Department of Cell Biology
Animals; Aphidicolin; Cell Cycle; Cell Cycle Proteins; Cross-Linking Reagents; Cytoplasm; Enzyme Inhibitors; Female; Fertilization; Ficusin; Male; Mitosis; Models, Genetic; Nuclear Envelope; Phosphoprotein Phosphatases; Sea Urchins; Serum Albumin, Bovine; Sex Factors; Time Factors; Wheat Germ Agglutinins; *cdc25 Phosphatases
Cell Biology | Life Sciences | Medicine and Health Sciences
In sea urchin zygotes and mammalian cells nuclear envelope breakdown (NEB) is not driven simply by a rise in cytoplasmic cyclin dependent kinase 1-cyclin B (Cdk1-B) activity; the checkpoint monitoring DNA synthesis can prevent NEB in the face of mitotic levels of Cdk1-B. Using sea urchin zygotes we investigated whether this checkpoint prevents NEB by restricting import of regulatory proteins into the nucleus. We find that cyclin B1-GFP accumulates in nuclei that cannot complete DNA synthesis and do not break down. Thus, this checkpoint limits NEB downstream of both the cytoplasmic activation and nuclear accumulation of Cdk1-B1. In separate experiments we fertilize sea urchin eggs with sperm whose DNA has been covalently cross-linked to inhibit replication. When the pronuclei fuse, the resulting zygote nucleus does not break down for >180 minutes (equivalent to three cell cycles), even though Cdk1-B activity rises to greater than mitotic levels. If pronuclear fusion is prevented, then the female pronucleus breaks down at the normal time (average 68 minutes) and the male pronucleus with cross-linked DNA breaks down 16 minutes later. This male pronucleus has a functional checkpoint because it does not break down for >120 minutes if the female pronucleus is removed just prior to NEB. These results reveal the existence of an activity released by the female pronucleus upon its breakdown, that overrides the checkpoint in the male pronucleus and induces NEB. Microinjecting wheat germ agglutinin into binucleate zygotes reveals that this activity involves molecules that must be actively translocated into the male pronucleus.
Rights and Permissions
Citation: J Cell Sci. 1999 Apr;112 ( Pt 8):1139-48. Link to article on publisher's website