Cdc18p can block mitosis by two independent mechanisms.
The DNA replication checkpoint is required to maintain the integrity of the genome, inhibiting mitosis until S phase has been successfully completed. The checkpoint preventing premature mitosis in Schizosaccharomyces pombe relies on phosphorylation of the tyrosine-15 residue on cdc2p to prevent its activation and hence mitosis. The cdc18 gene is essential for both generating the DNA replication checkpoint and the initiation of S phase, thus providing a key role for the overall control and coordination of the cell cycle. We show that the C terminus of the protein is capable of both initiating DNA replication and the checkpoint function of cdc18p. The C terminus of cdc18p acts upstream of the DNA replication checkpoint genes rad1, rad3, rad9, rad17, hus1 and cut5 and requires the wee1p/mik1p tyrosine kinases to block mitosis. The N terminus of cdc18p can also block mitosis but does so in the absence of the DNA replication checkpoint genes and the wee1p/mik1p kinases therefore acting downstream of these genes. Because the N terminus of cdc18p associates with cdc2p in vivo, we suggest that by binding the cdc2p/cdc13p mitotic kinase directly, it exerts an effect independently of the normal checkpoint control, probably in an unphysiological manner.
Greenwood
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Cell Cycle Laboratory, Imperial Cancer Research Fund, London, WC2A 3PX, UK. e.greenwood@icrf.icnet.uk.
Nishitani
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Nurse
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D015153:Blotting, Western; D016203:CDC2 Protein Kinase / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D019926:Cyclin B / Q000378:metabolism; D004261:DNA Replication / Q000235:genetics; D004268:DNA-Binding Proteins; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D005800:Genes, Fungal; D008938:Mitosis; D009154:Mutation; D009687:Nuclear Proteins; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D012568:Schizosaccharomyces / Q000166:cytology* / Q000235:genetics; D029702:Schizosaccharomyces pombe Proteins; D011503:Transglutaminases
D018797:Cell Cycle Proteins; D019926:Cyclin B; D004268:DNA-Binding Proteins; D005656:Fungal Proteins; D009687:Nuclear Proteins; C082463:RAD4 protein, S pombe; D029702:Schizosaccharomyces pombe Proteins; C082462:cdc18 protein, S pombe; D011503:Transglutaminases; C090679:mik1 protein, S pombe; C068689:wee1 protein, S pombe; D011505:Protein-Tyrosine Kinases; D016203:CDC2 Protein Kinase