VIRAL TRANSACTIVATORS E1A AND VP16 INTERACT WITH A LARGE COMPLEX THATIS ASSOCIATED WITH CTD KINASE-ACTIVITY AND CONTAINS CDK8

Previously, we showed that the viral transactivator proteins E1A and V P16 specifically interact with a cellular CTD kinase activity in vitro . We now report that E1A and VP16 complexes contain human CDK8, a newl y identified member of the cyclin-dependent kinase family that has bee n shown to be a component of the RNA polymerase II (RNAP II) holoenzym e complex, The presence of CDK8 in the E1A- and VP16-containing comple xes is specific for a functional activation domain of these viral tran sactivators, strongly suggesting that this association is relevant for the transactivation function of E1A and VP16, We show that CDK8 is as sociated with CTD kinase activity and that CDK8 co-fractionates with E 1A- and VP16-associated CTD kinase activity over several chromatograph y columns. Therefore, CDK8 is likely responsible for the E1A- and VP16 -associated CTD kinase activity, Gel filtration chromatography indicat es that the E1A- and VP16-associated CTD kinase activity has a molecul ar size of similar to 1.5 MDa and contains cyclin C and the human homo log of SRB7 in addition to CDK8. This implies that E1A and VP16 associ ate with the RNAP II holoenyzme. We also looked at the transcriptional activity of CDK8 and found that CDK8 can function as a transcriptiona l activator when fused to the DNA binding domain of GAL4. Surprisingly , the ability of GAL4-CDK8 to activate transcription in this assay was not dependent on the kinase activity of CDK8, since a kinase-deficien t mutant of CDK8 stimulated transcription nearly as well as wild-type GAL4-CDK8. This suggests that CDK8 may play a role in transcription th at is distinct from its ability to function as a CTD kinase.