 is produced from CP44 by N-terminal processing and CP37 is produced from CP39 by C-terminal processing. We discuss the possible role of phosphorylation in the processing and assembly of CaMV capsid protein.

Translational control of gene expression: role of IRESs and consequences for cell transformation and angiogenesis

Translational control of gene expression has, over the last 10 years, become appreciated as an important process in its regulation in eukaryotes. Among a series of control mechanisms exerted at the translational level, the use of alternative codons provides a very subtle means of increasing gene diversity by expressing several proteins from a single mRNA. The internal ribosome entry sites (IRESs) act as specific translational enhancers that allow translation initiation to occur independently of the classic cap-dependent mechanism, in response to specific stimuli and under the control of different trans-acting factors. It is striking to observe that the two processes mostly concern genes coding for control proteins such as growth factors, protooncogenes, angiogenesis factors, and apoptosis regulators. Here, we focus on the translational regulation of four mRNAs, with both IRESs and alternative initiation codons, which are the messengers of retroviral murine leukemia virus, fibroblast growth factor 2, vascular endothelial growth factor, and protooncogene c-myc. Four of them are involved in cell transformation and/or angiogenesis, with important consequences for such translation regulations in these pathophysiological processes.

N-terminal cleavage fragment of glycosylated Gag is incorporated into murine oncornavirus particles

Glycosylated Gag (Glycogag) is a transmembrane protein encoded by murine and feline oncornaviruses. While the protein is dispensible for virus replication, Glycogag-null mutants of a neurovirulent murine oncornavirus are slow to spread in vivo and exhibit a loss of pathogenicity. The function of this protein in the virus life cycle, however, is not understood. Glycogag is expressed at the plasma membrane of infected cells but has not been detected in virions. In the present study we have reexamined this issue and have found an N-terminal cleavage fragment of Glycogag which was pelleted by high-speed centrifugation and sedimented in sucrose density gradients at the same bouyant density as virus particles. Its association with virions was confirmed by velocity sedimentation through iodixanol, which effectively separated membrane microvesicles from virus particles. Furthermore, the apparent molecular weight of the virion-associated protein was different from that of the protein extracted from the plasma membrane, suggesting some level of specificity or selectivity of incorporation.

Truncated forms of the dual function human ASCT2 neutral amino acid transporter/retroviral receptor are translationally initiated at multiple alternative CUG and GUG codons

The sodium-dependent neutral amino acid transporter type 2 (ASCT2) was recently identified as a cell surface receptor for endogenously inherited retroviruses of cats, baboons, and humans as well as for horizontally transmitted type-D simian retroviruses. By functional cloning, we obtained 10 full-length 2.9-kilobase pair (kbp) cDNAs and two smaller identical 2.1-kbp cDNAs that conferred susceptibility to these viruses. Compared with the 2.9-kbp cDNA, the 2.1-kbp cDNA contains exonic deletions in its 3' noncoding region and a 627-bp 5' truncation that eliminates sequences encoding the amino-terminal portion of the full-length ASCT2 protein. Although expression of the truncated mRNA caused enhanced amino acid transport and viral receptor activities, the AUG codon nearest to its 5' end is flanked by nucleotides that are incompatible with translational initiation and the next in-frame AUG codon is far downstream toward the end of the protein coding sequence. Interestingly, the 5' region of the truncated ASCT2 mRNA contains a closely linked series of CUG(Leu) and GUG(Val) codons in optimal consensus contexts for translational initiation. By deletion and site-directed mutagenesis, cell-free translation, and analyses of epitope-tagged ASCT2 proteins synthesized intracellularly, we determined that the truncated mRNA encodes multiple ASCT2 isoforms with distinct amino termini that are translationally initiated by a leaky scanning mechanism at these CUG and GUG codons. Although the full-length ASCT2 mRNA contains a 5'-situated AUG initiation codon, a significant degree of leaky scanning also occurred in its translation. ASCT2 isoforms with relatively short truncations were active in both amino acid transport and viral reception, whereas an isoform with a 79-amino acid truncation that lacked the first transmembrane sequence was active only in viral reception. We conclude that ASCT2 isoforms with truncated amino termini are synthesized in mammalian cells by a leaky scanning mechanism that employs multiple alternative CUG and GUG initiation codons.

The human immunodeficiency virus type 1 gag gene encodes an internal ribosome entry site

Several retroviruses have recently been shown to promote translation of their gag gene products by internal ribosome entry. In this report, we show that mRNAs containing the human immunodeficiency virus type 1 (HIV-1) gag open reading frame (ORF) exhibit internal ribosome entry site (IRES) activity that can promote translational initiation of Pr55(gag). Remarkably, this IRES activity is driven by sequences within the gag ORF itself and is not dependent on the native gag 5'-untranslated region (UTR). This cap-independent mechanism for Pr55(gag) translation may help explain the high levels of translation of this protein in the face of major RNA structural barriers to scanning ribosomes found in the gag 5' UTR. The gag IRES activity described here also drives translation of a novel 40-kDa Gag isoform through translational initiation at an internal AUG codon found near the amino terminus of the Pr55(gag) capsid domain. Our findings suggest that this low-abundance Gag isoform may be important for wild-type replication of HIV-1 in cultured cells. The activities of the HIV-1 gag IRES may be an important feature of the HIV-1 life cycle and could serve as a novel target for antiretroviral therapeutic strategies.

Characterization of glycosylated Gag expressed by a neurovirulent murine leukemia virus: identification of differences in processing in vitro and in vivo

The neuroinvasiveness of a chimeric murine retrovirus, CasFrKP (KP), is dependent on the expression of glycosylated Gag (gp85gag). This viral protein is the product of alternate translation initiation 88 codons upstream of and in frame with the initiation codon of pr65gag, the precursor of the viral core proteins. Although expression of glycosylated Gag affects virus spread in the spleen, it appears not to affect virus spread in vitro in fibroblast cell lines (J. L. Portis et al., J. Virol. 68:3879-3887, 1994). The differential effects of this protein in vitro and in vivo have not been explained, and its function is unknown. We have here compared the in vitro processing of this molecule with that expressed in spleens of infected mice. In vitro, gp85gag was cleaved near the middle of the molecule, releasing the C-terminal half (containing capsid and nucleocapsid domains of pr65gag) as a secreted glycoprotein. The N-terminal half of the protein was associated with the plasma membrane as a approximately 55-kDa glycoprotein bearing the matrix domain of pr65gag as well as the N-terminal 88 residue L domain. This processing scheme was also observed in vivo, although two differences were seen. There were differences in N-linked glycosylation of the secreted form of the protein expressed in the spleen. In addition, whereas the membrane-associated species assumed the orientation of a type II integral membrane protein (N(cyto) C(exo)) in fibroblasts in vitro, a subpopulation of spleen cells was detected in which the N terminus of the protein was exposed at the cell surface. These results suggest that the differential effects of glycosylated Gag expression in vivo and in vitro may be related to differences in posttranslational processing of the protein.

Alternative translation initiation of the Moloney murine leukemia virus mRNA controlled by internal ribosome entry involving the p57/PTB splicing factor

Moloney murine leukemia virus (Mo-MuLV) genomic mRNA codes for two gag precursors by alternative initiations of translation. An AUG codon governs the synthesis of the retroviral capsid proteins precursor, whereas a CUG codon directs the synthesis of a glycosylated cell surface antigen, the gross cell surface antigen. Control of the relative synthesis of the two precursors is crucial for MuLV infectivity and pathology. Furthermore, the MuLV mRNA leader sequence is very long and should inhibit translation according to the classical scanning model. This suggests a different translation initiation mechanism allowing gag efficient expression. We demonstrate, by using bicistronic vectors expressed in COS-7 cells, that the Mo-MuLV mRNA leader drives translation initiation by internal ribosome entry. We have localized the internal ribosome entry site (IRES) between the two initiation codons. This 126 nucleotide long IRES implies an oligopyrimidine tract located 45 nucleotides upstream of AUG codon. UV cross-linking and affinity chromatography experiments show that the PTB/p57 splicing factor specifically interacts with this oligopyrimidine tract. The MuLV IRES controls alternative translation initiation by activating the capsid protein precursor expression. This gag translational enhancer could exist in other retroviruses.

Identification of a sequence in the unique 5' open reading frame of the gene encoding glycosylated Gag which influences the incubation period of neurodegenerative disease induced by a murine retrovirus

Neonatal inoculation of the wild-mouse ecotropic retrovirus CasBrE (clone 15-1) causes a noninflammatory spongiform neurodegenerative disease with an incubation period of > or = 6 months. Introduction of sequences from Friend murine leukemia virus (clone FB29) into the genome of CasBrE results in a marked shortening of the incubation period. The FB29 sequences which influence the incubation period were previously localized to the 5' leader sequence of the viral genome (M. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 66:3298-3305, 1992). In the current study, we constructed a series of chimeric viruses consisting of the genome of CasBrE containing various segments of the leader sequence from FB29. A 41-nucleotide element (positions 481 through 521) near the 3' end of the leader was found to have a strong influence on the incubation period. This element influenced the kinetics of virus replication and/or spread in nonneuronal tissues, a property which was shown previously to determine the extent of central nervous system infection (M. Czub, F. J. McAtee, and J. L. Portis, J. Virol. 66:3298-3305, 1992). Curiously, this sequence had no demonstrable effect on virus replication in vitro in a fibroblastic cell line from Mus dunni. This segment encodes 14 of the unique 88-amino-acid N terminus of pr75gag, the precursor of a glycosylated form of the gag polyprotein which is expressed at the cell surface. Previous in vitro studies of mutants of Moloney murine leukemia virus lacking expression of glycosylated Gag failed to reveal a function for this protein in virus replication. We mutated the Kozak consensus sequence around the initiation codon for this protein in the chimeric virus CasFrKP, a virus which induces neurologic disease with a short (18- to 23-day) incubation period. M. dunni cells infected with the mutants lacked detectable cell surface Gag, but, compared with CasFrKP, no effect on replication kinetics in vitro was observed. In contrast, there was a marked slowing of the replication kinetics in vivo and a dramatic attenuation of neurovirulence. These studies indicate that glycosylated Gag has an important function in virus replication and/or spread in the mouse and further suggest that the sequence of its N terminus is a critical, though likely indirect, determinant of neurovirulence.

A nonstructural gag-encoded glycoprotein precursor is necessary for efficient spreading and pathogenesis of murine leukemia viruses

In addition to the Gag-Pol and Env precursors whose translation initiates at AUG codons, murine, feline, and simian type C oncoviruses also express glycosylated Gag-Pol precursors (glycoGag), glycoGag translation is initiated at CUG codons located upstream of the Gag AUG initiation codon. In contrast to Gag, glycoGag is translocated into the endoplasmic reticulum and is absent from virions. Since glycoGag has been described to be dispensable ex vivo, we investigated the in vivo effects of a glycoGag- mutation in the Friend murine leukemia virus (F-MuLV). F-MuLV induces severe early hemolytic anemia and subsequent erythroleukemia within 2 months after inoculation of newborn mice. We obtained a glycoGag- F-MuLV, strain H5, by inserting an octanucleotide linker downstream of the CUG codon leading to the reading of a stop codon in all reading frames upstream of the Gag AUG. F-MuLV H5 did not induce severe early hemolytic anemia, and latency of erythroleukemia was significantly increased most likely because of an approximately 1-week delay in the in vivo spreading. Accordingly, induction of recombinant polytropic viruses was also significantly delayed. Close examination of ex vivo spreading kinetics also showed a slower dissemination of F-MuLV H5. Western blot (immunoblot) performed after inoculation of newborn mice with this glycoGag- virus indicated the emergence of new glycoGag+ viruses. PCR analyses with F-MuLV-specific primers demonstrated in vivo pseudoreversions restoring the glycoGag reading frame. Our results demonstrated that glycoGag expression is positively selected and essential for full spreading and pathogenic abilities.

Characterization of monoclonal antibodies against the human immunodeficiency virus matrix protein, p17gag: identification of epitopes exposed at the surfaces of infected cells

Eight monoclonal antibodies reactive with the matrix protein of human immunodeficiency virus type 1 (HIV-1), p17gag, were isolated from rats which had been immunized with solubilized HIV-1 lysate. The epitope specificities of these antibodies were determined with a series of synthetic peptides representing overlapping regions of p17. Six of the antibodies were mapped to three distinct regions of p17, while two antibodies (G11g1 and G11h3) reacted only with intact recombinant p17, suggesting that they were directed against conformational or discontinuous epitopes. All the antibodies bound to HIV-infected cells which had been permeabilized with acetone, but only G11g1 and G11h3 reacted with live HIV-infected cells. Specificity studies with diverse virus strains demonstrated that these two antibodies recognized distinct epitopes, one which was group specific for HIV-1, and one which was shared with HIV type 2 and simian immunodeficiency virus. Binding competition studies indicated that these epitopes were proximal in native p17. Despite their reactivity with intact cells, these two antibodies did not possess appreciable virus-neutralizing activity. These results indicate that a form of p17 is expressed on the surfaces of live HIV-infected cells which is accessible to some, but not all, antibodies against p17. These cell surface molecules may play a role in the generation of antibodies against p17gag that are characteristic of early stages of HIV infection, and they may act as natural targets for the immune system and as potential targets for immunotherapy of HIV-infected cells.

Mechanism of escape of endogenous murine leukemia virus emv-14 from recognition by anti-AKR/Gross virus cytolytic T lymphocytes

It was previously shown that spleen cells from endogenous ecotropic murine leukemia virus emv-14+ AKXL-5 mice fail to stimulate an anti-AKR/Gross virus cytolytic T-lymphocyte (CTL) response in a mixed lymphocyte culture with primed C57BL/6 responder spleen cells, whereas spleen cells from AKXL strains carrying the very similar emv-11 provirus do stimulate a response (Green and Graziano, Immunogenetics 23:106-110, 1986). We wished to determine whether the lack of response with AKXL-5 spleen cells was at the level of recognition between effector cell and target cell and whether the relevant mutation was within the emv-14 provirus. It is shown here that EMV-negative SC-1 fibroblast cells transfected with the major histocompatibility complex class I Kb gene and infected with virus isolated from the AKXL-5 strain (SC.Kb/5 cells) were not lysed by H-2b-restricted anti-AKR/Gross virus CTL. SC.Kb cells infected with virus isolated from emv-11+ strains, however, were efficiently lysed by anti-AKR/Gross virus CTL, indicating that there is nothing intrinsic to EMV-infected SC.Kb cells that would prevent them from being recognized and lysed efficiently by anti-AKR/Gross virus CTL. Analysis of virus expression for the infected SC.Kb cells by XC plaque assay and by flow cytometry indicated that emv-14 virus expression for SC.Kb/5 cells was not significantly different from that for emv-11-containing SC.Kb/9 or SC.Kb/21 cells. These data show that the mutation responsible for the lack of CTL recognition and lysis is at the level of recognition between target cell and effector cell. Furthermore, these data strongly suggest that the mutation is within the emv-14 genome. Flow cytometry experiments with monoclonal antibodies against a number of viral determinants indicated that there was no gross mutation detectable in the viral determinants analyzed. The data suggest that the relevant mutation may be a point mutation or a small insertion or deletion within a coding sequence that is critical for CTL recognition.

The secretory core protein of human hepatitis B virus is expressed on the cell surface

Human hepatitis B virus (HBV) causes acute and chronic liver disease, which can result in tumor formation. An as yet unexplained phenomenon is that virus elimination usually correlates with the development of antibodies directed against the HBeAg, a secretory HBV core gene product which can be detected in the serum of infected patients. Expression of HBeAg in a human hepatoma cell line by using recombinant vaccinia viruses revealed that the HBeAg is not only secreted from HBeAg-producing cells but also incorporated into the outer cell membrane. No membrane-expressed core gene product could be detected when the cytoplasmic core protein (HBcAg) was expressed. Immune sera from patients who developed anti-HBe antibodies efficiently recognized the membrane-bound HBeAg, suggesting that surface-expressed HBeAg can serve as a target for an antibody-mediated elimination of HBV-infected cells.

CUG initiation codon used for the synthesis of a cell surface antigen coded by the murine leukemia virus

Murine leukemia virus (MuLV) codes for two precursors of the group-specific antigens, Pr65gag and Pr75gag, in vivo. While Pr65gag is the precursor to the virion structural proteins, Pr75gag undergoes glycosylation and is found on the surface of the infected cell as gp85gag, and it is thought to play a role in virus maturation and spread. Pr65gag synthesis starts at an AUG codon within a favourable initiation context (AAUAUGG at positions 618 to 624). The gp85gag start codon is upstream but its precise location is not known. To map the initiation codon of gp85gag, we used deletion and site-directed mutagenesis of the leader sequence of MuLV RNA and in vitro translation of the RNAs. Synthesis of the MuLV gp85gag protein appears to be initiated at a CUG codon located within a favourable context (ACCCUGG at positions 354 to 359 for Moloney-MuLV). The possible function of gp85gag was investigated by expressing Moloney-MuLV and Friend-MuLV proviral DNA and mutants deficient for gp85gag synthesis in mouse and rat cells. The results indicate that the gp85gag protein probably facilitates the spread of virus infection in tissue culture.

Syngeneic monoclonal antibodies directed against Rauscher virus-induced myeloid leukemic cells: isolation and characterization

Hybridomas producing syngeneic monoclonal antibodies (MAbs) were prepared by fusion of spleen cells of BALB/c mice, which were immunized with sublethal doses of RMB-I cells. This cell line originates from a Rauscher virus (R-MuLV)-induced myeloid leukemia and forms tumors when re-inoculated into mice. MAbs were characterized as regards their reactivity against virally and non-virally induced cell lines. Two selected MAbs, IC5F5 and 4D2B4, were analyzed further. Their binding to subcellular structures was determined, and so were the properties of the antigens to which they are directed. MAb IC5F5 is of the IgG2A and 4D2B4 of the IgG2b subclass. Both bind to R-MuLV-infected or -transformed cell lines and are not mutually competitive. The antibodies do not react with other murine and human myeloid leukemic cells. As shown by immuno-electron microscopy, these MAbs have affinity to the cell membrane of non-virus producing RMB-I cells. When lysates of purified virus were analyzed, the MAbs were found to be directed to the gag precursor protein Pr65, and one of them (IC5F5) also to be directed to the core protein p12. In RMB-I cells, binding occurs to a 50-kDa glycoprotein and 2 proteins of 26 and 29 kDa. Since RMB-I cells do not produce virus, but express aberrant viral proteins, these MAbs are tumor-specific and useful for immunotherapy.

Structural study of adenovirus type 2 fibre using anti-fibre and anti-peptide sera

Peptides corresponding to the N- and C-extremities of the adenovirus 2 fibre polypeptide were synthesized, coupled to tetanus toxoid and injected into rabbits. Two sera were obtained: the anti-NTT serum and the anti-CTT serum. These sera and an anti-native-fibre serum were used to study fragments generated by hydrochloric acid cleavage of the fibre. The 44-Kd fragment corresponding to the 2/3 N-terminal part of the molecule retained its antigenic reactivity. This is consistent with a shaft structure for this part of the fibre. The anti-peptide sera were used to orientate the fibre, i.e., to determine the site of anchorage of this protein in the penton base. First, immunorevelation of blots of enzymatic digests of native or dissociated penton suggested that the N-extremity of the fibre was involved in the assembly of this protein in the penton base. Second, attempts were made to determine the accessibility of the fibre ends in the penton structure by ELISA assays and by immunorevelation of penton in Western blots. The results agreed with the proposed orientation derived from study of the enzymatic digests. Since the 2 anti-peptide sera and the peptides were unable to affect viral adsorption, it was not possible to determine how the fibre is orientated with respect to the cell receptor. However, the anti-peptide sera were found to inhibit viral production slightly.

Hepatitis B and AIDS and the promise for their control by vaccines

Human hepatitis B virus and human immunodeficiency virus (HIV), that causes AIDS, share attributes and possible evolutionary relationships. The specific requirements for inducing effective immunity against hepatitis B and HIV are discussed. Lessons learned in studies of hepatitis B and its vaccine may find application in creating a vaccine against HIV.

Homing receptors and metastasis

As discussed in the preceding sections, there are several indications that the lymphocyte homing receptors involved in the normal process of lymphocyte recirculation are also relevant to the behavior of metastatic cells. Cell fusion experiments indicate that previously nonmetastatic cells can acquire metastatic capacity from fusion with normal lymphocytes. Murine T lymphomas that bear high levels of functional homing receptors can metastasize to peripheral lymphoid organs, whereas those lymphomas lacking homing receptors cannot. Virtually all lymph node metastases of lymphomas contain a high proportion of MEL-14hi cells, even if the primary tumor has been selected to be relatively deficient in these cells. Further investigations of the biology of lymphocyte homing receptors will reveal whether or not there are additional lymphocyte homing receptors and will clarify the role of lymphocyte homing receptors in metastasis. Antibodies against three lymphocyte homing receptors could therefore be useful for diagnosis and treatment of metastatic disease.

The duck hepatitis B virus pre-C region encodes a signal sequence which is essential for synthesis and secretion of processed core proteins but not for virus formation

Analysis of the serum of duck hepatitis B virus (DHBV)-infected ducks has revealed the presence of C-terminally truncated viral core proteins (e antigens). These proteins are glycosylated and therefore were not released from infected cells by lysis but rather by active secretion, indicating that the DHBV core protein can be synthesized alternatively as a cytoplasmic or a secretory protein. Transient expression of cloned wild-type DHBV DNA and of a specifically designed viral mutant in a human hepatoma cell line (Hep-G2) showed that the DHBV core gene promoter is active in differentiated human liver cells and that synthesis and secretion of the processed core proteins are dependent on the expression of the pre-C region, a small open reading frame which precedes the core gene. In addition, these experiments showed that the mechanism of core protein processing and secretion is conserved between DHBV and the human hepatitis B virus and therefore might be important for the hepatitis B virus life cycle in general. In spite of this, intrahepatic injection of the pre-C mutant into uninfected ducks resulted in viremia without concomitant e-antigen synthesis, indicating that virus formation is independent of pre-C expression.