Identification of a precursor protein to the major glycoproteins of mouse mammary tumor virus

Expression of intracisternal A-particle-related retroviral element-encoded envelope proteins detected in cell lines

Intracisternal A-particle (IAP) retrotransposons of rodents express gag and pol proteins for assembly of intracellular viruslike particles but lack an env gene. The recently described IAP-related family of retroviral elements contains a reading frame with close resemblance to retroviral env genes (IAPEs) (F. U. Reuss and H. C. Schaller, J. Virol. 65:5702-5709, 1991). I now report the analysis of cellular IAPE mRNAs and detection of IAPE env proteins. IAPE elements are transcribed in cell lines NH15-CA2 and AtT20. Four major transcripts of 4.2, 3.9, 2.8, and 1.3 kb are detected and characterized by probes specific for defined regions of the cloned IAPE-1 cDNA. The 2.8-kb mRNA is shown to lack gag and pol genes but comprises an env gene and U3 region, as expected for a subgenomic env mRNA. Polymerase chain reaction amplification and cloning of such mRNAs confirmed the absence of gag and pol genes 5' from the env gene and implicates env mRNA generation by a splicing event. A polyclonal anti-IAPE env antiserum, raised against a bacterial IAPE-env fusion protein, specifically detects N-glycosylated env proteins of 91 kDa or less in cell lines positive for IAPE mRNA. IAPE env proteins of different sizes represent independent translation products. After inhibition of N-glycosylation, env proteins in the size predicted from the env gene sequence or smaller are present. These results provide evidence that putative IAPE env proteins are synthesized in vivo. Envelope protein expression by an IAP-related retroviral element identifies IAPEs as a possible missing link between IAP retrotransposons and retroviruses.

Dexamethasone-stimulated expression of a proviral copy of mouse mammary tumor virus env mRNA

The kinetics of dexamethasone induction of mouse mammary tumor virus RNAs in a cell line (designated line 8) which contains a deleted proviral DNA has been examined. This DNA is an apparent reverse transcription product of the env mRNA. A detectable increase in the 3.8-kilobase RNA (mouse mammary tumor virus env mRNA) was present within 15 min after hormone addition. The 2.5-kilobase RNA detected previously during hormone induction studies was also observed in these cells. Both RNAs increased in concentration up to about 4 h after hormone addition and thereafter attained the steady-state level. Pr73env was detected by immunoprecipitation and appeared to be identical to bona fide Pr73env isolated from wild-type mouse mammary tumor virus-infected cells. An additional 5.7 kilobases of RNA was detected in augmented quantities after hormone addition, but the structure of this RNA is not known.

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.

Candidate product of the FBJ murine osteosarcoma virus oncogene: characterization of a 55,000-dalton phosphoprotein

Sera from rat bearing tumors induced by inoculation of FBJ murine osteogenic sarcoma virus (FBJ-MSV) nonproducer rat cells precipitate two proteins with molecular weights of 55,000 (p55) and 39,000 (p39) from FBJ-MSV-transformed cells. These proteins cannot be precipitated from uninfected cells or cells transformed by other strains of murine sarcoma virus, nor can they be precipitated by sera specific for the viral structural proteins. A methionine tryptic peptide mapping analysis showed that p55 and p39 have little or no homology and that they are not related to the helper virus gag and env gene products. p55 could also be detected among the in vitro translation products of 70S RNA from FBJ murine leukemia virus plus FBJ-MSV virions but not among those from FBJ murine leukemia virus alone. This suggests that p55 is encoded by the FBJ-MSV genome, whereas p39, which was not detected among the in vitro translation products, may not be virus encoded. Another difference between p55 and p39 is that p55 is phosphorylated, with most of the phosphate on a serine residue(s), whereas p39 is phosphorylated to a much lesser extent, if at all. No protein kinase activity was associated with p55 and p39 immune complexes under standard conditions. Our data suggest that p55 is a strong candidate for the FBJ-MSV oncogene product.

Processing and amino acid sequence analysis of the mouse mammary tumor virus env gene product

The envelope proteins of mouse mammary tumor virus (MMTV) are synthesized from a subgenomic 24S mRNA as a 75,000-dalton glycosylated precursor polyprotein which is eventually processed to the mature glycoproteins gp52 and gp36. In vivo synthesis of this env precursor in the presence of the core glycosylation inhibitor tunicamycin yielded a precursor of approximately 61,000 daltons (P61env). However, a 67,000-dalton protein (P67env) was obtained from cell-free translation with the MMTV 24S mRNA as the template. To determine whether the portion of the protein cleaved from P67env to give P61env was removed from the NH2-terminal end of P67env and as such would represent a leader sequence, the NH2-terminal amino acid sequence of the terminal peptide gp52 was determined. Glutamic acid, and not methionine, was found to be the amino-terminal residue of gp52, indicating that the cleaved portion was derived from the NH2-terminal end of P67env. The NH2-terminal amino acid sequences of gp52's from endogenous and exogenous C3H MMTVs were determined though 46 residues and found to be identical. However, amino acid composition and type-specific gp52 radioimmunoassays from MMTVs grown in heterologous cells indicated primary structure differences between gp52's of the two viruses. The nucleic acid sequence of cloned MMTV DNA fragments (J. Majors and H. E. Varmus, personal communication) in conjunction with the NH2-terminal sequence of gp52 allowed localization of the env gene in the MMTV genome. Nucleotides coding for the NH2 terminus of gp52 begin approximately 0.8 kilobase to the 3' side of the single EcoRI cleavage site. Localization of the env gene at that point agrees with the proposed gene order -gag-pol-env- and also allows sufficient coding potential for the glycoprotein precursor without extending into the long terminal repeat.

Dexamethasone induction of the intracellular RNAs of mouse mammary tumor virus

We have studied the kinetics of dexamethasone induction of mouse mammary tumor virus (MMTV) RNAs and proteins in virus-infected rat XC cells and GR mouse mammary tumor cells. A detectable increase in viral RNA in infected XC cells was present within 10 min after hormone addition, and half-maximal induction was achieved in less than 2 h. The increase in viral RNA concentration was apparent first in nuclear RNA and later in the cytoplasm. Within the first 15 min of induction, only genome-sized RNA (35S, 7.8 kilobases) was present in augmented amounts, whereas the major subgenomic RNA (24S, 3.8 kilobases) did not appear until at least 30 to 60 min postinduction. The sequential appearance of these RNAs, the probable mRNA's for the gag and env proteins, paralleled the order of appearance of the gag and env proteins, respectively, after hormone treatment. An additional species of viral RNA (20S, 2.5 kilobases) was detected during these induction experiments, but the role of this RNA is not known. Both subgenomic RNAs contain sequences derived from both the 5' and 3' termini of genomic RNA and are presumably spliced. After dexamethasone induction of infected XC cells, we detected two smaller env-related proteins which were not found in full hormone induction. The functional role of these smaller proteins is not known. A previously reported smaller species of RNA (13S, 1.0 kilobase) did not appear to be induced and was shown to be cellular rather than viral in origin. In the fully induced infected XC and GR mammary tumor cells, the only viral RNAs present were the 35S and 24S RNAs. In addition, mammary tumors contained only these two viral RNAs. Thus, tumor cells appear to contain only the viral RNAs which direct the synthesis of the gag, pol, and env proteins of the virion.

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.

Protein-coding potential of mouse mammary tumor virus genome RNA as examined by in vitro translation

The protein-coding capacity of the mouse mammary tumor virus genome has been examined by in vitro translation of genome length and polyadenylated subgenomic fragments of viral RNA. Intact genome RNA of about 35S programmed synthesis of the Pr77gag, Pr110gag and Pr160gag/pol precursors seen in infected cells in vivo. Polyadenylated RNA fragments of 18 to 28S encoded products whose tryptic peptide maps resembled those of the nonglycosylated precursor to the envelope glycoproteins, confirming the gene order 5'-gag-pol-env-3'. Translation of polyadenylated RNA fragments smaller than 18S yielded a series of related proteins whose peptide maps bore no resemblance to any of the virion structural proteins. Thus, a region of the mouse mammary tumor virus genome distal to the env gene appears to have an open reading frame sufficient to encode at least 36,000 daltons of protein as of yet unknown function.

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.

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.

Polyprotein precursors to mouse mammary tumor virus proteins

Mouse mammary tumor virus (MMTV) derived from the culture medium of GR cells contained seven proteins, identified as gp55, gp33, p25, pp20, p16, p12, and p10. The major viral phosphoprotein was the 20,000-molecular-weight protein, pp20. Immunoprecipitation of cytoplasmic extracts from pulse-labeled GR cells identified three MMTV gag-specific proteins, termed Pr78(gag), Pr110(gag), and Pr180(gag+). These intracellular polyproteins were precipitable from cytoplasmic extracts by antisera to virions p25 and p12 but not by antisera to gp55. The major intracellular gag-specific precursor polyprotein, Pr78(gag), contained antigenic determinants and tryptic peptides characteristic of p25, p12, p10, and presumably pp20. This precursor is presumably derived from nascent chain cleavage or rapid posttranslational cleavage of the larger intracellular precursor-like protein, designated Pr110(gag). Pr110(gag) contained all but one of the leucine-containing tryptic peptides of Pr78(gag), plus several additional peptides. In addition to Pr78(gag) and Pr110(gag), monospecific antisera to virion p12 and p25 were also capable of precipitating from pulse-labeled cells a small amount of a 180,000-molecular-weight precursor-like protein, designated Pr180(gag+). This large polyprotein contained nearly all of the leucine-containing tryptic peptides of Pr78(gag) and Pr110(gag) plus several additional peptides. By analogy to type C viral systems, Pr180(gag+) is presumed to represent a gag-pol common precursor which is the major pathway for synthesis of MMTV polymerase. Immunoprecipitation of cytoplasmic extracts from pulse-labeled cells with antisera to gp55 identified two env-specific proteins, designated gPr76(env) and gP79(env). The major env precursor, gPr76(env), could be labeled with radioactive glucosamine and was shown to contain antigenic determinants and tryptic peptides characteristic of gp55 and gp33. A minor glycoprotein, gP79(env), contained both fucose and glucosamine and was precipitable from cytoplasmic extracts with monospecific serum to gp55. It is suggested that gP79(env) represents fucosylated gPr76(env) which is transiently synthesized and cleaved rapidly into gp55 and gp33.

Impaired maturation of mouse mammary tumor virus precursor polypeptides in lymphoid leukemia cells, producing intracytoplasmic A particles and no extracellular B-type virions

Processing of polypeptides of the mouse mammary tumor virus, a type B retrovirus, was investigated in a transplanted thymic lymphoma cell line of the GR strain (GRSL). This cell line was maintained in vivo in ascites form and in vitro as a suspension culture. GRSL cells produce clusters of intracytoplasmic A particles and are virtually deficient in the production of mature extracellular B-type particles. As control, a mammary tumor cell line of the same mouse strain capable of complete virion synthesis was used. The kinetics of viral polypeptide synthesis were studied by pulse labeling with various isotopes (including (35)S and (32)P), followed by immunoprecipitation of cell lysates with monospecific antisera to the major mouse mammary tumor virus gag and env proteins, p27 and gp52, respectively. Both the primary gag and env precursor polypeptides were synthesized in the GRSL cells, but their conversion into viral proteins was impaired. The major gag precursor, Pr73(gag), was stable over a period of 8 h, and mature viral core polypeptides could not be detected. Also, the highly phosphorylated intermediates in the proteolytic processing of Pr73(gag) in virus-producing cells were absent in GRSL cells. By immunoprecipitation, Pr73(gag) was detected in a GRSL particle fraction with the density of intracytoplasmic A particles. The precursor for envelope proteins, Pr73(env), was turned over without the generation of mature viral envelope components gp52 and gp36. The in vivo-transplanted ascites GRSL cells, however, were shown to express gp52 on the cell surface together with a 73,000-dalton polypeptide, as indicated by cell surface iodination and immunoprecipitation.

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.

Structural analysis of the intracellular RNAs of murine mammary tumor virus

We have characterized murine mammary tumor virus (MuMTV)-specific RNA in several types of cells in which viral DNA is transcribed into RNA: cultured GR mouse mammary tumor cells, S49 lymphoma cells from BALB/c mice, lactating mammary glands from C57BL/6 mice, and mink lung cells infected in vitro with MuMTV. In all cell types studied, there are three distinct species of intracellular viral RNA, with sedimentation coefficients of 35S, 24S, and 13S (or molecular weights of 3.1 X 10(6), 1.5 X 10(6), and 0.37 X 10(6), as determined by rate-zonal sedimentation in sucrose gradients and by electrophoresis in agarose gels under denaturing conditions. These three viral RNA species appear to be present regardless of viral RNA concentration, responsiveness to glucocorticoid hormones, production of extracellular virus, and use of either endogenous or acquired MuMTV proviral DNA as template. The three viral RNAs display characteristics of mRNAs in that they are polyadenylated, associated with polyribosomes, and released from polyribosomes by treatment with EDTA; hence all three species presumably direct the synthesis of virus-coded proteins. The two larger species of viral RNA are probably responsible for synthesis of the structural proteins of the virion, but the function of the 13S RNA is not known. Both of the subgenomic RNAs contain sequences found at the 3' terminus of 35S (or genomic) RNA. However, only the 24S RNA (not the 13S RNA) contains sequences which are located at the 5' terminus of 35S RNA and are apparently transposed during RNA synthesis of maturation, as described for subgenomic mRNA's of other retroviruses.

Regulation of protein synthesis in rat ventral prostate: cell-free translation of mRNA

Androgenic steroids regulate both a class of abundant poly(A)-containing RNA and four major proteins secreted by the rat ventral prostate. That this class of RNA was mRNA coding for the major proteins was demonstrated by translating total poly(A)-containing RNA in a wheat-germ cell-free system and using the technique of hybrid arrested translation. RNA alone resulted in the synthesis of four major translation products, whereas RNA hybridized with DNA complementary to the class of abundant poly(A)-containing RNA failed to direct their cell-free synthesis. The synthesis of the polypeptides was androgen dependent, but their relative mobilities on denaturing acrylamide gels were not identical to those of the polypeptides synthesized in whole cells. Because these polypeptides are secreted to form part of the prostatic fluid, the possibility that the immediate translation products are precursors to the polypeptides was investigated. Prostatic RNA, translated in an L-cell cell-free system supplemented with microsomal membranes isolated from dog pancreas and rat prostate, caused the appearance of four translation products with relative mobilities similar to those of the androgen-dependent polypeptides synthesized in whole cells. The relationship of one of the polypeptides synthesized in minced tissue with the immediate translation products and processed polypeptides was further investigated by tryptic peptide fingerprint analysis, which revealed identical peptide maps.

Identification of mouse mammary tumor virus-specific mRNA

Complementary DNA corresponding to the RNA genome of mouse mammary tumor virus was used to identify viral RNA contained in polysomes of a virus-producing mammary tumor cell line. Separation of polysomal mRNA by agarose gel electrophoresis, transfer of the RNA to diazobenzyloxymethyl paper, and hybridization with 32P-labeled mouse mammary tumor virus complementary DNA revealed three viral RNA size classes of 10, 8.8, and 4.4 kilobases in length, respectively.

Immunological characterization of mouse mammary tumor virus p10 and its presence in mammary tumors and sera of tumor-bearing mice

Mouse mammary tumor virus (MMTV) p10 and gp52 were purified and used as radiolabeled antigens in sensitive radioimmunoassays. These radioimmunoassays were specific for MMTV proteins since detergent-disrupted MMTV from C3H/HeN, RIII, and GR/N mice gave complete competition, whereas C3H/HeNf liver extracts and other lysed retroviruses did not. Both gp52 and p10 are coded by the viral genome, since MMTV grown in a heterologous cell line (feline kidney cells) competed in these assays. Sera from mammary tumor-bearing mice and mammary tumors from C3H/HeN and C3H/HeNf mice competed in both the gp52 and the p10 assays. Although these radioimmunoassays detected predominantly group-specific antigenic determinants in C3H/HeN and C3H/HeNf tumor extracts, type specificity was also found with gp52. Absorption of the anti-MMTV serum with C3H/HeNf tumor extracts removed all antibodies directed against p10 and decreased the anti-gp52 titer approximately 30-fold. When this absorbed antiserum was used at limiting dilution in the gp52 radioimmunoassay, C3H/HeN tumor extracts gave complete competition, whereas no competition was found with C3H/HeNf tumor extracts.

Cell-free synthesis of mouse mammary tumor virus Pr77 from virion and intracellular mRNA

Mouse mammary tumor virus (MuMTV) was purified from two cell lines (GR and Mm5MT/c1), and the genomic RNA was isolated and translated in vitro in cell-free systems derived from mouse L cells and rabbit reticulocytes. The major translation product in both systems was a protein with the molecular weight 77,000. Several other products were also detected, among them a 110,000-dalton and in minor amounts a 160,000-dalton protein. All three polypeptides were specifically immunoprecipitated by antiserum raised against the major core protein of MuMTV (p27), but they were not precipitated by antiserum against the virion glycoprotein gp52. Analysis of the in vitro products by tryptic peptide mapping established their relationship to the virion non-glycosylated structural proteins. The 77,000-dalton polypeptide was found to be similar, if not identical, to an analogous precursor isolated from MuMTV-producing cells. Peptide mapping of the 110,000-dalton protein shows that it contains all of the methionine-labeled peptides found in the 77,000-dalton protein plus some additional peptides. We conclude that the products synthesized in vitro from the genomic MuMTV RNA are related to the non-glycosylated virion structural proteins. Polyadenylic acid-containing RNA from MuMTV-producing cells also directed the synthesis of the 77,000-dalton polypeptide in the L-cell system. If this RNA preparation was first fractionated by sucrose gradient centrifugation the 77,000-dalton protein appeared to be synthesized from mRNA with a sedimentation coefficient between 25 and 35S.

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.

Modulation of mouse mammary tumor virus production in the MJY-alpha cell line

Implantation of the mouse mammary tumor virus (MMTV)-producing mammary tumor cell line MJY-alpha into isogeneic mice elicited both humoral and T-cell responses against MMTV virion antigens. The carcinosarcomas which developed from the implanted cells showed a significant decrease in MMTV synthesis, compared with cells remaining in culture, which was detectable as early as 7 days after implantation and for five transplant generations. Electron microscopic examination of thin sections of the tumors revealed that intracytoplasmic A particles, budding particles, and cell-free MMTV B particles were all affected. However, immunofluorescence assays of tumor sections demonstrated the presence of MMTV viral antigens in the cells. Cell cultures initiated from first-, third-, and fourth-generation tumors were morphologically identical to the original in vitro cell line, although virus production was barely detectable. Analysis of the cultures by electron microscopy revealed a significant increase in MMTV virions after in vitro passage 3. Polypeptide profiles obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of virions purified from these cultures were identical to MMTV. Immunodiffusion demonstrated the cross-reactivity between these virions and MMTV particles obtained from mouse milk. In vitro treatment of MJY-alpha cell cultures with rabbit anti-MMTV antiserum resulted in a reduction of extracellular MMTV virions, as well as alterations in their sodium dodecyl sulfate-polyacrylamide gel electrophoretic polypeptide patterns.

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.

Polyproteins related to the major core protein of mouse mammary tumor virus

The mouse mammary tumor virus (MuMTV) contains several low-molecular-weight proteins which, together with the genomic RNA, constitute the core structure of the virion. The most abundant protein in the core is the 27,000-dalton protein (p27), and, by analogy to the type C viruses, this protein probably forms the core shell. In mouse mammary tumor cell lines (GR and Mm5MT) producing MuMTV the major p57 antigenic specificity resides in a large protein, which migrates in polyacrylamide gels as a doublet of 77,000 and 75,000 daltons (p 77/75). A series of lower-molecular-weight proteins, p61, p48, p38, and p34, is also present in small amounts and is probably derived by proteolytic cleavage of the p 77/75. These proteins have been identified by immunoprecipitation with monospecific antiserum, and their sequence relatedness to p27 has been determined by an analysis of the peptides after trypsin digestion. After a 15-min pulse with [35S]-methionine, all of the p27-related proteins in these cell lines were labelled and, during a subsequent chase, progressively disappeared. The p27 was labeled poorly during the pulse, but the amount of label in this protein increased during the chase. A quantitation of these experiments suggested that the majority of the p27-related proteins were quite rapidly turned over in these cell lines. Hence, if p27 is derived by a progressive proteolytic cleavage mechanism, then the process is inefficient in the GR cells and only moderately efficient in the Mm5MT cells. When MuMTV was isolated from the culture medium of these cells harvested at 5-min intervals, the major p27-related protein was p34. The p27 accounted for only 29% of the anti-p27 serum immunoprecipitable proteins compared to 95% in virus isolated from an 18-h harvest. Incubation of the rapid-harvest virus at 37 degrees C for 2 h resulted in some conversion of p34 to p27. These results suggest that some of the p27 in MuMTV is formed in the virions by proteolytic cleavage of p34.

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.

Analysis of precursors to the envelope glycoproteins of avian RNA tumor viruses in chicken and quail cells

Immune precipitation with monospecific antiserum was employed to study the intracellular synthesis of viral glycoproteins gp85 and gp37. Labeled gp85 and gp37 were detected from lysates of cells transformed with Rous sacroma virus, strain B77, after long-term labeling with radioactive glucosamine or phenylalanine. Immune precipitates prepared from lysates of cells pulse-labeled for a short time resulted in a glycoprotein of 92,000 molecular weight (gp92). This precursor was stable in B77-transformed Japanese quail cells for several hours, whereas in chicken cells it could be chased within a few hours into virion glycoproteins gp85 and gp37. Similarly, the precursor for the structural viral proteins, pr76, persisted in quail cells much longer than in chicken cells. During very short pulses or in the presence of a glucosamine block (25 mM glucosamine), the antiserum against the viral envelope glycoproteins detected a precursor of higher electrophoretic mobility of approximately 70,000 molecular weight, "p70." Fucose label entered gp92 and gp85 as well as "p70." Proteolytic treatment of virion-bound gp85 in vitro generated two discrete glycoproteins of 62,000 and 45,000 molecular weight, but did not result in an increase in the amount of gp37.