Regulation of protein synthesis by phosphorylation of ribosomal protein S6 and aminoacyl-tRNA synthetases

Expression profiling of bioactive genes from Moringa oleifera

Plants are under constant assault by biotic and abiotic agents. When an elicitor is prologued, an immense reprogramming of plant gene expression and defense responses are initiated, which could be a natural source for potential drug development and insertional mutagenesis. In this regard, differential expression analysis of a medicinal plant Moringa oleifera was performed for bioactive genes at seedling stage, using differential display-RT-PCR technique. Infected seedlings with a fungus Fusarium solani collected at different time intervals, showed a massive change in their gene expression profile. The data analysis revealed that at least 150 pathogen-induced and about 60 suppressed genes were differentially expressed at 8-h postinoculation of the biotic stress. Fifty-five selective genes were disunited and reamplified. Sequence analysis of these potential genes illustrated that these genes had properties of some induced peroxidase mRNA, cell proliferation, others were mitogen activated protein kinases, ribosomal protein genes, defense regulating genes, and a few also had structural properties. Further studies about the utility of these genes in plant metabolism could assist to develop improved transgenic breeds with enhanced value of infection tolerance not only of M. oleifera but of other cultivars also.

Multifunctional glycoprotein receptors for insulin and the insulin-like growth factors

Insulin and the insulin-like growth factors (IGF) I and II are structurally related peptides that elicit a large number of similar biological effects in target cells. Three well-characterized receptor complexes bind one or more of these peptides with high affinity. Two of these receptors, denoted as type I, are ligand-activated tyrosine kinases with similar heterotetrameric alpha 2 beta 2 subunit structures which bind insulin or IGF-I, respectively, with highest affinity. Ligand-stimulated tyrosine autophosphorylation of these receptors further activates their intrinsic tyrosine kinase activities both in vitro and in intact cells. Rapid signal transduction follows such receptor autophosphorylation and tyrosine kinase activation, leading to increased serine phosphorylation of many cellular proteins and decreased serine phosphorylation of several others. Experiments in our laboratory have identified three distinct insulin-activated serine kinase activities in cell-free extracts that appear to account for the insulin-stimulated serine phosphorylation of the insulin receptor itself, ATP citrate lyase, and acetyl CoA carboxylase, respectively. A third receptor in this group binds IGF-I and II, lacks kinase activity and is denoted as type II IGF receptor. Amino acid sequences of this receptor deduced from isolated rat cDNA clones show a high degree of homology with those of the bovine cation-independent mannose 6-phosphate (Man-6-P) receptor. We demonstrated that these receptors are indeed identical. The IGF-II/Man-6-P receptor rapidly recycles between the cell surface membrane and intracellular membrane compartments, providing for the rapid uptake of both IGF-II and mannose 6-phosphate-linked lysosomal enzymes. Insulin action markedly increases the proportion of receptors in the plasma membrane and the uptake of bound ligands. We also observe that large amounts of the extracellular domain of the IGF-II/Man-6-P receptor are released into the serum of fetal, neonatal and adult rats. The biological role of this receptor in IGF-II function is yet to be determined.

Molecular signature linked to acquired resistance to cisplatin in esophageal cancer cells

To clarify the molecular basis of acquired cisplatin (CDDP) resistance in esophageal squamous cell carcinoma (ESCC), we used cDNA microarray technology. A CDDP-resistant cell line (YES-2/CDDP), which shows a 7.5-fold increase in resistance to CDDP and a 3-fold decrease in CDDP accumulation compared with the parental YES-2 ESCC cell line, was generated from YES-2 by exposure to increased concentrations of CDDP. By cDNA microarray analysis, we identified 44 genes with significantly different expression levels between YES-2/CDDP and YES-2 cells. Interestingly, 15 of these 44 genes encoded ribosome-related proteins, almost all of which were underexpressed in YES-2/CDDP cells. Our present data suggest that many ribosome-related genes may be involved in the acquired resistance to CDDP in ESCC and that such information may allow us to better understand the mechanism of CDDP resistance.

Towards the cellular functions of tumour suppressors

Many of the documented changes in cellular DNA that occur during tumour development involve activation of proto-oncogenes, but newer evidence has shown that oncogenesis can involve loss or inactivation of a different group of genes, called tumour suppressor genes (TSGs). Molecular analysis of TSGs is revealing that their protein products are involved in cell adhesion, signal transduction, transcription, translation and cell cycle control. Surprisingly, most of the TSG products had not been previously identified in studies of normal cells, so their analysis is contributing not only to our understanding of oncogenesis, but also to basic cell biology. The 'comment' articles in this issue discuss progress towards understanding the cellular functions of TSG products.

Purification and characterization of a dimer form of the cAMP-dependent protein kinase from mouse liver cytosol

A protein kinase that phosphorylates histones and polysomal proteins was partially purified from mouse liver cytosol. The active enzyme has a molecular mass of 100 kDa and a phosphorylatable subunit of 54 kDa. Biochemical as well as immunological data suggest that the enzyme is a heterodimer composed of the catalytic subunit of cyclic AMP-dependent protein kinase and the RII regulatory subunit. This RC form does not seem to dissociate upon activation with 3', 5' cyclic AMP and exhibits identical specificity as the classical cAMP-dependent protein kinase (2.7.1.37). The enzyme is affected by the 3', 5' cyclic phosphates of adenosine mainly, but also of guanosine, uridine and cytidine in a substrate-dependent manner. Cyclic nucleotides slightly stimulate phosphate incorporation into histones, while phosphorylation of polysomal proteins in intact polysomes is dramatically increased. The substrate- specific stimulatory effects of 3', 5' cyclic nucleotides are due to repression of the inhibition exerted upon the reaction, by negatively charged macromolecules such as RNA, DNA and to a lesser extent heparin.

Regulation of translational initiation during cellular responses to stress

Chemicals and conditions that damage proteins, promote protein misfolding, or inhibit protein processing trigger the onset of protective homeostatic mechanisms resulting in "stress responses" in mammalian cells. Included in these responses are an acute inhibition of mRNA translation at the initiation step, a subsequent induction of various protein chaperones, and the recovery of mRNA translation. Separate, but closely related, stress response systems exist for the endoplasmic reticulum (ER), relating to the induction of specific "glucose-regulated proteins" (GRPs), and for the cytoplasm, pertaining to the induction of the "heat shock proteins" (HSPs). Activators of the ER stress response system, including Ca(2+)-mobilizing and thiol-reducing agents, are discussed and compared to activators of the cytoplasmic stress system, such as arsenite, heavy metal cations, and oxidants. An emerging integrative literature is reviewed that relates protein chaperones associated with cellular stress response systems to the coordinate regulation of translational initiation and protein processing. Background information is presented describing the roles of protein chaperones in the ER and cytoplasmic stress response systems and the relationships of chaperones and protein processing to the regulation of mRNA translation. The role of chaperones in regulating eIF-2 alpha kinase activities, eIF-2 cycling, and ribosomal loading on mRNA is emphasized. The putative role of GRP78 in coupling rates of translation to processing is modeled, and functional relationships between the HSP and GRP chaperone systems are discussed.

Inhibition of translational initiation by activators of the glucose-regulated stress protein and heat shock protein stress response systems. Role of the interferon-inducible double-stranded RNA-activated eukaryotic initiation factor 2alpha kinase

Depletion of endoplasmic reticulum (ER) Ca2+ perturbs protein folding and processing within the organelle while inhibiting translational initiation through activation of the double-stranded RNA-activated eukaryotic initiation factor (eIF)-2alpha kinase (PKR) (Prostko, C. R., Dholakia, J. N., Brostrom, M. A., and Brostrom, C. O. (1995) J. Biol. Chem. 270, 6211-6215). The glucose-regulated stress protein (GRP) chaperones are subsequently induced. We now report that sodium arsenite, a prototype for stressors fostering cytoplasmic protein misfolding, also inhibits translational initiation through activation of PKR while subsequently inducing the heat shock protein (HSP) chaperones. Arsenite neither mobilized ER-associated Ca2+ nor slowed peptide chain elongation. Various HSP-inducing chemicals caused rapid phosphorylation of eIF-2alpha. When incubated with double-stranded RNA, extracts derived from arsenite-treated cells displayed greater degrees of phosphorylation of PKR and eIF-2alpha than did control extracts. Cells overexpressing a dominant negative PKR mutation resisted translational inhibition and eIF-2alpha phosphorylation in response to ER or cytoplasmic stressors. Induction of either the HSP or GRP chaperones was accompanied by development of translational tolerance to either Ca2+-mobilizing agents or arsenite. Following induction of the HSPs by arsenite, cells remained susceptible to induction of the GRPs by Ca2+-mobilizing agents. Conversely, cells possessing induced GRP contents in response to Ca2+-mobilizing agents readily induced the HSPs in response to arsenite. It is concluded that the two chaperone systems function independently except for their mutual suppression of PKR.

A membrane-bound protein kinase from rabbit reticulocytes is an active form of multipotential S6 kinase

An active ribosomal protein S6 kinase has been highly purified from the membranes of rabbit reticulocytes by chromatography of the Triton X-100 extract on DEAE-cellulose, SP-Sepharose Fast Flow, and by FPLC on Mono Q and Superose-12. The S6 kinase elutes around 40 000 daltons upon gel filtration on Superose-12 or Sephacryl S-200. It has a subunit molecular weight of 40-43 kDa as determined by protein kinase activity following denaturation/renaturation in SDS-polyacrylamide gels containing S6 peptide. It also phosphorylates translational initiation factors eIF-2 and eIF-4F, glycogen synthase, histone 1, histone 2B, myelin basic protein, but not prolactin, skeletal myosin light chain, histone 4, tubulin, and casein. Apparent Km values have been determined to be 15 microM for ATP, 1.2 microM for S6 and 10 microM for S6 peptide. Two-dimensional tryptic phosphopeptide mapping shows the same sites on S6 are phosphorylated as those identified previously with proteolytically activated multipotential S6 kinase from rabbit reticulocytes, previously denoted as protease activated kinase II. Examination of relative rates of phosphorylation and kinetic constants of synthetic peptides based on previously identified phosphorylation sites, indicates a minimum substrate recognition sequence to be arginine at the n - 3 position. Based on these characteristics, including molecular weight and an expanded substrate specificity, the membrane S6 kinase can be distinguished from the p90 (Type I) and p70 (Type II) S6 kinases, and from protein kinase C and the catalytic subunit of cAMP-dependent protein kinase.

Nuclear and nucleolar targeting of human ribosomal protein S6

Chimeric proteins were constructed to define the nuclear localization signals (NLSs) of human ribosomal protein S6. The complete cDNA sequence, different cDNA fragments and oligonucleotides of the human ribosomal proteins S6, respectively, were joined to the 5' end of the entire LacZ gene of Escherichia coli by using recombinant techniques. The hybrid genes were transfected into L cells, transiently expressed, and the intracellular location of the fusion proteins was determined by their beta-galactosidase activity. Three NLSs were identified in the C-terminal half of the S6 protein. Deletion mutagenesis demonstrated that a single NLS is sufficient for targeting the corresponding S6-beta-galactosidase chimera into the nucleus. Removal of all three putative NLSs completely blocked the nuclear import of the resulting S6-beta-galactosidase fusion protein, which instead became evenly distributed in the cytoplasm. Chimeras containing deletion mutants of S6 with at least one single NLS or unmodified S6 accumulated in the nucleolus. Analysis of several constructs reveals the existence of a specific domain that is essential but not sufficient for nucleolar accumulation of S6.

The mammalian RPS6 gene, homolog of the Drosophila air8 tumor suppressor gene: is it an oncosuppressor gene?

The mammalian gene encoding the S6 ribosomal protein is the homolog of the Drosophila air8 tumor suppressor gene. We assigned the rat Rps6 gene to chromosome 5q22-33. The rat 5q22-33 chromosome region, previously shown to bear a malignant transformation suppressor gene, is homologous to the human 9p2l region, frequently deleted in various kinds of cancers and also containing at least one tumor suppressor (oncosuppressor) gene. To test the possibility that the Rps6 gene could be an oncosuppressor gene in mammals, we analysed its sequence and expression in normal and malignantly transformed cells. In mouse hepatoma cells (BWTG3), the Rps6 gene is hemizygously deleted but the remaining copy shows no sequence anomaly in the coding region, indicating that Rps6 is not oncosuppressor and that another gene acting as an oncosuppressor is located in its vicinity. In human tumor cells, the RPS6 gene is retained in cells showing deletion of the near-by gene, IFNB. Our results do not support the possibility that the RPS6 gene acts as an oncosuppressor gene in mammalian cells.

Lysyl-tRNA synthetase

Lysyl-tRNA synthetase catalyses the formation of lysyl-transfer RNA, Lys-tRNA(Lys), which then is ready to insert lysine into proteins. Lysine is important for proteins since it is one of only two proteinogenic amino acids carrying an alkaline functional group. Seven genes of lysyl-tRNA synthetases have been localized in five organisms, and the nucleotide and the amino acid sequences have been established. The lysyl-tRNA synthetase molecules are of average chain lengths among the aminoacyl-tRNA synthetases, which range from about 300 to 1100 amino acids. Lysyl-tRNA synthetases act as dimers; in eukaryotes they can be localized in multienzyme complexes and can contain carbohydrates or lipids. Lysine tRNA is recognized by lysyl-tRNA synthetase via standard identity elements, namely anticodon region and acceptor stem. The aminoacylation follows the standard two-step mechanism. However the accuracy of selecting lysine against the other amino acids is less than average. The first threedimensional structure of a lysyl-tRNA synthetase worked out very recently, using the enzyme from the Escherichia coli lysU gene which binds one molecule of lysine, is similar to those of other class II synthetases. However, none of the reaction steps catalyzed by the enzyme is clarified to atomic resolution. Thus surprising findings might be possible. Lysyl-tRNA synthetase and its precursors as well as its substrates and products are targets and starting points of many regulation circuits, e.g. in multienzyme complex formation and function, dinucleoside polyphosphate synthesis, heat shock regulation, activation or deactivation by phosphorylation/dephosphorylation, inhibition by amino acid analogs, and generation of antibodies against lysyl-tRNA synthetase. None of these pathways is clarified completely.

S6 phosphorylation results from prothoracicotropic hormone stimulation of insect prothoracic glands: a role for S6 kinase

The insect prothoracic glands are the source of steroidal molting hormone precursors and the glands are stimulated by a brain neuropeptide, prothoracicotropic hormone (PTTH). Previous work from this laboratory revealed that PTTH acts via a cascade including Ca2+/calmodulin activation of adenylate cyclase, protein kinase A, and the subsequent phosphorylation of a 34 kDa protein (p34) hypothesized, but not proven, to be the S6 protein of the 40S ribosomal subunit. The immunosuppressive macrolide, rapamycin, is a potent inhibitor of cell proliferation, a signal transduction blocker, and also prevents ribosomal S6 phosphorylation in mammalian systems. We demonstrate here that rapamycin inhibited PTTH-stimulated ecdysteroidogenesis in vitro by the prothoracic glands of the tobacco hornworm, Manduca sexta, with half-maximal inhibition at a concentration of about 5 nM. At concentrations above 5 nM, there was a 75% inhibition of ecdysteroid biosynthesis. Similar results were observed with the calcium ionophore (A23187), a known stimulator of ecdysteroidogenesis. Most importantly, the inhibition of ecdysteroid biosynthesis was accompanied by the specific inhibition of the phosphorylation of p34, indicating that p34 indeed is ribosomal protein S6. In vivo assays revealed that injection of rapamycin into day 6 fifth instar larvae resulted in a decreased hemolymph ecdysteroid titer and a dose-dependent delay in molting and metamorphosis. When S6 kinase (S6K) activity was examined using rapamycin-treated prothoracic glands as the enzyme source and a synthetic peptide (S6-21) or a 40S ribosomal subunit fraction from Manduca tissues as substrate, the date revealed that rapamycin inhibited S6K activity.(ABSTRACT TRUNCATED AT 250 WORDS)

The nucleotide sequence of a cloned cDNA encoding ribosomal protein S6 from Drosophila melanogaster

A cDNA containing the complete coding sequence for a homolog of eukaryotic ribosomal protein S6 was isolated from a library derived from mRNAs expressed in embryos of Drosophila melanogaster. The cloned cDNA would encode a protein of 248 amino acid residues displaying 74.5% identity with its human homolog.

Drosophila homolog of the human S6 ribosomal protein is required for tumor suppression in the hematopoietic system

The tumor suppressor gene lethal(1)aberrant immune response 8 (air8) of Drosophila melanogaster encodes a homolog of the human S6 ribosomal protein. P element insertions that prevent expression of this gene cause overgrowth of the lymph glands (the hematopoietic organs), abnormal blood cell differentiation, and melanotic tumor formation. They also cause delayed development, inhibit growth of most of the larval organs, and lead to larval lethality. Mitotic recombination experiments indicate that the normal S6 gene is required for clone survival in the germ line and imaginal discs. The S6 gene produces a 1.1-kilobase transcript that is abundant throughout development in wild-type animals and in revertants derived from the insertional mutants but is barely detectable in the mutant larvae. cDNAs corresponding to this transcript show a 248-amino acid open reading frame with 75.4% identity and 94.8% similarity to both human and rat S6 ribosomal protein sequences. The results reveal a regulatory function of this ribosomal protein in the hematopoietic system of Drosophila that may be related to its developmentally regulated phosphorylation.

Signal transduction and regulation of translation initiation

Regulation of the rate of protein synthesis is important in the control of cellular proliferation. Changes in the rate of protein translation are brought about primarily at the level of initiation, which is usually rate limiting. This regulation involves the reversible phosphorylation of key initiation factors. Translation initiation factors eIF-4F, eIF-4B, and ribosomal protein S6 are phosphorylated in response to a wide variety of mitogens, growth factors, and tyrosine kinase oncogenes. Thus, translation initiation factors are important components of signal transduction pathways activated by extracellular factors and oncogenes. Of particular interest is the messenger RNA 5' cap-binding protein, eIF-4E. Overexpression of eIF-4E in fibroblasts results in malignant transformation, suggesting that it is an important transducer of growth signals, and that aberrant expression of a translation factor can cause malignancy. Elucidation of the components of the signalling pathways which regulate initiation factor activity should increase our understanding of how extracellular factors and oncogenes effect cellular proliferation, and the role that translation plays in this process.

Monoclonal antibody to the human insulin receptor, but not insulin, stimulates S6 kinase via human insulin receptors mutated at three major tyrosine autophosphorylation sites

Studies were carried out to examine the role of the major insulin receptor tyrosine autophosphorylation sites in stimulation of S6 kinase activity. For these studies, we employed HTC rat hepatoma cells transfected with and expressing human insulin receptors. In cells transfected with and expressing a large number of normal human insulin receptors (HTC-IR cells), the sensitivity of cells to insulin to stimulate S6 kinase was increased tenfold when compared to untransfected wild type HTC cells (HTC-WT cells). However, in cells transfected with and expressing a large number of mutated human insulin receptors where the tyrosines at three major autophosphorylation sites (1158, 1162, and 1163) were mutated to phenylalanines (HTC-F3 cells), there was no change in insulin sensitivity when compared to HTC-WT cells. We next studied the effect of a human-specific monoclonal antibody to the human insulin receptor, MA-5, on S6 kinase activation. In HTC-WT cells, MA-5 did not interact with endogenous rat insulin receptors and thus did not stimulate S6 kinase. In HTC-IR cells expressing normal human insulin receptors, MA-5 stimulated S6 kinase. Interestingly, MA-5, unlike insulin, was also able to stimulate S6 kinase in HTC-F3 cells expressing mutated receptors. In order to further understand the signaling mechanisms by MA-5 and insulin, two potential intermediate protein kinases were investigated. Neither insulin nor MA-5 appears to activate either microtubule-associated protein 2 (MAP-2) kinase or protein kinase C in these cells. These studies suggest therefore that: 1) insulin and MA-5 may signal S6 kinase activation by independent mechanisms that do not employ either MAP-2 kinase or protein kinase C; and 2) under certain circumstances, S6 kinase appears to be activated by mechanisms that are independent of insulin receptor tyrosine autophosphorylation.

Polyamines modulate multiple protein phosphorylation pathways in the insect prothoracic gland

Multiple endogenous substrates phosphorylated by four distinct protein kinases were identified in particulate and cytosolic fractions from the larval prothoracic gland of the tobacco hornworm, Manduca sexta. Three prominent particulate-associated phosphoprotein substrates (19, 21, and 34 kDa) were of particular interest. The in vitro phosphorylation of the 19 and 21 kDa peptides was markedly enhanced by cAMP, Ca2+/calmodulin, as well as Ca2+/phospholipids, presumably via cAMP-dependent protein kinase (cAMP-PK), Ca2+/calmodulin-dependent protein kinase (Ca2+/CaM-PK), and protein kinase C (PKC), respectively. The polyamine spermine markedly inhibits both PKC- and cAMP-PK-mediated phosphorylation of the 19 and 21 kDa peptides but had no effect on the Ca2+/CaMP-PK-mediated phosphorylation. Spermine also inhibits the phosphorylation of the 34 kDa peptide via cAMP-PK but does not affect PKC-promoted phosphorylation. In contrast to this differential inhibition of phosphorylation by a polyamine, four cytosolic and three particulate-associated peptides from the prothoracic glands undergo enhanced phosphorylation in the presence of spermine, presumably by stimulating casein kinase II activity. Therefore, polyamines appear to have multiple effects on protein phosphorylation pathways in this important endocrine gland, perhaps representing an important new regulatory control mechanism.

A comparative study of microsomal and cytosolic S6 phosphatase activities in rat liver

Spontaneous S6 phosphatase activities dephosphorylating Ser(P)-235 and Ser(P)-236 of the ribosomal protein S6 were measured and compared in microsomes and cytosol of rat liver. The substrate used, small (40S) ribosomal subunits 32P-labelled in vitro by protein kinase A, contained phosphorylated S6 (mainly in the diphosphorylated form) and some minor phosphorylated species. The microsomal and cytosolic S6 phosphatase activities displayed a number of distinct properties. The microsomal activity, representing ca 20% of the S6 phosphatase activity in the post-mitochondrial supernatant, was mainly due to a type-1 phosphatase and dephosphorylated only S6. The remaining post-mitochondrial S6 phosphatase activity, which was fully recovered in the cytosol, and appeared to result from a combination of type-1 (43%) and type 2 (57%) phosphatases, acted on S6 as well as on the minor phosphorylated species. The microsomal activity was 50% inhibited by MgCl2 (10 mM) and was stimulated at least 4.3 fold by MnCl2 (1 mM), while the cytosolic activity was inhibited only 18% by Mg2+ (10 mM) and was increased 2.2 fold by Mn2+ (1 mM). The microsomal activity was increased 10% (P less than 0.06) by lower doses of insulin (25 U/Kg) and 14% (P less than 0.05) by vanadate, but was not significantly (P greater than 0.10) affected by larger doses of insulin (100 U/kg), hepatectomy or cycloheximide. By comparison the cytosolic S6 phosphatase activity was unresponsive to insulin and vanadate, but was decreased 14% and 17% (P less than 0.05) by hepatectomy and cycloheximide.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced in vitro phosphorylation of synapsin I (site 1) in Alzheimer's disease postmortem tissues

Homogenates prepared from the temporal cortex and hippocampus of individuals who had histopathologically confirmed Alzheimer's disease exhibited reduced in vitro cyclic AMP-dependent phosphorylation of synapsin I, neuronal phosphoprotein. One specific phosphorylation site (site 1) was affected while two other sites, which are phosphorylated by calcium/calmodulin kinase II, exhibited no such differences. Other phosphoproteins such as pyruvate dehydrogenase, did not show these differences. The reductions were not observed in either cerebellum or thalamus of Alzheimer's disease brain. Analysis by immunoblots indicated that the reductions were not caused by a decrease in absolute amounts of the protein. The reduced AD synapsin I phosphorylation was not overcome by the addition of purified cyclic AMP-dependent protein kinase. No differences were detected in total cyclic AMP-dependent protein kinase activity between the control and Alzheimer samples. However, dephosphorylation of the synapsin I prior to the in vitro phosphorylation reversed the differences observed between the control and AD homogenates. Thus, the reduced in vitro phosphorylation of the synapsin I in the Alzheimer homogenate reflects a reduced phosphorylatability of the protein due to either an increased phosphate content or some other alteration of the phosphorylation site.

A ribosomal protein S6 kinase copurifies with phosphatase-1 activating factor

A highly purified preparation of phosphatase-activating kinase (Fa) from rabbit skeletal muscle phosphorylated ribosomal protein S6. The two activities copurified on DEAE-Sephadex, CM-Sephadex, and phosphocellulose chromatography and upon further chromatography on Sephacryl S-300 and FPLC Mono-S and Mono-Q columns. On the latter column, two separate peaks of Fa activity were observed when it was developed in Tris buffer as opposed to beta-glycerophosphate. S6 kinase activity was obtained only with the Fa which adhered to the resin. The Mr of the Fa and S6 activities was determined to be 83,200 by gel permeation on a Sephacryl S-300 column. The Fa preparation phosphorylated serine residues on S6; two tryptic phosphopeptides, A and C, were identified by two-dimensional phosphopeptide analysis. The enzyme also showed good activity toward initiation factor eIF-4B. Based on specificity toward ribosomal proteins and initiation factors, the Fa and a mitogen-stimulated S6 kinase purified from insulin-stimulated 3T3-L1 cells were similar. These results suggest that a form of Fa and an insulin-stimulated S6 kinase may be related or closely associated.

Rapid induction of polyadenylate binding protein and stimulation of translational initiation in pituitary tumor cells exposed to phorbol ester

1. GH3 pituitary cells treated for 1-2 hr with phorbol myristate acetate exhibited accumulation of large polysomes and increased incorporation of amino acids into all discrete protein populations. 2. Preferential incorporation into a basic 74 kDa polypeptide preceded significant augmentation of protein synthesis. Cellular content of this polypeptide correlated directly with the increase in protein synthesis. 3. Stimulations of incorporation, of polysome accumulation, and of preferential synthesis of the 74 kDa protein were eliminated by inhibitors of transcription. 4. The rapidly induced protein was identical with the ubiquitous polyadenylate-binding protein on the bases of size, isoelectric point, distribution with polysomes, and association with poly(A) + mRNA.

The biology of Coxiella burnetti and the pathobiochemistry of Q fever and its endotoxicosis

C. burnetii possesses a battery of host-independent enzymes which mediate endergonic and exergonic reactions. The biochemical and biophysical lesions responsible for the organism's obligate intracellular parasitic state have not been identified. Clues to this fundamental problem may lie in the agent's acidophilic metabolism and proliferation within the host cell's phagolysomal vacuole. What are the modifiers of transcription and translation? What constituents of the lysosomal vacuole contribute to the parasite's metabolism? Does the parasite have intrinsic cell-wall synthesizing capability? The isolation of plasmids from C. burnetii opens lines of investigation which should lead to solution of the problem, thereby helping to answer the general question of what is the nature of obligate parasitism. An understanding of the pathobiochemistry of Q fever and of its endotoxicosis rests on elucidating closely interrelated regulatory events. Stimulated hepatic transcription and translation of certain RNA and protein species attend the development of the disease, as do phosphorylation and dephosphorylation of central RNA and protein species. Phosphorylation and dephosphorylation are central regulators in protein synthesis. Whole animal experiments differ from those with cultured cells in important responses. Glycogenolysis, hepatic steatogenesis, and lipase activation are obvious examples of such differences. Stimulation of the production of lymphokines and of hormones is absent in HepG2 cells, and insulin seems to be critical in regulating the phosphorylation-dephosphorylation equilibrium which leads to regulation of protein synthesis. Newly synthesized, so far unidentified proteins may be involved in convalescence, re-establishing the homeostasis of the uninfected state. The model of pathobiochemical regulation in Q fever and endotoxicosis may be applicable to other febrile infections and endotoxicoses.

Does protein phosphorylation play a role in translational control by eukaryotic aminoacyl-tRNA synthetases?

In addition to their primary role in tRNA charging, aminoacyl-tRNA synthetases can regulate protein synthesis in eukaryotic cells. Although the phosphorylation of these enzymes themselves has little effect on their catalytic activity, there may be a role for protein phosphorylation in mediating their regulatory effects.

TPA stimulates S6 phosphorylation but not protein synthesis in Ehrlich cells

Increased phosphorylation of ribosomal protein S6 has been extensively correlated with an increased rate of protein synthesis. We report here that under two separate conditions in Ehrlich cells an increase in the level of S6 phosphorylation does not result in any increase in the rate of protein synthesis. 1) In glutamine-deprived cells TPA stimulates S6 phosphorylation but has no effect on the rate of protein synthesis, 2) In cells deprived of serum growth factors, addition of serum stimulates both S6 phosphorylation and protein synthesis while TPA stimulates only S6 phosphorylation. These results show that increased phosphorylation of S6 is not sufficient to cause increased rates of protein synthesis, and suggest that additional factors may play a more direct role.

Early events in peptide-stimulated ecdysteroid secretion by the prothoracic glands of Manduca sexta

The prothoracic glands of the tobacco hornworm, Manduca sexta, have been an advantageous model for investigating the cellular mechanisms underlying hormone-stimulated ecdysteroid secretion in insects. The cerebral neuropeptide prothoracicotropic hormone (PTTH) is currently thought to activate the prothoracic glands via a calcium-dependent increase in cAMP synthesis, activation of cAMP-dependent protein kinase, and protein phosphorylation (Gilbert et al.: Bioessays, 8:153-158, '88). The present paper discusses current research regarding early changes in cell function elicited by PTTH, with emphasis on the regulation of cAMP synthesis and degradation and the involvement of translational events in PTTH action.

Mitogen-responsive S6 kinase

Many cell lines respond to mitogenic stimuli (serum, growth factors) with rapid phosphorylation of the ribosomal protein S6 at several serine sites. We have tried to identify the protein kinase(s) mediating this effect of growth stimuli. Examining post-DEAE chromatography fractions of S49 kin- cell extracts, we could detect a highly active effector-independent S6 kinase with specificity for serine residues. The study was extended to the presumably homologous human enzyme, using HeLa S3 cells as model system. Activity yields increased up to sevenfold when exhausted HeLa cells were supplied with fresh medium plus serum. The enzyme uses ATP, not GTP, as cosubstrate, 40-S or 80-S (reassociated from subunits) ribosomal particles being substrate. The optimal K+ concentration, measured at 3 mM Mg2+, is 35 mM. Under optimized assay conditions S6 phosphorylation proceeded faster in vitro than it appeared to do in vivo. The apparent Mr of the enzyme, as estimated by gel filtration on Sephadex G-100, is 56,000 (determination in the presence of 200 mM KCl in 25 mM phosphate buffer). Tighter binding to DEAE-Sephacel and higher specificity for S6 distinguishes this enzyme from the following S6-phosphorylating protein kinases: protein kinase C, protease-activated kinase II, histone-4 phosphotransferase and an enzyme with the properties of casein kinase I. In published summaries of observations shown here and in a follow-up study with chick embryo fibroblasts, the enzyme(s) has been referred to as mitogen-responsive S6 kinase(s) [Martini, O. H. W. and Lawen, A. (1985) in Hormones and cell regulation (Dumont, J. E., Hamprecht, B. and Nunez, J., eds) vol. 9, pp. 411-412, Elsevier Company, North-Holland, Amsterdam; Lawen, A. and Martini, O. H. W. (1985) FEBS Lett. 185, 272-276].

Insulin-induced S6 kinase activation in HeLa cells and its reversal by hyperthermic stress

Insulin treatment of HeLa S3 cells activates an S6-phosphorylating protein kinase. Although this enzyme has chromatographic properties resembling those of described proteolytic fragments of other protein kinases, namely protein kinase C, protease-activated kinase II and histone-4 protein kinase, and although insulin has been proposed by others to cause S6 phosphorylation via proteolytic protein kinase activation, the insulin-induced increase in S6-kinase activity described here is probably not due to proteolysis. Rather, the activity indicates the existence, in HeLa cells, of an interconvertible S6 kinase, since the insulin-induced activity increase was rapidly reversed under hyperthermic stress, and since this effect of hyperthermia was itself reversible. The S6-kinase activities from serum- and from insulin-stimulated HeLa cells resemble each other closely and are likely to represent the same enzyme. The enzyme may therefore mediate both signals delivered by mitogens and the insulin signal. Analysed at an in vitro transfer of 1 mol phosphate/mol S6, this S6 kinase activity does not phosphorylate the (principal) S6 site recognized by the cAMP-dependent protein kinase.

Polyamines differentially inhibit cyclic AMP-dependent protein kinase-mediated phosphorylation in the brain of the tobacco hornworm, Manduca sexta

The effects of the naturally occurring polyamines spermine and spermidine on phosphorylation promoted by cyclic AMP (cAMP)-dependent protein kinase (PK) (cAMP-PK; EC 2.7.1.37) were studied using the brain of the tobacco hornworm, Manduca sexta. Four particulate-associated peptides (280, 34, 21, and 19 kilodaltons) in day 1 pupal brains are endogenous substrates for a particulate type II cAMP-PK. These phosphoproteins are present in brain synaptosomal, as well as microsomal, particulate fractions but are not present in the cytosol. They are distributed throughout the CNS and PNS and are present in several nonneuronal tissues as well. Phosphorylation of these proteins via cAMP-PK was inhibited markedly by micromolar concentrations of spermine and spermidine. Other particulate-associated peptides phosphorylated via a Ca2+/calmodulin-PK or Ca2+ and cAMP-independent PKs were unaffected by polyamines, whereas the phosphorylation of a 260-kilodalton peptide was markedly enhanced. Spermine did not exert its inhibitory effect indirectly by enhancement of cAMP or ATP hydrolysis or via proteolysis, but its action appears to involve a substrate-directed inhibition of cAMP-PK-promoted phosphorylation as well as enhanced dephosphorylation. Although addition of spermine resulted in marked ribosome aggregation in synaptosomal and microsomal particulate fractions, this phenomenon was not involved in the inhibition of cAMP-PK-promoted phosphorylation.

Insulin-stimulated protein kinase activity in rat skeletal muscle that phosphorylates ribosomal protein S6

Treatment of rats with a single high dose of insulin leads to rapid stimulation of cytosolic protein kinase activity in skeletal muscle that phosphorylates ribosomal protein S6. This stimulation is maximal within 15 minutes after insulin treatment, and the activity remains elevated for at least 90 minutes. The insulin-stimulated protein kinase activity elutes as two peaks from DEAE-Sepharose. Peak I elutes at 0.04-0.06 M KCl and is stimulated by insulin approximately 1.4-fold above the control. Peak II elutes at 0.09-0.11 M KCl and is stimulated 2.8-fold above the control. The peak II activity, which is most strongly stimulated by insulin, is resolved from cyclic AMP-dependent protein kinase on DEAE-Sepharose and appears to be distinct from protein kinase C. These results represent a novel finding of the stimulation of S6 kinase activity by insulin in skeletal muscle tissue in vivo.