p21 Is a Component of Active Cell Cycle Kinases

Much of our current understanding of the regulation of the cell division cycle has emerged from studies of a family of protein kinases (cdc2, CDC28, and generically CDK) and their inhibitors and activators (for review, see Sherr 1993). A critical step in understanding cell cycle control was the discovery that CDKs interact with cyclins, proteins that serve as essential activating subunits and specificity determinants of the kinases (Draetta 1990; Sherr 1993). In human cells, multiple cyclins and CDKs interact in a relatively promiscuous fashion to form a large family of related cyclin kinases, each of which is presumed to play a specific role in cell cycle progression.