title: Phosphorylation-dependent degradation of the cyclin-dependent kinase inhibitor p27.
abstract: The p27(Kip1) protein associates with G1-specific cyclin-CDK complexes and inhibits their catalytic activity. p27(Kip1) is regulated at various levels, including translation, degradation by the ubiquitin/proteasome pathway and non-covalent sequestration. Here, we describe point mutants of p27 deficient in their interaction with either cyclins (p27(c-)), CDKs (p27(k-)) or both (p27(ck-)), and demonstrate that each contact is critical for kinase inhibition and induction of G1 arrest. Through its intact cyclin contact, p27(k-) associated with active cyclin E-CDK2 and, unlike wild type p27, p27(c-) or p27(ck-), was efficiently phosphorylated by CDK2 on a conserved C-terminal CDK target site (TPKK). Retrovirally expressed p27(k-) was rapidly degraded through the proteasome in Rat1 cells, but was stabilized by secondary mutation of the TPKK site to VPKK. In this experimental setting, exogenous wild-type p27 formed inactive ternary complexes with cellular cyclin E-CDK2, was not degraded through the proteasome, and was not further stabilized by the VPKK mutation. p27(ck-), which was not recruited to cyclin E-CDK2, also remained stable in vivo. Thus, selective degradation of p27(k-) depended upon association with active cyclin E-CDK2 and subsequent phosphorylation. Altogether, these data show that p27 must be phosphorylated by CDK2 on the TPKK site in order to be degraded by the proteasome. We propose that cellular p27 must also exist transiently in a cyclin-bound non-inhibitory conformation in vivo.
authors.affiliation: Swiss Institute for Experimental Cancer Research, CH-1066 Epalinges, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Vlach
authors.forename: S
authors.initials: S
authors.lastname: Hennecke
authors.forename: B
authors.initials: B
authors.lastname: Amati
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D042846:CDC2-CDC28 Kinases; D018797:Cell Cycle Proteins; D002478:Cells, Cultured; 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*; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000378:metabolism; D016193:G1 Phase / Q000502:physiology*; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D017354:Point Mutation; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D051381:Rats; D017386:Sequence Homology, Amino Acid; D025521:Tumor Suppressor Proteins
chemical_list: C493962:Cdkn1b protein, rat; D018797:Cell Cycle Proteins; D019927:Cyclin E; D004791:Enzyme Inhibitors; D008869:Microtubule-Associated Proteins; D009097:Multienzyme Complexes; D010750:Phosphoproteins; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495892:Cdk2 protein, rat; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex