The Cln3-Cdc28 kinase complex of S. cerevisiae is regulated by proteolysis and phosphorylation.

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Abstract
In Saccharomyces cerevisiae, several of the proteins involved in the Start decision have been identified; these include the Cdc28 protein kinase and three cyclin‐like proteins, Cln1, Cln2 and Cln3. We find that Cln3 is a very unstable, low abundance protein. In contrast, the truncated Cln3‐1 protein is stable, suggesting that the PEST‐rich C‐terminal third of Cln3 is necessary for rapid turnover. Cln3 associates with Cdc28 to form an active kinase complex that phosphorylates Cln3 itself and a co‐precipitated substrate of 45 kDa. The cdc34‐2 allele, which encodes a defective ubiquitin conjugating enzyme, dramatically increases the kinase activity associated with Cln3, but does not affect the half‐life of Cln3. The Cln‐‐Cdc28 complex is inactivated by treatment with non‐specific phosphatases; prolonged incubation with ATP restores kinase activity to the dephosphorylated kinase complex. It is thus possible that phosphate residues essential for Cln‐Cdc28 kinase activity are added autocatalytically. The multiple post‐translational controls on Cln3 activity may help Cln3 tether division to growth.