FIG. 7.

Enhanced activity of Cdk4 and Cdk6 in the presence of Tax. Cells with induced or repressed Tax genes were lysed, and holoenzyme complexes containing Cdk6 or Cdk4 were immunoprecipitated. The kinase activity was assessed in vitro by using recombinant Rb protein as a substrate. (A) Cdk4 and Cdk6 activity of the cell line Tesi in the presence (−Tet) and absence (+Tet) of Tax. Quantification of the kinase activity from two independent experiments is shown graphically at the bottom of panel A. The kinase activities were normalized to the activities in untreated Tesi cells. The T-cell line SS8BPT was used as a tax-negative control. (B) Immunoblot analysis of Rb protein in Tesi and SS8tetTax cells. The upper band represents hyperphosphorylated forms (ppRb), while the lower band represents the hypophosphorylated protein (pRb).