Differential regulation of histone H1 and ribosomal S6 kinases during sea star oocyte maturation

In the preceding paper [Pelech, S.L., Meijer, L., & Krebs, E.G. (1987) Biochemistry (preceding paper in this issue)], at least three activated kinases were detected in soluble extracts from sea star oocytes induced to undergo maturation by 1-methyladenine (1-MeAde). Coincident with nuclear envelope breakdown (20 min after exposure to 1-MeAde), there was a rapid activation of a histone H1 kinase that eluted from DEAE-Sephacel with a conductivity of approximately 6 mmho. By contrast, 60-min treatment of the oocytes with 1-MeAde was required for maximal activation of two kinases, each of which phosphorylated a synthetic peptide, Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala (RRLSSLRA), patterned after a phosphorylation site sequence from ribosomal protein S6. These RRLSSLRA kinases were released from DEAE-Sephacel with elution conductivities of approximately 6 and approximately 10.5 mmho. The 1-MeAde dose-response curves for maturation induction and activation of the histone H1 and RRLSSLRA kinases were superimposable. Both oocyte maturation and the activation of the kinases required the presence of 1-MeAde during the hormone-dependent period. When 1-MeAde was removed after this period, full histone H1 kinase activation still occurred and maturation was induced. Forskolin pretreatment of the oocytes, by elevating the basal cAMP level more than 35-fold, doubled the hormone-dependent period and similarly delayed the onset of histone H1 kinase activation by 1-MeAde. However, postmaturation activation of the RRLSSLRA kinases was completely blocked by forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of maturation-activated histone H1 and ribosomal S6 kinases in sea star oocytes

DEAE-Sephacel chromatography of cytosolic extracts from sea star oocytes resolved at least two distinct peaks of maturation-activated protein kinase activity, each of which catalyzed the phosphorylation of histone H1, ribosomal protein S6, and Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala (RRLSSLRA), a synthetic peptide based on the sequence of a phosphorylation site in the latter protein. The first peak (elution conductivity approximately equal to 6 mmho) contained the major activated kinase with respect to the phosphorylation of histone H1, and the second peak (elution conductivity approximately equal to 10.5 mmho) contained the major activated kinase with respect to the phosphorylation of S6 and RRLSSLRA. These kinase activities were barely detectable in extracts from immature oocytes. The major stimulated histone H1 kinase exhibited an apparent Mr of approximately 90 000 on Sephacryl S-300 but eluted from TSK-400 with an apparent Mr of approximately 10 000. After DEAE-Sephacel fractionation, this kinase was shown to utilize both ATP (apparent Km approximately equal to 45 microM) and GTP (apparent Km approximately equal to 10 microM), although the Vmax was 8-fold higher with ATP than with GTP. The enzyme phosphorylated histone H1 with an apparent Km approximately equal to 50 micrograms/mL. Its properties resembled those of the growth-associated histone kinase. The major stimulated RRLSSLRA kinase had an apparent Mr of approximately 84 000 on Sephacryl S-300 and approximately 40 000 on TSK-400. After DEAE-Sephacel chromatography, this kinase selectively utilized ATP (apparent Km approximately equal to 25 microM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterizations of two distinct Ca2+-dependent phospholipid-binding proteins of 68-kDa isolated from human placenta

Two distinct 68-kDa proteins, named 68K-I (pI 6.4) and 68K-II (pI 5.6), were solubilized from human placenta by treatment with 5 mM EGTA. On DE52 cellulose column chromatography at pH 7.4, 68K-I in the EGTA eluate was recovered in the unadsorbed fractions, whereas 68K-II was retained on the column and eluted with 0.2 M NaCl. The 68K-I protein was obtained in more than 95% purity by further hydroxylapatite and cation exchange chromatographies, while the 68K-II protein was purified further by gel filtration and hydroxylapatite chromatographies. Partial amino acid sequence data showed that 68K-I protein was a novel protein which shared the same sequences as lipocortin I and that 68K-II was the same as human p68/67-kDa calelectrin (Crompton, M. R., Owens, R. J., Totty, N. F., Moss, S. E., Waterfield, M.D., and Crumpton, M. J. (1988) EMBO J. 7, 21-27; Südhof, T. C., Slaughter, C. A., Leznicki, I., Barjon, P., and Reynolds, G. A. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 664-668). The two proteins bound to acidic phospholipids, phosphatidylserine, and/or phosphatidylinositol, but not to phosphatidylcholine, in the presence of micromolar levels of Ca2+. 68K-I bound to phosphatidylinositol preferentially to phosphatidylserine, whereas 68K-II bound only to phosphatidylserine. Both 68K-I and 68K-II inhibited phospholipase A2 activity, and the inhibition by 68K-II was detectable only in the presence of 100 mM KCl. 68K-I, but not 68K-II, was found to bind to F-actin in a Ca2+-dependent (1 mM) manner. Moreover 68K-I, but not 68K-II, was phosphorylated in vitro at tyrosine residues by fps kinase and by epidermal growth factor receptor/kinase, the latter reaction being dependent on Ca2+ and epidermal growth factor. Western blot analysis with affinity purified anti-68K-I and anti-68K-II antibodies showed that 68K-I was located in only certain tissues, especially human placenta, whereas 68K-II was present in many human and rat tissues.     

Isolation and characterization of three forms of 36-kDa Ca2+-dependent actin- and phospholipid-binding proteins from human placenta membrane

We purified three forms of 36-kDa proteins, two monomeric 36-kDa proteins, which had pIs of 7.5 (36K-I) and 6.4 (36K-II), and one 36-kDa complex (36K-C) consisting of two subunits, 36-kDa (pI 7.5) and 12-kDa (pI 5.8), from human placenta membrane. The 36-kDa subunit of 36K-C was identical to 36K-I as judged by pI, cyanogen bromide peptide mapping and immunological cross-reactivity. The three proteins showed F-actin- and phosphatidylserine-binding abilities dependent on Ca2+ concentrations at millimolar and micromolar levels, respectively. They all had phospholipase A2 inhibitory activity. Only 36K-II was phosphorylated extensively at tyrosine residue in Ca2+- and EGF- dependent manners in the membrane fraction of A431 cells. 36K-I was the best substrate for src kinase, whereas 36K-II was the best for fps kinase. However, 36K-C was not phosphorylated by any kinases used here.     

A 32-kDa protein associated with phospholipase A2-inhibitory activity from human placenta

Two monomeric 32-kDa proteins, termed 32K-I (pI 5.8) and 32K-II (pI 5.1), were isolated from human placenta, which was solubilized by a Ca2+-chelator. Only 32K-I was associated with PLA2-inhibitory activity. CNBr peptide mapping indicated that 32K-I was distinct from 32K-II and two 36-kDa proteins, called calpactin I and II or lipocortin II and I, which have been shown to possess PLA2-inhibitory activity. 32K-I bound to PS in a Ca2+-dependent manner. 32K-I was detected in many tissues except brain, cardiac and skeletal muscle.     

Twenty-eight-kilodalton phosphorylatable calcium- and lipid-binding proteins purified from Physarum plasmodium.

Two 28-kDa calcium- and lipid-binding proteins were isolated from a detergent-insoluble fraction of the Physarum plasmodium. Both proteins have molecular masses of approximately 28 kDa by SDS-PAGE. The protein designated 28K-I has a slightly lower mobility than that designated 28K-II. The purified 28K-I has a dissociation constant of 1.0 microM for Ca2+ ions, while the 28K-II has two different dissociation constants: one of 0.32 microM and the other of 3.2 mM. The 28K-I binds to liposomes at Ca2+ concentrations higher than 1.0 microM and has a dissociation constant for lipid of 34 micrograms/ml at 10 microM Ca2+. The 28K-II binds to liposomes at concentrations of Ca2+ above the mM range and has a dissociation constant of 36 micrograms/ml for lipid at 2 mM Ca2+. There is no evidence of actin-binding activity by either of the 28-kDa (28K) proteins. The 28K proteins crossreacted with an antiserum against chicken brush border calpactin I. The two proteins have quite different phosphorylation levels between a fraction prepared from the cytosolic endoplasm and a fraction prepared from the whole cell. The 28K proteins may play some role in the membrane structure dynamics of the cortical gel layer.     

Cyclic activation of histone H1 kinase during sea urchin egg mitotic divisions

Fertilized sea urchin eggs undergo a series of rapid and synchronized mitotic divisions. Extracts were made at various times throughout the first three mitotic divisions and assayed for phosphorylating activity toward histone H1. Histone H1 kinase (HH1K) undergoes a transient activation (8- to 10-fold increase) 20 min before each cleavage. The amplitude of the HH1K peak strongly depends on the synchrony of the egg population. Concomitant cytological observations show that the time-course of HH1K correlates with the time-course of nuclear envelope breakdown and of metaphase. This correlation is observed at each cell division cycle. HH1K from each of the three first mitoses show identical time- and concentration-dependence curves as well as identical dose-inhibition curves with 6-dimethylaminopurine and quercetin, suggesting that the same (group of) kinase(s) is (are) activated before each cleavage. Ionophore A23187 does not trigger, but inhibits, HH1K activation; however, partial activation of the eggs with ammonia at pH 9.0 (but not at pH 8.0) triggers the transient HH1K activation. Appearance of the HH1K cycle requires protein synthesis since it is completely abolished in emetine-treated eggs. Although cytochalasin B blocks egg cleavage, it does not inhibit HH1K activation nor nuclear divisions. A prolonged HH1K activation cycle is observed in eggs arrested in metaphase with colchicine or nocodazole. Despite the existence of a cycle in cAMP concentration during mitosis, forskolin, an activator of adenylate cyclase, does not modify the time-course of HH1K activation and of cell division. The cycling HH1K is independent of calcium-calmodulin, calcium-phospholipids, or cyclic AMP. It clearly resembles the mammalian "growth-associated histone kinase." The relationship between the transient activation of HH1K and the intracellular mitotic factors driving the cell cycle is discussed.     

Structural and immunological properties of the phenolic glycolipids from Mycobacterium gastri and Mycobacterium kansasii.

Mycobacterial species-specific antigens belong to the three following classes: phenolic glycolipids (Phe Gl), acyltrehalose-containing lipooligosaccharides and polar glycopeptidolipids. These antigens have been chemically defined and alkali-labile epitopes were found to characterize the lipooligosaccharide antigen type. In the present study the major Mycobacterium kansasii phenolic glycolipid epitope namely Phe Gl K-I was delineated as the distal monoacetylated disaccharidic residue: 2,6-dideoxy-4-O-methyl-alpha-D-arabino-hexopyranosyl-(1----3)-2-O-methyl -4-O- acetyl-alpha-L-fucopyranose. This acetoxy group is required for K-I epitope recognition demonstrating that alkali-labile epitopes also occur in the phenolic glycolipid antigen class. Using immunoelectron microscopy, the Phe Gl K-I epitope was localized around the electron-transparent layer on the M. kansasii cell-wall surface. Furthermore, two new phenolic glycolipids namely Phe Gl K-III and Phe Gl K-IV were discovered in minute amounts. They were purified and characterized by their retention time in direct-phase column HPLC. These molecules are also M. kansasii antigens, whose epitopes differ from that of Phe Gl K-I. The complete family of phenolic glycolipids Phe Gl K-I, K-II, K-III and K-IV was found in both rough and smooth variants of both M. kansasii and Mycobacterium gastri species.     

Studies of a plasma membrane steroid receptor in Xenopus oocytes using the synthetic progestin RU 486

A steroid binding protein (Mr = 110,000) has previously been identified in the plasma membrane of Xenopus laevis oocytes by photoaffinity labeling with [3H]R5020. In order to further characterize this steroid receptor, the photoaffinity labeled receptor protein was solubilized with 0.1% Brij 35. The solubilized labeled receptor yielded an approximate mol. wt of 102,000 +/- 2,000 by sucrose density gradient centrifugation, suggesting that the solubilized receptor exists as a monomer. RU 486, a synthetic progestin antagonist for mammalian cytosolic receptor systems, inhibited up to 70% of [3H] R5020 photoaffinity binding to the 110,000-Dalton receptor with an IC50 of 5 microM and induced germinal vesicle breakdown (GVBD) with an EC50 of 9.0 +/- 0.6 microM. GVBD induced by RU 486 was slower than with progesterone, and RU 486 was less powerful than progesterone. Micromolar concentrations of RU 486 also potentiated GVBD induced by sub-optimal concentrations of progesterone or R5020. Furthermore, RU 486 inhibited oocyte plasma membrane adenylate cyclase with an apparent IC50 of 7.5 +/- 2.5 microM. The close correlation of the EC50 value for RU 486 induction of GVBD with the IC50 values for inhibition of [3H]R5020 photoaffinity labeling of the 110,000-Dalton receptor and inhibition of adenylate cyclase activity further supports the physiological significance of the oocyte plasma membrane steroid receptor.     

Identification of a conserved motif required for mTOR signaling

BACKGROUND: The mammalian target of rapamycin (mTOR) controls the translation machinery via activation of S6 kinases 1 and 2 (S6K1/2) and inhibition of the eukaryotic initiation factor 4E (eIF4E) binding proteins 1, 2, and 3 (4E-BP1/2/3). S6K1 and 4E-BP1 are regulated by nutrient-sensing and mitogen-activated pathways. The molecular basis of mTOR regulation of S6K1 and 4E-BP1 remains controversial. RESULTS: We have identified a conserved TOR signaling (TOS) motif in the N terminus of all known S6 kinases and in the C terminus of the 4E-BPs that is crucial for phosphorylation and regulation S6K1 and 4E-BP1 activities. Deletion or mutations within the TOS motif significantly inhibit S6K1 activation and the phosphorylation of its hydrophobic motif, Thr389. In addition, this sequence is required to suppress an inhibitory activity mediated by the S6K1 C terminus. The TOS motif is essential for S6K1 activation by mTOR, as mutations in this motif mimic the effect of rapamycin on S6K1 phosphorylation, and render S6K1 insensitive to changes in amino acids. Furthermore, only overexpression of S6K1 with an intact TOS motif prevents 4E-BP1 phosphorylation by a common mTOR-regulated modulator of S6K1 and 4E-BP1. CONCLUSIONS: S6K1 and 4E-BP1 contain a conserved five amino acid sequence (TOS motif) that is crucial for their regulation by the mTOR pathway. mTOR seems to regulate S6K1 by two distinct mechanisms. The TOS motif appears to function as a docking site for either mTOR itself or a common upstream activator of S6K1 and 4E-BP1.     

Purification and characterization of a protein kinase from Xenopus eggs highly specific for ribosomal protein S6

In Xenopus oocytes ribosomal protein S6 becomes phosphorylated on serine residues in response to hormones or growth factors and following microinjection of the tyrosine-specific protein kinases associated with Rous sarcoma virus or Abelson murine leukemia virus. To begin characterization of the enzymes responsible for S6 phosphorylation in this system, we have undertaken the purification of S6 protein kinases from unfertilized Xenopus eggs. DEAE-Sephacel chromatography of crude extracts revealed two peaks of S6 kinase activity, and the peak eluting at 160 mM NaCl was chosen for further purification. Successive chromatography on Mono S, Sephacryl S-200, Mono Q, and heparin-Sepharose resulted in purification of the enzyme to a single protein migrating at Mr = 92,000 on polyacrylamide gels. The final preparation was purified about 500-fold from the DEAE-Sephacel peak with a recovery of 10%. Apparent Km values of the enzyme for ATP and 40 S subunits were 28 and 5 microM, respectively, and the specific activity with 330 microM ATP and 5.6 microM 40 S subunits was 300 nmol/min/mg. The enzyme was inhibited by beta-glycerophosphate, sodium fluoride, potassium phosphate, ADP, heparin, quercetin, and spermine. The availability of a purified S6 protein kinase should facilitate elucidation of the molecular mechanism of S6 phosphorylation during growth stimulation.     

Beta-adrenergic receptors (betaAR) regulate cardiomyocyte proliferation during early postnatal life.

Cardiomyocyte development switches from hyperplasmic to hypertrophic growth between postnatal days 3 and 4 in rats. The mechanisms responsible for this transition have been controversial. beta-Adrenergic receptor (betaAR) activation of mitogenic responses in vitro has been reported. We hypothesized that tonic activation of the betaAR signaling regulates cell division in neonatal cardiomyocytes via effects on signaling kinases known to be important in cell cycle regulation. The purpose of the current study was to elucidate the roles of betaAR in rat cardiomyocyte growth in vivo. We demonstrated that betaAR blockade induced a significant reduction in cardiomyocyte proliferation as measured by the BrdU labeling index. Blockade of betaAR did not affect p38 or p44/42 MAPK activities. We further demonstrated that betaAR blockade induced a prompt deactivation of the p70 ribosomal protein S6 kinase (p70 S6K). To confirm these results, we measured p70 S6K activity directly. Basal activity of p70 S6K in neonatal cardiomyocytes was fourfold higher than that of insulin-treated adult rat liver. The activity of p70 S6K was reduced by 60% within 1 min after betaAR blockade. We conclude that the betaAR are involved in regulation of neonatal cardiomyocyte proliferation and that this mitogenic control may be mediated via the p70 S6K pathway.     

Purification of a human alternative complement pathway inhibitor from hemolymph of larval fall armyworm (Spodoptera frugiperda)

Larval Spodoptera frugiperda hemolymph contains a specific inhibitor of the alternative pathway of human complement. This inhibitor was purified from larval hemolymph (HL) by 50% (NH4)2SO4 precipitation, DEAE-Sephacel chromatography and sequential gel-filtration on Bio-Gel 1.5m, 0.5m and Sephacryl S-200. Purified HL protein (Mr = 110,000) was composed of two Mr 55,000 polypeptide chains. Addition of purified HL protein to human complement resulted in a dose-dependent inhibition of RaRBC lysis and clumping of cells. The protein inhibitor provides a new tool for investigating the regulation of human alternative complement pathway.     

Mammalian target of rapamycin as a therapeutic target in leukemia

Reflecting its critical role in integrating cell growth and division with the cellular nutritional environment, the mammalian target of rapamycin *(mTOR) is a highly conserved downstream effector of the phosphatidylinositol 3-kinase (PI3K)/Akt (protein kinase B) signaling pathway. mTOR activates both the 40S ribosomal protein S6 kinase (p70s6k) and the eukaryotic initiation factor 4E-binding protein-1. As a consequence of inhibiting its downstream messengers, mTOR inhibitors prevent cyclin-dependent kinase (CDK) activation, inhibit retinoblastoma protein phosphorylation, and accelerate the turnover of cyclin D1, leading to a deficiency of active CDK4/cyclin D1 complexes, all of which may help cause GI phase arrest. Constitutive activation of the PI3K/Akt kinases occur in human leukemias. FLT3, VEGF, and BCR-ABL mediate their activities via mTOR. New rapamycin analogs including CCI-779, RAD001, and AP23573, are entering clinical studies for patients with hematologic malignancies.     

Thapsigargin induces meiotic maturation in surf clam oocytes.

I report here that thapsigargin, an inhibitor of Ca(2+)-ATPase activities in internal Ca2+ stores, induces meiotic maturation in prophase I-arrested surf clam (Spisula solidissima) oocytes. The half-maximal dose for triggering germinal vesicle breakdown (GVBD) is 120 nM. Thapsigargin-induced GVBD is followed by all normal subsequent steps of meiotic maturation including extrusions of first and second polar bodies, with almost normal timing as compared with K(+)-induced activation. Thapsigargin-induced GVBD requires the presence of external Ca2+ at a half-maximal concentration of 0.6 mM. In normal sea water, thapsigargin-induced activation is accompanied by a slightly increased 45Ca2+ uptake by the oocytes and by an intracellular pH rise of 0.3 U. These results show that thapsigargin-sensitive Ca2+ pools regulating Ca2+ fluxes exist in surf clam oocytes, and they also further establish that Ca2+ ions are the major initial trigger for meiosis resumption in this species.     

Purification and partial characterization of bovine heart phosphorylase kinase

Bovine heart phosphorylase kinase has been isolated by a procedure involving precipitation with polyethylene glycol, DEAE-Sephacel chromatography and calmodulin-Sepharose affinity chromatography. The isolated enzyme had a specific activity of 8.3 IU/mg of protein at pH 8.2 at 30 degrees C in the presence of 1% glycogen. The native enzyme had a sedimentation coefficient of 23 S and the Mr of the alpha', beta, gamma, and delta subunits, were 140,000, 130,000, 46,000, and 18,000, respectively. Activation of the phosphorylase kinase by the catalytic subunit of bovine heart cAMP-dependent protein kinase increases the pH 6.8/8.2 activity ratio from 0.01 to 0.32-0.38. Glycogen (1%) decreased the Km of the activated phosphorylase kinase at pH 6.8 for phosphorylase b from 5.5 to 1.25 mg/ml. Trypsin treatment increased the pH 6.8 activity but decreased the pH 8.2 activity. During this process the alpha' subunit was converted to a Mr 110,000 polypeptide and the enzyme activity was converted essentially to a 5.9 S species having an apparent Mr of 100,000 as determined by gel filtration. On extended trypsin treatment only one major polypeptide corresponding to the beta subunit remained. The same polypeptide was present in the active fractions following gel filtration of the trypsinized kinase.     

Comparative biochemistry of mammalian arylsulfatase C and steroid sulfatase

1. Hepatic arylsulfatase C (ASC) and steroid sulfatase (SS) from six of eleven mammals (rat, dog, baboon, cow, goat, and sheep) coeluted from DEAE-Sephacel as a single anionic species. A minor cationic peak of ASC and SS activity was also recovered from solubilized microsomes derived from the domestic cat. Characterization of the cationic activities indicated they were most likely contributed by a protein structurally related to the anionic isozyme. Properties of ASC and SS activities occurring in these seven species were most consistent with the presence of both activities in the same enzyme. 2. Guinea-pig liver SS activity was partitioned between an alkylsulfatase (hydrolyzing dehydroepiandrosterone sulfate (DHEAS)) and an arylsulfatase (hydrolyzing both estrone sulfate (E1S) and 4-methylumbelliferyl sulfate (4MUS) at a common active site). These enzymes were physically separable by ion-exchange chromatography and possessed distinct immunological and chemical properties. 3. Porcine, squirrel, and human livers possessed a major isozyme of ASC that lacked both E1S- and DHEAS-sulfatase activities. The human hepatic ASC was separable from SS by electrophoresis and was partially resolved from SS by DEAE-Sephacel chromatography. The ASC isozyme lacking SS activity was heat-labile in all three species.     

Mitogen-activated S6 kinase is stimulated via protein kinase C-dependent and independent pathways in Swiss 3T3 cells

Soluble extracts prepared from quiescent Swiss mouse 3T3 cells that had been briefly exposed to various mitogens exhibited a 2- to 3-fold elevation in phosphorylating activities toward ribosomal protein S6 and a synthetic peptide, Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala (RRLSSLRA), patterned after a phosphorylation site sequence from S6. Optimal activation of the phosphorylating activity occurred within 15-20 min of exposure of the cells to platelet-derived growth factor (10 ng/ml), epidermal growth factor (100 nM), and insulin (100 nM), and 2-5 min after 12-O-tetradecanoylphorbol-13-acetate (TPA) (100 nM) treatment. Fractionation of the cytosolic extracts from mitogen- or TPA-treated cells on Sephacryl S-300, TSK-400, and DEAE-Sephacel columns gave results suggesting that a single stimulated kinase accounted for the enhanced S6 and RRLSSLRA phosphorylating activities. The mitogen-activated kinase had an apparent Mr of about 85,000 as determined with Sephacryl S-300, but eluted with an apparent Mr of 26,000 from a TSK-400 high pressure liquid chromatography column. The S6 kinase was also stimulated in cytosols from insulin-like growth factor 1- (100 nM), vasopressin- (250 nM), prostaglandin F2 alpha- (250 nM), and 10% fetal calf serum-treated cells but not from quiescent cells exposed to beta-transforming growth factor (2 ng/ml). TPA, vasopressin and prostaglandin F2 alpha appeared to stimulate this kinase via a protein kinase C-dependent mechanism, since the responses to these hormones, but not to platelet-derived growth factor, epidermal growth factor, and insulin, were lost in protein kinase C-depleted cells.     

Ribosomal S6 kinase signaling and the control of translation

The highly homologous 40S ribosomal protein S6 kinases (S6K1 and S6K2) play a key role in the regulation of cell growth by controlling the biosynthesis of translational components which make up the protein synthetic apparatus, most notably ribosomal proteins. In the case of S6K1, at least eight phosphorylation sites are believed to mediate kinase activation in a hierarchical fashion. Activation is initiated by phosphatidylinositide-3OH kinase (PI3K)-mediated phosphorylation of key residues in the carboxy-terminus of the kinase, allowing phosphorylation of a critical residue residing in the activation loop of the catalytic domain by phosphoinositide-dependent kinase 1 (PDK1). The kinases responsible for phosphorylating the carboxy-terminal sites have yet to be identified. Additionally, S6 kinases are under the control of the PI3K relative, mammalian Target Of Rapamycin (mTOR), which may serve an additional function as a checkpoint for amino acid availability. In this review we set out to discuss the present state of knowledge regarding upstream signaling components which have been implicated in the control of S6K1 activation and the role of the kinase in controlling cell growth through regulating ribosome biogenesis at the translational level.     

The orientation of transhydrogenase in the inner mitochondrial membrane of rat liver

The orientation of the transmembranous enzyme, pyridine dinucleotide transhydrogenase, in the inner mitochondrial membrane of rat liver has been determined by evaluating effects of proteases on the integrity of the enzyme in mitoplasts and submitochondrial particles. Following treatment of these membranes with the nonspecific protease, proteinase K, antigenic proteolytic products were detected by immunoblot analysis using polyclonal antibody prepared against purified bovine heart enzyme. Proteinase K treatment of mitoplasts converted the 110,000 transhydrogenase monomer into a single immunoreactive species having Mr 75,000. This proteolytic product is stable to further incubation with the protease. Treatment of submitochondrial particles with proteinase K resulted in the disappearance of the 110,000 monomer and the transient formation of an intermediate product with Mr 52,000. Information from these proteolysis studies was used to construct a model of the orientation of transhydrogenase in the inner mitochondrial membrane. This model indicates that transhydrogenase (Mr 110,000) contains a core of proteolytically inaccessible proteins within the membrane (Mr 23,000) bounded by extramembranous domains on the matrix (Mr 52,000) and cytoplasmic (Mr 35,000) face of the inner mitochondrial membrane.