In vivo regulation of the early embryonic cell cycle in Xenopus
Abstract
We report here the first extensive in vivo study of cell cycle regulation in the Xenopus embryo. Cyclin A1, B1, B2, and E1 levels, Cdc2 and Cdk2 kinase activity, and Cdc25C phosphorylation states were monitored during early Xenopus embryonic cell cycles. Cyclin B1 and B2 protein levels were high in the unfertilized egg, declined upon fertilization, and reaccumulated to the same level during the first cell cycle, a pattern repeated during each of the following 11 divisions. Cyclin A1 showed a similar pattern, except that its level was lower in the egg than in the cell cycles after fertilization. Cyclin B1/Cdc2 kinase activity oscillated, peaking before each cleavage, and Cdc25C alternated between a highly phosphorylated and a less phosphorylated form that correlated with high and low cyclin B1/Cdc2 kinase activity, respectively. Unlike the mitotic cyclins, the level of cyclin E1 did not oscillate during embryogenesis, although its associated Cdk2 kinase activity cycled twice for each oscillation of cyclin B1/Cdc2 activity, consistent with a role for cyclin E1 in both S-phase and mitosis. Although the length of the first embryonic cycle is regulated by both the level of cyclin B and the phosphorylation state of Cdc2, cyclin accum...Continue Reading
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