The Tumor Rejection Antigen Separated from Rous Sarcoma Virus‐induced Murine Fibrosarcoma Exhibits a Molecular Weight of Approximately 60 kD but Differs from Functional pp60(src)

The tumor antigen capable of inducing tumor resistance (tumor rejection antigen; TRA) was obtained in a solubilized form by sodium dodecyl sulfate (SDS) extraction of plasma membrane fraction from Rous sarcoma virus (RSV)‐induced CSA1M fibrosarcoma cells (BALB/c origin). Analyses by Sephacryl S‐300 gel filtration and SDS‐polyacrylamide gel electrophoresis revealed that TRA activity was recovered in the fraction with a molecular weight of approximately 60 kD. Unfractionated crude SDS‐solubilized preparation contained gp70 as detected by rabbit anti‐gp70 antiserum, whereas such reactivity was lost in the fraction exhibiting the molecular weight of about 60 kD. Since this fraction retained pp60(src) activity, the relation of TRA to pp60(src) was further investigated. pp60(v‐src) was also obtained from the lysate of v‐src‐expressing yeast transformant. Immunization of BALB/c mice with such pp60(v‐src)‐containing lysate failed to induce any significant tumor protection. The above 60 kD fraction of CSA1M solubilized antigens was allowed to bind to Sepharose beads coupled with anti‐pp60(src) monoclonal antibody and separated into the bead‐bound and bead‐unbound fractions. The bead‐bound fraction that was recovered from pp60(src)‐binding beads (pp60(src)‐positive fraction) did not exhibit the TRA activity. In contrast, immunization with the fraction depleted of pp60(src) activity (bead‐unbound fraction) resulted in potent tumor protection. These results indicate that the solubilized membranous component(s) of CSA1M with a molecular weight of approximately 60 kD, which is distinct from functional pp60(src), functions as the TRA against RSV‐induced CSA1M tumor cells.