Ribosome conformational changes associated with protein S6 phosphorylation

The Importance of Protein Phosphorylation for Signaling and Metabolism in Response to Diel Light Cycling and Nutrient Availability in a Marine Diatom

Diatoms are major contributors to global primary production and their populations in the modern oceans are affected by availability of iron, nitrogen, phosphate, silica, and other trace metals, vitamins, and infochemicals. However, little is known about the role of phosphorylation in diatoms and its role in regulation and signaling. We report a total of 2759 phosphorylation sites on 1502 proteins detected in Phaeodactylum tricornutum. Conditionally phosphorylated peptides were detected at low iron (n = 108), during the diel cycle (n = 149), and due to nitrogen availability (n = 137). Through a multi-omic comparison of transcript, protein, phosphorylation, and protein homology, we identify numerous proteins and key cellular processes that are likely under control of phospho-regulation. We show that phosphorylation regulates: (1) carbon retrenchment and reallocation during growth under low iron, (2) carbon flux towards lipid biosynthesis after the lights turn on, (3) coordination of transcription and translation over the diel cycle and (4) in response to nitrogen depletion. We also uncover phosphorylation sites for proteins that play major roles in diatom Fe sensing and utilization, including flavodoxin and phytotransferrin (ISIP2A), as well as identify phospho-regulated stress proteins and kinases. These findings provide much needed insight into the roles of protein phosphorylation in diel cycling and nutrient sensing in diatoms.

Point mutation in essential genes with loss or mutation of the second allele: relevance to the retention of tumor-specific antigens

Antigens that are tumor specific yet retained by tumor cells despite tumor progression offer stable and specific targets for immunologic and possibly other therapeutic interventions. Therefore, we have studied two CD4(+) T cell-recognized tumor-specific antigens that were retained during evolution of two ultraviolet-light-induced murine cancers to more aggressive growth. The antigens are ribosomal proteins altered by somatic tumor-specific point mutations, and the progressor (PRO) variants lack the corresponding normal alleles. In the first tumor, 6132A-PRO, the antigen is encoded by a point-mutated L9 ribosomal protein gene. The tumor lacks the normal L9 allele because of an interstitial deletion from chromosome 5. In the second tumor, 6139B-PRO, both alleles of the L26 gene have point mutations, and each encodes a different tumor-specific CD4(+) T cell-recognized antigen. Thus, for both L9 and L26 genes, we observe "two hit" kinetics commonly observed in genes suppressing tumor growth. Indeed, reintroduction of the lost wild-type L9 allele into the 6132A-PRO variant suppressed the growth of the tumor cells in vivo. Since both L9 and L26 encode proteins essential for ribosomal biogenesis, complete loss of the tumor-specific target antigens in the absence of a normal allele would abrogate tumor growth.

Intracellular messengers and the control of protein synthesis

The molecular events responsible for controlling cell growth and development, as well as their coordinate interaction is only beginning to be revealed. At the basis of these controlling events are hormones, growth factors and mitogens which, through transmembrane signalling trigger an array of cellular responses, initiated by receptor-associated tyrosine kinases, which in turn either directly or indirectly mediate their effects through serine/threonine protein kinases. Utilizing the obligatory response of activation of protein synthesis in cell growth and development, we describe efforts to work backwards along the regulatory pathway to the receptor, identifying those molecular components involved in modulating the rate of translation. We begin by describing the components and steps of protein synthesis and then discuss in detail the regulatory pathways involved in the mitogenic response of eukaryotic cells and during meiotic maturation of oocytes. Finally we discuss possible future work which will further our understanding of these systems.

Influence of sulphate ions on the structure of AA amyloid fibrils

To explore the possible interaction of sulphated GAG with AA amyloid peptides, human AA amyloid fibrils were exposed to buffers containing various salts, and the accessibility of free amino groups on the peptides to reductive methylation was examined. Sodium chloride had little effect except at concentrations of 1 M, where it reduced the accessibility of AA peptides to labelling. In contrast 70 mM Na2SO4 led to a significant increase in peptide accessibility to labelling. The results suggest that, at least in part, GAG interact with AA peptides through their sulphate moieties.

Inhibition of the initiation of hepatic protein synthesis during ethionine mediated ATP depletion in vivo: modification to ribosomal subunits, evidence of impaired ternary complex formation and a subcellular redistribution of eIF-2 alpha

Acute ethionine intoxication is known to induce a reversible hepatic injury in female rats by reducing the level of hepatic ATP. The injury indirectly impairs the initiation of hepatic protein synthesis, with resultant polysome disaggregation. Administration of adenine rapidly restores the ATP levels and protein synthesis. Analysis of liver polysome and ribosomal subunits reveals that polysome disaggregation occurs following 3 h of the intoxication, and reaggregation occurs following the administration of adenine. Inactive hepatic ribosomes accumulate as monomers and disomes when analysed by sucrose gradient sedimentation in low-salt buffers. High-salt buffers dissociate the inactive ribosomes into the component 40 S and 60 S subunits. The level of higher density, 1.48 g/cc, 40 S subunit increases during the inhibition of protein synthesis, while the lower density, 1.41 g/cc, 40 S subunit species does not change significantly. Hepatic microsomal and cytosolic extracts examined for their ability to support the formation of the ternary complex of eIF-2-GTP and [35S]Met-tRNAi demonstrate that during acute ethionine intoxication, ternary complex formation in the two extracts decrease 65% and 85%, respectively. These changes are coincident with polysome disaggregation. Administration of adenine to reverse the intoxication restores the ternary complex forming ability of the cytosolic extract, but does not affect the activity of the microsomal salt wash extracts. Mixing experiments indicate the accumulation of an inhibitor of ternary complex formation in the microsomal salt wash fraction. The application of quantitative western blotting demonstrates that the level of antigenic eIF-2 alpha in the microsomal salt wash extract increases 31% during the inhibition. These observations are consistent with the idea that the inhibition of the initiation of hepatic protein synthesis induced by ethionine is mediated by eIF-2 alpha phosphorylation. The latter results in an inhibition of ternary complex formation, redistribution of eIF-2 to the microsome fraction, polysomal disaggregation, and accumulation of inactive ribosomal subunits.

Ribosome and protein synthesis modifications after infection of human epidermoid carcinoma cells with herpes simplex virus type 1

Modifications of ribosomes have been investigated in human epidermoid carcinoma-2 cells at different stages of herpes simplex virus type 1 infection. Very early in infection, there is an increase in ribosomal protein S6 phosphorylation even in the absence of serum. The same result is obtained in the presence of actinomycin D. At early infection time, ribosomal proteins S2, S3a and Sa are newly phosphorylated. At early and early-late times, three phosphorylated non-ribosomal proteins (v1, v2 and v3) are differently associated temporally to ribosomes. Analyses of proteins extracted from 40S subunits, 80S ribosomes and polysomes show that v1 and v2 are distributed differently among the different ribosomal populations. S6 phosphopeptides were found to be identical after serum stimulation and after viral infection. In every case phosphoserine and phosphothreonine were identified in S6. Only phosphoserine was found in other phosphorylated proteins. Our results indicate that herpes simplex virus type 1 is able to modify pre-existing ribosomes: (i) by stimulating a pre-existing kinase for S6 phosphorylation even in the absence of serum and of viral genome expression; (ii) by inducing new specific kinase activity(ies); and (iii) by association of new, phosphorylated proteins to ribosomes. These ribosomal modifications are correlated with changes in protein synthesis, as shown by two-dimensional electrophoretic analyses of newly synthesized 35S-labelled proteins.

Differential stimulation of growth related metabolism in cultured smooth muscle cells from SHR and WKY rats by combinations of EGF and LDL

We have reported previously that vascular smooth muscle cells from spontaneously hypertensive rats (SHR) were more responsive to epidermal growth factor (EGF) than their normotensive derived Wistar Kyoto (WKY) controls. This differential responsiveness is evident for several cellular processes including activation of S6-kinase, elevation of intracellular pH and stimulation of both phosphoinositide metabolism and DNA synthesis. Quiescent smooth muscle cells exposed to low density lipoprotein (LDL) exhibited a similar differential responsiveness (SHR greater than WKY) in terms of S6-kinase activation, which was time- and dose-dependent (10(-10)-10(7) M), but neither cell type responded appreciably to LDL in terms of a stimulation in [3H]-thymidine incorporation. Exposure of the same cells to EGF and LDL in combination elicited a marked synergistic stimulation in DNA synthesis, the extent of which was greater for SHR than WKY. The sensitivity of both cell types to EGF was increased in the presence of LDL, although cells from hypertensive animals still exhibited their greater (vs. WKY) sensitivity. In both cell types, activation of nuclear protooncogenes c-fos and c-myc by LDL was minimal, whereas oncogene induction by EGF was approximately five-fold greater for SHR-derived cells compared to those from WKY animals. No marked synergistic effect on the time-dependent induction of either entity was observed for cells exposed to EGF and LDL simultaneously, and the response of SHR-cells remained greater than WKY-cells.

The Drosophila proteasome undergoes changes in its subunit pattern during development

The two-dimensional electrophoretic protein subunit pattern of the proteasome, which is a mulifunctional non-lysosomal proteinase, was analyzed throughout the development of Drosophila melanogaster. The experiments show that the proteasome is already present in early embryos and its characteristic gross morphology as judged by the outer diameter of 12 nm and the inner depression of 3 nm remains unaltered. The electrophoretic analysis of the enzyme subunits demonstrates that the proteasome undergoes, dependent on development, alterations in its protein composition. The most simple subunit pattern is observed in Schneider's S-3 tissue culture cells and early embryos while with ongoing fly development the subunit pattern of the proteasome becomes increasingly complex. 32P-Labeling and immunoblotting experiments indicate that post-translational modification of the subunits must in part be responsible for the development-dependent diversification of the subunit pattern. Our data raise the possibility that the in vivo proteolytic activity and the in vivo substrate specificity of the proteasome may be regulated by modification of its subunit composition during fly development.

Amiloride sensitive activation of S6 kinase by angiotensin II in cultured vascular smooth muscle cells

Angiotensin II was shown to activate S6-kinase in cultured vascular smooth muscle cells (VSMC) in a dose- (10(-9)-10(-6) M) and time-dependent manner. Pretreatment of quiescent cells with 12-O-Tetradecanoylphorbol-13-acetate had no effect on the activation levels of the kinase at the hormone levels used. However, stimulation of S6-kinase activity by angiotensin II was markedly inhibited by the inclusion of amiloride hydrochloride in serum-free medium during activation procedures. Angiotensin was not mitogenic for VSMC at even the highest doses used (10(-6) M). These findings support the notion that raised intracellular pH results in the activation of protein synthesis in quiescent cells.

Activation of S6 kinase in cultured vascular smooth muscle cells by submitogenic levels of thrombospondin

Purified human platelet thrombospondin was shown to activate S6 kinase in cultured vascular smooth muscle cells in a dose- (1-9 micrograms/ml) and time-dependent manner. Down regulation of epidermal growth factor and somatomedin C receptors by prior treatment of cells with their respective growth factors did not reduce this effect. Kinase activation by thrombospondin was only marginally reduced in the presence of platelet-derived growth factor specific antibody at levels that totally inhibited platelet-derived growth factor (5 ng/ml) induced activation. Additionally, thrombospondin elicits a rapid dose-dependent phosphoinositide turnover response analogous to that of platelet-derived growth factor, epidermal growth factor and somatomedin C. Prior treatment of cells with phorbol ester for 48 hrs in serum-free culture medium resulted in a small enhancement of S6 kinase activation by thrombospondin and the above mentioned growth factors but a complete loss in the ability of phorbol ester to activate this enzyme. These findings with cultured smooth muscle cells suggest a growth factor-like role for thrombospondin.

A comparison of accessibility of ribosomal proteins on free and membrane-bound ribosomes: the ribosomal proteins potentially involved in ribosome-membrane binding

The relative accessibility of rat liver ribosomal proteins to reductive methylation was examined using membrane-bound and free ribosomes. Comparisons indicated that 12-13 large ribosomal proteins are masked by ribosomal association with membranes. These consisted of L8, L10, L17, L26-28, L31 and L36, and probably also include L4, L5, L7 and L29. These proteins seem to surround a region centered about L3 and may partly define a ribosomal channel through which the nascent peptide emerges. Approx. 10-20% of the large ribosomal subunit surface area is shielded by the membrane.

Phosphorylation of ribosomal protein S6 during lens cell differentiation: correlation with translational efficiency

In vitro differentiation of embryonic chicken lens epithelial explants to form lens fiber cells is accompanied by an increase in protein synthesis without a corresponding increase in mRNA levels. This apparent increase in translational efficiency is correlated with a specific enhancement of phosphorylation of a 32K protein, which we identify as ribosomal protein S6 by two dimensional gel electrophoresis of purified ribosomal proteins. Serum, insulin, and chicken vitreous humor, three agents known to initiate differentiation in this system, all lead to enhanced S6 phosphorylation. Maximal enhancement of phosphorylation is reached within the first hour after the onset of differentiation, and is not blocked by inhibitors of RNA and protein synthesis.

Intracellular pH controls growth factor-induced ribosomal protein S6 phosphorylation and protein synthesis in the G0----G1 transition of fibroblasts

Mitogen-induced intracellular alkalinization mediated by activation of a Na+/H+ antiporter is a common feature of eukaryotic cells stimulated to divide. A Chinese hamster fibroblast mutant (PS120) lacking Na+/H+ antiport activity (Pouysségur et al., Proc natl acad sci US 81 (1984) 4833) [42] possesses an intracellular pH (pHi) 0.2-0.3 units lower than the wild type (CCL39) and requires a more alkaline pHout (pHo) for growth. Here, we show that serum-stimulated ribosomal protein S6 phosphorylation, protein synthesis activation and DNA synthesis re-initiation are pH-regulated events that display a similar threshold pHo value (6.60) in CCL39 cells. pH-Dependencies for initiation of all three events are shifted toward higher pHo values in the mutant PS120, indicating that growth factor-induced alkalinization has a permissive effect on the pleiotypic response. However, cytoplasmic alkalinization per se is insufficient to trigger S6 phosphorylation, polysome formation, and subsequent DNA synthesis. Transient exposure to a non-permissive pHo (6.5) inhibits both the rate of leucine incorporation into proteins and the progression through the G1 phase of the cell cycle. In contrast, cells committed to DNA synthesis are unaltered by the acidic pHo. These observations suggest that pHi by controlling the rate of protein synthesis play a determinant role in the control of cell division.

An altered ribosomal protein in an edeine-resistant mutant of Saccharomyces cerevisiae

The r-proteins of an edeine-resistant mutant of Saccharomyces cerevisiae were compared to those of the wild-type strain by using two different two-dimensional electrophoretic techniques: (1) the Kaltschmidt-Wittmann method and, (2) the Kaltschmidt-Wittmann system, in the first dimension and the Na Dodecyl-SO4 system in the second. With the first technique, the results indicate that the patterns of basic ribosomal proteins are similar in the two strains. However, the pattern of acidic ribosomal proteins of the mutant revealed an additional protein band with respect to the normal one. Using the other technique, the patterns of basic and acidic ribosomal proteins of the mutant demonstrated a similarity to the corresponding pattern of the wild-type strain. The data disclose that an acidic ribosomal protein of the mutant may have two forms with different electrophoretic mobilities and similar molecular weights.

Insulin-stimulated protein synthesis in adipocytes. Enhanced rate of initiation associated with increased phosphorylation of ribosomal protein S6

The mechanisms of insulin stimulation of protein synthesis in adipocytes are presently unknown. Addition of 10 nM insulin to isolated rat adipocytes caused a 1.5-2.5-fold increase in the protein synthetic rate and a corresponding increase in nascent chain level, indicating that the effect of insulin on protein synthesis in adipocytes is mediated by a stimulation of ribosomal initiation. The effect on protein synthesis exhibited a lag time of 6-8 min after insulin addition. A similar time dependence was also observed for the insulin-induced phosphorylation of ribosomal protein S6. This supports the proposal that these two phenomena are causally linked.

Isoproterenol induces a ribosomal modification in the heart and the skeletal muscle of hamsters

The protein synthesis activity of heart, skeletal muscle and liver polysomes from isoprotenerol-treated and control hamsters has been compared in an in vitro non-initiating translation system. Heart and skeletal muscle polysomes from treated hamsters were less active than controls and required a higher magnesium concentration for optimal protein synthesis. These results suggest that there is a conformational modification in heart and skeletal muscle ribosomes from isoprotenerol-treated hamsters. No such change was observed with ribosomes from the liver of isoproterenol-treated hamsters.

Cyclic AMP binding activity in liver supernatants during acute ethionine intoxication

There is a significant increase in hepatic cAMP binding activity 4 hr following ethionine intoxication. This activity is localized to the 100, 000 g supernatant. The binding of cAMP by the 100, 000 g supernatant of controls can be increased to experimental levels simply by dialysis. The increased binding activity seen during ethionine intoxication can be reversed by the administration of adenine. Neither S-adenosyl ethionine, S-adenosyl methionine, S-adenosyl homocysteine, nor AMP compete effectively with cAMP for the binding protein. Increasing ADP concentrations stimulates cAMP binding whereas increasing ATP concentration inhibits cAMP binding. At concentrations seen during ethionine intoxication the effects of ADP and ATP were equal but opposite in direction.