Glycosylation and intracellular transport of membrane glycoproteins encoded by murine leukemia viruses. Inhibition by amino acid analogues and by tunicamycin

Glycosylation in HIV-1 envelope glycoprotein and its biological implications

Glycosylation of HIV-1 envelope proteins (Env gp120/gp41) plays a vital role in viral evasion from the host immune response, which occurs through the masking of key neutralization epitopes and the presentation of the Env glycosylation as ‘self’ to the host immune system. Env glycosylation is generally conserved, yet its continual evolution plays an important role in modulating viral infectivity and Env immunogenicity. Thus, it is believed that Env glycosylation, which is a vital part of the HIV-1 architecture, also controls intra- and inter-clade genetic variations. Discerning intra- and inter-clade glycosylation variations could therefore yield important information for understanding the molecular and biological differences between HIV clades and may assist in effectively designing Env-based immunogens and in clearly understanding HIV vaccines. This review provides an in-depth perspective of various aspects of Env glycosylation in the context of HIV-1 pathogenesis.

Unique N-linked glycosylation of CasBrE Env influences its stability, processing, and viral infectivity but not its neurotoxicity

The envelope protein (Env) from the CasBrE murine leukemia virus (MLV) can cause acute spongiform neurodegeneration analogous to that induced by prions. Upon central nervous system (CNS) infection, Env is expressed as multiple isoforms owing to differential asparagine (N)-linked glycosylation. Because N-glycosylation can affect protein folding, stability, and quality control, we explored whether unique CasBrE Env glycosylation features could influence neurovirulence. CasBrE Env possesses 6/8 consensus MLV glycosylation sites (gs) but is missing gs3 and gs5 and contains a putative site (gs*). Twenty-nine mutants were generated by modifying these three sites, individually or in combination, to mimic the amino acid sequence in the nonneurovirulent Friend 57 MLV. Three basic viral phenotypes were observed: replication defective (dead; titer < 1 focus-forming unit [FFU]/ml), replication compromised (RC) (titer = 10(2) to 10(5) FFU/ml); and wild-type-like (WTL) (titer > 10(5) FFU/ml). Env protein was undetectable in dead mutants, while RC and WTL mutants showed variations in Env expression, processing, virus incorporation, virus entry, and virus spread. The newly introduced gs3 and gs5 sites were glycosylated, whereas gs* was not. Six WTL mutants tested in mice showed no clear attenuation in disease onset or severity versus controls. Furthermore, three RC viruses tested by neural stem cell (NSC)-mediated brainstem dissemination also induced acute spongiosis. Thus, while unique N-glycosylation affected structural features of Env involved in protein stability, proteolytic processing, and virus assembly and entry, these changes had minimal impact on CasBrE Env neurotoxicity. These findings suggest that the Env protein domains responsible for spongiogenesis represent highly stable elements upon which the more variable viral functional domains have evolved.

Solution structure of the monovalent lectin microvirin in complex with Man(alpha)(1-2)Man provides a basis for anti-HIV activity with low toxicity

Lectins that bind surface envelope glycoprotein gp120 of HIV with high avidity can potently inhibit viral entry. Yet properties such as multivalency that facilitate strong interactions can also cause nonspecific binding and toxicity. The cyanobacterial lectin microvirin (MVN) is unusual as it potently inhibits HIV-1 with negligible toxicity compared with cyanovirin-N (CVN), its well studied antiviral homolog. To understand the structural and mechanistic basis for these differences, we solved the solution structure of MVN free and in complex with its ligand Manα(1-2)Man, and we compared specificity and time windows of inhibition with CVN and Manα(1-2)Man-specific mAb 2G12. We show by NMR and analytical ultracentrifugation that MVN is monomeric in solution, and we demonstrate by NMR that Manα(1-2)Man-terminating carbohydrates interact with a single carbohydrate-binding site. Synchronized infectivity assays show that 2G12, MVN, and CVN inhibit entry with distinct kinetics. Despite shared specificity for Manα(1-2)Man termini, combinations of the inhibitors are synergistic suggesting they recognize discrete glycans and/or dynamic glycan conformations on gp120. Entry assays employing amphotropic viruses show that MVN is inactive, whereas CVN potently inhibits both. In addition to demonstrating that HIV-1 can be inhibited through monovalent interactions, given the similarity of the carbohydrate-binding site common to MVN and CVN, these data suggest that gp120 behaves as a clustered glycan epitope and that multivalent-protein interactions achievable with CVN but not MVN are required for inhibition of some viruses.

Activation of the N-terminally truncated form of the Stk receptor tyrosine kinase Sf-Stk by Friend virus-encoded gp55 is mediated by cysteine residues in the ecotropic domain of gp55 and the extracellular domain of Sf-Stk

Friend virus induces an erythroleukemia in susceptible mice that is initiated by the interaction of the Friend virus-encoded glycoprotein gp55 with the erythropoietin (Epo) receptor and the product of the host Fv2 gene, a naturally occurring truncated form of the Stk receptor tyrosine kinase (Sf-Stk). We have previously demonstrated that the activation of Sf-Stk, recruitment of a Grb2/Gab2/Stat3 signaling complex, and induction of Pu.1 expression by Stat3 are required for the development of the early stage of Friend disease both in vitro and in vivo. Here we demonstrate that the interaction of gp55 with Sf-Stk is dependent on cysteine residues in the ecotropic domain of gp55 and the extracellular domain of Sf-Stk. Point mutation of these cysteine residues or deletion of these domains inhibits the ability of gp55 to interact with Sf-Stk, resulting in the inability of these proteins to promote the Epo-independent growth of erythroid progenitor cells. We also demonstrate that the interaction of gp55 with Sf-Stk does not promote dimerization of Sf-Stk but results in enhanced phosphorylation of Sf-Stk and the relocalization of Sf-Stk from the cytosol to the plasma membrane. Finally, we demonstrate that a constitutively active form of Sf-Stk (Sf-StkM330T), as well as its human counterpart, Sf-Ron, promotes Epo-independent colony formation in the absence of gp55 and that this response is also dependent on the cysteines in the extracellular domains of Sf-StkM330T and Sf-Ron. These data suggest that the cysteines in the extracellular domains of Sf-Stk and Sf-Ron may also mediate the interaction of these truncated receptors with other cellular factors that regulate their ability to promote cytokine-independent growth.

Mutational library analysis of selected amino acids in the receptor binding domain of envelope of Akv murine leukemia virus by conditionally replication competent bicistronic vectors

The envelope protein of retroviruses is responsible for viral entry into host cells. Here, we describe a mutational library approach to dissect functional domains of the envelope protein involving a retroviral vector, which expresses both the envelope protein of Akv murine leukemia virus (MLV) and the neomycin phosphotransferase II (Neo) selection marker from the same transcript. Envelope expression was achieved by inserting an internal ribosome entry site (IRES) between the neo and the env genes. We found the structure of the linker between the IRES element and env to be critical for sufficient envelope expression. This vector functions as a replication competent mini-virus in a culture of NIH 3T3 derived semi-packaging cells that express the viral Gag and Pol proteins. Titers comparable to those of wild type virus were achieved by this system. To test this vector system, we created a random mutational library of Arg 85 and Asp 86 in the first variable region of Akv envelope protein. Homologous amino acids to Asp 86 in Moloney and Friend murine leukemia viruses are thought to be directly involved in receptor binding. Subsequent selection of mutants capable of infecting murine NIH 3T3 cells indicated that the wild type aspartic acid or another hydrophilic residue at position 86 is an important determinant for envelope function.

Differential glycosylation of the Cas-Br-E env protein is associated with retrovirus-induced spongiform neurodegeneration

The wild mouse ecotropic retrovirus, Cas-Br-E, induces progressive, noninflammatory spongiform neurodegenerative disease in susceptible mice. Functional genetic analysis of the Cas-Br-E genome indicates that neurovirulence maps to the env gene, which encodes the surface glycoprotein responsible for binding and fusion of virus to host cells. To understand how the envelope protein might be involved in the induction of disease, we examined the regional and temporal expression of Cas-Br-E Env protein in the central nervous systems (CNS) of mice infected with the highly neurovirulent chimeric virus FrCas(E). We observed that multiple isoforms of Cas-Br-E Env were expressed in the CNS, with different brain regions exhibiting unique patterns of processed Env glycoprotein. Specifically, the expression of gp70 correlated with regions showing microglial infection and spongiform neurodegeneration. In contrast, regions high in neuronal infection and without neurodegenerative changes (the cerebellum and olfactory bulb) were characterized by a gp65 Env protein isoform. Sedimentation analysis of brain region extracts indicated that gp65 rather than gp70 was incorporated into virions. Biochemical analysis of the Cas-Br-E Env isoforms indicated that they result from differential processing of N-linked sugars. Taken together, these results indicate that differential posttranslational modification of the Cas-Br-E Env is associated with a failure to incorporate certain Env isoforms into virions in vivo, suggesting that defective viral assembly may be associated with the induction of spongiform neurodegeneration.

Mutations affecting the cytoplasmic domain of the Moloney murine leukemia virus envelope protein: rapid reversion during replication

Five premature termination mutations and five missense mutations were introduced into the portion of cloned Moloney murine leukemia virus (M-MuLV) DNA encoding the Env cytoplasmic domain. All of the mutant DNAs gave rise to replication-competent virus after transfection of NIH/3T3 cells, but several of the mutant DNAs scored as replication-defective when introduced into Rat2 cells. Cell lines stably expressing the mutant DNAs all released virion particles, and in all but one case infectious virus were generated. These viable mutants were all found to have reverted to the wild-type sequence. To generate fully mutant virus stocks, the mutant DNAs were introduced transiently into COS cells, which are resistant to infection with MuLV, thus prohibiting reversion by error-prone mechanisms involving reverse transcription. Virions harvested from the COS cells were confirmed as mutant by analyzing both virion proteins and the viral DNA they generated, and were then tested for infectivity in NIH/3T3 cells. The mutant viruses were infectious, but still rapidly gave rise to revertants. We conclude that the mutations within the cytoplasmic domain do not provide an absolute block to virus replication, but that the mutants replicate more slowly than the wild-type and quickly give rise to revertants with selective advantage for replication.

Analysis of mutations in the envelope gene of Moloney murine leukemia virus: separation of infectivity from superinfection resistance

Six deletion mutations and an insertion were generated in the env gene of cloned copies of Moloney murine leukemia virus DNA. All seven mutants were replication-defective as tested by transformation of NIH/3T3 cells. The mutant DNAs were introduced into NIH/3T3 cells to generate stable producer lines; all released virion particles into the medium, suggesting that none of the mutations affected overall viral gene expression, gag and pol gene expression, gag and pol gene functions, or virion budding. Several of the mutations reduced the lifetime of the env protein or blocked its export to the cell surface. One mutation altering the membrane-spanning region and the cytoplasmic tail of the TM protein had no effect on export of the protein, proteolytic processing, or incorporation into virion particles, but still blocked the infectivity of the resulting virus. The results suggest that alterations in the transmembrane region can affect early steps of infection, such as the fusion of virion and host membranes. Cells expressing this mutant env protein were fully resistant to superinfection by wild-type virus. Thus, induction of virus resistance, presumably reflecting blocking the virus receptor, can be separated from virus infectivity.

Inhibition of HIV and SIV infectivity by blockade of alpha-glucosidase activity

Processing of HIV and SIV envelope oligosaccharides is critical for proper intracellular trafficking and function. An inhibitor of alpha-glucosidases I and II, N-butyl deoxynojirimycin (N-BuDNJ), retards HIV-1 and SIVmac spread in lymphocytes and monocytes by diminishing virus infectivity, and also causes a reduction in syncytia formation between infected cells and uninfected lymphocytes. N-BuDNJ retards envelope processing from the precursor form to the mature surface (SU) and transmembrane proteins in HIV-1- and SIVmac-infected cells, as well as in cells infected with vaccinia-HIV-1 envelope recombinant virus. However, no significant reduction is seen in the amount of SU in released virus particles, though the virus particle-associated SU from N-BuDNJ-treated cells has an altered electrophoretic mobility. In contrast, N-BuDNJ had no effect on GAG protein synthesis and processing. These findings demonstrate a critical requirement for oligosaccharide processing by alpha-glucosidases I and II for HIV-1 and SIVmac envelope processing and fusogenicity.

N-linked glycosylation and reticuloendotheliosis retrovirus envelope glycoprotein function

Different properties of the spleen necrosis virus (SNV) envelope glycoprotein were analyzed following biosynthesis in the presence of glycosylation inhibitors. Tunicamycin, which inhibits all asparagine N-linked glycosylation, prevented intracellular processing and translocation to the cell surface of the envelope protein. In contrast, castanospermine or deoxymannojirimycin, which block glycosidase trimming of the early high-mannose chains and subsequent complex type N-glycosylation, did not inhibit proteolytic cleavage or cellular translocation. The ability of unglycosylated and partially glycosylated envelope protein to bind the viral receptor was assayed using an infection interference assay. Tunicamycin abrogated SNV envelope glycoprotein-induced receptor interference, whereas the trimming glycosidase inhibitors had no effect on interference. Similarly, tunicamycin but not the glycosidase inhibitors reduced the titers of released virus 100-fold. We conclude that carbohydrate trimming and complex N-glycosylation are not essential for envelope glycoprotein translocation, proteolytic cleavage, receptor binding, or infectivity, whereas cotranslational high-mannose N-glycosylation is essential for all of the SNV envelope glycoprotein properties tested. Syncytia formation can be induced following transfection into D17 cells of an envelope glycoprotein expression plasmid. Unlike virus particle infectivity, cell fusion is strongly inhibited by the glycosidase inhibitors.

Glycosylation of the envelope glycoprotein from a polytropic murine retrovirus in two different host cells

A polytropic recombinant retrovirus containing the envelope gene of Friend mink cell focus-inducing virus plus the remainder of the genome of an amphoropic murine leukemia virus was propagated on mouse embryo fibroblasts and mink lung cells. Virus particles, metabolically labeled with [2-3H]mannose, were harvested from the culture supernatants and lysed with detergents. The viral envelope glycoprotein was isolated from the lysates by immunoaffinity chromatography and purified by preparative SDS/PAGE. Oligosaccharides were liberated by sequential treatment of tryptic glycopeptides with endo-beta-N-acetylglucosaminidase H and peptide-N4-(N-acetyl-beta-glucosaminyl) asparagine amidase F and fractionated by high-performance liquid chromatography. Individual glycans were characterized chromatographically, by methylation analyses and in part, by enzymic microsequencing. The results demonstrated that viral glycoproteins, synthesized in mouse embryo fibroblasts, carried as major constituents partially fucosylated diantennary, 2,4- and 2,6-branched triantennary and tetraantennary complex type N-glycans with 0-4 sialic acid residues and only small amounts of high-mannose type species with 5-9 mannose residues. As a characteristic feature, part of the complex type glycans contained additional Gal(alpha 1-3) substituents. Glycoprotein obtained from virions propagated on mink lung cells, contained partially fucosylated diantennary and 2,4-branched triantennary oligosaccharides with 1-3 sialic acid residues, in addition to trace amounts of high-mannose type species with 8 or 9 mannose residues. Thus, the results reveal that predominantly, the complex type N-glycans of the retroviral envelope glycoprotein display cell-specific variations including differences in oligosaccharide branching, sialylation and substitution by additional Gal(alpha 1-3) residues.

Oligosaccharides at individual glycosylation sites in glycoprotein 71 of Friend murine leukemia virus

Glycoprotein 71 from Friend murine leukemia virus was digested with proteases and the glycopeptides obtained were isolated and assigned, by amino acid sequencing, to the eight N-glycosylated asparagines in the molecule; only Asn334 and Asn341 could not be separated. The oligosaccharides liberated from each glycopeptide by endo-beta-N-acetylglucosaminidase H, or by peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F, were fractionated and subjected to structural analysis by one- and two-dimensional 1H NMR, as well as by methylation/gas-liquid-chromatography/mass-fragmentography. At each glycosylation site, the substituents were found to be heterogeneous including, at Asn334/341 and Asn410, substitution by different classes of N-glycans: oligomannosidic oligosaccharides, mainly Man alpha 1----6(Man alpha 1----3)Man alpha 1----6(Man alpha 1----3)Man beta 1----4GlcNAc beta 1----4GlcNAc beta 1----, were detected at Asn168, Asn334/341 and Asn410. Hybrid species, partially sialylated, intersected and (proximally) funcosylated Man alpha 1----6(Man alpha 1----3)Man alpha 1----6 and Man alpha 1----3Man alpha 1----6 and Man alpha 1----3Man alpha 1----6(Gal beta 1----4GlcNAc beta 1----2Man alpha 1----3)Man beta 1----4GlcNAc beta 1----4GlcNAc beta 1----, were found at Asn12, as previously published [Schlüter, M., Linder, D., Geyer, R., Hunsmann, H., Schneider, J. & Stirm, S. (1984) FEBS Lett. 169, 194-198] and at Asn334/341. N-Acetyllactosaminic glycans, mainly partially intersected and fucosylated NeuAc alpha 2----3 or Gal alpha 1----3Gal beta 1----4GlcNAc beta 1----2Man alpha 1----6(NeuAc alpha 2----6 or NeuAc alpha 2----3Gal-beta 1----4GlcNAc beta 1----2Man alpha 1----3)Man beta 1----4GlcNac beta 1----4GlcNAc beta 1---- with some bifurcation at ----6Man alpha 1----6, were obtained from Asn266, Asn302, Asn334/341, Asn374 and Asn410. In addition, Thr268, Thr277, Thr279, Thr304/309, as well as Ser273 and Ser275, were found to be O-glycosidically substituted by Gal beta 1----3GalNAc alpha 1----, monosialylated or desialylated at position 3 of Gal or/and position 6 of GalNAc.

Glycosylation of glycoproteins 52 and 65 encoded by the polycythemia-inducing strain of Friend spleen focus-forming virus. Isolation of glycopeptides containing individual glycosylation sites

The primary envelope gene product of the polycythemia-inducing strain of Friend spleen focus-forming virus, glycoprotein 52 (gp52), as well as its processed form, glycoprotein 65 (gp65), were isolated from virus-infected normal rat kidney cells metabolically labeled with [2-3H]mannose. Following digestion with trypsin, glycopeptides containing individual N-glycosylation sites were obtained by gel filtration and subsequent reversed-phase high-performance liquid chromatography. N-terminal amino acid sequencing of the glycopeptides demonstrated that only asparagine residues 11 and 26, located in the N-terminal domains of gp52 and gp65, carry carbohydrate substituents, while the potential N-glycosylation sites in the C-terminal portions of the molecules are not used. Carbohydrates attached were liberated by treatment with endo-beta-N-acetylglucosaminidase H or peptide: N-glycosidase F and characterized by high-performance liquid chromatography. The results demonstrated that gp52 carries similar patterns of oligomannosidic glycans in both positions. In gp65, however, asparagine residue 11 is almost exclusively substituted by complete, fucosylated N-acetyllactosaminic oligosaccharides, whereas asparagine residue 26 carries oligomannosidic or truncated N-acetyllactosaminic glycans.

Carbohydrate structure of glycoprotein 65 encoded by the polycythemia-inducing strain of Friend spleen focus-forming virus

The secondary envelope-gene product, glycoprotein 65 (gp65), of the polycythemia-inducing variant of Friend spleen focus-forming virus (F-SFFVp) was isolated from F-SFFVp-infected normal rat kidney cells cultivated in the presence or absence (-Glc) of glucose. Oligosaccharide side chains present were sequentially liberated by treatment of tryptic glycopeptides with endo-beta-N-acetylglucosaminidase H and peptide N-glycosidase F and fractionated by high-performance liquid chromatography. The glycans were characterized by digestion with exoglycosidases, by chromatographic comparison with oligosaccharide standards and by methylation analysis. The results demonstrate that gp65 contains oligomannosidic, hybrid and N-acetyllactosaminic glycans. The oligomannosidic glycans represent the same partially glucosylated species with six to nine mannose residues present in F-SFFVp gp52, the biosynthetic precursor of gp65 [Strube, K.-H. Schott, H.-H. and Geyer, R. (1988) J. Biol. Chem. 263, 3762-3771]. Oligosaccharides of the hybrid type were found to comprise one sialylated lactosamine unit and three or four alpha-linked mannose residues. Analysis of the N-acetyllactosaminic glycans revealed that gp65 carries fucosylated, partially sialylated bi-antennary, tri-antennary and tetra-antennary oligosaccharides, in addition to incomplete species. The glycosylation of gp65(-Glc) is characterized by the presence of oligomannosidic glycans with five to nine mannose residues, similar hybrid-type species and by increased amounts of incomplete N-acetyllactosaminic oligosaccharides, a decrease in sialylation and the lack of tetra-antennary species.

Molecular biology of Friend viral erythroleukemia

Friend virus clearly provides an important model for understanding the molecular biology of cancer. Moreover, the most important aspects of the erythroleukemia can be caused by a single SFFV infection in the absence of any helper virus. The SFFV env gene encodes a membrane glycoprotein, gp55. This glycoprotein, when expressed on erythroblast surfaces, causes a constitutive mitogenesis. However, SFFV infections only rarely increase the cell's self-renewal capability or abrogate its commitment to differentiate. Therefore, the consequence of infection is initially a polyclonal erythroblastosis. This polyclonal proliferation usually leads to cell differentiation and to recovery unless helper virus is present to cause continuing infection of new erythroblasts. Extremely rare SFFV proviral integrations, however, result in abrogation of the cell's commitment to differentiate and in the concomitant acquisition of cell immortality. These immortalizing proviral integrations occur at only a small number of sites in the mouse genome. Therefore, the mitogenic and immortalizing stages of erythroleukemia are now known to be caused by discrete genetic events--the first involving the SFFV env gene and the second involving the rare proviral integration sites. In early investigations of Friend virus, the first stage always preceded the second stage by at least several weeks. Now it is known that this delay in onset of the second stage is caused solely by statistics. Every SFFV-infected erythroblast is mitogenically activated, yet only rarely does the SFFV proviral integration produce immortality. Both steps in leukemogenesis can be caused simultaneously in an erythroblast by a rare single SFFV proviral integration. There has been an explosion of interest in retroviral env gene-mediated pathogenesis. Such pathogenesis has been recently associated with most of the naturally transmitted retroviral diseases including AIDS. Such pathogenesis involves in different viruses immunosuppression, anemia, neuropathy, and leukemia (Mathes et al. 1978; Simon et al. 1984, 1987; Weiss et al. 1985; Lifson et al. 1986; Riedel et al. 1986; Sitbon et al. 1986; Sodroski et al. 1986; Mitani et al. 1987; Schmidt et al. 1987; Klase et al. 1988; Overbaugh et al. 1988a, b). The shuffling and dynamic env gene rearrangements that have been associated with murine retroviral leukemogenesis have also now been seen in FeLV-FAIDS and HIV (Fisher et al. 1988; Overbaugh et al. 1 t88b; Saag et al. 1988; Tersmette et al. 1988). Friend virus provides an important established example of such env gene pathogenesis. Although we still do not understand precisely how gp55 causes erythroblast mitosis, workers in this field have discovered important clues that may lead to answers.(ABSTRACT TRUNCATED AT 400 WORDS)

Anti-invasive activities of experimental chemotherapeutic agents

We have discussed a number of agents that affect invasion and we have grouped them according to their most probable targets. This strategy is based on the following hypothesis. Invasion is the result of cellular responses to extracellular signals. Candidate signals are components of the extracellular matrix, which are rendered inactive by the flavonoid (+)-catechin (see Section III). Signals are recognized by receptors on the plasma membrane, possibly glycoproteins, that may lose their recognition function through alteration of the oligosaccharide side chains by inhibitors of protein glycosylation (see Section IV) and possibly also by alkyllysophospholipids (see Section V). Synthetic oligopeptides reflecting sequences from cell-binding domains of extracellular matrix molecules are also effective tools for blocking specific receptors (see Section VI). GTP-binding proteins (G proteins) act as signal transducers and can be inactivated by pertussis toxin (see Section VII). An intriguing aspect of both alkyllysophospholipids and pertussis toxin is that they can either inhibit the invasion of constitutively invasive cells or induce invasion of constitutively noninvasive cells. Without doubt, cellular responses implicated in invasion are many-fold. Discussed here are cell motility and directional migration with inhibition through dipyridamole and its analogs and through microtubule inhibitors, respectively (see Section VIII). Alternative hypotheses and alternative strategies for the dissection of the invasion process do exist, and alternative cellular and molecular mechanisms of action may explain the anti-invasive activity of the agents discussed earlier. The latter are mentioned in each section. It is the authors' opinion that the possibilities for exploiting the battery of anti-invasive agents have by no means been exhausted. Introducing researchers to experiments that may lead to an understanding of the mechanisms of invasion and metastasis and to new rationales for cancer treatment has been the purpose of our review.

Novel glycosylation pathways of retroviral envelope proteins identified with avian reticuloendotheliosis virus

Previously, we identified two mature glycoproteins, gp90, the surface glycoprotein, and gp20, the transmembrane protein, from avian reticuloendotheliosis virus and an avian reticuloendotheliosis virus env gene-encoded intracellular polyprotein gPr77env, but the precise relationship of gPr77env to the mature envelope proteins was not determined (W.-P. Tsai, T.D. Copeland, and S. Oroszlan, Virology 155:567-583, 1986). In the present study, using metabolic labeling of viral proteins with [35S]cysteine, radioimmunoprecipitation, and carbohydrate structure analysis, we have identified a higher-molecular-weight endo-H-resistant env gene-encoded polyprotein designated gPr115env in addition to the endo-H-sensitive gPr77env. It appears that gPr77env is the primary polyprotein precursor, modified with mannosyloligosaccharides that are processed into sialic-acid-rich extraordinarily large complex-type carbohydrates (up to 17 kilodaltons for each N-linked site) on the gp90 domain but not on the gPr22 domain. In this process, gPr77env is converted into the apparently endo-H-resistant secondary polyprotein, gPr115env, which is rapidly processed into gp90 and gPr22. The proteolytic processing which occurs only after the appearance of an endo-H resistant precursor is now clearly demonstrated for a retrovirus. Some important aspects of carbohydrate structure, including the site-specific glycosylation, as well as the intracellular location and nature of the potential enzyme involved in the proteolytic cleavage of gPr115env are discussed.

Allele and locus-specific differences in cell surface expression and the association of HLA class I heavy chain with beta 2-microglobulin: differential effects of inhibition of glycosylation on class I subunit association

The assembly of HLA class I antigens, and the contribution of the single N-linked glycan to this process were examined. We observed a requirement for N-linked glycosylation in the proper assembly and surface expression of HLA-B locus products in particular, although considerable variation was seen within the allelic series of the HLA-A and B loci. We conclude that the single N-linked glycan can contribute in a major way to that conformation of the heavy (H) chain which is competent to associate with the light chain beta 2-microglobulin, and that the presence, rather than the type, of carbohydrate chain is important in this respect. The association of human class I H chains with beta 2-microglobulin shows biphasic kinetics, where an initially rapid phase is followed by a prolonged period during which no further association can be measured. It appears that HLA-C H chains are initially synthesized in amounts similar to HLA-A and B H chains, but associate inefficiently with beta 2-microglobulin, resulting in low expression of HLA-C at the cell surface. The individual stages of assembly and maturation of class I antigens including the transfer from Golgi to cell surface were found to display characteristic allelic variation.

O-linked glycosylation of retroviral envelope gene products

Treatment of [3H]glucosamine-labeled Friend mink cell focus-forming virus (FrMCF) gp70 with excess peptide:N-glycanase F (PNGase F) resulted in removal of the expected seven N-linked oligosaccharide chains; however, approximately 10% of the glucosamine label was retained in the resulting 49,000-Mr (49K) product. For [3H]mannose-labeled gp70, similar treatment led to removal of all the carbohydrate label from the protein. Prior digestion of the PNGase F-treated gp70 with neuraminidase resulted in an additional size shift, and treatment with O-glycanase led to the removal of almost all of the PNGase F-resistant sugars. These results indicate that gp70 possesses sialic acid-containing O-linked oligosaccharides. Analysis of intracellular env precursors demonstrated that O-linked sugars were present in gPr90env, the polyprotein intermediate which contains complex sugars, but not in the primary translation product, gPr80env, and proteolytic digestion studies allowed localization of the O-linked carbohydrates to a 10K region near the center of the gp70 molecule. Similar substituents were detected on the gp70s of ecotropic and xenotropic murine leukemia viruses and two subgroups of feline leukemia virus, indicating that O-linked glycosylation is a conserved feature of retroviral env proteins.

Inhibition of herpesvirus-induced thymidine kinase and DNA polymerase by beta-hydroxynorvaline

Treatment of HSV-infected cells with 5-10 mM beta-hydroxynorvaline (Hnv), a threonine analog, specifically affects herpesvirus DNA replication: both the rate of and total DNA synthesis are reduced, the former approximately 15-fold by Hnv (6 h post-infection) and the latter by 12-fold (between 3 and 12 h post-infection). The effect on DNA replication was due to inhibition of HSV-1 thymidine kinase (TK) and DNA polymerase (DP) activities; the former is reduced by 75% and whereas DP returns to baseline levels (when compared to untreated and/or uninfected cells). Host cell TK and DP activities are unaffected. It is suggested that beta-hydroxynorvaline is incorporated into these enzyme(s), either close to or at the active site thus perturbing viral DNA synthesis. beta-Hydroxynorvaline should have unique utility as a targeted antiviral compound, acting on both membrane-mediated phenomena (fusion, penetration and attachment) and DNA replication.

Minor modifications to the structure of tunicamycin lead to loss of the biological activity of the antibiotic

Three analogues of tunicamycin, each with minor alterations in structure in different regions of the molecule, have been employed to study the effects of such modifications upon the biological activity of the antibiotic. The data indicate that any modification of structure results in loss of the ability of the antibiotic to inhibit N-glycosylation of proteins. In contrast to tunicamycin itself, none of the analogues had any deleterious effects upon cellular macromolecule synthesis, nor upon the kinetics of export of de novo synthesised IgM or IgG molecules from treated rat hybridoma cells. In addition, the incorporation of tritiated sugars into acid-precipitable macromolecules was not inhibited. Endoglycosidase H digestion of isolated IgG molecules further suggested that the analogues employed did not interfere with qualitative glycosylation at the level of N-acetylglucosamine transferases I and II in the golgi apparatus. The data are consistent with the interpretation that tunicamycin has very precise structural requirements for expression of inhibitory effects upon protein glycosylation, and that small variations of structure can lead to loss of its inhibitory effects.

Glycosylation of simian virus 40 T antigen and localization of glycosylated T antigen in the nuclear matrix

Evidence has been obtained for the glycosylation of simian virus 40 (SV40) T antigen in SV40-infected TC7 cells. Both [3H]mannose and [3H]glucosamine are incorporated into T antigen in cells grown and labeled in medium containing fructose instead of glucose. In addition, T antigen is visualized by a carbohydrate stain specific for mannose and/or glucose residues. Finally, lectin binding studies suggest that T antigen contains galactose and/or galactosamine, since T antigen is specifically eluted from soybean lectin by 0.2 M galactose. When gel-purified, [3H]glucosamine-labeled T antigen is subjected to tryptic peptide mapping, label is found in only one peptide, thought to correspond to the methionine-containing peptide extending from Asn-653 to Arg-691, near the carboxy-terminal end of T antigen. Insensitivity to tunicamycin and the localization of the glycosylation site in the carboxy-terminus of T antigen, and not at Asn-153, suggest that T antigen is not N-glycosylated. Cell fractionation studies show that [3H]glucosamine-labeled T antigen is preferentially associated with the nuclear matrix of SV40-infected TC7 cells.

The effects of 2-deoxyglucose and tunicamycin on the biosynthesis of the murine mammary tumor virus proteins, and on the assembly and release of the virus

The role of glycosylation in the biosynthesis, processing, and shedding of the murine mammary tumor virus (MuMTV) glycoproteins and in virus production was investigated in a clonal mammary tumor cell line, GR-3A, using two inhibitors of protein glycosylation, 2-deoxyglucose (2-DG) and tunicamycin (TM). It was found that both 2-DG and TM completely inhibited the synthesis of the MuMTV envelope precursor polyprotein, Pr70env, and, as a consequence, the synthesis of the viral glycoproteins gp52 and gp36. By contrast, the synthesis of Pr73gag, the polyprotein precursor of the internal structural proteins of the virus, was only inhibited by 10-15% by 2-DG and TM. Although 2-DG and TM blocked the synthesis of Pr70env, a new polypeptide, related to gp52 and gp36, with a mol wt of 60,000 (P60env) was found to be synthesized in the treated cells. The P60env molecules appeared to be degraded intracellularly since they were not found to (1) undergo site-specific cleavage; (2) accumulate inside the cell or on the cell surface; (3) be secreted into the culture medium; and (4) be incorporated into the virions produced during the drug treatment. In spite of the lack of gp52 and gp36 synthesis in the presence of TM and 2-DG, mature MuMTV particles containing the characteristic surface projections known to be composed of gp52 and gp36 continued to be assembled and released at a reduced rate for at least 30 hr. In addition, the buoyant density and the polypeptide composition of the particles were found to be identical to virions produced by untreated cells. Thus, the virions assembled and released during 2-DG and TM treatment were not defective. Our investigations into the origin of gp52 and gp36 in these particles revealed that both molecules were synthesized prior to 2-DG and TM treatment and continued to be incorporated, along with the newly synthesized viral core proteins, into budding virions during the drug treatment. Furthermore, we found that gp52 and P75env (an aberrant form of Pr70env) that were not incorporated into virions continued to be shed normally from the cell during drug treatment. In conclusion, our results suggest that MuMTV assembly is not dependent on the synchronized synthesis of the viral core and envelope polypeptides, and that the assembled virions contain the correct ratio of these polypeptides, even when their ratio in the cell varies.

Role of a membrane glycoprotein in Friend virus-induced erythroleukemia: studies of mutant and revertant viruses

We previously reported the isolation and characterization of spontaneous, transmissible mutants of Friend spleen focus-forming virus (SFFV) that are nonpathogenic in adult NIH/Swiss mice and that contain abnormalities in nonoverlapping regions of their envelope glycoprotein (env) genes (M. Ruta, R. Bestwick, C. Machida, and D. Kabat, 1983, Proc. Natl. Acad. Sci. USA 80, 4704-4708). In newborn NIH/Swiss mice, these mutant SFFVs form revertants that are pathogenic in mice of all ages. At least two of three studied revertants contain second site env mutations which affect the sizes and proteolytic fragmentation patterns of their encoded glycoproteins. A variety of structural and genetic evidence suggests that the xenotropic- and ecotropic-related regions of the SFFV glycoprotein fold into separate globular domains that are connected by a flexible proline-rich joint. A glutamyl peptide bond within this joint is exceptionally susceptible to cleavage with Staphylococcus aureus V8 protease. Moreover, disulfide bonds occur within the xenotropic-related domain, but not between the globular domains. These results provide strong additional evidence that the env gene is required for SFFV pathogenesis, and they provide a new system for identifying the features of glycoprotein structure and localization which are essential for its leukemogenic activity.

Effect of inhibitors of glycosylation and carbohydrate processing on invasion of malignant mouse MO4 cells in organ culture

Inhibitors of glycosylation and carbohydrate processing were used to investigate the role of carbohydrates exposed at the cell surface in invasion. Malignant mouse MO4 cells were confronted with embryonic chick heart in organ culture, an assay shown to be relevant for a number of aspects of invasion in vivo. Tunicamycin (1.0 microgram/ml), 2-deoxy-D-glucose (100 mM), beta-OH-norvaline (1.0 mM), and Monensin (0.1 microgram/ml) reversibly inhibited the invasion of MO4 cells. At these concentrations the drugs also inhibited the growth of MO4 cells. 1-Deoxynojirimycin (10mM), swainsonine (0.4 microgram/ml), and Marcellomycin (0.1 microgram/ml) permitted invasion. Marcellomycin also reversibly inhibited the growth of MO4 cells. These results show that drugs known to interfere with the glycosylation or processing of carbohydrate chains of glycoproteins in different ways have different effects on the invasion of MO4 cells in vitro.

Modification of blood-borne arrest properties of lymphoma cells by inhibitors of protein glycosylation suggests the existence of endogenous lectins

The requirement for intact carbohydrates of glycoproteins at the cell surface was investigated after treatment of lymphoma cells with compounds which interfere at different steps in N-linked glycosylation: swainsonine and 1-deoxynojirimycin act at different levels during the processing, so that complex oligosaccharides cannot be formed; 2-deoxyglucose, beta-hydroxynorvaline, and tunicamycin completely prevent the formation of N-linked (high-mannose as well as complex) oligosaccharides. The role of sialic acid was investigated by treating the cells with neuraminidase. These treatments resulted in altered patterns of surface-labelled glycoproteins after SDS-polyacrylamide gel electrophoresis. Blood-borne arrest of lymphoma cells in the spleen was sensitive to neuraminidase and to treatments interfering with the processing of complex N-linked oligosaccharides. It is suggested that carbohydrates are signals for cellular interactions involved in the recirculation and homing behaviour of lymphoid cells and probably interact with endogenous lectins at their site of homing.

Inhibition of synthesis of herpesvirus (HSV-1) glycoproteins and endogenous fusion by beta-hydroxynorvaline in BHK-21 cells

Treatment of HSV-infected BHK-21 cells with 5-10 mM of beta-hydroxynorvaline (Hnv), an analog of threonine which blocked attachment of oligosaccharides at the Asn-X-Thr sites, markedly inhibited the synthesis of all viral glycoproteins as well as the major capsid protein. However, the synthesis of host-specific dolichol-linked oligosaccharides was not significantly affected by Hnv. Treatment of cells with 10 mM reduced the yield of virus greater than 95% and completely blocked endogenous fusion. Inhibition of Hnv could be reversed by simultaneous addition of threonine to the culture medium. It is likely that the incorporation of Hnv into HSV polypeptides at Asn-X-Thr (in place of Thr) sites blocked transfer of N-linked oligosaccharides.

Studies with inhibitors of oligosaccharide processing indicate a functional role for complex sugars in the transport and proteolysis of Friend mink cell focus-inducing murine leukemia virus envelope proteins

The functions of asparagine-linked oligosaccharides on the PrENV protein of Friend mink cell focus-inducing (FrMCF-1) murine leukemia virus were investigated by examining the effect of two inhibitors of different stages of the biosynthetic pathway of these sugar substituents on the synthesis and processing of the viral proteins. Treatment of virus-producing cells with tunicamycin totally inhibited the glycosylation of PrEnv, and resulted in the formation of a nonglycosylated form of the protein of molecular weight 62 kDa. This component was not proteolytically processed inside the cells, and neither it nor any derivative proteins were incorporated into extracellular virions. Treatment of cells with 1-deoxynojirimycin (DNM), which inhibits the cellular glucosidases normally involved in removal of the three glucose residues present on the initially transferred oligosaccharide chains, resulted in the intracellular accumulation of a slightly larger than normal form of PrENV, and decreased levels of cell-associated gp70. Only gp70 was detected on the cell surface. The bulk of the gp70 produced in the presence of the drug was aberrantly glycosylated, and contained decreased levels of complex and increased numbers of high mannose oligosaccharides; almost all of the gp70 molecules however, contained at least one complex sugar chain. Decreased incorporation of both env and gag proteins into extracellular virions was observed, despite the fact that the gag proteins were processed normally intracellularly; in contrast, DNM treatment of Gazdar murine sarcoma virus-infected HTG2 cells, which produce only gag but not env proteins, did not inhibit the release of extracellular virus. Ultrastructural examination of FrMCF-infected cells treated with DNM indicated the presence of large numbers of intracytoplasmic vacuoles, many of which contained viral particles. These studies indicate that the normal maturation process involved in the formation of complex oligosaccharides is necessary to obtain efficient transport to the plasma membrane and proteolysis of PrEnv, and also provide evidence suggesting a role for the env proteins in regulating assembly of gag proteins into virions.

Enhanced degradation of cathepsin D synthesized in the presence of the threonine analog beta-hydroxynorvaline

The threonine analog beta-hydroxynorvaline is an inhibitor of asparagine-linked glycosylation. In the presence of the analog human fibroblasts synthesized cathepsin D molecules containing two, one, or no oligosaccharides. The nonglycosylated cathepsin D precursor was but a minor species and was degraded within 45 min of its synthesis, presumably in the lumen of the endoplasmic reticulum. The polypeptides with one or two oligosaccharides were normally segregated into lysosomes and their proteolytic maturation was not affected. The stability of mature glycosylated and nonglycosylated cathepsin D polypeptides within the lysosomes, however, was markedly decreased. The recovery of cathepsin D polypeptides was increased in the presence of inhibitors of cysteine and aspartyl-proteinases. These data suggest that the absence of carbohydrate side chains in cathepsin D results in an enhancement of the degradation rate of the precursor in the endoplasmic reticulum, and the replacement of threonine by beta-hydroxynorvaline in an enhanced degradation of the mature cathepsin D in lysosomes.

Structural analysis of the spleen focus-forming virus envelope gene product

The structure of the envelope gene product (gp52) of the spleen focus-forming virus was analyzed and compared to that of envelope proteins (gp70 and p15E) of another pathogenic Friend virus recombinant, Friend mink cell focus-inducing virus (F-MCF). This has enabled us to confirm and extend previous nucleotide sequence data regarding the make up of specific domains of the SFFV glycoprotein. Amino-terminal 23,000-Da V8 protease fragments from gp52 of the Lilly-Steeves strain of SFFV and from gp70 of a pathogenic F-MCF isolate produced tryptic peptide fingerprints in which the mobilities of the trypsin-generated peptides were identical. The carboxyl-terminal, 21,000 Da, V8 protease fragment of gp52, however, has a unique fingerprint that contained a single highly charged trypsin-generated peptide. This peptide migrated to the same position as a peptide in F-MCF p15E, thus indicating that the p15E-related nucleotide sequences, that follow the large envelope deletion, are translated in the same reading frame as those in the standard p15E. Although R peptide determinants can be detected in F-MCF Pr15E, they could not be detected in gp52. The amino-terminal 23K domain of gp52, like that of MCF gp70, contains two oligosaccharide attachment sites. The other two attachment sites are located within the 21K carboxyl-terminal domain.

The expression of the env gene-related gp66 in mouse cells infected with the helper independent Friend leukemia virus is restricted to the myelomonocytic and mastocytic lineages

The presence of a gp66 closely related to the Friend ecotropic murine leukemia virus gp70 (F-MuLV) has recently been reported (D. Mathieu-Mahul, J. M. Heard, S. Fichelson, S. Gisselbrecht, B. Sola, and C. J. Larsen (1982) virology 119, 59-67). In the present work, characterization of this gp66 was continued. First, immunoprecipitation tests, using cytoplasmic membrane subfractions from one of the myelomonocytic cell lines in which gp66 was first detected, indicated that most of it was associated with rough endoplasmic reticulum. Second, to define the limits of gp66 expression, a variety of hemopoietic cell lines were analyzed for gp66 content. These lines were obtained (a) from various tumors (including erythroleukemias, chloroleukemias, and lymphatic leukemias) induced in susceptible mice by F-MuLV and (b) from long-term bone marrow cultures (LTBMC) infected with F-MuLV. In the latter case, lines of adherent fibroblastoid cells and nonadherent cells with myelomonocytic and mastocytic characteristics were obtained. Although several F-MuLV isolates were used, gp66 was only expressed in myelomonocytic and mastocytic cells. This did not result from in vitro culture conditions as gp66 was also found in fresh cells. These data suggested that a particular processing of the env gene product may exist in both myelomonocytic and mastocytic cell lines. In agreement with this hypothesis, a metabolically unstable gp62 related to MCF gp70 was found in one myelomonocytic cell line expressing MCF virus.

Dolichyl phosphate supplementation increases N-linked protein glycosylation in rat parotid acinar cells without increasing glycoprotein secretion

N-linked protein glycosylation was increased three- to five-fold, in dispersed rat parotid acinar cells in vitro, by supplementation with exogenous dolichylphosphate. Despite this increase, glycoprotein secretion from both control and dolichylphosphate-supplemented cells was comparable.