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Phosphorylation of Sic1p by G1 Cdk required for its degradation and entry into S phase.

G1 cyclin-dependent kinase (Cdk)-triggered degradation of the S-phase Cdk inhibitor Sic1p has been implicated in the transition from G1 to S phase in the cell cycle of budding yeast. A multidimensional electrospray mass spectrometry technique was used to map G1 Cdk phosphorylation sites in Sic1p both in vitro and in vivo. A Sic1p mutant lacking three Cdk phosphorylation sites did not serve as a substrate for Cdc34p-dependent ubiquitination in vitro, was stable in vivo, and blocked DNA replication. Moreover, purified phosphoSic1p was ubiquitinated in cyclin-depleted G1 extract, indicating that a primary function of G1 cyclins is to tag Sic1p for destruction. These data suggest a molecular model of how phosphorylation and proteolysis cooperate to bring about the G1/S transition in budding yeast.

Pubmed ID: 9334303


  • Verma R
  • Annan RS
  • Huddleston MJ
  • Carr SA
  • Reynard G
  • Deshaies RJ


Science (New York, N.Y.)

Publication Data

October 17, 1997

Associated Grants

  • Agency: NIGMS NIH HHS, Id: R01 GM52466-01

Mesh Terms

  • Amino Acid Sequence
  • Anaphase-Promoting Complex-Cyclosome
  • Cyclin G
  • Cyclin-Dependent Kinase Inhibitor Proteins
  • Cyclin-Dependent Kinases
  • Cyclins
  • DNA Replication
  • Enzyme Inhibitors
  • Fungal Proteins
  • G1 Phase
  • Ligases
  • Molecular Sequence Data
  • Mutagenesis
  • Phenotype
  • Phosphopeptides
  • Phosphorylation
  • Recombinant Fusion Proteins
  • S Phase
  • Saccharomyces cerevisiae Proteins
  • Ubiquitin-Protein Ligase Complexes
  • Ubiquitin-Protein Ligases
  • Ubiquitins
  • Yeasts