Involvement of Chk1 kinase in prophase I arrest of Xenopus oocytes

Chk1 kinase, a DNA damage/replication G2 checkpoint kinase, has recently be en shown to phosphorylate and inhibit Cdc25C, a Cdc2 Tyr-15 phosphatase, th ereby directly linking the G2 checkpoint to negative regulation of Cdc2. Im mature Xenopus oocytes are arrested naturally at the first meiotic prophase (prophase I) or the late G2 phase, with sustained Cdc2 Tyr-15 phosphorylat ion. Here we have cloned a Xenopus homolog of Chk1, determined its developm ental expression, and examined its possible role in prophase I arrest of oo cytes. Xenopus Chk1 protein is expressed at approximately constant levels t hroughout oocyte maturation and early embryogenesis. Overexpression of wild -type Chk1 in oocytes prevents the release from prophase I arrest by proges terone. Conversely, specific inhibition of endogenous Chk1 either by overex pression of a dominant-negative Chk1 mutant or by injection of a neutralizi ng anti-Chk1 antibody facilitates prophase I release by progesterone. Moreo ver, when ectopically expressed in oocytes, a Chk1-nonphosphorylatable Cdc2 5C mutant alone can induce prophase I release much more efficiently than wi ld-type Cdc25C; if endogenous Chk1 function is inhibited, however, even wil d-type Cdc25C can induce the release very efficiently. These results sugges t strongly that Chk1 is involved in physiological prophase I arrest of Xeno pus oocytes via the direct phosphorylation and inhibition of Cdc25 C. We di scuss the possibility that Chk1 might function either as a G2 checkpoint ki nase or as an ordinary cell cycle regulator in prophase-I-arrested oocytes. (C) 1999 Academic Press.