STUDY OF THE HIGHER EUKARYOTIC GENE-FUNCTION CDK2 USING FISSION YEAST

In the fission yeast Schizosaccharomyces pombe, cdc2 function is requi red both in G1 to enter the cell cycle and in G2 to initiate mitosis. In higher eukaryotes, these functions appeared to be shared between se veral cdc2-like genes including CDK2. Temperature-sensitive mutations in S. pombe cdc2 that arrest the cell cycle in both G1 and G2 phases a re not complemented by CDK2. We have used S. pombe to investigate what functions CDK2 can perform. We found that overexpression of the human homologue (HsCDK2) caused cell cycle arrest in G2/M showing that HsCD K2 interfered with mitotic events. Xenopus CDK2 (XlCDK2) overexpressio n did not cause cell cycle arrest and could rescue the G1 block but no t the G2 block of a cdc2-M26 ts strain. A mutant XlCDK2-R33, which is inactive as a kinase, failed to rescue the G1 block, suggesting that t he protein kinase activity of CDK2 is required to enter the cell cycle in these circumstances. We designed screens to select mutants that wo uld require XlCDK2 expression for viability, hoping to isolate new gen e functions interacting with, or that could be replaced by, XlCDK2 in G1, or new cdc2 mutants altered solely in their G1 role. From these sc reens several cell cycle mutants were selected that were XlCDK2-depend ent. These were all cdc2 mutants altered only in their G2/M function. Therefore XlCDK2 can influence both the G1/S and G2/M transition point s of cdc2 in S. pombe.