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© The Rockefeller University Press, 0021-9525/2000//963 $5.00
The Journal of Cell Biology, Volume 150, Number 5, , 2000 963-974

Nuclei and Microtubule Asters Stimulate Maturation/M Phase Promoting Factor (Mpf) Activation in Xenopus Eggs and Egg Cytoplasmic Extracts

^a UMR 7009, Centre National de la Recherche Scientifique/Université Paris VI, Station Zoologique, 06230 Villefranche-sur-mer, France
^b School of Biological Sciences, University of Sussex, Falmer, Brighton BN1 9QG, United Kingdom
UMR 7009, CNRS/Université Paris VI, Station Zoologique, 06230 Villefranche-sur-mer, France.33-4-93-76-37-9233-4-93-96-37-91

houliston{at}obs-vlfr.fr

Although maturation/M phase promoting factor (MPF) can activate autonomously in Xenopus egg cytoplasm, indirect evidence suggests that nuclei and centrosomes may focus activation within the cell. We have dissected the contribution of these structures to MPF activation in fertilized eggs and in egg fragments containing different combinations of nuclei, centrosomes, and microtubules by following the behavior of Cdc2 (the kinase component of MPF), the regulatory subunit cyclin B, and the activating phosphatase Cdc25. The absence of the entire nucleus–centrosome complex resulted in a marked delay in MPF activation, whereas the absence of the centrosome alone caused a lesser delay. Nocodazole treatment to depolymerize microtubules through first interphase had an effect equivalent to removing the centrosome. Furthermore, microinjection of isolated centrosomes into anucleate eggs promoted MPF activation and advanced the onset of surface contraction waves, which are close indicators of MPF activation and could be triggered by ectopic MPF injection. Finally, we were able to demonstrate stimulation of MPF activation by the nucleus–centriole complex in vitro, as low concentrations of isolated sperm nuclei advanced MPF activation in cycling cytoplasmic extracts. Together these results indicate that nuclei and microtubule asters can independently stimulate MPF activation and that they cooperate to enhance activation locally.

Key Words: cell cycle • centrosome • maturation promoting factor • microtubule • nucleus • Xenopus

© 2000 The Rockefeller University Press

Abbreviations used in this paper: MPF, maturation/M phase promoting factor; NT, normalized time; SCW, surface contraction wave.

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