FIG. 3.
In vitro phosphorylation of threonine 124 in class 4 and 5 mutant T antigens is strongly enhanced by pretreatment with acid phosphatase. (A) Each of the indicated T antigens was purified and analyzed by electrophoresis in 10% denaturing gels and then Coomassie blue stained. (B and C) Each of the purified T antigens was treated with alkaline (B) or acid (C) phosphatase, immunoprecipitated, and then phosphorylated with purified cyclin A-cdk2 and [γ-32P]ATP. Proteins were separated by electrophoresis on 10% denaturing gels, Coomassie stained (a), and visualized by autoradiography (b). The positions of molecular weight marker proteins (lanes 1), T antigen (Tag) (lanes 2 to 9), and the immunoglobulin heavy chain (IgG) are indicated.