Cdc37 Promotes the Stability of Protein Kinases Cdc28 and Cak1

Abstract

In the budding yeast Saccharomyces cerevisiae, Cdc37 is required for the productive formation of Cdc28-cyclin complexes. Thecdc37-1 mutant arrests at Start with low levels of Cdc28 protein, which is predominantly unphosphorylated at Thr169, fails to bind cyclin, and has little protein kinase activity. We show here that Cdc28 and not cyclin is specifically defective in the cdc37-1 mutant and that Cdc37 likely does not act as an assembly factor for Cdc28-cyclin complex formation. We have also found that the levels and activity of the protein kinase Cak1 are significantly reduced in the cdc37-1 mutant. Pulse-chase analysis indicates that Cdc28 and Cak1 proteins are both destabilized when Cdc37 function is absent during but not after translation. In addition, Cdc37 promotes the production of Cak1, but not that of Cdc28, when coexpressed in insect cells. We conclude that budding yeast Cdc37, like its higher eukaryotic homologs, promotes the physical integrity of multiple protein kinases, perhaps by virtue of a cotranslational role in protein folding.

ACKNOWLEDGMENTS

We thank Doug Kellogg and Holly Chamberlin for reagents, Hernan Espinoza for assistance with Cdc37-Cak1 coexpression in insect cells, Julia Charles and Andrew Murray for comments on the manuscript, and Ira Herskowitz and members of the Morgan and Murray labs for helpful advice throughout the course of this work.

This work was supported by funding from the National Institute of General Medical Sciences (to D.O.M.) and a postgraduate scholarship from the Natural Sciences and Engineering Research Council of Canada (to A.F.).