Murine leukemia virus env-gene expression in preleukemic thymocytes and leukemia cells of AKR strain mice

Somatically acquired recombinant murine leukemia proviruses in thymic leukemias of AKR/J mice

We have probed the structure and arrangement of murine leukemia virus genomes in eight spontaneous AKR thymic leukemias by Southern hybridization with one ecotropic pol and four ecotropic env probes. These probes revealed many (in 2 cases over 15) somatically acquired proviruses that had undergone complex patterns of recombination. The large majority were not deleted and were structurally analogous to the oncogenic mink cell focus-inducing murine leukemia viruses isolated from AKR tumors in that the amino-terminal p15E-coding region derived from ecotropic AKR murine leukemia virus sequences, whereas certain gp70-coding sequences were nonecotropic. Nevertheless, we observed a few proviruses which did not appear to be gp70 recombinants; however, these proviruses were in general clearly recombinant within the p15E-coding sequences. Although the proviral recombination patterns were quite variable, in general the large majority of recombinant proviruses within each tumor appeared structurally identical, indicating that they originate from a common parent. Each tumor contained a unique pattern of provirus integrations; densitometer tracings of the Southern hybridizations indicated that many of the integrated proviruses were present at one copy per cell, suggesting that the tumors derive from a single cell which contained multiple integrated copies of a unique recombinant virus structurally similar to the mink cell focus-inducing viruses.

Genome structure of mink cell focus-forming murine leukemia virus in epithelial mink lung cells transformed vitro by iododeoxyuridine-induced C3H/MuLV cells

We characterized mink cell focus-forming murine leukemia viruses that were isolated from C3H/MCA-5 cells after induction with 5-iododeoxyuridine in culture. Mink lung epithelial cells malignantly transformed in vitro by induced virus were the source of four molecular clones of mink cell focus-forming virus. CI-1, CI-2, CI-3, and CI-4. Three clones, CI-1, CI-2, and CI-3, had full-length mink cell focus-forming viral genomes, one of which (CI-3) was infectious. In addition, we obtained a defective viral genome (CI-4) which had a deletion in the envelope gene. A comparison between the envelope genes of CI-4 and those of spleen focus-forming virus by heteroduplex mapping showed close homology in the substitution region and defined the deletion as being identical to the p15E deletion of spleen focus-forming virus. The recombinant mink cell focus-forming genomes are not endogenous in C3H/MCA-5 cells and therefore must have been formed in culture after induction by 5-iododeoxyuridine. CI-3, the infectious clone of mink cell focus-forming murine leukemia virus, was dualtropic, and mink cells infected with CI-3 were altered in their response to epidermal growth factor. In the presence of epidermal growth factor at 10 ng/ml, uninfected mink cells retained their epithelial morphology in monolayer culture and did not form colonies in soft agar. In contrast, CI-3 virus-infected mink cells grew with fibroblastic morphology in monolayer culture and showed an increased growth rate in soft agar in the presence of epidermal growth factor.

Analysis of the env gene of a molecularly cloned and biologically active Moloney mink cell focus-forming proviral DNA

A biologically active molecular clone of BALB/Moloney mink cell focus-forming (Mo-MCF) proviral DNA has been reconstructed in vitro. It contains the 5' half of BALB/Moloney murine leukemia virus (Mo-MuLV) DNA and the 3' half of BALB/Mo-MCF DNA. The complete nucleotide sequence of the env gene and the 3' long terminal repeat (LTR) of the cloned Mo-MCF DNA has been determined and compared with the sequence of the corresponding region of parental Mo-MuLV DNA. The substitution in the Mo-MCF DNA encompasses 1,159 base pairs, beginning in the carboxyl terminus of the pol gene and extending to the middle of the env gene. The Mo-MCF env gene product is predicted to be 29 amino acids shorter than the parental Mo-MuLV env gene product. The portion of the env gene encoding the p15E peptide is identical in both viral DNAs. There is an additional A residue in the Mo-MCF viral DNA in a region just preceding the 3' LTR. The nucleotide sequence of the 3' LTR of Mo-MCF DNA is similar to that of the 5' LTR of BALB/Mo-MuLV DNA with the exception of two single base substitutions. We conclude that the sequence substitution in the env gene is responsible for the dual-tropic properties of Mo-MCF viruses.

Leukemogenesis by Gross passage A murine leukemia virus: expression of viruses with recombinant env genes in transformed cells

Gross passage A murine leukemia virus (MuLV) derived from extracts of C3Hf/Bi mouse leukemias has been shown to be a virus complex consisting of ecotropic, xenotropic, and recombinant, dualtropic MuLV components. The three virus components were distinguished biochemically by differences in the molecular weights and peptide maps of their primary env gene products synthesized in infected cells in vivo and in vitro. Virus expression was studied in primary leukemias induced in C3Hf/Bi mice by Gross passage A virus extracts and by the individual ecotropic and recombinant MuLV components that were isolated in vitro. Our findings suggest that expression of the recombinant MuLV component of the Gross passage A virus complex is necessary and sufficient for the induction of leukemias in C3Hf/Bi mice. In contrast, induction of leukemias by the ecotropic virus component appears to involve generation of a second virus with characteristics of recombinant, dualtropic MuLV.

Env gene products of AKR dual-tropic viruses: examination of peptide maps and cell surface expression

The env gene products of nine AKR dual-tropic murine leukemia viruses were compared by peptide mapping and were assayed for expression on the cell surface of infected fibroblasts. Seven virus isolates expressed the env gene polyprotein on the cell surface. The env gene products of six of the seven viruses had identical peptide maps. The analysis of structure and expression of env gene products carried out in this study characterizes a subset of dual-tropic murine leukemia viruses shown by others to be thymotropic.

Glycoproteins of murine leukemia viruses. III. Glycosylation of env precursor glycoproteins

We have compared the glycopeptides obtained after extensive pronase digestion of the env precursors (PrENV proteins) of ecotropic, xenotropic, and dual-tropic murine leukemia viruses. Two glycopeptide size classes, having molecular weights of approximately 2,200 and 1,500, were shown to be associated with the PrENV proteins of all murine leukemia viruses studied. Glycopeptides associated with the env precursors were totally susceptible to endo-beta-N-acetyglucosaminidase H. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of partial endo-beta-N-acetylglucosaminidase H digestion products of the env precursor of dual-tropic mink cell focus-forming virus (MCF 247) revealed the presence of seven bands, suggesting that six glycosylation sites were present on the precursor molecule. The MCF 247 PrENV protein had been previously shown to be accessible to lactoperoxidase-catalyzed radioiodination on the surface of infected cells. The cell surface PrENV molecules had the same electrophoretic mobility as pulse-labeled PrENV protein, and after endo-beta-N-acetylglucosaminidase H treatment a similar shift in electrophoretic mobility was observed for the cell surface PrENV protein and the pulse-labeled precursors, a finding which indicated that the PrENV protein located on the cell surface also possessed only mannose-rich oligosaccharides. These results indicated that the env precursor glycoproteins of dual-tropic viruses had the unusual property of migrating to the cell surface without undergoing the normal oligosaccharide processing and proteolytic cleavage events that had been observed for ecotropic and xenotropic murine leukemia virus glycoproteins.