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.
|Evidence ID||Analyze ID||Interactor||Interactor Systematic Name||Interactor||Interactor Systematic Name||Type||Assay||Annotation||Action||Modification||Phenotype||Source||Reference||Note|
|Evidence ID||Analyze ID||Gene||Gene Systematic Name||Gene Ontology Term||Gene Ontology Term ID||Qualifier||Aspect||Method||Evidence||Source||Assigned On||Reference||Annotation Extension|
|Evidence ID||Analyze ID||Gene||Gene Systematic Name||Phenotype||Experiment Type||Experiment Type Category||Mutant Information||Strain Background||Chemical||Details||Reference|
|Evidence ID||Analyze ID||Regulator||Regulator Systematic Name||Target||Target Systematic Name||Experiment||Conditions||Strain||Source||Reference|