Identification of a ribosomal protein S6 kinase regulated by transformation and growth-promoting stimuli

Peroxisome Proliferator-Activated Receptors and Caloric Restriction-Common Pathways Affecting Metabolism, Health, and Longevity

Caloric restriction (CR) is a traditional but scientifically verified approach to promoting health and increasing lifespan. CR exerts its effects through multiple molecular pathways that trigger major metabolic adaptations. It influences key nutrient and energy-sensing pathways including mammalian target of rapamycin, Sirtuin 1, AMP-activated protein kinase, and insulin signaling, ultimately resulting in reductions in basic metabolic rate, inflammation, and oxidative stress, as well as increased autophagy and mitochondrial efficiency. CR shares multiple overlapping pathways with peroxisome proliferator-activated receptors (PPARs), particularly in energy metabolism and inflammation. Consequently, several lines of evidence suggest that PPARs might be indispensable for beneficial outcomes related to CR. In this review, we present the available evidence for the interconnection between CR and PPARs, highlighting their shared pathways and analyzing their interaction. We also discuss the possible contributions of PPARs to the effects of CR on whole organism outcomes.

Ribosomal Protein S6 Phosphorylation: Four Decades of Research

The phosphorylation of ribosomal protein S6 (rpS6) has been described for the first time about four decades ago. Since then, numerous studies have shown that this modification occurs in response to a wide variety of stimuli on five evolutionarily conserved serine residues. However, despite a large body of information on the respective kinases and the signal transduction pathways, the physiological role of rpS6 phosphorylation remained obscure until genetic manipulations were applied in both yeast and mammals in an attempt to block this modification. Thus, studies based on both mice and cultured cells subjected to disruption of the genes encoding rpS6 and the respective kinases, as well as the substitution of the phosphorylatable serine residues in rpS6, have laid the ground for the elucidation of the multiple roles of this protein and its posttranslational modification. This review focuses primarily on newly identified kinases that phosphorylate rpS6, pathways that transduce various signals into rpS6 phosphorylation, and the recently established physiological functions of this modification. It should be noted, however, that despite the significant progress made in the last decade, the molecular mechanism(s) underlying the diverse effects of rpS6 phosphorylation on cellular and organismal physiology are still poorly understood.

Using phospho-motif antibodies to determine kinase substrates

Phosphorylation of substrates by protein kinases regulates a myriad of cellular processes, ranging from proliferation and migration to autophagy, senescence, and apoptosis. Kinase substrate selectivity is largely dependent on the amino acid sequence surrounding the phosphorylation site; therefore, substrate-directed, phosphorylation-state-sensitive, motif-specific ("phospho-motif") antibodies represent powerful tools to identify novel kinase substrates and to investigate mechanisms of substrate phosphorylation in many signaling pathways typically associated with human malignancies. Phospho-motif antibodies are engineered to recognize proteins that contain a phosphorylated residue in the context of a specific motif. They are raised against a library of phospho-peptides comprising both the phosphorylated residue and the surrounding residues that determine kinase specificity, with degenerate residues taking up the remaining positions. Currently, several categories of phospho-motif antibody are commercially available, which may be used to specifically detect Ser, Thr, Ser/Thr, or Tyr residues phosphorylated by different protein kinase families. These antibodies are commonly used in immunoprecipitation and/or immunoblotting protocols to determine kinase-induced substrate phosphorylation. This unit describes the use of phospho-motif antibodies to elucidate the kinase(s) responsible for phosphorylating substrate proteins.

AMPK and mTOR in cellular energy homeostasis and drug targets

The mammalian target of rapamycin (mTOR) is a central controller of cell growth and proliferation. mTOR forms two distinct complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2). mTORC1 is regulated by multiple signals such as growth factors, amino acids, and cellular energy and regulates numerous essential cellular processes including translation, transcription, and autophagy. The AMP-activated protein kinase (AMPK) is a cellular energy sensor and signal transducer that is regulated by a wide array of metabolic stresses. These two pathways serve as a signaling nexus for regulating cellular metabolism, energy homeostasis, and cell growth, and dysregulation of each pathway may contribute to the development of metabolic disorders such as obesity, type 2 diabetes, and cancer. This review focuses on our current understanding of the relationship between AMPK and mTORC1 signaling and discusses their roles in cellular and organismal energy homeostasis.

Ribosomal protein S6 kinase from TOP mRNAs to cell size

Ribosomal protein S6 kinase (S6K) has been implicated in the phosphorylation of multiple substrates and is subject to activation by a wide variety of signals that converge at mammalian target of rapamycin (mTOR). In the course of the search for its physiological role, it was proposed that S6K activation and ribosomal protein S6 (rpS6) phosphorylation account for the translational activation of a subgroup of transcripts, the TOP mRNAs. The structural hallmark of these mRNAs is an oligopyrimidine tract at their 5'-terminus, known as the 5'-TOP motif. TOP mRNAs consists of about 90 members that encode multiple components of the translational machinery, such as ribosomal proteins and translation factors. The translation efficiency of TOP mRNAs indeed correlates with S6K activation and rpS6 phosphorylation, yet recent biochemical and genetic studies have established that, although S6K and TOP mRNAs respond to similar signals and are regulated by mTOR, they maintain no cause and effect relationship. Instead, S6K is primarily involved in regulation of cell size, and affects glucose homeostasis, but is dispensable for global protein synthesis, whereas translational efficiency of TOP mRNAs is a determinant of the cellular protein synthesis capacity. Despite extensive studies of their function and mode of regulation, the mechanism underlying the effect of S6K on the cell size, as well as the trans-acting factor that mediates the translational control of TOP mRNAs, still await their identification.

Physiological roles of ribosomal protein S6: one of its kind

The phosphorylation of ribosomal protein S6 (rpS6), which occurs in response to a wide variety of stimuli on five evolutionarily conserved serine residues, has attracted much attention since its discovery more than three decades ago. However, despite a large body of information on the respective kinases and the signal transduction pathways, the role of this phosphorylation remained obscure. It is only recent that targeting the genes encoding rpS6, the phosphorylatable serine residues or the respective kinases that the unique role of rpS6 and its posttranslational modification have started to be elucidated. This review focuses primarily on the critical role of rpS6 for mouse development, the pathways that transduce various signals into rpS6 phosphorylation, and the physiological functions of this modification. The mechanism(s) underlying the diverse effects of rpS6 phosphorylation on cellular and organismal physiology has yet to be determined. However, a model emerging from the currently available data suggests that rpS6 phosphorylation operates, at least partly, by counteracting positive signals simultaneously induced by rpS6 kinase, and thus might be involved in fine-tuning of the cellular response to these signals.

Rheb activation of mTOR and S6K1 signaling

More than 10 years ago, Rheb (Ras homolog enriched in brain) was identified as a highly conserved protein that is a member of the Ras superfamily of small GTPases, which play critical roles in cell growth and proliferation. Recently, a convergence of genetic and biochemical evidence from yeast, Drosophila, and mammalian cells has placed Rheb upstream of the mammalian target of rapamycin (mTOR) and immediately downstream of the tumor suppressors TSC1 (hamartin) and TSC2 (tuberin). Rheb plays a key role in the regulation of cell growth in response to growth factors, nutrients, and amino acids linking PI3K and TOR signaling. Rheb activation of the nutrient and energy-sensitive TOR pathway leads to the direct phosphorylation of two known downstream translational control targets by mTOR, the 40S ribosomal S6 kinase 1 (S6K1) and the eukaryotic translation initiation factor 4E (eIF4E)- binding protein 1 (4E-BP1). Appropriate regulation of this pathway is crucial for the proper control of cell growth, proliferation, survival, and differentiation. Inappropriate regulation of these signaling molecules, therefore, can lead to a variety of human diseases. In this chapter, we describe cell biological and biochemical methods commonly used to study Rheb activation and dissect its role in the mTOR-signaling pathway.

Rapamycin: an anti-cancer immunosuppressant?

Rapamycin and its derivatives are promising therapeutic agents with both immunosuppressant and anti-tumor properties. These rapamycin actions are mediated through the specific inhibition of the mTOR protein kinase. mTOR serves as part of an evolutionarily conserved signaling pathway that controls the cell cycle in response to changing nutrient levels. The mTOR signaling network contains a number of tumor suppressor genes including PTEN, LKB1, TSC1, and TSC2, and a number of proto-oncogenes including PI3K, Akt, and eIF4E, and mTOR signaling is constitutively activated in many tumor types. These observations point to mTOR as an ideal target for anti-cancer agents and suggest that rapamycin is such an agent. In fact, early preclinical and clinical studies indicate that rapamycin derivatives have efficacy as anti-tumor agents both alone, and when combined with other modes of therapy. Rapamycin appears to inhibit tumor growth by halting tumor cell proliferation, inducing tumor cell apoptosis, and suppressing tumor angiogenesis. Rapamycin immunosuppressant actions result from the inhibition of T and B cell proliferation through the same mechanisms that rapamycin blocks cancer cell proliferation. Therefore, one might think that rapamycin-induced immunosuppression would be detrimental to the use of rapamycin as an anti-cancer agent. To the contrary, rapamycin decreases the frequency of tumor formation that occurs in organ transplant experiments when combined with the widely used immunosuppressant cyclosporine compared with the tumor incidence observed when cyclosporine is used alone. The available evidence indicates that with respect to tumor growth, rapamycin anti-cancer activities are dominant over rapamycin immunosuppressant effects.

Nuclear translocation of p90Rsk and phosphorylation of CREB is induced by ionomycin in a Ras-independent manner in PC12 cells

In the present study we examined the possible role of p90Rsk in pathways leading to neuronal differentiation of PC12 cells induced by nerve growth factor (NGF) and the calcium ionophore ionomycin. PC12-M17 cells, expressing a dominant inhibitory Ras protein, do not undergo neuronal differentiation in response to NGF like wild-type PC12 cells, but exhibit neurite outgrowth when treated with NGF in combination with ionomycin. However, the blockade of Ras in these cells results in failure of activation of mitogen-activated protein kinase (MAPK)/extracellular signal regulation kinase (ERK) (MEK) and ERK activation as well, therefore kinases other than those of the ERK pathway might play a role in the induction of neuronal differentiation in this case. Here we show that p90Rsk translocates to the nucleus in response to ionomycin in both wild-type PC12 and PC12-M17 cells, and this spatial distribution is followed by increased phosphorylation of the cAMP response element binding protein (CREB). Since CREB is believed to be the transcription factor that can integrate Ca2+, growth factor and cAMP-induced signals, we suggest that p90Rsk may be one of the kinases which is able to replace ERKs under certain circumstances, thereby participating in Ras-independent neuronal differentiation induced by NGF plus ionomycin.

Hitting the target: emerging technologies in the search for kinase substrates

Through phosphorylation, protein kinases can alter the activity, localization, protein association, and stability of their targets. Despite the importance to our understanding of all aspects of cell biology, progress toward identifying bona fide substrates of specific protein kinases has been slow. Traditionally used techniques to identify true kinase substrates, such as genetics, yeast two-hybrid screens, and biochemical purification, are often laborious and unreliable. However, several new approaches have recently been developed and used successfully to identify genuine in vivo substrates of certain protein kinases. These methods include screening for phosphorylation of proteins from phage expression libraries, peptide library screens to determine optimal motifs favored by specific kinases, the use of phospho-motif antibodies, and an approach that uses structurally altered kinases and allele-specific adenosine triphosphate analogs and kinase inhibitors. We describe these approaches and discuss their utility and inherent caveats.

Salicylate-induced growth arrest is associated with inhibition of p70s6k and down-regulation of c-myc, cyclin D1, cyclin A, and proliferating cell nuclear antigen

Salicylate and its pro-drug form aspirin are widely used medicinally for their analgesic and anti-inflammatory properties, and more recently for their ability to protect against colon cancer and cardiovascular disease. Despite the wide use of salicylate, the mechanisms underlying its biological activities are largely unknown. Recent reports suggest that salicylate may produce some of its effects by modulating the activities of protein kinases. Since we have previously shown that the farnesyltransferase inhibitor l-744, 832 inhibits cell proliferation and p70(s6k) activity, and salicylate inhibits cell proliferation, we examined whether salicylate affects p70(s6k) activity. We find that salicylate potently inhibits p70(s6k) activation and phosphorylation in a p38 MAPK-independent manner. Interestingly, low salicylate concentrations (</=250 microm) inhibit p70(s6k) activation by phorbol myristate acetate, while higher salicylate concentrations (>/=5 mm) are required to block p70(s6k) activation by epidermal growth factor + insulin-like growth factor-1. These data suggest that salicylate may selectively inhibit p70(s6k) activation in response to specific stimuli. Inhibition of p70(s6k) by salicylate occurs within 5 min, is independent of the phosphatidylinositol 3-kinase pathway, and is associated with dephosphorylation of p70(s6k) on its major rapamycin-sensitive site, Thr(389). A rapamycin-resistant mutant of p70(s6k) is resistant to salicylate-induced Thr(389) dephosphorylation.

Inhibition of DNA synthesis by a farnesyltransferase inhibitor involves inhibition of the p70(s6k) pathway

Previously, the protein farnesyltransferase inhibitor (FTI), L-744, 832, has been shown to inhibit the proliferation of a number of tumor cell lines in vitro in a manner that correlated with the inhibition of the mitogen-activated protein kinase cascade. Here we show that FTI inhibits p70(s6k) phosphorylation in mammary tumors in vivo in transgenic mice. Furthermore, in a mouse keratinocyte cell line, FTI inhibits p70(s6k) phosphorylation and activity and inhibits PHAS-1 phosphorylation in vitro in both rapidly growing cells and in growth factor-stimulated quiescent cells. Dominant-negative Ras expression inhibits p70(s6k) stimulation by epidermal growth factor, demonstrating a requirement for Ras activity during p70(s6k) activation. FTI does not inhibit protein kinase B phosphorylation on Ser473, indicating that FTI does not act by inhibiting phosphatidylinositol 3-kinase. FTI also inhibits DNA synthesis in keratinocytes, and inhibition of DNA synthesis correlates closely with p70(s6k) inhibition. Rapamycin, an inhibitor of p70(s6k) and PHAS-1 phosphorylation, causes a 30-45% reduction in DNA synthesis in keratinocytes, while FTI induces an 80-90% reduction in DNA synthesis. These observations suggest that alteration of p70(s6k) and PHAS-1 function by FTI are responsible for a substantial portion of the growth-inhibitory properties of FTI. Together, these data demonstrate that p70(s6k) and PHAS-1 are novel downstream targets of FTI and suggest that the anti-tumor properties of FTI are probably due to the inhibition of multiple mitogenic pathways.

Targeted disruption of p70(s6k) defines its role in protein synthesis and rapamycin sensitivity

Here, we disrupted the p70 S6 kinase (p70(s6k)) gene in murine embryonic stem cells to determine the role of this kinase in cell growth, protein synthesis, and rapamycin sensitivity. p70(s6k-/-) cells proliferated at a slower rate than parental cells, suggesting that p70(s6k) has a positive influence on cell proliferation but is not essential. In addition, rapamycin inhibited proliferation of p70(s6k-/-) cells, indicating that other events inhibited by the drug, independent of p70(s6k), also are important for both cell proliferation and the action of rapamycin. In p70(s6k-/-) cells, which exhibited no ribosomal S6 phosphorylation, translation of mRNA encoding ribosomal proteins was not increased by serum nor specifically inhibited by rapamycin. In contrast, rapamycin inhibited phosphorylation of initiation factor 4E-binding protein 1 (4E-BP1), general mRNA translation, and overall protein synthesis in p70(s6k-/-) cells, indicating that these events proceed independently of p70(s6k) activity. This study localizes the function of p70(s6k) to ribosomal biogenesis by regulating ribosomal protein synthesis at the level of mRNA translation.

Regulation of neuronal survival by the serine-threonine protein kinase Akt

A signaling pathway was delineated by which insulin-like growth factor 1 (IGF-1) promotes the survival of cerebellar neurons. IGF-1 activation of phosphoinositide 3-kinase (PI3-K) triggered the activation of two protein kinases, the serine-threonine kinase Akt and the p70 ribosomal protein S6 kinase (p70(S6K)). Experiments with pharmacological inhibitors, as well as expression of wild-type and dominant-inhibitory forms of Akt, demonstrated that Akt but not p70(S6K) mediates PI3-K-dependent survival. These findings suggest that in the developing nervous system, Akt is a critical mediator of growth factor-induced neuronal survival.

Upregulated expression of the genes encoding translation initiation factors eIF-4E and eIF-2alpha in transformed cells

Increased protein synthesis is necessary for the transition of cells from quiescence to proliferation. It is shown in this paper that the induction of expression of the translation initiation factor eIF-4E in normal cells requires serum growth factors, while this requirement is abrogated in tumor cells analyzed in this study. Further, the expression of eIF-4E and eIF-2alpha is increased in c-myc, v-src, and v-abl-transformed cells. It is demonstrated that an increase in c-myc function leads to elevated expression of eIF-4E and eIF-2alpha, increases in net protein synthesis and cell proliferation. It may be suggested that constitutive activation of translational machinery may be one common mechanism by which various oncogenes exert their transforming function.

Deregulation of protein synthesis as a mechanism of neoplastic transformation

Early research on the cell cycle revealed correlations between protein accumulation and cell proliferation. In this review, I describe the data showing that abnormality of cell growth and tumor development are dependent upon oncogene-induced increases in the levels and activity of factors that determine the rate of protein synthesis. It is proposed that the establishment of a vicious circle, namely oncoproteins-->increase in translation-->oncoproteins, is a major biological mechanism that fuels neoplastic growth. The constitutively high rates of protein synthesis and accumulation of proteins, including those necessary for DNA replication and mitosis, would drive cells to excessive proliferation.

ERKs, extracellular signal-regulated MAP-2 kinases

A family of protein kinases, known alternatively as microtubule-associated protein-2/myelin basic protein kinases or extracellular signal-regulated kinases, is activated by numerous hormones, growth factors and other extracellular stimuli. At least two members of this family function as intermediate kinases in protein phosphorylation cascades. Their mechanisms of activation may involve autophosphorylation, which occurs on both threonine and tyrosine residues.

Heat shock induces two distinct S6 protein kinase activities in quiescent mammalian fibroblasts

The regulation of S6 kinase activity was used to monitor perturbations of intracellular signaling activity during heat shock of quiescent murine and human fibroblasts. Previous reports on exponentially growing insect and plant cells had indicated that 40S ribosomal protein S6 is dephosphorylated during heat shock; thus inhibition of S6 kinase activity by heat shock was anticipated in NIH 3T3 fibroblasts and human cells (HeLa, diploid embryonic fibroblasts MRC-5, and skin-derived fibroblasts). Unexpectedly, two distinct S6 protein kinases were activated in quiescent fibroblasts after heat exposure. One of the enzymes was partially purified by sequential column chromatography and was determined to be equivalent to the enzyme activated by serum and other growth factors, referred to here as pp70-S6 protein kinase. The other protein S6 kinase, pp90rsk, was identified by a specific immunoprecipitation assay. Monitoring both enzymatic activities during heat shock revealed a temporal pattern of activation that was reversed when compared to non-stressed, mitogen-stimulated cells. Finally, heat shock stimulated protein S6 phosphorylation in cultured, quiescent mammalian cells. These data demonstrate that specific protein kinases can be activated during heat shock, and that some early mitogenic signals may also participate in the response of cells to physiologic stress.

MAP2 kinase and 70K S6 kinase lie on distinct signalling pathways

Activation of protein synthesis is required for quiescent cells to transit the cell cycle, and seems to be mediated in part by phosphorylation of the 40S ribosomal protein, S6. A mitogen-activated S6 kinase of relative molecular mass 70,000 (70K) has been isolated from mouse fibroblasts as well as from avian, rat and rabbit tissues. Comparison of complementary DNA sequences shows that this enzyme is distinct from S6 kinase II (92K) found in Xenopus eggs and fibroblasts. Both kinases are activated by serine/threonine phosphorylation, suggesting that at least one serine/threonine kinase links receptor tyrosine kinases with S6 kinases. A candidate for this link is MAP2 kinase, which is rapidly activated by tyrosine/threonine phosphorylation following mitogenic stimulation. Incubation of MAP2 kinase from insulin-treated 3T3-L1 adipocytes with phosphatase-inactivated S6 kinase II from Xenopus leads to partial reactivation and phosphorylation of the enzyme. These and other findings have led to the suggestion that MAP2 kinase also activates the 70K S6 kinase. Here we refute this idea by showing that the two kinases lie on distinct signalling pathways.

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.

Site-specific phosphorylation of a synthetic peptide derived from ribosomal protein S6 by human placenta protein kinases

A synthetic peptide S6-21 (AKRRRLSSLRASTSKSESSQK) which contains the phosphorylated residues in the ribosomal protein S6 has been used as a substrate for two partially purified human placenta protein kinases. Two distinct classes of protein kinases which catalyze either amino terminal (AKRRRLSS) or carboxyl terminal (LRASTSKSESSQK) peptide phosphorylation were identified. Multiple sites were phosphorylated in each domain. A single protein kinase which catalyzed phosphorylation of sites in both domains was identified. Although growth factors are known to promote phosphorylation of S6 at five serine sites, no enzyme which could modify S6-21 to that extent was observed.

cDNA encoding a 59 kDa homolog of ribosomal protein S6 kinase from rabbit liver

We have isolated cDNA molecules encoding a protein with the characteristic sequence elements that are conserved between the catalytic domains of protein kinases. This protein is apparently a serine/threonine kinase and is most closely related to the amino-terminal half of the ribosomal protein S6 kinase II first characterized in Xenopus eggs (42% overall identity and 56% identity in the predicted catalytic domain). However, it clearly differs from S6 kinase II in that it has only one, rather than two predicted catalytic domains and a deduced molecular mass of 59,109 Da. We propose that is may be more related to, or identical, with, the mitogen-inducible S6 kinase of molecular mass 65-70 kDa described in mammalian liver, mouse 3T3 cells and chicken embryos. Remarkable structural features of the cDNA-encoded polypeptide are a section rich in proline, serine and threonine residues that resemble the multiphosphorylation domains of glycogen synthase and phosphorylase kinase alpha subunit, and a characteristic tyrosine residue in the putative nucleotide-binding glycine cluster which, by analogy to cdc2 kinase, is a potential tyrosine phosphorylation site.

Xenopus oocytes and the biochemistry of cell division

The control of cell proliferation involves both regulatory events initiated at the plasma membrane that control reentry into the cell cycle and intracellular biochemical changes that direct the process of cell division itself. Both of these aspects of cell growth control can be studied in Xenopus oocytes undergoing meiotic maturation in response to mitogenic stimulation. All mitogenic signaling pathways so far identified lead to the phosphorylation of ribosomal protein S6 on serine residues, and the biochemistry of this event has been investigated. Insulin and other mitogens activate ribosomal protein S6 kinase II, which has been cloned and sequences in oocytes and other cells. This enzyme is activated by phosphorylation on serine and threonine residues by an insulin-stimulated protein kinase known as MAP-2 kinase. MAP kinase itself is also activated by direct phosphorylation on threonine and tyrosine residues in vivo. These results reconstitute one step of the insulin signaling pathway evident shortly after insulin receptor binding at the membrane. Several hours after mitogenic stimulation, a cell cycle cytoplasmic control element is activated that is sufficient to cause entry into M phase. This control element, known as maturation-promoting factor or MPF, has been purified to near homogeneity and shown to consist of a complex between p34cdc2 protein kinase and cyclin B2. In addition to apparent phosphorylation of cyclin, regulation of MPF activity involves synthesis of the cyclin subunit and its periodic degradation at the metaphase----anaphase transition. The p34cdc2 kinase subunit is regulated by phosphorylation/dephosphorylation on threonine and tyrosine residues, being inactive when phosphorylated and active when dephosphorylated. Analysis of phosphorylation sides in histone H1 for p34cdc2 has revealed a consensus sequence of (K/R)S/TP(X)K/R, where the elements in parentheses are present in some but not all sites. Sites with such a consensus are specifically phosphorylated in mitosis and by MPF in the protooncogene pp60c-src. These results provide a link between cell cycle control and cell growth control and suggest that changes in cell adhesion and the cytoskeleton in mitosis may be regulated indirectly by MPF via protooncogene activation. S6 kinase II is also activated upon expression of MPF in cells, indicating that MPF is upstream of S6 kinase on the mitogenic signaling pathway. Further study both of the signaling events that lead to MPF activation and of the substrates for phosphorylation by MPF should lead to a comprehensive understanding of the biochemistry of cell division.

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.

Novel characteristics of a 33KDa protein (pp33) rapidly phosphorylated in IL3 dependent cells by stimulation with IL3

We report the novel properties of a 33 KDa cellular protein rapidly phosphorylated by stimulation of growth by IL3 in IL3 dependent lines. Although pp33 is readily soluble in SDS, SDS-solubilised pp33 is insoluble in non-ionic detergents and is excluded from electrophoretic analysis (IEF, NEPHGE) employing such detergents. Native pp33 is not extracted by non-ionic detergents with or without cation chelation. pp33 is concentrated in a cell fraction containing endoplasmic reticulum where it is associated with a specific trypsin-sensitive degredative enzyme, active at 4 degrees. Its unusual characteristics and kinetics of phosphorylation suggest pp33 may be a novel molecule, explain its absence in studies elsewhere where non-ionic detergent extraction has been exclusively used and suggest it is intimately related to the signal transduced by IL3.

Properties of the 12-O-tetradecanoylphorbol-13-acetate-stimulated S6 kinase from rat astroglial cells

The S6 kinase activity of astroglial cells in primary culture stimulated by 12-O-tetradecanoylphorbol-13-acetate (TPA) has been studied. This activity was eluted as a single peak at 0.15 M NaCl from a DEAE-Sephacel column. The chromatography of this peak on phosphocellulose revealed an activity eluted at 0.15 M NaCl. This partially purified enzyme had a sedimentation coefficient of 3.7S; Km values were 2 X 10(-5) M for ATP and 10(-6) M for 40S ribosomal subunits. The optimal Mg2+ concentration requirement was 2-3 mM. Mn2+ and Co2+ could substitute for Mg2+ (optimum concentrations 1.5 and 0.8 mM, respectively), but these cations were strong inhibitors in the presence of Mg2+. The enzyme was inhibited by N-ethylmaleimide, indicating that it contained thiol groups. This S6 kinase used ATP, but not GTP, as a phosphate donor, and exhibited great specificity for S6 as phosphate acceptor. Whole histones and protamine were slightly phosphorylated whereas phosvitin, histone H1, and surprisingly the peptide Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala were not phosphorylated. The TPA-stimulated S6 kinase resembles the insulin-, fibroblast growth factor- and cyclic AMP-stimulated enzymes, suggesting that several pathways might activate the same entity.

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.