Ubiquitination of cyclin-dependent kinase inhibitor, Xic1, is mediated by the Xenopus F-box protein xSkp2

Horng Ru Lin, Li Chiou Chuang, Hector Boix-Perales, Anna Philpott, P. Renee Yew

Research output: Contribution to journalArticlepeer-review

10 Scopus citations


In the frog, Xenopus laevis, the Cip/Kip-type cyclin-dependent kinase (CDK) inhibitor, Xic1, inhibits DNA replication in interphase egg extracts through the binding of CDK2-cyclins and Proliferating Cell Nuclear Antigen (PCNA). During DNA polymerase switching in the replicating Xenopus egg extract, Xic1 is targeted for ubiquitination and degradation when localized to chromatin through its binding to PCNA. To date, the machinery responsible for Xic1 ubiquitination is unknown and although it is predicted that the E3 called SCF may mediate Xic1 ubiquitination, characterization of the SCF in Xenopus is lacking. In this study, we describe the identification and characterization of Xenopus Skp2 (xSkp2) and the role of xSkp2 in the ubiquitination of Xic1. Our results indicate that the expression of xSkp2 appears to be developmentally regulated with low protein levels found in the egg and increased levels found in the developing embryo. We also demonstrate that when ectopically expressed, a xSkp2 F-box deletion mutant inhibits the initiation of DNA replication suggesting a role for the SCF in the onset of S phase in Xenopus egg extracts. We further show that xSkp2 binds to C-terminal residues of Xic1 and when coexpressed with Skp1, promotes the proteolysis of Xic1 in the egg extract. Moreover, the xSkp2 F-box deletion mutant inhibits the DNA-dependent ubiquitination and proteolysis of Xic1 when added to the interphase egg extract. Importantly, our studies demonstrate that SCFxSkp2 supports the ubiquitination of Xic1 in a reconstituted in vitro ubiquitination assay and that this Xic1 ubiquitination does not require either CDK2-cyclins or Cks1. These studies provide the first characterization of the SCF in Xenopus and its role in the ubiquitination of CDK inhibitor, Xic1, during DNA replication initiation.

Original languageEnglish (US)
Pages (from-to)304-314
Number of pages11
JournalCell Cycle
Issue number3
StatePublished - Feb 1 2006


  • CDK inhibitor
  • Cell cycle
  • F-box
  • SCF
  • Ubiquitin
  • Xenopus
  • p27Xic1
  • xSkp2

ASJC Scopus subject areas

  • Molecular Biology
  • Developmental Biology
  • Cell Biology


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