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Published August 15, 2000 | Published
Journal Article Open

Cks1 Is Required for G1 Cyclin-Cyclin-Dependent Kinase Activity in Budding Yeast


p13suc1 (Cks) proteins have been implicated in the regulation of cyclin-dependent kinase (CDK) activity. However, the mechanism by which Cks influences the function of cyclin-CDK complexes has remained elusive. We show here that Cks1 is required for the protein kinase activity of budding yeast G1 cyclin-CDK complexes. Cln2 and Cdc28 subunits coexpressed in baculovirus-infected insect cells fail to exhibit protein kinase activity towards multiple substrates in the absence of Cks1. Cks1 can both stabilize Cln2-Cdc28 complexes and activate intact complexes in vitro, suggesting that it plays multiple roles in the biogenesis of active G1 cyclin-CDK complexes. In contrast, Cdc28 forms stable, active complexes with the B-type cyclins Clb4 and Clb5 regardless of whether Cks1 is present. The levels of Cln2-Cdc28 and Cln3-Cdc28 protein kinase activity are severely reduced in cks1-38 cell extracts. Moreover, phosphorylation of G1 cyclins, which depends on Cdc28 activity, is reduced in cks1-38 cells. The role of Cks1 in promoting G1 cyclin-CDK protein kinase activity both in vitro and in vivo provides a simple molecular rationale for the essential role of CKS1 in progression through G1 phase in budding yeast.

Additional Information

© 2000, American Society for Microbiology. Received 17 March 2000/Returned for modification 26 April 2000/Accepted 19 May 2000. We acknowledge M. Weinreich and B. Stillman for providing recombinant baculoviruses encoding Clb4 and Clb5. We also thank M. Olson for MBP-Clb2 and anti-Clb2 serum, R. Booher for Cks1 plasmids, S. Reed for cks1-38, F. Cross for the Far1 fragment and Cln33×HA strains, Wade Harper for baculoviruses encoding Cks1 and GST-Cdc28HA, members of W. Dunphy's laboratory for advice on baculovirus expression, and W. Dunphy, R. Feldman, G. Turner, R. Verma, and D. Patra for discussions and comments on the manuscript. R.J.D. is a Searle and Markey Scholar, and this work was supported by the Searle Program/The Chicago Community Trust, The Lucille P. Markey Charitable Trust, and the National Institutes of Health (NIH RO1 GM52466).

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