Entry into mitosis requires the activation of cdk1/cyclin B, while mitotic exit is achieved when the same kinase activity decreases, as cyclin B is degraded. Cyclin B proteolysis is mediated by the anaphase promoting complex, or APC, an E3 ligase that is active at anaphase in mitosis through G1. We have identified a G1 substrate of the APC that we have termed Tome-1, for trigger of mitotic entry. Tome-1 is a cytosolic protein required for proper activation of cdk1/cyclin B and mitotic entry. Tome-1 associates with Skp-1 and is required for degradation of the cdk1 inhibitory tyrosine kinase wee1; Tome-1 therefore appears to be acting as part of an SCF-type E3 for wee1. Degradation of Tome-1 during G1 allows for wee 1 accumulation during interphase, thereby providing a critical link between the APC and SCF pathways in regulation of cdk1/cyclin B activity and thus mitotic entry and exit.

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the induction of zygotic transcription. In addition, Wee1 is positively regulated by tyrosine autophosphorylation in early gastrula embryos and this upregulation of Wee1 activity is required for normal gastrulation. We also show that overexpression of Cdc25C, a phosphatase that activates the CyclinB/Cdc2 complex, induces gastrulation defects that can be rescued by Wee1, providing additional evidence that cell cycle inhibition is crucial for the gastrulation process. Together, these findings further elucidate the developmental function of Wee1 and demonstrate the importance of cell cycle regulation in vertebrate morphogenesis.

Several key developmental events occur in the first mitotic cell cycle of Xenopus; consequently this cycle has two gap phases and is ∼60-75 min in length. In contrast, embryonic cycles 2-12 consist only of S and M phases and are 30 min in length. Xe-Wee1 and Mos are translated and degraded in a developmentally regulated manner. Significantly, both proteins are present in the first cell cycle. We showed previously that the expression of nondegradable Mos, during early interphase, delays the onset of M phase in the early embryonic cell cycles. Here we report that Xe-Wee1 is required for the Mos-mediated M-phase delay. We find that Xe-Wee1 tyrosine autophosphorylation positively regulates Xe-Wee1 and is only detected in the first 30 min of the first cell cycle. The level and duration of Xe-Wee1 tyrosine phosphorylation is elevated significantly when the first cell cycle is elongated with nondegradable Mos. Importantly, we show that the tyrosine phosphorylation of Xe-Wee1 is required for the Mos-mediated M-phase delay. These findings indicate that Mos positively regulates Xe-Wee1 to generate the G 2 phase in the first cell cycle and establish a direct link between the MAPK signal transduction pathway and Wee1 in vertebrates.[Key Words: Mos; Xe-Wee1; Xenopus; embryonic cell cycle]Received December 22, 1998; revised version accepted January 15, 1999.The oocytes, eggs, and embryos of Xenopus undergo an extraordinary series of cell-cycle transitions that are critical for the propagation and development of the organism (Gerhart 1980;. In oocytes, the meiotic cell cycle is initiated in response to progesterone and consists of two consecutive M phases without an intervening S phase. The resultant eggs are arrested at metaphase of meiosis II until fertilization, which releases the arrest and initiates the series of mitotic cycles. The first mitotic cell cycle following fertilization is ∼60-75 min long and has two gap phases. In contrast, mitotic cycles 2-12 are ∼25-30 minutes long and consist only of alternating S and M phases. The gap phases in the first cycle are necessary to accommodate the completion of meiosis II, the fusion of the sperm and egg pro-nuclei and the cortical rotation that determines the second axis of asymmetry (Gerhart 1980;. The early embryonic cell cycles proceed in the absence of growth and transcription, therefore the gap phases in the first cycle are unlike those found in somatic cell cycles (Gerhart 1980;Newport and Kirschner 1982). We have studied the mechanism that regulates the length of the first mitotic cell cycle and propose that the G 2 phase in this cycle is generated by the developmentally regulated translation of Xe-Wee1 and degradation of Mos.The cyclin B/cdc2 complex regulates the entry into M phase (for review, see Norbury and Nurse 1992; Morgan 1997). The Wee1 and Myt1 protein kinases inhibit entry into M phase by mediating the phosphorylation of cdc2 on two inhibitory sites, T14 and Y15. Wee1 is localized to the nucleus and phosphorylates only the Y15 residue, whereas Myt 1 is local...