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.
Mesh Terms:
Amino Acid Sequence, 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
Amino Acid Sequence, 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
Science
Date: Oct. 17, 1997
PubMed ID: 9334303
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