CDK9/CyclinK, Active

CDK9/CyclinK is a member of the cyclin-dependent protein kinase (CDK) family. CDK9 is closely related to cdc28 and cdc2 and is an important regulator of the cell cycle (1). CDK9 is a component of the multiprotein complex TAK/P-TEF?. CDK9 can modulate RNA polymerase II-directed transcription by phosphorylating the C-terminal domain of the largest subunit of RNA polymerase II. CDK9 forms a complex with and is regulated by its regulatory subunit cyclin T or cyclin K. CDK9 also interacts with the HIV-1 Tat protein which suggested a possible involvement of this protein in AIDS (2).