Effect of constitutively active 4EBP-1 on p27Kip1 association with CDK2 and CDK4. MCF7-4EBP-1-5A cells were cultured with or without pon A for 2 days. Cell extracts were prepared as described in Experimental Procedures. A portion of each extract, containing equal amounts of protein, was subjected to immunoprecipitation with anti-CDK2 (left) or anti-CDK4 (right) antibodies. After immunoprecipitation a portion of the sample was used to detect p27Kip1 and CDK2 by Western blotting. For the CDK2 immunoprecipitate the remainder of the precipitate was used to assay CDK2 kinase activity with histone H1 as a substrate.