The role of Cdc6 and geminin in Xenopus laevis oocytes during meiosis
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In oocytes three important tasks are accomplished during meiosis: 1) establishment and maintenance of replication competence 2) inhibition of DNA replication between meiosis I (MI) and meiosis II (MII) and 3) proper segregation of chromosomes. Failure to perform these tasks results in abnormal gamete formation, defective development and spontaneous abortions. From the perspective of achieving these tasks, the role of two proteins, Cdc6 and geminin during meiosis has been addressed in this work. An oocyte that has all the factors necessary to support DNA replication is said to have replication competence. Earlier, it was noticed that replication competence of oocytes fluctuates during meiosis. The immature oocyte lacks replication competence, however, regains it upon re-entering meiosis, particularly before the completion of MI. Studies have shown that expression of Cdc6 protein is necessary and sufficient to acquire the lost replication competence during meiosis. However, maintenance of this competence throughout meiosis and after fertilization requires the maintenance of other replication factors along with Cdc6. Steady state level of replication factors is dependent on their rate of synthesis. This study reveals that the level of one particular factor, Cdt1 not only depends on the rate of synthesis, but also requires stabilization by geminin. As a result geminin is necessary to maintain replication competence of oocytes during meiosis. Cdc6 protein is synthesized in oocytes to establish replication competence prior to completion of MI, although DNA replication does not occur between MI and MII. The timing of Cdc6 protein expression led to the hypothesis that Cdc6 may be required for chromosome segregation during meiosis. To this end, the preliminary data present in this work indicates that Cdc6 is localized to spindle poles in both MI and MII and is required for spindle rotation and attachment to the animal cortex. Given that Cdc6 is required for both establishing replication competence and segregating chromosomes, regulation of Cdc6 expression was studied. It is noteworthy that two different Cdc6 protein isoforms, namely Cdc6A and Cdc6B are present in Xenopus oocytes. This work indicates that the expression of both the isoforms is regulated at the translational level, which is mediated by the conserved cytoplasmic polyadenylation elements present in the 3' UTR of the Cdc6 transcript.