Akamatsu E, Tanaka T, Kato J Y
Graduate School of Biological Sciences, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan.
J Biol Chem. 1998 Jun 26;273(26):16494-500. doi: 10.1074/jbc.273.26.16494.
Although no chromosomal DNA replication actually occurs during Xenopus oocyte maturation, the capability develops during the late meiosis I (MI) phase in response to progesterone. This ability, however, is suppressed by Mos proteins and maturation/mitosis promoting factor during the second meiosis phase (meiosis II; MII) until fertilization. Inhibition of RNA synthesis by actinomycin D during early MI prevented induction of the replication ability, but did not interfere with initiation of the meiotic cell cycle progression characterized by oscillation of the maturation/mitosis promoting factor activity and germinal vesicle breakdown. Microinjection of recombinant proteins such as dominant-negative E2F or universal Cdk inhibitors, p21 and p27, but not wild type human E2F-1 or Cdk4-specific inhibitor, p19, into maturing oocytes during MI abolished induction of the DNA replication ability. Co-injection of human E2F-1 and cyclin E proteins into immature oocytes allowed them to initiate DNA replication even in the absence of progesterone treatment. Injection of cyclin E alone, which was sufficient to activate endogenous Cdk2 kinase, failed to induce DNA replication. Moreover, the activation of Cdk2 was not affected under the conditions where DNA replication was blocked by actinomycin D. Thus, like somatic cells, both activities of E2F and cyclin E-Cdk2 complex are required for induction of the DNA replication ability in maturing Xenopus oocytes, and enhancement of both activities enables oocytes to override DNA-replication inhibitory mechanisms that specifically lie in maturing oocytes.
虽然在非洲爪蟾卵母细胞成熟过程中实际上不会发生染色体DNA复制，但这种能力在减数第一次分裂（MI）后期会因孕酮的作用而形成。然而，在第二次减数分裂阶段（减数分裂II；MII）直到受精期间，这种能力会被Mos蛋白和成熟/有丝分裂促进因子所抑制。在MI早期用放线菌素D抑制RNA合成可阻止复制能力的诱导，但不干扰以成熟/有丝分裂促进因子活性振荡和生发泡破裂为特征的减数分裂细胞周期进程的启动。在MI期间向成熟卵母细胞中显微注射重组蛋白，如显性负性E2F或通用的Cdk抑制剂p21和p27，但不注射野生型人E2F-1或Cdk4特异性抑制剂p19，可消除DNA复制能力的诱导。将人E2F-1和细胞周期蛋白E蛋白共同注射到未成熟卵母细胞中，即使在没有孕酮处理的情况下也能使其启动DNA复制。单独注射足以激活内源性Cdk2激酶的细胞周期蛋白E未能诱导DNA复制。此外，在放线菌素D阻断DNA复制的条件下，Cdk2的激活不受影响。因此，与体细胞一样，E2F和细胞周期蛋白E-Cdk2复合物的活性对于成熟非洲爪蟾卵母细胞中DNA复制能力的诱导都是必需的，并且这两种活性的增强能使卵母细胞克服成熟卵母细胞中特有的DNA复制抑制机制。
