Identification of a 39,000-dalton protein in cells transformed by the FBJ murine osteosarcoma virus

Two components of the FBJ murine osteosarcoma virus complex have been isolated separately in tissue culture; the FBJ murine leukemia virus (FBJ-MLV) by dilution and the FBJ murine sarcoma virus (FBJ-MSV) by the establishment of nonproducer transformed rat cells. Analysis of these cells using MLV antisera indicated that there were no new proteins related to viral structural proteins specifically associated with the presence of the FBJ-MSV genome. The FBJ-MSV nonproducer cells were used to induce tumors in syngeneic and allogeneic F₁ rats. Sera from tumor-bearing rats were examined for activity against FBJ-MSV-specific antigens. A number of sera were found to precipitate a 39,000-dalton protein, p39, from several producer and nonproducer FBJ-MSV transformed rodent cells, but not from cells transformed by other strains of MSV or cells infected with MLV. Precipitation of p39 was not blocked by the presence of excess viral proteins, indicating that p39 is not related to the viral structural proteins. This conclusion was confirmed by methionine tryptic peptide analysis which showed that the fingerprint of p39 was distinct from those of the viral gag or env gene proteins. The data demonstrate the presence of a unique antigenic protein, unrelated to the MLV proteins, in FBJ-MSV transformed cells.