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

Intracellular degradation of unassembled asialoglycoprotein receptor subunits: a pre-Golgi, nonlysosomal endoproteolytic cleavage

The human asialoglycoprotein receptor is a heterooligomer of the two homologous subunits H1 and H2. As occurs for other oligomeric receptors, not all of the newly made subunits are assembled in the RER into oligomers and some of each chain is degraded. We studied the degradation of the unassembled H2 subunit in fibroblasts that only express H2 (45,000 mol wt) and degrade all of it. After a 30 min lag, H2 is degraded with a half-life of 30 min. We identified a 35-kD intermediate in H2 degradation; it is the COOH-terminal, exoplasmic domain of H2. After a 90-min chase, all remaining intact H2 and the 35-kD fragment were endoglycosidase H sensitive, suggesting that the cleavage generating the 35-kD intermediate occurs without translocation to the medial Golgi compartment. Treatment of cells with leupeptin, chloroquine, or NH4Cl did not affect H2 degradation. Monensin slowed but did not block degradation. Incubation at 18-20 degrees C slowed the degradation dramatically and caused an increase in intracellular H2, suggesting that a membrane trafficking event occurs before H2 is degraded. Immunofluorescence microscopy of cells with or without an 18 degrees C preincubation showed a colocalization of H2 with the ER and not with the Golgi complex. We conclude that H2 is not degraded in lysosomes and never reaches the medial Golgi compartment in an intact form, but rather degradation is initiated in a pre-Golgi compartment, possibly part of the ER. The 35-kD fragment of H2 may define an initial proteolytic cleavage in the ER.

The soluble glycoprotein of vesicular stomatitis virus is formed during or shortly after the translation process

Gs protein is a shorter, soluble form of the viral G protein of vesicular stomatitis virus (VSV) lacking the membrane-anchoring domain. Production of Gs protein appears to be a general property of VSV because infection of BHK-21 cells by five different isolates of the VSV serotype Indiana led in all cases to the synthesis of Gs protein. Moreover, it is formed in a variety of eucaryotic cell lines after VSV infection. In pulse-chase experiments, we observed a time-dependent change in the ratio of G to Gs protein released into the growth medium, suggesting that Gs is formed intracellularly rather than on the cell surface. Further experiments revealed that Gs protein can be synthesized in vitro in the reticulocyte lysate system after addition of a viral mRNA fraction and in a coupled transcription-translation system with VSV core particles. In the presence of microsomal membranes both G and Gs protein were glycosylated in the reticulocyte lysate, confirming that the authentic Gs protein is synthesized in vitro. The addition of various protease inhibitors to the cell-free system and variation of the incubation conditions did not alter the ratio of G to Gs formation. Taken together, these experiments suggest strongly that Gs protein is not a product of a membrane-associated proteolytic activity but is formed during or shortly after the translation process. Our attempts to detect a specific, shorter mRNA coding for the Gs protein by molecular hybridization procedures did not reveal the existence of such a mRNA species.

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.

Murine mammary tumor virus related antigen in human male mammary carcinoma

An antigen immunologically related to mouse mammary tumor virus (MuMTV) and the major envelope glycoprotein, gp52 of MuMTV, was identified in tissue sections of human male and female mammary carcinomas using the peroxidase-antiperoxidase technique. The specificity of the reaction was established by absorption studies. Positive reactions with the gp52 antiserum were seen in mouse and human mammary carcinomas, but not in normal mammary tissues, mammary tissues with benign diseases and in other primary malignant neoplasms. Almost all (32/36, 89%) male mammary carcinomas were positive for the gp52 related antigen. A lesser proportion of tumors among female patients (14/50, 28%) were positive. The gp52 positive tumors were significantly larger than the gp52 negative tumors in female patients (P less than 0.05). Gp52 positive tumors were also larger than gp52 negative tumors in male patients, but the difference was not statistically significant. Gp52 reactivity was also detected in metastatic mammary carcinoma in axillary lymph nodes of male and female patients. The presence of gp52 related antigen was not apparently related to tumor grade or lymphocytic infiltrate in the primary tumor. The data do not permit a firm conclusion regarding nodal status in men; no correlation of gp52 activity and nodal status in women was evident. These results indicate that mammary carcinomas in men as well as in women have an antigen related immunologically to MuMTV gp52. Other than tumor size, the antigen seems to be unrelated to major prognostic factors. The significance of the antigen with respect to etiologic features and prognosis in breast cancer remains to be determined.

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.

Phosphorylation of fibronectin in quiescent and growing cell cultures

Phosphorylation of fibronectin was studied in quiescent vs growing cells from several species. Fibronectin secreted by actively growing cells exhibits a significantly higher level of phosphorylation than does the fibronectin secreted by quiescent cells of the same species.

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.

Precursor polypeptides to structural proteins of visna virus

Visna virus is a retrovirus which replicates in fibroblast-like cells of the sheep choroid plexus through a lytic cycle. Visna virions contain three major low-molecular-weight proteins (p30, p16, and p14) which, together with the genomic RNA and several molecules of reverse transcriptase, constitute the core structure of the virions. The core is surrounded by an envelope containing a major glycoprotein (gp135). By analogy with the oncoviruses, these three groups of structural proteins (i.e., the internal proteins, the envelope glycoprotein, and the reverse transcriptase) are probably encoded by the gag, env, and pol genes, respectively. To elucidate the genetic organization of the visna virus genome and its expression, we studied the synthesis of viral proteins in infected sheep choroid plexus cells. Intracellular viral proteins were detected by immunoprecipitation of pulse-labeled cell extracts with monospecific sera raised against p30, p16, and gp135 and resolution of the proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunoprecipitation with anti-p30 and anti-p16 sera allowed the characterization of the 55,000-dalton polypeptide precursor to internal virion proteins p30, p16, and p14 (Pr55(gag)). Tryptic peptide mapping confirmed the precursor-product relationship between Pr55(gag) and the three internal proteins. In addition, a gag-related polypeptide of 150,000 daltons was also detected. This polypeptide, which was less abundant than Pr55(gag), is a likely precursor to the viral reverse transcriptase (Pr150(gag-pol)). Pr55(gag) and Pr150(gag-pol) are not glycosylated. The precursor related to major envelope protein gp135 is a glycosylated polypeptide with an average molecular weight of 150,000 (gPr150(env)). Pulse-chase experiments indicated that gPr150(env) matures into glycoprotein gp135 intracellularly; however, gp135 was never preponderant in cell extracts. The non-glycosylated from of gPr150(env), which accumulated in the presence of 2-deoxy-d-glucose, appeared as a polypeptide of about 100,000 daltons. These results indicated that visna virus codes for the largest non-glycosylated env-related precursor among all of the retroviruses and therefore probably contains the largest env gene.

ML antigen of DBA/2 mouse leukemias: expression of an endogenous murine mammary tumor virus

Spontaneous, transplantable leukemias of DBA/2 mice express an antigen (ML) which cross-reacts with antigens of murine mammary tumor virus (MuMTV). The MuMTV cross-reactive antigen of the DBA/2 leukemias (ML cells) was found to be a glycoprotein of 78,000 molecular weight containing antigenic determinants of the major MuMTV glycoprotein gp52. No MuMTV particles were produced by the ML cells, although they did contain type A particles--the pronucleocapsids of MuMTV. The ML antigen appeared to be an aberrant form of the intracellular MuMTV env precursor molecular prgp70, which was not processed properly but instead acquired extra carbohydrate groups and was expressed in uncleaved form on the cell surface. Isolation of MuMTV core protein p28 from the leukemic cells and subsequent tryptic peptide mapping analysis showed that the p28 from leukemia cells differed from the p28 of MuMTV isolated from DBA/2 mouse milk. These observations indicate that the MuMTV expressed in DBA/2 leukemic spleen cells is of a different strain than the virus secreted in lactating mammary glands of DBA/2 mice and probably represents the expression of an endogenous DBA/2 provirus.

Shedding of vesicular stomatitis virus soluble glycoprotein by removal of carboxy-terminal peptide

A comparison of partial NH2-terminal sequences of vesicular stomatitis viral glycoprotein G (molecular weight, 69,000) and the soluble extracellular glycoprotein antigen Gs (molecular weight, 57,000) shows that both of the sequences are identical. Tryptic fingerprint analyses show that Gs lacks the carboxy-terminal region containing the membrane-anchoring hydrophobic domain of G. These results suggest that Gs is formed by cleavage in the carboxy-terminal region of G.

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.

Synthesis of murine mammary tumor viral proteins in vitro

The coding potential of murine mammary tumor viral genomic RNA was investigated by in vitro translation of various size classes of RNAs isolated from the virions. The major products of translation of full-size 35S polyadenylylated virion RNA were gag-related polyproteins of 75,000, 105,000, and 180,000 daltons (P75, P105, and P180, respectively). Studies on the kinetics of translation of these three proteins established that they were synthesized independently and that the smaller proteins were not post-translational cleavage products of the larger proteins. Tryptic peptide mapping showed that almost all of the P75 sequences were contained within P105 and almost all of the P105 sequences were contained within P180. The syntheses of all three proteins were inhibited by m7GTP, indicating that they were translated from capped mRNA's. Although a 24S polyadenylylated RNA had been identified as the intracellular mRNA for env precursor polyprotein, no such protein could be translated from the 24S polyadenylylated RNA isolated from the virions. However, translation of a 14S size class of polyadenylylated virion yielded four prominent proteins of about 36,000, 23,000, 21,000, and 20,000 daltons. These proteins were unrelated to murine mammary tumor viral structural proteins, as suggested from tryptic peptide mapping and immunoprecipitation data. They might be the products of an as-yet-unidentified gene located near the 3' terminus of the murine mammary tumor viral genomic RNA.

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.

The effect of diet on autogenous immunity to mouse mammary tumor virus in C3H/Bi mice

Antibody levels to mouse mammary tumor virus (MuMTV) in sera of C3H/Bi, C57BL/6 and AKR mice were assayed by an enzyme-linked immunosorbent assay (ELISA). It was found that antibody levels to MuMTV were highest in sera of C3H/Bi females which have a high incidence of breast cancer and low in male C3H/Bi and female C3H/Bi foster-nursed on C57BL/6 females. Sera of C57BL/6 and AKR mice showed very low or undetectable levels of antibody to MuMTV. Reduction of caloric intake sufficient to inhibit mammary tumor formation strikingly inhibited formation of antibody against MuMTV. The lowest levels of antibody were observed in sera of mice fed 10 cal/day of a defined diet. Significantly higher levels developed in the sera of mice fed 16 cal/day of the same defined diet. The highest antibody levels against MuMTV, however, were seen in sera of mice fed lab chow ad libitum but no correlation was evident between levels of antibody and tumor size. These studies indicate that dietary caloric restriction has a profound influence on the development of both mammary adenocarcinoma and antibody responses to MuMTV.

Structure of and alterations to defective murine sarcoma virus particles lacking envelope proteins and core polyprotein cleavage

HTG2 hamster cells produce a defective murine sarcoma virus lacking gp70 and, consequently, viral surface projections (knobs), but the lack of knobs appears to have no effect on intramembrane particle distribution. In addition, it has been noted that the core of the virus remains in the "immature" form as a result of the failure of the polyprotein precursor (p65) to undergo cleavage. However, incubation of HTG2 virus with avian myoblastosis virus was found to yield specific cleavage products of p65.

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.

Analysis of secondary modifications of mouse mammary tumor virus proteins by two-dimensional gel electrophoresis

The structural proteins of mouse mammary tumor virus (MMTV) were analyzed by two-dimensional electrophoresis on isoelectric focusing and sodium dodecyl sulfate gels. Many of the viral proteins displayed heterogeneity in charge due to variable contents of carbohydrates (in particular, sialic acid) and phosphate residues. Neuraminidase treatment of the virions influenced the isoelectric pattern of the envelope glycoproteins. The glycoproteins of an MMTV variant which was attenuated by replication in feline kidney cells had different isoelectric points. This suggested that the acquisition of an altered carbohydrate configuration had changed the host range of the virus. The major MMTV structural core protein, p27, consisted of two species, which had identical iodinated tryptic peptide compositions but differed in phosphate contents. Another MMTV phosphoprotein, p21, was separated into four different phosphorylated species. Phosphorylation of p21 could be performed in vitro by the MMTV virion-associated protein kinase. This enzyme also has a high affinity for MMTV p30 as a substrate. Possible functions of this enzyme are discussed.

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.

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.

Virolysis of mouse mammary tumor virus by sera from breast cancer patients

All type C retroviruses are lysed by human serum in apparently antibody-independent, complement-mediated reactions. In contrast, we have now determined that the mouse mammary tumor virus (MMTV), a type B retrovirus, is not disrupted by normal human serum. MMTV was lysed, however, when rabbit antibody to whole MMTV was added to the serum. By taking advantage of this dependence of MMTV lysis on specific antibody, a virolytic assay was developed, based on the measurement of reverse transcriptase released from disrupted virions, to search for evidence of antibodies to MMTV in human sera. Significantly greater virolytic activity was detected in the sera of patients with breast cancer than in sera of patients with benign disease (P less than 0.001) or colorectal cancer (P less than 0.001) or in sera from apparently healthy individuals (P less than 0.002). This assay thus appears to be able to detect a unique attribute, possibly the presence of an antibody crossreacting with MMTV, in serum in patient with breast cancer.

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.

Phosphorylation of murine mammary tumor virus precursor polypeptides

Phosphorylation of the murine mammary tumor virus (MuMTV) structural proteins was studied in an MuMTV-infected epithelial cell line derived from a BALB/cf C3H mouse mammary tumor. Immunoprecipitation of 32P-labeled cell extracts with monospecific anti-p27 serum revealed that phosphorylation occurred at the stage of the core-protein polyprotein precursor prp75. Two forms of phosphorylated prp75 were found: one migrating with an apparent molecular weight of 80,000, and the other with a molecular weight of 76,000. The 80,000-molecular-weight species was found to be the most heavily phosphorylated. In addition, a relatively stable phosphorylated processing intermediate of 34,000 molecular weight was observed as well. Tryptic peptide mapping analysis of the 32P-labeled viral proteins indicated a precursor product relationship between the intracellular phosphorylated, high-molecular-weight peptides and the mature MuMTV phosphoproteins p23 and p27. Phosphopeptide analysis also suggested that phosphorylation of the viral proteins occurred in discrete steps and that the attached phosphate groups were conserved throughout the processing steps.

Identification of the messenger RNAs coding for the gag and env gene products of the murine mammary tumor virus

Full-length (35S) genomic RNA from murine mammary tumor virus (MuMTV) was translated in vitro, using a reticulocyte lysate system, into proteins of 105,000, 75,000, 65,000, 35,000, and 27,000 daltons. These proteins were all immunoprecipitable with a monospecific antiserum to the major viral core protein, p27, but not with antiserum to the major viral envelope glycoprotein, gp47. Translation in vitro of RNA of about 24S size extracted from MuMTV yielded proteins similar in size and immunoreactivity to the products of the 35S RNA translation. Polyadenylylated RNA isolated from an MuMTV-producing cell line was fractionated according to size by velocity sedimentation and subsequently hybridized to MuMTV complementary DNA probes. These studies identified at least three size classes (35S, 24S, and 14-18S) of intracellular MuMTV-specific RNA. The 35S intracellular RNA was translated into MuMTV-specific proteins identical in size and immunoreactivity to the products of the virion-derived 35S RNA. On the other hand, translation of the intracellular 24S RNA fraction resulted in the synthesis of proteins, of which two (of about 70,000 daltons) could be immunoprecipitated with anti-gp47 serum, but not with anti-p27 serum. From these data we conclude that MuMTV core and envelope proteins are synthesized from two different mRNAs with approximate sizes of 35S and 24S, respectively. Our results also imply that the intracellular 24S mRNA is synthesized by a process more complex than simple cleavage of the 35S RNA.

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.