Role of nuclear material in the early cell cycle of Xenopus embryos.

Activated Xenopus eggs show periodic surface contraction waves and oscillations in endogenous protein phosphorylation, MPF, and kinase activities timed with the cleavage cycle of control fertilized eggs. In this paper, we show that in activated eggs lacking the material that originates from the oocyte nucleus, MPF and kinase oscillations occur in the absence of surface contraction waves. Two mitotic phosphoproteins (M116 and M46), previously described by 32P labeling in nucleated eggs, are no longer detected in the enucleated eggs. We conclude that a cytoplasmic temporal control of MPF and kinase activities is likely to be the essential cell cycle oscillator. The oocyte nuclear components normally stored in the cytoplasm of the embryos are not involved in the clock although they appear to be required for the generation of surface contraction waves.