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First published online March 22, 2006
doi: 10.1242/10.1242/jcs.02919

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Cytostatic factor: an activity that puts the cell cycle on hold

¹ Chemical Genetics, Independent Research Group, Max Planck Institute of Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Germany
² Department of Cell Biology, Max Planck Institute of Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Germany

View larger version (30K): [in a new window]   Fig. 1. MPF and CSF are key activities in vertebrate oocytes. The upper panel summarizes stages important for oocyte development and the onset of embryonic development. Oogenesis leads to the production of an immature oocyte arrested in prophase of meiosis I. After resumption of meiosis in response to hormonal stimulation, the oocyte progresses until the second arrest point in meiosis II (CSF arrest). Fertilization releases the oocyte from this arrest and triggers exit from meiosis II. The lower panel shows a simplified scheme illustrating the cytoplasmic injection experiments that led to the identification of MPF and CSF in frog oocytes. Small amounts of cytoplasm taken from mature oocytes were injected into immature oocytes or one blastomere of two-cell embryos. After injection immature oocytes resume meiosis whereas injected blastomeres arrest in metaphase. These observations led Masui and Markert to postulate that cytoplasm of mature oocytes contains two distinct biochemical activities, MPF and CSF, that regulate the oocyte maturation process (Masui and Markert, 1971).

View larger version (10K): [in a new window]   Fig. 2. XErp1 and Xenopus Emi1. Domains and positions of amino acid sequences are depicted approximately to scale. XErp1/Emi2 and Xenopus Emi1 share 39% identity in their C-terminal domains. The C-terminus of both proteins contains the F-box followed by a zinc-binding region (ZBR). Note that the XErp1 N-terminal extension is not shared by Emi1.

View larger version (22K): [in a new window]   Fig. 3. Model for meiosis II exit after fertilization-induced destruction of XErp1. During CSF arrest, the APC/C is kept inactive by XErp1. Fertilization leads to the production of intracellular Ca2+ transients that activate CaMKII. CaMKII phosphorylation on Thr195 directs Plx1 to its substrate XErp1. Plx1 phosphorylation then triggers SCFß-TRCP-dependent degradation of XErp1 by the ubiquitin/proteasome system. As a result, the APC/C is liberated from its repression and triggers meiosis II exit.

View larger version (19K): [in a new window]   Fig. 4. Models of CSF arrest. The establishment and maintenance of CSF may involve many different, seemingly parallel pathways (left panel). However, we speculate that future investigation and reinvestigation of all proposed components could lead to a more straight-forward model in which all pathways converge onto the APC/C inhibitor Erp1 (right panel).