Structure of the mouse mammary tumor virus: polypeptides and glycoproteins

Effects of cytoskeletal disrupting agents on mouse mammary tumor virus replication

Alterations in mouse mammary tumor virus (MMTV) production and composition were induced by exposure of mammary tumor cells to cytodisruptive agents. Treatment with 2.1 microM cytochalasin D (CD) for 24 h reduced MMTV yield by 80% and electron microscopic examination of these cells did not reveal budding virions. Immune precipitation and quantitative immunofluorescence studies demonstrated that CD had no significant effect on MMTV polypeptide synthesis or surface expression suggesting that CD inhibited late steps in MMTV maturation. Decreases in MMTV production were also observed as a result of 24 h exposure of the cells to 2.1 microM cytochalasin B (CB). However, an initial 70% increase in the levels of extracellular virions within the first 18 h of treatment preceded diminution of virus production. In addition, CB was unable to abrogate maturation and release of MMTV particles as revealed by electron microscopic evaluation of thin sections of treated cells. Colcemid at 0.28 microM had no effect on virus production during the first 24 h of exposure although MMTV yield was reduced by 60-70% after 36 h of treatment. Polypeptide profiles of MMTV purified from cell cultures treated with any of the three cytodisruptive agents were altered and included 5-7 polypeptides not typically present in MMTV from untreated cells. These cytodisruptive agents did not significantly affect viability and protein metabolism of MJY-alpha cells; the data suggest that alterations in MMTV replication were due to disruption of the cellular cytoskeleton.

Exogenous mouse mammary tumor virus proviral DNA isolated from a kidney adenocarcinoma cell line contains alterations in the U3 region of the long terminal repeat

Mouse mammary tumor virus (MMTV) is a B-type retrovirus which induces predominantly mammary carcinomas after a relatively long latency period. To date, very little is known about the reasons for the strict tissue specificity of MMTV. The BALB/cf/Cd strain of mice, which was infected with milk-borne MMTV (C3H), shows a high incidence of kidney adenocarcinomas, and our data suggest that MMTV might be involved in the formation of these tumors. Newly integrated exogenous MMTV proviruses were found in the genome of transplanted tumor cells as well as in the DNA of a cell line derived from one tumor, but not in normal cells of BALB/cf/Cd mice. The MMTV DNA in these tumor cells was transcribed and viral RNA synthesis was strongly stimulated by glucocorticoid hormones. Viral structural polypeptides, comparable in size and antigenicity to MMTV polypeptides of infected mammary tumor cells were synthesized and processed normally in the cell line and were organized correctly into intracytoplasmic particles. Heteroduplex analysis of the molecularly cloned MMTV proviral DNAs of kidney and mammary tumor origin revealed a high degree of homology in the gag, pol, and env genes. A striking difference, however, was observed in the U3 region of the two LTRs that might relate to the different tissue specificity of the two viruses.

Analysis of structural polypeptides of the lymphoproliferative disease virus (LPDV) of turkeys

The polypeptide composition of the lymphoproliferative disease virus (LPDV) of turkeys was shown to comprise several polypeptides with apparent molecular weights of 76, 31, 28, 20 and 15 kDa. This polypeptide pattern is distinctly different from the protein profiles of avian leukosis viruses, reticuloendotheliosis virus, or murine leukemia viruses. Moreover, LPD virions contain 2 major structural proteins (p31 and p28), in contrast to only one major internal protein present in most other retroviruses. The 76 kDa protein was established as the major viral envelope glycoprotein. The uniqueness of the LPDV polypeptide pattern is consistent with the lack of genetic relatedness of LPDV genome to other retroviruses, establishing LPDV as a representative of a distinct group of retroviridae.

Immunological selection of variant mouse lymphoid cells with altered glucocorticoid responsiveness

We have devised an immunological procedure to separate cells on the basis of expression of mouse mammary tumor virus (MMTV) gene products. Plastic petri dishes coated with specific antibodies against MMTV proteins bind cells with an efficiency that correlates with the level of MMTV gene expression. Glucocorticoid-sensitive mouse thymoma cell line W7 was infected with MMTV. Clones from the infected population retain the relatively slow cytolytic glucocorticoid response and, in addition, exhibit a rapid induction of MMTV-specific RNA and proteins. By combining our immunological selection with the selection for resistance to hormone-mediated cytolysis, we have isolated variant cells which are resistant to the cytotoxic effect of glucocorticoids but which retain the induction of viral gene products and must therefore have a functional glucocorticoid receptor protein.

Immunological selection of variant mouse lymphoid cells with altered glucocorticoid responsiveness

We have devised an immunological procedure to separate cells on the basis of expression of mouse mammary tumor virus (MMTV) gene products. Plastic petri dishes coated with specific antibodies against MMTV proteins bind cells with an efficiency that correlates with the level of MMTV gene expression. Glucocorticoid-sensitive mouse thymoma cell line W7 was infected with MMTV. Clones from the infected population retain the relatively slow cytolytic glucocorticoid response and, in addition, exhibit a rapid induction of MMTV-specific RNA and proteins. By combining our immunological selection with the selection for resistance to hormone-mediated cytolysis, we have isolated variant cells which are resistant to the cytotoxic effect of glucocorticoids but which retain the induction of viral gene products and must therefore have a functional glucocorticoid receptor protein.

Expression and disposition of the murine mammary tumor virus (MuMTV) envelope gene products by murine mammary tumor cells

Three murine mammary tumor virus (MuMTV)-producing epithelial cell lines derived from murine mammary tumors were examined in order to identify the MuMTV-specific cell surface antigens and their distribution on the cell surface, to study the kinetics of the MuMTV envelope precursor processing, virus assembly, and release, and to characterize the soluble MuMTV antigens that are shed into culture medium. Cell surface labeling experiments showed that only the mature MuMTV envelope glycoproteins gp52 and gp36 were exposed on the cell surface, and that gp52 was more abundant than gp36. In cells producing large quantities of MuMTV, expression of gp52 on the cell surface was shown by immunoelectron microscopy to be localized predominantly on the surface of budding virions and not on smooth areas of the cell surface where virus was not budding. The cell surface associated gp36 was found not to be incorporated into budding virions. A few cells in all three cell lines were found to produce only a few or no MuMTV particles and in these cells, unlike in the high virus-producing cells, considerable quantities of gp52 were expressed on the surface membrane. All three cell lines were found to shed large amounts of the MuMTV env precursor polyprotein as well as the mature non-virion-associated glycoprotein, gp52, into the culture medium. The envelope precursor protein (P75env) that was shed into the culture medium was found to differ from the predominant form of the cellular env precursor (Pr70env) in that (1) P75env migrated with an apparent higher molecular weight than Pr70env in SDS gels; (2) Pr70env contained only the core oligosaccharide, whereas P75env contained fucose in addition to the core sugars; (3) two-dimensional gel electrophoretic analysis showed that Pr70env could be resolved into three to four components migrating in the basic region of the isoelectric focussing gel (pH 7-8), whereas P75env was resolved into 9-13 components migrating in a more acidic region of the gel (pH 5-7). The molecular structure of the exfoliated gp52 was found to be similar to that of the gp52 that was incorporated into the virions although the virion-associated gp52 was not the source of the gp52 in the medium. Our quantitative pulse-chase studies suggest that of the two populations of MuMTV env precursors that are present in MuMTV-producing cells, only Pr70env is processed intracellularly to give rise to the mature MuMTV envelope proteins gp52 and gp36.

Strain-specific determinants on the major phosphoprotein of murine mammary tumor viruses

The major phosphoprotein (p23) of murine mammary tumor virus (MuMTV) was purified for C3H virus, using a combination of alkyl agarose chromatography and gel filtration. Monospecific antiserum was raised against glutaraldehyde crosslinked polymers of purified p23, and the antiserum was then used with iodinated p23 to develop a competition radioimmunoassay. Phosphoprotein p23 was shown to contain both group- and type-specific antigenic determinants in assays using detergent-disrupted virions of MuMTV strains C3H, RIII, GR, and DBA/2 as competing antigens. Tryptic and chymotryptic peptide mapping analysis of p23 polypeptides isolated from MuMTV strains C3H, RIII, GR and DBA/2 showed that the p23s of each virus strain could be clearly distinguished from each other.

Cloned mouse mammary tumor virus DNA exhibits glucocorticoid-dependent expression in simian virus 40-transformed mink cells

Mink lung epithelial cells were transfected with two cloned mouse mammary tumor virus (MMTV) DNAs, a 9-kilobase clone derived from an unintegrated exogenous viral genome and a 14-kilobase clone containing an integrated endogenous provirus along with cellular flanking sequences. Mink lung cells were chosen because they do not contain endogenous MMTV sequences. On the basis of our observation that simian virus 40 DNA efficiently transforms these cells, we isolated cell clones containing MMTV DNA by using transformation with simian virus 40 DNA as a selective marker in cotransfection experiments. Levels of the 9-kilobase MMTV mRNA representing the entire viral genome and of the spliced 4.4-kilobase mRNA which codes for the viral envelope proteins were glucocorticoid dependent in transformed cells. Expression of low levels of Pr77gag, the precursor of the group-specific viral core proteins, and of gPr73env, the precursor of the viral envelope proteins, was also hormone dependent. We conclude that these cloned MMTV DNAs contain all the information necessary for the synthesis of normal viral RNAs and proteins. These findings also provide further evidence that the DNA sequences involved in the hormone responsiveness of MMTV expression are contained within the viral genome.

Immunological characterization of a low oncogenic mouse mammary tumor virus BALB/cNIV mice

Antigenic determinants of mouse mammary tumor virus (MuMTV) from the low-mammary-tumor-incidence strain BALB/cNIV were compared by competition radioimmunoassay with those of MuMTV's isolated from several high- and low-mammary-tumor-incidence mouse strains, using rabbit hyperimmune sera against BALB/cNIV MuMTV and against MuMTV from the high-mammary-tumor-incidence strain BALB/cfC3H. Using anti-BALB/cfC3H serum in competition radioimmunoassay, BALB/cNIV MuMTV lacked antigenic determinants present on MuMTV's from the BALB/cfC3H, C3H, and GR strains. With anti-BALB/cNIV serum in competition radioimmunoassay, type-specific antigenicity was detected with BALB/cNIV MuMTV. We found class-specific antigenicities on BALB/cNIV MuMTV that were shared with RIII and C3Hf MuMTV's BALB/cNIV MuMTV was reportedly derived from the C3Hf strain by infection of BALB/c mouse mammary tissue after transplantation into a (BALB/c X C3Hf)F1 hybrid followed by retransplantation into the BALB/c strain (D. R. Pitelka, K. B. DeOme, and H. A. Bern. Proc. Am Assoc. Cancer Res. 6:51, 1965). However, BALB/cNIV MuMTV contained type-specific antigens not present on C3Hf MuMTV. The possible origin of these determinants is discussed.

Cloned mouse mammary tumor virus DNA is biologically active in transfected mouse cells and its expression is stimulated by glucocorticoid hormones

We have cloned circular unintegrated mouse mammary tumor virus (MMTV) DNA from infected rat hepatoma cells in bacteriophage lambda. Seven independent clones containing MMTV DNA of homogeneous length of 9 kb (five) or 10 kb (two) were identified. The five 9 kb clones had identical restriction maps consistent with that of 9 kb unintegrated DNA; the other two were aberrant. MMTV DNA inserts were purified, ligated and used for cotransfection of Ltk- cells together with a plasmid containing the thymidine kinase gene of herpes simplex virus. All Tk+ cell clones acquired new MMTV sequences and those transfected with the 9 kb MMTV DNA synthesized normal viral RNA and proteins. Viral gene expression was increased by the addition of dexamethasone.

Murine mammary tumor virus structural protein interactions: formation of oligomeric complexes with cleavable cross-linking agents

Murine mammary tumor virus protein interactions in the intact virion structure were studied with the use of the cleavable cross-linking reagents dithiobis(succinimidyl propionate) and methyl 4-mercaptobutyrimidate hydrochloride. Cross-linked oligomeric complexes of murine mammary tumor virus proteins were analyzed by two-dimensional gel electrophoresis. Among the complexes most consistently formed were a heterodimer of the two glycoproteins gp36 and gp52, the homodimer of gp36, and the homotrimer of gp52. A very prominent oligomer formed at higher concentrations of dithiobis(succinimidyl propionate) was a complex of about 230,000 molecular weight, made up of three molecules each of gp36 and gp52. A number of lines of evidence, including electron microscopic analysis, suggest that the 230,000-molecular-weight complex actually represents the murine mammary tumor virus spike structure. Of the murine mammary tumor virus core proteins, p14 forms homooligomers most readily. Upon cross-linking with methyl 4-mercaptobutyrimidate hydrochloride a small amount of what seems to be a heterodimer made up of the N-terminal gag protein p10 and the hydrophobic membrane glycoprotein gp36 can be observed.

Structure and processing of the mouse mammary tumor virus glycoprotein precursor pr73env

The polyprotein precursor to the envelope glycoproteins of mouse mammary tumor virus was investigated by using subcellular fractionation procedures, pactomycin mapping techniques, tunicamycin inhibition of glycosylation, and endo-beta-N-acetyl glucosaminidase H-catalyzed removal of glycosylated residues in order to characterize the biosynthesis and processing of the precursor. The results suggest that the precursor (Pr73env) is synthesized on the rough endoplasmic reticulum as a transmembrane protein, with the carboxyl terminus remaining on the cytoplasmic side. The apoprotein as an estimated molecular weight of 60,000 and acquires five core oligosaccharide units during synthesis. Cleavage of the precursor precedes the secondary glycosylation steps and therefore probably occurs before transport to the plasma membrane. However, a minor population of Pr73env containing complex oligosaccharides was also found in the plasma membrane. The order of the glycoproteins in the precursor, as determined by pactomycin mapping, in NH2-gp52-gp36-COOH.

Feasibility studies of oncornavirus production in microcarrier cultures

Studies conducted with virus-infected monolayer cell cultures have demonstrated the feasibility of producing several tumor-associated viruses in microcarrier (mc) cultures (Sephadex G50 beads treated with DEAE-chloride). The efficiency of cell adherence to mc varied with the cell type, the pH of the growth medium, and the stirring force applied to keep the mc in suspension. Most cells attached firmly to the mc and could not be removed easily with routine trypsinization procedures. Techniques using Enzar-T and Pronase were effective in detaching cells from mc in 10 to 15 min while retaining 95% cell viability. After detachment, Ficoll gradients were used for rapid and complete separation of viable cell suspensions from the mc. Retrovirus production in large volumes of mc cultures was investigated with periodic harvesting of growth fluids. Physical, biochemical, and biological properties of the Mason-Pfizer monkey virus and the RD114 virus recovered from the mc cultures were identical to those produced in conventional cultures. The utilization of mc has several applications in research and short-term cultures, but the as-yet-unsolved technical problems met were found to be serious limitations when attempting mass cell culturing on a long-term basis.

Glucocorticoid regulation of mouse mammary tumor virus gene expression

Glucocorticoid hormones act rapidly and specifically to stimulate the synthesis of mouse mammary tumor virus RNA in a variety of mouse mammary tumor cells and infected heterologous cells. The increase in viral RNA production appears to be mediated by receptor proteins and requires the presence of basal levels of viral RNA. Infection of heterologous cells with MMTV may alter host cell responses to glucocorticoids; in addition, production of unintegrated viral DNA in these cells has provided reagents required for studying the structure and function of the viral DNA itself. The advent of new techniques for genetic manipulation of eukaryotic cells and for isolation of large amounts of specific DNA sequences should now permit detailed analyses of steroid hormone action in this system.

Mouse mammary tumor virus (MMTV) antigen(s) are present on B lymphocytes of BALB/c mice

Immunofluorescence studies using a polyvalent anti-MMTV serum revealed the presence of MMTV antigen(s) on the surface of spleen cells from the low mammary tumor incidence mouse strain BALB/cCrgl. Upon separation of the splenocytes on nylon-wool columns the populations of cells exhibiting positive fluorescence were nylon-adherent. These cells were considered to be B lymphocytes by the following criteria: presence of surface immunoglobulin (Slg), absence of Thy-1 antigens, enhanced responses to B-cell mitogens and decreased reactivity to T-cell mitogens. In order to ascertain the specificity of the cell surface immunofluorescence reaction, blocking studies were performed. Immunofluorescence was blocked by preincubation of the antiserum or with purified MMTV, but not by R-MuLV. Non-specific binding of the anti-MMTV to Fc receptors present on B lymphocytes was ruled out by the use of Fab'2 fragments of IgG from the various sera. Double label immunofluorescent studies with rhodamine-conjugated anti-mouse Slg Fab'2 fragments and rabbit anti-MMTV Fab'2 with fluorescein-conjugated goat anti-rabbit Fab'2 showed the simultaneous appearance of both labels in the nylon-adherent splenocytes. These results suggest the expression of viral polypeptide(s) encoded by an endogenous MMTV on the surface of B-type splenocytes in mice devoid of intact viral particles.

In vitro infectivity assay for mouse mammary tumor virus

Studies of mouse mammary tumor virus (MMTV) have been impeded by the lack of an in vitro infectivity assay. We have developed a rapid, quantitative in vitro assay for MMTV infectivity based on the detection of positively staining foci by immunoperoxidase. This assay and a 50% end-point titration of MMTV infectivity gave identical virus titers. Infection of a rat hepatoma cell line, a feline kidney cell line, and a normal murine mammary gland cell line by virus from the mouse mammary tumor GR3A cell line was linear with respect to virus concentration. The infectious titers obtained in both homologous and heterologous cell lines were not significantly different, demonstrating a lack of host range specificity. Virus infectivity was inactivated by heating at 55 degrees C and by ultraviolet irradiation. Rabbit anti-MMTV serum neutralized the infectivity with a 50% neutralization end point of 1:5000. Applications of this assay to the study of the immunological, biological, and biochemical characteristics of MMTV are discussed.

Simultaneous purification of murine mammary tumor virus structural proteins: analysis of antigenic reactivities of native gp34 by radioimmunocompetition assays

All the structural proteins (gp47, gp34, p27, p23, p16, and p12) of the murine mammary tumor virus (MuMTV) were simultaneously purified utilizing alkylagarose chromatography as the initial fractionation step. Least-hydrophobic MuMTV polypeptides (p23, p16) and the slightly hydrophobic p27 were separated from moderately hydrophobic proteins gp47 and p12 by passage through octylimino (C(8))-agarose; the gp47 and p12 could be removed from the matrix by elution with ethylene glycol, whereas the most hydrophobic MuMTV protein, gp34, was eluted using nonionic detergent together with ethylene glycol. Subsequent purification steps involved ion-exchange or gel filtration chromatography. The resulting protein preparations appeared near-homogeneous on analysis by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Recoveries of MuMTV proteins, based on their approximate individual contribution to total virus protein, ranged from about 20% for gp47 to greater than 100% for the minor structural component p23, the major phosphoprotein of MuMTV. Antiserum against purified C3H MuMTV gp34, together with purified, radioiodinated gp34, was used to develop a radioimmunoassay which showed that from 13 to 14% of total MuMTV protein by weight is gp34. Using this assay system, the group-specific antigenic reactivity of gp34 was also demonstrated. When solubilized preparations of C3H, RIII, and GR MuMTV's were used as competing antigens in gp34 radioimmunoassays with anti-C3H MuMTV serum, both group- and type-specific differences in antigenic reactivity were found.

Radioimmunoassays for the 36,000-dalton glycoprotein of murine mammary tumor viruses demonstrate type, group, and interspecies determinants

Mouse mammary tumor viruses (MMTVs) contain distinct membrane glycoproteins of 52,000 daltons (gp52) and 36,000 daltons (gp36). We report here the development of new radioimmunoassays for gp36, using gp36 purified by hydrophobic chromatography and gel filtration. These assays demonstrate that gp36 has both type-specific and group-specific antigenic determinants. The virus-coded nature of these determinants was shown by utilizing different MMTVs grown in the same feline cell line. Interspecies determinants on gp36 were demonstrated by the observations that (i) MC-MTV (a virus isolate from the Asian rodent Mus cervicolor, and morphologically identical to MMTVs) competed, with an altered slope, in the gp36 radioimmunoassay, and (ii) antisera raised against MC-MTV immunopreciptitated 125I-labeled gp36. The detection of gp36 in spontaneous mammary tumors of several strains of mice also facilitates further studies on the replication of MMTVs and the host's immune response to MMTV-mediated oncogenesis.

Murine mammary tumor virus expression during mammary tumorigenesis in BALB/c mice

Steady-state levels of murine mammary tumor virus (MuMTV) RNA were quantitated during mammary tumorigenesis in BALB/c mice by molecular hybridization with a representative MuMTV complementary DNA (cDNA) probe. Hyperplastic alveolar nodule (HAN) lines are preneoplastic mammary lesions that were induced in BALB/c mice by hormones alone or in combination with 7,12-dimethylbenz(a)anthracene and give rise to mammary tumors. The hormone-induced HAN lines D1 and D2 contained detectable amounts of hybridizable MuMTV sequences. MuMTV RNA sequences were also observed in five of the six transplanted BALB/c mammary tumors that were examined. Similar levels of hybridizable MuMTV RNA were observed between the D1 or D2 HAN line and mammary tumors derived from each HAN line. The D2 HAN line as well as D2, C4, and CD8 mammary tumors accumulated RNA that was apparently homologous to most of the MuMTV genome. Thermal denaturation of hybrids indicated extensive sequence homology between the MuMTV cDNA and hybridizable RNA in the BALB/c HAN lines and mammary tumors. A low level of type C viral RNA was observed in the BALB/c HAN lines and most mammary tumors by molecular hybridization with a cDNA to Moloney murine leukemia virus. These data demonstrate that MuMTV sequences are frequently expressed in hormone-induced BALB/c HAN lines and mammary tumors derived from HAN lines or ductal hyperplasias induced in BALB/c mice by hormones and/or a chemical carcinogen. The transition from the preneoplastic to the neoplastic state in BALB/c mice does not appear to be due to a change in the steady-state levels of MuMTV RNA since the hormone-induced HAN lines and mammary tumors had similar levels of hybridizable MuMTV RNA.

Large-scale production of mouse mammary tumor virus in the absence of endogenous murine leukemia virus

A system for the large-scale production and purification of mouse mammary tumor virus in the absence of detectable endogenous murine leukemia virus is described. By utilizing the Mm5mt/c1 cell line established from an adenocarcinoma of a C3H mouse, the continuous production of over 25,000 liters of mouse mammary tumor virus-containing tissue culture fluids has been achieved. By the strict adherence to well-defined tissue culture conditions, mammary tumor virus production was accomplished without the expression of murine leukemia virus. Various biochemical and immunological systems were established for the rapid and precise detection of the endogenous leukemia virus, the expression of which could be enhanced under conditions of culture stress.

Regulation of mouse mammary tumor virus gene expression by glucocorticoid hormones

Several laboratories have documented that glucocorticoid hormones markedly stimulate the expression of mouse mammary tumor virus genes in a variety of mouse mammary tumor cells and in infected heterologous cells. The effect of the hormone appears to be a rapid and specific augmentation of the synthesis of viral RNA, mediated by interaction with glucocorticoid receptor proteins. The availability of virus-specific reagents and recent developments in the molecular biology of RNA tumor viruses now permit a highly refined analysis of hormonal regulation in this experimental system.

Strain-specific markers for the major structural proteins of highly oncogenic murine mammary tumor viruses by tryptic peptide analyses

Tryptic peptide analyses were performed on the major structural 52,000- and 36,000-dalton glycoproteins (gp52 and gp36-38) and the nonglycosylated 28,000-, 14,000-, and 10,000-dalton proteins (p28, p14, and p10) of the highly oncogenic murine mammary tumor viruses (MMTVs) of C3H, RIII, and GR mice, i.e., MMTV(C3H), MMTV(RIII), and MMTV(GR), respectively. Each virus was grown in both murine and feline cells to ensure the virus-coded nature of each peptide analyzed. The gp36-38 peptide maps of all three MMTVs were indistinguishable, as were the p14 maps of the different MMTVs. Both the p28 and the gp52 of MMTV(C3H), however, could be clearly distinguished from the corresponding proteins of MMTV(RIII) and MMTV(GR), regardless of whether the viruses were grown in feline or murine cells. The p1o of MMTV(RIII) was clearly different from that of MMTV(C3H) and MMTV(GR). Therefore, tryptic peptide analysis of three proteins, gp52, p28, and p10, can serve to distinguish these three viruses from one another. These studies further characterize the heterogeneity in polypeptides among MMTVs.

Mammary leukaemia (ML) antigen isolated from L 1210 leukaemia cells

Crude 3 M KCl extracts from L 1210 cells and purified ML antigen were characterized and the presence of ML antigens was verified by various anti-ML sera, as well as by antisera to disrupted MMTV B particles. Selected ML-positive fractions from preparative polyacrylamide electrophoresis and purified ML antigen isolated by affinity chromatography were analyzed by polyacrylamide disc electrophoresis. With both procedures the same sharp band was obtained. The molecular weight of both preparations was found to be 73,500 as estimated. The relationship between ML antigen, MMTV antigenic products and cell-associated viral proteins is discussed.

Synthesis and processing of precursor polypeptides to murine mammary tumor virus structural proteins

Biosynthesis of murine mammary tumor virus (MuMTV) proteins was studied in the chronically MuMTV-infected epithelial cell line MuMT-73 by using monospecific antisera to the major MuMTV core protein p27 and the major envelope glycoprotein gp47. In pulse-labeling experiments using [35S]methionine, monospecific antisera to p27 precipitated a 75,000-molecular-weight protein as the major intracellular component. Analysis of the same cellular extracts with monospecific antisera to gp47 revealed that the gp47 precursor was a 70,000-dalton protein. After chase periods, there was a loss of label from the precursors and a concomitant increase of labeled extracellular mature viral proteins. The glycoprotein precursor incorporated labeled glucosamine and seemed to be processed more rapidly than the p27 precursor. Considerable amounts of apparently nonvirion-associated gp47 and glycoprotein precursor could be detected in the extracellular culture fluid.

Type-specific antigenic determinants on the major external glycoprotein of high- and low-oncogenic murine mammary tumor viruses

We recently showed that the 52,000-dalton external glycoprotein (gp52) of the highly oncogenic mouse mammary tumor viruses (MMTVs) of RIII, GR, and C3H mice contains both type- and group-specific antigenic determinants. This was demonstrated by using a competition radioimmunoassay with 125I-externally labeled virions and antisera to the gp52 of MMTV from RIII mice (Proc. Natl. Acad. Sci. U.S.A. 74:3564-3568, 1977). We report here that we were able to distinguish between the gp52's of the high-oncogenic MMTV of C3H mice [MMTV(C3H)] and the low-oncogenic MMTV of that same mouse strain [MMTV(C3Hf)]. This was accomplished by use of a competition radioimmunoassay with 125I-externally labeled virions of MMTV(C3H) and antisera prepared against MMTV(C3H). A comparison of the intact virion and purified gp52 radioimmunoassays showed that MMTV type-specific differences were enhanced with the intact virion radioimmunoassay. These differences were further magnified with appropriately absorbed antisera. These findings thus allow an immunological distinction between the surface glycoproteins of a low-oncogenic endogenous and a high-oncogenic exogenous MMTV of the same mouse strain.

Multiple antigenic determinants on the major surface glycoprotein of murine mammary tumor viruses

The major external protein of murine mammary tumor viruses (MMTVs) is a glycoprotein of molecular weight 52,000 (gp52). We report here that the gp52s of MMTVs from different mouse strains contain both type-specific and group-specific antigenic determinants. Competitive radioimmunoassays with externally labeled virion preparations were used to demonstrate that MMTV virions from C3H, RIII, and GR mice competed identically in "group-specific" assays but either competed with altered slopes or competed incompletely in the appropriate "type-specific" assays. Similar results were obtained with purified gp52 preparations as competitors. MMTVs of RIII, C3H, and GR mice, grown in feline cells, competed similarly to the corresponding murine-derived MMTVs, thus demonstrating that the observed type-specific and group-specific antigenic reactivities are viral coded. In competitive radioimmunoassays using an antiserum against MMTV of C3H mice that was preabsorbed with MMTV of GR mice, type-specific reactivities were further magnified; MMTV of C3H mice competed completely whereas no competition was observed with MMTV of GR mice as a competitor. These studies are in agreement with previous biological and molecular hybridization studies on the diversity of MMTVs from different mouse strains.

The major polypeptides of the murine-mammary-tumor virus isolated by plant-lectin affinity chromatography

Solubilized polypeptides of the murine mammary tumor virus (MuMT virus) were chromatographed on a column of immobilised concanavalin A. The unbound viral material was rechromatographed on phosphocellulose, resulting in the isolation of the major proteins with a molecular weight of 28000 (p28) and 12000 (p12) respectively. The adsorbed glycopolypeptides after elution with methyl alpha-D-mannopyranoside were subjected to gel filtration. The major glycoprotein with a molecular weight of 52000 (gp52) was obtained in an almost pure form. However, a considerable part of gp52 elutes together with a glycoprotein with a molecular weight of 36000 (gp36), suggesting that in addition to the free form of gp52 a complex exists of gp52 plus gp36.

Isolation and comparison of murine leukemia virus-related glycoproteins from AKR and New Zealand mice

The major glycoprotein (gp70) of murine leukemia virus occurs free of virus in the serum and body fluids of certain strains of mice. These glycoproteins were isolated from New Zealand Black mouse (NZB) ascites fluid and from AKR and New Zealand White mouse (NZW) serum by immunoaffinity chromatography and were compared by immunological tests and peptide mapping. Glycoproteins gp70-NZB and gp70-NZW were indistinguishable by all criteria tested and were more closely related to gp70 from Moloney leukemia virus than was gp70-AKR.

Proteins of bovine leukemia virus. I. Characterization and reactions with natural antibodies

The bovine leukemia virus (BLV) was purified from a chronically infected fetal lamb kidney cell line. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of this virus revealed the presence of eight distinguishable viral components with molecular weights ranging from 80,000 to 11,000. The major component is a non-glycosylated protein having a molecular weight of 24,000 (p24). At least three heavier polypeptides were found, one of them representing a glycoprotein (gp 60). In addition, four minor polypeptides with respective molecular weights of 19,000, 16,000, 13,000, and 11,000 were identified. In a complement fixation assay using naturally occurring antibodies of a leukemic cow, four polypeptides, which included gp 60, p35, p24, and p16, were found to be reactive.