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 th rough the cell cycle, but there has been relatively lesser success in identifying the specific biochemical pathways and cell cycle events th at are directly under CDK control. To identify physiologically signifi cant CDK substrates we generated mutations in cyclin E that altered th e ability of the cyclin to direct the cyclin-CDK holoenzyme to specifi c in vivo substrates. We show that one of these mutations defines a do main in cyclin E necessary for phosphorylation of the retinoblastoma p rotein (Rb). These observations confirm the idea that cyclins contribu te to substrate recognition by cyclin-CDK complexes, demonstrate the u tility 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.