Sequence and expression of the mouse mammary tumour virus env gene

We have determined the DNA sequence of the envelope gene region of the GR strain of mouse mammary tumour virus. The sequence extends for 3012 nucleotides from the single EcoRI site to beyond the PstI site in the 3' long terminal repeat (LTR) of the provirus. There is a major open reading frame from nucleotides 752 to 2818 which encompasses the entire env gene. This reading frame extends through a polypurine tract and into the LTR. There is another open reading frame from the first nucleotide to position 803, presumably corresponding to the end of the pol gene. The splice acceptor site which generates env mRNA has been mapped experimentally to nucleotide 750. The env gene products, gp52 and gp36, have been positioned on the sequence using the directly determined amino acid sequences of the amino terminus of gp52; and both the amino and carboxyl termini of gp36. The start of gp52 is preceded by a series of 19 uncharged amino acids which could function as a typical signal sequence, but this sequence is only part of a much longer leader peptide. The tetrad Arg-Ala-Lys-Arg is the presumed cleavage site in the gPr73env precursor, and occurs just before the gp36 amino terminus. There are five potential asparagine-linked glycosylation sites which agrees with previous experimental results. The gp36 has two long hydrophobic regions at its amino and carboxy termini, these are suggested to act as a fusion peptide and the trans-membrane anchor, respectively.     

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.     

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.     

Terminal amino acid sequences and proteolytic cleavage sites of mouse mammary tumor virus env gene products

The mature envelope glycoproteins of mouse mammary tumor virus (gp52 and gp36) were isolated by reversed-phase high-pressure liquid chromatography. The N-terminal amino acid sequence of gp36 was determined for 28 residues. The C-terminal amino acid sequences of gp52 and gp36 were determined by carboxypeptidase digestion. The N-terminal amino acid sequence of gp52 has been reported previously (L. O. Arthur et al., J. Virol. 41:414-422, 1982). These data were aligned with the predicted amino acid sequence of the env gene product obtained by translation of the DNA sequence (S. M. S. Redmond and C. Dickson, Eur. Mol. Biol. Org. J. 2:125-131, 1983). The amino acid sequences of the mature viral proteins were in agreement with the predicted amino acid sequence of the env gene product over the regions of alignment. This alignment showed the sites of proteolytic cleavages of the env gene product leading to the mature viral envelope glycoproteins. The N-terminal amino acid sequence of gp52 starts at residue 99 of the predicted structure indicating proteolytic cleavage of a signal peptide. A dipeptide (Lys-Arg) is excised between the C-terminus of gp52 and the N-terminus of gp36. The C-terminal amino acid sequence of gp36 is identical to the sequence predicted by the codons immediately preceding the termination codon for the env gene product. The data show that there is no proteolytic processing at the C-terminal of the murine mammary tumor virus env gene product and that the env gene coding region extends into the long terminal repeat.     

Glucocorticoid hormone interactions with cloned proviral DNA of mouse mammary tumor virus

The molecular details of glucocorticoid hormone regulation of expression of the mouse mammary tumor virus (MMTV) proviral gene have been investigated. Cloned proviral DNA was introduced into cultured cells by a gene transfer procedure. DNA acquired by transfection was shown to be expressed in a hormone regulated fashion. The proviral DNA was fragmented and recombined in vitro with an indicator gene to delimit the hormone response sequence. Inducibility of the indicator gene (thymidine kinase gene from Herpes Simplex Virus, tk) was observed upon recombination with the long terminal repeat (LTR) sequence of MMTV. Further delimitation of the LTR DNA demonstrated that 202 nucleotides located 5' of the RNA initiation site are sufficient to confer glucocorticoid regulation. In vitro interaction of LTR DNA with glucocorticoid hormone receptor complex, showed a preferential affinity to the same sequence which mediated hormonal regulation in transfected cells. Evidence for a direct receptor gene interaction in the process of gene induction was gained by the measurement of the kinetics of induction and the use of a glucocorticoid antagonist (RU 486). The induction of the transfected gene is very rapid, independent of simultaneous protein synthesis and requires a functional glucocorticoid receptor hormone complex.     

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.     

Characterization of endogenous and exogenous mouse mammary tumor virus proviral DNA with site-specific molecular clones.

Restriction fragments of the mouse mammary tumor virus (MMTV) proviral DNA were obtained by molecular cloning procedures. A 4-kilobase fragment delimited by two PstI sites was isolated from unintegrated, linear MMTV DNA and amplified in the pBr322 plasmid vector. EcoRI fragments of proviral DNA, integrated into the genome of a GR mammary tumor cell line, were isolated as lambda recombinant molecules. Five different recombinant phages which contained the 3' region of the MMTV proviral DNA and adjacent host DNA sequences were isolated. Heteroduplex analysis and S1 nuclease digestion suggested that there is no extensive sequence homology in the host DNA flanking the different proviral genes. The cloned DNA was fractionated into site-specific restriction fragments which served as molecular probes in the analysis of the endogenous MMTV proviral copies of C3H, GR, BALB/c, and feral mice. This allowed the correlation of MMTV-specific EcoRI fragments obtained from genomic DNA of these strains with the 5' and 3' ends of the proviral gene. Restriction fragments of two clones which contained the proviral sequences adjacent to the flanking host DNA as well as 1 to 2 kilobases of host DNA were used as hybridization probes, and the results allow the following conclusions: the proviral DNA of both clones contains nucleotide sequences complementary to the 5' and 3' ends of proviral DNA; and the host DNA flanking one clone belongs to the unique class of genomic DNA, whereas the DNA flanking the second clone is reiterated at least 15 times within the mouse genome.     

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.     

Transgenic mouse mammary tumor virus superantigen expression prevents viral infection.

Endogenous mouse mammary tumor virus (MMTV) proviruses have recently been shown to cosegregate genetically with the minor lymphocyte-stimulating loci, also termed self-superantigens. The antigenic activity has been localized to the open reading frame (ORF) protein encoded in the long terminal repeat of MMTV. We show here that unlike their nontransgenic littermates, transgenic mice expressing high levels of an ORF protein derived from the C3H exogenous MMTV specifically delete their V beta 14+ T cells and do not become infected with this virus when it is present in their mother's milk. Thus, it appears that MMTV utilizes cells of the immune system in its infection pathway, and mice that retain endogenous MMTVs should be immune to infection by exogenous virus. These results offer possible new approaches to anti-viral therapy or immunization.