Checkpoint controls ensure chromosomal integrity through the cell cycle. Chk1 and Cds1/Chk2 are effector kinases in the G(2)-phase checkpoint activated by damaged or unreplicated DNA, and they prevent entry into M-phase through inhibition of cyclin B-Cdc2 kinase activation. However, little is known about how the effector kinases are regulated when the checkpoint is attenuated. Recent studies indicate that Chk1 is also involved in the physiological G(2)-phase arrest of immature Xenopus oocytes

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... a direct phosphorylation and inhibition of Cdc25C, the activator of cyclin B-Cdc2 kinase. Bearing in mind the overlapping functions of Chk1 and Cds1, here we have studied the involvement of Xenopus Cds1 (XCds1) in the G(2)/M-phase transition of immature oocytes and the regulation of its activity during this period. Protein levels of XCds1 remained constant throughout oocyte maturation and early embryonic development. The levels of XCds1 kinase activity were high in immature oocytes and decreased at the meiotic G(2)/M-phase transition. Consistently, when overexpressed in immature oocytes, wild-type, but not kinase-deficient, XCds1 significantly delayed entry into M-phase after progesterone treatment. The inactivation of XCds1 depended on the activation of cyclin B-Cdc2 kinase, but not MAP kinase. Although XCds1 was not directly inactivated by cyclin B-Cdc2 kinase in vitro, XCds1 was inactivated by overexpression of cyclin B, which induces the activation of cyclin B-Cdc2 kinase without progesterone. Thus, the present study is the first indication of Cds1 activity in cells that are physiologically arrested at G(2)-phase, and of its downregulation at entry into M-phase.