Translationally mediated changes in patterns of protein synthesis during maturation of starfish oocytes

De novo assembly of a transcriptome from the eggs and early embryos of Astropecten aranciacus

Starfish have been instrumental in many fields of biological and ecological research. Oocytes of Astropecten aranciacus, a common species native to the Mediterranean Sea and the East Atlantic, have long been used as an experimental model to study meiotic maturation, fertilization, intracellular Ca²⁺ signaling, and cell cycle controls. However, investigation of the underlying molecular mechanisms has often been hampered by the overall lack of DNA or protein sequences for the species. In this study, we have assembled a transcriptome for this species from the oocytes, eggs, zygotes, and early embryos, which are known to have the highest RNA sequence complexity. Annotation of the transcriptome identified over 32,000 transcripts including the ones that encode 13 distinct cyclins and as many cyclin-dependent kinases (CDK), as well as the expected components of intracellular Ca²⁺ signaling toolkit. Although the mRNAs of cyclin and CDK families did not undergo significant abundance changes through the stages from oocyte to early embryo, as judged by real-time PCR, the transcript encoding Mos, a negative regulator of mitotic cell cycle, was drastically reduced during the period of rapid cleavages. Molecular phylogenetic analysis using the homologous amino acid sequences of cytochrome oxidase subunit I from A. aranciacus and 30 other starfish species indicated that Paxillosida, to which A. aranciacus belongs, is not likely to be the most basal order in Asteroidea. Taken together, the first transcriptome we assembled in this species is expected to enable us to perform comparative studies and to design gene-specific molecular tools with which to tackle long-standing biological questions.

Nuclear envelope breakdown may deliver an inhibitor of protein phosphatase 1 which triggers cyclin B translation in starfish oocytes

In vertebrates, enhanced translation of mRNAs in oocytes and early embryos entering M-phase is thought to occur through polyadenylation, involving binding, hyperphosphorylation and proteolytic degradation of Aurora-activated CPEB. In starfish, an unknown component of the oocyte nucleus is required for cyclin B synthesis following the release of G2/prophase block by hormonal stimulation. We have found that CPEB cannot be hyperphosphorylated following hormonal stimulation in starfish oocytes from which the nucleus has been removed. Activation of Aurora kinase, known to interact with protein phosphatase 1 and its specific inhibitor Inh-2, is also prevented. The microinjection of Inh-2 restores Aurora activation, CPEB hyperphosphorylation and cyclin B translation in enucleated oocytes. Nevertheless, we provide evidence that CPEB is in fact hyperphosphorylated by cdc2, without apparent involvement of Aurora or MAP kinase, and that cyclin B synthesis can be stimulated without previous degradation of phosphorylated CPEB. Thus, the regulation of cyclin B synthesis necessary for progression through meiosis can be explained by an equilibrium between CPEB phosphorylation and dephosphorylation, and both aspects of this control may rely on the sole activation of Cdc2 and subsequent nuclear breakdown.

Mutation of a conserved CDK site converts a metazoan Elongation Factor 1Bbeta subunit into a replacement for yeast eEF1Balpha

Elongation factor subunit eEF1Bbeta (formerly EF-1beta in plants and EF-1delta in animals) was identified and cloned in a screen for proteins from pea that interact with a cyclin-dependent kinase (CDK). CDKs are enzymes that regulate progression through meiotic and mitotic cell cycles in eukaryotes. eEF1Bbeta and the related protein eEF1Balpha (formerly EF-1beta' in plants and EF-1beta in animals and fungi) can catalyze GTP/GDP exchange on the G-protein eEF1A (formerly EF-1alpha in plants, animals and fungi) during the elongation phase of protein synthesis in eukaryotes. Recombinant Cdc2 and its native homologues from pea extracts associated both in vitro and in vivo with eEF1Bbeta. A Cdc2-cyclin B complex phosphorylated recombinant plant eEF1Bbetas, but not eEF1Balpha. These interactions between CDK and eEF1Bbeta prompted investigations into the in vivo consequences of this relationship. Expression of cDNAs encoding rice or pea eEF1Bbeta subunits failed to complement a Saccharomyces cerevisiae mutant deleted for the eEF1Balpha gene, as was previously observed for the human eEF1Bbeta. However, replacement of Thr91, the sole consensus CDK phosphorylation site in pea eEF1Bbeta, with alanine allowed the pea protein to substitute for eEF1Balpha function in vivo. In addition, this rescued strain was severely cold sensitive, and more sensitive to translational inhibitors than wild-type yeast. Taken together, these results suggest a physiological connection between the cyclin-dependent class of kinases and a translational elongation factor in mitotic cells, and provide the first in vivo evidence that an altered form of eEF1Bbeta can serve as the guanine nucleotide exchange factor for eEF1A.

Phosphorylation of protein kinase C-related kinase PRK2 during meiotic maturation of starfish oocytes

The resumption of meiosis in the developing starfish oocyte is the result of intracellular signaling events initiated by 1-methyladenine stimulation. One of the earliest detectable kinase activities during meiotic maturation of starfish oocytes is a protein kinase C or PKC-like activity. In this study, several isoforms of protein kinase C were cloned from the oocyte; however, the most abundant PKC-like maternal transcript corresponds to protein kinase C-related kinase 2 (PRK2). PRK2 is expressed in the immature oocyte and at least until germinal vesicle breakdown. Subcellular localization of PRK2 revealed a cytoplasmic distribution in the immature oocyte, which, during meiotic maturation, remained in the cytoplasm but also localized to the disintegrating germinal vesicle. Significantly, PRK2 is phosphorylated in vivo in response to 1-methyladenine which precedes MPF activation, making PRK2 a candidate regulator of early signaling events of meiotic maturation.

A nuclear factor required for specific translation of cyclin B may control the timing of first meiotic cleavage in starfish oocytes

In most animals, the rate of cyclin B synthesis increases after nuclear envelope breakdown during the first meiotic cell cycle. We have found that cyclin B-cdc2 kinase activity drops earlier in emetine-treated than in control starfish oocytes, although the protein synthesis inhibitor does not activate the cyclin degradation pathway prematurely. Moreover, protein synthesis is required to prevent meiotic cleavage to occur prematurely, sometimes before chromosomes have segregated on the metaphase plate. In normal conditions, increased synthesis of cyclin B after germinal vesicle breakdown (GVBD) balances cyclin degradation and increases the time required for cyclin B-cdc2 kinase to drop below the level that inhibits cleavage. Taken together, these results point to cyclin B as a possible candidate that could explain the need for increased protein synthesis during meiosis I. Although direct experimental evidence was not provided in the present work, cyclin B synthesis after GVBD may be important for correct segregation of homologous chromosomes at the end of first meiotic metaphase, as shown by a variety of cytological disorders that accompany premature cleavage. Although the overall stimulation of protein synthesis because of cdc2 kinase activation is still observed in oocytes from which the germinal vesicle has been removed before hormonal stimulation, the main increase of cyclin B synthesis normally observed after germinal vesicle breakdown is suppressed. The nuclear factor required for specific translation of cyclin B after GVBD is not cyclin B mRNA, as shown by using a highly sensitive reverse transcription followed by polymerase chain reaction procedure that failed to detect any cyclin B mRNA in isolated germinal vesicles.

Cyclin synthesis: who needs it?

Studies of the G2 to M transition in amphibian oocytes, in combination with in vitro mitotic systems and yeast genetic analysis, have significantly contributed to our understanding of the mechanisms by which M-phase is regulated. Historically, oocyte maturation has provided a number of valuable initial observations, but the biochemical elucidation of cell cycle control mechanisms has proved more tractable in cell-free extracts of frog eggs which reproduce aspects of early embryogenic mitosis. Recent experiments examining the importance of protein synthesis in the maturing oocyte have highlighted some important differences between mitosis and meiosis. Additional controls found in meiosis but not embryonic mitosis, are similar to controls found in somatic cells. This suggests that understanding the differences, as well as the similarities, between meiosis in the oocyte and mitosis in the early embryo will help us to learn more about the way in which cells enter and leave mitosis.

Sea urchin maternal mRNA classes with distinct development regulation

Previous studies of newly synthesized proteins during early development in sea urchins have revealed several different patterns of synthesis that can be used to predict the existence of mRNA classes with distinct regulatory controls. We have identified clones for abundant maternal mRNAs that are actively translated during early development by screening a cDNA library prepared from polysomal poly(A)+RNA isolated from 2-cell stage (2-hour) Strongylocentrotus purpuratus embryos. Probes prepared from these cDNA clones and several previously characterized maternal mRNA cDNAs were used to compare relative levels of individual mRNAs in eggs and embryos and their translational status at various developmental stages. These abundant mRNAs can be classified into two major groups which we have termed cleavage stage-specific (CSS) and post cleavage stage (PCS) mRNAs. The relative levels of the CSS mRNAs are highest during the rapid cleavage stage and decrease dramatically at the blastula stage (12-hours). In contrast, PCS mRNAs are present at relatively low levels during the rapid cleavage stage and then increase at the blastula stage. Polysome partition profiles reveal that CSS mRNAs are translated more efficiently than PCS mRNAs in the unfertilized egg, at fertilization, and during the cleavage stages. Following the blastula stage, some CSS transcripts move out of polysomes and accumulate as untranslated RNAs, while newly transcribed PCS mRNAs are recruited into polysomes. These data suggest that the rapid cell cycles following fertilization require high levels of specific cleavage stage proteins, and the synthesis of these proteins occurs preferentially over PCS mRNAs.

Regulation of M-phase progression in Chaetopterus oocytes by protein kinase C

We have examined the presence of protein kinase C in oocytes of Chaetopterus pergamentaceus and its role in the initiation of germinal vesicle breakdown (GVBD). First, we demonstrated that the oocytes contain a phospholipid- and calcium-dependent protein kinase, protein kinase C (PKC). Since PKC is the primary intracellular receptor for phorbol esters, we tested the ability of phorbol 12,13-dibutyrate (PDBu) to induce GVBD and compared several critical events and processes involved in GVBD induced by PDBu to those induced normally (by seawater). Seawater and 100-200 nM PDBu induced chromosome condensation, spindle formation, and spindle migration over a similar time course. Both treatments induced similar alterations in the SDS-PAGE pattern of newly synthesized proteins. The synthesis of polypeptides of approximately 46 and 54 kDa increased specifically. Both treatments increased oocyte protein phosphorylation, especially of proteins of 22, 32, 46, 55, 64, and 84 kDa. Both treatments resulted in the activation of an M-phase-specific histone H1 kinase activity, which demonstrates the appearance of maturation-promoting factor. Staurosporine, a potent protein kinase C inhibitor, blocked GVBD and the activation of M-phase-specific H1 kinase, whereas HA1004, which preferentially antagonizes protein kinase A, had no effect. The results of this study demonstrate that protein kinase C can activate a wide spectrum of essential biochemical and morphological processes involved in GVBD. Further, these studies suggest that protein kinase C elicits GVBD by activating maturation-promoting factor and support the hypothesis that protein kinase C plays an essential role in oocyte maturation in this species.

Nascent protein requirement for completion of meiotic maturation and pronuclear development: examination of fertilized and A-23187-activated surf clam (Spisula solidissima) eggs

The involvement of newly synthesized proteins and calcium in meiotic processes, sperm nuclear transformations, and pronuclear development was examined in emetine-treated, fertilized, and A-23187-activated Spisula eggs by observing changes in the morphogenesis of the maternal and paternal chromatin. Emetine treatment (50 micrograms/ml) initiated 30 min before fertilization or A-23187 activation inhibited incorporation of [3H]leucine into TCA-precipitable material and blocked second polar body formation. Sperm incorporation and the initial enlargement of the sperm nucleus were unaffected; however, the dramatic enlargement and transformation of the sperm nucleus into a male pronucleus, which normally follow polar body formation, were delayed 10 to 20 min. Unlike the situation in untreated, control eggs, male pronuclear development took place while the maternally derived chromosomes remained condensed. It was not until approximately 20 min after the normal period of pronuclear development that the maternal chromosomes dispersed and formed a female pronucleus in emetine-treated, fertilized eggs. Formation of pronuclei, however, was unaffected in both emetine-treated, A-23187-activated eggs and fertilized eggs incubated with A-23187. These observations indicate that germinal vesicle breakdown, first polar body formation, and initial transformations of the sperm nucleus are independent of newly synthesized proteins. Inhibition of second polar body formation and the delay in pronuclear development brought about by emetine, as well as the appearance of silver grains over pronuclei in autoradiographs of control eggs incubated with [3H]leucine demonstrate that nascent proteins are involved with the completion of meiotic maturation and the development of male and female pronuclei. The ability of A-23187 to override the inhibitory effects of emetine on pronuclear development suggests that both nascent protein and calcium signals are involved in regulating the status of the maternal and paternal chromatin during pronuclear development.

Cyclin A, cell cycle control and oncogenesis

One of the most fundamental questions in biology is how a cell is able to regulate its division cycle. Initially it was thought that in mammalian cells control over entry into the cell cycle is exerted at a restriction point in G1; once past this point the cell would be free to undergo all the steps needed until the following division. Hence, for many years research on tumorigenesis focused on the mitogenic activation of quiescent cells by growth factors, peptide hormones and oncogene products (for reviews see [1, 2]). These studies investigated the initial steps required to induce a quiescent, nondividing cell to proliferate, and led to the identification of many growth factor receptors, of both the tyrosine kinase family and the G-protein coupled family. Receptors bearing protein tyrosine phosphatase or serine kinase catalytic domains were also identified via this route (for reviews see [3, 4, 5]). However more recent studies on the cooperation between different growth factors for mitogenesis have shown that multiple requirements exist for a cell to proceed through the entire division cycle. Indeed studies in several different organisms, pioneered by investigators working with Ascomycetes [6, 7, 8], have now clearly shown that the eukaryotic cell cycle proceeds through multiple check-points. Furthermore, it now appears that many of the regulatory elements and even pathways have been conserved throughout evolution. In this review we discuss the possible involvement of one of the transducing molecules, cyclin A, in abnormal cell proliferation.

Cell-free translation systems prepared from starfish oocytes faithfully reflect in vivo activity; mRNA and initiation factors stimulate supernatants from immature oocytes

Meiotic maturation stimulates a change in the translation of stored mRNAs: mRNAs encoding proteins needed for growth of oocytes are translated before meiotic maturation, whereas those encoding proteins required for cleavage are translated after meiotic maturation. Studies of translational regulation during meiotic maturation have been limited by the lack of translationally active cell-free supernatants. Starfish oocytes are ideal for preparing cell-free translation systems because experimental application of the hormone 1-methyladenine induces their maturation, synchronizing meiosis. We have prepared such systems from both immature and mature oocytes of starfish. Changes in protein synthesis rates and the specificity of proteins synthesized in these cell-free translation supernatants mimic those seen in vivo. Supernatants both from immature and mature oocytes have a high capacity to initiate new translation because 90% of the proteins made are newly initiated from mRNAs. Cell-free supernatants from mature oocytes have a much higher rate of initiation of translation than those from immature oocytes and use the 43S preinitiation complexes more efficiently in initiation of translation. Similarly, we have shown that mRNAs and initiation factors are rate limiting in cell-free translation systems prepared from immature oocytes. In addition, cell-free translation systems prepared from immature oocytes are only slightly, if at all, inhibitory to cell-free translation systems from mature oocytes. Thus, soluble inhibitors, if they exist, are rapidly converted by cell-free supernatants from mature oocytes. The similarities between translation in our starfish cell-free translation systems and in intact oocytes suggests that the cell-free translation systems will be useful tools for further studies of maturation events and translational control during meiosis.

A review of echinoderm oogenesis

This review of the anatomical, histological, biochemical, and molecular biological literature on echinoderm oogenesis includes the entire developmental history of oocytes; from their inception to the time they become ova. This is done from a comparative perspective, with reference to members of the five extant echinoderm classes; crinoids, holothurians, asteroids, ophiuroids, and echinoids. I describe the anatomy and fine structure of the echinoderm ovary, with emphasis on both the cellular relationships of the germ line cells to the somatic cells of the inner epithelium, and on the neuromuscular systems. I review the literature on the growth of oogonia into fully formed oocytes, including the process of vitellogenesis, presenting an ultrastructural analysis of the organelles and extracellular structures found in fully formed echinoderm oocytes. Echinoderm oocyte maturation is reviewed and a description of the ultrastructural, biochemical and molecular biological changes thought to occur during this process is presented. Finally, I discuss oocyte ovulation, the severing of cellular connections between the oocyte and its surrounding somatic epithelial cells.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.

The role of cyclin B in meiosis I

In clams, fertilization is followed by the prominent synthesis of two cyclins, A and B. During the mitotic cell cycles, the two cyclins are accumulated and then destroyed near the end of each metaphase. Newly synthesized cyclin B is complexed with a small set of other proteins, including a kinase that phosphorylates cyclin B in vitro. While both cyclins can act as general inducers of entry into M phase, the two are clearly distinguished by their amino acid sequences (70% nonidentity) and by their different modes of expression in oocytes and during meiosis. In contrast to cyclin A, which is stored solely as maternal mRNA, oocytes contain a stockpile of cyclin B protein, which is stored in large, rapidly sedimenting aggregates. Fertilization results in the release of cyclin B to a more disperse, soluble form. Since the first meiotic division in clams can proceed even when new protein synthesis is blocked, these results strongly suggest it is the fertilization-triggered unmasking of cyclin B protein that drives cells into meiosis I. We propose that the unmasking of maternal cyclin B protein allows it to interact with cdc2 protein kinase, which is also stored in oocytes, and that the formation of this cyclin B/cdc2 complex generates active M phase-promoting factor.

Cdc2 protein kinase is complexed with both cyclin A and B: evidence for proteolytic inactivation of MPF

In the clam, Spisula, two previously described proteins known as cyclin A and B display the unusual property of selective proteolytic degradation at the end of each mitosis. We show here that clam oocytes and embryos contain a cdc2 protein kinase. This protein kinase is a component of the M phase promoting factor (MPF) in frog eggs and the M phase-specific histone H1 kinase in starfish. Clam cdc2 is found in association with both cyclin A and B, probably not as a trimolecular association, but as separate cdc2/cyclin A and cdc2/cyclin B complexes. Clam cdc2 and the associated cyclins bind to p13suc1-Sepharose. The p13-bound complex, and also anti-cyclin A or B immunoprecipitates, each display cell cycle-dependent histone H1 kinase activity. We suggest that in addition to the cdc2 protein kinase, the cyclins are further components of the M phase promoting factor and that cyclin proteolysis provides the mechanism of MPF inactivation and thus exit from mitosis.

Conserved tRNA gene cluster in starfish mitochondrial DNA

Partial sequencing of mtDNA from four long-diverged species of starfish reveals the existence of a conserved cluster of 13 tRNA genes, organized in a manner similar to that of the tRNA cluster of sea urchin mtDNA, but located at a position distant from the presumed replication origin. These findings suggest that a clustered organization of tRNA genes may have been present in the ancestral mitochondrial genome, and raise the possibility that tRNAs may have catalyzed the dispersal rather than the accumulation of the genes which encode them.

Cyclin synthesis, modification and destruction during meiotic maturation of the starfish oocyte

The pattern of protein synthesis in oocytes of starfish Marthasterias glacialis changes during 1-methyladenine-induced meiotic maturation. One of the newly synthesized proteins, a major 54-kDa polypeptide, was synthesized continuously after activation but was destroyed abruptly just before appearance of the polar bodies at each meiotic division. This protein thus resembles the cyclin proteins identified in cleaving sea urchin and clam embryos. RNA extracted from oocytes before and after maturation encoded virtually identical polypeptides when translated in the reticulocyte lysate. However, there was poor correspondence between the in vitro translation products and the labelling pattern of intact cells. There was no exact in vitro counterpart to the in vivo-labelled cyclin. Instead, a major polypeptide of 52 kDa was seen which appears to be a precursor of the 54-kDa form of cyclin. The 52-kDa polypeptide was identified as cyclin by hybrid arrest of translation. Cyclin mRNA is ot translated to a significant extent before oocyte activation and is present in oocytes as nonadenylated form. It becomes polyadenylated when the oocytes mature. This behavior is also seen in the case of the mRNA for the small subunit of ribonucleotide reductase, another abundant maternal mRNA whose translation is activated at maturation.

Three translationally regulated mRNAs are stored in the cytoplasm of clam oocytes

In situ hybridization was used to examine the spatial distributions of three translationally controlled maternal RNAs in oocytes and two-cell embryos of the clam Spisula. 3H-labeled single-stranded RNA probes were generated from SP6 recombinant clones containing DNA inserts encoding portions of histone H3 (the DNA sequence which is presented here), cyclin A, and the small subunit of ribonucleotide reductase. Hybridization of these probes to oocytes, in which the mRNAs are translationally inactive, shows that these mRNAs are stored in the cytoplasm. There is no evidence for sequestration of any of the RNAs within the nucleus or any other discrete structure. Instead they appear to be evenly distributed throughout the cytoplasm.

Changes in the pattern of protein synthesis during zoospore germination in Blastocladiella emersonii

Using two-dimensional gel electrophoresis, we analyzed the pattern of proteins synthesized during Blastocladiella emersonii zoospore germination in an inorganic solution, in both the presence and absence of actinomycin D. During the transition from zoospore to round cells (the first 25 min), essentially no qualitative differences were noticeable, indicating that the earliest stages of germination are entirely preprogrammed with stored RNA. Later in germination (after 25 min), however, changes in the pattern of protein synthesis were found. Some of these proteins (a total of 6 polypeptides) correspond possibly to a selective translation of stored messages, whereas the majority of the changed proteins (22 polypeptides) corresponds to newly synthesized mRNA. Thus, multiple levels of protein synthesis regulation seem to occur during zoospore germination, involving both transcriptional and translational controls. We also analyzed the pattern of protein synthesis during germination in a nutrient medium; synthesis of specific polypeptides occurred during late germination. During early germination posttranslational control was also observed, several labeled proteins from zoospores being specifically degraded or charge modified.

Quantitative assessment of actin transcript number in eggs, embryos, and tube feet of the sea star Pisaster ochraceus

Actin coding sequence cDNA probes were used to quantitate the number of transcripts in RNA from eggs, embryos, and tube feet of the sea star Pisaster ochraceus. Transcript concentrations were measured in both total RNA and in poly(A)+ RNA by titration and hybridization kinetic methods. Surprisingly, the actin transcript number in sea star eggs is two orders of magnitude greater than in sea urchin eggs. There are at least 2.9 X 10(5) actin transcripts per sea star egg, 1.2 X 10(5) per 48-h gastrula and 1.9 X 10(5) per 72-h gastrula. The number of actin transcripts per unit mass of extracted tube foot RNA is lower than in developmental stages. The relative abundance and size of actin transcripts was determined by Northern and dot blot analyses using probes containing actin coding DNA or 3'-untranslated-region sequences. The actin transcript in eggs and embryos is 2,300 nucleotides (nt) long and originates from the Cy (cytoplasmic) gene class. In tube feet, the most abundant actin transcript is 2,200 nt long and originates from the M (muscle) gene class. Tube feet also contain, at lower abundance, 2,300-nt transcripts of the Cy gene type expressed in eggs and embryos.

Patterns of maternal messenger RNA accumulation and adenylation during oogenesis in Urechis caupo

We have investigated the accumulation and adenylation of the maternal mRNA during oogenesis in the oocytes of the marine worm Urechis caupo. The analysis, using in vitro translation and cDNA probes to assay for specific mRNAs, demonstrates that different maternal mRNAs accumulate with different patterns during oogenesis. One class of maternal mRNAs accumulates throughout oogenesis and remains at a steady level in the full-grown oocyte. These mRNAs do not have a poly(A) tail long enough to mediate binding to oligo(dT)-cellulose in oocytes, but are rapidly adenylated immediately following fertilization. The other maternal mRNAs accumulate in growing oocytes as poly(A)+ RNA and undergo some deadenylation in full-grown oocytes and embryos. Some of these mRNAs attain their highest concentration fairly early in oogenesis, while others continue to accumulate during later stages. Many of the mRNAs that accumulate as poly(A)+ RNA in growing oocytes diminish dramatically in concentration in full-grown oocytes.

Transient synthesis of a specific set of proteins during the rapid cleavage phase of sea urchin development

The rapid cleavage stage of early sea urchin development is characterized by the transient synthesis of a specific set of proteins. These proteins were identified by comparing the pattern of newly synthesized proteins from fully grown sea urchin oocytes, unfertilized and fertilized eggs, several embryonic stages, and adult tissues by one- and two-dimensional gel electrophoresis. We found that, in contrast to fertilization, meiotic maturation results in major changes in the relative amounts and types of proteins synthesized. The synthesis of many proteins in the oocyte, including actin, tubulin, and other proteins which have accumulated during oogenesis, appears to be greatly reduced or eliminated in the unfertilized and newly fertilized egg. An examination of the proteins synthesized in unfertilized and fertilized eggs reveals several proteins that are synthesized only during this phase of development and may be required for the rapid cell divisions which occur during this period. At the midblastula stage, when the cell-doubling time shows down, the pattern of synthesis changes to resemble that of the oocyte. These results imply that many of the changes in the pattern of protein synthesis seen around the blastula stage may be associated with the return of the cell cycle to a normal length rather than synthetic changes associated with differentiation.

Quantitative assessment of actin transcript number in eggs, embryos, and tube feet of the sea star Pisaster ochraceus

Actin coding sequence cDNA probes were used to quantitate the number of transcripts in RNA from eggs, embryos, and tube feet of the sea star Pisaster ochraceus. Transcript concentrations were measured in both total RNA and in poly(A)+ RNA by titration and hybridization kinetic methods. Surprisingly, the actin transcript number in sea star eggs is two orders of magnitude greater than in sea urchin eggs. There are at least 2.9 X 10(5) actin transcripts per sea star egg, 1.2 X 10(5) per 48-h gastrula and 1.9 X 10(5) per 72-h gastrula. The number of actin transcripts per unit mass of extracted tube foot RNA is lower than in developmental stages. The relative abundance and size of actin transcripts was determined by Northern and dot blot analyses using probes containing actin coding DNA or 3'-untranslated-region sequences. The actin transcript in eggs and embryos is 2,300 nucleotides (nt) long and originates from the Cy (cytoplasmic) gene class. In tube feet, the most abundant actin transcript is 2,200 nt long and originates from the M (muscle) gene class. Tube feet also contain, at lower abundance, 2,300-nt transcripts of the Cy gene type expressed in eggs and embryos.

Autoregulation of tubulin synthesis in enucleated cells

The effects on tubulin messenger RNA levels and tubulin protein synthesis of treating cells with microtubule-depolymerizing drugs or directly microinjecting cells with tubulin have suggested that non-polymerized tubulin depresses its own synthesis. The precise level of this control is unclear. It has been shown that enucleated cells, termed cytoplasts, retain many properties of the original cell, including maintenance of cell shape, pinocytic activity and locomotion as well as biosynthetic activities such as protein synthesis and replication of cytoplasmic viruses. Furthermore, cytoplasts retain most of the components of the cytoskeleton including the centrioles. If cytoplasmic activities alone are responsible for regulating tubulin biosynthesis, cytoplasts should contain the necessary components. To distinguish between regulation which would occur in the nucleus, that is, alterations in mRNA synthesis or modifications of the mRNA, from alterations in mRNA stability and/or translatability which would take place in the cytoplasm, we examined the autoregulation of tubulin synthesis in enucleated cells. Here, we report that enucleated mouse fibroblasts retain the ability to turn off tubulin protein synthesis in response to microtubule depolymerization, the reduction in tubulin synthesis being accompanied by a corresponding decrease in tubulin mRNA levels. Thus, transcription, processing and transport of tubulin mRNA from the nucleus are not likely to be the loci of regulation. Instead, tubulin must reduce, either directly or indirectly, the translatability of its own mRNA.

Noncoordinate histone synthesis in heat-shocked Drosophila cells is regulated at multiple levels

Transferring Drosophila tissue culture cells from 25 to 37 degrees C (heat shock) causes histone protein synthesis to become noncoordinate. To determine the level at which this is controlled, the synthesis, degradation, and translation of individual histone mRNAs was studied under both heat shock and control conditions. The increased synthesis of histone H2b protein during heat shock appears to be controlled primarily at the level of translation. During heat shock, H2b mRNA is transcribed at about the same level as in the control. However, H2b mRNA is more stable under heat shock than under control conditions and is predominantly found in polysomes. The reduction in synthesis of H2a, H3, and H4 protein during heat shock appears to be controlled at both the transcriptional and translational levels. Although transcription of H2a, H3, and H4 mRNAs is reduced during heat shock, like H2b mRNA, they are more stable. However, unlike H2b mRNA, these mRNAs are not predominantly associated with polysomes during heat shock. Regulation of H1 synthesis during heat shock is completely different from that of the other histones. During heat shock, H1 mRNA is not transcribed, and unlike all of the other Drosophila mRNAs studied to date, its mRNA is not stable in heat-shocked cells. Results from in vitro translation studies support the conclusion that noncoordinate synthesis of the core histone proteins during heat shock is controlled at the level of translation.

Informational content of the echinoderm egg

The sea urchin egg contains a store of mRNA synthesized during oogenesis but translated only after fertilization, which accounts for a large, rapid increase in the rate of synthesis of largely the same set of proteins synthesized by eggs. Starfish oocytes contain a population of stored maternal mRNA that becomes actively translated upon GVBD and codes for a set of proteins distinct from that synthesized by oocytes. The sequence complexity of RNA in echinoderm eggs is about 3.5 x 10(8) nucleotides, enough to code for about 12,000 different mRNAs averaging 3 kb in length. About 2-4% of the egg RNA functions as mRNA during early embryonic development; most of the sequences are rare, represented in a few thousand copies per egg, but some are considerably more abundant. Many of the stored RNA sequences accumulate during the period of vitellogenesis, which lasts a few weeks. The mechanisms of storage and translational activation of maternal mRNA are not well understood. Histone mRNAs are sequested in the egg pronucleus until first cleavage, but other mRNAs are widely distributed in the cytoplasm. The population of maternal RNA includes many very large molecules having interspersed repetitive sequence transcripts colinear with single-copy sequences. The structural features of much of the cytoplasmic maternal RNA is thus reminiscent of incompletely processed nuclear precursors of mRNA. The functional role of these strange molecules is not understood, but many interesting possibilities have been considered. For instance, they may be segregated into different cell lineages during cleavage and/or they may become translationally activated by selective processing during development. Maternal mRNA appears to be underloaded with ribosomes when translated, possibly because the coding sequences are short relative to the size of the mRNA. Most abundant and many rare mRNA sequences persist during embryonic development. The rare sequence molecules are replaced by newly synthesized RNA, but some abundant maternal transcripts appear to persist throughout embryonic development. Most of the proteins present in the egg do not change significantly in mass during development, but a few decline or accumulate substantially. Together, these observations indicate that much of the information for embryogenesis is stored in the egg, although substantial changes in gene expression occur during development.

Noncoordinate histone synthesis in heat-shocked Drosophila cells is regulated at multiple levels

Transferring Drosophila tissue culture cells from 25 to 37 degrees C (heat shock) causes histone protein synthesis to become noncoordinate. To determine the level at which this is controlled, the synthesis, degradation, and translation of individual histone mRNAs was studied under both heat shock and control conditions. The increased synthesis of histone H2b protein during heat shock appears to be controlled primarily at the level of translation. During heat shock, H2b mRNA is transcribed at about the same level as in the control. However, H2b mRNA is more stable under heat shock than under control conditions and is predominantly found in polysomes. The reduction in synthesis of H2a, H3, and H4 protein during heat shock appears to be controlled at both the transcriptional and translational levels. Although transcription of H2a, H3, and H4 mRNAs is reduced during heat shock, like H2b mRNA, they are more stable. However, unlike H2b mRNA, these mRNAs are not predominantly associated with polysomes during heat shock. Regulation of H1 synthesis during heat shock is completely different from that of the other histones. During heat shock, H1 mRNA is not transcribed, and unlike all of the other Drosophila mRNAs studied to date, its mRNA is not stable in heat-shocked cells. Results from in vitro translation studies support the conclusion that noncoordinate synthesis of the core histone proteins during heat shock is controlled at the level of translation.

Translational changes induced by 1-methyladenine in anucleate starfish oocytes

Translationally mediated changes in the patterns of protein synthesis occur when meiotic maturation of oocytes of Asterias forbesi is induced by 1-methyladenine. These changes, which were detected by electrophoretic separation of proteins labeled with [35S]methionine, begin about the time of germinal vesicle breakdown. Changes in protein synthesis which are indistinguishable from those of intact oocytes occur when nucleate and anucleate oocyte fragments derived by centrifugation are stimulated with 1-methyladenine. Thus these changes in protein synthesis do not require new transcription, and are not the result of the release during maturation of maternal mRNAs or translation factors sequestered in the germinal vesicle. The changes in protein synthesis are due to changes during maturation in the cytoplasmic population of maternal mRNA being translated.

Differential mRNA accumulation and translation during Spisula development

The patterns of proteins synthesized in developing Spisula embryos and larvae were compared with in vitro translation products by one-dimensional gel electrophoresis. Major changes in the in vivo pattern occur at fertilization; these are regulated at the translational level (Rosenthal, Hunt, and Ruderman, 1980, Cell 20, 487-494). The pattern is further altered by midcleavage, and subsequent development is accompanied by frequent changes in the kinds of proteins made. By midcleavage many of the in vivo changes are paralleled by alterations in mRNA levels. Three cDNA clones containing developmentally regulated, nonmitochondrial sequences were isolated from a library constructed from veliger larval RNA. Clone 3v4 encodes alpha-tubulin. Clone 12v4 encodes a 35,000-D protein of unknown function. The protein product of clone 10v8 has not been identified. The concentration of alpha-tubulin RNA is relatively low through midcleavage, increases by the swimming gastrula stage, and is maintained at a moderately high level throughout larval development. 10v8 and 12v4 RNAs first appear in trochophore larvae; their concentrations peak 10-12 hr later, and then decline. The proportions of alpha-tubulin and 10v8 RNA that are translated vary with developmental stage. During early cleavage very little alpha-tubulin RNA is on polysomes; in swimming gastrulae 64% of this mRNA is polysomal. Seventy percent of 10v8 RNA is translated in the trochophore larva, while only approximately 40% is polysomal in the 21-hr veliger. These results show that translational regulation may be superimposed on changes in cytoplasmic mRNA concentrations to determine the level of gene expression during embryogenesis.

Cyclin: a protein specified by maternal mRNA in sea urchin eggs that is destroyed at each cleavage division

Cleavage in embryos of the sea urchin Arbacia punctulata consists of eight very rapid divisions that require continual protein synthesis to sustain them. This synthesis is programmed by stored maternal mRNAs, which code for three or four particularly abundant proteins whose synthesis is barely if at all detectable in the unfertilized egg. One of these proteins is destroyed every time the cells divide. Eggs of the sea urchin Lytechinus pictus and oocytes of the surf clam Spisula solidissima also contain proteins that only start to be made after fertilization and are destroyed at certain points in the cell division cycle. We propose to call these proteins the cyclins.

Sequence-specific adenylations and deadenylations accompany changes in the translation of maternal messenger RNA after fertilization of Spisula oocytes

A dramatic change in the pattern of protein synthesis occurs within ten minutes after fertilization of Spisula oocytes. This change is regulated entirely at the translational level. We have used DNA clones complementary to five translationally regulated messenger RNAs to follow shifts in mRNA utilization at fertilization and to characterize alterations in mRNA structure that accompany switches in translational activity in vivo. Four of the mRNAs studied are translationally inactive in the oocyte. After fertilization two of these mRNAs are completely recruited onto polysomes, and two are partially recruited. All four of these mRNAs have very short poly(A) tracts in the oocyte; after fertilization the poly(A) tails lengthen considerably. In contrast, a fifth mRNA, that encoding alpha-tubulin mRNA, is translated very efficiently in the oocyte and is rapidly lost from polysomes after fertilization. Essentially all alpha-tubulin mRNA in the oocyte is poly(A)+ and a large portion of this mRNA undergoes complete deadenylation after fertilization. These results reveal a striking relationship between changes in adenylation and translational activity in vivo. This correlation is not perfect, however. Evidence for and against a direct role for polyadenylation in regulating these translational changes is discussed. Changes in poly(A) tails are the only alterations in mRNA sizes that we have been able to detect. This indicates that, at least for the mRNAs studied here, translational activation is not due to extensive processing of larger translationally incompetent precursors. We have also isolated several complementary DNA clones to RNAs encoded by the mitochondrial genome. Surprisingly, the poly(A) tracts of at least two of the mitochondrial RNAs also lengthen in response to fertilization.

The control of protein synthesis during heat shock in Drosophila cells involves altered polypeptide elongation rates

When Drosophila tissue culture cells are shifted from 25 to 36 degrees C (heat shocked) the pre-existing mRNAs (25 degrees C mRNAs) remain in the cytoplasm but their translation products are underrepresented relative to the induced heat shock proteins. Many of these undertranslated 25 degrees C mRNAs are found in association with polysomes of similar size in heat-shocked and control cells. Furthermore, the messages encoding alpha-tubulin, beta-tubulin, and actin are found associated with one-third to one-half as many total ribosomes in heat-shocked cells as in cells incubated at 25 degrees C. Increased temperature should lead to increased output of protein per ribosome. However, the 25 degrees C proteins are actually synthesized at less than 10% of 25 degrees C levels in heat-shocked cells. Thus, the rates of both elongation and initiation of translation are significantly (15- to 30-fold) slower on 25 degrees C mRNAs than they are on heat shock mRNAs in heat-shocked cells.

Adenovirus VAI RNA is required for efficient translation of viral mRNAs at late times after infection

Two adenovirus type 5 mutants were constructed to probe the function of the virus-encoded RNA polymerase III transcripts (VA RNAs). Each mutant fails to synthesize one of the two VA RNA species. The variant that does not produce the minor VAII species grows normally. The mutant that cannot synthesize the major VAI species grows more poorly than its parent. Analysis of the mutant's growth defect indicates that the adenovirus VAI RNA is required for efficient translation of viral mRNAs at late times after infection.