Identification of a substrate-targeting domain in cyclin E necessary for phosphorylation of the retinoblastoma protein

Considerable advances have been made in characterizing the cyclins and cyclin-dependent kinases (CDKs) that are necessary for progression through the cell cycle, but there has been relatively lesser success in identifying the specific biochemical pathways and cell cycle events that are directly under CDK control. To identify physiologically significant CDK substrates we generated mutations in cyclin E that altered the ability of the cyclin to direct the cyclin–CDK holoenzyme to specific in vivo substrates. We show that one of these mutations defines a domain in cyclin E necessary for phosphorylation of the retinoblastoma protein (Rb). These observations confirm the idea that cyclins contribute to substrate recognition by cyclin–CDK complexes, demonstrate the utility of targeting mutants in the identification of essential cyclin–CDK substrates, and put cyclin E squarely into the family of proteins designed to regulate Rb.