The SIT4 Protein Phosphatase Is Required in Late G1 for Progression into S Phase

It is now well established that protein kinases are key regulators of the eukaryotic cell cycle. The main cell cycle clock seems to involve the precise regulation of the cdc2/CDC28 kinase (and perhaps closely related kinases). The cell has elaborate mechanisms that regulate the activation of the cdc2/CDC28 kinase and determine when the cell should initiate mitosis (G2/M regulation) and whether or not the cell should initiate a new cell cycle (commitment). For G2/M regulation in Schizosaccharomyces pombe, the cdc2 kinase activity is regulated in part by (1) association of cdc2 with a B-type cyclin (Booher and Beach 1988; Solomon et al. 1990); (2) two kinases, wee1 and mik1, that phosphorylate cdc2 to inhibit its kinase activity (Lundgren et al. 1991); and (3) dephosphorylation of cdc2 by a process that requires cdc25 (Gould and Nurse 1989; Kumagai and Dunphy 1991). The cdc25 protein may directly dephosphorylate cdc2. This dephosphorylation step...