Endogenous murine leukemia virus-encoded proteins in radiation leukemias of BALB/c mice

To explore the role of endogenous retroviruses in radiation-induced leukemogenesis in the mouse, we have examined virus-encoded proteins in nine BALB/c leukemias by pulse-chase labeling procedures and serological typing with monospecific and monoclonal antibodies. The major gag precursor protein, Pr65^gag, was observed in all cases, but only three leukemias expressed detectable amounts of the glycosylated gag species, gP95^gag, or its precursor, Pr75^gag. No evidence was found for synthesis of gag-host fusion proteins. None of the leukemias released infectious xenotropic or dualtropic virus, but all nine expressed at least one env protein with xenotropic properties. In two instances a monoclonal antibody, $\text{35}\text{56}$, which is specific for the MuLV G_IX antigen, displayed a distinctive reactivity with this class of env protein, although this antibody is unreactive with replicating xenotropic viruses. An ecotropic/xenotropic recombinant env protein with the same $\text{35}\text{56}$ phenotype was observed in a leukemia induced by a strongly leukemogenic virus isolated from a BALB/c radiation leukemia.