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SIC1 is ubiquitinated in vitro by a pathway that requires CDC4, CDC34, and cyclin/CDK activities.

Traversal from G1 to S-phase in cycling cells of budding yeast is dependent on the destruction of the S-phase cyclin/CDK inhibitor SIC1. Genetic data suggest that SIC1 proteolysis is mediated by the ubiquitin pathway and requires the action of CDC34, CDC4, CDC53, SKP1, and CLN/CDC28. As a first step in defining the functions of the corresponding gene products, we have reconstituted SIC1 multiubiquitination in DEAE-fractionated yeast extract. Multiubiquitination depends on cyclin/CDC28 protein kinase and the CDC34 ubiquitin-conjugating enzyme. Ubiquitin chain formation is abrogated in cdc4ts mutant extracts and assembly restored by the addition of exogenous CDC4, suggesting a direct role for this protein in SIC1 multiubiquitination. Deletion analysis of SIC1 indicates that the N-terminal 160 residues are both necessary and sufficient to serve as substrate for CDC34-dependent ubiquitination. The complementary C-terminal segment of SIC1 binds to the S-phase cyclin CLB5, indicating a modular structure for SIC1.

Pubmed ID: 9285816


  • Verma R
  • Feldman RM
  • Deshaies RJ


Molecular biology of the cell

Publication Data

August 16, 1997

Associated Grants

  • Agency: NIGMS NIH HHS, Id: GM-52466-02

Mesh Terms

  • Anaphase-Promoting Complex-Cyclosome
  • CDC28 Protein Kinase, S cerevisiae
  • Cell Cycle Proteins
  • Cyclin B
  • Cyclin-Dependent Kinase Inhibitor Proteins
  • Cyclins
  • Enzyme Inhibitors
  • F-Box Proteins
  • Fungal Proteins
  • Ligases
  • Phosphorylation
  • Saccharomyces cerevisiae Proteins
  • Ubiquitin-Conjugating Enzymes
  • Ubiquitin-Protein Ligase Complexes
  • Ubiquitin-Protein Ligases
  • Ubiquitins
  • Yeasts