Increased phosphorylation of ribosomal protein S6 during meiotic maturation of Xenopus oocytes

Pathologic tau conformer ensembles induce dynamic, liquid-liquid phase separation events at the nuclear envelope

Background

The microtubule-associated protein tau forms aggregates in different neurodegenerative diseases called tauopathies. Prior work has shown that a single P301L mutation in tau gene, MAPT, can promote alternative tau folding pathways that correlate with divergent clinical diagnoses. Using progressive chemical denaturation, some tau preparations from the brain featured complex transitions starting at low concentrations of guanidine hydrochloride (GdnHCl) denaturant, indicating an ensemble of differently folded tau species called conformers. On the other hand, brain samples with abundant, tangle-like pathology had simple GdnHCl unfolding profile resembling the profile of fibrillized recombinant tau and suggesting a unitary conformer composition. In studies here we sought to understand tau conformer progression and potential relationships with condensed liquid states, as well as associated perturbations in cell biological processes.

Results

As starting material, we used brain samples from P301L transgenic mice containing tau conformer ensembles that unfolded at low GdnHCl concentrations and with signatures resembling brain material from P301L subjects presenting with language or memory problems. We seeded reporter cells expressing a soluble form of 4 microtubule-binding repeat tau fused to GFP or YFP reporter moieties, resulting in redistribution of dispersed fluorescence signals into focal assemblies that could fuse together and move within processes between adjacent cells. Nuclear envelope fluorescent tau signals and small fluorescent inclusions behaved as a demixed liquid phase, indicative of liquid-liquid phase separation (LLPS); these droplets exhibited spherical morphology, fusion events and could recover from photobleaching. Moreover, juxtanuclear tau assemblies were associated with disrupted nuclear transport and reduced cell viability in a stable cell line. Staining for thioflavin S (ThS) became more prevalent as tau-derived inclusions attained cross-sectional area greater than 3 μm², indicating (i) a bipartite composition, (ii) in vivo progression of tau conformers, and (iii) that a mass threshold applying to demixed condensates may drive liquid-solid transitions.

Conclusions

Tau conformer ensembles characterized by denaturation at low GdnHCl concentration templated the production of condensed droplets in living cells. These species exhibit dynamic changes and develop in vivo, and the larger ThS-positive assemblies may represent a waystation to arrive at intracellular fibrillar tau inclusions seen in end-stage genetic tauopathies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01132-y.

The brominated flame retardant, TBCO, impairs oocyte maturation in zebrafish (Danio rerio)

The brominated flame retardant, 1,2,5,6-tetrabromocyclooctane (TBCO), has been shown to decrease fecundity in Japanese medaka (Oryzias latipes) and there is indirect evidence from analysis of the transcriptome and proteome that this effect might be due to impaired oogenesis. An assay for disruption of oocyte maturation by chemical stressors has not been developed in Japanese medaka. Thus, using zebrafish (Danio rerio) as a model, objectives of the present study were to determine whether exposure to TBCO has effects on maturation of oocytes and to investigate potential mechanisms. Sexually mature female zebrafish were given a diet of 35.3 or 628.8 μg TBCO / g food for 14 days after which, stage IV oocytes were isolated to assess maturation in response to maturation inducing hormone. To explore potential molecular mechanisms, abundances of mRNAs of a suite of genes that regulate oocyte maturation were quantified by use of quantitative real-time PCR, and abundances of microRNAs were determined by use of miRNAseq. Ex vivo maturation of oocytes from fish exposed to TBCO was significantly less than maturation of oocytes from control fish. The percentage of oocytes which matured from control fish and those exposed to low and high TBCO were 89, 71, and 67%, respectively. Among the suite of genes known to regulate oocyte maturation, mRNA abundance of insulin like growth factor-3 was decreased by 1.64- and 3.44-fold in stage IV oocytes from females given the low and high concentrations of TBCO, respectively, compared to the control group. Abundances of microRNAs regulating the expression of proteins that regulate oocyte maturation, including processes related to insulin-like growth factor, were significantly different in stage IV oocytes from fish exposed to TBCO. Overall, results of this study indicated that impaired oocyte maturation might be a mechanism of reduced reproductive performance in TBCO-exposed fish. Results also suggested that effects of TBCO on oocyte maturation might be due to molecular perturbations on insulin-like growth factor signaling and expression of microRNAs.

The Xenopus oocyte: a single-cell model for studying Ca2+ signaling

In the four decades since the Xenopus oocyte was first demonstrated to have the capacity to translate exogenous mRNAs, this system has been exploited for many different experimental purposes. Typically, the oocyte is used either as a "biological test tube" for heterologous expression of proteins without any particular cell biological insight or, alternatively, it is used for applications where cell biology is paramount, such as investigations of the cellular adaptations that power early development. In this article, we discuss the utility of the Xenopus oocyte for studying Ca(2+) signaling in both these contexts.

Regulation of the mTOR signaling network in hibernating thirteen-lined ground squirrels

For many small mammals, survival over the winter months is a serious challenge because of low environmental temperatures and limited food availability. The solution for many species, such as thirteen-lined ground squirrels (Ictidomys tridecemlineatus), is hibernation, an altered physiological state characterized by seasonal heterothermy and entry into long periods of torpor that are interspersed with short arousals back to euthermia. During torpor, metabolic rate is strongly reduced to achieve major energy savings, and a coordinated depression of non-essential ATP-expensive functions such as protein synthesis takes place. This study examines the mammalian target of rapamycin (mTOR) signaling pathway, a crucial component of the insulin receptor network, over six stages of the torpor–arousal cycle of hibernation. Immunoblots showed that the phosphorylation state of mTORSer2448 was strongly reduced in skeletal muscle (by 55%) during late torpor but increased by 200% during early arousal compared with euthermia. However, the phosphorylation state of this residue remained relatively constant in cardiac muscle during torpor but was enhanced during entrance into torpor and early arousal from torpor stages (by 2.9- and 3.2-fold, respectively). Phosphorylation states of upstream regulators of mTOR, p-AktThr473 and p-TSC2Thr1462, were also suppressed in skeletal muscle by 55 and 51%, respectively, during late torpor, as were selected downstream substrates – p-4E-BP1Thr46 and p-S6Ser235 contents dropped by 74 and 41%, respectively. Overall, the results indicate suppressed mTOR signaling in skeletal muscle, but not cardiac muscle, during torpor. By contrast, activation of mTOR and other components of the mTORC1 complex (p-PRAS40Thr246 and GβL) occurred during early arousal in both skeletal and cardiac muscle.

Regulation and function of the RSK family of protein kinases

The RSK (90 kDa ribosomal S6 kinase) family comprises a group of highly related serine/threonine kinases that regulate diverse cellular processes, including cell growth, proliferation, survival and motility. This family includes four vertebrate isoforms (RSK1, RSK2, RSK3 and RSK4), and single family member orthologues are also present in Drosophila and Caenorhabditis elegans. The RSK isoforms are downstream effectors of the Ras/ERK (extracellular-signal-regulated kinase) signalling pathway. Significant advances in the field of RSK signalling have occurred in the past few years, including several new functions ascribed to the RSK isoforms, the discovery of novel protein substrates and the implication of different RSK isoforms in cancer. Collectively, these new findings increase the diversity of biological functions regulated by RSK, and highlight potential new directions of research. In the present paper, we review the structure, expression and activation mechanisms of the RSK isoforms, and discuss their physiological roles on the basis of established substrates and recent discoveries.

Phosphatases driving mitosis: pushing the gas and lifting the brakes

Entry into and progression through mitosis depends critically on the establishment and maintenance of protein phosphorylation. For this reason, studies on mitotic progression have focused heavily on the activation of MPF (M phase promoting factor), a cyclin-dependent kinase responsible for phosphorylating proteins that execute the dynamic events of mitosis. Recent work, however, has significantly expanded our understanding of mechanisms that allow accumulation of phosphoproteins at M phase, suggesting that mitotic entry relies not only on MPF activation but also on the inhibition of antimitotic phosphatases. It is now clear that there exists a separate, albeit equally important, signaling pathway for the inactivation of protein phosphatases at the G2/M transition. This pathway, which is governed by the kinase Greatwall is essential for both entry into and maintenance of M phase. This chapter will outline the molecular events regulating entry into mitosis, specifically highlighting the role that protein phosphorylation plays in triggering both MPF activation and the inhibition of phosphatase activity that would otherwise prevent accumulation of mitotic phosphoproteins. These intricate regulatory pathways are essential for maintaining normal cell division and preventing inappropriate cell proliferation, a central hallmark of cancer cells.

Metabolic cooperation following fusion of starfish ootid and primary oocyte restores meiotic-phase-promoting activity

In the starfish Marthasterias glacialis, polyethylene glycol (PEG) homologous fused pairs consisting of two immature oocytes, blocked at the germinal vesicle stage, or two ootids, blocked at the female pronucleus stage, remain arrested at these specific stages, unless they are stimulated by the hormone 1-methyladenine. In contrast, heterologous pairs develop up to female pronucleus formation in the immature partner, indicating that maturation-promoting factor was formed under these conditions. Kinetics for this process, reconstitution of the nuclear envelopes after first polar body extrusion, and delaying effect of emetine argue for the existence of a true metabolic cooperation process requiring complementary factors present in each partner. The effect of inhibitors that penetrate the plasma membrane points to the possible involvement of endogenous proteases that may activate latent or neosynthesized maturation-promoting factor precursor and/or protein kinases.

The RSK family of kinases: emerging roles in cellular signalling

The 90 kDa ribosomal S6 kinase (RSK) family of proteins is a group of highly conserved Ser/Thr kinases that regulate diverse cellular processes, such as cell growth, cell motility, cell survival and cell proliferation. RSKs are downstream effectors of the Ras-extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) signalling cascade. Significant advances in the field of RSK and ERK/MAPK signalling have occurred in the past few years, including biological insights and the discovery of novel substrates and new RSK regulatory mechanisms. Collectively, these data expand the current models of RSK signalling and highlight potential directions of research in RSK-mediated survival, growth, proliferation and migration.

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.

Coordinate regulation of ribosome biogenesis and function by the ribosomal protein S6 kinase, a key mediator of mTOR function

Current understanding of the mechanisms by which cell growth is regulated lags significantly behind our knowledge of the complex processes controlling cell cycle progression. Recent studies suggest that the mammalian target of rapamycin (mTOR) pathway is a key regulator of cell growth via the regulation of protein synthesis. The key mTOR effectors of cell growth are eukaryotic initiation factor 4E-binding protein 1 (4EBP-1) and the ribosomal protein S6 kinase (S6K). Here we will review the current models for mTOR dependent regulation of ribosome function and biogenesis as well as its role in coordinating growth factor and nutrient signaling to facilitate homeostasis of cell growth and proliferation. We will place particular emphasis on the role of S6K1 signaling and will highlight the points of cross talk with other key growth control pathways. Finally, we will discuss the impact of S6K signaling and the consequent feedback regulation of the PI3K/Akt pathway on disease processes including cancer.

A 32 kDa protein?whose phosphorylation correlates with oncogenic Ras-induced cell cycle arrest in activatedXenopus egg extracts?is identified as ribosomal protein S6

Oncogenic Ras induces cell-cycle arrest in mammalian cells and in fertilized Xenopus eggs. How oncogenic Ras induces cell-cycle arrest remains unclear. We previously showed that oncogenic Ras induces cell-cycle arrest in activated Xenopus egg extracts (cycling extracts) and that the induced cell-cycle arrest correlates with hyperphosphorylation of a 32 kDa protein. However, the identity of the 32 kDa protein was not known. By using a sucrose density-gradient centrifugation, Triton X-100-acetic acid-urea (TAU)-gel electrophoresis, composite agarose-polyacrylamide gel electrophoresis (CAPAGE), SDS-PAGE, and partial tryptic peptide sequence analysis, the 32 kDa protein has now been identified as S6, a 40S subunit ribosomal protein. Hence, our results indicate that the oncogenic Ras-induced cell-cycle arrest is correlated with hyperphosphorylation of S6, suggesting that phosphorylation of S6 plays an important role in the induced cell-cycle arrest. It has been shown that conditional deletion of gene encoding S6 in mammalian cells prevents proliferation, demonstrating the importance of S6 in cell proliferation. The exact role S6 plays in cell proliferation is unclear. However, phosphorylation of S6 has been implicated in the regulation of protein synthesis. Thus, our results are consistent with the concept that oncogenic Ras induces S6 phosphorylation to influence protein synthesis, thereby contributing to the cell-cycle arrest. In addition, our results also demonstrate that composite agarose-polyacrylamide gel electrophoresis is suitable for the separation of large molecular complexes.

p70(S6K) controls selective mRNA translation during oocyte maturation and early embryogenesis in Xenopus laevis

In mammalian cells, p70(S6K) plays a key role in translational control of cell proliferation in response to growth factors. Because of the reliance on translational control in early vertebrate development, we cloned a Xenopus homolog of p70(S6K) and investigated the activity profile of p70(S6K) during Xenopus oocyte maturation and early embryogenesis. p70(S6K) activity is high in resting oocytes and decreases to background levels upon stimulation of maturation with progesterone. During embryonic development, three peaks of activity were observed: immediately after fertilization, shortly before the midblastula transition, and during gastrulation. Rapamycin, an inhibitor of p70(S6K) activation, caused oocytes to undergo germinal vesicle breakdown earlier than control oocytes, and sensitivity to progesterone was increased. Injection of a rapamycin-insensitive, constitutively active mutant of p70(S6K) reversed the effects of rapamycin. However, increases in S6 phosphorylation were not significantly affected by rapamycin during maturation. mos mRNA, which does not contain a 5'-terminal oligopyrimidine tract (5'-TOP), was translated earlier, and a larger amount of Mos protein was produced in rapamycin-treated oocytes. In fertilized eggs rapamycin treatment increased the translation of the Cdc25A phosphatase, which lacks a 5'-TOP. Translation assays in vivo using both DNA and RNA reporter constructs with the 5'-TOP from elongation factor 2 showed decreased translational activity with rapamycin, whereas constructs without a 5'-TOP or with an internal ribosome entry site were translated more efficiently upon rapamycin treatment. These results suggest that changes in p70(S6K) activity during oocyte maturation and early embryogenesis selectively alter the translational capacity available for mRNAs lacking a 5'-TOP region.

Inhibition of S6 kinase by rapamycin blocks maturation of Rana dybowskii oocytes

Studies were carried out to define the hormone-induced signal transduction pathway during maturation of Rana dybowskii oocytes. Rapamycin, a specific inhibitor of S6 kinase, blocked progesterone-induced oocyte germinal vesicle breakdown (GVBD) in a dose-dependent manner indicating that S6 kinase is required for meiotic maturation of Rana oocytes. Addition of rapamycin within 3 h, but not 6 h, of progesterone treatment inhibited GVBD. In contrast, cycloheximide, a general protein synthesis inhibitor, blocked GVBD even when added 9 h after progesterone addition. A twofold increase in S6 kinase activity occurred within 1 h of progesterone stimulation and rapamycin inhibited this activity. Rapamycin also suppressed, in a dose-dependent manner, progesterone-induced protein synthesis during the first 12 h of culture but less effectively later. Histone H1 kinase activity (maturation-promoting factor, MPF) was observed in oocyte extracts at two different times (between 6 and 9 h and at 24 h) following progesterone stimulation. Rapamycin blocked H1 kinase activity between 6 and 9 h of culture but not that observed at 24 h. In contrast, cycloheximide suppressed progesterone-induced H1 kinase activity as well as protein synthesis throughout the course of incubation. Such results indicate that rapamycin and cycloheximide have common and unique effects on oocyte maturation and suggest that progesterone-induced S6 kinase activity is closely associated with induction of protein synthesis and activation of MPF during oocyte maturation. Results in Rana contrast with those obtained in Xenopus where rapamycin inhibited S6 kinase but failed to inhibit GVBD or protein synthesis. Differences in the response of Rana and Xenopus oocytes to rapamycin are discussed in relation to seasonal, biochemical, and species variations.

p38 Mitogen-activated protein kinase is a critical component of the redox-sensitive signaling pathways activated by angiotensin II. Role in vascular smooth muscle cell hypertrophy

Angiotensin II induces an oxidant stress-dependent hypertrophy in cultured vascular smooth muscle cells. To investigate the growth-related molecular targets of H2O2, we examined the redox sensitivity of agonist-stimulated activation of the mitogen-activated protein kinase (MAPK) family. We show here that angiotensin II elicits a rapid increase in intracellular H2O2 and a rapid and robust phosphorylation of both p42/44MAPK (16-fold) and p38MAPK (15-fold). However, exogenous H2O2 activates only p38MAPK (14-fold), and diphenylene iodonium, an NADH/NADPH oxidase inhibitor, attenuates angiotensin II-stimulated phosphorylation of p38MAPK, but not p42/44MAPK. Furthermore, in cells stably transfected with human catalase, angiotensin II-induced intracellular H2O2 generation is almost completely blocked, resulting in inhibition of phosphorylation of p38MAPK, but not p42/44MAPK, and a subsequent partial decrease in angiotensin II-induced hypertrophy. Specific inhibition of either the p38MAPK pathway with SB203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H- imidaz ole) or the p42/44MAPK pathway with PD98059 (2-(2'-amino-3'-methoxyphenyl)oxanaphthalen-4-one) also partially, but significantly, attenuates angiotensin II-induced hypertrophy; however, simultaneous blockade of both pathways has an additive inhibitory effect, indicating that the hypertrophic response to angiotensin II requires parallel, independent activation of both MAPK pathways. These results provide the first evidence that p38MAPK is a critical component of the oxidant stress (H2O2)-sensitive signaling pathways activated by angiotensin II in vascular smooth muscle cells and indicate that it plays a crucial role in vascular hypertrophy.

Phosphorylation of elongation factor-1 (EF-1) by cdc2 kinase

Elongation factor-1 (EF-1) is a major substrate for cdc2 kinase in Xenopus oocytes. The guanine-nucleotide exchange factor EF-1 beta gamma delta, appears to have a highly complex macromolecular structure containing several GTP/GDP exchange proteins, valyl-tRNA synthetase, and a putative anchoring protein EF-1 gamma. During meiotic cell division, the factor becomes phosphorylated by cdc2 kinase, not only on EF-1 gamma, but also on two different phospho-acceptors on EF-1 delta. Phosphorylation is concomitant with changes in protein synthesis in vivo. Xenopus oocytes, and potentially all cells, contain a multitude of heteromeric forms of the complex which postulates that EF-1 beta gamma delta is not a "house keeping" factor but a sophisticated regulatory element.

Evidence that protein kinase C and mitogen activated protein kinase are not involved in the mechanism by which insulin stimulates translation in L6 myoblasts

Insulin stimulated a concentration-dependent increase in protein synthesis in L6 myoblasts which was significant at 1 nM. This response was not prevented by the transcription inhibitor, actinomycin D. The protein kinase C (PKC) inhibitor, Ro-31-8220, and downregulation of PKC by prolonged incubation of cells with 12-O-tetradecanoylphorbol-13-acetate (TPA), had no effect on the ability of insulin to stimulate protein synthesis whilst completely blocking the response to TPA. In contrast, insulin failed to enhance protein synthesis significantly in the presence of either ibuprofen, a selective cyclooxygenase inhibitor or rapamycin, an inhibitor of the 70 kDa S6 kinase. When cell extracts were prepared and assayed for total myelin basic protein kinase activity, a stimulatory effect of insulin was not observed until the concentration approached 100-fold (i.e. 100 nM) that required to elicit increases in protein synthesis. Upon fractionation on a Mono-Q column, 100 nM insulin increased the activity of 3 peaks which phosphorylated myelin basic protein. Two of these peaks were identified as the 42 and 44 kDa forms of Mitogen Activated Protein (MAP) kinase by immunoblotting. In contrast, 1 nM insulin had no effect on the activity of these peaks. The data suggest that physiologically relevant concentrations of insulin do not stimulate translation in L6 cells through either PKC or the 42/44 kDa isoforms of MAP kinase and that this response is, at least in part, mediated through the activation of the 70 kDa S6 kinase by cyclooxygenase metabolites.

S6 phosphorylation and the p70s6k/p85s6k

Activation of cell growth leads to the multiple phosphorylation of 40S ribosomal protein S6. The kinase responsible for controling this event is termed p70s6k/p85s6k. Both isoforms of the kinase are derived from a common gene activated by a complex set of phosphorylation events; each resides in a unique cellular compartment: the p70s6k in the cytoplasm and the p85s6k in the nucleus. Although p70s6k/p85s6k represent the first mitogen-activated serine/threonine kinase described, the signaling pathway leading to activation of both isoforms remains obscure. Recent studies have shown that this pathway is distinct from that of p21ras and the p42mapk/p44mapk, and that bifurcation of these pathways takes place at the level of the receptor. Experiments with point mutants of the PDGF receptor and inhibitors of phosphatidyl-inositol-3-OH kinase have implicated the latter molecule in this signaling event, but more recent findings suggest an alternative route may be employed. The p70s6k signaling pathway can also be ablated by the immunosuppressant rapamycin, which blocks p70s6k activation and S6 phosphorylation without affecting the other kinases whose activation is triggered by mitogen treatment. In parallel, rapamycin suppresses the translation of a family of mRNAs that contain a polypyrimidine tract at their 5' transcriptional start site. The implication is that this event is mediated by the phosphorylated form of S6 that may either (1) directly interact with the polypyrimidine tract or (2) alter the affinity of the 40S ribosome mRNA binding site for polypyrimidine tract mRNAs, or (3) recognize proteins that directly bind to the polypyrimidine tract.

A nuclear factor required for specific translation of cyclin B may control the timing of first meiotic cleavage in starfish oocytes

In most animals, the rate of cyclin B synthesis increases after nuclear envelope breakdown during the first meiotic cell cycle. We have found that cyclin B-cdc2 kinase activity drops earlier in emetine-treated than in control starfish oocytes, although the protein synthesis inhibitor does not activate the cyclin degradation pathway prematurely. Moreover, protein synthesis is required to prevent meiotic cleavage to occur prematurely, sometimes before chromosomes have segregated on the metaphase plate. In normal conditions, increased synthesis of cyclin B after germinal vesicle breakdown (GVBD) balances cyclin degradation and increases the time required for cyclin B-cdc2 kinase to drop below the level that inhibits cleavage. Taken together, these results point to cyclin B as a possible candidate that could explain the need for increased protein synthesis during meiosis I. Although direct experimental evidence was not provided in the present work, cyclin B synthesis after GVBD may be important for correct segregation of homologous chromosomes at the end of first meiotic metaphase, as shown by a variety of cytological disorders that accompany premature cleavage. Although the overall stimulation of protein synthesis because of cdc2 kinase activation is still observed in oocytes from which the germinal vesicle has been removed before hormonal stimulation, the main increase of cyclin B synthesis normally observed after germinal vesicle breakdown is suppressed. The nuclear factor required for specific translation of cyclin B after GVBD is not cyclin B mRNA, as shown by using a highly sensitive reverse transcription followed by polymerase chain reaction procedure that failed to detect any cyclin B mRNA in isolated germinal vesicles.

On the importance of protein phosphorylation in cell cycle control

This chapter is written as a contribution to a volume commemorating the work of Krebs and Fischer that led to awarding of the Nobel Prize in 1992. This award was made because of their fundamental discovery in the mid-1950s that protein phosphorylation was the underlying mechanism that accounted for the reversible modification of activity of glycogen phosphorylase in mammalian skeletal muscle. Although it could not be anticipated at the time that phosphorylation would turn out to be such a ubiquitous regulator of cellular functions, it is now evident that phosphorylation controls virtually every important reaction in cells and provides the basis for understanding how integrated cellular behavior is regulated by both extracellular signals and internal control mechanisms. This chapter relates the historical development in biochemical terms of protein phosphorylation as a regulator of the cell cycle in Xenopus oocytes and eggs.

The MAP kinase cascade. Discovery of a new signal transduction pathway

Using biochemical techniques similar to those used by Krebs and Fischer in elucidating the cAMP kinase cascade, a protein kinase cascade has been found that represents a new pathway for signal transduction. This pathway is activated in almost all cells that have been examined by many different growth and differentiation factors, suggesting control of different cell responses. At this writing, four tiers of growth factor regulated kinases, each tier represented by more than one enzyme, have been reconstituted in vitro to form the MAP kinase cascade. Preliminary findings suggesting multiple feedback or feedforward regulation of several components in the cascade predict higher complexity than a simple linear pathway.

Insulin and IGF-I receptor signaling in cultured neurons

The data presented in this chapter are summarized in the schematic shown in Figure 9. Insulin binds to and stimulates autophosphorylation of neuronal insulin receptors, whereas, IGF-I and IGF-II binds to and stimulate autophosphorylation of neuronal IGF-I receptors. IGF-II is also capable of binding to the insulin receptor. Whether or not it activates the insulin receptor kinase remains to be clarified. Activated insulin and IGF-I receptor kinases phosphorylate a 70-kDa protein at early times in culture. This protein may mediate some actions of insulin, but we speculate that there are other intermediary proteins involved in the transduction pathway resulting in the activation of S6 kinase and PKC epsilon. The stimulation of S6 kinase by insulin and IGF-I may be associated with the translational activation of protein synthesis by these peptides. The stimulation of PKC epsilon appears to be a necessary step in the transcriptional regulation of the c-fos gene by insulin and IGF-I. The regulation of neuronal protein synthesis at a translational step and the initiation of transcriptional programs regulated by AP-1 represent two mechanisms by which insulin and IGFs alter neuronal growth and differentiation.

Inhibition of mos-induced oocyte maturation by protein kinase A

The relationship between the mos protooncogene protein and cAMP-dependent protein kinase (PKA) during the maturation of Xenopus oocytes was investigated. Microinjection of the PKA catalytic subunit (PKAc) into Xenopus oocytes inhibited oocyte maturation induced by the mos product but did not markedly affect the autophosphorylation activity of injected mos protein. By contrast, PKAc did not inhibit maturation promoting factor (MPF) activation or germinal vesicle breakdown (GVBD) that was initiated by injecting crude MPF preparations. In addition, inhibiting endogenous PKA activity by microinjecting the PKA regulatory subunit (PKAr) induced oocyte maturation that was dependent upon the presence of the endogenous mos product. Moreover, PKAr potentiated mos protein-induced MPF activation in the absence of progesterone and protein synthesis. These data are consistent with the hypothesis that progesterone-induced release from G2/M is regulated via PKAc and that PKAc negatively regulates a downstream target that is positively regulated by mos.

Regulation of biological functions by an insulin receptor monoclonal antibody in insulin receptor beta-subunit mutants

We investigated the effects of MA-5, a human-specific monoclonal antibody to the insulin receptor alpha-subunit, on transmembrane signaling in cell lines transfected with and expressing both normal human insulin receptors and receptors mutated in their beta-subunit tyrosine kinase domains. In cell lines expressing normal human insulin receptors, MA-5 stimulated three biological functions: aminoisobutyric acid (AIB) uptake, thymidine incorporation, and S6 kinase activation. Under conditions where these biological functions were stimulated, there was no detectable stimulation of receptor tyrosine kinase. We then combined the use of this monoclonal antibody with cells expressing insulin receptors with mutations in the beta-subunit tyrosine kinase domain; two of ATP binding site mutants V1008 (Gly----Val) and M1030 (Lys----Met) and one triple-tyrosine autophosphorylation site mutant F3 (Tyr----Phe at 1158, 1162, and 1163). In cells expressing V1008 receptors, none of the three biological functions of insulin was stimulated. In cells expressing M1030 receptors, AIB uptake was stimulated to a small, but significant, extent whereas the other two functions were not. In cells expressing F3 receptors, AIB uptake and S6 kinase activation, but not thymidine incorporation, were fully stimulated. The data suggest, therefore, that (1) activation of insulin receptor tyrosine kinase may not be a prerequisite for signaling of all the actions of insulin and (2) there may be multiple signal transduction pathways to account for the biological actions of insulin.

tpr-met oncogene product induces maturation-producing factor activation in Xenopus oocytes

tpr-met, a tyrosine kinase oncogene, is the activated form of the met proto-oncogene that encodes the receptor for hepatocyte growth factor/scatter factor. The tpr-met product (p65tpr-met) was tested for its ability to induce meiotic maturation in Xenopus oocytes. While src and abl tyrosine kinase oncogene products have previously been shown to be inactive in this assay, p65tpr-met efficiently induced maturation-promoting factor (MPF) activation and germinal vesicle breakdown (GVBD) together with the associated increase in ribosomal S6 subunit phosphorylation. tpr-met-mediated MPF activation and GVBD was dependent on the endogenous c-mosxe, while the increase in S6 protein phosphorylation was not significantly affected by the loss of mos function. The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine inhibits tpr-met-mediated GVBD at concentrations that prevent insulin- but not progesterone-induced oocyte maturation. Moreover, maturation triggered by tpr-met is also inhibited by cyclic AMP-dependent protein kinase. This is the first demonstration that a tyrosine kinase oncogene product, p65tpr-met, can induce meiotic maturation in Xenopus oocytes and activate MPF through a mos-dependent pathway, possibly the insulin or insulinlike growth factor 1 pathway.

tpr-met oncogene product induces maturation-producing factor activation in Xenopus oocytes

tpr-met, a tyrosine kinase oncogene, is the activated form of the met proto-oncogene that encodes the receptor for hepatocyte growth factor/scatter factor. The tpr-met product (p65tpr-met) was tested for its ability to induce meiotic maturation in Xenopus oocytes. While src and abl tyrosine kinase oncogene products have previously been shown to be inactive in this assay, p65tpr-met efficiently induced maturation-promoting factor (MPF) activation and germinal vesicle breakdown (GVBD) together with the associated increase in ribosomal S6 subunit phosphorylation. tpr-met-mediated MPF activation and GVBD was dependent on the endogenous c-mosxe, while the increase in S6 protein phosphorylation was not significantly affected by the loss of mos function. The phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine inhibits tpr-met-mediated GVBD at concentrations that prevent insulin- but not progesterone-induced oocyte maturation. Moreover, maturation triggered by tpr-met is also inhibited by cyclic AMP-dependent protein kinase. This is the first demonstration that a tyrosine kinase oncogene product, p65tpr-met, can induce meiotic maturation in Xenopus oocytes and activate MPF through a mos-dependent pathway, possibly the insulin or insulinlike growth factor 1 pathway.

Tyrosine phosphorylation and activation of homologous protein kinases during oocyte maturation and mitogenic activation of fibroblasts

Meiotic maturation of Xenopus and sea star oocytes involves the activation of a number of protein-serine/threonine kinase activities, including a myelin basic protein (MBP) kinase. A 44-kDa MBP kinase (p44mpk) purified from mature sea star oocytes is shown here to be phosphorylated at tyrosine. Antiserum to purified sea star p44mpk was used to identify antigenically related proteins in Xenopus oocytes. Two tyrosine-phosphorylated 42-kDa proteins (p42) were detected with this antiserum in Xenopus eggs. Xenopus p42 chromatographs with MBP kinase activity on a Mono Q ion-exchange column. Tyrosine phosphorylation of Xenopus p42 approximately parallels MBP kinase activity during meiotic maturation. These results suggest that related MBP kinases are activated during meiotic maturation of Xenopus and sea star oocytes. Previous studies have suggested that Xenopus p42 is related to the mitogen-activated protein (MAP) kinases of culture mammalian cells. We have cloned a MAP kinase relative from a Xenopus ovary cDNA library and demonstrate that this clone encodes the Xenopus p42 that is tyrosine phosphorylated during oocyte maturation. Comparison of the sequences of Xenopus p42 and a rat MAP kinase (ERK1) and peptide sequences from sea star p44mpk indicates that these proteins are close relatives. The family members appear to be tyrosine phosphorylated, and activated, in different contexts, with the murine MAP kinase active during the transition from quiescence to the G1 stage of the mitotic cell cycle and the sea star and Xenopus kinases being active during M phase of the meiotic cell cycle.

Tyrosine phosphorylation and activation of homologous protein kinases during oocyte maturation and mitogenic activation of fibroblasts

Meiotic maturation of Xenopus and sea star oocytes involves the activation of a number of protein-serine/threonine kinase activities, including a myelin basic protein (MBP) kinase. A 44-kDa MBP kinase (p44mpk) purified from mature sea star oocytes is shown here to be phosphorylated at tyrosine. Antiserum to purified sea star p44mpk was used to identify antigenically related proteins in Xenopus oocytes. Two tyrosine-phosphorylated 42-kDa proteins (p42) were detected with this antiserum in Xenopus eggs. Xenopus p42 chromatographs with MBP kinase activity on a Mono Q ion-exchange column. Tyrosine phosphorylation of Xenopus p42 approximately parallels MBP kinase activity during meiotic maturation. These results suggest that related MBP kinases are activated during meiotic maturation of Xenopus and sea star oocytes. Previous studies have suggested that Xenopus p42 is related to the mitogen-activated protein (MAP) kinases of culture mammalian cells. We have cloned a MAP kinase relative from a Xenopus ovary cDNA library and demonstrate that this clone encodes the Xenopus p42 that is tyrosine phosphorylated during oocyte maturation. Comparison of the sequences of Xenopus p42 and a rat MAP kinase (ERK1) and peptide sequences from sea star p44mpk indicates that these proteins are close relatives. The family members appear to be tyrosine phosphorylated, and activated, in different contexts, with the murine MAP kinase active during the transition from quiescence to the G1 stage of the mitotic cell cycle and the sea star and Xenopus kinases being active during M phase of the meiotic cell cycle.

Regulation of pp90rsk phosphorylation and S6 phosphotransferase activity in Swiss 3T3 cells by growth factor-, phorbol ester-, and cyclic AMP-mediated signal transduction

Somatic cell homologs to the Xenopus laevis S6 protein kinases (referred to collectively as pp90rsk) have recently been identified and partially characterized. Here we examine alterations in pp90rsk phosphorylation and S6 phosphotransferase activity in response to regulators of multiple signal transduction systems: purified growth factors, phorbol ester, changes in cyclic AMP (cAMP) levels, and sodium vanadate. All reagents tested increased pp90rsk serine and threonine phosphorylation, but only those agents that regulate cell proliferation and sodium vanadate activated its S6 kinase activity. In addition to the cAMP-stimulated phosphorylation of pp90rsk, a simple correlation between the extent of growth-regulated pp90rsk phosphorylation and S6 phosphotransferase activity was not observed. Quantitative phosphorylation of pp90rsk continued to increase after its S6 kinase activity began its return towards basal levels. However, a close correlation between the appearance and disappearance of a slow-mobility form of phosphorylated pp90rsk (by electrophoresis) and pp90rsk activity was observed. In addition, pp90rsk was regulated by both protein kinase C-independent and -dependent signaling mechanisms. The extent of protein kinase C participation, however, varied depending on which growth factor receptor was activated. Furthermore, growth factor-specific differences in the temporal regulation of pp90rsk S6 phosphotransferase activity were also observed. These results support the notion that the complex regulation of the rsk gene product constitutes one of the primary responses of animal cells to mitogenic signals.

Regulation of pp90rsk phosphorylation and S6 phosphotransferase activity in Swiss 3T3 cells by growth factor-, phorbol ester-, and cyclic AMP-mediated signal transduction

Somatic cell homologs to the Xenopus laevis S6 protein kinases (referred to collectively as pp90rsk) have recently been identified and partially characterized. Here we examine alterations in pp90rsk phosphorylation and S6 phosphotransferase activity in response to regulators of multiple signal transduction systems: purified growth factors, phorbol ester, changes in cyclic AMP (cAMP) levels, and sodium vanadate. All reagents tested increased pp90rsk serine and threonine phosphorylation, but only those agents that regulate cell proliferation and sodium vanadate activated its S6 kinase activity. In addition to the cAMP-stimulated phosphorylation of pp90rsk, a simple correlation between the extent of growth-regulated pp90rsk phosphorylation and S6 phosphotransferase activity was not observed. Quantitative phosphorylation of pp90rsk continued to increase after its S6 kinase activity began its return towards basal levels. However, a close correlation between the appearance and disappearance of a slow-mobility form of phosphorylated pp90rsk (by electrophoresis) and pp90rsk activity was observed. In addition, pp90rsk was regulated by both protein kinase C-independent and -dependent signaling mechanisms. The extent of protein kinase C participation, however, varied depending on which growth factor receptor was activated. Furthermore, growth factor-specific differences in the temporal regulation of pp90rsk S6 phosphotransferase activity were also observed. These results support the notion that the complex regulation of the rsk gene product constitutes one of the primary responses of animal cells to mitogenic signals.

Phosphorylation of ribosomal proteins induced by auxins in maize embryonic tissues

The effect of auxin on ribosomal protein phosphorylation of germinating maize (Zea mays) tissues was investigated. Two-dimensional gel electrophoresis and autoradiography of [(32)P] ribosomal protein patterns for natural and synthetic auxin-treated tissues were performed. Both the rate of (32)P incorporation and the electrophoretic patterns were dependent on (32)P pulse length, suggesting that active protein phosphorylation-dephosphorylation occurred in small and large subunit proteins, in control as well as in auxin-treated tissues. The effect of ribosomal protein phosphorylation on in vitro translation was tested. Measurements of poly(U) translation rates as a function of ribosome concentration provided apparent K(m) values significantly different for auxin-treated and nontreated tissues. These findings suggest that auxin might exert some kind of translational control by regulating the phosphorylated status of ribosomal proteins.

In vivo activation of a microtubule-associated protein kinase during meiotic maturation of the Xenopus oocyte

We have characterized a serine/threonine protein kinase from Xenopus metaphase-II-blocked oocytes, which phosphorylates in vitro the microtubule-associated protein 2 (MAP2). The MAP2 kinase activity, undetectable in prophase oocytes, is activated during the progesterone-induced meiotic maturation (G2-M transition of the cell cycle). p-Nitrophenyl phosphate, a phosphatase inhibitor, is required to prevent spontaneous deactivation of the MAP2 kinase in crude preparations; conversely, the partially purified enzyme can be in vitro deactivated by the low-Mr polycation-stimulated (PCSL) phosphatase (also termed protein phosphatase 2A2), working as a phosphoserine/phosphothreonine-specific phosphatase and not as a phosphotyrosyl phosphatase indicating that phosphorylation of serine/threonine is necessary for its activity. S6 kinase, a protein kinase activated during oocyte maturation which phosphorylates in vitro ribosomal protein S6 and lamin C, can be deactivated in vitro by PCSL phosphatase. S6 kinase from prophase oocytes can also be activated in vitro in fractions known to contain all the factors necessary to convert pre-M-phase-promoting factor (pre-MPF) to MPF. Active MAP2 kinase can activate in vitro the inactive S6 kinase present in prophase oocytes or reactivate S6 kinase previously inactivated in vitro by PCSL phosphatase. These data are consistent with the hypothesis that the MAP2 kinase is a link of the meiosis signalling pathway and is activated by a serine/threonine kinase. This will lead to the regulation of further steps in the cell cycle, such as microtubular reorganisation and S6 kinase activation.

Effects of the v-mos oncogene on Xenopus development: meiotic induction in oocytes and mitotic arrest in cleaving embryos

Previous work has demonstrated that the Xenopus protooncogene mosxe can induce the maturation of prophase-arrested Xenopus oocytes. Recently, we showed that mosxe can transform murine NIH3T3 fibroblasts, although it exhibited only 1-2% of the transforming activity of the v-mos oncogene. In this study we have investigated the ability of the v-mos protein to substitute for the mosxe protein in stimulating Xenopus oocytes to complete meiosis. Microinjection of in vitro synthesized RNAs encoding either the mosxe or v-mos proteins stimulates resting oocytes to undergo germinal vesicle breakdown. Microinjection of an antisense oligonucleotide spanning the initiation codon of the mosxe gene blocked progesterone-induced oocyte maturation. When oocytes were microinjected first with the mosxe antisense oligonucleotide, and subsequently with in vitro synthesized v-mos RNA, meiotic maturation was rescued as evidenced by germinal vesicle breakdown. The v-mos protein exhibited in vitro kinase activity when recovered by immunoprecipitation from either microinjected Xenopus oocytes or transfected monkey COS-1 cells; however, in parallel experiments, we were unable to detect in vitro kinase activity associated with the mosxe protein. Microinjection of in vitro synthesized v-mos RNA into cleaving Xenopus embryos resulted in mitotic arrest, demonstrating that the v-mos protein can function like the mosxe protein as a component of cytostatic factor. These results exemplify the apparently conflicting effects of the v-mos protein, namely, its ability to induce maturation of oocytes, its ability to arrest mitotic cleavage of Xenopus embryo, and its ability to transform mammalian fibroblasts.

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.

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.

Ha-rasVal-12,Thr-59 activates S6 kinase and p34cdc2 kinase in Xenopus oocytes: evidence for c-mosxe-dependent and -independent pathways

Treatment with insulin or progesterone or microinjection of the transforming protein product of Ha-rasVal-12,Thr-59 (p21) is known to induce germinal vesicle breakdown in Xenopus oocytes. We have investigated the effect of p21 on S6 kinase and the H1 histone kinase of maturation-promoting factor in the presence and absence of antisense oligonucleotides against the c-mosxe proto-oncogene. Injection of p21 led to a rapid increase in S6 phosphorylation, with kinetics similar to those previously observed with insulin. Microinjection of c-mosxe antisense oligonucleotides inhibited germinal vesicle breakdown induced by p21 and totally abolished S6 kinase activation by insulin or progesterone but only partially inhibited activation by p21. However, the activation of p34cdc2 protein kinase by all three stimuli was blocked by antisense oligonucleotides. The results suggest that in oocyte maturation c-mosxe functions downstream of p21 but upstream of p34cdc2 and S6 kinase activation, although not all p21-induced events require c-mosxe.

Ha-rasVal-12,Thr-59 activates S6 kinase and p34cdc2 kinase in Xenopus oocytes: evidence for c-mosxe-dependent and -independent pathways

Treatment with insulin or progesterone or microinjection of the transforming protein product of Ha-rasVal-12,Thr-59 (p21) is known to induce germinal vesicle breakdown in Xenopus oocytes. We have investigated the effect of p21 on S6 kinase and the H1 histone kinase of maturation-promoting factor in the presence and absence of antisense oligonucleotides against the c-mosxe proto-oncogene. Injection of p21 led to a rapid increase in S6 phosphorylation, with kinetics similar to those previously observed with insulin. Microinjection of c-mosxe antisense oligonucleotides inhibited germinal vesicle breakdown induced by p21 and totally abolished S6 kinase activation by insulin or progesterone but only partially inhibited activation by p21. However, the activation of p34cdc2 protein kinase by all three stimuli was blocked by antisense oligonucleotides. The results suggest that in oocyte maturation c-mosxe functions downstream of p21 but upstream of p34cdc2 and S6 kinase activation, although not all p21-induced events require c-mosxe.

Sequence and expression of chicken and mouse rsk: homologs of Xenopus laevis ribosomal S6 kinase

We have previously reported the isolation of cDNAs encoding two closely related Xenopus ribosomal S6 kinases, S6KII alpha and -beta (S. W. Jones, E. Erikson, J. Blenis, J. L. Maller, and R. L. Erikson, Proc. Natl. Acad. Sci. USA 85:3377-3381, 1988). We report here the molecular cloning of one chicken and two mouse homologs of the Xenopus laevis cDNAs. As described for the Xenopus proteins, these cDNAs were found to predict polypeptides that contain two distinct kinase domains, of which one is most closely related to the catalytic subunit of cyclic AMP-dependent protein kinase and the other is most closely related to the catalytic subunit of phosphorylase b kinase. The three predicted proteins were more than 79% identical to the Xenopus S6KII alpha protein. The chicken and one of the mouse cDNAs were, respectively, 3.7 and 3.1 kilobase pairs in length, predicted proteins of 752 and 724 amino acids with molecular weights of 84.4 and 81.6 kilodaltons, and hybridized to mRNAs in fibroblasts and tissues of approximately 3.6 and 3.4 kilobases (kb). The second mouse cDNA was approximately 6.1 kilobase pairs and was not full length but predicted the C-terminal 633 amino acids of a protein that is similar to the C-terminal portion of Xenopus S6KII alpha. This clone hybridized to mRNA transcripts of 7.6 and 3.4 kb. In vitro transcription and translation of the chicken and the mouse cDNAs that predict complete proteins produced major products with apparent molecular weights of 96 and 84 kilodaltons. Analysis of mRNA levels in chicken tissues showed significant quantities of the 3.6-kb transcript in small and large intestine, spleen, and bursa. Both mouse cDNA were similarly expressed at significant levels in intestine, thymus, and lung; however, the 7.6-kb mRNA was differentially and more highly expressed in heart and brain. The two mouse cDNAs represent two different S6 kinase genes, as shown by comparison of their protein sequences, mRNA transcript sizes, genomic organizations, and nucleic acid sequences. We propose that this family of genes be named rsk, for ribosomal S6 kinase.

Sequence and expression of chicken and mouse rsk: homologs of Xenopus laevis ribosomal S6 kinase

We have previously reported the isolation of cDNAs encoding two closely related Xenopus ribosomal S6 kinases, S6KII alpha and -beta (S. W. Jones, E. Erikson, J. Blenis, J. L. Maller, and R. L. Erikson, Proc. Natl. Acad. Sci. USA 85:3377-3381, 1988). We report here the molecular cloning of one chicken and two mouse homologs of the Xenopus laevis cDNAs. As described for the Xenopus proteins, these cDNAs were found to predict polypeptides that contain two distinct kinase domains, of which one is most closely related to the catalytic subunit of cyclic AMP-dependent protein kinase and the other is most closely related to the catalytic subunit of phosphorylase b kinase. The three predicted proteins were more than 79% identical to the Xenopus S6KII alpha protein. The chicken and one of the mouse cDNAs were, respectively, 3.7 and 3.1 kilobase pairs in length, predicted proteins of 752 and 724 amino acids with molecular weights of 84.4 and 81.6 kilodaltons, and hybridized to mRNAs in fibroblasts and tissues of approximately 3.6 and 3.4 kilobases (kb). The second mouse cDNA was approximately 6.1 kilobase pairs and was not full length but predicted the C-terminal 633 amino acids of a protein that is similar to the C-terminal portion of Xenopus S6KII alpha. This clone hybridized to mRNA transcripts of 7.6 and 3.4 kb. In vitro transcription and translation of the chicken and the mouse cDNAs that predict complete proteins produced major products with apparent molecular weights of 96 and 84 kilodaltons. Analysis of mRNA levels in chicken tissues showed significant quantities of the 3.6-kb transcript in small and large intestine, spleen, and bursa. Both mouse cDNA were similarly expressed at significant levels in intestine, thymus, and lung; however, the 7.6-kb mRNA was differentially and more highly expressed in heart and brain. The two mouse cDNAs represent two different S6 kinase genes, as shown by comparison of their protein sequences, mRNA transcript sizes, genomic organizations, and nucleic acid sequences. We propose that this family of genes be named rsk, for ribosomal S6 kinase.

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].