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Xic1 degradation in Xenopus egg extracts is coupled to initiation of DNA replication
Zhongsheng You1, Kevin Harvey, Lindsay Kong
1Division of Biology, University of California, San Diego, La Jolla, California 92093-0349, USA.
Abstract:
CDK2 activity is regulated by phosphorylation/dephosphorylation, subcellular localization, cyclin levels, and cyclin dependent kinase inhibitors (CKIs). Using Xenopus egg extracts, we find that degradation of Xic1, a Xenopus p21(cip1)/p27(kip1) family member, is coupled to initiation of DNA replication. Xic1 turnover requires the formation of a prereplication complex (pre-RC). Additionally, downstream initiation factors including CDK2, Cdc7, and Cdc45, but not RPA or DNA polymerase alpha, are necessary for activating the degradation system. Xic1 degradation is attenuated following completion of DNA replication. Unlike degradation of p27(kip1) in mammalian cells, CDK2 activity is not directly involved in Xic1 degradation and interactions between Xic1 and CDK2/cyclin E are dispensable for Xic1 turnover. Interestingly, a C-terminal region (162-192) of Xic1 is essential and apparently sufficient for triggering Xic1 ubiquitination prior to degradation. These observations demonstrate that a direct link exists between DNA replication and CKI degradation.
Insights
Degradation of Xic1, a cyclin-dependent kinase inhibitor (CKI), is linked to DNA replication initiation in Xenopus egg extracts. This process requires prereplication complex formation and specific initiation factors, but not CDK2 activity.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Cyclin-dependent kinase 2 (CDK2) activity is a key regulator of the cell cycle.
- Cyclin-dependent kinase inhibitors (CKIs) like p21(cip1)/p27(kip1) control CDK2 activity.
- The regulation of CKI stability is crucial for cell cycle progression.
Purpose of the Study:
- To investigate the regulation of Xic1, a Xenopus CKI, degradation.
- To determine the relationship between Xic1 turnover and DNA replication initiation.
- To elucidate the molecular mechanisms governing Xic1 degradation.
Main Methods:
- Utilized Xenopus egg extracts to study CKI degradation.
- Investigated the role of prereplication complex (pre-RC) formation in Xic1 turnover.
- Assessed the necessity of various DNA replication initiation factors for Xic1 degradation.
Main Results:
- Xic1 degradation is coupled to the initiation of DNA replication.
- Prereplication complex formation is required for Xic1 turnover.
- Downstream initiation factors (CDK2, Cdc7, Cdc45) are necessary for activating Xic1 degradation.
- CDK2 activity is not directly involved in Xic1 degradation, unlike mammalian p27(kip1).
- A specific C-terminal region of Xic1 is essential for its ubiquitination and degradation.
Conclusions:
- A direct link exists between DNA replication initiation and CKI degradation.
- Xic1 degradation is a regulated process dependent on replication factors.
- The mechanism of Xic1 degradation differs from that of mammalian CKIs regarding CDK2 involvement.