The retinoblastoma-susceptibility gene product becomes phosphorylated in multiple stages during cell cycle entry and progression

The phosphorylation of the retinoblastoma-susceptibility gene product (RB) was monitored at different stages in the cell cycle of human T lymphocytes. The initial cell population was arrested at G(sub 0) and induced to divide synchronously after mitogenic treatment. The results showed that RB phosphorylation occurs first, at mid- G(sub 1) phase. The second phosphorylation occurs at the S phase, and the thirdat G(sub 2)/M. These events occur on different residues of RB. These results suggest that the different phosphorylated forms of RB may be involved in growthregulation at different stages in the cell cycle.