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 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.
Chevalier
S
S
Département de Biologie et Génétique du développement, CNRS URA 256, Université de Rennes I, France.
Couturier
A
A
Chartrain
I
I
Le Guellec
R
R
Beckhelling
C
C
Le Guellec
K
K
Philippe
M
M
Ford
C C
CC
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D042846:CDC2-CDC28 Kinases; D002467:Cell Nucleus / Q000378:metabolism; D003001:Cloning, Molecular; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D016213:Cyclins / Q000235:genetics / Q000378:metabolism*; D004261:DNA Replication; D018076:DNA, Complementary / Q000235:genetics; D005260:Female; D006801:Humans; D008969:Molecular Sequence Data; D009865:Oocytes / Q000166:cytology / Q000254:growth & development / Q000378:metabolism*; D010766:Phosphorylation; D011061:Poly A / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D016196:S Phase; D014981:Xenopus; D029867:Xenopus Proteins
D016213:Cyclins; D018076:DNA, Complementary; D012333:RNA, Messenger; D029867:Xenopus Proteins; D011061:Poly A; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; C495891:Cdk2 protein, Xenopus; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases