Molecular aspects of development in the brine shrimp Artemia

Regulation of promoter occupancy during activation of cryptobiotic embryos from the crustacean Artemia franciscana

Artemia franciscana embryos can suspend their development and metabolism at the gastrula stage to enter a state of cryptobiosis, forming cysts. Embryonic development and metabolism can be resumed under favorable environmental conditions to give rise to free-swimming larvae or nauplii. The mechanisms that mediate these processes are not completely known. Here, we report our studies of the mechanisms that regulate transcriptional activation upon exiting cryptobiosis. Regulatory regions of several A. franciscana gene promoters were identified. Functional analyses in mammalian cells allowed the identification of transcriptional activator regions in the Actin302 promoter and in promoter 2 of the sarco/endoplasmic reticulum Ca(2+)-ATPase-encoding gene. These regions were shown to specifically bind protein factors from nuclear extracts of A. franciscana nauplii by means of electrophoretic mobility shift assays. Several protein-binding regions were also detected by DNase I protection analysis in the promoters of the genes encoding the alpha1 subunit of Na(+)/K(+)-ATPase, actin 302 and sarco/endoplasmic reticulum Ca(2+)-ATPase. Specific DNA-binding proteins in nauplius nuclear extracts were detected for all the promoter regions analyzed. These proteins were either not present in cyst nuclear extracts or were present in much smaller concentrations. Three of the five regions analyzed also bound proteins present in cyst nuclear extracts. These data indicate that transcriptional activation upon exiting cryptobiosis in A. franciscana involves the expression/activation of DNA-binding transcription factors that are not present in cyst nuclei

In vivo downregulation of protein synthesis in the snail Helix apersa during estivation

Protein synthesis is downregulated during metabolic depression in a number of systems where the metabolic depression is effected by obvious extrinsic cues. The metabolic depression of the estivating land snail Helix apersa occurs in the absence of any obvious physiological stress and has an intrinsic component independent of temperature, pH, O(2) status, or osmolality. We show that this metabolic depression is accompanied by a downregulation of protein synthesis in vivo. The rate of protein synthesis decreases in two major tissues during estivation: to 23% and 53% of the awake rate in hepatopancreas and foot muscle, respectively. We show from calculations of the theoretical contribution of protein synthesis to total O(2) consumption that the depression of protein synthesis must be a significant, obligate, in vivo component of metabolic depression in H. aspersa.

Purification, structure and in vitro molecular-chaperone activity of Artemia p26, a small heat-shock/alpha-crystallin protein

Encysted brine-shrimp gastrulae bring their metabolism to a reversible standstill during diapause and quiescence, demonstrating a remarkable resistance to unfavourable environmental conditions. For example, mortality of Artemia embryos under normal temperature and hydration is very low, even after two years of anoxia, and embryos commonly experience complete desiccation as part of their developmental program. Previous evidence from our laboratories indicated that p26, an abundant low-molecular-mass cyst-specific protein capable of translocation into the nucleus, may have a protective function in Artemia cysts. p26 was purified to apparent homogeneity and a continuous sequence of 141 of its amino acids was determined by peptide sequencing, revealing that it is a member of the small-heat-shock/alpha-crystallin family of proteins. As determined by molecular-sieve chromatography and sucrose-density-gradient centrifugation, native p26 is a multimer of about 27 monomers with a molecular mass of approximately 700 kDa. Inactivation of citrate synthase was less when the enzyme was heated in the presence rather than the absence of p26. Additionally, the renaturation of heat-inactivated citrate synthase was promoted by p26. These results indicated that p26 possesses molecular-chaperone activity, a property of other small heat-shock/alpha-crystallin proteins. Our findings demonstrate that p26 has the potential to protect the macromolecular components of Artemia embryos, either as they encyst or upon exposure to environmental extremes. Protection may depend upon the ability of p26 to function as a molecular chaperone.

Elongation factor 2 from Artemia salina embryos and its affinity for ribosomes

Crude extracts from Artemia salina undeveloped embryos do not contain detectable elongation-factor-2 (EF2) kinase and endogenous ADP-ribosylating activities. Accordingly, EF2 purified from this source is an enzyme relatively free from phosphorylated and ADP-ribosylated forms. Endogenous ADP-ribosyltransferase activity appears only after purification of EF2. The affinities of EF2 and of ADP-ribosyl-EF2 for ribosomes from A. salina undeveloped embryos have been calculated by measuring the ability of the factors to inhibit the N-glycosidase activity of ricin on ribosomes.

Characterization of cell-free protein-synthesis systems from undeveloped and developing Artemia embryos

We have developed and characterized translationally active cell-free systems from Artemia embryos at different developmental times. The optimized lysates from 16 h-developed embryos incorporated radiolabelled amino acids into polypeptides for up to 120 min. The polypeptides synthesized ranged in Mr from 150,000 to 10,000, suggesting that the endogenous mRNA was capable of directing the synthesis of complete polypeptides. Similar results were obtained by using lysates from early developmental stages; even the cell-free system prepared from 1 h-developed embryos was partially active in protein synthesis. Furthermore, all these lysates were capable of re-initiation, as demonstrated by inhibition of initiation with the inhibitors edeine and 7-methylguanosine 5'-triphosphate. Because we found no endogenous protein-synthetic activity in the corresponding lysates from undeveloped embryos, we have used cell-free translation systems from 0 h- and 16 h-developed Artemia embryos to analyse the mechanisms limiting protein synthesis at very early developmental stages. Undeveloped-embryo lysates supplemented with nuclease-treated reticulocyte lysate were capable of translating endogenous mRNAs to give products with a wide spectrum of Mr values, but lysates of 16 h-developed embryos supplemented in this way were not further stimulated. The nuclease-treated lysate appeared to be unnecessary 5 h after resumption of development. These results suggested that a component(s) limiting translation in the undeveloped-embryo lysate was provided by the nuclease-treated reticulocyte lysate, and that this component(s) no longer limited protein synthesis after development. In view of these results, partially fractionated reticulocyte lysates were tested for restoration of protein-synthetic activity in the undeveloped embryo lysate. A high-salt ribosomal wash devoid of ribosomal subunits, which is considered a crude polypeptide-initiation-factor preparation, also restored translation activity in the undeveloped embryo lysate and made it capable of directing the synthesis of both endogenous mRNAs and exogenous (globin) mRNA.

Phosphorylation and guanine nucleotide exchange on polypeptide chain initiation factor-2 from Artemia embryos

Eukaryotic initiation factor-2 (eIF-2) from Artemia embryos is able to exchange guanine nucleotides at the same rate in the presence or absence of Mg2+ when the reaction is carried out with either purified eIF-2 at 30 degrees C or less purified preparations at any temperature (10-30 degrees C). No exchange factor appears to catalyze this reaction. However, with purified eIF-2 at lower temperatures (10 degrees C) the exchange is clearly impaired by Mg2+ and this impairment is overcome by the guanine nucleotide exchange factor (GEF) of rabbit reticulocytes. Thus, Artemia eIF-2 is able to exchange guanine nucleotides by two alternative mechanisms that may reflect two states of the protein. Phosphorylation of the eIF-2 alpha subunit by the heme-controlled inhibitor (HCI) of rabbit reticulocytes abolishes the GEF-dependent reaction, but has no effect on the factor-independent one. The search for eIF-2 alpha kinases in Artemia embryo led to the detection of only one such enzyme, which was identified as a casein kinase type II. None of the exchange reactions is affected by the phosphorylation of the eIF-2 alpha subunit by this kinase, suggesting that, irrespective of the kind of mechanism for guanine nucleotide exchange that is actually operating in Artemia, it might not be a target for regulation by eIF-2 alpha phosphorylation.

Sodium-calcium exchange in membrane vesicles from Artemia

Membrane vesicles were prepared from Artemia nauplii (San Francisco Bay variety) 45 h after hydration of the dry cysts. Na+-loaded vesicles accumulated up to 10 nmol Ca2+/mg protein when diluted 50-fold into 160 mM KCl containing 15 microM CaCl2. Practically no accumulation of Ca2+ was observed if the vesicles were diluted into 160 mM NaCl instead of KCl, or if they were treated with monensin, a Na+ ionophore, for 30 s prior to addition of CaCl2 to the KCl medium. These observations indicate that the Artemia vesicles exhibit Na-Ca exchange activity. The velocity of Ca2+ accumulation by the vesicles in KCl was stimulated 2.6-fold by the K+ ionophore valinomycin, suggesting that the exchange system is electrogenic, with a stoichiometry greater than 2Na+ per Ca2+. Km,Ca and Vmax values were 15 microM and 7.5 nmol/mg protein.s, respectively. Exchange activity in the Artemia vesicles was inhibited by benzamil (IC50 approximately equal to 100 microM) and by quinacrine (IC50 approximately equal to 250 microM), agents that also inhibit exchange activity in cardiac sarcolemmal vesicles. Unlike cardiac vesicles, however, exchange activity in Artemia was not stimulated by limited proteolysis, redox reagents, or intravesicular Ca2+. This indicates that the two exchange systems are regulated by different mechanisms. Vesicles were prepared from Artemia at various times after hydration of the dry cysts and examined for exchange activity. Activity was first observed at approximately 10 h after hydration and increased to a maximal value by 30-40 h; hatching of the free swimming nauplii occurred at 18-24 h. The results suggest that hatching Artemia nauplii might be a particularly rich source of mRNA coding for the Na+-Ca2+ exchange carrier.

Natural mRNA is required for directing Met-tRNA(f) binding to 40S ribosomal subunits in animal cells: involvement of Co-eIF-2A in natural mRNA-directed initiation complex formation

Two protein factors, eIF-2 as well as a high molecular weight protein complex from reticulocyte ribosomal high-salt wash which we term Co-eIF-2, promote Met-tRNA(f) binding to 40S ribosomes. This binding is dependent on the presence of an AUG codon or natural mRNAs [Roy et al. (1984) Biochem. Biophys. Res. Commun. 122, 1418-1425]. Co-eIF-2 contains two component activities, Co-eIF-2A and Co-eIF-2C. Previously, we have purified an 80-kDa polypeptide containing Co-eIF-2A activity and showed that this polypeptide is a component of Co-eIF-2 and is responsible for Co-eIF-2A activity in Co-eIF-2 [Chakravarty et al. (1985) J. Biol. Chem. 260, 6945-6949]. We now report purification of a protein complex (subunits of Mr 180K, 110K, 65K, 63K, 53K, 50K, 43K, and 40K) containing Co-eIF-2C activity and devoid of Co-eIF-2A activity. In SDS-PAGE, the purified Co-eIF-2C preparation and an eIF-3 preparation (purified in Dr. A. Wahba's laboratory) separated into seven similar major polypeptides (Mr 110K, 65K, 63K, 53K, 50K, 43K, and 40K). The 50-kDa polypeptide in Co-eIF-2C was immunoreactive with a monoclonal antibody against eIF-4A (50 kDa). We have studied the roles of purified Co-eIF-2A and Co-eIF-2C activities in ternary and Met-tRNA(f).40S ribosome complex formation. The results are as follows: (1) At low and presumably physiological factor concentration (30 nM), eIF-2 did not form detectable levels of ternary complex. Moreover, such complex formation was totally dependent on the presence of Co-eIF-2A and/or Co-eIF-2C.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of cDNA clones coding for mitochondrial 16S ribosomal RNA from the crustacean Artemia

cDNA clones coding for Artemia mitochondrial 16S ribosomal RNA (rRNA) have been isolated. The clones cover from nucleotide 650 of the RNA molecule to its 3' end. The comparison of Artemia sequence with both vertebrate and invertebrate mitochondrial 16S rRNA sequences has shown the existence of regions of high similarity between them. A model for the secondary structure of the 3' half of Artemia mitochondrial 16S rRNA is proposed. The size of the rRNA molecule has been estimated at 1.35 kb. Despite the similarity of the Artemia gene to insect rRNA in size, sequence and secondary structure, the G + C content of the Artemia gene (42%) is closer to that of mammals than to the insect genes. The number of mitochondria in Artemia has been estimated at 1500 per diploid genome in the cyst and 4000 in the nauplius. In contrast, the amount of mt 16S rRNA is constant at all stages of Artemia development.

Roles of a 67-kDa polypeptide in reversal of protein synthesis inhibition in heme-deficient reticulocyte lysate

During heme deficiency in reticulocyte lysates, the heme-regulated protein synthesis inhibitor, HRI, phosphorylates the alpha subunit of eukaryotic initiation factor 2 (eIF-2) and thus inhibits protein synthesis. Two factors, eIF-2 and a reticulocyte-lysate supernatant factor that we term RF, reverse this inhibition. We now report the following. (i) An active eIF-2 preparation contained, in addition to the three subunits (alpha, beta, and gamma), a 67-kDa polypeptide. Pretreatment of eIF-2 with polyclonal antibodies against either isolated alpha subunit or 67-kDa polypeptide almost completely inhibited the reversal activity. Upon further fractionation, three-subunit eIF-2 and the 67-kDa polypeptide were resolved. Neither the three-subunit eIF-2 nor the 67-kDa polypeptide alone was active in protein synthesis inhibition reversal. The activity was, however, restored by combining both the three-subunit eIF-2 and the 67-kDa polypeptide. (ii) Active RF preparations contained eIF-2 alpha (unphosphorylated) and beta subunits and the 67-kDa polypeptide. As with eIF-2, prior treatment of the RF preparation with antibodies to either the alpha subunit or the 67-kDa polypeptide almost completely inhibited the reversal activity. The RF preparation devoid of eIF-2 gamma subunit did not form ternary complex (Met-tRNA(fMet).eIF-2.GTP). The eIF-2 gamma subunit in the free form was isolated, and addition of this isolated gamma subunit to RF promoted significant ternary-complex formation. (iii) Purified HRI efficiently phosphorylated the alpha subunit in the three subunit eIF-2. However, the extent of such phosphorylation was significantly reduced when eIF-2 containing the 67-kDa polypeptide was used. The 67-kDa polypeptide apparently protected eIF-2 alpha subunit from HRI-catalyzed phosphorylation but did not inhibit HRI activity. Based on these results, we suggest that the protein synthesis inhibition reversal activity in both eIF-2 and RF is due to the same components--namely, eIF-2 alpha subunit and the 67-kDa polypeptide. The 67-kDa polypeptide protects eIF-2 alpha subunit from HRI-catalyzed phosphorylation and may also be a necessary component of the functioning eIF-2 molecule.

Expression of a gene for mouse eucaryotic elongation factor Tu during murine erythroleukemic cell differentiation

The eucaryotic elongation factor Tu (eEF-Tu) is a single polypeptide with an approximate Mr of 53,000. During protein synthesis eEF-Tu promotes the binding of aminoacyl-tRNA to the ribosome. To study the expression of the gene(s) for this factor, a genomic clone was isolated that contains a mouse eEF-Tu gene. We screened a phage genomic library with a synthetic oligonucleotide probe complementary to a region of the Saccharomyces cerevisiae and Artemia sp. eEF-Tu genes which codes for an area that is highly conserved between both yeast and Artemia sp. eEF-Tu. From approximately 75,000 phage plaques we obtained five isolates with apparently identical inserts. All five clones contained a 3.8-kilobase EcoRI fragment that hybridized to additional oligonucleotide probes corresponding to different conserved regions of eEF-Tu. We sequenced the 5' end of one genomic clone and determined the length of the cloned fragment that was protected by eEF-Tu mRNA in S1 nuclease protection assays. A quantitative S1 nuclease protection assay was used to compare the relative steady-state levels of eEF-Tu mRNA in total mRNA in total RNA isolated from hexamethylene-bisacetamide-induced murine erythroleukemia cells. The results show a dramatic reduction in the steady-state level of eEF-Tu mRNA as differentiation proceeds. A similar reduction in transcription of eEF-Tu mRNA was observed in isolated nuclei. Finally, we examined the in vivo synthesis of eEF-Tu during differentiation and found that it declined in a manner parallel to the decline in the steady-state level of eEF-Tu mRNA. In addition, we have isolated and sequenced a cDNA clone for mouse eEF-Tu. The derived amino acid sequence is compared with sequences from other eucaryotes.

Expression of a gene for mouse eucaryotic elongation factor Tu during murine erythroleukemic cell differentiation

The eucaryotic elongation factor Tu (eEF-Tu) is a single polypeptide with an approximate Mr of 53,000. During protein synthesis eEF-Tu promotes the binding of aminoacyl-tRNA to the ribosome. To study the expression of the gene(s) for this factor, a genomic clone was isolated that contains a mouse eEF-Tu gene. We screened a phage genomic library with a synthetic oligonucleotide probe complementary to a region of the Saccharomyces cerevisiae and Artemia sp. eEF-Tu genes which codes for an area that is highly conserved between both yeast and Artemia sp. eEF-Tu. From approximately 75,000 phage plaques we obtained five isolates with apparently identical inserts. All five clones contained a 3.8-kilobase EcoRI fragment that hybridized to additional oligonucleotide probes corresponding to different conserved regions of eEF-Tu. We sequenced the 5' end of one genomic clone and determined the length of the cloned fragment that was protected by eEF-Tu mRNA in S1 nuclease protection assays. A quantitative S1 nuclease protection assay was used to compare the relative steady-state levels of eEF-Tu mRNA in total mRNA in total RNA isolated from hexamethylene-bisacetamide-induced murine erythroleukemia cells. The results show a dramatic reduction in the steady-state level of eEF-Tu mRNA as differentiation proceeds. A similar reduction in transcription of eEF-Tu mRNA was observed in isolated nuclei. Finally, we examined the in vivo synthesis of eEF-Tu during differentiation and found that it declined in a manner parallel to the decline in the steady-state level of eEF-Tu mRNA. In addition, we have isolated and sequenced a cDNA clone for mouse eEF-Tu. The derived amino acid sequence is compared with sequences from other eucaryotes.

The function of proteins that interact with mRNA

Specific proteins are associated with mRNA in the cytoplasm of eukaryotic cells. The complement of associated proteins depends upon whether the mRNA is an integral component of the polysomal complex being translated, or, alternatively, whether it is part of the non-translated free mRNP fraction. By subjecting cells to ultraviolet irradiation in vivo to cross-link proteins to mRNA, mRNP proteins have been shown to be associated with specific regions of the mRNA molecule. Examination of mRNP complexes containing a unique mRNA has suggested that not all mRNA contain the same family of associated RNA binding proteins. The functions of mRNA associated proteins may include a role in providing stability for mRNA, and/or in modulating translation. With the recent demonstrations that both free and polysomal mRNPs are associated with the cytoskeletal framework, specific mRNP proteins may play a role in determining the subcellular localization of specific mRNPs.

Protein synthesis in yeast Saccharomyces cerevisiae. Purification of Co-eIF-2A and 'mRNA-binding factor(s)' and studies of their roles in Met-tRNAf.40S.mRNA complex formation

Antibodies prepared against a homogeneous preparation of Co-eIF-2A20 [Ahmad et al. (1985) J. Biol. Chem. 260, 6955-6959] reacted with several polypeptides including an 80-kDa polypeptide present in a crude yeast ribosomal salt wash. This 80-kDa polypeptide, containing Co-eIF-2A (Co-eIF-2A80) activity, has been extensively purified using a two-step purification procedure involving an immunoaffinity column chromatograph prepared using antibodies against Co-eIF-2A20 (fraction II) and hydroxyapatite chromatography (fraction III). The factors, eIF-2 + homogeneous Co-eIF-2A80 (fraction III) promoted Met-tRNAf.40S complex formation with an AUG codon but not with a physiological mRNA or a polyribonucleotide messenger poly(U,G) whereas eIF-2 + a partially purified Co-eIF-2A80 preparation (fraction II) promoted Met-tRNAf.40S complex formation with an AUG codon as well as with globin mRNA and poly(U,G) messenger. This factor-promoted Met-tRNAf binding to 40S ribosomes depends absolutely on the presence of a polyribonucleotide messenger containing an initiation codon (such as AUG or GUG). Other polyribonucleotide messengers tested, such as poly(U), poly(A) and poly(A,C) were completely ineffective in this binding reaction. This result indicates that the Met-tRNAf.40S.mRNA complex is formed by a direct interaction between Met-tRNAf, 40S ribosomes and the initiation site in mRNA. A mechanism has been proposed for Met-tRNAf.40S.mRNA complex formation in yeast.