Cdc4, a Protein Required for the Onset of S Phase, Serves an Essential Function during G2/M Transition in Saccharomyces cerevisiae

Saccharomyces cerevisiae proteins Cdc4 and Cdc20 contain WD40 repeats and participate in proteolytic processes. However, they are thought to act at two different stages of the cell cycle: Cdc4 is involved in the proteolysis of the Cdk inhibitor, Sic1, necessary for G1 /S transition, while Cdc20 mediates anaphase-promoting complex-dependent degradation of anaphase inhibitor Pds1, a process necessary for the onset of chromosome segregation. We have isolated three mutant alleles ofCDC4 (cdc4-10 ,cdc4-11 , andcdc4-16 ) which suppress the nuclear division defect ofcdc20-1 cells. However, the previously characterized mutationcdc4-1 and a new allele,cdc4-12 , do not alleviate the defect ofcdc20-1 cells. This genetic interaction suggests an additional role for Cdc4 in G2 /M. Reexamination of thecdc4-1 mutant revealed that, in addition to being defective in the onset of S phase, it is also defective in G2 /M transition when released from hydroxyurea-induced S-phase arrest. A second function forCDC4 in late S or G2 phase was further confirmed by the observation that cells lacking theCDC4 gene are arrested both at G1 /S and at G2 /M. We subsequently isolated additional temperature-sensitive mutations in theCDC4 gene (such ascdc4-12 ) that render the mutant defective in both G1 /S and G2 /M transitions at the restrictive temperature. While the G1 /S block in bothcdc4-12 andcdc4 Δ mutants is abolished by the deletion of theSIC1 gene (causing the mutants to be arrested predominantly in G2 /M), the preanaphase arrest in thecdc4-12 mutant is relieved by the deletion ofPDS1 . Collectively, these observations suggest that, in addition to its involvement in the initiation of S phase, Cdc4 may also be required for the onset of anaphase.