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Regulation of the G1 phase of the cell cycle by periodic stabilization and degradation of the p25rum1 CDK inhibitor

In fission yeast, the cyclin‐dependent kinase (CDK) inhibitor p25rum1 is a key regulator of progression through the G1 phase of the cell cycle. We show here that p25rum1 protein levels are sharply periodic. p25rum1 begins to accumulate at anaphase, persists in G1 and is destroyed during S phase. p25rum1 is stabilized and polyubiquitinated in a mutant defective in the 26S proteasome, suggesting that its degradation normally occurs through the ubiquitin‐dependent 26S proteasome pathway. Phosphorylation of p25rum1 by cdc2–cyclin complexes at residues T58 and T62 is important to target the protein for degradation. Mutation of one or both of these residues to alanine causes stabilization of p25rum1 and induces a cell cycle delay in G1 and polyploidization due to occasional re‐initiation of DNA replication before mitosis. The CDK–cyclin complex cdc2–cig1, which is insensitive to p25rum1 inhibition, seems to be the main kinase that phosphorylates p25rum1. Phosphorylation of p25rum1 in S phase and G2 serves as the trigger for p25rum1 proteolysis. Thus, periodic accumulation and degradation of the CDK inhibitor p25rum1 in G1 plays a role in setting a threshold of cyclin levels important in determining the length of the pre‐Start G1 phase and in ensuring the correct order of cell cycle events.