DISSOCIATION OF RETINOBLASTOMA GENE PROTEIN HYPERPHOSPHORYLATION AND COMMITMENT TO ENTER S-PHASE

Mitogenic activities of simian virus 40 large T and small t antigens w ere studied in serum-deprived human diploid fibroblasts. Wild-type lar ge T and small t cooperated in stimulating DNA synthesis and in induci ng hyperphosphorylation of the Rb gene product (pRb). In contrast. a T antigen mutant defective for pRb binding (Rb-T) possessed no detectab le mitogenic activity alone and failed to complement small t in stimul ating DNA synthesis. Surprisingly, Rb-T and small t cooperated as stro ngly as wild-type T and small t with respect to pRb hyperphosphorylati on. As a consequence, in two closely related conditions (i.e., stimula tion by small t plus wild-type T versus small t plus Rb-T), the fracti on of pRb in hyperphosphorylated forms dissociated from the fraction o f cells in the S phase. These results indicate that pRb hyperphosphory lation is not always tightly coupled with a commitment to initiate DNA replication.