Requirement of the self-glucosylating initiator proteins Glg1p and Glg2p for glycogen accumulation in Saccharomyces cerevisiae

Glycogen, a branched polymer of glucose, is a storage molecule whose accumulation is under rigorous nutritional control in many cells. We report the identification of two Saccharomyces cerevisiae genes, GLG1 and GLG2, whose products are implicated in the biogenesis of glycogen. These genes encode self-glucosylating proteins that in vitro can act as primers for the elongation reaction catalyzed by glycogen synthase. Over a region of 258 residues, the Glg proteins have 55% sequence identify to each other and approximately 33% identity to glycogenin, a mammalian protein postulated to have a role in the initiation of glycogen biosynthesis. Yeast cells defective in either GLG1 or GLG2 are similar to the wild type in their ability to accumulate glycogen. Disruption of both genes results in the inability of the cells to synthesize glycogen despite normal levels of glycogen synthase. These results suggest that a self-glucosylating protein is required for glycogen biosynthesis in a eukaryotic cell. The activation state of glycogen synthase in glg1 glg2 cells is suppressed, suggesting that the Glg proteins may additionally influence the phosphorylation state of glycogen synthase.

Role of COOH-terminal phosphorylation in the regulation of casein kinase I delta

Casein kinase I delta is a member of the casein kinase I (CKI) family, a group of second messenger independent protein kinases. We present evidence that the COOH-terminal domain of CKI delta has regulatory properties. CKI delta expressed in Escherichia coli was activated by heparin, as found previously, and by treatment with the catalytic subunit of type-1 protein phosphatase (CS1). Concomitant with activation by CS1, there was a reduction in the apparent molecular weight of CKI delta from 55,000 to 49,000 as judged by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Truncation of CKI delta by removal of the COOH-terminal 110 amino acids eliminated the ability of CS1 to activate or to increase electrophoretic mobility. Casein kinase I alpha, a 37-kDa isoform that lacks an extended COOH-terminal domain, was not activated by CS1 or the presence of heparin. However, a chimeric enzyme consisting of CKI alpha fused to the COOH-terminal domain of CKI delta was activated by both heparin and CS1. Analysis of the effects of CS1 on a series of CKI delta COOH-terminal truncation mutants identified an inhibitory region between His317 and Pro342, which contained six potential phosphorylation sites. From analysis of the specific activites of these truncation mutants, removal of the same region resulted in enzyme with a specific activity nearly 10-fold greater than wild-type. Thus, CKI delta activity can be regulated by phosphorylation of its COOH terminus, which may serve to create an autoinhibitory domain. This mechanism of regulation could have important consequences in vivo.

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.

The value of bronchodilator administration in asthmatic patients before lung imaging

A 17-year-old boy with chest pain and severe bronchospasm underwent lung imaging to exclude pulmonary embolism. This showed widespread nonsegmental defects in both the ventilation and perfusion scans. A repeat study performed immediately after nebulized bronchodilator administration was normal apart from changes consistent with left lower lobe pneumonia and atelectasis which had subsequently evolved on the chest radiograph. Bronchodilator administration is valuable in improving the accuracy of lung scanning in asthmatic patients by reducing the perfusion and ventilation abnormalities caused by hypoxia-induced vasoconstriction.

Phosphorylation of sites 3a and 3b (Ser640 and Ser644) in the control of rabbit muscle glycogen synthase

Glycogen synthase kinase-3 inactivates rabbit muscle glycogen synthase by sequential phosphorylation of four COOH-terminal residues Ser652 (site 4), Ser648 (site 3c), Ser644 (site 3b), and Ser640 (site 3a). Effective recognition of glycogen synthase by glycogen synthase kinase-3 occurs only after the phosphorylation of Ser656 (site 5) catalyzed by casein kinase II. The present study addresses specifically the role of sites 3a and 3b in the regulation of glycogen synthase expressed in COS cells. Simultaneous Ser-->Ala substitutions at sites 3 a, b and c, 4, and 5 in the same protein molecule eliminated 32P labeling in the proteolytic fragment Arg634-Lys682, which contains these sites. This mutant enzyme (which also had a Ser-->Ala substitution at site 2 in the NH2 terminus) had a -/+ glucose-6-P activity ratio of approximately 0.8, similar to that of totally dephosphorylated enzyme. Reinstating serine residues at either site 3a or site 3b restored labeling in the Arg634-Lys682 peptide and caused a decrease in the activity ratio to 0.4-0.6. When both sites 3a and 3b were reintroduced, there was complete inactivation of the enzyme. Thus, sites 3a and 3b are sufficient for the inactivation of glycogen synthase and act synergistically to control activity. This investigation demonstrates the existence of an alternate mechanism for the phosphorylation of sites 3a and 3b that does not depend on prior phosphorylation of site 5.

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.

Mechanism of glycogenin self-glucosylation

Glycogenin, the proposed initiator of mammalian glycogen biosynthesis, transfers glucose residues from UDP-glucose to an oligosaccharide chain attached to Tyr-194 in a self-glucosylation reaction. Mutation of Tyr-194 to either Phe or Thr residues results in the loss of this self-glucosylating activity since the site of oligosaccharide attachment has been lost (Y. Cao, A. M. Mahrenholz, A. A. DePaoli-Roach, and P. J. Roach (1993) J. Biol. Chem. 268, 14687-14693). We describe here that Phe-194 and Thr-194 mutants of glycogenin, as well as wild-type protein, were active in transferring glucose to an exogenous acceptor, maltose, a known inhibitor of the self-glucosylation reaction. The reaction product was exclusively maltotriose with no evidence for further elongation to maltotetraose or maltopentaose. The values of Vmax/Km for maltotriose synthesis for the mutant proteins were 1.5-3.5 times greater than that of the wild type. Analysis of crystals of wild-type glycogenin by X-ray diffraction gives a tetragonal unit cell of a = b = 130 A and c = 174 A in space group I4 with four glycogenin molecules in one asymmetric unit. Considerations of the symmetry and the crystal packing indicate the existence of dimers of glycogenin which may further associate to form a tetramer. The existence of oligomeric forms of glycogenin, together with the idea that glucose transfer to an exogenous acceptor is possible, raises the possibility that the intramolecular self-glucosylation of glycogenin could involve an intersubunit transfer of glucose.

Novel Saccharomyces cerevisiae gene, MRK1, encoding a putative protein kinase with similarity to mammalian glycogen synthase kinase-3 and Drosophila Zeste-White3/Shaggy

A Saccharomyces cerevisiae gene was identified that would encode a protein similar to the mammalian protein kinase glycogen synthase kinase-3 (GSK-3) and the Drosophila Zeste-White3/Shaggy gene product. The open reading frame predicts a 375 amino acid polypeptide with a putative protein kinase domain that displays 70% and 39% identity, respectively, to two known yeast proteins, Mds1p and Mck1p. The new gene, designated MRK1 (Mds1p Related Kinase), is located on chromosome IV. Disruption of MRK1 was not lethal and did not elicit any alteration in glycogen accumulation. In addition, an mck1 mds1 mrk1 triple disruptant was viable.

Phosphorylation and activation of p90rsk by glycogen synthase kinase-3

Recombinant p90rsk expressed from baculovirus was found to be phosphorylated and activated by glycogen synthase kinase-3 (GSK-3) in vitro. Phosphorylation of p90rsk by both GSK-3 alpha and GSK-3 beta isoforms was predominantly on threonine residues. Activated p90rsk, resulting from co-expression in insect cells with the oncogenic protein tyrosine kinase p60v-src, was able to phosphorylate GSK-3 but was a poor GSK-3 substrate. These results suggest a potentially novel regulatory connection in the signal transduction cascades in which p90rsk participates.

Renal dysplasia in infants: appearance on 99mTc DMSA scintigraphy

Infantile renal dysplasias, including multicystic dysplastic kidneys (MCDK), are reported rarely to accumulate radiopharmaceuticals on renal scintigraphy. 99mTc DMSA is a highly sensitive tracer for detecting functioning renal cortical tissue and may be more suited to studying renal dysplasia than 99mTc DTPA. We reviewed the ultrasound studies and 99mTc DMSA scintigrams of 42 infants (age range 1-12 months) with known or suspected MCDK. Overall, uptake on 99mTc DMSA scintigraphy was evident in 6/41 (15%) dysplastic kidneys. Of the 18 patients who underwent nephrectomy, histopathological examination revealed that uptake correlated closely with the presence of mature renal cortical tissue in the affected kidney. Our study shows that a small, but significant number of MCDK will show low-grade uptake on DMSA scintigraphy. This finding may be relevant given the reliance placed on renal scintigraphy in planning treatment for infants with suspected MCDK, particularly with the increasing trend for the non-operative management of this condition.

A secondary phosphorylation of CREB341 at Ser129 is required for the cAMP-mediated control of gene expression. A role for glycogen synthase kinase-3 in the control of gene expression

The cAMP-dependent protein kinase (PKA) phosphorylates CREB327/341 at a single serine residue, Ser119/133, respectively. Phosphorylation at this site creates the sequence motif SXXXS(P), a consensus site of the glycogen synthase kinase-3 (GSK-3) enzyme (Fiol, C.J., Mahrenholz, A.M., Wang, Y., Roeske, R.W., and Roach, P.J. (1987) J. Biol. Chem. 262, 14042-14048). We examined the phosphorylation of CREB at the SXXXS(P) consensus site and its role in CREB transactivation to cAMP induction. Neither isoform of the GSK-3 enzyme (GSK-3 alpha or beta) utilizes CREB as its substrate unless CREB is already phosphorylated at Ser119/133. A 13-amino acid peptide containing the sequence surrounding Ser119/133 was phosphorylated by GSK-3, at Ser115/129, only after the primary phosphorylation of the peptide by PKA (at Ser119/133), suggesting that Ser115/129 is a GSK-3 phosphoacceptor site. Mutant CREB327/341 proteins containing Ser-->Ala substitutions confirmed Ser115/129 as the only GSK-3 phosphorylation site. Transfection assays of wild type and mutant Gal4-CREB fusion proteins in PC12 cells demonstrated that Ser-->Ala substitution of residue 129 of CREB341 impairs the transcriptional response to cAMP induction. Analogous mutation in CREB327 results in 70% decrease in its transactivation response to cAMP. In undifferentiated F9 cells, which are refractory to cAMP induction, transfected GSK-3 beta kinase induces a 60-fold increase in cyclic AMP response element-dependent transcription, mediated via the endogenous CREB protein. We propose that the hierarchical phosphorylation at the PKA and GSK-3 sites of CREB are essential for cAMP control of CREB.

Interactions between cAMP-dependent and SNF1 protein kinases in the control of glycogen accumulation in Saccharomyces cerevisiae

The synthesis of glycogen in Saccharomyces cerevisiae is stimulated by nutrient limitation and requires both glycogen synthase and the glycogen branching enzyme. Of the two glycogen synthase genes present in yeast, GSY2 appears to be more important for the accumulation of glycogen upon entry into stationary phase. In cells grown on glucose, GSY2 mRNA levels increased approximately 10-fold during the transition from logarithmic to stationary phase. Growth of cells in glycerol, however, resulted in constitutive expression of GSY2 mRNA and the corresponding protein, GS-2, suggestive of glucose repression of GSY2. Mutants defective in the SNF1 gene, which encodes a protein kinase important in glucose repression mechanisms, are known not to accumulate glycogen. A modest 2-4-fold decrease in total GS-2 level was observed, and upon entry into stationary phase, the enzyme was blocked in the inactive, phosphorylated state in snf1 strains. The GS-2 protein is thought to be regulated by covalent phosphorylation of three COOH-terminal sites (Hardy, T.A., and Roach, P.J. (1993) J. Biol. Chem. 268, 23799-23805), removal of which results in constitutively active glycogen synthase that bypasses phosphorylation controls. Expression of COOH-terminally truncated GS-2 in snf1 cells restored glycogen accumulation, and so we propose that the SNF1 kinase controls the phosphorylation state of GS-2. Cyclic AMP pathways also exert control over glycogen accumulation. In bcy1 cells, which have constitutively active cyclic AMP-dependent protein kinase, greatly reduced levels of both GS-2 message and protein were observed. With wild type GSY2 placed under control of the ADH1 promoter, bcy1 cells did not accumulate glycogen despite increased GS-2. Overexpression of truncated GS-2, however, resulted in definite though reduced glycogen accumulation; the glycogen synthesized was structurally distinct from wild type with properties characteristic of less branched polysaccharide. We conclude that the cAMP pathway controls both the expression and the phosphorylation state of GS-2. Furthermore, other factor(s) necessary for glycogen biosynthesis, such as the branching enzyme GLC3, must also be under negative control by the cAMP pathway. The results demonstrate interactive controls of GS-2 by the cAMP-dependent and SNF1 protein kinases.

Rabbit skeletal muscle glycogen synthase expressed in COS cells. Identification of regulatory phosphorylation sites

Rabbit skeletal muscle glycogen synthase contains multiple sites for phosphorylation. To investigate the relative importance of these sites, the enzyme was overexpressed in COS M9 cells, and Ser-->Ala mutations were introduced singly, or in combinations, at nine known phosphorylation sites. Overexpressed wild-type enzyme had a very low +/- glucose-6-P activity ratio of approximately 0.01, indicative that the glycogen synthase is in a highly phosphorylated state. No single Ser-->Ala mutation was able to cause a substantial increase in activity ratio; rather, simultaneous mutation at both NH2- and COOH-terminal sites was needed. The most effective combinations were mutations at site 3a (Ser-640) or site 3b (Ser-644) together with site 2 (Ser-7). The results were consistent with site 2 phosphorylation being a prerequisite for phosphorylation of site 2a (Ser-10). Mutation of site 5 (Ser-656) perturbed COOH-terminal phosphorylation but did not prevent inactivation. Expression of the most active mutants correlated with increased glycogen accumulation in the COS M9 cells. In summary, we conclude that (i) the sites most important for activating the enzyme are sites 2, 2a, 3a, and 3b; (ii) removal of phosphate from both NH2- and COOH-terminal sites is required for activation; and (iii) sites 3a and/or 3b can be phosphorylated in COS cells by mechanisms that do not depend on phosphorylation of site 5.

Technetium-99m HmPAO brain SPECT and outcome of hemispherectomy for intractable seizures

With recent descriptions of the modified hemispherectomies and hemicorticectomy, there has been renewed interest in hemispherectomy for treatment of intractable seizures with hemiparesis. Because long-term outcome remains uncertain, patient selection remains difficult. 99mTc-HmPAO brain SPECT has been a helpful adjunct in the evaluation of epilepsy surgery candidates. We report SPECT scan findings in 7 patients who underwent hemispherectomy and compare these results with scalp EEG findings. Six patients had unilateral SPECT findings and all had a favorable outcome, regardless of surface EEG findings.

Use of a synthetic peptide as a selective substrate for glycogen synthase kinase 3

Glycogen synthase kinase 3 (GSK-3) is involved in the regulation of several metabolic enzymes and transcription factors in response to extracellular signals. Here we report the use of a synthetic peptide derived from the sequence of the cyclic AMP responsive element binding protein (CREB) as a specific substrate for GSK-3 isoforms. The 13-amino acid peptide, KRREILSRRPSYR, was phosphorylated by the catalytic subunit of cAMP-dependent protein kinase (PKA) and purified on a C18 cartridge. Phosphorylation of the COOH-terminal serine of the peptide by PKA creates a phosphorylation site for GSK-3 since GSK-3 recognizes the consensus motif -S-X-X-X-S(P)-. Although the COOH-terminal serine of the peptide can be phosphorylated by PKA and several other kinases, the phospho-CREB peptide is specific for GSK-3 with Kms of 140 and 200 microM for GSK-3 alpha and GSK-3 beta isoforms, respectively. Using the phospho-CREB peptide, we have successfully purified GSK-3 activity from rabbit skeletal muscle and Escherichia coli cells transformed with a GSK-3 expression vector. The assay described provides a convenient and specific determination of GSK-3 activity.

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.

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)

MDS1, a dosage suppressor of an mck1 mutant, encodes a putative yeast homolog of glycogen synthase kinase 3

The yeast gene MCK1 encodes a serine/threonine protein kinase that is thought to function in regulating kinetochore activity and entry into meiosis. Disruption of MCK1 confers a cold-sensitive phenotype, a temperature-sensitive phenotype, and sensitivity to the microtubule-destabilizing drug benomyl and leads to loss of chromosomes during growth on benomyl. A dosage suppression selection was used to identify genes that, when present at high copy number, could suppress the cold-sensitive phenotype of mck1::HIS3 mutant cells. Several unique classes of clones were identified, and one of these, designated MDS1, has been characterized in some detail. Nucleotide sequence data reveal that MDS1 encodes a serine/threonine protein kinase that is highly homologous to the shaggy/zw3 kinase in Drosophila melanogaster and its functional homolog, glycogen synthase kinase 3, in rats. The presence of MDS1 in high copy number rescues both the cold-sensitive and the temperature-sensitive phenotypes, but not the benomyl-sensitive phenotype, associated with the disruption of MCK1. Analysis of strains harboring an mds1 null mutation demonstrates that MDS1 is not essential during normal vegetative growth but appears to be required for meiosis. Finally, in vitro experiments indicate that the proteins encoded by both MCK1 and MDS1 possess protein kinase activity with substrate specificity similar to that of mammalian glycogen synthase kinase 3.

Inactivation of rabbit muscle glycogen synthase by glycogen synthase kinase-3. Dominant role of the phosphorylation of Ser-640 (site-3a)

Rabbit skeletal muscle glycogen synthase, a rate-limiting enzyme for glycogen biosynthesis, is regulated by multisite phosphorylation. The protein kinase glycogen synthase kinase 3 (GSK-3) phosphorylates 4 Ser residues (Ser-640, Ser-644, Ser-648, and Ser-652; also known as sites 3a, 3b, 3c, and 4, respectively) at the COOH terminus of the subunit. Phosphorylation of these sites by GSK-3 is sequential, from COOH- to NH2-terminal, and is wholly dependent on prior phosphorylation by casein kinase II at Ser-656 (site 5). Expression in Escherichia coli was used to generate mutant forms of glycogen synthase, S640A, S644A, and S648A, in which site 3a, site 3b, or site 3c was changed to Ala, respectively. The purified enzymes had -/+ glucose-6-P activity ratios in the range of 0.8-0.9. Phosphorylation by casein kinase II and GSK-3 gave results consistent with the model of obligate sequential action of GSK-3. Phosphorylation at site 5, sites 4 + 5, or sites 3c + 4 + 5 had no measurable effect on activity. When sites 3b + 3c + 4 + 5 were phosphorylated, modest inactivation resulted. Additional phosphorylation at site 3a, however, was potently inactivating, reducing the -/+ glucose-6-P activity ratio to 0.1 and increasing the glucose-6-P concentration needed for half-maximal activation by an order of magnitude. Introduction of each additional phosphate, in the order site 4, 3c, 3b, and 3a, caused an incremental reduction in the mobility of the subunit when analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The results of this study demonstrate that GSK-3 phosphorylation of site 3a (Ser-640), and to a lesser extent, site 3b, correlates with inactivation of glycogen synthase by GSK-3. Evidence is also presented for an allosteric mechanism of inactivation whereby modification of one subunit influences the activity state of adjacent subunits.

Control of yeast glycogen synthase-2 by COOH-terminal phosphorylation

The budding yeast Saccharomyces cerevisiae expresses two isoforms of glycogen synthase, of which glycogen synthase-2 (GS-2) appears to be the most important determinant of glycogen accumulation (Farkas, I., Hardy, T. A., Goebl, M. G., and Roach, P. J. (1991) J. Biol. Chem. 266, 15602-15607). Partial proteolysis of purified yeast glycogen synthase activated the enzyme, mimicking the effects of dephosphorylation. The cleavage was localized to the COOH terminus of the molecule and trypsin treatment released 32P from enzyme labeled in vivo with 32P or in vitro by cyclic AMP-dependent protein kinase. Similarly, when cells were labeled with 32P, no radioactivity was incorporated into a mutant form of GS-2 truncated at residue 643 while the wild type enzyme was phosphorylated at both Ser and Thr residues. The 9 Ser and Thr residues COOH-terminal to position 643 were mutated individually to Ala, and the GS-2 mutants were expressed from a low copy plasmid in yeast that lacked functional chromosomal copies of the two glycogen synthase genes. Mutations at Ser-650, Ser-654, and Thr-667 resulted in significant activation of yeast glycogen synthase and elevation in the level of accumulated glycogen as compared with wild type. Likewise, expression of the truncated GS-2 resulted in hyperactive enzyme and the overaccumulation of glycogen. None of the other Ser or Thr mutations substantially affected glycogen synthase activity and glycogen storage. We conclude that Ser-650, Ser-654, and Thr-667 are regulatory phosphorylation sites in vivo. However, in vitro, cyclic AMP-dependent protein kinase modified Ser residue(s) COOH-terminal to position 659, and so the identity of the physiological GS-2 kinases is unclear. Yeast strains bearing glc7 and gac1 mutations are defective in genes encoding type 1 protein phosphatase components and are impaired in their ability to accumulate glycogen. Expression of the truncated GS-2 in these strains restored glycogen accumulation, as did the presence of GS-2 mutated at Ser-650, Ser-654, or Thr-667. These data are consistent with the hypothesis that type 1 phosphatase regulates GS-2 by controlling its phosphorylation state.

Asystole and bradycardia during dipyridamole stress testing in patients receiving beta blockers

Only rarely have serious side effects been reported with the use of intravenous dipyridamole. We describe two cases of severe bradycardia, of which one led to asystole, in patients undergoing dipyridamole-thallium studies. The association between beta blocker therapy and the seven reported cases of asystole with dipyridamole is discussed and mechanisms postulated. Some caution is advised when patients on beta blockers or similar medications have dipyridamole-thallium studies.