Protein Phosphatases and Cell Division Cycle Control

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Abstract
Fission yeast has at least ten protein phosphatase genes that appear to play distinct roles in cell cycle control. Because of functional overlap, a clear lethal phenotype can be obtained only after multiple genetic alterations. Cells that have lost the protein phosphatase 1 (PP1)-like dis2/sds21 phosphatase activities prematurely enter mitosis and remain in a defective mitotic state with high H1 kinase activity and without sister chromatid disjunction. The same phenotype can be obtained in the presence of hydroxyurea. Overexpression of PP1-like phosphatase, on the other hand, delays the entry into mitosis. Cells that have lost PP2A-like ppa2 phosphatase activity also prematurely enter mitosis with a reduction in cell size. This semi-wee phenotype is enhanced in delta ppa2 mutants treated with the phosphatase inhibitor, okadaic acid. Genetic interactions between ppa2 and mitotic regulators suggest that ppa1/ppa2 phosphatase may directly or indirectly inhibit p34cdc2/cyclin kinase. Thus both PP1- and PP2A-like phosphatases in fission yeast may negatively regulate entry into mitosis. The major property of the dis2/sds21 mutant which is distinct from those of the ppa2/ppa1 mutant is its failure to inactivate the p34cdc2/cyclin complex after entry into mitosis. A novel phosphatase regulator encoded by sds22+ binds to dis2 phosphatase and controls the substrate specificity which appears to become essential in the progression from metaphase to anaphase.