Figure 6

CDK9 is dephosphorylated by PP1 in cultured cells. A, Immunoprecipitation of CDK9. Lane 1, CDK9 was precipitated from HeLa cell extract with anti-cyclin T1 antibodies, resolved on 10% SDS-PAGE and immunoblotted with anti-CDK9 antibodies; Lane 2, immunoprecipitation of recombinant CDK9/cyclin T1; Lanes 3, input recombinant CDK9/cyclin T1; lane 4, input HeLa cell extract. B, HeLa cells were labelled with (³²P) orthophosphate in the absence (lane 1) and in the presence of 1 μM okadaic acid (lane 2) and cellular extracts were immunoprecipitated with anti-cyclin T1 antibodies, resolved by 10% SDS-PAGE and transferred to PVDF membrane. Position of CDK9 was determined by probing the membrane with anti-CDK9 antibodies using 3,3'-Diaminobenzidine enhancer system. The picture is autoradiogram of the membrane exposed to phosphor imager screen. C, 293T cells were labeled with (³²P) orthophosphate in the absence (lane 1) and in the presence of 100 nM okadaic acid (lane 2) and cellular extracts were immunoprecipitated with anti-CDK9 antibodies, resolved by 10% SDS-PAGE and transferred to PVDF membrane. Position of CDK9 was determined by probing the membrane with anti-CDK9 antibodies using 3,3'-Diaminobenzidine enhancer system. The picture is an autoradiogram of the membrane exposed to phosphor imager screen. D, 293T-cdNIPP1 cells stably expressing central domain of NIPP1 (143–224) were transfected with Flag-CDK9 expression vector and labeled with (³²P) orthophosphate in the absence (lane 1) and in the presence of 100 nM okadaic acid (lane 2). Cellular extracts were immunoprecipitated with anti-Flag antibodies, resolved by 10% SDS-PAGE and transferred to PVDF membrane. Position of CDK9 was determined by probing the membrane with anti-CDK9 antibodies using 3,3'-Diaminobenzidine enhancer system. The picture is autoradiogram of the membrane exposed to phosphor imager screen.