Cdc4, a protein required for the onset of S phase, serves an essential function during G(2)/M transition in Saccharomyces cerevisiae

Saccharomyces cerevisiae proteins Cdc4 and Cdc20 contain WD40 repeats and p articipate in proteolytic processes. However, they are thought to act at tw o different stages of the cell cycle: Cdc4 is involved in the proteolysis o f the Cdk inhibitor, Sic1, necessary for G(1)/S transition,while Cdc20 medi ates anaphase-promoting complex-dependent degradation of anaphase inhibitor Pds1, a process necessary for the onset of chromosome segregation. We have isolated three mutant alleles of CDC4 (cdc4-10, cdc4-11, and cdc4-16) whic h suppress the nuclear division defect of cdc20-1 cells. However, the previ ously characterized mutation cdc4-1 and a new allele, cdc4-12, do mt allevi ate the defect of cdc20-1 cells. This genetic interaction suggests an addit ional role for Cdc4 in G(2)/M. Reexamination of the cdc4-1 mutant revealed that, in addition to being defective in the onset of S phase, it is also de fective in G(2)/M transition when released from hydroxyurea-induced S-phase arrest. A second function for CDC4 in late S or G(2) phase was further con firmed by the observation that cells lacking the CDC4 gene are arrested bot h at G(1)/S and at G(2)/M. We subsequently isolated additional temperature- sensitive mutations in the CDC4 gene (such as cdc4-12) that render the muta nt defective in both G(1)/S and G(2)/M. transitions at the restrictive temp erature. While the G(1)/S block in both cdc4-12 and cdc4 Delta mutants is a bolished by the deletion of the SIC1 gene (causing the mutants to be arrest ed predominantly in G(2)/M), the preanaphase arrest in the cdc4-12 mutant i s relieved by the deletion of PDS1. Collectively, these observations sugges t that, in addition to its involvement in the initiation of S phase, Cdc4 m ay also be required for the onset of anaphase.