The mechanism of protein synthesis activation after fertilization of sea urchin eggs

A molecular network of conserved factors keeps ribosomes dormant in the egg

Ribosomes are produced in large quantities during oogenesis and are stored in the egg. However, the egg and early embryo are translationally repressed1-4. Here, using mass spectrometry and cryo-electron microscopy analyses of ribosomes isolated from zebrafish (Danio rerio) and Xenopus laevis eggs and embryos, we provide molecular evidence that ribosomes transition from a dormant state to an active state during the first hours of embryogenesis. Dormant ribosomes are associated with four conserved factors that form two modules, consisting of Habp4-eEF2 and death associated protein 1b (Dap1b) or Dap in complex with eIF5a. Both modules occupy functionally important sites and act together to stabilize ribosomes and repress translation. Dap1b (also known as Dapl1 in mammals) is a newly discovered translational inhibitor that stably inserts into the polypeptide exit tunnel. Addition of recombinant zebrafish Dap1b protein is sufficient to block translation and reconstitute the dormant egg ribosome state in a mammalian translation extract in vitro. Thus, a developmentally programmed, conserved ribosome state has a key role in ribosome storage and translational repression in the egg.

Identification and characterization of PlAlix, the Alix homologue from the Mediterranean sea urchin Paracentrotus lividus

The sea urchin provides a relatively simple and tractable system for analyzing the early stages of embryo development. Here, we use the sea urchin species, Paracentrotus lividus, to investigate the role of Alix in key stages of embryogenesis, namely the egg fertilization and the first cleavage division. Alix is a multifunctional protein involved in different cellular processes including endocytic membrane trafficking, filamentous (F)-actin remodeling, and cytokinesis. Alix homologues have been identified in different metazoans; in these organisms, Alix is involved in oogenesis and in determination/differentiation events during embryo development. Herein, we describe the identification of the sea urchin homologue of Alix, PlAlix. The deduced amino acid sequence shows that Alix is highly conserved in sea urchins. Accordingly, we detect the PlAlix protein cross-reacting with monoclonal Alix antibodies in extracts from P. lividus, at different developmental stages. Focusing on the role of PlAlix during early embryogenesis we found that PlAlix is a maternal protein that is expressed at increasingly higher levels from fertilization to the 2-cell stage embryo. In sea urchin eggs, PlAlix localizes throughout the cytoplasm with a punctuated pattern and, soon after fertilization, accumulates in larger puncta in the cytosol, and in microvilli-like protrusions. Together our data show that PlAlix is structurally conserved from sea urchin to mammals and may open new lines of inquiry into the role of Alix during the early stages of embryo development.

Isolation and distribution of elongation factor 2 in eggs and embryos of sea urchins

The subcellular distribution of elongation factor 2 (EF-2) in eggs and early embryos of the sea urchin, Strongylocentrotus purpuratus, was studied by employing the diphtheria toxin dependent ADP-ribosylation of EF-2. When egg and embryo homogenates were fractionated by sedimentation, EF-2 was found associated with a low-speed pellet containing yolk, nuclei, and mitochondria. It also sedimented at 80 S and 5 S. No significant amounts of EF-2 were found on polyribosomes. The 5S form of EF-2 probably represents a monomeric unit of the factor as EF-2 had a molecular weight of 95 000 on sodium dodecyl sulfate-polyacrylamide gels. EF-2 could only be isolated intact if soybean trypsin inhibitor or EGTA was present. The total amount of EF-2 was similar in eggs and embryos. However, the distributions of the factor between the various fractions were substantially different for eggs and embryos. Also, a marked difference in the physical association of EF-2 with material in the low-speed pellet occurred after fertilization. Specifically, in eggs, 23% of the EF-2 was associated with the low-speed pellet; in cleavage-stage embryos, only 11% of the EF-2 was associated with the pellet. In eggs, 65% of the EF-2 sedimented as 80 S; by the 16-cell stage, this amount decreased to 44%. Concomitantly, the amount of EF-2 in the 5S fraction increased from about 8% in eggs to 44% in the 16-cell embryos. In addition, Triton X-100 was required for the extraction of EF-2 from the low-speed pellet of eggs, but not of embryos.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of a factor from dystrophic mouse muscle inhibitory towards protein synthesis

A substance inhibitory to protein synthesis was purified from mouse skeletal muscle by gel filtration and ion-exchange chromatography, as well as by centrifugation on sucrose gradients. The molecular weight of the inhibitor, determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, was 71000. The inhibitory activity was insensitive to ribonuclease A, deoxyribonuclease I and phospholipase C. It was sensitive to Pronase treatment but insensitive to heat-treatment and trypsin degradation. The present results, taken together with previous studies, indicate that the site of action of the inhibitor is not on the initiation phase of protein synthesis but rather at a step after the binding of aminoacyl-tRNA to ribosomes. The increased inhibitor activity found in dystrophic muscle is discussed.

Regulation of protein synthesis at the translational level in neuroblastoma cells

Protein synthesis in reuroblastoma cells has been studied in a cell-free system. The activity of lysates from cells grown insuspension and monolayer has been compared. A higher level of activity has been found in monolayer cells. The activity of some components of the lysate that are involved in protein synthesis has been analyzed. The data suggest that the controlling step of protein synthesis in this system might be in the initiation process. The correlation between activation of protein synthesis and neurite outgrowth in monolayer cultures is discussed.

Preformed mRNA and the programming of early embryo development

A significant feature of the early development of fertilized echinoderm and amphibian eggs and germinating seed embryos is the utilization of genetic information that has been previously transcribed during oogenesis and seed ripening. When RNA synthesis is suppressed in the early developing embryos by actinomycin D, cordycepin, or alpha-amanitin, there is no effect on the translation of the "preformed mRNA", only a limited number have been thus far identified; microtubule and histone proteins in the fertilized sea urchin egg and carboxypeptidase and isocritric lyase in germinating cottonseed. Data obtained on the protein synthetic pattern at different times after the onset of development suggest that preformed mRNAs are made available to the translational system in a gradual process, thereby providing a molecular basis for the regulation of development. The possibility is considered that polyadenylation of mRNA, a reaction known to occur early after sea urchin fertilization, is responsible for regulating the release of preformed mRNA. It is shown that this reaction (polyadenylation) can be completely suppressed with little effect on the function of preformed mRNA. Finally, it is suggested, at least for the seed embryo system, that the formation of ATP may be a prerequisite for the activation of protein synthesis.

Increased efficiency of exogenous messenger RNA translation in a Krebs ascites cell lysate

Addition of a 0.5 M KCl wash fraction from rabbit reticulocyte ribosomes causes a 3- to 10-fold increase in the extent of translation of natural mRNAs by Krebs-cell lysates. In the presence of the wash fraction, 1 pmol of rabbit or mouse 10S RNA directs the incorporation of 80 pmol of leucine into rabbit globin. The addition of human 10S RNA results in the synthesis of equal amounts of human alpha and beta chains, identified by column chromatography. The stimulation by the wash fraction is almost completely dependent on added mammalian tRNA. In contrast to the wash fraction from rabbit reticulocytes, the wash fraction isolated from Krebs-cell ribosomes is inhibitory to both endogenous and exogenous mRNA translation. The stimulation by the wash fraction from rabbit ribosomes is not specific for globin mRNAs, but also increases endogenous, phage Qbeta, and viral RNA-directed protein synthesis.