CPEB3 and CPEB4 in neurons: analysis of RNA-binding specificity and translational control of AMPA receptor GluR2 mRNA

CPEB is a sequence-specific RNA-binding protein that promotes polyadenylation-induced translation in oocytes and neurons. Vertebrates contain three additional genes that encode CPEB-like proteins, all of which are expressed in the brain. Here, we use SELEX, RNA structure probing, and RNA footprinting to show that CPEB and the CPEB-like proteins interact with different RNA sequences and thus constitute different classes of RNA-binding proteins. In transfected neurons, CPEB3 represses the translation of a reporter RNA in tethered function assays; in response to NMDA receptor activation, translation is stimulated. In contrast to CPEB, CPEB3-mediated translation is unlikely to involve cytoplasmic polyadenylation, as it requires neither the cis-acting AAUAAA nor the trans-acting cleavage and polyadenylation specificity factor, both of which are necessary for CPEB-induced polyadenylation. One target of CPEB3-mediated translation is GluR2 mRNA; not only does CPEB3 bind this RNA in vitro and in vivo, but an RNAi knockdown of CPEB3 in neurons results in elevated levels of GluR2 protein. These results indicate that CPEB3 is a sequence-specific translational regulatory protein.     

Activity-dependent polyadenylation in neurons

Activity-dependent changes in protein synthesis modify synaptic efficacy. One mechanism that regulates mRNA translation in the synapto-dendritic compartment is cytoplasmic polyadenylation, a process controlled by CPEB, the cytoplasmic polyadenylation element (CPE)-specific RNA binding protein. In neurons, very few mRNAs are known CPEB substrates, and none appear to be responsible for the effects on plasticity that are found in the CPEB knockout mouse. These results suggest that the translation of other mRNAs is regulated by CPEB. To identify them, we have developed a functional assay based on the polyadenylation of brain-derived mRNAs injected into Xenopus oocytes, a surrogate system that carries out this 3' end processing event in an efficient manner. The polyadenylated RNAs were isolated by binding to and thermal elution from poly(U) agarose and identified by microarray analysis. Selected sequences that were positive for polyadenylation were cloned and retested for polyadenylation by injection into oocytes. These sequences were then examined for activity-dependent polyadenylation in cultured hippocampal neurons. Finally, the levels of two proteins encoded by polyadenylated mRNAs were examined in glutamate-stimulated synaptoneurosomes. These studies show that many mRNAs undergo activity-dependent polyadenylation in neurons and that this process coincides with increased translation in the synapto-dendritic compartment.     

CPEB phosphorylation and cytoplasmic polyadenylation are catalyzed by the kinase IAK1/Eg2 in maturing mouse oocytes

In both vertebrates and invertebrates, the expression of several maternal mRNAs is regulated by cytoplasmic polyadenylation. In Xenopus oocytes, where most of the biochemical details of this process have been examined, polyadenylation is controlled by CPEB, a sequence-specific RNA binding protein. The activity of CPEB, which is to recruit cleavage and polyadenylation specificity factor (CPSF) and poly(A) polymerase (PAP) into an active cytoplasmic polyadenylation complex, is controlled by Eg2-catalyzed phosphorylation. Soon after CPEB phosphorylation and resulting polyadenylation take place, the interaction between maskin, a CPEB-associated factor, and eIF4E, the cap-binding protein, is destroyed, which results in the recruitment of mRNA into polysomes. Polyadenylation also occurs in maturing mouse oocytes, although the biochemical events that govern the reaction in these cells are not known. In this study, we have examined the phosphorylation of CPEB and have assessed the necessity of this protein for polyadenylation in maturing mouse oocytes. Immunohistochemistry has revealed that all the factors that control polyadenylation and translation in Xenopus oocytes (CPEB, CPSF, PAP, maskin, and IAK1, the murine homologue of Eg2) are also present in the cytoplasm of mouse oocytes. After the induction of maturation, a kinase is activated that phosphorylates CPEB on a critical regulatory residue, an event that is essential for CPEB activity. A peptide that competitively inhibits the activity of IAK1/Eg2 blocks the progression of meiosis in injected oocytes. Finally, a CPEB protein that acts as a dominant negative mutation because it cannot be phosphorylated by IAK1/Eg2, prevents cytoplasmic polyadenylation. These data indicate that cytoplasmic polyadenylation in mouse oocytes is mediated by IAK1/Eg2-catalyzed phosphorylation of CPEB.     

CPEB4 is a cell survival protein retained in the nucleus upon ischemia or endoplasmic reticulum calcium depletion

The RNA binding protein CPEB (cytoplasmic polyadenylation element binding) regulates cytoplasmic polyadenylation and translation in germ cells and the brain. In neurons, CPEB is detected at postsynaptic sites, as well as in the cell body. The related CPEB3 protein also regulates translation in neurons, albeit probably not through polyadenylation; it, as well as CPEB4, is present in dendrites and the cell body. Here, we show that treatment of neurons with ionotropic glutamate receptor agonists causes CPEB4 to accumulate in the nucleus. All CPEB proteins are nucleus-cytoplasm shuttling proteins that are retained in the nucleus in response to calcium-mediated signaling and alpha-calcium/calmodulin-dependent kinase protein II (CaMKII) activity. CPEB2, -3, and -4 have conserved nuclear export signals that are not present in CPEB. CPEB4 is necessary for cell survival and becomes nuclear in response to focal ischemia in vivo and when cultured neurons are deprived of oxygen and glucose. Further analysis indicates that nuclear accumulation of CPEB4 is controlled by the depletion of calcium from the ER, specifically, through the inositol-1,4,5-triphosphate (IP3) receptor, indicating a communication between these organelles in redistributing proteins between subcellular compartments.     

Transient CPEB dimerization and translational control

During oocyte development, the cytoplasmic polyadenylation element-binding protein (CPEB) nucleates a set of factors on mRNA that controls cytoplasmic polyadenylation and translation. The regulation of polyadenylation is mediated in part through serial phosphorylations of CPEB, which control both the dynamic integrity of the cytoplasmic polyadenylation apparatus and CPEB stability, events necessary for meiotic progression. Because the precise stoichiometry between CPEB and CPE-containing RNA is responsible for the temporal order of mRNA polyadenylation during meiosis, we hypothesized that, if CPEB production exceeded the amount required to bind mRNA, the excess would be sequestered in an inactive form. One attractive possibility for the sequestration is protein dimerization. We demonstrate that not only does CPEB form a dimer, but dimerization requires its RNA-binding domains. Dimer formation prevents CPEB from being UV cross-linked to RNA, which establishes a second pool of CPEB that is inert for polyadenylation and translational control. During oocyte maturation, the dimers are degraded much more rapidly than the CPEB monomers, due to their greater affinity for polo-like kinase 1 (plx1) and the ubiquitin E3 ligase β-TrCP. Because dimeric CPEB also binds cytoplasmic polyadenylation factors with greater affinity than monomeric CPEB, it may act as a hub or reservoir for the polyadenylation machinery. We propose that the balance between CPEB and its target mRNAs is maintained by CPEB dimerization, which inactivates spare proteins and prevents them from inducing polyadenylation of RNAs with low affinity binding sites. In addition, the dimers might serve as molecular hubs that release polyadenylation factors for translational activation upon CPEB dimer destruction.     

CPEB controls the cytoplasmic polyadenylation of cyclin, Cdk2 and c-mos mRNAs and is necessary for oocyte maturation in Xenopus.

Cytoplasmic polyadenylation is a key mechanism controlling maternal mRNA translation in early development. In most cases, mRNAs that undergo poly(A) elongation are translationally activated; those that undergo poly(A) shortening are deactivated. Poly(A) elongation is regulated by two cis-acting sequences in the 3'-untranslated region (UTR) of responding mRNAs, the polyadenylation hexanucleotide AAUAAA and the U-rich cytoplasmic polyadenylation element (CPE). Previously, we cloned and characterized the Xenopus oocyte CPE binding protein (CPEB), showing that it was essential for the cytoplasmic polyadenylation of B4 RNA. Here, we show that CPEB also binds the CPEs of G10, c-mos, cdk2, cyclins A1, B1 and B2 mRNAs. We find that CPEB is necessary for polyadenylation of these RNAs in egg extracts, suggesting that this protein is required for polyadenylation of most RNAs during oocyte maturation. Our data demonstrate that the complex timing and extent of polyadenylation are partially controlled by CPEB binding to multiple target sites in the 3' UTRs of responsive mRNAs. Finally, injection of CPEB antibody into oocytes not only inhibits polyadenylation in vivo, but also blocks progesterone-induced maturation. This is due to inhibition of polyadenylation and translation of c-mos mRNA, suggesting that CPEB is critical for early development.     

An Unusual Two-Step Control of CPEB Destruction by Pin1

Cytoplasmic polyadenylation is a conserved mechanism that controls mRNA translation and stability. A key protein that promotes polyadenylation-induced translation of mRNAs in maturing Xenopus oocytes is the cytoplasmic polyadenylation element binding protein (CPEB). During this meiotic transition, CPEB is subjected to phosphorylation-dependent ubiquitination and partial destruction, which is necessary for successive waves of polyadenylation of distinct mRNAs. Here we identify the peptidyl-prolyl cis-trans isomerase Pin1 as an important factor mediating CPEB destruction. Pin1 interacts with CPEB in an unusual manner in which it occurs prior to CPEB phosphorylation and prior to Pin1 activation by serine 71 dephosphorylation. Upon induction of maturation, CPEB becomes phosphorylated, which occurs simultaneously with Pin1 dephosphorylation. At this time, the CPEB-Pin1 interaction requires cdk1-catalyzed CPEB phosphorylation on S/T-P motifs. Subsequent CPEB ubiquitination and destruction are mediated by a conformational change induced by Pin1 isomerization of CPEB. Similar to M phase progression in maturing Xenopus oocytes, the destruction of CPEB during the mammalian cell cycle requires Pin1 as well. These data identify Pin1 as a new and essential factor regulating CPEB degradation.     

Aurora Kinase A Is Not Involved in CPEB1 Phosphorylation and cyclin B1 mRNA Polyadenylation during Meiotic Maturation of Porcine Oocytes

Regulation of mRNA translation by cytoplasmic polyadenylation is known to be important for oocyte maturation and further development. This process is generally controlled by phosphorylation of cytoplasmic polyadenylation element binding protein 1 (CPEB1). The aim of this study is to determine the role of Aurora kinase A in CPEB1 phosphorylation and the consequent CPEB1-dependent polyadenylation of maternal mRNAs during mammalian oocyte meiosis. For this purpose, we specifically inhibited Aurora kinase A with MLN8237 during meiotic maturation of porcine oocytes. Using poly(A)-test PCR method, we monitored the effect of Aurora kinase A inhibition on poly(A)-tail extension of long and short cyclin B1 encoding mRNAs as markers of CPEB1-dependent cytoplasmic polyadenylation. Our results show that inhibition of Aurora kinase A activity impairs neither cyclin B1 mRNA polyadenylation nor its translation and that Aurora kinase A is unlikely to be involved in CPEB1 activating phosphorylation.     

Poly(A) polymerase and the regulation of cytoplasmic polyadenylation

Translational activation in oocytes and embryos is often regulated via increases in poly(A) length. Cleavage and polyadenylation specificity factor (CPSF), cytoplasmic polyadenylation element binding protein (CPEB), and poly(A) polymerase (PAP) have each been implicated in cytoplasmic polyadenylation in Xenopus laevis oocytes. Cytoplasmic polyadenylation activity first appears in vertebrate oocytes during meiotic maturation. Data presented here shows that complexes containing both CPSF and CPEB are present in extracts of X. laevis oocytes prepared before or after meiotic maturation. Assessment of a variety of RNA sequences as polyadenylation substrates indicates that the sequence specificity of polyadenylation in egg extracts is comparable to that observed with highly purified mammalian CPSF and recombinant PAP. The two in vitro systems exhibit a sequence specificity that is similar, but not identical, to that observed in vivo, as assessed by injection of the same RNAs into the oocyte. These findings imply that CPSFs intrinsic RNA sequence preferences are sufficient to account for the specificity of cytoplasmic polyadenylation of some mRNAs. We discuss the hypothesis that CPSF is required for all polyadenylation reactions, but that the polyadenylation of some mRNAs may require additional factors such as CPEB. To test the consequences of PAP binding to mRNAs in vivo, PAP was tethered to a reporter mRNA in resting oocytes using MS2 coat protein. Tethered PAP catalyzed polyadenylation and stimulated translation approximately 40-fold; stimulation was exclusively cis-acting, but was independent of a CPE and AAUAAA. Both polyadenylation and translational stimulation required PAPs catalytic core, but did not require the putative CPSF interaction domain of PAP. These results demonstrate that premature recruitment of PAP can cause precocious polyadenylation and translational stimulation in the resting oocyte, and can be interpreted to suggest that the role of other factors is to deliver PAP to the mRNA.     

Subdividing cell populations in the developing limbs of Drosophila: do wing veins and leg segments define units of growth control?

In maturing mouse oocytes, protein synthesis is required for meiotic maturation subsequent to germinal vesicle breakdown (GVBD). While the number of different proteins that must be synthesized for this progression to occur is unknown, at least one of them appears to be cyclin B1, the regulatory subunit of M-phase-promoting factor. Here, we investigate the mechanism of cyclin B1 mRNA translational control during mouse oocyte maturation. We show that the U-rich cytoplasmic polyadenylation element (CPE), a cis element in the 3' UTR of cyclin B1 mRNA, mediates translational repression in GV-stage oocytes. The CPE is also necessary for cytoplasmic polyadenylation, which stimulates translation during oocyte maturation. The injection of oocytes with a cyclin B1 antisense RNA, which probably precludes the binding of a factor to the CPE, delays cytoplasmic polyadenylation as well as the transition from GVBD to metaphase II. CPEB, which interacts with the cyclin B1 CPE and is present throughout meiotic maturation, becomes phosphorylated at metaphase I. These data indicate that CPEB is involved in both the repression and the stimulation of cyclin B1 mRNA and suggest that the phosphorylation of this protein could be involved in regulating its activity.     

A Role of CPEB1 in the Modulation of Proliferation and Neuronal Maturation of Rat Primary Neural Progenitor Cells

Cytoplasmic polyadenylation binding protein 1 (CPEB1) is a RNA binding protein, which regulates translation of target mRNAs by regulating polyadenylation status. CPEB1 plays important roles in the regulation of germline cell development by modulating cell cycle progression through the polyadenylation of target mRNAs such as cyclin B1. Similar mechanism is reported in proliferating astrocytes by us, although CPEB1 is involved in the transport of target mRNAs as well as local translation at dendritic spines. In this study, we found the expression of CPEB1 in cultured rat primary neural progenitor cells (NPCs). EGF stimulation of cultured NPCs induced rapid phosphorylation of CPEB1, a hallmark of CPEB1-dependent translational control along with cyclin B1 polyadenylation and translation. EGF-induced activation of ERK1/2 and Aurora A kinase was responsible for CPEB1 phosphorylation. Pharmacological inhibition studies suggested that ERK1/2 is involved in the activation of Aurora A kinase and regulation of CPEB1 phosphorylation in cultured NPCs. Long-term incubation in EGF resulted in the down-regulation of CPEB1 expression, which further increased expression of cyclin B1 and cell cycle progression. When we down-regulated the expression of CPEB1 in NPCs by siRNA transfection, the proliferation of NPCs was increased. Increased NPCs proliferation by down-regulation of CPEB1 resulted in eventual up-regulation of neuronal differentiation with increase in both pre- and post-synaptic proteins. The results from the present study may suggest the importance of translational control in the regulation of neuronal development, an emerging concept in many neurodevelopmental and psychiatric disorders such as autism spectrum disorder.     

Differential mRNA translation and meiotic progression require Cdc2-mediated CPEB destruction

Translational activation of several dormant mRNAs in vertebrate oocytes is mediated by cytoplasmic polyadenylation, a process controlled by the cytoplasmic polyadenylation element (CPE) and its binding protein CPEB. The translation of CPE-containing mRNAs does not occur en masse at any one time, but instead is temporally regulated. We show here that in Xenopus, partial destruction of CPEB controls the temporal translation of CPE-containing mRNAs. While some mRNAs, such as the one encoding Mos, are polyadenylated at prophase I, the polyadenylation of cyclin B1 mRNA requires the partial destruction of CPEB that occurs at metaphase I. CPEB destruction is mediated by a PEST box and Cdc2-catalyzed phosphorylation, and is essential for meiotic progression to metaphase II. CPEB destruction is also necessary for mitosis in the early embryo. These data indicate that a change in the CPEB:CPE ratio is necessary to activate mRNAs at metaphase I and drive the cells' entry into metaphase II.     

Meiosis requires a translational positive loop where CPEB1 ensues its replacement by CPEB4

Meiotic progression is driven by the sequential translational activation of maternal messenger RNAs stored in the cytoplasm. This activation is mainly induced by the cytoplasmic elongation of their poly(A) tails, which is mediated by the cytoplasmic polyadenylation element (CPE) present in their 3′ untranslated regions. Although polyadenylation in prophase I and metaphase I is mediated by the CPE‐binding protein 1 (CPEB1), this protein is degraded during the first meiotic division. Thus, raising the question of how the cytoplasmic polyadenylation required for the second meiotic division is achieved. In this work, we show that CPEB1 generates a positive loop by activating the translation of CPEB4 mRNA, which, in turn, replaces CPEB1 and drives the transition from metaphase I to metaphase II. We further show that CPEB1 and CPEB4 are differentially regulated by phase‐specific kinases, generating the need of two sequential CPEB activities to sustain cytoplasmic polyadenylation during all the meiotic phases. Altogether, this work defines a new element in the translational circuit that support an autonomous transition between the two meiotic divisions in the absence of DNA replication.     

Novel RNA recognition motif domain in the cytoplasmic polyadenylation element binding protein 3

The family of cytoplasmic polyadenylation element binding proteins CPEB1, CPEB2, CPEB3, and CPEB4 binds to the 3′‐untranslated region (3′‐UTR) of mRNA, and plays significant roles in mRNA metabolism and translation regulation. They have a common domain organization, involving two consecutive RNA recognition motif (RRM) domains followed by a zinc finger domain in the C‐terminal region. We solved the solution structure of the first RRM domain (RRM1) of human CPEB3, which revealed that CPEB3 RRM1 exhibits structural features distinct from those of the canonical RRM domain. Our structural data provide important information about the RNA binding ability of CPEB3 RRM1. Proteins 2014; 82:2879–2886. © 2014 Wiley Periodicals, Inc.     

Specificity of RNA Binding by CPEB: Requirement for RNA Recognition Motifs and a Novel Zinc Finger

CPEB is an RNA binding protein that interacts with the maturation-type cytoplasmic polyadenylation element (CPE) (consensus UUUUUAU) to promote polyadenylation and translational activation of maternal mRNAs in Xenopus laevis. CPEB, which is conserved from mammals to invertebrates, is composed of three regions: an amino-terminal portion with no obvious functional motif, two RNA recognition motifs (RRMs), and a cysteine-histidine region that is reminiscent of a zinc finger. In this study, we investigated the physical properties of CPEB required for RNA binding. CPEB can interact with RNA as a monomer, and phosphorylation, which modifies the protein during oocyte maturation, has little effect on RNA binding. Deletion mutations of CPEB have been overexpressed in Escherichia coli and used in a series of RNA gel shift experiments. Although a full-length and a truncated CPEB that lacks 139 amino-terminal amino acids bind CPE-containing RNA avidly, proteins that have had either RRM deleted bind RNA much less efficiently. CPEB that has had the cysteine-histidine region deleted has no detectable capacity to bind RNA. Single alanine substitutions of specific cysteine or histidine residues within this region also abolish RNA binding, pointing to the importance of this highly conserved domain of the protein. Chelation of metal ions by 1,10-phenanthroline inhibits the ability of CPEB to bind RNA; however, RNA binding is restored if the reaction is supplemented with zinc. CPEB also binds other metals such as cobalt and cadmium, but these destroy RNA binding. These data indicate that the RRMs and a zinc finger region of CPEB are essential for RNA binding.     

A reaction–diffusion mechanism influences cell lineage progression as a basis for formation, regeneration, and stability of intestinal crypts

Background

Translation efficiency of certain mRNAs can be regulated through a cytoplasmic polyadenylation process at the pre-initiation phase. A translational regulator controls the polyadenylation process and this regulation depends on its posttranslational modifications e.g., phosphorylation. The cytoplasmic polyadenylation binding protein (CPEB1) is one such translational regulator, which regulates the translation of some mRNAs by binding to the cytoplasmic polyadenylation element (CPE). The cytoplasmic polyadenylation process can be turned on or off by the phosphorylation or dephosphorylation state of CPEB1. A specific example could be the regulation of Calcium/Calmodulin-dependent protein kinase II (??CaMKII) translation through the phosphorylation/dephosphorylation cycle of CPEB1.

Result

Here, we show that CPEB1 mediated polyadenylation of ??CaMKII mRNA can result in a bistable switching mechanism. The switch for regulating the polyadenylation is based on a two state model of ??CaMKII and its interaction with CPEB1. Based on elementary biochemical kinetics a high dimensional system of non-linear ordinary differential equations can describe the dynamic characteristics of the polyadenylation loop. Here, we simplified this high-dimensional system into approximate lower dimension system that can provide the understanding of dynamics and fixed points of original system. These simplified equations can be used to develop analytical bifurcation diagrams without the use of complex numerical tracking algorithm, and can further give us intuition about the parameter dependence of bistability in this system.

Conclusion

This study provides a systematic method to simplify, approximate and analyze a translation/activation based positive feedback loop. This work shows how to extract low dimensional systems that can be used to obtain analytical solutions for the fixed points of the system and to describe the dynamics of the system. The methods used here have general applicability to the formulation and analysis of many molecular networks.     

Phosphorylation of CPEB by Eg2 mediates the recruitment of CPSF into an active cytoplasmic polyadenylation complex

The release of Xenopus oocytes from prophase I arrest is largely driven by the cytoplasmic polyadenylation-induced translation of dormant maternal mRNAs. Two cis elements, the CPE and the hexanucleotide AAUAAA, and their respective binding factors, CPEB and a cytoplasmic form of CPSF, control polyadenylation. The most proximal stimulus for polyadenylation is Eg2-catalyzed phosphorylation of CPEB serine 174. Here, we show that this phosphorylation event stimulates an interaction between CPEB and CPSF. This interaction is direct, does not require RNA tethering, and occurs through the 160 kDa subunit of CPSF. Eg2-stimulated and CPE-dependent polyadenylation is reconstituted in vitro using purified components. These results demonstrate that the molecular function of Eg2-phosphorylated CPEB is to recruit CPSF into an active cytoplasmic polyadenylation complex.     

The 36-kilodalton embryonic-type cytoplasmic polyadenylation element-binding protein in Xenopus laevis is ElrA, a member of the ELAV family of RNA-binding proteins.

The translational activation of several maternal mRNAs in Xenopus laevis is dependent on cytoplasmic poly(A) elongation. Messages harboring the UUUUUAU-type cytoplasmic polyadenylation element (CPE) in their 3' untranslated regions (UTRs) undergo polyadenylation and translation during oocyte maturation. This CPE is bound by the protein CPEB, which is essential for polyadenylation. mRNAs that have the poly(U)12-27 embryonic-type CPE (eCPE) in their 3' UTRs undergo polyadenylation and translation during the early cleavage and blastula stages. A 36-kDa eCPE-binding protein in oocytes and embryos has been identified by UV cross-linking. We now report that this 36-kDa protein is ElrA, a member of the ELAV family of RNA-binding proteins. The proteins are identical in size, antibody directed against ElrA immunoprecipitates the 36-kDa protein, and the two proteins have the same RNA binding specificity in vitro. C12 and activin receptor mRNAs, both of which contain eCPEs, are detected in immunoprecipitated ElrA-mRNP complexes from eggs and embryos. In addition, this in vivo interaction requires the eCPE. Although a number of experiments failed to define a role for ElrA in cytoplasmic polyadenylation, the expression of a dominant negative ElrA protein in embryos results in an exogastrulation phenotype. The possible functions of ElrA in gastrulation are discussed.     

CPEB interacts with an ovary-specific eIF4E and 4E-T in early Xenopus oocytes

CPEB (cytoplasmic polyadenylation element-binding protein) is an important regulator of translation in oocytes and neurons. Although previous studies of CPEB in late Xenopus oocytes involve the eIF4E-binding protein maskin as the key factor for the repression of maternal mRNA, a second mechanism must exist, since maskin is absent earlier in oogenesis. Using co-immunoprecipitation and gel filtration assays, we show that CPEB specifically interacts, via protein/protein interactions, with the RNA helicase Xp54, the RNA-binding proteins P100(Pat1) and RAP55, the eIF4E-binding protein 4E-T, and an eIF4E protein. Remarkably, these CPEB complex proteins have been characterized, in one or more organism, as P-body, maternal, or neuronal granule components. We do not detect interactions with eIF4E1a, the canonical cap-binding factor, eIF4G, or eIF4A or with proteins expressed late in oogenesis, including maskin, PARN, and 4E-BP1. The eIF4E protein was identified as eIF4E1b, a close homolog of eIF4E1a, whose expression is restricted to oocytes and early embryos. Although eIF4E1b possesses all residues required for cap and eIF4G binding, it binds m(7)GTP weakly, and in pull-down assays, rather than binding eIF4G, it binds 4E-T, in a manner independent of the consensus eIF4E-binding site, YSKEELL. Wild type and Y-A mutant 4E-T (which binds eIF4E1b but not eIF4E1a), when tethered to a reporter mRNA, represses its translation in a cap-dependent manner, and injection of eIF4E1b antibody accelerates meiotic maturation. Altogether, our data suggest that CPEB, partnered with several highly conserved RNA-binding partners, inhibits protein synthesis in oocytes using a novel pairing of 4E-T and eIF4E1b.