Transcriptional and translational control of cytoplasmic proteins after serum stimulation of quiescent Swiss 3T3 cells

Mutations in the Drosophila gene encoding ribosomal protein S6 cause tissue overgrowth

We have characterized two P-element-induced, lethal mutations in Drosophila melanogaster which affect the larval hemocytes, mediators of the insect immune response. Each mutant displays larval melanotic tumors characteristic of mutations affecting the insect cellular immune system, and the moribund animals develop grossly hypertrophied hematopoietic organs because of increased cell proliferation and extra rounds of endoreduplication in some hematopoietic cells. Surprisingly, these mutations are due to P element insertions in the 5' regulatory region of the Drosophila gene encoding ribosomal protein S6 and cause a reduction of S6 transcript abundance in mutant larvae.

Mutations in the Drosophila gene encoding ribosomal protein S6 cause tissue overgrowth

We have characterized two P-element-induced, lethal mutations in Drosophila melanogaster which affect the larval hemocytes, mediators of the insect immune response. Each mutant displays larval melanotic tumors characteristic of mutations affecting the insect cellular immune system, and the moribund animals develop grossly hypertrophied hematopoietic organs because of increased cell proliferation and extra rounds of endoreduplication in some hematopoietic cells. Surprisingly, these mutations are due to P element insertions in the 5' regulatory region of the Drosophila gene encoding ribosomal protein S6 and cause a reduction of S6 transcript abundance in mutant larvae.

Alteration of translation and stability of mRNA for the poly(A)-binding protein during myogenesis

The regulation of synthesis of various factors involved in mRNA translation during differentiation of muscle cells was examined. The steady-state levels of mRNAs coding for eukaryotic initiation factor (eIF) 2 alpha, 2 beta and elongation factor (eEF)-1 alpha were measured in both proliferating rat L6 myoblast and differentiated myotubes. The steady-state levels of these mRNAs were not altered during myogenesis. Furthermore, the distribution of these mRNAs between repressed and translated populations remained unchanged. Recent studies suggest a role for poly(A)-binding protein (PABP) in translation initiation. Therefore, we also examined the expression of PABP mRNA during myogenesis. The PABP mRNA was less abundant in myotubes compared to myoblasts. However, the synthesis of PABP remained unchanged. In myoblasts, approximately 50-60% of the total mRNA was associated with polyribosomes, whereas in myotubes more than 80% of the mRNA was associated with polyribosomes. These results, therefore, suggest that the PABP mRNA was more efficiently translated in differentiated myotubes than in the proliferating myoblasts. Measurement of the stability and transcription of PABP mRNA showed that, while transcription was not affected during myogenesis, the stability of the mRNA was reduced in differentiated cells. The t1/2 of PABP mRNA in myoblasts was 13 h compared to 7.5 h in myotubes. This observation suggests that the reduced steady-state level of PABP mRNA in myotube were largely due to the change in stability of this mRNA during myogenesis.

cDNA cloning and sequence analysis of beta ig-h3, a novel gene induced in a human adenocarcinoma cell line after treatment with transforming growth factor-beta

Transforming growth factor-beta (TGF-beta) is capable of affecting the proliferation of many cell types. To identify novel genes whose protein products may mediate cellular responses to this factor, a cDNA library was made from mRNA isolated from a human lung adenocarcinoma cell line (A549) that had been treated for 3 days with TGF-beta. The library was screened by differential hybridization and a cDNA clone, beta ig-h3, was isolated. This gene was induced up to 20-fold in A549 cells after 2 days of treatment with TGF-beta 1. It was also induced in several other cell lines, including PC-3 and H2981. DNA sequence analysis of beta ig-h3 indicated that it encoded a novel protein, beta IG-H3, of 683 amino acids, which contained an amino-terminal secretory sequence and a carboxy-terminal Arg-Gly-Asp (RGD) sequence that can serve as a ligand recognition site for several integrins. beta IG-H3 also contained short amino acid regions homologous to similar regions in Drosophila fasciclin-I and four homologous internal domains, which can be folded into a potential bivalent structure and could act as a bridge between cells expressing the appropriate ligand. beta ig-h3 RNA was detected in several cell lines and tissues. COS cells transfected with plasmids encoding beta IG-H3 secreted a major 68-kD protein that was detected by immunoblotting using antipeptide antibodies. Since beta ig-h3 is induced in several cell lines whose proliferation is affected by TGF-beta 1, it may be involved in mediating some of the signals of this multifunctional growth modulator.

Cloning of the mitogen-activated S6 kinase from rat liver reveals an enzyme of the second messenger subfamily

Recently we reported the purification of a mitogen-activated S6 kinase from Swiss mouse 3T3 fibroblasts and rat liver. The rat liver protein was cleaved with cyanogen bromide or trypsin and 17 of the resulting peptides were sequenced. DNA primers were generated from 3 peptides that had homology to sequences of the conserved catalytic domain of protein kinases. These primers were used in the polymerase chain reaction to obtain a 0.4-kilobase DNA fragment. This fragment was either radioactively labeled and hybridized to Northern blots of poly(A)+ mRNA or used to screen a rat liver cDNA library. Northern blot analysis revealed four transcripts of 2.5, 3.2, 4.0, and 6.0 kilobases, and five S6 kinase clones were obtained by screening the library. Only two of the clones, which were identical, encoded a full-length protein. This protein had a molecular weight of 56,160, which correlated closely to that of the dephosphorylated kinase determined by SDS/PAGE. The catalytic domain of the kinase resembles that of other serine/threonine kinases belonging to the second messenger subfamily of protein kinases.

Identical Mr 70,000 S6 kinase is activated biphasically by epidermal growth factor: a phosphopeptide that characterizes the late phase

Mitogenic stimulation of quiescent mouse 3T3 cells with epidermal growth factor leads to biphasic S6 kinase activation. The kinases present in both phases of the response have been purified from 32P-labeled cells and shown to contain a phosphoprotein of equivalent Mr 70,000. Chromatographic analysis of the purified S6 kinases on a Mono Q column reveals that (i) all 32P-labeled protein coelutes with S6 kinase activity, (ii) only those fractions containing S6 kinase autophosphorylate, (iii) autophosphorylation is restricted to a single Mr 70,000 protein, and (iv) the extent of autophosphorylation directly parallels the degree of S6 kinase activation. Analysis of the two autophosphorylated S6 kinases by two-dimensional tryptic phosphopeptide mapping indicates that they are the same protein. Both in vivo 32P-labeled S6 kinases contain phosphoserine and phosphothreonine but no detectable phosphotyrosine. Two-dimensional tryptic peptide maps of the in vivo 32P-labeled S6 kinases are essentially identical, except for a single qualitative change in the late-phase S6 kinase.

Control of protein synthesis by extracellular Na+ in cultured fibroblasts

In chick embryo fibroblasts (CEFs), a partial substitution of extracellular Na+ with other cations or carbohydrates decreased the intracellular Na+ content without altering the K+ level. Concomitantly, a significant decrease in the serum-dependent rate of protein synthesis occurred. This phenomenon appeared to be quickly reversible upon reconstitution of the correct extracellular Na+ concentration in the culture medium. The presence of a transcriptional inhibitor such as actinomycin D during the treatment did not inhibit the reversibility of the phenomenon. The presence in the culture medium of K+ in such excess as to dissipate the membrane potential did not alter the observed relationship between the protein synthesis rate and the internal Na+ content. Analysis of the amino acid pool indicated that the observed inhibition of the rate of protein synthesis in CEFs incubated in low Na+ medium was not caused by an unbalanced availability of intracellular amino acids. In addition, intracellular pH, as estimated by the measurement of the equilibrium distribution of benzoic acid, did not show any significant alteration in cells incubated in the presence of bicarbonate buffer and in low extracellular Na+. Moreover, the relationship between the rate of protein synthesis and the internal Na+ content was still observed in CEFs cultured in bicarbonate-containing media, but at lower or higher than physiological pH. Analysis by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) of the proteins synthesized by CEFs cultured at a reduced extracellular Na+ concentration showed that specific alterations of gene expression occurred.

Effects of serum, cycloheximide and actinomycin D on protein secretion by quiescent mouse embryo fibroblasts

Quiescent secondary cultures of Swiss mouse embryo fibroblasts secrete several proteins in response to the addition of 20% fetal calf serum (FCS). Of these proteins, a polypeptide of molecular weight (Mr) 48,000 (48 K) was identified in the medium within an hour of mitogenic stimuli. In the next hour an additional protein of Mr26,000 (26 K) appeared in the medium. These two proteins were absent in the conditioned medium of quiescent cells. A third protein of molecular weight 45,000 (45 K) was found in small quantities in the conditioned medium of quiescent cells but a 2-3 fold increase in the level of this protein was observed in the medium of stimulated cells. The level of the serum-induced 45 K protein was much higher in the medium of cells that were treated with cycloheximide (CH) and FCS than that found in the medium of cells treated with FCS alone. A 40,000 dalton protein was found to be a quiescence specific protein which was observed in large amounts in the medium of quiescent cells; the level of this protein gradually declined in the conditioned medium as the cells entered into the proliferative phase. Actinomycin D specifically inhibited the level of the 45 K secreted protein and a 29 K intracellular protein when added along with CH. In contrast to the inhibition of the synthesis of mitogen induced proteins, actinomycin D super-induced the intracellular and extracellular levels of the matrix proteins fibronectin and procollagens.

Maturation-related gene expression of rat astroblasts in vitro studied by two-dimensional polyacrylamide gel electrophoresis

Maturation of rat astroblasts was induced in vitro by acidic fibroblast growth factor (aFGF), hydrocortisone and dibutyryl cyclic AMP. Cells grown for 20 days were treated for 48 h and labelled with [35S]methionine only during the last 18 h of treatment. Cell proteins solubilized in lithium dodecyl sulfate were submitted to two-dimensional polyacrylamide gel electrophoresis. About 300 radioactive proteins could be analysed visually and compared. All treatments induced visible quantitative and sometimes qualitative changes. A total of 81 proteins had their rate of biosynthesis modified. For some proteins, this rate was changed by only one treatment, while for others it was changed by two or even by the three treatments, mostly in the same way. These results are consistent with the hypothesis that the proteins involved in the maturation process are organized in sets, proteins belonging to one set always being regulated together under a common control. Some sets would be regulated by only one effector while others would be regulated by several effectors.

The stability of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells varies with growth rate

The decay rates of eucaryotic elongation factor Tu (eEF-Tu) mRNA and eucaryotic initiation factor 4A (eIF-4A) mRNA in Friend erythroleukemia (FEL) cells were determined under several different growth conditions. In FEL cells which were no longer actively dividing (stationary phase), eEF-Tu mRNA was found to be rather stable, with a t1/2 of about 24 h. In rapidly growing FEL cells eEF-Tu mRNA was considerably less stable, with a t1/2 of about 9 h. In both cases a single rate of mRNA decay was observed. However, when stationary-phase cells resumed growth after treatment with fresh medium, we observed that eEF-Tu mRNA decay followed a biphasic process. The faster of the two decay rates involved approximately 50% of the eEF-Tu mRNA and had a t1/2 of about 1 h. The decay rates for eIF-4A (t1/2 = 2 h) and total poly(A)+ RNA (t1/2 = 3 h) were unaffected by changes in growth conditions. The t1/2 for polysomal eEF-Tu mRNA was found to be about 8 h when stationary FEL cells were treated with fresh medium. Previous work in this laboratory has shown (T. R. Rao and L. I. Slobin, Mol. Cell. Biol. 7:687-697, 1987) that when FEL cells are allowed to grow to stationary phase, approximately 60% of the mRNA for eEF-Tu is found in a nontranslating postpolysomal messenger ribonucleoprotein (mRNP) particle. eEF-Tu mRNP was rapidly cleared from stationary cells after treatment with fresh medium. The data presented in this report indicate that the stability of eEF-Tu mRNP is rapidly altered and the particle is targeted for degradation when stationary FEL cells resume growth.

The stability of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells varies with growth rate

The decay rates of eucaryotic elongation factor Tu (eEF-Tu) mRNA and eucaryotic initiation factor 4A (eIF-4A) mRNA in Friend erythroleukemia (FEL) cells were determined under several different growth conditions. In FEL cells which were no longer actively dividing (stationary phase), eEF-Tu mRNA was found to be rather stable, with a t1/2 of about 24 h. In rapidly growing FEL cells eEF-Tu mRNA was considerably less stable, with a t1/2 of about 9 h. In both cases a single rate of mRNA decay was observed. However, when stationary-phase cells resumed growth after treatment with fresh medium, we observed that eEF-Tu mRNA decay followed a biphasic process. The faster of the two decay rates involved approximately 50% of the eEF-Tu mRNA and had a t1/2 of about 1 h. The decay rates for eIF-4A (t1/2 = 2 h) and total poly(A)+ RNA (t1/2 = 3 h) were unaffected by changes in growth conditions. The t1/2 for polysomal eEF-Tu mRNA was found to be about 8 h when stationary FEL cells were treated with fresh medium. Previous work in this laboratory has shown (T. R. Rao and L. I. Slobin, Mol. Cell. Biol. 7:687-697, 1987) that when FEL cells are allowed to grow to stationary phase, approximately 60% of the mRNA for eEF-Tu is found in a nontranslating postpolysomal messenger ribonucleoprotein (mRNP) particle. eEF-Tu mRNP was rapidly cleared from stationary cells after treatment with fresh medium. The data presented in this report indicate that the stability of eEF-Tu mRNP is rapidly altered and the particle is targeted for degradation when stationary FEL cells resume growth.

Stimulation of DNA replication by growth factor and hormones in Swiss 3T3 cells: comparison of the rate of entry into S phase with in vitro DNA synthesis and DNA polymerase alpha activity

An approach to the investigation how growth factors and hormones regulate mammalian cell proliferation is to study the activity of enzymes involved in DNA replication. Quiescent cultures of Swiss mouse 3T3 cells were stimulated with prostaglandin F2 alpha, insulin, and/or hydrocortisone for a time at which less than 50% of the cells had initiated DNA synthesis. Such cells were lysed with a Ca++-containing hypotonic buffer and incubated with a nucleotide mixture including [3H]thymidine-triphosphate for 1 hr at 37 degrees C. The amount of radioactive label incorporated into the trichloroacetic acid (TCA)-precipitate and the percentage of labeled nuclei correlated with the in vivo stimulation. Analysis of radioactively and density-labeled DNA in sucrose and CsC gradients indicated that the incorporation of label reflected semiconservative replication. DNA polymerase activities were assayed in supernatants from whole-cell lysates prepared with a hypotonic buffer not containing Ca++. Using various templates, it was shown that the increase in activity of DNA polymerase alpha correlated with the percentage of cells in S phase upon the different stimulation, while DNA polymerase beta activity after various times of stimulation showed that this activity increased only when cells began to enter S phase, regardless of the combination of growth factor and hormones.

Phosphorylation of partially purified 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine:acetyl-CoA acetyltransferase from rat spleen

A new improved method for purification of the enzyme 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine: acetyl-CoA acetyltransferase (EC 2.3.1.67) from rat spleen is described. The catalytic subunit of cyclic AMP-dependent protein kinase in the presence of MgATP stimulated about 3-fold the activity of this partially purified enzyme activity. When [gamma-32P]ATP was included in the assay mixture, the analysis of phosphoprotein products by SDS/polyacrylamide-gel electrophoresis and autoradiography showed the incorporation of [32P]phosphate into a single protein band of about 30 kDa. Analysis of the phosphorylated amino acids indicated that the phosphate was incorporated into a serine residue. Activation of the acetylation reaction by the protein kinase was reversible. The reversal of the activation was coincident with the loss of the [32P]phosphate incorporated into the 30 kDa protein band, which suggests that the acetyltransferase is regulated by a phosphorylation-dephosphorylation mechanism dependent on cyclic AMP.

Regulation of the utilization of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells

When Friend erythroleukemia cells were allowed to grow to stationary phase (2 X 10(6) to 3 X 10(6) cells per ml), approximately 60% of the mRNA for eucaryotic elongation factor Tu (eEF-Tu) sedimented at less than or equal to 80S, and most of the remaining factor mRNA was associated with small polysomes. Under the same growth conditions, greater than 90% of the mRNA for eucaryotic initiation factor 4A remained associated with polysomes. The association of eEF-Tu mRNA with polysomes changed dramatically when stationary-phase cells were treated with fresh medium. After 1 h in fresh medium, approximately 90% of eEF-Tu mRNA in Friend cells was found in heavy polysomes. Associated with the shift of eEF-Tu mRNA into heavy polysomes, we found at least a 2.6-fold increase in the synthesis of eEF-Tu in vivo as well as a remarkable 40% decrease in the total amount of eEF-Tu mRNA per cell. Our data raise the possibility that eEF-Tu mRNA that has accumulated in ribonucleoprotein particles in stationary-phase cells is degraded rather than reutilized for eEF-Tu synthesis.

Starvation-induced alterations of ribosomal protein phosphorylation in Bombyx mori L. Evidence for different phosphorylation kinetics in free and membrane-bound ribosomes

In the posterior silk gland of Bombyx mori, ribosomal protein S1, homologous to S6 in mammals, is partially phosphorylated in a normally fed animal. Before the first meal of the fifth larval instar, S1 is completely dephosphorylated. Likewise, starvation induces rapid dephosphorylation of the protein in both free and membrane-bound ribosomes. Upon refeeding after 48 h of starvation, S1 becomes phosphorylated again, first on membrane-bound ribosomes, then on free ribosomes, with a lag time of about 3 h. Following 48 h of refeeding, the most highly phosphorylated form of S1 predominates in both populations of ribosomes. These variations in phosphorylation are correlated with the level of protein synthesis in the posterior silk gland, 70% of the ribosomes occurring in polysomes upon feeding and only 30% upon starvation [Prudhomme, J.-C. & Couble, P. (1979) Biochimie (Paris) 61, 215-227]. After in vivo 32P labelling, the phosphopeptides of S1 from free and membrane-bound ribosomes were found to be identical and phosphoserine (only) was found in each S1. These results suggest the involvement of S1 phosphorylation in the regulation of protein synthesis at the translational level and the existence of at least two different pathways controlling this phosphorylation: one for the free ribosomes, the other for the membrane-bound ribosomes.

Effect of essential amino acids on the phosphorylation of a 40S ribosomal protein and protein synthesis in Acanthamoeba castellanii

Reversible and multiple phosphorylation of a 40S ribosomal protein is observed in a variety of eukaryotic cells. In the primitive eukaryote Acanthamoeba, one or three phosphorylated S3 derivatives are observed during growth phase in nondefined nutrient medium (ND cells) or in chemically defined nutrient medium (D cells), respectively. In both cases, stationary phase cells exhibit nonphosphorylated S3; however, transfer of these cells into the respective fresh nutrient media results in a transient accumulation of four phosphorylated S3 derivatives. Transfer of D cells into nutrient medium, deficient in all or any single essential amino acids, leads to reversible inhibition of S3 phosphorylation and growth arrest. The low level of phosphorylated S3 is not simply the consequence of growth arrest, since in cells where growth is arrested differently, the level of phosphorylated S3 can be high. In response to amino acid deficiency, a number of other changes can be observed. These include a 2-3-fold decrease of total protein synthesis, 13 changes in the cellular protein pattern, and specific alterations in the ribosome absorbance profiles and in the distribution of poly-A+ RNA within subribosomal and ribosomal fractions. While the rate of total protein synthesis seems to be associated with the level of phosphorylated S3, the level of the synthesis of at least 10 of the particular proteins can be dissociated from the level of S3 phosphorylation.

Expression of a set of growth-related immediate early genes in BALB/c 3T3 cells: coordinate regulation with c-fos or c-myc

We have previously identified by cDNA cloning 5 mRNAs that appear in resting BALB/c 3T3 cells soon after growth stimulation by serum or platelet-derived growth factor. Five additional mRNAs of this class are described in this report. The mRNAs reached peak levels between 40 and 120 min after serum addition and rapidly decayed thereafter. All 10 RNAs were superinduced in the presence of cycloheximide. Nuclear run-on experiments indicated that the increase in the mRNAs is the result of rapid transcriptional activation of their genes on stimulation by serum or platelet-derived growth factor. Superinducibility by cycloheximide is due to two effects: prolonged transcription and stabilization of mRNAs. This overall pattern of regulation is similar to that of the c-fos or c-myc protooncogenes reported previously. We hypothesize that these newly identified "immediate early" genes play a role in the proliferative response induced by growth factors.

Regulation of the utilization of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells

When Friend erythroleukemia cells were allowed to grow to stationary phase (2 X 10(6) to 3 X 10(6) cells per ml), approximately 60% of the mRNA for eucaryotic elongation factor Tu (eEF-Tu) sedimented at less than or equal to 80S, and most of the remaining factor mRNA was associated with small polysomes. Under the same growth conditions, greater than 90% of the mRNA for eucaryotic initiation factor 4A remained associated with polysomes. The association of eEF-Tu mRNA with polysomes changed dramatically when stationary-phase cells were treated with fresh medium. After 1 h in fresh medium, approximately 90% of eEF-Tu mRNA in Friend cells was found in heavy polysomes. Associated with the shift of eEF-Tu mRNA into heavy polysomes, we found at least a 2.6-fold increase in the synthesis of eEF-Tu in vivo as well as a remarkable 40% decrease in the total amount of eEF-Tu mRNA per cell. Our data raise the possibility that eEF-Tu mRNA that has accumulated in ribonucleoprotein particles in stationary-phase cells is degraded rather than reutilized for eEF-Tu synthesis.

Translational control of mRNA expression during the early mitogenic response in Swiss mouse 3T3 cells: identification of specific proteins

Addition of serum or epidermal growth factor to quiescent Swiss mouse 3T3 cells in culture leads to a number of specific changes in the pattern of protein synthesis. Earlier experiments with actinomycin D suggested that the altered expression of these proteins was controlled at either the pretranslational or translational level. Here we have identified and further characterized the regulation of mRNA expression for ten of these proteins, including protein synthesis elongation factor eEF-1 alpha, poly A binding protein, vimentin, the multiple forms of the actin protein family, and alpha- and beta-tubulin. Using an in vitro translation system, we determined the change in the level of mRNA encoding for each of these proteins after serum stimulation. The results showed that the amount of mRNA coding for eEF-1 alpha, poly A binding protein, vimentin, and alpha- and beta-tubulin remains unchanged during this time, whereas that of the actin family increases. Thus, with the exception of the actin family, the results argue that the expression of all the proteins identified is regulated at the translational level. The importance of this latter group of proteins in cell growth and the abundance of their cognate mRNAs should prove them useful tools in elucidating the mechanisms involved in the activation of translationally repressed mRNA during the mitogenic response.

Synthesis and stability of nuclear matrix proteins in resting and serum-stimulated Swiss 3T3 cells

The major [35S]methionine-radiolabeled nuclear matrix proteins of mouse 3T3 cells were isolated, and most of these were found to be similar in molecular weight, charge, and protease fingerprint to the nuclear matrix proteins of African green monkey kidney cells, which are found tightly bound to simian virus 40 chromosomes. These nuclear matrix proteins were found to be synthesized in quiescent and serum-stimulated cells and to be turned over slowly during pulse-chase experiments. In contrast, a 70-Kd (kilodalton) neutral protein identified as lamin a was found to be turned over rapidly, producing a 68-Kd protein with a similar isoelectric point. In addition, we observed a decrease in the amounts of two chromatin-bound matrix proteins and a relative increase in lamin a following labeling in the presence of 2 micrograms/ml actinomycin D. However, these effects do not appear to be a result of inhibition of transcription, since they were not observed with other inhibitors (alpha-amanitin and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole).

Genes coding for the elongation factor EF-1 alpha in Artemia

The elongation factor EF-1 alpha is one of the most abundant proteins in eukaryotic cells, where it catalyzes the binding of aminoacyl-tRNA to ribosomes. The genes coding for this protein in the brine shrimp Artemia were analyzed by gene cloning, electron microscopy and chromosomal blot hybridization. There are only a few (about four) copies of one type of gene per haploid genome. These genes contain five exons divided over 10(4) base pairs. Local rearrangements give rise to a number of gene variants. Cross-hybridizations of Artemia cDNA probes with yeast and Drosophila DNA revealed two different yeast EF-1 alpha genes and one or two different Drosophila genes, respectively. Nucleotide sequencing revealed signals for synthesis and processing of EF-1 alpha transcripts as well as the exact location of exons. One interruption in the coding sequence corresponds closely to a splice junction in the gene coding for the homologous chloroplast protein EF-Tu from Euglena gracilis, presumably of prokaryotic origin. The first exon in the chloroplast gene codes for the region of EF-Tu that is homologous to regions of the elongation factor EF-G and of the initiation factor IF2, respectively.

Identification of a set of genes expressed during the G0/G1 transition of cultured mouse cells

To identify previously undetected genes that may be involved in the transition from a resting state (G0) to a proliferative state (G1) of mammalian cells, we set out to isolate cDNA clones derived from mRNAs that appear in serum-stimulated cells in the absence of protein synthesis. A lambda cDNA library was prepared using poly(A)+ RNA from BALB/c 3T3 cells that had been brought to quiescence and subsequently stimulated with serum in the presence of cycloheximide. Approximately 50 000 recombinant phage plaques were screened, and 357 clones were isolated that hybridized to probes derived from stimulated-cell RNA but not to probes from resting-cell RNA. Cross hybridization analysis showed that four RNA sequence families account for approximately 90% of these clones. One of the clones hybridized to an actin probe; none hybridized to any of 13 oncogene probes tested. Five different RNAs that appear to be previously uncharacterized have been further analyzed. These RNAs accumulate and decay rapidly following stimulation by serum or purified growth factors, or by a tumor promoter, and they are superinduced by serum in the presence of cycloheximide. Three of the RNAs could be enriched by hybridization to cDNAs and translated in vitro, yielding proteins of approximately 43, 40 and 35 kd, respectively.

Effect of heparin on vascular smooth muscle cells. II. Specific protein synthesis

Heparin suppresses the proliferation of vascular smooth muscle cells both in vivo and in vitro. The mechanism of action of the antiproliferative activity of heparin is not known. We have detected differences in the synthesis of specific proteins when vascular smooth muscle cells are exposed to heparin and report here that many characteristics of these protein alterations parallel the properties of the antiproliferative activity. The induction into the culture medium of a pair of proteins of approximately 35,000 dalton mw in heparin-treated smooth muscle cell cultures and the antiproliferative effect of heparin share the following characteristics: 1) the effect is reversible, 2) the effect is specific for smooth muscle cells, 3) anticoagulant and non-anticoagulant heparin are equally effective, 4) the effect is lost with time in culture and, 5) heparin is the most potent glycosaminoglycan in producing the effect. Furthermore, heparin causes a transient suppression of a 48,000 dalton substrate-attached protein, whereas chondroitin sulfate A and C and dermatan sulfate had much less effect. Dextran sulfate was almost as effective as heparin in suppressing the synthesis of the substrate-attached protein. These proteins appear to be noncollagenous and the induced synthesis of the 35,000 dalton proteins is inhibited by actinomycin D. Although a direct relationship between these specific protein changes and the antiproliferative effect of heparin has not been proven, these protein alterations may play a crucial role in the effect of heparin on smooth muscle cell growth.

A protein kinase from Xenopus eggs specific for ribosomal protein S6

A protein kinase specific for ribosomal protein S6 has been purified from eggs of Xenopus laevis. As visualized on a silver-stained polyacrylamide gel, the major protein in the preparation migrated with a Mr of 90,000. Incubation of the enzyme preparation with [gamma-32P]ATP led to phosphorylation of this protein on serine residues. Upon glycerol gradient centrifugation, the S6 kinase activity and the Mr 90,000 protein both sedimented with a Mr of 50,000-55,000. Two-dimensional gel electrophoresis demonstrated that up to 4-5 phosphate groups per S6 molecule could be incorporated with this enzyme in vitro, and two-dimensional peptide mapping demonstrated that the phosphopeptides from S6 labeled in vitro with the enzyme comigrated with those from highly phosphorylated S6 labeled in vivo in response to progesterone treatment. The purified S6 protein kinase did not phosphorylate at a significant rate ribosomal protein S10, histone H1, histone H4, mixed histones, casein, or phosvitin, indicating a high degree of substrate specificity. These results indicate that activation of a single S6 protein kinase may be sufficient to account for increased S6 phosphorylation after a growth stimulus.

Platelet-derived growth factor-modulated translatable mRNAs

The treatment of density-arrested BALB/c 3T3 cells with electrophoretically homogeneous or highly purified preparations of the platelet-derived growth factor (PDGF) stimulated the rapid and selective accumulation of several species of abundant mRNA identified by cell-free translation. These translatable mRNAs appeared long before entry into the S phase. Less PDGF was required for selective mRNA accumulation than for PDGF-modulated DNA synthesis. The translatable mRNAs also accumulated after addition of the epidermal growth factor but not after addition of insulin or platelet-poor plasma. Their selective accumulation was blocked by addition of actinomycin D. Three classes of PDGF-modulated mRNAs were defined. An early (primary) RNA appeared within 30 to 60 min of PDGF addition; its accumulation was not blocked by cycloheximide. Another early mRNA also appeared within 60 min, but treatment with both PDGF and cycloheximide was required for optimal accumulation. A third class, secondary RNAs, began to accumulate later at 90 to 120 min; the appearance of this class was inhibited by cycloheximide. One- and two-dimensional gel electrophoresis of translation products demonstrated that a spontaneously transformed BALB/c 3T3 (ST2-3T3) cell line, which does not require PDGF or epidermal growth factor for growth, constitutively accumulated the secondary growth factor-regulated mRNAs. The accumulation of these translatable mRNAs may be required for PDGF-modulated DNA synthesis.

Characterization of the epidermal-growth-factor-dependent phosphorylation system from normal mouse-liver sinusoidal plasma membranes

Blood sinusoidal plasma membrane subfractions were isolated from normal mouse liver in the presence of the proteinase inhibitors PhMeSO2F and iodoacetamide. They were purified from smooth microsomal and Golgi vesicle contaminants. The phosphorylation reaction was studied at 33 degrees C, in the presence of 2 mM MnCl2. Addition of epidermal growth factor (EGF) to the preparations stimulated 32P incorporation from [gamma-32P]ATP or [gamma-32P]GTP essentially into one 170 000 Mr protein. Some incorporation was observed in a minor 120 000-Mr component which appears to be a degradation product of the 170 000-Mr component. No EGF-dependent phosphorylation of other membrane proteins or various exogenous proteins could be detected in vitro. The dephosphorylation of the 170 000-Mr component was observed after 4 min of incubation at 33 degrees C. This dephosphorylation reaction was inhibited by addition of 5 mM p-nitrophenyl phosphate but not by addition of micromolar Zn2+, Be2+ or orthovanadate. The 170 000-Mr protein specifically bound 125I-labeled EGF and thus appeared to be the hepatic EGF receptor. The EGF stimulatable kinase activity considerably enhances incorporation of 32P into tyrosine residues of the 170 000-Mr EGF receptor at 33 degrees C. Tryptic peptide maps of the 32P-labeled 170 000-Mr protein revealed a multiplicity of phosphorylated sites. Seven 32P-labeled phosphopeptides were observed after EGF stimulation, three of them being largely prominent. Tryptic peptide maps of the 170 000-Mr protein after it was covalently linked to 125I-labeled EGF showed only one 125I-labeled peptide, the migration of which appeared different from that of 32P-labeled phosphopeptides. These findings were confirmed by V8 protease unidimensional peptide mapping of the 170 000-Mr protein, labeled with 32P or 125I-EGF.

The translation of the messenger for the poly(A)-binding protein-associated with translated mRNA is suppressed. A case of cytoplasmic repression in duck erythroblasts

In vivo protein synthesis in duck erythroblasts was compared to in vitro translation of polyribosomal and free cytoplasmic mRNA. The in vivo study showed the absence of de novo synthesis of the Mr 73 000 poly(A)-binding protein found associated with all polyribosomal mRNA. In vitro translation demonstrated that the mRNA for this protein is absent from the polyribosomal mRNA fraction but constitutes a medium frequency messenger among the repressed free mRNA. This result confirms the existence of a qualitative translational control in terminal differentiating duck erythroblasts leading eventually to the arrest of the protein synthesizing machinery.

Rapid selective effects by a growth inhibitor and epidermal growth factor on the incorporation of [35S]methionine into proteins secreted by African green monkey (BSC-1) cells

Confluent African green monkey kidney (BSC-1) cells secrete a protein (Mr approximately equal to 24,000) that inhibits DNA synthesis and growth of the same cells. Using [35S]methionine to metabolically label proteins, we have found that this growth inhibitor selectively induces the BSC-1 cells to synthesize and secrete another protein with a relative Mr of 48,000 on NaDodSO4/polyacrylamide gels. We have called this protein "inhibitor-inducible protein" (IIP48). The maximal increase in rate of labeling of IIP48 due to treatment with the growth inhibitor averages 12-fold over the control. IIP48 is an N-glycosidically linked glycoprotein, and it is not a major intracellular protein. This protein is maximally induced within 4 to 6 hr of adding the growth inhibitor to the cells. This is an early response of these cells to the growth inhibitor and may represent a primary response to the growth inhibitor. Epidermal growth factor (EGF) increases the rate of labeling of three other secreted proteins (MrS 28,000, 59,000, and 61,000), which we have called "mitogen-inducible proteins" (MIP28, MIP59, and MIP61). The specific effects of both EGF and the growth inhibitor on the secreted levels of these proteins are inhibited if actinomycin D is added with the growth effectors. Thus, RNA synthesis appears necessary for the inductions. EGF and the growth inhibitor induce these secreted proteins by independent and noninteracting pathways.

Platelet-derived growth factor-modulated translatable mRNAs

The treatment of density-arrested BALB/c 3T3 cells with electrophoretically homogeneous or highly purified preparations of the platelet-derived growth factor (PDGF) stimulated the rapid and selective accumulation of several species of abundant mRNA identified by cell-free translation. These translatable mRNAs appeared long before entry into the S phase. Less PDGF was required for selective mRNA accumulation than for PDGF-modulated DNA synthesis. The translatable mRNAs also accumulated after addition of the epidermal growth factor but not after addition of insulin or platelet-poor plasma. Their selective accumulation was blocked by addition of actinomycin D. Three classes of PDGF-modulated mRNAs were defined. An early (primary) RNA appeared within 30 to 60 min of PDGF addition; its accumulation was not blocked by cycloheximide. Another early mRNA also appeared within 60 min, but treatment with both PDGF and cycloheximide was required for optimal accumulation. A third class, secondary RNAs, began to accumulate later at 90 to 120 min; the appearance of this class was inhibited by cycloheximide. One- and two-dimensional gel electrophoresis of translation products demonstrated that a spontaneously transformed BALB/c 3T3 (ST2-3T3) cell line, which does not require PDGF or epidermal growth factor for growth, constitutively accumulated the secondary growth factor-regulated mRNAs. The accumulation of these translatable mRNAs may be required for PDGF-modulated DNA synthesis.

Characterization of a cycloheximide-resistant Tetrahymena thermophila mutant which also displays altered growth properties

A cycloheximide-resistant strain of Tetrahymena thermophila, expressing a mutant chx-B gene (Ares and Bruns, Genetics 90:463-474, 1978), displayed very different temperature-dependent growth characteristics than either wild-type cells or another cycloheximide-resistant strain expressing a different mutant gene. Whereas wild-type cells showed an immediate decline in ribosome translocation rates when shifted from 30 to 38 or 40 degrees C, this mutant strain (X-8) showed no such decline. These results directly correlated with the growth rate differences we found for these cells at these temperatures. By genetic analysis, we showed that the phenotype of cycloheximide resistance cosegregated with the ability to grow rapidly at 40 degrees C. Analyses, both direct and indirect, suggested that a number of functional and structural characteristics of the ribosomes from strain X-8 cells are most likely conformationally different from those of wild-type ribosomes.

Growth-related changes in specific mRNAs of cultured mouse cells

A cDNA plasmid library has been constructed from the poly(A)+ RNA present in BALB/c3T3 cells after serum stimulation. Of 3,500 clones tested, approximately 0.5% contained inserts corresponding to mRNAs present at higher levels in serum-stimulated BALB/c 3T3 cell cultures than in quiescent cultures. Most of these RNA species increased 2- to 5-fold, and the kinetics of increase for various RNAs differed. One clone (28H6) hybridized to a 1-kilobase RNA species that is present at barely detectable levels in resting cells but is increased at least 15- to 20-fold after serum stimulation, reaching a maximal level coincident with the onset of DNA synthesis. This RNA was at a high level in proliferating cells but decreased rapidly as cells reached confluence. 28H6 RNA was also increased in resting cells infected with simian virus 40 or stimulated with platelet-derived growth factor.

Identification of a BALB/c-3T3 cell protein modulated by platelet-derived growth factor

The platelet-derived growth factor (PDGF) stimulates density-arrested BALB/c-3T3 cells to synthesize a protein (pII; Mr, 35,000) that is constitutively synthesized by spontaneously transformed BALB/c-3T3 (ST2-3T3) cells which do not require PDGF for growth. Antisera against a major excreted protein family (MEP) of retrovirus-transformed cells quantitatively precipitated cellular pII. PDGF-stimulated pII has the same molecular weight, a similar charge, and similar antigenic determinants as authentic MEP isolated from ST2-3T3 or retrovirus-transformed cells. MEP represented about 2% of the nonnuclear proteins synthesized by ST2-3T3 cells and 0.3 to 0.6% of the proteins synthesized by PDGF-treated BALB/c-3T3 cells, a three- to sixfold increase over the background. In BALB/c-3T3 cells, less PDGF was required for pII (MEP) synthesis than for DNA synthesis. PDGF induced a selective increase in pII (MEP) within 40 min. Such preferential synthesis was inhibited by brief treatment with actinomycin D, suggesting a requirement for newly formed RNA. The constitutive synthesis of pII (MEP) by ST2-3T3 cells was not inhibited by actinomycin D. Five spontaneously or chemical carcinogen-transformed tumorigenic BALB/c-3T3 cell lines were studied; they neither required PDGF for growth nor responded to it. These cell lines became arrested at confluence with a G1 DNA content. Each of these independently isolated lines synthesized pII (MEP) constitutively. Thus, the synthesis of pII (MEP) may be required, but is not sufficient, for PDGF-modulated DNA synthesis.