Phosphorylation of Xenopus cyclins B1 and B2 is not required for cell cycle transitions.
The cdc2 kinase and B-type cyclins are known to be components of maturation- or M-phase-promoting factor (MPF). Phosphorylation of cyclin B has been reported previously and may regulate entry into and exit from mitosis and meiosis. To investigate the role of cyclin B phosphorylation, we replaced putative cdc2 kinase phosphorylation sites in Xenopus cyclins B1 and B2 by using oligonucleotide site-directed mutagenesis. We found that Ser-90 of cyclin B2 and Ser-94 or Ser-96 of cyclin B1 are the main phosphorylation sites both in functional Xenopus egg extracts and after phosphorylation with purified MPF in vitro. Microtubule-associated protein (MAP) kinase from Xenopus eggs phosphorylated cyclin B1 significantly at Ser-94 or Ser-96, whereas it was largely inactive against cyclin B2. The substitutions that ablated phosphorylation at these sites, however, resulted in no functional differences between mutant and wild-type cyclin, as judged by the kinetics of M-phase degradation, induction of mitosis in egg extracts, or induction of oocyte maturation. These results indicate that the phosphorylation of Xenopus B-type cyclins by cdc2 kinase or MAP kinase is not required for the hallmark functions of cyclin.
Izumi
T
T
Howard Hughes Medical Institute, University of Colorado School of Medicine, Denver 80262.
Maller
J L
JL
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D002453:Cell Cycle; D003001:Cloning, Molecular; D016213:Cyclins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D004926:Escherichia coli / Q000235:genetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D015345:Oligonucleotide Probes; D009865:Oocytes / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D010766:Phosphorylation; D011374:Progesterone / Q000494:pharmacology; D014176:Protein Biosynthesis; D011494:Protein Kinases / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D014158:Transcription, Genetic; D014981:Xenopus
D016213:Cyclins; D015345:Oligonucleotide Probes; D012333:RNA, Messenger; D011374:Progesterone; D011494:Protein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases