title: Xic1 degradation in Xenopus egg extracts is coupled to initiation of DNA replication.
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.
authors.affiliation: Division of Biology, University of California, San Diego, La Jolla, California 92093-0349, USA.
authors.forename: Zhongsheng
authors.initials: Z
authors.lastname: You
authors.forename: Kevin
authors.initials: K
authors.lastname: Harvey
authors.forename: Lindsay
authors.initials: L
authors.lastname: Kong
authors.forename: John
authors.initials: J
authors.lastname: Newport
mesh_terms: D000818:Animals; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D002843:Chromatin / Q000235:genetics / Q000378:metabolism; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D004256:DNA Polymerase I / Q000378:metabolism; D004261:DNA Replication / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000378:metabolism; D004625:Embryo, Nonmammalian / Q000166:cytology / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000378:metabolism; D005260:Female; D008025:Ligases / Q000235:genetics / Q000378:metabolism; D008297:Male; D064168:Minichromosome Maintenance Complex Component 7; D016297:Mutagenesis, Site-Directed; D009687:Nuclear Proteins / Q000378:metabolism; D009865:Oocytes / Q000166:cytology / Q000378:metabolism*; D011233:Precipitin Tests; D011994:Recombinant Proteins / Q000378:metabolism; D051716:Replication Protein A; D016196:S Phase; D013094:Spermatozoa / Q000502:physiology; D025521:Tumor Suppressor Proteins / Q000235:genetics / Q000378:metabolism*; D014452:Ubiquitins / Q000235:genetics / Q000378:metabolism*; D029867:Xenopus Proteins; D014982:Xenopus laevis
chemical_list: D018797:Cell Cycle Proteins; D002843:Chromatin; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D009687:Nuclear Proteins; D011994:Recombinant Proteins; D051716:Replication Protein A; D025521:Tumor Suppressor Proteins; D014452:Ubiquitins; D029867:Xenopus Proteins; C493963:Xicl protein, Xenopus; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases; D004256:DNA Polymerase I; C509836:Mcm7 protein, Xenopus; D064168:Minichromosome Maintenance Complex Component 7; D008025:Ligases