Isolation and Characterization of C-type Viral Gene Products of Virus-negative Mouse Cells* By

The genetic information for RNA C-type viruses is naturally integrated within the DNA of mouse cells (1-4). In tissue culture, unexpressed virus can be activated spontaneously (5) or by treatment with different chemical inducers (6-8). I t has recently been shown that one class of induced virus is oncogenic when inoculated into animals (9). Thus, the study of cellular controls over the expression of integrated C-type viral genes may help in determining the mechanisms of development of naturally occurring cancer. The identification of translational products of endogenous viral genetic information in virus-negative mouse cells would be useful in an analysis of the cellular restrictions to virus expression. I t is known that in the absence of virus production, normal mouse cells contain detectable levels of an antigen that cross-reacts immunologically with the major structural polypeptide (p30) of mouse C-type viruses (10-12). However, it has yet to be shown biochemically that this activity represents the p30 polypeptide. Recently, a sensitive immunologic test for another virion polypeptide, p12 (13), has been reported. Further, type-specific immunologic assays for p12 and p30 polypeptides allow classification of mouse C-type viruses into different subgroups (14). In the present report, methods have been developed to purify these antigens from virus-negative mouse cells. I t has been possible to identify them imunologically and biochemically as virion structural polypeptides, and to compare their antigenic specificities with those of known murine leukemia virus (MuLV) 1 isolates.