Bovine leukemia virus post-envelope gene coded protein: evidence for expression in natural infection

Partial sequence analysis of a 14 kilodalton protein (p14), synthesized by in vitro translation of bovine leukemia virus genomic RNA, showed that it is encoded in the 'X' region of proviral DNA, located between the env gene and the 3' long terminal repeat. The 'X' gene contains a short and a long open reading frame (X-SORF and X-LORF) which overlap. BLV p14x is specified by X-SORF and not X-LORF as seen with the related human T-cell leukemia virus which expresses p38-40x. Antibodies in sera from animals with BLV induced tumors were shown to recognize p14x. Expression of this protein in natural infection might be important for virus replication and/or for BLV induced oncogenesis.     

Comparison of the entire genomes of bovine leukemia virus and human T-cell leukemia virus and characterization of their unidentified open reading frames.

We have compared the sequence of the entire genomes of bovine leukemia virus (BLV) and human T-cell leukemia virus type I (HTLV-I). Both the gag and pol genes show overall strong homologies indicating the close evolutionary relationship of the two retroviruses. However, a surface glycoprotein portion of the env gene shows no appreciable homology, which probably reflects a difference in their host ranges. The 3' end portion of the BLV genome (designated as pXBL) contains an unidentified long open reading frame that has a typical protein-coding property. The potential product of this open reading frame may be a glycoprotein of approximately 40 000 daltons. We note that its amino acid sequence shows low but appreciable homology, especially in its N-terminal quarter, to that of the HTLV-I counterpart (pX product), and we thus suggest that BLV pXBL and HTLV-I pX have diverged from a common ancestral gene. It is tentatively concluded that both the putative pXBL and pX products are respectively produced from a spliced mRNA.     

Expression of the bovine leukemia virus X region in virus-infected cells.

Bovine leukemia virus, like its closest relatives the human T-cell leukemia virus types I and II, contains a 1.8-kilobase X region between the env gene and the 3' long terminal repeat. In this communication, we report the detection and characterization of a subgenomic mRNA from which this X region is presumably translated. This mRNA was produced by a complex splicing mechanism which resulted in juxtaposition of the 5' end of the env gene and the two overlapping X-region open reading frames. Translation of this mRNA could yield at least two distinct proteins depending on which initiation codon is used. Detection of the protein encoded by the BLV X-region long open reading frame has been reported (N. Sagata, J. Tsuzuku-Kawamura, M. Nagayoshi-Aida, F. Shimizu, K.-I. Imagawa, and Y. Ikawa, Proc. Natl. Acad. Sci. USA 82:7879-7883, 1985). Using synthetic peptide antisera, we detected a protein encoded by the short open reading frame in virus-infected cells. The protein migrated in sodium dodecyl sulfate-polyacrylamide gels with an apparent molecular weight of 19,000. It is a nuclear phosphoprotein.     

Two distinct polypeptides may be translated from a single spliced mRNA of the X genes of human T-cell leukemia and bovine leukemia viruses

Human T-cell leukemia and bovine leukemia viruses have a potential transforming gene, termed X. In addition to the major open reading frame known to encode a functional protein, the X gene harbors another short open reading frame which overlaps this major one. Both of these open reading frames are found on a single spliced X mRNA in a potentially functional form. Circumstantial evidence strongly suggests that they are both translated from the single X mRNA molecule, showing striking similarity to the translation mechanism of an adenovirus Elb gene mRNA. We note that the short open reading frame has the capability to encode a putative nuclear protein with structural features similar to those of an AIDS virus trans-acting protein.     

Role of the 3'' long open reading frame region of bovine leukemia virus in the maintenance of cell transformation.

Viral RNA expression was studied by dot blot hybridization with polyadenylated RNAs extracted from a bovine (YR-1) and an ovine (YR-2) tumor cell clone. Both clones were derived from in vivo bovine leukemia virus-induced tumors. The probes used were either the bovine leukemia virus information or only the long open reading frame sequences. No viral RNA corresponding to the bovine leukemia virus long open reading frame region was detected in YR-2, and a very limited amount of bovine leukemia virus messages was unraveled in YR-1. These results strongly suggest that viral expression, even in the long open reading frame region, is not required to maintain transformation of at least some tumor cells.     

Isolation and characterization of a 2.3-kilobase-pair cDNA fragment encoding the binding domain of the bovine leukemia virus cell receptor.

An immunoscreening strategy was used to isolate a cDNA clone encoding the binding domain for the external glycoprotein gp51 of the bovine leukemia virus (BLV). Three recombinant phages demonstrating BLV binding activity and containing 2.3-kbp cDNA inserts with identical nucleotide sequences were isolated from a lambda gt11 cDNA library of bovine kidney cells (MDBK). One clone, BLVRcp1, hybridized with a 4.8-kb mRNA from cells of bovine origin and was also found to be conserved as a single-copy gene in murine, bovine, ovine, primate, canine, feline, and porcine DNAs. The same gene is amplified in caprine DNA isolated from a BLV-induced tumor. The longest open reading frame of BLVRcp1 encodes a protein fragment of 729 amino acids with a putative receptor structure. BLVRcp1 cDNA was cloned in the eucaryotic expression vector pXT-1 and transfected into murine NIH 3T3 and human HEp-2 cells. Cells expressing BLVRcp1 mRNA became susceptible to BLV infection. BLVRcp1 has no known physiological function and has no significant homology with sequences registered in the GenBank and EMBL data libraries (31 July 1992). Expression of deleted constructs of BLVRcp1 indicates that the BLV binding region is encoded at the 5' side of the receptor clone.     

Expression of bovine leukemia virus protein p24 in Escherichia coli and its use in the immunoblotting assay.

The gag gene encoded protein, p24 of bovine leukemia virus (BLV), was cloned and expressed as thioredoxin-6xHis-p24 protein in Escherichia coli. The bacterial cells carrying plasmid pT7THis-p24 expressed the protein of 38 kDa that was detected by immunoblotting analysis using anti-p24 monoclonal antibodies and sera from BLV infected cattle and sheep. The purified p24 fusion protein was shown to be sensitive and specific for detection of BLV antibodies in the infected cattle.     

Solid phase synthesis of the proteinase of bovine leukemia virus. Comparison of its specificity to that of HIV-2 proteinase.

The 126-residue proteinase (PR) of bovine leukemia virus (BLV) was synthesized by solid-phase peptide synthesis and its activity was shown using various oligopeptide substrates representing cleavage sites in BLV, human T-cell leukemia virus type 1 (HTLV-1), murine leukemia virus (MuLV) and human immunodeficiency virus type 1 (HIV-1). The specificity of the BLV PR was also compared to that of chemically synthesized human immunodeficiency virus type 2 (HIV-2) PR. Many of the peptides were cleaved at the expected site, however, 6 out of 15 were hydrolyzed only by one of the PRs. Furthermore, one BLV peptide was processed differently by the two enzymes. These results, together with the relative activities and the lack of inhibition of BLV PR by two HIV-1 PR inhibitors, suggest that the BLV PR specificity is substantially different from that of HIV PRs.     

Detection of px gene product of bovine leukemia virus in infected cells

Bovine leukemia virus (BLV), like its closest relatives human T-cell leukemia virus-I and II, contain a 'px' gene, between the 'env' gene and the 3' long terminal repeat in its genome. A monoclonal antibody prepared against a synthetic oligopeptide whose sequence was deduced from highly conserved region of 'px' gene of BLV, was used to detect the presence of 'px' gene product in chronically BLV infected synchronised cells. By immunoperoxidase staining the 'px' gene product was detected maximum after 6-9 hr after synchronization in the nucleus of the cells which demonstrated the close interaction of it with viral DNA which is integrated with host cell genome.     

The structure of cloned 3'-terminal RNA region of bovine leukemia virus (BLV)

cDNA synthesized on the bovine leukemia virus RNA template has been cloned in the pBR322 Pst I site. Colony hybridization with BLV RNA fragments and oligo (dT) has revealed a clone with cDNA insert containing 660 3'-terminal nucleotides of the BLV genome. The nucleotide sequence of the insert corresponding to U3 and R regions of the long terminal repeats (LTR) of viral genome has been determined. BLV U3, like U3 of other retroviruses, presumably contains promoter. The unusually long R region (about 230 bp), a certain homology with ATLV U3-R and some other structural features allow to group BLV LTR together with ATLV LTR in a separate class of retroviral LTR.     

Construction of a recombinant bovine leukemia virus vector for analysis of virus infectivity.

A recombinant bovine leukemia virus (BLV) was constructed in which the X region was replaced with the bacterial neomycin resistance gene controlled by the simian virus 40 early promoter. This virus, termed BLV-SVNEO, is a self-packaging, activator-dependent retroviral vector. Introduction of the plasmid pBLV-SVNEO into mammalian cells resulted in constitutive expression of the neo gene, whereas the BLV structural genes, gag, pol, and env, were expressed only in the presence of the two regulatory proteins, Tax and Rex. The production and release of recombinant virus by cells transfected with pBLV-SVNEO were proportional to the number of G418-resistant colonies that developed after susceptible cells were exposed to the filtered culture medium. BLV-SVNEO was able to infect cell lines of human, bovine, canine, feline, and murine origin. BLV-producing cell lines were resistant to superinfection with BLV-SVNEO. This cell-virus system should facilitate molecular genetic studies of BLV and will provide a rapid, quantitative measure of BLV infectivity in a variety of cell types. These studies also demonstrate the feasibility of using activator-dependent retroviral vectors such as BLV-SVNEO to deliver foreign genes into cells and eventually animals.     

Molecular cloning of bovine leukemia virus DNA integrated into the bovine tumor cell genome

The bovine leukemia virus (BLV) DNA harbored in the bovine tumor cell genome was cloned in lambda Charon 4A phage. Using either representative or 3' half-enriched BLV cDNA as a blot hybridization probe, clone lambda BLV-1 was shown to carry 9 kb of the BLV genome, flanked by cellular sequences at both ends. Restriction mapping with twelve endonucleases and hybridization of the DNA fragments to BLV cDNA representing a 3'-end portion of the viral genome revealed the presence and precise location of two long terminal repeats (LTRs) and virus-cell junctions. Thus, lambda BLV-1 appears to contain the complete BLV genome and flanking tumor cellular sequences. The restriction map of the cloned BLV proviral DNA closely resembles that previously reported for unintegrated linear proviral DNA, but differs significantly from that of the integrated provirus of another BLV isolate, the difference occurring preferentially in the putative gag and pol genes.     

The bovine leukemia virus p34 is a transactivator protein.

Recombinant Moloney murine retroviruses containing the BLV post-envelope long open reading frame were constructed and transfected into the psi 2 packaging cell line. They were shown to encode and to express a 34-kd protein able to transactivate the BLV long terminal repeat-directed gene expression in the respective transfected cells. These data demonstrate that the BLV X-LOR gene encodes a p34 transactivator product. Furthermore, the different cell lines produced infectious recombinant retroviruses capable of transferring X-LOR genes into recipient cells. The availability of the BLV transactivator protein should allow us to understand the role of the transactivator protein in BLV-induced leukemogenesis.