Structural proteins of RNA tumor viruses as probes for viral gene expression

Monoclonal antibody to the amino-terminal L sequence of murine leukemia virus glycosylated gag polyproteins demonstrates their unusual orientation in the cell membrane

To analyze cell surface murine leukemia virus gag protein expression, we have prepared monoclonal antibodies against the spontaneous AKR T lymphoma KKT-2. One of these antibodies, 43-13, detects an AKR-specific viral p12 determinant. A second monoclonal antibody, 43-17, detects a novel murine leukemia virus-related antigen found on glycosylated gag polyproteins (gp95gag, gp85gag, and gp55gag) on the surface of cells infected with and producing ecotropic endogenous viruses, but does not detect antigens within these virions. The 43-17 antibody immunoprecipitates the precursor of the cell surface gag protein whether in its glycosylated or unglycosylated state, but does not detect the cytoplasmic precursor of the virion gag proteins (Pr65gag). Based on these findings, we have localized the 43-17 determinant to the unique amino-terminal part of the glycosylated gag polyprotein (the L domain). We have determined that gp95gag contains L-p15-p12-p30-p10 determinants, whereas gp85gag lacks the carboxyterminal p10 determinant, and gp55gag lacks both p30 and p10 carboxy terminal determinants. Analysis of cell surface gag expression with the 43-17 antibody leads us to propose that the L domain plays a crucial role in (i) the insertion and orientation of murine leukemia virus gag polyproteins in the cell membrane and (ii) the relative abundance of expression of AKR leukemia virus versus Moloney murine leukemia virus glycosylated gag polyproteins in infected cells.

Structure of endogenous murine leukemia virus DNA in mouse genomes

By using molecularly cloned ecotropic AKR murine leukemia virus (MuLV) DNA, a 400-base-pair ecotropic type-specific segment in the env region has been identified. This DNA segment and other defined viral subgenomic fragments have been used as 32P-labeled probes to identify and analyze the structure of integrated ecotropic viral DNA sequences in uninfected mouse genomes. Those mice from which endogenous ecotropic MuLV of the AKR type have been isolated contained at least one virtually complete linear copy of the viral genome. Strains from which ecotropic MuLV has not been isolated lacked ecotropic-specific sequences. All inbred mouse strains tested also contained MuLV DNAs of genomic length whose restriction endonuclease digestion pattern was characteristic of xenotropic viruses.

Specificity of serum from rodents immune to Moloney C-type virus-induced tumours

When cells are infected by C-type viruses such as Moloney sarcoma/leukaemia virus, new antigens appear on the cell membrane. Mice and rats will respond immunologically to the antigen(s). It was uncertain whether the antigens were related to the viral structural proteins or to non-virion, tumour-specific surface antigens (TSSA) or both. In an 125I-antiglobulin binding assay, Moloney virus completely blocked the binding of mouse and rat sera to virus shedding target cells, thus suggesting that mice and rats recognise only viral proteins. Mice responded to type-specific and rats mainly to group-specific determinants on the virus. Individual Moloney viral proteins were prepared using the guanidine HCl method and were used to block the binding of the rat anti-Moloney immune serum to Moloney virus shedding target cells. By this method, it was demonstrated that the rat serum contains specificities for the viral proteins gp70 and p30, but it was not possible to detect any antibodies directed towards non-virion or TSSA-like molecules.