Cell cycle dependent subcellular distribution of Cdc25B subtypes

The dual specificity phosphatase and oncogene Cdc25B has been implicated in the G2/M cell cycle checkpoint, but the mode by which it is regulated rema ins poorly understood, Regional subcellular redistribution of proteins repr esents a unique potential regulatory mechanism. Thus, we examined in live c ells the subcellular localization characteristics of Cdc25B(2) and Cdc25B(3 ) fused to green fluorescent protein. Cdc25B(2) partitioned primarily in th e cytoplasm during G1 and progressively migrated to the nucleus as cells tr ansited from S to G2/M phase. In contrast, Cdc25B(3) maintained a homogeneo usly staining diffuse phenotype irrespective of cell cycle phase. Treatment of the Cdc25B(2)-green fluorescent protein stable transfectants with vanad ate inhibited the cell cycle dependency of intracellular distribution, whil e okadaic acid had little effect except in G1, suggesting regulation by at least one phosphorylation-dependent pathway. The DNA topoisomerase II poiso n and DNA damaging agent, etoposide, inhibited nuclear localization of Cdc2 5B(2) in S phase, possibly by invoking a sequestration cascade. Thus, diffe rences in the spatial distribution of Cdc25B subtypes exist within cells an d the 41 amino acid insert in the N-terminus of the Cdc25B(3) splice varian t encodes an important inhibitory determinant for such regulation. The subc ellular redistribution of Cdc25B(2) could be functionally important for G2/ M checkpoint regulation.