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Cell. 1994 Oct 21;79(2):233-44.

The B-type cyclin kinase inhibitor p40SIC1 controls the G1 to S transition in S. cerevisiae.

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I. M. P. Research Institute of Molecular Pathology, Vienna, Austria.

Erratum in

  • Cell 1996 Jan 12;84(1):following 174.


When yeast cells reach a critical size, they initiate bud formation, spindle pole body duplication, and DNA replication almost simultaneously. All three events depend on activation of Cdc28 protein kinase by the G1 cyclins Cln1, -2, and -3. We show that DNA replication also requires activation of Cdc28 by B-type (Clb) cyclins. A sextuple clb1-6 mutant arrests as multibudded G1 cells that resemble cells lacking the Cdc34 ubiquitin-conjugating enzyme. cdc34 mutants cannot enter S phase because they fail to destroy p40SIC1, which is a potent inhibitor of Clb but not Cln forms of the Cdc28 kinase. In wild-type cells, p40SIC1 protein appears at the end of mitosis and disappears shortly before S phase. Proteolysis of a cyclin-specific inhibitor of Cdc28 is therefore an essential aspect of the G1 to S phase transition.

[Indexed for MEDLINE]

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