XENOPUS CYCLIN-E, A NUCLEAR PHOSPHOPROTEIN, ACCUMULATES WHEN OOCYTES GAIN THE ABILITY TO INITIATE DNA-REPLICATION

The capacity to initiate DNA replication appears during oocyte maturat ion in Xenopus. Initiation of S phase is driven by several components which include active cyclin/cdk complexes, We have identified three Xe nopus cyclin E clones showing 59% amino acid identity with human cycli n E, The recruitment of cyclin E mRNA, like cdk2 mRNA, into the polyso mal fraction during oocyte maturation, results in the accumulation of the corresponding proteins in unfertilized eggs, Cyclin E mRNA remains polyadenylated during cleavage and anti-cyclin E antibodies detect Xl cyclin E in embryonic nuclei at this time. Cdk2 protein is necessary f or the phosphorylation of radiolabelled cyclin E added to egg extracts . Radiolabelled Xlcyclin E enters interphase nuclei and, though stable through interphase and mitosis, is not associated with condensed mito tic chromatin, In egg extracts, endogenous Xlcyclin E rapidly associat es with nuclei before S phase and remains nuclear throughout interphas e, becoming nucleoplasmic in G(2)/prophase, Under conditions where ini tiation of replication is limiting in extracts, Xlcyclin E associates only with those nuclei that undergo S phase, These features are entire ly consistent with the view that Xlcyclin E is required for initiation of S phase.