The capacity to initiate DNA replication appears during oocyte maturation in Xenopus. Initiation of S phase is driven by several components which include active cyclin/cdk complexes. We have identified three Xenopus cyclin E clones showing 59% amino acid identity with human cyclin E. The recruitment of cyclin E mRNA, like cdk2 mRNA, into the polysomal 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 Xlcyclin E in embryonic nuclei at this time. Cdk2 protein is necessary for 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 mitotic chromatin. In egg extracts, endogenous Xlcyclin E rapidly associates with nuclei before S phase and remains nuclear throughout interphase, becoming nucleoplasmic in G2/prophase. Under conditions where initiation of replication is limiting in extracts, Xlcyclin E associates only with those nuclei that undergo S phase. These features are entirely consistent with the view that Xlcyclin E is required for initiation of S phase.
Xenopus cyclin E, a nuclear phosphoprotein, accumulates when oocytes gain the ability to initiate DNA replication
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S. Chevalier, A. Couturier, I. Chartrain, R. Le Guellec, C. Beckhelling, K. Le Guellec, M. Philippe, C.C. Ford; Xenopus cyclin E, a nuclear phosphoprotein, accumulates when oocytes gain the ability to initiate DNA replication. J Cell Sci 1 June 1996; 109 (6): 1173–1184. doi: https://doi.org/10.1242/jcs.109.6.1173
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