Conservation of protein coding potential in the long terminal repeats of exogenous and endogenous mouse mammary tumor viruses

Polyclonal and monoclonal antibodies monospecific to MMTV LTR orf protein produced in E. coli

Monoclonal and polyclonal antibodies specific to an open reading frame of the mouse mammary tumor virus long terminal repeat were generated using an open reading frame-beta-galactosidase fusion protein produced in E. coli. Both antibodies reacted with the open reading frame-beta-galactosidase fusion protein but not with beta-galactosidase alone using an immunoblotting technique. It is concluded that these antibodies were specific for the protein encoded by the open reading frame of the mouse mammary tumor virus long terminal repeat.

Expression of the mouse mammary tumor virus ORF gene in cultured cells

We have recently shown that expression vectors harboring the open reading frame of the long terminal repeat region of mouse mammary tumor virus direct the synthesis of a product which acts as a superantigen in transgenic mice. The detection of the ORF protein has been hampered by the extremely low levels of expression observed in these mice, as estimated from the low levels of specific mRNA. To study the properties of the ORF protein, we attempted its expression in different cell types in culture. The experiments performed in yeast show that the ORF gene product is a glycoprotein of approximately 45 kDA. As expected from the derived primary sequence, the unglycosylated product made in the presence of tunicamycin has a molecular weight of 36 kDA. No secretion of the glycosylated protein was observed. Curiously, the full-length molecule was made in lower amounts than a truncated version which contains only the C-terminal half of the protein. Transfection experiments in different mammalian cells suggest that high expression of the ORF protein might have an adverse effect on survival of cells in culture.

Endogenous ligands selecting T cells expressing particular V beta elements

It has recently become clear that the minor lymphocyte stimulatory antigens (Mls) and other endogenous ligands which lead to the partial or total deletion of T cells bearing particular V beta segments are encoded by mouse mammary tumor virus (MMTV). We review here the genetic analyses of multiple V beta 11 and V beta 3 deletion ligands and demonstrate the involvement of MMTV in all examples. Several features of Mls and the V beta 11/V beta 3 deleting ligands identify them as members of the superantigen family. Bacterial superantigens are known to bind both MHC class II and the TCR in regions distinct from conventional peptide antigens. Within the MMTV genome, the 3' LTR has been identified as encoding superantigen function. We present data demonstrating that in vitro translation identifies the major product of the open reading frame (ORF) within the 3' LTR as a type II integral membrane glycoprotein. It is proposed that the type II membrane glycoprotein interacts with MHC and TCR in a manner analogous to the bacterial superantigens and distinct from conventional peptide antigen. Several unanswered questions regarding superantigen action remain; what determines total or partial deletion? How is Mls transferred between cells? These questions are addressed in the discussion.

Detection and characterization of a glycoprotein encoded by the mouse mammary tumor virus long terminal repeat gene

Mouse mammary tumor virus (MMTV) is a retrovirus that causes mammary tumors in susceptible mice. MMTV contains a unique open reading frame (ORF) in the unique 3' region of the proviral long terminal repeat (LTR) with the potential to encode a 36-kDa protein. However, the ORF gene product has not been detected in murine mammary tissues or cell lines. We utilized the baculovirus expression vector system to generate large amounts of the ORF protein. Putative ORF gene products of 36 and 45 kDa were detected as unique proteins in extracts of insect cells infected with recombinant baculovirus (LTR-ORF BV), and the identities of these proteins as viral gene products were confirmed immunologically. Antipeptide antisera were generated in rabbits against peptides chosen from computer-predicted hydrophilic regions of the ORF coding sequence. These antisera reacted specifically by immunoprecipitation and by immunoblot with the proteins expressed in LTR-ORF BV-infected insect cells, as well as with MMTV LTR ORF in vitro translation products. Polyclonal antisera were raised against two putative ORF protein species partially purified from insect cells. These sera specifically immunoprecipitated viral protein products translated in vitro. In vitro translation of MMTV LTR ORF transcripts in the presence of canine pancreatic microsomal membranes generated a higher-molecular-weight ORF gene product, indicating that the ORF protein is modified by N-linked glycosylation. This glycosylated ORF product comigrated with the larger ORF protein species produced in infected insect cells. The gp45 product was metabolically labeled with [3H] mannose, [3H] galactose, and [3H] N-acetyl-D-glucosamine in insect cells, whereas this incorporation was inhibited in the presence of tunicamycin. Digestion of gp45 with endoglycosidase H yielded the lower-molecular-weight ORF protein p36. These observations suggest that the ORF glycoprotein contains hybrid N-linked oligosaccharides. Demonstration of the modified nature of the ORF gene product will facilitate characterization of ORF protein expression in murine tissues.

Transactivating potential of the 3' open reading frame of murine mammary tumor virus

The procaryotic chloramphenicol acetyltransferase (CAT) gene controlled by the murine mammary tumor virus (MMTV) promoter shows reduced activity in a rat mammary tumor cell line after infection with MMTV but to a considerably lesser extent than the CAT gene controlled by a heterologous promoter, indicating trans-acting regulation of promoter activity by MMTV. Cotransfection of vectors capable of expressing RNA from the 3' open reading frame (orf) of MMTV with the CAT-MMTV construct resulted in enhanced CAT activity, suggesting that orf carries a transactivating potential. Since transactivation was also found with a vector containing only orf and part of the viral env gene, it was concluded that a separate transcriptional unit exists for the orf message.

Expression of the protein product of the mouse mammary tumor virus long terminal repeat gene in phorbol ester-treated mouse T-cell-leukemia cells

Exposure of C57BL/6 mouse EL-4 T-cell leukemia cells to phorbol ester (12-O-tetradecanoylphorbol-13-acetate) (TPA) induced the synthesis of protein products encoded by the mouse mammary tumor virus (MMTV) long terminal repeat (LTR) region. Analysis of TPA-treated EL-4 cells with antiserum raised against a synthetic peptide predicted by the MMTV LTR open reading frame sequence detected a polypeptide migrating in gels with an apparent molecular weight of 37,000 Mr, as well as three less prominent proteins with apparent molecular weights of 31,000, 34,000, and 39,000. Tryptic peptide analysis established the identity of the immunoprecipitated cellular proteins with the LTR proteins obtained from in vitro translation of MMTV genomic RNA. All four proteins were glycosylated and were derived from one initial nonglycosylated translation product of 21,000 Mr. The 21,000-Mr apoprotein could be detected after digestion with endoglycosidase F or pretreatment of cells with tunicamycin. Untreated EL-4 cells synthesized three species of MMTV mRNA: 35S, 24S, and 20S. TPA treatment resulted in an increased level of transcription of the three mRNAs and the appearance of a new 1-kilobase mRNA. At least 10 acquired MMTV proviruses are present in the EL-4 genome, and examination of the degree of proviral methylation revealed extensive demethylation. However, no qualitative differences in the state of proviral methylation were apparent between TPA-treated and untreated cells.

Proteins encoded by the long terminal repeat region of mouse mammary tumor virus: identification by hybrid-selected translation

The long terminal repeat (LTR) region of mouse mammary tumor virus (MMTV) is known to contain an open reading frame of sufficient length to code for a protein of 36,000 Mr. The coding capacity of the 3' sequences of MMTV genomic RNA has been demonstrated by in vitro translation studies, which have reported the synthesis of four related proteins: p36, p24, p21, and p18. These proteins are overlapping translation products of the same open reading frame, with the smaller ones initiating at internal methionine codons. From the predicted amino acid sequence of the LTR protein, we have selected a region likely to be antigenic, obtained a synthetic peptide of that region, and raised antiserum to the peptide. The antipeptide serum specifically immunoprecipitated all four proteins from in vitro translated genomic 3' MMTV RNA, plus an additional one of 32,000 Mr. Published sequence data of MMRV LTRs show an internal AUG codon at a position which could initiate a protein of 32,000 Mr. The three smaller in vitro translation products (p24, p21, and p18) were consistently synthesized in much greater amounts than the p36 or p32 protein. The relative amount of each in vitro synthesized protein from genomic MMTV RNA could be predicted and was in good agreement with the postulated effect of flanking nucleotides on the efficiency of the respective AUG initiation codon. Polyadenylated RNAs, isolated from various mouse tissues, were selected by hybridization to plasmid DNA containing MMTV LTR sequences immobilized on nitrocellulose. In vitro translation of hybrid-selected mRNAs isolated from BALB/c mouse lactating mammary glands and carcinogen-induced mammary tumors, followed by immunoprecipitation with antipeptide serum, revealed that only one polypeptide was synthesized by the MMTV LTR-specific mRNA, the 36,000 Mr species.

Evidence for a prokaryotic promoter in the murine mammary tumor virus long terminal repeat

The long terminal repeat (LTR) of C3H murine mammary tumor virus (MuMTV) is approx. 1.3 kb long. HaeIII digestion of a cloned PstI fragment containing the left-end LTR generated four fragments of sizes 0.56, 0.41, 0.34 and 0.14 kb, one of which (0.41 kb) had a promoter activity in Escherichia coli. This was demonstrated by replacing the bacterial promoter for the neomycin-resistance (NmR) gene in the plasmid pKC56 with the HaeIII fragments. Only the 0.41-kb fragment that contains sequences from the U3 region of the LTR was found to contain a promoter, as shown by the expression of the drug-resistance phenotype in the recombinant plasmid. The strength of this promoter was comparable to or greater than that found with the parental NmR gene promoter. S1 nuclease mapping of the NmR gene transcript indicated that the initiation of this transcript occurred within the 0.41-kb LTR fragment from a site approx. 10 bp upstream from the 3' end. A comparison of the known DNA sequences in the MuMTV LTR with those found in bacterial promoters revealed that a 'Pribnow box', the initiation signal for the prokaryotic promoters, is present in the 0.41-kb LTR fragment upstream from the initiation site. Furthermore, in a recombinant plasmid that contained the complete LTR the same promoter sequences appeared to be involved in the initiation of RNA transcription. The 0.34-kb LTR fragment, which contains sequences derived from the U3 and U5 regions of the LTR, did not possess promoter activity in E. coli. However, it was found to induce deletions of adjacent plasmid DNA sequences. The deletions were specifically initiated from the downstream end of the LTR-fragment insert. The presence of a prokaryotic promoter in the MuMTV LTR, together with the observation that certain LTR sequences can induce deletions, analogous to those caused by transposable elements, in recombinant plasmids suggest that the MuMTV LTR may have evolved from such elements.

Tumorigenesis by mouse mammary tumor virus: evidence for a common region for provirus integration in mammary tumors

We have prepared specific probes for unique-sequence cellular DNA adjacent to each of the newly integrated proviruses in tumors induced by mouse mammary tumor virus (MMTV). The use of such probes to screen a large number of independent mammary tumors in the BR6 strain of mouse has indicated that in at least 17 out of the 40 tumors examined so far, an MMTV provirus has integrated into a common chromosomal domain. A 10 kb Eco RI fragment of single copy DNA from this region has been isolated and partially characterized by restriction enzyme mapping. Of the proviruses located within this fragment in different tumors, all but one are complete, in the same orientation, and clustered within about 3 kb of cellular DNA. These findings are consistent with an insertional mutagenesis model for tumorigenesis by MMTV, in which the integration of a provirus in a particular region of cellular DNA may activate a neighboring oncogene. The region we describe here appears to be different from that reported for mammary tumors in the C3H strain of mouse.