tokens: The Cdk2 - cyclin - D1 complex did not phosphorylate any tested substrates , such as H1 histone , pRB , SV40 large T antigen , p53 , E2F - 1 or a preparation of nuclear proteins from HeLa cells ; in contrast , Cdk2 - cyclin - E and Cdk2 - cyclin - A phosphorylated these proteins .