Figure 2.

Effect of γ-irradiation on G₁ cdk activity and pRB phosphorylation. (A) Normalized protein extracts from untreated or γ-irradiated (18 hr) cells were precleared with protein A beads, immunoprecipitated with the indicated antibodies, and then assayed for kinase activity by incubation with an excess of [γ-³²P]ATP and substrate (histone H1 or a C-terminal fragment of pRB). (B) SDS/PAGE analysis of [³²P]pRB from wild-type and p21−/− cells 18 hr after irradiation. (C) Western blot analysis of [³²P]pRB with an α-pRB mAb.