REGULATION OF THE G(1) PHASE OF THE CELL-CYCLE BY PERIODIC STABILIZATION AND DEGRADATION OF THE P25(RUM1) CDK INHIBITOR

In fission yeast, the cyclin-dependent kinase (CDK) inhibitor p25(rum1 ) is a key regulator of progression through the G(1) phase of the cell cycle, We show here that p25(rum1) protein levels are sharply periodi c, p25(rum1) begins to accumulate at anaphase, persists in G(1) and is destroyed during S phase, p25(rum1) is stabilized and polyubiquitinat ed in a mutant defective in the 26S proteasome, suggesting that its de gradation normally occurs through the ubiquitin-dependent 26S proteaso me pathway. Phosphorylation of p25(rum1) by cdc2-cyclin complexes at r esidues T58 and T62 is important to target the protein for degradation . Mutation of one or both of these residues to alanine causes stabiliz ation of p25(rum1) and induces a cell cycle delay in G(1) and polyploi dization due to occasional re-initiation of DNA replication before mit osis, The CDK-cyclin complex cdc2-cig1, which is insensitive to p25(ru m1) inhibition, seems to be the main kinase that phosphorylates p25(ru m1), Phosphorylation of p25(rum1) in S phase and G(2) serves as the tr igger for p25(rum1) proteolysis, Thus, periodic accumulation and degra dation of the CDK inhibitor p25(rum1) in G(1) plays a role in setting a threshold of cyclin levels important in determining the length of th e pre-Start G(1) phase and in ensuring the correct order of cell cycle events.