A novel regulatory element determines the timing of Mos mRNA translation during Xenopus oocyte maturation

Identification of post-transcriptionally regulated Xenopus tropicalis maternal mRNAs by microarray

Cytoplasmic control of the adenylation state of mRNAs is a critical post-transcriptional process involved in the regulation of mRNAs stability and translational efficiency. The early development of Xenopus laevis has been a major model for the study of such regulations. We describe here a microarray analysis to identify mRNAs that are regulated by changes in their adenylation state during oogenesis and early development of the diploid frog Xenopus tropicalis. The microarray data were validated using qRT–PCR and direct analysis of the adenylation state of endogenous maternal mRNAs during the period studied. We identified more than 500 mRNAs regulated at the post-transcriptional level among the 3000 mRNAs potentially detected by the microarray. The mRNAs were classified into nine different adenylation behavior categories. The various adenylation profiles observed during oocyte maturation and early development and the analyses of 3′-untranslated region sequences suggest that previously uncharacterized sequence elements control the adenylation behavior of the newly identified mRNAs. These data should prove useful in identifying mRNAs with important functions during oocyte maturation and early development.

Translational regulation during oogenesis and early development: the cap-poly(A) tail relationship

Metazoans rely on the regulated translation of select maternal mRNAs to control oocyte maturation and the initial stages of embryogenesis. These transcripts usually remain silent until their translation is temporally and spatially required during early development. Different translational regulatory mechanisms, varying from cytoplasmic polyadenylation to localization of maternal mRNAs, have evolved to assure coordinated initiation of development. A common feature of these mechanisms is that they share a few key trans-acting factors. Increasing evidence suggest that ubiquitous conserved mRNA-binding factors, including the eukaryotic translation initiation factor 4E (eIF4E) and the cytoplasmic polyadenylation element binding protein (CPEB), interact with cell-specific molecules to accomplish the correct level of translational activity necessary for normal development. Here we review how capping and polyadenylation of mRNAs modulate interaction with multiple regulatory factors, thus controlling translation during oogenesis and early development.

Phosphorylation of Maskin by Aurora-A Participates in the Control of Sequential Protein Synthesis during Xenopus laevis Oocyte Maturation

At the end of oogenesis, Xenopus laevis stage VI oocytes are arrested at the G2/M transition (prophase) waiting for progesterone to release the block and begin maturation. Progesterone triggers a cascade of phosphorylation events such as a decrease of pK(a) and an increase of maturating-promoting factor activity. Progression through meiosis was controlled by the sequential synthesis of several proteins. For instance, the MAPK kinase kinase c-Mos is the very first protein to be produced, whereas cyclin B1 appears only after meiosis I. After the meiotic cycles, the oocyte arrests at metaphase of meiosis II with an elevated c-Mos kinase activity (cytostatic factor). By using a two-hybrid screen, we have identified maskin, a protein involved in the control of mRNA sequential translation, as a binding partner of Aurora-A, a protein kinase necessary for oocyte maturation. Here we showed that, in vitro, Aurora-A directly binds to maskin and that both proteins can be co-immunoprecipitated from oocyte extracts, suggesting that they do associate in vivo. We also demonstrated that Aurora-A phosphorylates maskin on a Ser residue conserved in transforming acidic coiled coil proteins from Drosophila to human. When the phosphorylation of this Ser was inhibited in vivo by microinjection of synthetic peptides that mimic the maskin-phosphorylated sequence, we observed a premature maturation. Under these conditions, proteins such as cyclin B1 and Cdc6, which are normally detected only in meiosis II, were massively produced in meiosis I before the occurrence of the nuclear envelope breakdown. This result strongly suggests that phosphorylation of maskin by Aurora-A prevents meiosis II proteins from being produced during meiosis I.

Roles for cytoplasmic polyadenylation in cell cycle regulation

Polyadenylation of eukaryotic mRNAs in the nucleus promotes their translation following export to the cytoplasm and is an important determinant of mRNA stability. An additional level of control of gene expression is provided by cytoplasmic polyadenylation, which activates translation of a number of mRNAs important in orchestrating cell cycle events in oocytes. Recent studies indicate that cytoplasmic polyadenylation may be a mechanism of translational activation that is more widespread in eukaryotic cells. Here we discuss the roles of a recently identified family of nucleotidyl transferases (encoded by the cid1 gene family) in cell cycle regulation. To date, this family has been characterised mainly in yeasts, but it is conserved throughout the eukaryotes. Biochemical studies have indicated that a subset of members of this family function as cytoplasmic poly(A) polymerases targeting specific mRNAs for translation. This form of translational control appears to be particularly important for cell cycle regulation following inhibition of DNA synthesis.

Cytoplasmic polyadenylation element (CPE)- and CPE-binding protein (CPEB)-independent mechanisms regulate early class maternal mRNA translational activation in Xenopus oocytes

Meiotic cell cycle progression during vertebrate oocyte maturation requires the correct temporal translation of maternal mRNAs encoding key regulatory proteins. The mechanism by which specific mRNAs are temporally activated is unknown, although both cytoplasmic polyadenylation elements (CPE) within the 3'-untranslated region (3'-UTR) of mRNAs and the CPE-binding protein (CPEB) have been implicated. We report that in progesterone-stimulated Xenopus oocytes, the early cytoplasmic polyadenylation and translational activation of multiple maternal mRNAs occur in a CPE- and CPEB-independent manner. We demonstrate that polyadenylation response elements, originally identified in the 3'-UTR of the mRNA encoding the Mos proto-oncogene, direct CPE- and CPEB-independent polyadenylation of an early class of Xenopus maternal mRNAs. Our findings refute the hypothesis that CPE sequences alone account for the range of temporal inductions of maternal mRNAs observed during Xenopus oocyte maturation. Rather, our data indicate that the sequential action of distinct 3'-UTR-directed translational control mechanisms coordinates the complex temporal patterns and extent of protein synthesis during vertebrate meiotic cell cycle progression.

Involvement of Xenopus Pumilio in the translational regulation that is specific to cyclin B1 mRNA during oocyte maturation

Protein synthesis of cyclin B by translational activation of the dormant mRNA stored in oocytes is required for normal progression of maturation. In this study, we investigated the involvement of Xenopus Pumilio (XPum), a cyclin B1 mRNA-binding protein, in the mRNA-specific translational activation. XPum exhibits high homology to mammalian counterparts, with amino acid identity close to 90%, even if the conserved RNA-binding domain is excluded. XPum is bound to cytoplasmic polyadenylation element (CPE)-binding protein (CPEB) through the RNA-binding domain but not to its phosphorylated form in mature oocytes. In addition to the CPE, the XPum-binding sequence of cyclin B1 mRNA acts as a cis-element for translational repression. Injection of anti-XPum antibody accelerated oocyte maturation and synthesis of cyclin B1, and, conversely, over-expression of XPum retarded oocyte maturation and translation of cyclin B1 mRNA, which was accompanied by inhibition of poly(A) tail elongation. The injection of antibody and the over-expression of XPum, however, had no effect on translation of Mos mRNA, which also contains the CPE. These findings provide the first evidence that XPum is a translational repressor specific to cyclin B1 in vertebrates. We propose that in cooperation with the CPEB-maskin complex, the master regulator common to the CPE-containing mRNAs, XPum acts as a specific regulator that determines the timing of translational activation of cyclin B1 mRNA by its release from phosphorylated CPEB during oocyte maturation.

Dissection of c-MOS degron

c-MOS, a MAP kinase kinase kinase, is a regulator of oocyte maturation. The concentration of c-MOS is controlled in part through its conditional degradation. Previous studies proposed the "second-codon rule", according to which the N-terminal proline (Pro) of c-MOS is a destabilizing residue that targets c-MOS for degradation. We analyzed the degradation signal (degron) of c-MOS in Xenopus oocytes, found it to be a portable degron, and demonstrated that, contrary to the model above, the N-terminal Pro residue of c-MOS is entirely dispensable for its degradation if Ser-2 (encoded Ser-3) of c-MOS is replaced by a small non-phosphorylatable residue such as Gly. The dependence of c-MOS degradation on N-terminal Pro is shown to be caused by a Pro-mediated downregulation of the net phosphorylation of Ser-2, a modification that halts c-MOS degradation in oocytes. Thus, the N-terminal Pro residue of c-MOS is not a recognition determinant for a ubiquitin ligase, in agreement with earlier evidence that Pro is a stabilizing residue in the N-end rule.