Maturation-specific deadenylation in Xenopus oocytes requires nuclear and cytoplasmic factors

Poly(A)-specific ribonuclease (PARN): an allosterically regulated, processive and mRNA cap-interacting deadenylase

Deadenylation of eukaryotic mRNA is a mechanism critical for mRNA function by influencing mRNA turnover and efficiency of protein synthesis. Here, we review poly(A)-specific ribonuclease (PARN), which is one of the biochemically best characterized deadenylases. PARN is unique among the currently known eukaryotic poly(A) degrading nucleases, being the only deadenylase that has the capacity to directly interact during poly(A) hydrolysis with both the m(7)G-cap structure and the poly(A) tail of the mRNA. In short, PARN is a divalent metal-ion dependent poly(A)-specific, processive and cap-interacting 3'-5' exoribonuclease that efficiently degrades poly(A) tails of eukaryotic mRNAs. We discuss in detail the mechanisms of its substrate recognition, catalysis, allostery and processive mode of action. On the basis of biochemical and structural evidence, we present and discuss a working model for PARN action. Models of regulation of PARN activity by trans-acting factors are discussed as well as the physiological relevance of PARN.

Inhibition of human poly(A)-specific ribonuclease (PARN) by purine nucleotides: kinetic analysis

Poly(A)-specific ribonuclease (PARN) is a cap-interacting and poly(A)-specific 3'-exoribonuclease that efficiently degrades mRNA poly(A) tails. Based on the enzyme's preference for its natural substrates, we examined the role of purine nucleotides as potent effectors of human PARN activity. We found that all purine nucleotides tested can reduce poly(A) degradation by PARN. Detailed kinetic analysis revealed that RTP nucleotides behave as non-competitive inhibitors while RDP and RMP exhibit competitive inhibition. Mg(2 + ) which is a catalytically important mediator of PARN activity can release inhibition of RTP and RDP but not RMP. Although many strategies have been proposed for the regulation of PARN activity, very little is known about the modulation of PARN activity by small molecule effectors, such as nucleotides. Our data imply that PARN activity can be modulated by purine nucleotides in vitro, providing an additional simple regulatory mechanism.

Expression of p27BBP/eIF6 is highly modulated duringXenopus laevis embryogenesis

Protein p27BBP/eIF6 is necessary for ribosomal function of all cells. Previous data showed that from mammals to yeast p27BBP/eIF6 is involved in the biogenesis of ribosomal subunit 60S and its association with the 60S prevents premature 80S formation regulated by PKC signaling, indicating that phosphorylation of p27BBP/eIF6 is needed for translation to occur. While in vitro p27BBP/eIF6 is constitutively expressed, and it has a high level of expression in cycling cells, in vivo its expression varies according to tissues and appears regulated by factors up to now unknown. p27BBP/eIF6 has never been investigated in developing organisms where its upregulation can be correlated with tissue growth and differentiation. In this study we have sequenced p27BBP/eIF6 cDNA and studied its expression during development of Xenopus laevis, as the first step for studying its regulation. The amino acid sequence is highly conserved with two putative PKC phosphorylation sites in serine, one site being typical of Xenopus. At the end of gastrulation, the p27BBP/eIF6 riboprobe localizes in the neural plate and in the paraxial mesoderm. In particular, from stage 24, a clear-cut localization occurs in the perspective head. In embryos exposed to teratogens, the localization of p27BBP/eIF6 riboprobe varies according to the change of head size caused by the treatment. p27BBP/eIF6 expression is particularly evident in differentiating olfactory pits, the lens, otic vesicles, and in branchial arches. Features of particular interest are p27BBP/eIF6 high level of expression in the eye field, and in the mid-hindbrain-boundary, two regions with high proliferative activity. Altogether, data indicate that a modulated expression of p27BBP/eIF6 occurs in developing anlagens in addition to a basal level of expression, and may suggest a correlation between p27BBP/eIF6 and proliferative activity. Moreover, the X. laevis cDNA isolation and characterization offer new hints for further studies in relation to potential p27BBP/eIF6 phosphorylation.

Inhibition of mRNA deadenylation by the nuclear cap binding complex (CBC)

Poly(A)-specific ribonuclease (PARN) is a cap-interacting and poly(A)-specific 3'-exoribonuclease. Here we have investigated how the cap binding complex (CBC) affects human PARN activity. We showed that CBC, via its 80-kDa subunit (CBP80), inhibited PARN, suggesting that CBC can regulate mRNA deadenylation. The CBC-mediated inhibition of PARN was cap-independent, and in keeping with this, the CBP80 subunit alone inhibited PARN. Our data suggested a new function for CBC, identified CBC as a potential regulator of PARN, and emphasized the importance of communication between the two extreme ends of the mRNA as a key strategy to regulate mRNA degradation. Based on our data, we have proposed a model for CBC-mediated regulation of PARN, which relies on an interaction between CBP80 and PARN. Association of CBC with PARN might have importance in the regulated recruitment of PARN to the nonsense-mediated decay pathway during the pioneer round of translation.

RNA-binding proteins in early development

RNA-binding proteins play a major part in the control of gene expression during early development. At this stage, the majority of regulation occurs at the levels of translation and RNA localization. These processes are, in general, mediated by RNA-binding proteins interacting with specific sequence motifs in the 3'-untranslated regions of their target RNAs. Although initial work concentrated on the analysis of these sequences and their trans-acting factors, we are now beginning to gain an understanding of the mechanisms by which some of these proteins function. In this review, we will describe a number of different families of RNA-binding proteins, grouping them together on the basis of common regulatory strategies, and emphasizing the recurrent themes that occur, both across different species and as a response to different biological problems.

Poly(A) RNA Is Reduced by Half During Bovine Oocyte Maturation but Increases when Meiotic Arrest Is Maintained with CDK Inhibitors1

Variations in the amount of different RNA species were investigated during in vitro maturation of bovine oocytes. Total RNA content was estimated to be 2 ng before meiosis, and after meiosis resumption, no decrease was observed. Ribosomal RNA did not appear to be degraded either, whereas poly(A) RNA was reduced by half after meiosis resumption, from 53 pg to 25 pg per oocyte. Real-time polymerase chain reaction was performed on growth and differentiation factor-9 (GDF-9), on cyclin B1, and on two genes implicated in the resistance to oxidative stress, glucose-6-phosphate-dehydrogenase (G6PD) and peroxiredoxin-6 (PRDX6). When these transcripts were reverse-transcribed with hexamers, the amplification results were not different before or after in vitro maturation. But when reverse transcription was performed with oligo(dT), amplification was dramatically reduced after maturation, except for cyclin B1 mRNA, implying deadenylation without degradation of three transcripts. Although calf oocytes have a lower developmental competence, their poly(A) RNA contents were not different from that of cow oocytes, nor were they differently affected during maturation. When bovine oocytes were maintained in vitro under meiotic arrest with CDK inhibitors, their poly(A) RNA amount increased, but this rise did not change the poly(A) RNA level once maturation was achieved. The increase could not be observed under transcription inhibition and, when impeding transcription and adenylation, the poly(A) RNA decreased to a level normally observed after maturation, in spite of the maintenance of meiotic arrest. These results demonstrate the importance of adenylation and deadenylation processes during in vitro maturation of bovine oocytes.

The oligo(A) tail on histone mRNA plays an active role in translational silencing of histone mRNA during Xenopus oogenesis

Metazoan replication-dependent histone mRNAs end in a stem-loop sequence. The one known exception is the histone mRNA in amphibian oocytes, which has a short oligo(A) tail attached to the stem-loop sequence. Amphibian oocytes also contain two proteins that bind the 3' end of histone mRNA: xSLBP1, the homologue of the mammalian SLBP, and xSLBP2, which is present only in oocytes. xSLBP2 is an inhibitor of histone mRNA translation, while xSLBP1 activates translation. The short A tail on histone mRNAs appears at stage II to III of oogenesis and is present on histone mRNAs throughout the rest of oogenesis. At oocyte maturation, the oligo(A) tail is removed and the xSLBP2 is degraded, resulting in the activation of translation of histone mRNA. Both SLBPs bind to the stem-loop with the oligo(A) tail with similar affinities. Reporter mRNAs ending in the stem-loop with or without the oligo(A) tail are translated equally well in a reticulocyte lysate, and their translation is stimulated by the presence of xSLBP1. In contrast, translation of the reporter mRNA with an oligo(A) tail is not activated in frog oocytes in response to the presence of xSLBP1. These results suggest that the oligo(A) tail is an active part of the translation repression mechanism that silences histone mRNA during oogenesis and that its removal is part of the mechanism that activates translation.

AtPARN is an essential poly(A) ribonuclease in Arabidopsis

Deadenylation is the first and rate-limiting step in the degradation of many mRNAs in a wide-range of organisms from yeast to higher eukaryotes. It can also play a regulatory role in early development. In this study, we examined the Arabidopsis homolog of poly(A) ribonuclease (PARN), a deadenylase first identified in mammals and absent from yeast. Consistent with the conservation of domains and residues important for catalytic activity, Arabidopsis PARN (AtPARN) expressed in Escherichia coli has poly(A) degradation activity in vitro. Protein localization experiments in plant cells indicate that AtPARN resides in both the nucleus and cytoplasm. To address the importance of the enzyme in vivo, we identified three independent T-DNA insertion mutants of AtPARN which interrupt the gene at different positions between the ATG and the stop codon. All three alleles cause lethality prior to seed germination, indicating that AtPARN is an essential gene first required during early development. Although homologous genes have yet to be inactivated in any other organism, our observations argue for the critical importance of PARN and suggest that it may be essential in many other multicellular eukaryotes.

Role of cdc2 kinase phosphorylation and conserved N-terminal proteolysis motifs in cytoplasmic polyadenylation-element-binding protein (CPEB) complex dissociation and degradation

Cytoplasmic polyadenylation-element-binding protein (CPEB) is a well-characterized and important regulator of translation of maternal mRNA in early development in organisms ranging from worms, flies and clams to frogs and mice. Previous studies provided evidence that clam and Xenopus CPEB are hyperphosphorylated at germinal vesicle breakdown (GVBD) by cdc2 kinase, and degraded shortly after. To examine the conserved features of CPEB that mediate its modification during meiotic maturation, we microinjected mRNA encoding wild-type and mutated clam CPEB into Xenopus oocytes that were subsequently allowed to mature with progesterone. We observed that (i) ectopically expressed clam CPEB is phosphorylated at GVBD and subsequently degraded, mirroring the fate of the endogenous Xenopus CPEB protein, (ii) mutation of nine Ser/Thr Pro-directed kinase sites prevents phosphorylation and degradation and (iii) deletion of the PEST box, and to a lesser extent of the putative cyclin destruction box, generates a stable and phosphorylated version of CPEB. We conclude that phosphorylation of both consensus and non-consensus sites by cdc2 kinase targets clam CPEB for PEST-mediated destruction. We also show that phosphorylation of CPEB mediates its dissociation from ribonucleoprotein complexes, prior to degradation. Our findings reinforce results obtained in Xenopus, and have implications for CPEB from other invertebrates including Drosophila, Caenorhabditis elegans and Aplysia, which lack PEST boxes.

Visualization of molecular and cellular events with green fluorescent proteins in developing embryos: a review

During the past 5 years, green fluorescent protein (GFP) has become one of the most widely used in vivo protein markers for studying a number of different molecular processes during development, such as promoter activation, gene expression, protein trafficking and cell lineage determination. GFP fluorescence allows observation of dynamic developmental processes in real time, in both transiently and stably transformed cells, as well as in live embryos. In this review, we include the most up-to-date use of GFP during embryonic development and point out the unique contribution of GFP visualization, which resulted in novel discoveries.

Comparative analysis of protein synthesis, transcription and cytoplasmic polyadenylation of mRNA during maturation of bovine oocytes in vitro

In many species, large numbers of macromolecules are accumulated during oocyte growth. The messenger and ribosomal RNAs produced in these cells are far in excess of those necessary to support protein synthesis. Thus, the aim of this study was to elucidate the processes of translational regulation during meiotic maturation. The relationship between transcription, translation and polyadenylation of mRNA during in-vitro maturation (IVM) of bovine oocytes was investigated. The results presented here show that overall protein synthesis is stimulated during meiotic maturation (approximately three times) concomitantly with the onset of germinal vesicle breakdown after 6 to 10 h of IVM. However, in metaphase II, the incorporation of [35S]methionine into proteins showed only basal levels, as in the germinal vesicle (GV) stage. Furthermore, in the course of IVM, de-novo transcription strongly declines as determined by measuring the incorporation of [3H]uridine into RNA. In contrast to this finding, the incorporation of [3H]adenosine increased and showed a peak during the time interval from 6 to 10 h of IVM, parallel with the onset of germinal vesicle breakdown (GVBD) and translation. In the further course of maturation, only a moderate decrease of [3H]adenosine incorporation was observed. These results indicate that (i) translation increased at the time of GVBD; (ii) these processes were accompanied by polyadenylation of mRNA; and (iii) although transcription declines, polyadenylated mRNA is accumulated until metaphase II (as shown by poly(U)-hybridization), in which protein synthesis is low. The correlation of these processes is discussed here. A detailed knowledge of the biochemical and molecular processes which occur during oocyte maturation can be useful for the improvement of IVM conditions.

Translational control in vertebrate development

Translational control plays a large role in vertebrate oocyte maturation and contributes to the induction of the germ layers. Translational regulation is also observed in the regulation of cell proliferation and differentiation. The features of an mRNA that mediate translational control are found both in the 5' and in the 3' untranslated regions (UTRs). In the 5' UTR, secondary structure, the binding of proteins, and the presence of upstream open reading frames can interfere with the association of initiation factors with the cap, or with scanning of the initiation complex. The 3' UTR can mediate translational activation by directing cytoplasmic polyadenylation and can confer translational repression by interference with the assembly of initiation complexes. Besides mRNA-specific translational control elements, the nonspecific RNA-binding proteins contribute to the modulation of translation in development. This review discusses examples of translational control and their relevance for developmental regulation.

RNA-binding proteins in mouse oocytes and embryos: expression of genes encoding Y box, DEAD box RNA helicase, and polyA binding proteins

Growth and differentiation of early embryos depends almost entirely on information which is maternally inherited in the form of macromolecules accumulated by the female gamete during its growth phase. Most of the maternal mRNAs synthesized by growing oocytes are not immediately recruited onto polysomes but are stored as translationally dormant messenger ribonucleoprotein (mRNP) particles. mRNA binding proteins which have been associated with masked mRNP complexes in Xenopus oocytes fall into two main categories, those having affinity for a variety of RNA sequences (members of the Y box and DEAD box RNA helicase families) and those which interact more specifically with 3' polyA tails (the polyA binding proteins or PABPs). The objective of this study was to determine whether mouse oocytes and embryos express sequences encoding a Y box protein, (MSY1); on RNA helicase, (RCK/p54); and a universally expressed PABP and testis specific isoform (PABP1 and PABPt, respectively). RNAs were amplified by RT/PCR and the identities of targeted cDNAs were confirmed by restriction analysis and/or direct sequencing. Relative steady state levels and time courses of accumulation/decay were compared by Northern hybridization. All of the sequences are transcribed as maternal mRNAs. MSY1 transcripts accumulated during the growth phase appear to be degraded in parallel with the bulk of maternal mRNAs by the mid-late two-cell stage. RCK/p54 mRNAs are most abundant in growing oocytes; steady state levels decline in primary and secondary oocytes, and degradation appears to be complete by the mid-late two-cell stage. Zygotic transcription of MSY1 and RCK/p54 is evident in four-cell stage embryos. Most of the PABP1 message accumulated by growing oocytes decays during meiotic maturation with transcription resuming in two-cell embryos. PABPt is expressed at very low levels in oocytes and embryos. Based on the temporal patterns of expression and the reported activities of homologous sequences in other systems, we suggest that these RNA binding proteins may participate in the post-transcriptional regulation of gene expression during the period of maternal control of development in the mouse.

Constitutive and stress-inducible expression of the endoplasmic reticulum heat shock protein 70 gene family member, immunoglobulin-binding protein (BiP), during Xenopus laevis early development

We have characterized the constitutive and stress-inducible pattern of immunoglobulin-binding protein (BiP) gene expression during Xenopus early development. Whole mount in situ hybridization analysis revealed that BiP mRNA was detected in unfertilized eggs, cleavage and blastula stage embryos. In gastrulae, BiP mRNA was present across the surface of the embryo, while in neurulae BiP mRNA was enriched in the neural plate, neural fold, and around the blastopore. In early and late tailbud embryos, BiP mRNA was found primarily in the dorsal region. Tunicamycin and A23187, the calcium ionophore, enhanced BiP mRNA accumulation first at the neurula stage, while heat shock induced BiP mRNA accumulation first at the gastrula stage. Compared to control, A23187- and heat shock-treated neurulae displayed relatively high levels of BiP mRNA in selected tissues, including the neural plate, neural folds, around the blastopore, and ectoderm. At the early tailbud stage, A23187 and heat shock enhanced BiP mRNA accumulation primarily in the head, somites, tail, and along the spinal cord. A similar situation was found with A23187- and heat shock-treated late tailbud embryos, except that heat-shocked embryos also displayed enhanced BiP mRNA accumulation in the epidermis. These studies demonstrate a preferential accumulation of BiP mRNA in selected tissues during development and in response to stress.

Cap-dependent deadenylation of mRNA

Poly(A) tail removal is often the initial and rate-limiting step in mRNA decay and is also responsible for translational silencing of maternal mRNAs during oocyte maturation and early development. Here we report that deadenylation in HeLa cell extracts and by a purified mammalian poly(A)-specific exoribonuclease, PARN (previously designated deadenylating nuclease, DAN), is stimulated by the presence of an m(7)-guanosine cap on substrate RNAs. Known cap-binding proteins, such as eIF4E and the nuclear cap-binding complex, are not detectable in the enzyme preparation, and PARN itself binds to m(7)GTP-Sepharose and is eluted specifically with the cap analog m(7)GTP. Xenopus PARN is known to catalyze mRNA deadenylation during oocyte maturation. The enzyme is depleted from oocyte extract with m(7)GTP-Sepharose, can be photocross-linked to the m(7)GpppG cap and deadenylates m(7)GpppG-capped RNAs more efficiently than ApppG-capped RNAs both in vitro and in vivo. These data provide additional evidence that PARN is responsible for deadenylation during oocyte maturation and suggest that interactions between 5' cap and 3' poly(A) tail may integrate translational efficiency with mRNA stability.

Cytoplasmic polyadenylation in development and beyond

Maternal mRNA translation is regulated in large part by cytoplasmic polyadenylation. This process, which occurs in both vertebrates and invertebrates, is essential for meiosis and body patterning. In spite of the evolutionary conservation of cytoplasmic polyadenylation, many of the cis elements and trans-acting factors appear to have some species specificity. With the recent isolation and cloning of factors involved in both poly(A) elongation and deadenylation, the underlying biochemistry of these reactions is beginning to be elucidated. In addition to early development, cytoplasmic polyadenylation is now known to occur in the adult brain, and there is circumstantial evidence that this process occurs at synapses, where it could mediate the long-lasting phase of long-term potentiation, which is probably the basis of learning and memory. Finally, there may be multiple mechanisms by which polyadenylation promotes translation. Important questions yet to be answered in the field of cytoplasmic polyadenylation are addressed.

The deadenylating nuclease (DAN) is involved in poly(A) tail removal during the meiotic maturation of Xenopus oocytes

Exonucleolytic degradation of the poly(A) tail is often the first step in the decay of eukaryotic mRNAs and is also used to silence certain maternal mRNAs translationally during oocyte maturation and early embryonic development. We previously described the purification of a poly(A)-specific 3'-exoribonuclease (deadenylating nuclease, DAN) from mammalian tissue. Here, the isolation and functional characterization of cDNA clones encoding human DAN is reported. Recombinant DAN overexpressed in Escherichia coli has properties similar to those of the authentic protein. The amino acid sequence of DAN shows homology to the RNase D family of 3'-exonucleases. DAN appears to be localized in both the nucleus and the cytoplasm. It is not stably associated with polysomes or ribosomal subunits. Xenopus oocytes contain nuclear and cytoplasmic DAN isoforms, both of which are closely related to the human DAN. Anti-DAN antibody microinjected into oocytes inhibits default deadenylation during progesterone-induced maturation. Ectopic expression of human DAN in enucleated oocytes rescues maturation-specific deadenylation, indicating that amphibian and mammalian DANs are functionally equivalent.

Masking, unmasking, and regulated polyadenylation cooperate in the translational control of a dormant mRNA in mouse oocytes

The mechanisms responsible for translational silencing of certain mRNAs in growing oocytes, and for their awakening during meiotic maturation, are not completely elucidated. We show that binding of a approximately 80-kD protein to a UA-rich element in the 3' UTR of tissue-type plasminogen activator mRNA, a mouse oocyte mRNA that is translated during meiotic maturation, silences the mRNA in primary oocytes. Translation can be triggered by injecting a competitor transcript that displaces this silencing factor, without elongation of a pre-existing short poly(A) tail, the presence of which is mandatory. During meiotic maturation, cytoplasmic polyadenylation is necessary to maintain a poly(A) tail, but the determining event for translational activation appears to be the modification or displacement of the silencing factor.

A dependent pathway of cytoplasmic polyadenylation reactions linked to cell cycle control by c-mos and CDK1 activation

During oocyte maturation and early development, mRNAs receive poly(A) in the cytoplasm at distinct times relative to one another and to the cell cycle. These cytoplasmic polyadenylation reactions do not occur during oogenesis, but begin during oocyte maturation and continue throughout early development. In this report, we focus on the link between cytoplasmic polyadenylation and control of the cell cycle during meiotic maturation. Activation of maturation promoting factor, a complex of CDK1 and cyclin, is required for maturation and dependent on c-mos protein kinase. We demonstrate here that two classes of polyadenylation exist during oocyte maturation, defined by their dependence of c-mos and CDK1 protein kinases. Polyadenylation of the first class of mRNAs (class I) is independent of c-mos and CDK1 kinase activities, whereas polyadenylation of the second class (class II) requires both of these activities. Class I polyadenylation, through its effects on c-mos mRNA, is required for class II polyadenylation. cis-acting elements responsible for this distinction reside in the 3'-untranslated region, upstream of the polyadenylation signal AAUAAA. Cytoplasmic polyadenylation elements (CPEs) are sufficient to specify class I polyadenylation, and subtle changes in the CPE can substantially, though not entirely, shift an RNA from class I to class II. Activation of class I polyadenylation events is independent of hyperphosphorylation of CPE-binding protein or poly(A) polymerase, and requires cellular protein synthesis. The two classes of polyadenylation and of mRNA define a dependent pathway, in which polyadenylation of certain mRNAs requires the prior polyadenylation of another. We propose that this provides one method of regulating the temporal order of polyadenylation events, and links polyadenylation to the control of the meiotic cell cycle.

Regulated unmasking of in vivo synthesized maternal mRNA at oocyte maturation. A role for the chaperone nucleoplasmin

We examine the translational regulation of histone H4 mRNA when Xenopus laevis oocytes are induced to mature with progesterone. Histone H4 mRNA synthesized from plasmid templates microinjected into oocyte nuclei is translationally silenced (masked). This masked mRNA becomes translationally active only after oocyte maturation. In contrast, histone H4 mRNA injected into the oocyte cytoplasm is translationally active both before and after oocyte maturation. Thus, transcription in vivo is required to mask histone H4 mRNA and to allow subsequent translational regulation. Protein association with histone H4 mRNA synthesized in vivo was determined before and after oocyte maturation. UV cross-linking of radiolabeled RNA to protein and immunoprecipitation of cross-linked proteins reveals an increased association of the chaperone nucleoplasmin with ribonucleoprotein particles dependent on the oocyte maturation process. The Y-box protein FRGY2 inhibits translation of histone H4 mRNA in vitro. Nucleoplasmin is able to partially relieve this repression. We discuss the potential role of nuleoplasmin in the remodeling of repressive ribonucleoprotein particles containing maternal mRNA to facilitate translational activation.

Isolation and characterization of a cDNA encoding a Xenopus immunoglobulin binding protein, BiP (grp78)

We have isolated a full-length cDNA clone encoding a Xenopus laevis immunoglobulin binding protein (BiP; also called glucose-regulated protein or grp78). The Bip cDNA sequence includes an open reading frame of 1,965 bp encoding a 655 amino acid protein with an N-terminal hydrophobic leader sequence and a C-terminal KDEL tetrapeptide which has been found in other lumenal proteins of the endoplasmic reticulum. The 3' untranslated region contains a polyadenylation and an adenylation control element (ACE) as well as a putative mRNA instability sequence. The Xenopus BiP amino acid sequence displayed high identity with BiP from other vertebrates including chicken (91.3%), rat (90.7%), and human (89.9%). Northern hybridization analysis demonstrated that BiP mRNA was present constitutively in the Xenopus A6 kidney epithelial cell line and that BiP mRNA levels could be enhanced by treatment of the cells with galactose-free media, 2-deoxyglucose, 2-deoxygalactose, glucosamine, tunicamycin, heat shock, dithiothreitol, and the calcium ionophore, A23187. Finally, while BiP mRNA was detected in all of the adult tissues examined, the relative level of BiP mRNA differed dramatically between organs. For example, relatively high levels of BiP mRNA were detected in liver with moderate levels in testis, ovary and heart and reduced levels in eye and muscle tissue.

Characteristics of poly(A)-degrading factor present in the avian oocytes and early embryos

The presence of poly(A)-degrading activity was studied in vitro in the quail and mouse oocytes and early embryos using 3H-poly(A) as a substrate. The activity was measured by adsorption of the undegraded substrate to DE-81 filter paper discs, by chromatographic separation on Sephadex G-50 column and by agarose gel electrophoresis followed by transfer onto a Zeta-probe membrane (BioRad, Richmond, CA) and autoradiography. High poly(A)-degrading activity was found in the quail previtellogenic and vitellogenic oocytes and lower activity in the early embryos from cleavage stage to gastrulation. This activity is localized predominantly in the nucleus and, to a lesser degree, in the cytoplasm and in the vitellus of vitellogenic oocytes. The length of the poly(A) degradation product was estimated to be of about (A)10. Optimum activity was at pH 8.7 and at Mn2+ concentration of 0.5 mM. This makes the deadenylating enzyme from the quail oocytes similar to endoribonuclease IV from the chick and quail oviducts (Müller [1976] Eur. J. Biochem., 70:241-248; Müller [1976], Eur. J. Biochem., 70:249-258). We suggest that the poly(A)-degrading enzyme, similar to endoribonuclease IV found in the quail oocytes, might be the "deadenylating factor" reported in Xenopus oocytes (Varnum et al. [1992] Dev. Biol., 153:283-290). Such poly(A)-degrading activity is undetectable in unfertilized mouse eggs; however, a slight, statistically insignificant tendency for poly(A) degradation was seen in two-cell embryos.

XB/U-cadherin mRNA contains cytoplasmic polyadenylation elements and is polyadenylated during oocyte maturation in Xenopus laevis

Cytoplasmic polyadenylation elements (CPE) are distinct sequence motifs in the 3'-untranslated region of mRNAs. They control translation of these RNAs by cytoplasmic polyadenylation. We show that the mRNA of the cell adhesion molecule XB/U-cadherin contains two CPE motifs. With oocyte maturation this mRNA becomes polyadenylated and increasingly recruited into the polysomal fraction. Our results give evidence that CPEs of the XB/U-cadherin mRNA are responsible for the XB/U-cadherin protein increase during oocyte maturation.

Multiple sequence elements and a maternal mRNA product control cdk2 RNA polyadenylation and translation during early Xenopus development

Cytoplasmic poly(A) elongation is one mechanism that regulates translational recruitment of maternal mRNA in early development. In Xenopus laevis, poly(A) elongation is controlled by two cis elements in the 3' untranslated regions of responsive mRNAs: the hexanucleotide AAUAAA and a U-rich structure with the general sequence UUUUUAAU, which is referred to as the cytoplasmic polyadenylation element (CPE). B4 RNA, which contains these sequences, is polyadenylated during oocyte maturation and maintains a poly(A) tail in early embryos. However, cdk2 RNA, which also contains these sequences, is polyadenylated during maturation but deadenylated after fertilization. This suggests that cis-acting elements in cdk2 RNA signal the removal of the poly(A) tail at this time. By using poly(A) RNA-injected eggs, we showed that two elements which reside 5' of the CPE and 3' of the hexanucleotide act synergistically to promote embryonic deadenylation of this RNA. When an identical RNA lacking a poly(A) tail was injected, these sequences also prevented poly(A) addition. When fused to CAT RNA, the cdk2 3' untranslated region, which contains these elements, as well as the CPE and the hexanucleotide, promoted poly(A) addition and enhanced chloramphenicol acetyltransferase activity during maturation, as well as repression of these events after fertilization. Incubation of fertilized eggs with cycloheximide prevented the embryonic inhibition of cdk2 RNA polyadenylation but did not affect the robust polyadenylation of B4 RNA. This suggests that a maternal mRNA, whose translation occurs only after fertilization, is necessary for the cdk2 deadenylation or inhibition of RNA polyadenylation. This was further suggested when poly(A)+ RNA isolated from two-cell embryos was injected into oocytes that were then allowed to mature. Such oocytes became deficient for cdk2 RNA polyadenylation but remained proficient for B4 RNA polyadenylation. These data show that CPE function is developmentally regulated by multiple sequences and factors.

Multiple sequence elements and a maternal mRNA product control cdk2 RNA polyadenylation and translation during early Xenopus development

Cytoplasmic poly(A) elongation is one mechanism that regulates translational recruitment of maternal mRNA in early development. In Xenopus laevis, poly(A) elongation is controlled by two cis elements in the 3' untranslated regions of responsive mRNAs: the hexanucleotide AAUAAA and a U-rich structure with the general sequence UUUUUAAU, which is referred to as the cytoplasmic polyadenylation element (CPE). B4 RNA, which contains these sequences, is polyadenylated during oocyte maturation and maintains a poly(A) tail in early embryos. However, cdk2 RNA, which also contains these sequences, is polyadenylated during maturation but deadenylated after fertilization. This suggests that cis-acting elements in cdk2 RNA signal the removal of the poly(A) tail at this time. By using poly(A) RNA-injected eggs, we showed that two elements which reside 5' of the CPE and 3' of the hexanucleotide act synergistically to promote embryonic deadenylation of this RNA. When an identical RNA lacking a poly(A) tail was injected, these sequences also prevented poly(A) addition. When fused to CAT RNA, the cdk2 3' untranslated region, which contains these elements, as well as the CPE and the hexanucleotide, promoted poly(A) addition and enhanced chloramphenicol acetyltransferase activity during maturation, as well as repression of these events after fertilization. Incubation of fertilized eggs with cycloheximide prevented the embryonic inhibition of cdk2 RNA polyadenylation but did not affect the robust polyadenylation of B4 RNA. This suggests that a maternal mRNA, whose translation occurs only after fertilization, is necessary for the cdk2 deadenylation or inhibition of RNA polyadenylation. This was further suggested when poly(A)+ RNA isolated from two-cell embryos was injected into oocytes that were then allowed to mature. Such oocytes became deficient for cdk2 RNA polyadenylation but remained proficient for B4 RNA polyadenylation. These data show that CPE function is developmentally regulated by multiple sequences and factors.

Unmasking the role of the 3' UTR in the cytoplasmic polyadenylation and translational regulation of maternal mRNAs

The poly(A)-dependent translational regulation of maternal mRNAs is an important mechanism to execute stage-specific programs of protein synthesis during early development. This control underlies many crucial developmental events including the meiotic maturation of oocytes and activation of the mitotic cell cycle at fertilization. A recent report demonstrates that the 3' untranslated region of the cyclin A1, B1, B2 and c-mos mRNAs determines the timing and extent of their cytoplasmic polyadenylation and translational activation during Xenopus oocyte maturation. These studies further establish that protein synthesis can be temporally and quantitatively controlled by developmentally regulated changes in the polyadenylation of maternal mRNAs.

Regulation of translation by specific protein/mRNA interactions

This review will focus on cases of specific translational control by protein/RNA interactions in the 5'- or 3'-UTR of eukaryote mRNA where either the cis-acting RNA determinant or the trans-acting protein (or preferably both) have been identified with fair certainty. Examples of messages that are regulated by 5' motifs, which are proposed to occlude ribosome binding when bound by their specific factors, include ferritin and ribosomal protein mRNAs and the autoregulated thymidylate synthase and poly(A)-binding mRNAs. However, it has become increasingly evident recently that 3' UTR determinants and their specific binding proteins also regulate translation efficiency either directly, or indirectly via an influence on the polyadenylation status of the mRNA. It is still unclear how events at the 3' end of mRNA influence ribosome binding. Most, if not all, of the mRNAs known to be regulated by 3' UTR motifs are subject to regulation during early development or during differentiation such as several spermatocyte and oocyte mRNAs and erythroid lipoxygenase mRNA. To date, in all cases where translation is controlled directly by specific protein/mRNA interactions, the protein seems to act as a negative regulator, a translational repressor, whose binding to the specific site on the mRNA results in inhibition of initiation. The only cases of translational activation known so far concern internal initiation of translation of picornaviral RNAs, but this topic is beyond the scope of this review.

Poly(A) and translation: development control

The stage-specific translational control of maternal mRNAs is determined by their differential polyadenylation and deadenylation. In the past year, a growing number of cis-acting elements that both positively and negatively regulate polyadenylation and deadenylation have been delineated. Considerable progress has been made on the biochemical characterization and regulation of trans-acting polyadenylation and deadenylation factors. This review summarizes these advances and their relevance to the roles of polyadenylation and deadenylation in translational control.