title: Kip1 degradation via the ubiquitin-proteasome pathway.
abstract: The cell cycle has been the object of extensive studies for the past years. A complex network of molecular interactions has been identified. In particular, a class of cell cycle inhibitory proteins has been identified but details of the molecular mechanism of their action have yet to be resolved. These inhibitors regulate the progression through G1 and the G1/S transition via the inhibition of the cyclin-dependent kinase (Cdk) activity. The potential function of these negative regulators as tumor suppressors provides new insights into the link between the cell cycle and oncogenesis. Kip1 is a potent inhibitor of Cdks. In quiescent cells Kip1 accumulates without an increase in mRNA or protein synthesis. We demonstrated that cell cycle regulation of Kip1 levels, both in normal and transformed human cells, occurs via the ubiquitin-proteasome pathway. In a crude in vitro system, Kip1 is ubiquitinated and degraded in an ATP dependent manner and inhibition or depletion of the proteasome blocks Kip1 degradation. Human Ubc2 and Ubc3, the homologs of yeast Rad6 and Cdc34 gene products respectively, are specifically involved in the ubiquitination of Kip1. Compared to proliferating cells, quiescent cells contain a far lower amount of Kip1 ubiquitinating activity. These results represent the first demonstration that the ubiquitin-proteasome pathway plays a role in the regulation of a cell cycle protein in human cells, namely the Cdk inhibitor Kip1. The specific proteolysis of Kip1 may be involved in the pathway of inactivation of Cdks.
authors.affiliation: Mitotix Inc., Cambridge, MA 02139, USA.
authors.forename: S W
authors.initials: SW
authors.lastname: Tam
authors.forename: A M
authors.initials: AM
authors.lastname: Theodoras
authors.forename: M
authors.initials: M
authors.lastname: Pagano
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D064173:Anaphase-Promoting Complex-Cyclosome; D000818:Animals; D002453:Cell Cycle / Q000502:physiology; D018797:Cell Cycle Proteins; D002460:Cell Line; D002471:Cell Transformation, Neoplastic; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors*; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000378:metabolism*; D005656:Fungal Proteins / Q000378:metabolism; D005786:Gene Expression Regulation; D016147:Genes, Tumor Suppressor; D006801:Humans; D008025:Ligases / Q000378:metabolism; D051379:Mice; D008869:Microtubule-Associated Proteins / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D025521:Tumor Suppressor Proteins; D044763:Ubiquitin-Conjugating Enzymes; D043743:Ubiquitin-Protein Ligase Complexes; D044767:Ubiquitin-Protein Ligases; D014452:Ubiquitins / Q000378:metabolism*
chemical_list: C493961:Cdkn1b protein, mouse; D018797:Cell Cycle Proteins; D004791:Enzyme Inhibitors; D005656:Fungal Proteins; D008869:Microtubule-Associated Proteins; D009097:Multienzyme Complexes; D029701:Saccharomyces cerevisiae Proteins; D025521:Tumor Suppressor Proteins; D014452:Ubiquitins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D000255:Adenosine Triphosphate; C577449:CDC34 protein, S cerevisiae; C577447:CDC34 protein, human; C471606:RAD6 protein, S cerevisiae; D044763:Ubiquitin-Conjugating Enzymes; D043743:Ubiquitin-Protein Ligase Complexes; D064173:Anaphase-Promoting Complex-Cyclosome; D044767:Ubiquitin-Protein Ligases; D018844:Cyclin-Dependent Kinases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D008025:Ligases