title: PP2A(Cdc55) regulates G1 cyclin stability.
abstract: Maintaining accurate progression through the cell cycle requires the proper temporal expression and regulation of cyclins. The mammalian D-type cyclins promote G1-S transition. D1 cyclin protein stability is regulated through its ubiquitylation and resulting proteolysis catalyzed by the SCF E3 ubiquitin ligase complex containing the F-box protein, Fbx4. SCF E3-ligase-dependent ubiquitylation of D1 is trigged by an increase in the phosphorylation status of the cyclin. As inhibition of ubiquitin-dependent D1 degradation is seen in many human cancers, we set out to uncover how D-type cyclin phosphorylation is regulated. Here we show that in S. cerevisiae, a heterotrimeric protein phosphatase 2A (PP2A(Cdc55)) containing the mammalian PPP2R2/PR55 B subunit ortholog Cdc55 regulates the stability of the G1 cyclin Cln2 by directly regulating its phosphorylation state. Cells lacking Cdc55 contain drastically reduced Cln2 levels caused by degradation due to cdk-dependent hyperphosphorylation, as a Cln2 mutant unable to be phosphorylated by the yeast cdk Cdc28 is highly stable in cdc55-null cells. Moreover, cdc55-null cells become inviable when the SCF(Grr1) activity known to regulate Cln2 levels is eliminated or when Cln2 is overexpressed, indicating a critical relationship between SCF and PP2A functions in regulating cell cycle progression through modulation of G1-S cyclin degradation/stability. In sum, our results indicate that PP2A is absolutely required to maintain G1-S cyclin levels through modulating their phosphorylation status, an event necessary to properly transit through the cell cycle.
authors.affiliation: Venenum Biodesign, Genesis Biotechnology Group, Hamilton, NJ, USA.
authors.forename: Paula
authors.initials: P
authors.lastname: McCourt
authors.forename: Christina
authors.initials: C
authors.lastname: Gallo-Ebert
authors.forename: Yan
authors.initials: Y
authors.lastname: Gonghong
authors.forename: Yu
authors.initials: Y
authors.lastname: Jiang
authors.forename: Joseph T
authors.initials: JT
authors.lastname: Nickels
mesh_terms: D015153:Blotting, Western; D002453:Cell Cycle / Q000502:physiology*; D018797:Cell Cycle Proteins / Q000378:metabolism*; D056766:Cyclin G1 / Q000378:metabolism*; D016213:Cyclins / Q000378:metabolism*; D047468:Immunoprecipitation; D016296:Mutagenesis; D010766:Phosphorylation; D054648:Protein Phosphatase 2 / Q000378:metabolism*; D055550:Protein Stability; D012441:Saccharomyces cerevisiae / Q000378:metabolism / Q000502:physiology*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*; D000091346:Tripeptidyl-Peptidase 1
chemical_list: C071246:CDC55 protein, S cerevisiae; C082721:CLN2 protein, S cerevisiae; D018797:Cell Cycle Proteins; D056766:Cyclin G1; D016213:Cyclins; D029701:Saccharomyces cerevisiae Proteins; D000091346:Tripeptidyl-Peptidase 1; D054648:Protein Phosphatase 2
keywords: cell cycle; cyclin; phosphorylation; protein phosphatase; ubiquitin