| Abstract: | A molecular complex of simian virus 40 large tumor antigen (T-Ag) and p53 cellular protein is present on the surface of simian virus 40-transformed mouse cells. The stability of the association of the two proteins with the cell surface was characterized. Cells were either surface iodinated by the lactoperoxidase technique or metabolically labeled with [35S]methionine, and surface antigens were detected by differential immunoprecipitation with specific antibodies immediately after labeling or after incubation at 37 degrees C. A rapid, concomitant disappearance of T-Ag and p53 from the cell surface was observed. The half-life of iodinated surface T-Ag was less than 30 min, whereas that of [35S]methionine-labeled surface T-Ag was 1 to 2 h. Although T-Ag and p53 were rapidly lost, both were also rapidly replaced on the cell surface, since newly exposed molecules could be detected when cells were reiodinated after a 2-h chase period. Control experiments established that the loss of the surface molecules was not induced by the iodination reaction. The appearance of surface T-Ag was prevented when cellular protein synthesis was inhibited with cycloheximide. The disappearance and replacement of T-Ag and p53 appeared to be energy-independent processes, as neither was inhibited by sodium azide or 2,4-dinitrophenol. Incubation of iodinated cells at 4 degrees C did block the loss of T-Ag and p53. These observations suggest that T-Ag and p53 are coordinately turned over in the plasma membrane. The nature of the association of the T-Ag-p53 complex with the cell surface can best be described as highly dynamic. |
