Aurora-A kinase and inhibitor-2 regulate the cyclin threshold for mitotic entry in Xenopus early embryonic cell cycles

Cell Cycle. 2006 Oct;5(19):2268-74. doi: 10.4161/cc.5.19.3316. Epub 2006 Oct 1.


The abrupt activation of CDK1 during mitotic entry requires suppression of CDK activity until a threshold concentration of cyclin B is synthesized, triggering the activation of a large pool of CDK. The cellular mechanisms that define the concentration of cyclin B at which the threshold occurs are unknown. Here we demonstrate that this threshold is regulated by Aurora-A kinase and phosphatase Inhibitor-2. In Xenopus CSF extracts that actively translate cyclin B1, immunodepletion of either endogenous xInhibitor-2 or endogenous xAurora-A caused delayed mitotic entry and normal timing was restored by addition of the respective recombinant proteins. Aurora-A depleted extracts also could be rescued by the addition of full-length xInhibitor-2, but not an xInhibitor-2 truncated of its PP1 binding motif. This demonstrates that inhibition of PP1 was required to compensate for the absence of Aurora-A. To test the hypothesis that the delays in mitotic entry in CSF extracts were due to increases in cyclin B thresholds, we employed interphase extracts, which are driven into mitosis by the addition of recombinant cyclin B in a nonlinear (threshold) dose-response. Neutralization of endogenous xInhibitor-2 or xAurora with antibodies increased the cyclin B threshold concentration. Alternatively, the addition of exogenous Aurora-A or Inhibitor-2 lowered the concentration of cyclin B that triggered CDK activation. Because the cyclin B threshold could be raised or lowered by changing the amount of either Aurora-A or Inhibitor-2, the results demonstrate these regulatory proteins are involved in a signaling loop required to create the switching behavior characteristic of mitotic entry.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Aurora Kinases
  • Cell Cycle*
  • Cyclin B / metabolism*
  • Cyclin-Dependent Kinases / metabolism
  • Embryo, Nonmammalian
  • Mitosis*
  • Phosphoprotein Phosphatases / antagonists & inhibitors
  • Protein-Serine-Threonine Kinases / pharmacology
  • Protein-Serine-Threonine Kinases / physiology*
  • Proteins / pharmacology
  • Proteins / physiology*
  • Recombinant Proteins / pharmacology
  • Xenopus
  • Xenopus Proteins / physiology*


  • Cyclin B
  • Proteins
  • Recombinant Proteins
  • Xenopus Proteins
  • protein phosphatase inhibitor-2
  • Aurora Kinases
  • Protein-Serine-Threonine Kinases
  • Cyclin-Dependent Kinases
  • Phosphoprotein Phosphatases