Cyclin A/cdk2 coordinates centrosomal and nuclear mitotic events
L De Boer1, V Oakes, H Beamish
1Cancer Biology Program, Diamantina Institute for Cancer, Immunology and Metabolic Medicine, University of Queensland, Princess Alexandra Hospital, Brisbane, Queensland, Australia.
Abstract:
Cyclin A/cdk2 has a role in progression through S phase, and a large pool is also activated in G2 phase. Here we report that this G2 phase pool regulates the timing of progression into mitosis. Knock down of cyclin A by siRNA or addition of a specific cdk2 small molecule inhibitor delayed entry into mitosis by delaying cells in G2 phase. The G2 phase-delayed cells contained elevated levels of inactive cyclin B/cdk1. However, increased microtubule nucleation at the centrosomes was observed, and the centrosomes stained for markers of cyclin B/cdk1 activity. Both microtubule nucleation at the centrosomes and the phosphoprotein markers were lost with short-term treatment of the cdk1/2 inhibitor roscovitine but not the Mek1/2 inhibitor U0126. Cyclin A/cdk2 localized at the centrosomes in late G2 phase after separation of the centrosomes but before the start of prophase. Thus G2 phase cyclin A/cdk2 controls the timing of entry into mitosis by controlling the subsequent activation of cyclin B/cdk1, but also has an unexpected role in coordinating the activation of cyclin B/cdk1 at the centrosome and in the nucleus.
Insights
The G2 phase cyclin A/cdk2 complex controls entry into mitosis by regulating cyclin B/cdk1 activation. This finding reveals a novel role for cyclin A/cdk2 in coordinating cell cycle progression.
Area of Science:
- Cell Biology
- Molecular Biology
- Cell Cycle Regulation
Background:
- Cyclin A/cdk2 is crucial for S phase progression and also active in G2 phase.
- The precise function of the G2 phase cyclin A/cdk2 pool in cell cycle timing remains incompletely understood.
Purpose of the Study:
- To investigate the role of G2 phase cyclin A/cdk2 in regulating the timing of entry into mitosis.
- To elucidate the molecular mechanisms by which cyclin A/cdk2 influences mitotic entry.
Main Methods:
- RNA interference (siRNA) to knock down cyclin A.
- Small molecule inhibitors targeting cdk2 and cdk1/2 (roscovitine).
- Immunofluorescence microscopy to assess centrosome maturation and protein localization.
Main Results:
- Depletion of cyclin A or inhibition of cdk2 delayed mitotic entry by arresting cells in G2 phase.
- Delayed G2 cells exhibited elevated inactive cyclin B/cdk1 but increased centrosomal microtubule nucleation.
- Cyclin A/cdk2 localized to centrosomes in late G2 phase, coordinating cyclin B/cdk1 activation.
Conclusions:
- G2 phase cyclin A/cdk2 is essential for timely mitotic entry by regulating cyclin B/cdk1 activation.
- Cyclin A/cdk2 plays an unexpected role in coordinating cyclin B/cdk1 activation at both centrosomes and the nucleus.
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