title: Degradation of p27(Kip1) at the G(0)-G(1) transition mediated by a Skp2-independent ubiquitination pathway.
abstract: Targeting of the cyclin-dependent kinase inhibitor p27(Kip1) for proteolysis has been thought to be mediated by Skp2, the F-box protein component of an SCF ubiquitin ligase complex. Degradation of p27(Kip1) at the G(0)-G(1) transition of the cell cycle has now been shown to proceed normally in Skp2(-/-) lymphocytes, whereas p27(Kip1) proteolysis during S-G(2) phases is impaired in these Skp2-deficient cells. Degradation of p27(Kip1) at the G(0)-G(1) transition was blocked by lactacystin, a specific proteasome inhibitor, suggesting that it is mediated by the ubiquitin-proteasome pathway. The first cell cycle of stimulated Skp2(-/-) lymphocytes appeared normal, but the second cycle was markedly inhibited, presumably as a result of p27(Kip1) accumulation during S-G(2) phases of the first cell cycle. Polyubiquitination of p27(Kip1) in the nucleus is dependent on Skp2 and phosphorylation of p27(Kip1) on threonine 187. However, polyubiquitination activity was also detected in the cytoplasm of Skp2(-/-) cells, even with a threonine 187 --> alanine mutant of p27(Kip1) as substrate. These results suggest that a polyubiquitination activity in the cytoplasm contributes to the early phase of p27(Kip1) degradation in a Skp2-independent manner, thereby promoting cell cycle progression from G(0) to G(1).
authors.affiliation: Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Hara
authors.forename: T
authors.initials: T
authors.lastname: Kamura
authors.forename: K
authors.initials: K
authors.lastname: Nakayama
authors.forename: K
authors.initials: K
authors.lastname: Oshikawa
authors.forename: S
authors.initials: S
authors.lastname: Hatakeyama
authors.forename: K
authors.initials: K
authors.lastname: Nakayama
mesh_terms: D000111:Acetylcysteine / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism*; D002458:Cell Fractionation; D002478:Cells, Cultured; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors; D003546:Cysteine Endopeptidases / Q000737:chemistry / Q000378:metabolism; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D005434:Flow Cytometry; D016193:G1 Phase; D016195:G2 Phase; D015759:Ionomycin / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D008214:Lymphocytes / Q000187:drug effects / Q000502:physiology; D051379:Mice; D018345:Mice, Knockout; D008954:Models, Biological; D009097:Multienzyme Complexes / Q000737:chemistry / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D016196:S Phase; D044786:S-Phase Kinase-Associated Proteins; D025521:Tumor Suppressor Proteins / Q000378:metabolism*; D025801:Ubiquitin / Q000235:genetics / Q000378:metabolism*
chemical_list: C493961:Cdkn1b protein, mouse; D018797:Cell Cycle Proteins; D015853:Cysteine Proteinase Inhibitors; D007476:Ionophores; D009097:Multienzyme Complexes; D011993:Recombinant Fusion Proteins; D044786:S-Phase Kinase-Associated Proteins; D025521:Tumor Suppressor Proteins; D025801:Ubiquitin; C067713:lactacystin; D050760:Cyclin-Dependent Kinase Inhibitor p27; D015759:Ionomycin; D018844:Cyclin-Dependent Kinases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D000111:Acetylcysteine