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

Science (New York, N.Y.) | Oct 17, 1997

http://www.ncbi.nlm.nih.gov/pubmed/9334303

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 RIS Download

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