The muscle-specific protein phosphatase PP1G/R(GL)(G(M))is essential for activation of glycogen synthase by exercise

In skeletal muscle both insulin and contractile activity are physiological stimuli for glycogen synthesis, which is thought to result in part from the dephosphorylation and activation of glycogen synthase (GS). PP1G/R(GL)(G(M)) is a glycogen/sarcoplasmic reticulum-associated type 1 phosphatase that was originally postulated to mediate insulin control of glycogen metabolism. However, we recently showed (Suzuki, Y., Lanner, C., Kim, J.-H., Vilardo, P. G., Zhang, H., Jie Yang, J., Cooper, L. D., Steele, M., Kennedy, A., Bock, C., Scrimgeour, A., Lawrence, J. C. Jr., L., and DePaoli-Roach, A. A. (2001) Mol. Cell. Biol. 21, 2683-2694) that insulin activates GS in muscle of R(GL)(G(M)) knockout (KO) mice similarly to the wild type (WT). To determine whether PP1G is involved in glycogen metabolism during muscle contractions, R(GL) KO and overexpressors (OE) were subjected to two models of contraction, in vivo treadmill running and in situ electrical stimulation. Both procedures resulted in a 2-fold increase in the GS -/+ glucose-6-P activity ratio in WT mice, but this response was completely absent in the KO mice. The KO mice, which also have a reduced GS activity associated with significantly reduced basal glycogen levels, exhibited impaired maximal exercise capacity, but contraction-induced activation of glucose transport was unaffected. The R(GL) OE mice are characterized by enhanced GS activity ratio and an approximately 3-4-fold increase in glycogen content in skeletal muscle. These animals were able to tolerate exercise normally. Stimulation of GS and glucose uptake following muscle contraction was not significantly different as compared with WT littermates. These results indicate that although PP1G/R(GL) is not necessary for activation of GS by insulin, it is essential for regulation of glycogen metabolism under basal conditions and in response to contractile activity, and may explain the reduced muscle glycogen content in the R(GL) KO mice, despite the normal insulin activation of GS.

Gene structure and expression of the targeting subunit, RGL, of the muscle-specific glycogen-associated type 1 protein phosphatase, PP1G

The type I phosphatase associated with glycogen, PP1G, plays an important role in glycogen metabolism. PP1G is targeted to glycogen by the R(GL) subunit, which regulates the function of the enzyme. We report the cloning and characterization of the gene as well as the pattern of expression of the R(GL) subunit from mouse. The gene covers more than 37 kb, is composed of four exons and three introns, and codes for a 1089 residue polypeptide with a calculated molecular weight of 121,000. The amino acid sequence has 60% identity with the human and rabbit R(GL). The 5' flanking region of the gene contains a TATA box, c-Myc sites, and a potential cAMP-responsive element. Muscle specific motifs, such as MyoD and MEF-2, were also found. The A-T rich 3'-UTR contained several polyadenylation signals, two associated with poly(A) down-stream consensus motifs. ARE elements, which regulate mRNA stability, were dispersed throughout the 3'-UTR. Northern analysis of poly(A) mRNA from various murine tissues indicates a major transcript of 7.5 kb in skeletal muscle and heart. Western analysis demonstrates that R(GL) protein is present in skeletal and cardiac muscle from mouse, rat, and rabbit but not in L6 myoblasts, L6 myotubes, 3T3 L1 fibroblasts, 3T3 L1 or rat primary adipocytes, confirming that expression of the gene is specific to striated muscle. Analysis of skeletal muscle from rats made diabetic by streptozotocin treatment reveals that the level of R(GL) protein is the same as in control animals, indicating that expression is not regulated by insulin.

Insulin control of glycogen metabolism in knockout mice lacking the muscle-specific protein phosphatase PP1G/RGL

The regulatory-targeting subunit (RGL), also called GM) of the muscle-specific glycogen-associated protein phosphatase PP1G targets the enzyme to glycogen where it modulates the activity of glycogen-metabolizing enzymes. PP1G/RGL has been postulated to play a central role in epinephrine and insulin control of glycogen metabolism via phosphorylation of RGL. To investigate the function of the phosphatase, RGL knockout mice were generated. Animals lacking RGL show no obvious defects. The RGL protein is absent from the skeletal and cardiac muscle of null mutants and present at approximately 50% of the wild-type level in heterozygotes. Both the level and activity of C1 protein are also decreased by approximately 50% in the RGL-deficient mice. In skeletal muscle, the glycogen synthase (GS) activity ratio in the absence and presence of glucose-6-phosphate is reduced from 0.3 in the wild type to 0.1 in the null mutant RGL mice, whereas the phosphorylase activity ratio in the absence and presence of AMP is increased from 0.4 to 0.7. Glycogen accumulation is decreased by approximately 90%. Despite impaired glycogen accumulation in muscle, the animals remain normoglycemic. Glucose tolerance and insulin responsiveness are identical in wild-type and knockout mice, as are basal and insulin-stimulated glucose uptakes in skeletal muscle. Most importantly, insulin activated GS in both wild-type and RGL null mutant mice and stimulated a GS-specific protein phosphatase in both groups. These results demonstrate that RGL is genetically linked to glycogen metabolism, since its loss decreases PP1 and basal GS activities and glycogen accumulation. However, PP1G/RGL is not required for insulin activation of GS in skeletal muscle, and rather another GS-specific phosphatase appears to be involved.

Distinctive regulatory and metabolic properties of glycogen-targeting subunits of protein phosphatase-1 (PTG, GL, GM/RGl) expressed in hepatocytes

Glycogen-targeting subunits of protein phosphatase-1 facilitate interaction of the phosphatase with enzymes of glycogen metabolism. We have shown that overexpression of one member of the family, protein targeting to glycogen (PTG), causes large increases in glycogen storage in isolated hepatocytes or intact rat liver. In the current study, we have compared the metabolic and regulatory properties of PTG (expressed in many tissues), with two other members of the gene family, G(L) (expressed primarily in liver) and G(M)/R(Gl) (expressed primarily in striated muscle). Adenovirus-mediated expression of these proteins in hepatocytes led to the following key observations. 1) G(L) has the highest glycogenic potency among the three forms studied. 2) Glycogen synthase activity ratio is much higher in G(L)-overexpressing cells than in PTG or G(M)/R(Gl)-overexpressing cells. Thus, at moderate levels of G(L) overexpression, glycogen synthase activity is increased by insulin treatment, but at higher levels of G(L) expression, insulin is no longer required to achieve maximal synthase activity. In contrast, cells with high levels of PTG overexpression retain dose-dependent regulation of glycogen synthesis and glycogen synthase enzyme activity by insulin. 3) G(L)- and G(M)/R(Gl)-overexpressing cells exhibit a strong glycogenolytic response to forskolin, whereas PTG-overexpressing cells are less responsive. This difference may be explained in part by a lesser forskolin-induced increase in glycogen phosphorylase activity in PTG-overexpressing cells. Based on these results, we suggest that expression of either G(L) or G(M)/R(Gl) in liver of diabetic animals may represent a strategy for lowering of blood glucose levels in diabetes.

Interaction of inhibitor-2 with the catalytic subunit of type 1 protein phosphatase. Identification of a sequence analogous to the consensus type 1 protein phosphatase-binding motif

Inhibitor-2 (I-2) is the regulatory subunit of a cytosolic type 1 Ser/Thr protein phosphatase (PP1) and potently inhibits the activity of the free catalytic subunit (CS1). Previous work from the laboratory had proposed that the interaction of I-2 with CS1 involved multiple sites (Park, I. K., and DePaoli-Roach, A. A. (1994) J. Biol. Chem. 269, 28919-28928). The present study refines the earlier analysis and arrives at a more detailed model for the interaction between I-2 and CS1. Although the NH(2)-terminal I-2 regions containing residues 1-35 and 1-64 have no inhibitory activity on their own, they increase the IC(50) for I-2 by approximately 30-fold, indicating the presence of a CS1-interacting site. Based on several experimental approaches, we have also identified the sequence Lys(144)-Leu-His-Tyr(147) as a second site of interaction that corresponds to the RVXF motif present in many CS1-binding proteins. The peptide I-2(135-151) significantly increases the IC(50) for I-2 and attenuates CS1 inhibition. Replacement of Leu and Tyr with Ala abolishes the ability to counteract inhibition by I-2. The I-2(135-151) peptide, but not I-2(1-35), also antagonizes inhibition of CS1 by DARPP-32 in a pattern similar to that of I-2. Furthermore, a peptide derived from the glycogen-binding subunit, R(GL)/G(M)(61-80), which contains a consensus CS1-binding motif, completely counteracts CS1 inhibition by I-2 and DARPP-32. The NH(2)-terminal 35 residues of I-2 bind to CS1 at a site that is specific for I-2, whereas the KLHY sequence interacts with CS1 at a site shared with other interacting proteins. Other results suggest the presence of yet more sites of interaction. A model is presented in which multiple "anchoring interactions" serve to position a segment of I-2 such that it sterically occludes the catalytic pocket but need not make high affinity contacts itself.

Decreased protein phosphatase 2A activity in hippocampal long-term potentiation

Using autophosphorylated Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) as substrate, we now find that long-term potentian (LTP) induction and maintenance are also associated with a significant decrease in calyculin A-sensitive protein phosphatase (protein phosphatase 2A) activity, without changes in Mg2+-dependent protein phosphatase (protein phosphatase 2C) activity. This decrease in protein phosphatase 2A activity was prevented when LTP induction was inhibited by treatment with calmidazolium or D-2-amino-5-phosphonopentanoic acid. In addition, the application of high-frequency stimulation to 32P-labeled hippocampal slices resulted in increases in the phosphorylation of a 55-kDa protein immunoprecipitated with anti-phosphatase 2A antibodies. Use of a specific antibody revealed that the 55-kDa protein is the B'alpha subunit of protein phosphatase 2A. Following purification of brain protein phosphatase 2A, the B'alpha subunit was phosphorylated by CaM kinase II, an event that led to the reduction of protein phosphatase 2A activity. These results suggest that the decreased activity in protein phosphatase 2A following LTP induction contributes to the maintenance of constitutively active CaM kinase II and to the long-lasting increase in phosphorylation of synaptic components implicated in LTP.

Structure and chromosomal localization of the human glycogenin-2 gene GYG2

Glycogenin-2 is one of two self-glucosylating proteins involved in the initiation phase of the synthesis of the storage polysaccharide glycogen. Cloning of the human glycogenin-2 gene, GYG2, has revealed the presence of 11 exons and a gene of more than 46 kb in size. The structure of the gene explains much of the observed diversity in glycogenin-2 cDNA sequences as being due to alternate exon usage. In some cases, there is variation in the splice junctions used. Over regions of protein sequence similarity, the GYG2 gene structure is similar to that of the other glycogenin gene, GYG. A genomic GYG2 clone was used to localize the gene to Xp22.3 by fluorescence in-situ hybridization. Localization close to the telomere of the short arm of the X chromosome is consistent with mapping information obtained from glycogenin-2 STS sequences. Glycogenin-2 maps between the microsatellite anchor markers AFM319te9 (DXS7100) and AFM205tf2 (DXS1060), and its 3' end is 34.5 kb from the 3' end of the arylsulphatase gene ARSD. GYG2 is outside the pseudoautosomal region PAR1 but still in a region of X-Y shared genes. As is true for several other genes in this location, an inactive remnant of GYG2, consisting of exons 1-3, may be present on the Y chromosome.

Phosphorylation-dependent inhibition of protein phosphatase-1 by G-substrate. A Purkinje cell substrate of the cyclic GMP-dependent protein kinase

G-substrate, a specific substrate of the cGMP-dependent protein kinase, has previously been localized to the Purkinje cells of the cerebellum. We report here the isolation from mouse brain of a cDNA encoding G-substrate. This cDNA was used to localize G-substrate mRNA expression, as well as to produce recombinant protein for the characterization of G-substrate phosphatase inhibitory activity. Brain and eye were the only tissues in which a G-substrate transcript was detected. Within the brain, G-substrate transcripts were restricted almost entirely to the Purkinje cells of the cerebellum, although transcripts were also detected at low levels in the paraventricular region of the hypothalamus and the pons/medulla. Like the native protein, the recombinant protein was preferentially phosphorylated by cGMP-dependent protein kinase (Km = 0.2 microM) over cAMP-dependent protein kinase (Km = 2.0 microM). Phospho-G-substrate inhibited the catalytic subunit of native protein phosphatase-1 with an IC50 of 131 +/- 27 nM. Dephospho-G-substrate was not found to be inhibitory. Both dephospho- and phospho-G-substrate were weak inhibitors of native protein phosphatase-2A1, which dephosphorylated G-substrate 20 times faster than the catalytic subunit of protein phosphatase-1. G-substrate potentiated the action of cAMP-dependent protein kinase on a cAMP-regulated luciferase reporter construct, consistent with an inhibition of cellular phosphatases in vivo. These results provide the first demonstration that G-substrate inhibits protein phosphatase-1 and suggest a novel mechanism by which cGMP-dependent protein kinase I can regulate the activity of the type 1 protein phosphatases.

Targeted overexpression of phospholamban to mouse atrium depresses Ca2+ transport and contractility

Phospholamban is a small phosphoprotein regulator of the Ca2+-pump of cardiac sarcoplasmic reticulum. Dephosphorylated phospholamban inhibits the Ca2+-pump and depresses contractility, whereas phosphorylation of phospholamban by cAMP-activated mechanisms relieves this inhibition and increases contractility. In order to better understand the function of phospholamban in living systems, a transgenic mouse model was established employing targeted overexpression of phospholamban to the atrium, which normally expresses low levels of the protein. Overexpression was achieved by fusing the alpha-MHC-promoter or the ANF-promoter to the phospholamban gene. Double transgenic mice were created by mating mice positive for each transgene. In single transgenic lineages, phospholamban was overexpressed four to six-fold in left atrium. In the double transgenic mice, phospholamban was overexpressed eight- to nine-fold. In the three transgenic strains. Ca2+ uptake by the sarcoplasmic reticulum was depressed to 22-30% of control values at low ionized calcium. This depression of Ca2+ uptake was largely reversed by addition of a phospholamban monoclonal antibody. In the atrial muscle strips, the time course of contraction was increased in a concentration-dependent manner by overexpression of phospholamban, whereas the basal developed tension was decreased up to 85% by phospholamban-overexpression. In all transgenic lineages, isoproterenol, a beta-adrenoceptor agonist, reversed the depression of contractility caused by overexpression of phospholamban and significantly shortened time parameters to levels approaching control values. These data demonstrate that overexpression of phospholamban in a mammalian myocardial tissue normally deficient in the protein substantially inhibits basal contractility, and furthermore suggest that in myocardial tissues containing high levels of the protein, phosphorylation of phospholamban can account for many of the positive inotropic and lusitropic effects of beta-adrenergic stimulation.

Binding of CTP:phosphocholine cytidylyltransferase to lipid vesicles: diacylglycerol and enzyme dephosphorylation increase the affinity for negatively charged membranes

The regulation of membrane binding and activity of purified CDP:phosphocholine cytidylyl-transferase (CT) by lipid activators and enzyme dephosphorylation was examined. The binding of CT to membranes was analyzed using sucrose-loaded vesicles (SLVs). Binding to phosphatidylcholine vesicles was not detected even at a lipid:protein ratio of approximately 2000 (1 mM PC). CT bound to vesicles containing anionic lipids with apparent molar partition coefficients between 2 x 10(5) and 2 x 10(6), depending on the vesicle charge. The vesicle binding and activation of CT showed very similar sigmoidal dependencies on the lipid negative charge. In addition, diacylglycerol interacted synergistically with anionic phospholipids to stimulate both binding and activation at lower mole percent anionic lipid. These results demonstrate parallel requirements for binding and activity. Dephosphorylation of CT without destabilization was accomplished using the catalytic subunit of protein phosphatase 1. Dephosphorylated CT required a lower mole percent anionic phospholipid than phosphorylated CT for binding to and activation by SLVs. The combination of 10 mol % diacylglycerol and enzyme dephosphorylation shifted the mole percent phosphatidic acid required for half-maximal activation from 25% to 12%. These results suggest a mechanism whereby large changes in CT activity can result from changes in the phosphorylation state combined with small alterations in the membrane content of diacylglycerol. We propose a mechanism whereby dephosphorylation on the domain adjacent to the membrane binding domain increases the affinity of the latter for a negatively charged membrane surface.

Saccharomyces cerevisiae homologs of mammalian B and B' subunits of protein phosphatase 2A direct the enzyme to distinct cellular functions

Protein phosphatase 2A (PP2A) is a major cellular serine/threonine protein phosphatase, present in the cell in a variety of heterotrimeric forms that differ in their associated regulatory B-subunit. Cloning of the mammalian B' subunit has allowed the identification of a highly homologous Saccharomyces cerevisiae gene, RTS1. Disruption of the gene results in a temperature-sensitive growth defect that can be suppressed by expression of rabbit B'alpha or B'gamma isoforms. The B'alpha subunit is much more effective in restoring normal growth at 37 degrees C than B'gamma. Immunoprecipitated Rts1p was found associated with type 2A-specific protein phosphatase activity that is sensitive to 2 nM okadaic acid, but not to 100 nM phosphatase inhibitor-2, and to be phosphorylated in vivo. However, overexpression of RTS1 was unable to suppress the cold sensitivity, defective cytokinesis, and abnormal cell morphology resulting from defects in the CDC55 gene, which encodes the yeast homolog of a different B subunit of another form of 2A phosphatase, PP2A1. These results indicate that Rts1p is a yeast homolog of the mammalian B' subunit and that the various regulatory B-subunits of PP2A are not functionally redundant but direct the enzyme to distinct cellular functions.

Dephosphorylation of Sp1 by protein phosphatase 1 is involved in the glucose-mediated activation of the acetyl-CoA carboxylase gene

When mouse 30A5 preadipocytes are exposed to high glucose concentrations, acetyl-CoA carboxylase is induced through glucose activation of promoter II of the acetyl-CoA carboxylase gene. Glucose treatment of the cells increases Sp1 binding to two GC-rich glucose response elements in promoter II. We have investigated the mechanism by which glucose increases Sp1 binding and transactivation of promoter II in 30A5 cells. DNA mobility shift assays have shown that nuclear extracts from glucose-treated cells exhibit increased Sp1 binding activity. This increase in the binding activity is not due to glucose-mediated changes in the amount of Sp1 in the nucleus but to an increase in the activity that modifies Sp1 so that it binds more effectively to the promoter sequence. This Sp1 modifying activity is inhibited by okadaic acid and phosphatase inhibitor 2, and has a molecular mass of 38-42 kDa. The catalytic subunit of type 1 protein phosphatase, whose molecular mass is 38 kDa, also increased the ability of Sp1 to bind to promoter II. Treatment of nuclear extract with antibodies against the catalytic subunit partially suppressed the nuclear activity for Sp1 activation. From these results, we conclude that the Sp1 transcription factor exhibits enhanced binding to promoter II and transcriptional activation is the result of glucose-induced dephosphorylation by type 1 phosphatase.

Regulation of both glycogen synthase and PHAS-I by insulin in rat skeletal muscle involves mitogen-activated protein kinase-independent and rapamycin-sensitive pathways

Incubating rat diaphragm muscles with insulin increased the glycogen synthase activity ratio (minus glucose 6-phosphate/plus glucose 6-phosphate) by approximately 2-fold. Insulin increased the activities of mitogen-activated protein (MAP) kinase and the Mr = 90,000 isoform of ribosomal protein S6 kinase (Rsk) by approximately 1.5-2.0-fold. Epidermal growth factor (EGF) was more effective than insulin in increasing MAP kinase and Rsk activity, but in contrast to insulin, EGF did not affect glycogen synthase activity. The activation of both MAP kinase and Rsk by insulin was abolished by incubating muscles with the MAP kinase kinase (MEK) inhibitor, PD 098059; however, the MEK inhibitor did not significantly reduce the effect of insulin on activating glycogen synthase. Incubating muscles with concentrations of rapamycin that inhibited activation of p70S6K abolished the activation of glycogen synthase. Insulin also increased the phosphorylation of PHAS-I (phosphorylated heat- and acid-stable protein) and promoted the dissociation of the PHAS-I*eIF-4E complex. Increasing MAP kinase activity with EGF did not mimic the effect of insulin on PHAS-I phosphorylation, and the effect of insulin on increasing MAP kinase could be abolished with the MEK inhibitor without decreasing the effect of insulin on PHAS-I. The effects of insulin on PHAS-I were attenuated by rapamycin. Thus, activation of the MAP kinase/Rsk signaling pathway appears to be neither necessary nor sufficient for insulin action on glycogen synthase and PHAS-I in rat skeletal muscle. The results indicate that the effects of insulin on increasing the synthesis of glycogen and protein in skeletal muscle, two of the most important actions of the hormone, involve a rapamycin-sensitive mechanism that may include elements of the p70S6K signaling pathway.

High complexity in the expression of the B' subunit of protein phosphatase 2A0. Evidence for the existence of at least seven novel isoforms

Association of the catalytic subunit (C2) with a variety of regulatory subunits is believed to modulate the activity and specificity of protein phosphatase 2A (PP2A). In this study we report the cloning and expression of a new family of B-subunit, the B', associated with the PP2A0 form. Polymerase chain reactions and cDNA library screening have identified at least seven cDNA isotypes, designated alpha, beta 1, beta 2, beta 3, beta 4, gamma, and delta. The different beta subtypes appear to be generated by alternative splicing. The deduced amino acid sequences of the alpha, beta 2, beta 3, beta 4 and gamma isoforms predict molecular weights of 57,600, 56,500, 60,900, 52,500, and 68,000, respectively. The proteins are 60-80% identical and differ mostly at their termini. Two of the isoforms, B' beta 3 and B' gamma, contain a bipartite nuclear localization signal in their COOH terminus. No homology was found with other B- or B- related subunits. Northern analyses indicate a tissue-specific expression of the isoforms. Expression of B' alpha protein in Escherichia coli generated a polypeptide of approximately 53 kDa, similar to the size of the B' subunit present in the purified PP2A0. The recombinant protein was recognized by antibody raised against native B' and interacted with the dimeric PP2A (A.C2) to generate a trimeric phosphatase. The deduced amino acid sequences of the B' isoforms show significant homology to mammalian, fungal, and plant nucleotide sequences of unknown function present in the data bases. Notably, a high degree of homology (55-66%) was found with a yeast gene, RTS1, encoding a multicopy suppressor of a rox3 mutant. Our data indicate that at least seven B' subunit isoforms may participate in the generation of a large number of PP2A0 holoenzymes that may be spatially and/or functionally targeted to different cellular processes.

The G-protein-coupled receptor phosphatase: a protein phosphatase type 2A with a distinct subcellular distribution and substrate specificity

Phosphorylation of G-protein-coupled receptors plays an important role in regulating their function. In this study the G-protein-coupled receptor phosphatase (GRP) capable of dephosphorylating G-protein-coupled receptor kinase-phosphorylated receptors is described. The GRP activity of bovine brain is a latent oligomeric form of protein phosphatase type 2A (PP-2A) exclusively associated with the particulate fraction. GRP activity is observed only when assayed in the presence of protamine or when phosphatase-containing fractions are subjected to freeze/thaw treatment under reducing conditions. Consistent with its identification as a member of the PP-2A family, the GRP is potently inhibited by okadaic acid but not by I-2, the specific inhibitor of protein phosphatase type 1. Solubilization of the membrane-associated GRP followed by gel filtration in the absence of detergent yields a 150-kDa peak of latent receptor phosphatase activity. Western blot analysis of this phosphatase reveals a likely subunit composition of AB alpha C. PP-2A of this subunit composition has previously been characterized as a soluble enzyme, yet negligible soluble GRP activity was observed. The subcellular distribution and substrate specificity of the GRP suggests significant differences between it and previously characterized forms of PP-2A.

Phosphorylation and activation of the ATP-Mg-dependent protein phosphatase by the mitogen-activated protein kinase

Inhibitor-2 (I-2) is the regulatory subunit of the cytosolic ATP-Mg-dependent form of type 1 serine/threonine protein phosphatase and its phosphorylation at Thr-72 by glycogen synthase kinase-3 results in phosphatase activation. Activation of cytosolic type 1 phosphatase has been observed in cells treated with growth factors. Reported here is the phosphorylation and activation of the ATP-Mg-dependent phosphatase by mitogen-activated protein kinase (MAPK). Recombinant I-2 was phosphorylated by activated MAPK to an extent (approximately 0.3 mol of phosphate/mol of polypeptide) similar to that reported for phosphorylation by the alpha isoform of glycogen synthase kinase-3. The phosphorylation of I-2 by MAPK was exclusively at Thr-72, the site involved in the activation of phosphatase. Incubation of MAPK with purified ATP-Mg-dependent phosphatase resulted in phosphorylation of the I-2 component and activation of the phosphatase. Ribosomal S6 protein kinase II (p90rsk) was also able to phosphorylate the recombinant I-2; however, this phosphorylation occurred on serines and had no effect on phosphatase activation. Our data may explain growth factor-induced activation of the ATP-Mg-dependent phosphatase and suggest that MAPK may of cytosolic type 1 phosphatase in response to insulin and/or other growth factors.

Casein kinase I gamma subfamily. Molecular cloning, expression, and characterization of three mammalian isoforms and complementation of defects in the Saccharomyces cerevisiae YCK genes

Casein kinase I, one of the first protein kinases identified biochemically, is known to exist in multiple isoforms in mammals. Using a partial cDNA fragment corresponding to an isoform termed CK1 gamma, three full-length rat testis cDNAs were cloned that defined three separate members of this subfamily. The isoforms, designated CK1 gamma 1, CK1 gamma 2, and CK1 gamma 3, have predicted molecular masses of 43,000, 45,500, and 49,700. CK1 gamma 3 may also exist in an alternatively spliced form. The proteins are more than 90% identical to each other within the protein kinase domain but only 51-59% identical to other casein kinase I isoforms within this region. Messages for CK1 gamma 1 (2 kilobases (kb)), CK1 gamma 2 (1.5 and 2.4 kb), and CK1 gamma 3 (2.8 kb) were detected by Northern hybridization of testis RNA. Message for CK1 gamma 3 was also observed in brain, heart, kidney, lung, liver, and muscle whereas CK1 gamma 1 and CK1 gamma 2 messages were restricted to testis. All three CK1 gamma isoforms were expressed as active enzymes in Escherichia coli and partially purified. The enzymes phosphorylated typical in vitro casein kinase I substrates such as casein, phosvitin, and a synthetic peptide, D4. Phosphorylation of the D4 peptide was activated by heparin whereas phosphorylation of the protein substrates was inhibited. The known casein kinase I inhibitor CK1-7 also inhibited the CK1 gamma s although less effectively than the CK1 alpha or CK1 delta isoforms. All three CK1 gamma s underwent autophosphorylation when incubated with ATP and Mg2+. The YCK1 and YCK2 genes in Saccharomyces cerevisiae encode casein kinase I homologs, defects in which lead to aberrant morphology and growth arrest. Expression of mammalian CK1 gamma 1 or CK1 gamma 3 restored growth and normal morphology to a yeast mutant carrying a disruption of YCK1 and a temperature-sensitive allele of YCK2, suggesting overlap of function between the yeast Yck proteins and these CK1 isoforms.

In vivo regulation of protein kinase C by trans-phosphorylation followed by autophosphorylation

Dephosphorylation by the catalytic subunits of protein phosphatases 1 (CS1) and 2A (CS2) reveals that mature protein kinase C is phosphorylated at two distinct sites. Treatment of protein kinase C beta II with CS1 causes a significant increase in the protein's electrophoretic mobility (approximately 4 kDa) and a coincident loss in catalytic activity. The CS1-dephosphorylated enzyme cannot autophosphorylate or be phosphorylated by mature protein kinase C, indicating that a different kinase catalyzes the phosphorylation at this site. The loss of activity is consistent with dephosphorylation on protein kinase C's activation loop (Orr, J. W., and Newton, A. C., (1994) J. Biol. Chem. 269, 27715-27718). Treatment with CS2 results in a smaller shift in electrophoretic mobility (approximately 2 kDa) and no loss in catalytic activity. Furthermore, the CS2-dephosphorylated form can autophosphorylate and thus regain the electrophoretic mobility of mature enzyme, consistent with dephosphorylation at protein kinase C's carboxyl-terminal autophosphorylation site, which is modified in vivo (Flint, A. J., Paladini, R. D., and Koshland, D. E., Jr. (1990) Science 249, 408-411). In summary, two phosphorylations process protein kinase C to generate the mature form: a transphosphorylation that renders the kinase catalytically competent and an autophosphorylation that may be important for the subcellular localization of the enzyme.

Domains of phosphatase inhibitor-2 involved in the control of the ATP-Mg-dependent protein phosphatase

Inhibitor-2 (I-2) inhibits the free catalytic subunit of type 1 phosphatase (CS1) and controls the cyclic inactivation/activation of CS1 in the ATP-Mg-dependent protein phosphatase complex. We report here the effect of mutations on these two properties of I-2. Substitution of Thr-72 with Ala, Asp, or Glu generated complexes with CS1 that could not be activated. Mutation of Ser-86 did not affect activation by glycogen synthase kinase-3 (GSK-3) alone but impaired synergistic activation by casein kinase II and GSK-3. Mutations in the region between Thr-72 and Ser-86 did not alter the inhibitory potency of I-2 but prevented complete inactivation of CS1. A mutant without the 35 NH2-terminal residues exhibited an IC50 for CS1 200-fold higher than that of wild-type I-2. However, it formed an inactive phosphatase complex with CS1, which was activated by GSK-3. A mutant with the 59 COOH-terminal residues deleted retained full inhibitory activity and formed an inactive complex that could not be activated by GSK-3. We conclude that the NH2-terminal region of I-2 is involved in inhibition, that the sequence between Thr-72 and Ser-86 is necessary for the conversion of CS1 from an active to an inactive conformation, and that the COOH terminus is required for activation by GSK-3. Thus, different functional domains of I-2 may interact with distinct regions of CS1.

Diversity in the regulatory B-subunits of protein phosphatase 2A: identification of a novel isoform highly expressed in brain

The physiological role of type 2A protein phosphatases (PP2A) is dependent upon the association of the catalytic subunit with a variety of regulatory subunits. In order to understand the function of PP2A, we have undertaken purification of the holoenzymes and molecular cloning of the regulatory subunits. Two trimeric forms containing distinct B-subunits, PP2A0 and PP2A1, have been purified from rabbit skeletal muscle. The B-subunits associated with PP2A0 and PP2A1 migrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with slightly different mobility, approximately 52.5 and approximately 51.5 kDa, respectively and showed distinct immunological properties. The B' form of B-subunit associated with PP2A0 was recognized by antibodies against the B-subunit present in bovine heart PP2A but not by antibodies specific to the B subunit isoforms of rabbit PP2A1. Cloning of cDNAs encoding the B subunit of PP2A1 resulted in the isolation of a cDNA highly homologous to, but distinct from, the B alpha subunit isoform. The deduced amino acid sequence of this novel isoform, which was designated B gamma, encoded a protein which was 81% and 87% identical to the B alpha and B beta isoforms, respectively. Northern blot analysis indicated that the B gamma isoform is highly expressed in rabbit brain as a transcript of 3.9 kb. Analysis of B-subunit expression by Western blot indicated a general parallel with the message levels. In conclusion, our data reveal even greater complexity of PP2A trimeric holoenzymes due to the identification of a novel B regulatory subunit isoform of PP2A1 and a distinct B' subunit associated with PP2A0.

Characterization of the PP2A alpha gene mutation in okadaic acid-resistant variants of CHO-K1 cells

Okadaic acid (OA)-resistant variants of Chinese hamster ovary cells, clones CHO/OAR6-6 and CHO/OAR2-3, were isolated from a CHO-K1 culture. These variant cells were 17- to 26-fold more resistant to OA than the parental cells. The phosphorylase phosphatase activity of the variant cell extracts was 2- to 4-fold more resistant to OA than that of the parental cells in the presence of inhibitor 2, a specific inhibitor of type 1 protein serine/threonine phosphatase (PP1). Nucleotide sequencing of PP2A alpha (an isotype of PP2A catalytic subunit) cDNA demonstrated that both variants have a T-->G transversion at the first base of codon 269 (805 nt), which results in substitution of glycine for cysteine. We expressed in COS-1 cells a mutant PP2A alpha tagged with the influenza hemagglutinin epitope. The recombinant mutant PP2A alpha protein immunoprecipitated with an anti-influenza hemagglutinin antibody was more resistant than the wild type to OA, their IC50 values being 0.65 nM and 0.15 nM, and their IC80 values being 4.0 nM and 0.45 nM, respectively. The cysteine at residue 269 present only in highly OA-sensitive protein serine/threonine phosphatase catalytic subunit isozymes, PP2A alpha, PP2A beta, and PPX, is suggested to be involved in the binding of OA. CHO/OAR6-6 and CHO/OAR2-3 cells also overexpressed the P-glycoprotein, and the efflux of OA was more rapid. It is suggested that the PP2A alpha mutation in cooperation with a high level of P-glycoprotein makes the CHO-K1 variants highly resistant to OA.

Glycogen synthase kinase-3 beta is a dual specificity kinase differentially regulated by tyrosine and serine/threonine phosphorylation

The enzyme glycogen synthase kinase-3 (GSK-3) has been implicated in the control of several metabolic enzymes and transcription factors in response to extracellular signals. In the past, the enzyme has been considered to be a protein Ser/Thr kinase although it was recently reported to contain Tyr(P) (Hughes, K., Nikolakaki, E., Plyte, S. E., Totty, N. F., and Woodgett, J. R. (1993) EMBO J. 12, 803-808). A cDNA encoding rabbit skeletal muscle GSK-3 beta was cloned and expressed in Escherichia coli as an active protein kinase, with apparent M(r) 46,000, capable of phosphorylating several known GSK-3 substrates. Recombinant GSK-3 beta autophosphorylated on Ser, Thr, and Tyr residues although the enzyme already contained Tyr(P) as judged by its recognition by anti-Tyr(P) antibodies. The net result of the autophosphorylation was a 3-5-fold reduction in enzyme activity. GSK-3 alpha, purified from rabbit muscle, also underwent autophosphorylation but only on Ser and Thr residues. In this case, the autophosphorylation stabilized the enzyme activity compared with the control lacking ATP/Mg2+. Of several phosphatases tested, the lambda-phage phosphatase was the most effective in dephosphorylating at Ser and Thr residues but did not dephosphorylate at Tyr residues. The action of the lambda-phosphatase caused a reactivation of GSK-3 beta to approximately 80% of the starting activity. The protein tyrosine phosphatase PTP1B was able to dephosphorylate at Tyr residues leading to a reduction in enzyme activity. A truncated form of GSK-3 beta, apparent M(r) 40,000, had a significantly higher specific activity, was defective in autophosphorylation, and was not inactivated in the autophosphorylation reaction. We conclude that GSK-3 beta is a dual specificity protein kinase in the same sense as the mitogen-activated protein kinase/ERK family of enzymes. Phosphorylation at different residues differentially controls enzyme activity, Ser/Thr phosphorylation causing inactivation and Tyr phosphorylation resulting in increased activity.

Native cytosolic protein phosphatase-1 (PP-1S) containing modulator (inhibitor-2) is an active enzyme

In vitro, the modulator protein (inhibitor-2) slowly converts the catalytic subunit of protein phosphatase-1 (PP-1C) into an inactive 'MgATP-dependent form' that can be reactivated by the transient phosphorylation of modulator with GSK-3/FA. We report here that this modulator-induced inactivation of PP-1C can be blocked by addition (at pH 7.5) of either 0.3 mM NaF or 150 mM NaCl, or by raising the pH to 8.5. Making use of a combination of the latter conditions, we have partially purified a soluble modulator-associated form of PP-1 (PP-1S) from rabbit skeletal muscle as a spontaneously active enzyme that cannot be further activated by kinase GSK-3/FA. These observations argue against a role for the 'MgATP-dependent' form of PP-1S as an inactive reservoir of PP-1C. PP-1S was separated on aminohexyl Sepharose from another active, cytosolic species of PP-1, which appears to be a proteolytic product of the glycogen-bound PP-1G.

Molecular mechanism of the synergistic phosphorylation of phosphatase inhibitor-2. Cloning, expression, and site-directed mutagenesis of inhibitor-2

Inhibitor-2 (I-2) is the regulatory subunit of the ATP-Mg-dependent phosphatase, a cytosolic form of type 1 protein phosphatase. Phosphorylation of I-2 at Thr-72 by the protein kinase glycogen synthase kinase-3 (GSK-3) leads to activation of the enzyme. Casein kinase II action was shown to synergistically enhance phosphorylation and activation by GSK-3 (DePaoli-Roach, A.A. (1984) J. Biol. Chem. 259, 12144-12152). Rabbit skeletal muscle and liver I-2 cDNA clones have been isolated. Rabbit skeletal muscle cDNAs could be placed in two subtypes, differing in the length of the 3'-untranslated region. The coding sequence of 612 nucleotides was identical in the two skeletal muscle and the liver cDNAs and predicted a protein of 204 amino acids, consistent with analysis of the purified protein. Northern hybridization analysis indicated that the two mRNAs of 1.7 and 2.7 kilobase pairs were present in all rabbit tissues examined, except in liver, where only the larger transcript was detected, and in testis, where additional transcripts were present. Expression in Escherichia coli of wild-type and phosphorylation site mutants resulted in the production of I-2 polypeptides with apparent M(r) values of approximately 31,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The inhibitory activity of the recombinant proteins was similar to that of native rabbit skeletal muscle I-2 and was unaffected by the substitution of alanine for the GSK-3 site (Thr-72) and for the casein kinase II sites (Ser-86 and Ser-120/121) or by substitution of glutamic acid and aspartic acid for Thr-72 and Ser-86. Recombinant wild-type I-2 and the Ala-120/121 mutant were phosphorylated synergistically by GSK-3 and casein kinase II. The Thr-72 and Ser-86 mutants, however, did not undergo this synergistic phosphorylation. Our studies indicate that Thr-72 is the only GSK-3 site and that Ser-86 is the casein kinase II site required for the potentiation of GSK-3 action. Furthermore, acidic residues cannot substitute for the phosphate group either in enhancing GSK-3 phosphorylation or in activating the phosphatase.

Isoform differences in substrate recognition by glycogen synthase kinases 3 alpha and 3 beta in the phosphorylation of phosphatase inhibitor 2

Phosphorylation of inhibitor 2, the regulatory subunit of the ATP-Mg-dependent protein phosphatase, by glycogen synthase kinase 3 (GSK-3) causes activation of the phosphatase. Prior phosphorylation by casein kinase II has been shown to enhance both phosphorylation and activation of the phosphatase by GSK-3 (DePaoli-Roach, A. A. (1984) J. Biol. Chem. 259, 12144-12152). Reported here is a comparison of the phosphorylation of inhibitor 2 by two defined isoforms of GSK-3, GSK-3 alpha and GSK-3 beta. GSK-3 beta was a significantly better inhibitor 2 kinase than was GSK-3 alpha. The Vmax/Km value for GSK-3 beta was approximately 10-fold higher than that for GSK-3 alpha. GSK-3 beta phosphorylated inhibitor 2 to a stoichiometry of approximately 1.0 mol of phosphate/mol of inhibitor 2. The phosphorylation by GSK-3 beta was determined to be exclusively at Thr-72 on the basis of the inability of the enzyme to modify a mutant inhibitor 2 in which Thr-72 was changed to alanine. Prior phosphorylation by casein kinase II promoted the action of GSK-3 alpha in keeping with earlier reports using undefined GSK-3 preparations. Phosphorylation by GSK-3 beta, in contrast, was unaffected by the previous action of casein kinase II. These results suggest that there can be important differences in substrate recognition by different isoforms of the same protein kinase and may help explain why some reported GSK-3 substrates require prior phosphorylation whereas other do not.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase A modulates an endogenous calcium channel, but not the calcium-activated chloride channel, in Xenopus oocytes

In Xenopus oocytes, Ca2+ influx through an endogenous voltage-gated Ca2+ channel activates a transient outward Cl- current (ICl(Ca)), which is potentiated by cAMP increase. The site of cAMP effect appears to be the Ca2+ channel instead of the Ca(2+)-activated Cl- channel, because cAMP potentiates the Ba2+ current through the Ca2+ channel in a similar way to the ICl(Ca), and cAMP does not potentiate the Ca(2+)-dependent Cl- current in cells treated with Ca2+ ionophore. Using the catalytic subunit of protein kinase A (PKA) and PKA inhibitors, it was shown that PKA is both necessary and sufficient for the cAMP effect on ICl(Ca). Furthermore, the cAMP/PKA-mediated potentiation of ICl(Ca) was inhibited by both type 1 and type 2A protein phosphatases.

Initiation of glycogen synthesis. Control of glycogenin by glycogen phosphorylase

Glycogen biosynthesis involves a specific initiation event, mediated by a specialized protein, glycogenin. Glycogenin undergoes self-glucosylation to generate an oligosaccharide primer, which, when long enough, supports the action of glycogen synthase to elongate the polysaccharide chain, leading ultimately to the formation of glycogen. We report that primed glycogenin is also a substrate for glycogen phosphorylase. Phosphorylase removed glucose from the oligosaccharide attached to glycogenin in a phosphorolysis reaction that required phosphate and produced glucose 1-phosphate. The phosphorylated form, phosphorylase a, was much more effective than the dephosphorylated phosphorylase b. However, in the presence of the allosteric effector AMP, phosphorylase b also catalyzed the phosphorolysis reaction. Glucose, an allosteric inhibitor of phosphorylase, inhibited the reaction. Glycogen, but not a short oligosaccharide (maltopentaose), also inhibited the reaction. Treatment of fully primed glycogenin with phosphorylase converted the glycogenin to a form with slightly lower apparent molecular weight, which was less effective as a substrate for glycogen synthase. These results suggest a novel role for phosphorylase in the control of glycogen biosynthesis. We propose that the glucosylation level of glycogenin would be determined by the balance between the self-glucosylation reaction and the opposing action of phosphorylase. The level of glucosylation would in turn determine whether or not glycogenin was an effective primer for glycogen synthase. In this way, several known controls of phosphorylase activity, such as epinephrine, glucagon, and insulin, could influence not only the elongation/degradation stage of glycogen metabolism but also its initiation.

Characterization of rabbit skeletal muscle glycogenin. Tyrosine 194 is essential for function

The biogenesis of glycogen involves a specific initiation event mediated by the initiator protein, glycogenin, which undergoes self-glucosylation to generate an oligosaccharide primer from which the glycogen molecule grows. Rabbit muscle glycogenin was expressed at high levels in Escherichia coli and purified close to homogeneity in a procedure that involved binding to a UDP-agarose affinity column. The resulting protein had subunit molecular weight of 38,000 as judged by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Analysis of peptide fragments by mass spectroscopy indicated that the recombinant glycogenin was already glucosylated at Tyr-194 and contained from 1 to 8 glucose residues attached. The enzyme was active as a glucosyl transferase and could incorporate a further approximately 5 mol of glucose/mol. The apparent Km for the glucosyl donor UDP-glucose was 4.5 microM, and the pH optimum was pH 8. Of a number of nucleotides and related compounds surveyed, UDP and UTP were the most effective inhibitors. There was also a correlation between inhibition and the presence of a pyrophosphate group. Of several oligosaccharides of glucose, only maltose caused significant inhibition. The glucosylation reaction was first order with respect to glycogenin suggesting that it was intramolecular. The efficacy of the purified glycogenin as a substrate for the elongation reaction catalyzed by glycogen synthase was significantly enhanced if glycogenin was first allowed to undergo self-glucosylation. The length of the priming oligosaccharide is thus important for glycogen synthase action. A mutant of glycogenin, in which Tyr-194 was changed to Phe, behaved identically to the wild-type through purification and in particular bound to the UDP-agarose affinity matrix. Despite these indications of the protein's overall structural integrity, it was unable to self-glucosylate. This result indicates that Tyr-194 is necessary for glycogenin function and is consistent with Tyr-194 being the sole site of glucosylation.

Mechanisms of multisite phosphorylation and inactivation of rabbit muscle glycogen synthase

Glycogen synthase, a rate-determining enzyme for glycogen biosynthesis, is regulated by complex multisite phosphorylation of its subunit. Previous work has suggested that phosphorylation by some protein kinases, casein kinase II and cyclic AMP-dependent protein kinase, potentiates the ability of other protein kinases, glycogen synthase kinase 3 and casein kinase I, respectively, to modify the enzyme. In the present study, active glycogen synthase was expressed in Escherichia coli using a pET vector. The purified recombinant glycogen synthase had specific activity and subunit M(r) similar to enzyme isolated from rabbit muscle. Prior phosphorylation by casein kinase II was found to be an obligate requirement for phosphorylation by glycogen synthase kinase 3, which introduced 4 mol phosphate/mol subunit. Casein kinase II action did not affect activity, whereas the phosphorylation catalyzed by glycogen synthase kinase 3 caused a potent inactivation, reducing the +/- glucose 6-phosphate activity ratio from 0.7 to 0.10. Casein kinase I alone phosphorylated the recombinant glycogen synthase, indicating that substrate phosphorylation was not an absolute requirement. However, the prior action of cyclic AMP-dependent protein kinase significantly potentiated the ability of casein kinase I to phosphorylate and inactivate glycogen synthase. All previous analyses of glycogen synthase phosphorylation have used enzyme purified from mammalian sources and containing residual covalent phosphate. By using recombinant substrate, the present study represents a rigorous assessment of the role of prior phosphorylation in the recognition of mammalian glycogen synthase by glycogen synthase kinase 3 and casein kinase I. The conclusion is that phosphorylation of glycogen synthase can involve the concerted action of multiple protein kinases.

Molecular cloning, expression, and characterization of a 49-kilodalton casein kinase I isoform from rat testis

We report the molecular cloning and characterization of a 49-kDa form of casein kinase I from rat testis. A cDNA clone encoding the enzyme, designated casein kinase I delta, contained an open reading frame of 1284 nucleotides that predicts a polypeptide of 428 amino acids with a M(r) of 49,121. The predicted amino acid sequence shares 76% identity with casein kinase I alpha, a 37-kDa form recently cloned from bovine brain (Rowles, J., Slaughter, C., Moomaw, C., Hsu, J., and Cobb, M. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 9548-9552), and 65% identity with HRR25, a 57-kDa form of casein kinase I from yeast shown to be involved in DNA repair (Hoekstra, M. F., Liskay, R. M., Ou, A. C., DeMaggio, A. J., Burbee, D. G., and Heffron, F. (1991) Science 253, 1031-1034). Northern analysis of rat or rabbit RNA revealed three hybridizing species of 3.5-4.1, 2.2, and 1.9 kilobase pairs (kb). The largest message was detected in all tissues examined, whereas the 1.9- and 2.2-kb species were found predominantly in testis. A probe corresponding to the 3'-untranslated region of the casein kinase I delta cDNA hybridized only to the 1.9-kb transcript. Expression of the casein kinase I delta cDNA in Escherichia coli resulted in active enzyme that phosphorylated casein, phosvitin, and the peptide substrate DDDDVASLPGLRRR. Enzyme activity was associated with a predominant polypeptide of 55-kDa, although COOH-terminal degradation products of 50 and 42 kDa were also present in partially purified enzyme. Recombinant casein kinase I delta was inhibited by the specific casein kinase I inhibitor, CKI-7, half-maximally at 12 microM. Heparin inhibited recombinant casein kinase I delta when phosvitin was the substrate, with half-maximal inhibition at 11.5 micrograms/ml. However, if the peptide substrate was used, heparin activated recombinant casein kinase I delta 4-5-fold, with half-maximal activation at 9.5 micrograms/ml. A truncated form of casein kinase I delta, lacking the COOH-terminal 111 amino acids, was no longer activated by heparin. Casein kinase I delta therefore represents a separate member of the casein kinase I family distinguished by its larger size and unique kinetic behavior with respect to heparin.

Rabbit skeletal muscle glycogenin. Molecular cloning and production of fully functional protein in Escherichia coli

Glycogenin is a self-glucosylating protein involved in the initiation reactions of glycogen synthesis. Initiation occurs in two stages, requiring first the covalent attachment of a glucose residue to Tyr-194 of glycogenin and then elongation to form an oligosaccharide chain. The latter reaction is known to be catalyzed by glycogenin itself. The glycogenin sequence determined from the protein by Campbell and Cohen (Campbell, D. G., and Cohen, P. (1989) Eur. J. Biochem. 185, 119-125) was used to design oligonucleotide probes to screen a rabbit muscle lambda gt11 library. A cDNA was isolated that predicted an amino acid sequence identical to that of Campbell and Cohen, except that Cys residues replaced Ser-88 and Leu-97. Northern analysis indicated a strongly hybridizing message of 1.8 kilobases, present in most tissues including skeletal muscle, but much weaker in kidney and scarcely detectable in liver. A much weaker 3-kilobase message was also detected in muscle. Polymerase chain reaction was used to isolate DNA fragments encoding a portion of glycogenin from rat and cow. The sequence of this segment was > 90% identical at the amino acid level across the three species, indicating that glycogenin is a highly conserved protein. Using the pET-8c vector, the glycogenin protein was expressed in Escherichia coli. Incubation of the recombinant glycogenin with UDP-[14C]glucose and Mn2+ resulted in labeling of the glycogenin protein, indicating that the recombinant glycogenin was enzymatically active and capable of self-glucosylation. Furthermore, after incubation with UDP-glucose, the recombinant glycogenin could serve as a substrate for glycogen synthase, leading to the production of high M(r) polysaccharide. Therefore, production of functional glycogenin did not require the intervention of any other mammalian protein.

Recombinant rabbit muscle casein kinase I alpha is inhibited by heparin and activated by polylysine

The casein kinase I (CKI) family consists of widely distributed monomeric Ser/Thr protein kinases that have a preference for acidic substrates. Four mammalian isoforms are known. A full length cDNA encoding the CKI alpha isoform was cloned from a rabbit skeletal muscle cDNA library and was utilized to construct a bacterial expression vector. Active CKI alpha was expressed in Escherichia coli as a polypeptide of Mr 36,000. The protein kinase phosphorylated casein, phosvitin and a specific peptide substrate (D4). The enzyme was inhibited by the isoquinolinesulfonamide CKI-7, half-maximally at 70 microM. Heparin inhibited phosphorylation of the D4 peptide or phosvitin by CKI alpha. Polylysine activated when the D4 peptide was the substrate but had no effect on phosvitin phosphorylation. It is becoming clear that the individual CKI isoforms have different kinetic properties and hence could have quite distinct cellular functions.

Multiple roles for protein phosphatase 1 in regulating the Xenopus early embryonic cell cycle

Using cytostatic factor metaphase II-arrested extracts as a model system, we show that protein phosphatase 1 is regulated during early embryonic cell cycles in Xenopus. Phosphatase 1 activity peaks during interphase and decreases shortly before the onset of mitosis. A second peak of activity appears in mitosis at about the same time that cdc2 becomes active. If extracts are inhibited in S-phase with aphidicolin, then phosphatase 1 activity remains high. The activity of phosphatase 1 appears to determine the timing of exit from S-phase and entry into M-phase; inhibition of phosphatase 1 by the specific inhibitor, inhibitor 2 (Inh-2), causes premature entry into mitosis, whereas exogenously added phosphatase 1 lengthens the interphase period. Analysis of DNA synthesis in extracts treated with Inh-2, but lacking the A- and B-type cyclins, shows that phosphatase 1 is also required for the process of DNA replication. These data indicate that phosphatase 1 is a component of the signaling pathway that ensures that M-phase is not initiated until DNA synthesis is complete.

Yeast casein kinase I homologues: an essential gene pair

We report the isolation of an essential pair of Saccharomyces cerevisiae genes that encode protein kinase homologues. The two genes were independently isolated as dosage-dependent suppressors. Increased dosage of YCK1 suppressed defects caused by reduced SNF1 protein kinase activity, and increased dosage of YCK2 relieved sensitivity of wild-type cells to salt stress. The two genes function identically in the two growth assays, and loss of function of either gene alone has no discernible effect on growth. However, loss of function of both genes results in inviability. The two predicted protein products share 77% overall amino acid identity and contain sequence elements conserved among protein kinases. Partial sequence obtained for rabbit casein kinase I shares 64% identity with the two yeast gene products. Moreover, an increase in casein kinase I activity is observed in extracts from cells overexpressing YCK2. Thus YCK1 and YCK2 appear to encode casein kinase I homologues.

CDC55, a Saccharomyces cerevisiae gene involved in cellular morphogenesis: identification, characterization, and homology to the B subunit of mammalian type 2A protein phosphatase

Microscopic screening of a collection of cold-sensitive mutants of Saccharomyces cerevisiae led to the identification of a new gene, CDC55, which appears to be involved in the morphogenetic events of the cell cycle. CDC55 maps between CDC43 and CHC1 on the left arm of chromosome VII. At restrictive temperature, the original cdc55 mutant produces abnormally elongated buds and displays a delay or partial block of septation and/or cell separation. A cdc55 deletion mutant displays a cold-sensitive phenotype like that of the original isolate. Sequencing of CDC55 revealed that it encodes a protein of about 60 kDa, as confirmed by Western immunoblots using Cdc55p-specific antibodies. This protein has greater than 50% sequence identity to the B subunits of rabbit skeletal muscle type 2A protein phosphatase; the latter sequences were obtained by analysis of peptides derived from the purified protein, a polymerase chain reaction product, and cDNA clones. An extragenic suppressor of the cdc55 mutation lies in BEM2, a gene previously identified on the basis of an apparent role in bud emergence.

Molecular cloning and expression of the regulatory (RG1) subunit of the glycogen-associated protein phosphatase

DNA clones encoding the glycogen-binding (RG1) subunit of glycogen-associated protein phosphatase were isolated from rabbit skeletal muscle lambda gt11 cDNA libraries. Overlapping clones provided an open reading frame of 3327 nucleotides that predicts a polypeptide of 1109 amino acids with a molecular weight of 124,257. Northern hybridization of rabbit RNA identified a major mRNA transcript of 7.5 kilobases present in skeletal, diaphragm, and cardiac muscle, but not in brain, kidney, liver, and lung. Southern analysis of rabbit genomic DNA digested with various restriction endonucleases gave rise to a single hybridizing fragment, suggesting that a single gene is present. Expression of the complete RG1 subunit coding sequence in Escherichia coli generated a protein of apparent molecular weight on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of approximately 160,000, similar to the size of the polypeptide detected by Western immunoblot in rabbit skeletal muscle extracts. The RG1 subunit shares significant homology with the Saccharomyces cerevisiae GAC1 gene product which is involved in activation of glycogen synthase and glycogen accumulation. The homology with GAC1 substantiates the role of this enzyme in control of glycogen metabolism. Hydropathy analysis of the RG1 subunit amino acid sequence revealed the presence of a hydrophobic region in the COOH terminus, suggesting a potential association with membrane. This result suggests that the same phosphatase regulatory component may be involved in targeting the enzyme both to membranes and to glycogen.

Glycogen metabolism and signal transduction in mammals and yeast

Mammalian glycogen synthase, with its complex multisite phosphorylation mechanisms, continues to provide interesting and novel examples of the regulation of protein function. The mammalian enzyme is phosphorylated in a hierarchal manner such that modification of certain sites requires the prior phosphorylation of other sites. Yeast contains two glycogen synthases that have extensive similarities to their mammalian counterpart but the greatest divergence in amino acid sequence is seen precisely in the regions likely to be involved in covalent control. We hope that examination of the control of the yeast glycogen synthase will be as informative as study of the mammalian enzymes, whether by revealing important parallels with the mammalian system or by uncovering major differences in mechanism.

Isolation of the GSY1 gene encoding yeast glycogen synthase and evidence for the existence of a second gene

Glycogen synthase preparations from Saccharomyces cerevisiae contained two polypeptides of molecular weights 85,000 and 77,000. Oligonucleotides based on protein sequence were utilized to clone a S. cerevisiae glycogen synthase gene, GSY1. The gene would encode a protein of 707 residues, molecular mass 80,501 daltons, with 50% overall identity to mammalian muscle glycogen synthases. The amino-terminal sequence obtained from the 85,000-dalton species matched the NH2 terminus predicted by the GSY1 sequence. Disruption of the GSY1 gene resulted in a viable haploid with glycogen synthase activity, and purification of glycogen synthase from this mutant strain resulted in an enzyme that contained the 77,000-dalton polypeptide. Southern hybridization of genomic DNA using the GSY1 coding sequence as a probe revealed a second weakly hybridizing fragment, present also in the strain with the GSY1 gene disrupted. However, the sequences of several tryptic peptides derived from the 77,000-dalton polypeptide were identical or similar to the sequence predicted by the GSY1 gene. The data are explained if S. cerevisiae has two glycogen synthase genes encoding proteins with significant sequence similarity The protein sequence predicted by the GSY1 gene lacks the extreme NH2-terminal phosphorylation sites of the mammalian enzymes. The COOH-terminal phosphorylated region of the mammalian enzyme over-all displayed low identity to the yeast COOH terminus, but there was homology in the region of the mammalian phosphorylation sites 3 and 4. Three potential cyclic AMP-dependent protein kinase sites are located in this region of the yeast enzyme. The region of glycogen synthase likely to be involved in covalent regulation are thus more variable than the catalytic center of the molecule.

Primary structure of rabbit skeletal muscle glycogen synthase deduced from cDNA clones

The complete amino acid sequence of rabbit skeletal muscle glycogen synthase was deduced from cDNA clones with a composite length of 3317 bp. An mRNA of 3.6 kb was identified by Northern blot analysis of rabbit skeletal muscle RNA. The mRNA coded for a protein of 734 residues with a molecular weight of 83,480. The deduced NH2-terminal and COOH-terminal sequences corresponded to those reported for the purified protein, indicating the absence of any proteolytic processing. At the nucleotide level, the 5' untranslated and coding regions were 79 and 90% identical for rabbit and human muscle glycogen synthases, whereas the 3' untranslated regions were significantly less similar. The enzymes had 97% amino acid sequence identity. Interestingly, the NH2 and COOH termini of rabbit and human muscle glycogen synthase, the regions of phosphorylation, showed the greatest sequence variation (15 of 19 mismatches and two insertion/deletion events), which may indicate different evolutionary constraints in the regulatory and catalytic regions of the molecule.

Phosphoserine as a recognition determinant for glycogen synthase kinase-3: phosphorylation of a synthetic peptide based on the G-component of protein phosphatase-1

Prior phosphorylation of its substrate has been shown to be important for substrate recognition by the protein kinase glycogen synthase kinase-3 (GSK-3). Phosphorylation of glycogen synthase by GSK-3 is known to be enhanced by the previous action of casein kinase II and the sequence -SXXXS(P)- was proposed as the minimal recognition determinant for GSK-3. The glycogen binding subunit of type 1 phosphoprotein phosphatase has been shown to be phosphorylated by cyclic AMP-dependent protein kinase at serine-13 in the sequence KPGFS(5)PQPS(9)RRGS(13)ESSEEVYV (F.B. Caudwell, A. Hiraga, and P. Cohen (1986) FEBS Lett. 194, 85-89). Inspection of the sequence revealed potential GSK-3 sites at residues 5 and 9. Using a synthetic peptide with the above sequence, we found that phosphorylation of serine-13 by cyclic AMP-dependent protein kinase permitted the recognition of serine-9 and serine-5 by GSK-3. The work provides another example of a substrate for GSK-3 and demonstrates that the action of GSK-3 is linked to the presence of phosphate in the substrate and not the action of any particular protein kinase. In the course of the analyses, a novel feature of trypsin cleavage of phosphopeptides was noted. In the sequence -SRRGS(P)- trypsin acted uniquely after the first arginine whereas in the sequence -S(P)RRGS(P)- it cleaved randomly at either arginine residue. The fact that GSK-3 could phosphorylate a peptide derived from a phosphatase subunit also raises the possibility that GSK-3 might be involved in controlling glycogen-associated type 1 phosphatase and, more generally, in mediating cyclic AMP control of protein phosphorylation in cells.

Identification of phosphorylation sites in peptides using a gas-phase sequencer

A simple procedure is described for determining the location of phosphorylation sites in phosphopeptides. The method employs measurement of 32P-labeled inorganic phosphate release during Edman degradation cycles using a gas-phase sequencer. The procedure is based on extracting peptides and inorganic phosphate from portions of the sample filter at strategic cycles in the sequence analysis followed by determination of the relative amounts of phosphate and phosphopeptide. One advantage of this technique is the very high recovery of the phosphate associated with the peptide, 80-97% in this study. In the course of this work, it was also found that phosphoserine residues themselves caused reduced efficiency of the Edman degradation as compared with unesterified serine residues. The present procedure has the merit of being simple and easy to apply.

Insulin action inhibits insulin-like growth factor-II (IGF-II) receptor phosphorylation in H-35 hepatoma cells. IGF-II receptors isolated from insulin-treated cells exhibit enhanced in vitro phosphorylation by casein kinase II

Insulin caused a rapid, dose-dependent increase in the binding of 125I-insulin-like growth factor-II (IGF-II) to the surface of cultured H-35 hepatoma cells. The [32P]phosphate content of the IGF-II receptors, immunoprecipitated from extracts of H-35 cell monolayers previously incubated with [32P]phosphate for 24 h, was decreased after brief exposure of the cells to insulin. Analysis of tryptic digests of labeled IGF-II receptors by bidimensional peptide mapping revealed that the decrease in the content of [32P]phosphate occurred to varying degrees on three tryptic phosphopeptides. Thin layer electrophoresis of an acid hydrolysate of isolated IGF-II receptors revealed the presence of [32P] phosphoserine and [32P]phosphothreonine. Insulin treatment of cells caused a decrease in the labeled phosphoserine and phosphothreonine content of IGF-II receptors. The ability of a number of highly purified protein kinases (cAMP-dependent protein kinase, protein kinase C, phosphorylase kinase, and casein kinase II) to catalyze the phosphorylation of purified IGF-II receptors was examined. Casein kinase II was the only kinase capable of catalyzing the phosphorylation of the IGF-II receptor on serine and threonine residues under the conditions of our assay. Bidimensional peptide mapping revealed that the kinase catalyzed phosphorylation of the IGF-II receptor on a tryptic phosphopeptide which comigrated with the main tryptic phosphopeptide found in receptors obtained from cells labeled in vivo with [32P]phosphate. IGF-II receptors isolated by immunoadsorption from insulin-treated H-35 cells were phosphorylated in vitro by casein kinase II to a greater extent than the receptors isolated from control cells. Similarly, IGF-II receptors from plasma membranes obtained from insulin-treated adipocytes were phosphorylated by casein kinase II to a greater extent than the receptors from control adipocyte plasma membranes. Thus, the insulin-regulated phosphorylation sites on the IGF-II receptor appear to serve as substrates in vivo for casein kinase II or an enzyme with similar substrate specificity.

Phosphorylation of phosphoprotein phosphatase inhibitor-2 (I-2) in rat fat cells

Fat cells were incubated with 32Pi for 2 h before the [32P]I-2 was immunoprecipitated, subjected to SDS/PAGE, and detected by autoradiography. [32P]I-2 (Mr = 32,000) was not recovered when excess purified I-2 was added with the antiserum or when nonimmune serum was used. Immunoprecipitated I-2 was heat-stable, inhibited phosphatase activity, and could be synergistically phosphorylated by casein kinase II and FA/GSK-3. Several times more [32P]phosphoserine than [32P]phosphothreonine was found in I-2 from 32P-labeled cells. Insulin increased the 32P-content of I-2 by as much as 40%, suggesting that phosphorylation of I-2 might be involved in the effect of insulin on stimulating protein dephosphorylation.

Multiple phosphorylation of mouse muscle glycogen synthase

Glycogen synthase was isolated from extracts of mouse diaphragm muscle by immunoprecipitation with specific antibodies raised against the rabbit muscle enzyme. A procedure was developed which permitted phosphorylation of the immunoprecipitated enzyme by several purified protein kinases. Peptide mapping techniques (including reverse-phase HPLC and thin-layer electrophoresis and chromatography) were used to compare tryptic phosphopeptides of the rabbit and mouse muscle enzymes. The results demonstrated a high degree of similarity in the chemical properties of these peptides, suggesting significant homology around the phosphorylation sites in these proteins. Thus, mouse peptides corresponding to the rabbit muscle peptides containing sites 1a, 1b, 2, 3, and 5 were identified, with protein kinase recognition specificities identical to those of the rabbit enzyme. The study indicates significant conservation in the muscle isozymes of glycogen synthase between mouse and rabbit as well as a similar distribution of phosphorylation sites throughout the enzyme subunit.

Heparin-activated protein kinase from rabbit muscle: relationship to enzymes of the glycogen synthase kinase-3 category

A heparin-activated protein kinase has been previously identified in rabbit skeletal muscle extracts (Z. Ahmad et al. (1985) FEBS Lett. 179, 96-100). Further study has indicated that this enzyme phosphorylates rabbit muscle glycogen synthase in the same tryptic peptide(s) as the protein kinase FA/GSK-3 (glycogen synthase kinase-3) and is able to activate the ATP-Mg2+-dependent protein phosphatase. These results indicate similarities in properties between the two protein kinases. Exposure of the heparin-activated enzyme to trypsin resulted in loss of heparin activation, from 3-fold to 1.3-fold. One hypothesis suggested by this result is that the enzyme FA/GSK-3 could be a derivative of the heparin-activated enzyme that has lost heparin sensitivity. The conceptual importance of this hypothesis is that it may provide a clue to the mode of regulation of this important class of protein kinases.

Multiple phosphorylation sites of rat liver glycogen synthase

Rat liver glycogen synthase was purified to homogeneity by an improved procedure that yielded enzyme almost exclusively as a polypeptide of Mr 85,000. The phosphorylation of this enzyme by eight protein kinases was analyzed by cleavage of the enzyme subunit followed by mapping of the phosphopeptides using polyacrylamide gel electrophoresis in the presence of SDS, reverse-phase high-performance liquid chromatography and thin-layer electrophoresis. Cyclic AMP-dependent protein kinase, phosphorylase kinase, protein kinase C and the calmodulin-dependent protein kinase all phosphorylated the same small peptide (approx. 20 amino acids) located in a 14 kDa CNBr-fragment (CB-1). Calmodulin-dependent protein kinase and protein kinase C also modified second sites in CB-1. A larger CNBr-fragment (CB-2) of approx. 28 kDa was the dominant site of action for casein kinases I and II, FA/GSK-3 and the heparin-activated protein kinase. The sites modified were all localized in a 14 kDa species generated by trypsin digestion. Further proteolysis with V8 proteinase indicated that FA/GSK-3 and the heparin-activated enzyme recognized the same smaller peptide within CB-2, which may also be phosphorylated by casein kinase 1. Casein kinase 1 also modified a distinct peptide, as did casein kinase II. The results lead us to suggest homology to the muscle enzyme with regard to CB-1 phosphorylation and the region recognized by FA/GSK-3, which in rabbit muscle is characterized by a high density of proline and serine residues. A striking difference with the muscle isozyme is the apparent lack of phosphorylations corresponding to the muscle sites 1a and 1b. These results provide further evidence for the presence of liver- and muscle-specific glycogen synthase isozymes in the rat. That the isozymes differ subtly as to phosphorylation sites may provide a clue to the functional differences between the isozymes.

Dephosphorylation of myosin by the catalytic subunit of a type-2 phosphatase produces relaxation of chemically skinned uterine smooth muscle

It is now well-established that phosphorylation of the 20,000-dalton light chain of smooth muscle myosin (LC20) is a prerequisite for muscle contraction. However, the relationship between myosin dephosphorylation and muscle relaxation remains controversial. In the present study, we utilized a highly purified catalytic subunit of a type-2, skeletal muscle phosphoprotein phosphatase (protein phosphatase 2A) and a glycerinated smooth muscle preparation to determine if myosin dephosphorylation, in the presence of saturating calcium and calmodulin, would cause relaxation of contracted uterine smooth muscle. Addition of the phosphatase catalytic subunit (0.28 microM) to the muscle bath produced complete relaxation of the muscle. The phosphatase-induced relaxation could be reversed by adding to the muscle bath either purified, thiophosphorylated, chicken gizzard 20,000-dalton myosin light chains or purified, chicken gizzard myosin light chain kinase. Incubation of skinned muscles with adenosine 5'-O-(thiotriphosphate) prior to the addition of phosphatase resulted in the incorporation of 0.93 mol of PO4/mol of LC20 and prevented phosphatase-induced relaxation. Under all of the above conditions, changes in steady-state isometric force were associated with parallel changes in myosin light chain phosphorylation over a range of phosphorylation extending from 0.01 to 0.97 mol of PO4/mol of LC20. We found no evidence that dephosphorylation of contracted uterine smooth muscles, in the presence of calcium and calmodulin, could produce a latch-state where isometric force was maintained in the absence of myosin light chain phosphorylation. These results show that phosphorylation or dephosphorylation of the 20,000-dalton myosin light chain is adequate for the regulation of contraction or relaxation, respectively, in glycerinated uterine smooth muscle.