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Nuclear accumulation of cyclin E/Cdk2 triggers a concentration-dependent switch for the destruction of p27Xic1
C Swanson1, J Ross, P K Jackson
1Departments of Pathology and Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305-5324, USA.
Abstract:
The action of cyclin-dependent kinases (CDKs) is regulated by phosphorylation, cyclin levels, the abundance of CDK inhibitors, and, as recently has been shown for cyclin B/cdc2, their localization. It is unclear how localization regulates the action of cyclin E/Cdk2 and its inhibitors. Here, we show that the closest known Xenopus laevis homolog of mammalian Cdk2 inhibitors p27(Kip1) and p21(CIP1), Xic1, is concentrated, ubiquitinated, and destroyed in the nucleus. Furthermore, Xic1 destruction requires nuclear import, but not nuclear export, and requires the formation of a transport-competent nuclear envelope, but not interactions between the lamina and chromatin. We show that (i) cyclin E/Cdk2 and Xic1 are transported into the nucleus as a complex and that Xic1 destruction requires the activity of cyclin E, (ii) that phosphorylation of Xic1 by cyclin E/Cdk2 bypasses the requirement for nuclear formation, and (iii) that the phosphorylation of Xic1 by cyclin E/Cdk2 is concentration dependent and likely realized through second-order interactions between stable cyclin E/Cdk2/Xic1 ternary complexes. Based on these results we propose a model wherein nuclear accumulation of the cyclin E/Cdk2/Xic1 complex triggers a concentration-dependent switch that promotes the phosphorylation of Xic1 and, consequently, its ubiquitination and destruction, thus allowing subsequent activation of cyclin E/Cdk2.
Insights
Nuclear import of cyclin E/Cdk2 and Xic1 triggers Xic1 destruction, enabling cyclin E/Cdk2 activation. This process involves concentration-dependent phosphorylation of Xic1 by cyclin E/Cdk2, leading to its ubiquitination and degradation.
Area of Science:
- Cell Biology
- Molecular Biology
- Developmental Biology
Background:
- Cyclin-dependent kinases (CDKs) control cell cycle progression.
- CDK activity is regulated by various factors including inhibitors.
- Localization's role in regulating cyclin E/Cdk2 and its inhibitors is not well understood.
Purpose of the Study:
- To investigate how the localization of cyclin E/Cdk2 and its inhibitor Xic1 regulates their activity in Xenopus laevis.
- To elucidate the mechanism of Xic1 destruction in the nucleus.
Main Methods:
- Studied the localization, ubiquitination, and degradation of Xic1, a Xenopus homolog of p27(Kip1) and p21(CIP1).
- Investigated the requirements for Xic1 destruction, including nuclear import, nuclear envelope formation, and cyclin E/Cdk2 activity.
- Analyzed the role of Xic1 phosphorylation by cyclin E/Cdk2 in its degradation.
Main Results:
- Xic1 is concentrated, ubiquitinated, and destroyed in the nucleus.
- Xic1 destruction requires nuclear import and the activity of cyclin E/Cdk2.
- Phosphorylation of Xic1 by cyclin E/Cdk2 is concentration-dependent and promotes its ubiquitination and destruction.
- Nuclear accumulation of the cyclin E/Cdk2/Xic1 complex triggers a switch for Xic1 degradation.
Conclusions:
- Nuclear import and subsequent concentration-dependent phosphorylation by cyclin E/Cdk2 are critical for Xic1 destruction.
- This mechanism allows for the activation of cyclin E/Cdk2, promoting cell cycle progression.