Fig. 3. A, effect of p16Ink4a expression on the in vitro kinase activity of cdk4, cdk6, cyclin D1, cyclin D3, and cdk2 immunoprecipitates. The indicated proteins were immunoprecipitated from uninfected, control-infected (Ad-GFP), and Ad-p16Ink4a-infected NCI-H460 cell extracts and used in an in vitro kinase assay with [-32P]ATP and GST-Rb as substrate. B, coimmunoprecipitation of p16Ink4a with cdk4, cdk6, and cdk2 from uninfected, control-infected (Ad-GFP), and Ad-p16Ink4a-infected NCI-H460 cell extracts. cdk4, cdk6, and cdk2 immunoprecipitates were boiled in SDS-PAGE sample buffer, resolved by gel electrophoresis, and Western blotted with antibodies to the immunoprecipitated protein and p16Ink4a.