Cks1 is required for G(1) cyclin-cyclin-dependent kinase activity in budding yeast.
p13(suc1) (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 G(1) 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 G(1) 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 G(1) cyclins, which depends on Cdc28 activity, is reduced in cks1-38 cells. The role of Cks1 in promoting G(1) 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 G(1) phase in budding yeast.
Division of Biology, California Institute of Technology, Pasadena, California 91125, USA.
G J
GJ
Reynard
W
W
Reynolds
R
R
Verma
R J
RJ
Deshaies
D048868:Adaptor Proteins, Signal Transducing; D018797:Cell Cycle Proteins; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000378:metabolism*; D004789:Enzyme Activation; D005656:Fungal Proteins / Q000378:metabolism*; D016193:G1 Phase; D011485:Protein Binding; D029701:Saccharomyces cerevisiae Proteins; D004718:Saccharomycetales / Q000166:cytology / Q000378:metabolism*; D015398:Signal Transduction
D048868:Adaptor Proteins, Signal Transducing; C081185:CKS1 protein, S cerevisiae; D018797:Cell Cycle Proteins; D016213:Cyclins; D005656:Fungal Proteins; D029701:Saccharomyces cerevisiae Proteins; D018844:Cyclin-Dependent Kinases