title: Regulation of nuclear transport and degradation of the Xenopus cyclin-dependent kinase inhibitor, p27Xic1.
abstract: The regulation of the vertebrate cell cycle is controlled by the function of cyclin-dependent kinases (CDKs), cyclins, and CDK inhibitors. The Xenopus laevis kinase inhibitor, p27(Xic1) (Xic1) is a member of the p21(Cip1)/p27(Kip1)/p57(Kip2) CDK inhibitor family and inhibits CDK2-cyclin E in vitro as well as DNA replication in Xenopus egg extracts. Xic1 is targeted for degradation in interphase extracts in a manner dependent on both the ubiquitin conjugating enzyme, Cdc34, and nuclei. Here we show that ubiquitination of Xic1 occurs exclusively in the nucleus and that nuclear localization of Xic1 is necessary for its degradation. We find that Xic1 nuclear localization is independently mediated by binding to CDK2-cyclin E and by nuclear localization sequences within the C terminus of Xic1. Our results also indicate that binding of Xic1 to CDK2-cyclin E is dispensable for Xic1 ubiquitination and degradation. Moreover, we show that amino acids 180-183 of Xic1 are critical determinants of Xic1 degradation. This region of Xic1 may define a motif of Xic1 essential for recognition by the ubiquitin conjugation machinery or for binding an alternate protein required for degradation.
authors.affiliation: University of Texas Health Science Center at San Antonio, Department of Molecular Medicine, Institute of Biotechnology, San Antonio, Texas 78245-3207, USA.
authors.forename: L C
authors.initials: LC
authors.lastname: Chuang
authors.forename: P R
authors.initials: PR
authors.lastname: Yew
mesh_terms: D000818:Animals; D001665:Binding Sites; D042846:CDC2-CDC28 Kinases; D018797:Cell Cycle Proteins; D002467:Cell Nucleus / Q000502:physiology; D002474:Cell-Free System; D002843:Chromatin / Q000502:physiology; D019927:Cyclin E / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D004261:DNA Replication; D004791:Enzyme Inhibitors / Q000378:metabolism; D005260:Female; D007399:Interphase; D008297:Male; D008869:Microtubule-Associated Proteins / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D009865:Oocytes / Q000166:cytology / Q000502:physiology; D017354:Point Mutation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D013094:Spermatozoa / Q000502:physiology; D025521:Tumor Suppressor Proteins; D014452:Ubiquitins; D029867:Xenopus Proteins; D014982:Xenopus laevis
chemical_list: D018797:Cell Cycle Proteins; D002843:Chromatin; D019927:Cyclin E; D004791:Enzyme Inhibitors; D008869:Microtubule-Associated Proteins; D011994:Recombinant Proteins; D025521:Tumor Suppressor Proteins; D014452:Ubiquitins; D029867:Xenopus Proteins; C493963:Xicl protein, Xenopus; D050760:Cyclin-Dependent Kinase Inhibitor p27; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495891:Cdk2 protein, Xenopus; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases