Envelope polypeptides of Friend leukemia virus: purification and structural analysis

Moloney murine leukemia virus envelope protein subunits, gp70 and Pr15E, form a stable disulfide-linked complex

The nature and stability of the interactions between the gp70 and Pr15E/p15E molecules of murine leukemia virus (MLV) have been disputed extensively. To resolve this controversy, we have performed quantitative biochemical analyses on gp70-Pr15E complexes formed after independent expression of the amphotropic and ecotropic Moloney MLV env genes in BHK-21 cells. We found that all cell-associated gp70 molecules are disulfide linked to Pr15E whereas only a small amount of free gp70 is released by the cells. The complexes were resistant to treatment with reducing agents in vivo, indicating that the presence and stability of the disulfide interaction between gp70 and Pr15E are not dependent on the cellular redox state. However, disulfide-bonded Env complexes were disrupted in lysates of nonalkylated cells in a time-, temperature-, and pH-dependent fashion. Disruption seemed not to be caused by a cellular factor but is probably due to a thiol-disulfide exchange reaction occurring within the Env complex after solubilization. The possibility that alkylating agents induce the formation of the intersubunit disulfide linkage was excluded by showing that disulfide-linked gp70-Pr15E complexes exist in freshly made lysates of nonalkylated cells and that disruption of the complexes can be prevented by lowering the pH. Together, these data establish that gp70 and Pr15E form a stable disulfide-linked complex in vivo.

Biochemical and immunological characterization of micellar complexes of the envelope glycoprotein of a simian immunodeficiency virus isolated from an African green monkey

The external envelope glycoprotein gp130 of a simian immunodeficiency virus isolated from an African green monkey (SIVagmTYO-7) was purified as micellar complexes. The molecular weight of the gp130 micelles was about 700 K. On electron microscopy, the micelles appeared as spherical particles with a diameter of 15 to 20 nm. Such aggregates consisted of about 4 to 5 gp130 monomers. Hyperimmune sera raised in rabbits and rhesus monkeys against these gp130 micelles exhibited titers between 10(5) and 10(6). Such sera inhibit the CD4 binding of gp130 and neutralize SIVagmTYO-7 and SIVmac251 but not HIV-2ben.

CD4-binding of gp130 micelles isolated from SIVagmTYO-7

The binding and binding inhibition of the SIVagmTYO-7 external glycoprotein gp130 in micellar form to the CD4 molecule on human Molt-4 clone 8 cells was investigated. The best binding of gp130 to Molt-4 clone 8 cells occurred at pH 5.5 to 6.5 at 37 degrees C after 4 h or at room temperature after 10 h. The dissociation constant of this reaction was 0.2-0.4 nM, with both soluble CD4 or CD4 on Molt-4 clone 8 cells. This value is close to 0.15 nM determined for the antihuman CD4 monoclonal antibody 30F16H5. After partial deglycosylation of gp130, a 90 kD product arose which still bound to CD4. Fully deglycosylated gp130 of 60 kD was still immunoprecipitable, but had lost the CD4 binding activity. Lens culinaris agglutinin was able to inhibit the gp130-CD4 interaction very efficiently, while the agglutinin of Phaseolus vulgaris was half as efficient and Canavalia ensiformis was inefficient. CD4 binding of gp130 micelles was also inhibited with several anti CD4 monoclonal antibodies directed against the OKT4a epitope as well as with soluble recombinant CD4.

A general model for the transmembrane proteins of HIV and other retroviruses

A hypothetical model of the transmembrane (TM) protein of human immunodeficiency virus (HIV) is proposed that is derived from the known structure of the influenza TM protein HA2. This model is consistent with computer algorithms of predicted protein structure and with known properties of TM proteins determined by sequence homology, site-directed mutations, peptide analogs, immunochemistry, or other biologic means. It is applicable to a wide variety of retroviral TM proteins differing widely in overall molecular weight.

Possible immunoenhancement of persistent viremia by feline leukemia virus envelope glycoprotein vaccines in challenge-exposure situations where whole inactivated virus vaccines were protective

Kittens immunized with purified native FeLV-gp70 or -gp85 envelope proteins developed ELISA, but not virus neutralizing, antibodies in their serum to both whole FeLV and FeLV-gp70. Kittens vaccinated with envelope proteins and infected with feline sarcoma virus (FeSV) developed smaller tumors than nonvaccinates, but a greater incidence of persistent retroviremia. Similarly, FeLV-gp70 and -gp85 vaccinated kittens were more apt to become persistently retroviremic following virulent FeLV challenge exposure than nonvaccinates. Kittens vaccinated with inactivated whole FeLV developed smaller tumors after FeSV inoculation and had a lower incidence of persistent retroviremia than nonvaccinates. The protective effect of inactivated whole FeLV vaccine against persistent retroviremia was also seen with FeLV challenge-exposed cats. Protection afforded by inactivated whole FeLV vaccine was not associated with virus neutralizing antibodies, although ELISA antibodies to both whole FeLV and FeLV-gp70 were induced by vaccination.

Coordination of cleavage of gag and env gene products of murine leukemia virus: implications regarding the mechanism of processing

Mouse 3T6 cells infected with Murine Leukemia Virus (MuLV) were cloned to yield several sublines producing viruses distinct from one another with respect to the ratio of uncleaved to cleaved gag gene-coded polyprotein, Pr65gag. The virus produced by the cloned sublines also differed in the ratio of the env gene-coded protein, p15E, to its product, p12E. The two ratios, Pr65gag/p30 and p15E/p12E, were found to be highly correlated among the cloned cell lines. Velocity gradient separation of the virions produced by individual sublines, followed by polypeptide analysis, demonstrated that the particles were inhomogeneous with respect to extent of cleavage both of PR65gag and of p15E. The two cleavages were again highly correlated. These data indicate that the gag and env gene product cleavages are not independent events but are tightly coupled.

Subunit vaccines against enveloped viruses: virosomes, micelles and other protein complexes

The envelope proteins (the peplomers) of enveloped viruses are the components that are important for induction of protective immunity. This article reviews methods and problems of making subunit vaccines of peplomers. In the first section, the solubilization of enveloped viruses with detergent is discussed. The preparation of envelope proteins into defined different physical forms is described, i.e. monomeric and micelle forms and the reconstitution of the protein into lipid vesicles (virosomes). Finally, the preparation of a new type of complex is described (named iscom), which is highly immunogenic. In the following sections the efficacy of the different physical forms are reviewed and it is concluded that monomeric forms must be avoided since they are poorly immunogenic and they may even have a suppressive effect on the immune response. The multimeric micelles, virosomes and iscoms are all immunogenic. The iscom is an interesting new concept that can be used to produce efficient subunit vaccines.

Isolation of glycopeptides containing individual glycosylation sites of Friend murine leukemia virus glycoprotein: studies of glycosylation by methylation analysis

Glycopeptides containing individual N-glycosylation sites of the glycoprotein from Friend murine leukemia virus were isolated by digestion of the viral glycoprotein with protease of S. aureus (V8) or with trypsin followed by fractionation of the resulting (glyco)peptides by gel filtration and reversed-phase, high-performance liquid chromatography at pH 6. Isolated glycopeptides were assigned to the known amino acid sequence of the protein by amino acid analysis and by determination of the NH2-termini. The carbohydrate moieties of each glycosylation site were analysed by methylation analysis. A high selectivity of the glycoprotein glycosylation was found with regard to the distribution of oligomannosidic, mixed, and N-acetyl-lactosaminic oligosaccharides.

Major oligosaccharides in the glycoprotein of Friend murine leukemia virus: structure elucidation by one- and two-dimensional proton nuclear magnetic resonance and methylation analysis

The highly microheterogeneous, N-glycosidically linked oligosaccharides in the glycoproteins of Friend murine leukemia virus (as produced by Eveline cells) were liberated with endo-beta-N-acetylglucosaminidase H and by alkaline hydrolysis. They were fractionated (as desialylated oligosaccharitols) by gel filtration and by concanavalin A affinity chromatography, and the major fractions were analyzed by methylation-gas chromatography-mass spectrometry, by digestion with exoglycosidases, and, especially, by one- and two-dimensional proton nuclear magnetic resonance spectroscopy. Guidelines for qualitative and quantitative analysis of complex oligosaccharide mixtures by NMR were worked out and the results compared with those obtained by methylation analysis. It was found that these major fractions consist of bi-, tri-, and tetraantennary oligosaccharitols of the "complex" type (comprising a minority of species with N-acetyllactosamine repeating units), which are, in part, substituted by nonreducing terminal Gal alpha (1----3) and/or bisecting GlcNAc beta (1----4) residues.

Structure of the oligosaccharides sensitive to endo-beta-n-acetylglucosaminidase H in the glycoprotein of Friend murine leukemia virus

The surface glycoprotein (mixture of isoglycoproteins with Mr 69 000 and 71 000) was isolated from the particles of Friend murine leukemia virus, and was successively digested with protease and with endo-beta-N-acetylglucosaminidase from Streptomyces griseus. Roughly 20% (w/w) of the carbohydrates in this glycoprotein were thus released, and they were fractionated by high-performance liquid chromatography after reduction with KB3H4/NaBH4. The radioactive oligosaccharitol fractions obtained were analyzed by exoglycosidase digestion, by acetolysis, and, after permethylation, by fast atom-bombardment mass spectrometry, as well as by capillary gas-liquid chromatography/mass spectrometry following hydrolysis, reduction and peracetylation. Around 85% (mol/mol) of the endo-H-sensitive viral glycans were thus found to be oligomannosidic oligosaccharitols of size classes Man5GlcNAcOH, Man6GlcNAcOH, Man8GlcNAcOH, Man7GlcNAcOH, and Man9GlcNAcOH (in order of prevalence), and the major structural isomers of each size class were identified. About another 15% (mol/mol) of the oligosaccharitols were shown to be of the 'mixed type', comprising mainly four species in which the Man(alpha 1----6)-branch of the Man alpha 1----6 (Man alpha 1----3) Man beta 1----4GlcNAcOH core is substituted by one or two additional alpha-mannoses, while the Man(alpha 1----3)-branch carries an N-acetyllactosamine unit substituted by sialic acid, or by Gal(alpha 1----3).

Method to map antigenic determinants recognized by monoclonal antibodies: localization of a determinant of virus neutralization on the feline leukemia virus envelope protein gp70

A method is presented whereby antigenic determinants recognized by specific monoclonal antibodies can be mapped to specific sites on a protein sequence with high resolution. Short DNase I-generated DNA fragments encoding portions of the protein of interest are molecularly cloned into the EcoRI site of the beta-galactosidase gene of phage lambda Charon 16 so as to obtain expression of random protein fragments as fusion proteins. The monoclonal antibody is used to screen the phage library to isolate phage expressing the specific antigenic determinant. DNA of immunoreactive phage can be analyzed rapidly and subcloned to allow DNA sequence determination. The method is generally applicable and permits antigenic determinants of functionally interesting monoclonal antibodies to be mapped and related to specific protein sequences. We have used this procedure to determine the region of the feline leukemia virus envelope protein gp70 recognized by a virus-neutralizing monoclonal antibody, cl.25. Antibody binding was mapped to a 14-amino acid region in the amino-terminal half of gp70. This region may be directly involved in an essential function of the gp70 protein, perhaps in gp70-mediated host recognition functions. Synthetic peptides derived from this region may provide useful vaccine antigens for the prevention of feline leukemia virus-associated disease in cats.

Mitogenic effect of anti-Friend leukemia virus antiserum on T cells

It has been previously reported that goat- and rat antisera directed against Friend leukemia virus (aFLV) are mitogenic for some B cells, but not for T cells. Here we report that activation of T cells by concanavalin A (Con A) rendered T cells responsive to the mitogenic activity of aFLV. This activity was contained in the immunoglobulin fraction and could be absorbed by purified FLV preparations. Optimal conditions for measuring the mitogenc activity of aFLV include 48 h preincubation of thymocytes with 3 micrograms/ml Con A followed by reculturing the activated thymocytes for 28 h with aFLV. The acquisition of an aFLV-responsive state was dependent on early protein synthesis during the Con A-induced activation period. aFLV did not substitute for interleukin 2 (IL2) in a costimulator assay. Evidence is presented that aFLV acted in a cocultivation assay via growth factor(s). In contrast to control cultures, aFLV-treated lymphoblasts contained, in their supernatants, IL2 activity as demonstrated by their effect on an IL2-dependent T cell line. The data suggest that aFLV acted upon activated T cells by enhancing the endogenous production and/or release of IL2.

Monoclonal antibodies derived during graft-versus-host reaction. II. Antibodies detect unique determinants common to many MCF viruses

Hybridoma cell lines were recovered from the spleens of 6-week-old (B6 X D2)F1 mice undergoing graft-versus-host reaction induced by the transfer of 5-week-old B6 parental spleen cells. These cell lines produced antibodies reactive with envelope polypeptides of a variety of MuLV. The viral specificity assessed by membrane immunofluorescence and virus-binding radioimmunoassay indicated that the reactivity of these antibodies was distinctly different from monoclonal antibodies recovered from (B6 X D2)F1 recipients of D2 spleen cells reported previously (Portis et al., Virology 118, 181-190, 1982). Ten out of 17 monoclonal antibodies in the current study reacted exclusively with MCF viruses and three of these antibodies detected envelope determinants which were shared by a broad panel of MCF viruses of diverse origin. These common MCF determinants were expressed by the gp70 molecule as determined by Western blot analysis. The production of these antibodies by young mice in the absence of exogenous virus inoculation suggests that these antigens may be encoded by endogenous MCF-like sequences.

Active immunization with feline leukemia virus envelope glycoprotein suppresses growth of virus-induced feline sarcoma

The efficiency was examined of immunization with feline leukemia virus glycoprotein complexes (gp85 rosettes) to protect cats against tumors induced by feline sarcoma virus (FeSV). The glycoprotein was isolated from feline leukemia virus (FeLV). Young cats were vaccinated with the purified viral glycoprotein and challenged with FeSV (FeLV). FeLV gp85 antibody levels were measured by enzyme-linked immunosorbent assay and tumor volumes were determined. In immunized animals tumor development was reduced. Gp85 antibody levels before challenge were correlated inversely with tumor size (r2 = 0.79). This method appears to be suitable for fast and efficient testing of future FeLV vaccines.

Nucleotide sequence of the envelope gene of Friend murine leukemia virus

The envelope gene of the helper-independent, highly leukemogenic virus Friend murine leukemia virus was sequenced by using a molecular clone of a Friend murine leukemia provirus. The deduced amino acid sequences of the envelope proteins gp70 and p15env were homologous to the sequences of Moloney murine leukemia virus (86%) and Akv (76%). However, a stretch of about 40 amino acid residues near the middle of gp70 was dissimilar in Friend and Moloney murine leukemia viruses and Akv. In this type-specific region the gp70s of all three viruses contained more than 30% proline residues, giving this sequence a very rigid conformation. We suggest that this rigid and highly variable region of gp70 participates in infection by recognition of cell surface receptors and, in addition, might contribute to the different oncogenic spectra of murine leukemia viruses.

Complete amino acid sequence and glycosylation sites of glycoprotein gp71A of Friend murine leukemia virus

The complete amino acid sequence of glycoprotein gp71A of Friend murine leukemia virus (F-MuLV) is presented. The protein moiety of gp71A was digested with Staphylococcus aureus (SV8) protease, trypsin, and thermolysin. The sequences of the peptides were determined by the micro dansyl Edman procedure. gp71A is composed of 445 amino acid residues and contains eight oligosaccharide side chains, which are attached exclusively to asparagine by N-glycosyl bonds primarily in the COOH-terminal half of the polypeptide. gp71A is rich in proline (49 residues), tryptophan (16 residues), and cysteine (19 residues). Proline has the highest molar content (11%) of all amino acids. The prolines cluster in two segments. The most interesting one stretches between residue 233 and residue 283 and contains 18 prolines within 51 amino acids. This proline-rich domain most likely forms a flexible polyproline helix. The comparison of gp70 of Moloney murine leukemia virus (Mo-MuLV gp70) with F-MuLV gp71A revealed that 70 amino acids have been exchanged and 9 residues have been deleted from Mo-MuLV gp70. The most striking alterations have taken place within the large polyproline segment (residues 247 to 281). In this part of the molecule 7 amino acids have been deleted in Mo-MuLV and 18 residues have been replaced. This evidence supports the proposal of Shinnick et al. [Shinnick, T. M., Lerner, R. A. & Sutcliffe, J. G. (1981) Nature (London) 293, 543-548] that this area is a "hot spot" for recombination.

Separation procedure and sugar composition of oligosaccharides in the surface glycoprotein of Friend murine leukemia virus

The sugar composition of the surface glycoprotein from Friend murine leukemia virus was determined by gas-liquid chromatography of the alditol acetates and by the thiobarbituric acid method, respectively. N-Acetylglucosamine, mannose, galactose, sialic acid and fucose were found in a molar ratio around 15.2:11.6:7.4:3.3:1.0. Ten oligosaccharide fractions were obtained from glycoprotein preparations by a suitable sequence of degradation (with pronase, endo-beta-N-acetylglucosaminidase H, neuraminidase, and by hydrazinolysis) and separation procedures (concanavalin A-affinity chromatography and gel filtration). The qualitative sugar composition of these fractions was analyzed by in vivo labelling with D-[6-(3)H]glucosamine, D-[2-(3)H]mannose, D-[6-(3)H]galactose, or L-[6-(3)H]fucose, and their molecular weights were estimated from the gel elution volumina. Four fractions of N-glycosidically linked oligosaccharides of the oligomannosidic ('high mannose') type oligomannosidic7-oligomannosidic10, about seven to ten sugar residues), two of the mixed (M11 and M12), and four of the N-acetyllactosaminic ('complex') type (N-acetyllactosaminic9, probably nine sugar residues; N-acetyllactosaminica-N-acetyllactosaminic c, size unknown) were thus identified.

Glycoproteins of friend murine leukemia virus: separation and NH2-terminal amino acid sequences of gp69 and gp71

The NH2-terminal amino acid sequences (initial 23 residues) of Friend murine leukemia virus gp71 and gp69 were determined and found to be different but highly related. Friend murine leukemia virus gp71 differed from Rauscher murine leukemia virus gp70 in only one position. Friend murine leukemia virus gp69 showed approximately 41% homology to these glycoproteins but lacked the glycosylation site (sequon) occurring at position 12 in Rauscher murine leukemia virus gp70.

Possible cell surface receptor for Friend murine leukemia virus isolated with viral envelope glycoprotein complexes

Water-soluble multimeric complexes of Friend leukemia virus envelope glycoprotein gp85 bind specifically to C57BL/6 mouse spleen leukocytes. Such complexes were used to isolate cell surface receptors for the virus, using an immunoprecipitation technique. The putative rceptor has a molecular weight of 14,000. Mouse H-2 histocompatibility antigens, which are receptors for Semliki Forest virus, are not receptors for Friend leukemia virus.