Analysis of respiratory mutants reveals new aspects of the control of glycogen accumulation by the cyclin-dependent protein kinase Pho85p

The PHO85 gene of Saccharomyces cerevisiae encodes a cyclin-dependent protein kinase that can interact with 10 different cyclins (Pcls). In conjunction with Pcl8p and Pcl10p, Pho85p phosphorylates and regulates glycogen synthase. Respiratory-deficient strains, such as coq3 mutants, have reduced glycogen stores and contain hyperphosphorylated and inactive glycogen synthase. We show here that pho85 coq3 mutants have dephosphorylated and active glycogen synthase yet do not maintain glycogen reserves. In contrast, deletion of PCL8 and PCL10 in the coq3 mutant background partially restores glycogen accumulation. This suggested the existence of inputs from Pho85p into glycogen storage, independent of Pcl8p and Pcl10p, and acting antagonistically.

Glycogen and its metabolism

Glycogen is a branched polymer of glucose which serves as a reservoir of glucose units. The two largest deposits in mammals are in the liver and skeletal muscle but many cells are capable synthesizing glycogen. Its accumulation and utilization are under elaborate controls involving primarily covalent phosphorylation and allosteric ligand binding. Both muscle and liver glycogen reserves are important for whole body glucose metabolism and their replenishment is linked hormonally to nutritional status. Control differs between muscle and liver in part due to the existence of different tissue-specific isoforms at key steps. Control of synthesis is shared between transport into the muscle and the step catalyzed by glycogen synthase. Breakdown of liver glycogen, as part of blood glucose homeostasis, is also in response to nutritional cues. Muscle glycogen serves only to fuel muscular activity and its utilization is controlled by muscle contraction and by catecholamines. Though the number of enzymes directly involved in the metabolism of glycogen is quite small, many more proteins act indirectly in a regulatory capacity. Defects in the basic metabolizing enzymes lead to severe consequences whereas, with some exceptions, mutations in the regulatory proteins appear to cause a more subtle phenotypic change.

Systematic identification of the genes affecting glycogen storage in the yeast Saccharomyces cerevisiae: implication of the vacuole as a determinant of glycogen level

At the onset of nutrient limitation, the yeast Saccharomyces cerevisiae synthesizes glycogen to serve as a carbon and energy reserve. We undertook a systematic survey for the genes that affect glycogen accumulation by taking advantage of the strain deletion set generated by the Saccharomyces Genome Deletion Project. The strain collection analyzed contained some 4600 diploid homozygous null deletants, representing approximately 88% of all viable haploid disruptants. We identified 324 strains with low and 242 with elevated glycogen stores, accounting for 12.4% of the genes analyzed. The screen was validated by the identification of many of the genes known already to influence glycogen accumulation. Many of the mutants could be placed into coherent families. For example, 195 or 60% of the hypoaccumulators carry mutations linked to respiratory function, a class of mutants well known to be defective in glycogen storage. The second largest group consists of approximately 60 genes involved in vesicular trafficking and vacuolar function, including genes encoding 13 of 17 proteins involved in the structure or assembly of the vacuolar ATPase. These data are consistent with our recent findings that the process of autophagy has a significant impact on glycogen storage (Wang, Z., Wilson, W. A., Fujino, M. A., and Roach, P. J. (2001) Antagonistic controls of autophagy and glycogen accumulation by Snf1p, the yeast homolog of AMP-activated protein kinase, and the cyclin-dependent kinase Pho85p. Mol. Cell. Biol. 21, 5742-5752). Autophagy delivers glycogen to the vacuole, and we propose that the impaired vacuolar function associated with ATPase mutants (vma10 or vma22) results in reduced degradation and subsequent hyperaccumulation of glycogen.

Mutations of muscle glycogen synthase that disable activation by glucose 6-phosphate

Glycogen synthase, an enzyme of historical importance in the field of reversible protein modification, is inactivated by phosphorylation and allosterically activated by glucose 6-phosphate (glucose-6-P). Previous analysis of yeast glycogen synthase had identified a conserved and highly basic 13-amino-acid segment in which mutation of Arg residues resulted in loss of activation by glucose-6-P. The equivalent mutations R578R579R581A (all three of the indicated Arg residues mutated to Ala) and R585R587R590A were introduced into rabbit muscle glycogen synthase. Whether expressed transiently in COS-1 cells or produced in and purified from Escherichia coli, both mutant enzymes were insensitive to activation by glucose-6-P. The effect of phosphorylation was studied in two ways. Purified, recombinant glycogen synthase was directly phosphorylated by casein kinase 2 and glycogen synthase kinase 3, under conditions that inactivate the wild-type enzyme. In addition, phosphorylation sites were converted to Ala by mutagenesis in wild-type and in the glucose-6-P desensitized mutants expressed in COS-1 cells. Phosphorylation inactivated the R578R579R581A mutant but had little effect on the R585R587R590A. This result was surprising since phosphorylation had the opposite effects on the corresponding yeast enzyme mutants. The results confirm that the region of glycogen synthase, Arg-578-Arg-590, is required for activation by glucose-6-P and suggest that it is part of a sensitive and critical switch involved in transitions between different conformational states. However, the role must differ subtly between the mammalian and the yeast enzymes.

Optimal pinhole techniques for preoperative localization with Tc-99m MIBI for primary hyperparathyroidism

PURPOSE

Technetium-99m-labeled 2-methoxyisobutylisonitrile (Tc-99m MIBI) has been used extensively to localize parathyroid adenomas before operation. Imaging techniques vary widely, and the aim of this study was to determine the optimal time of delayed imaging and the value of routine correlative pertechnetate thyroid imaging.

MATERIALS AND METHODS

In this study, preoperative parathyroid localization was performed using pinhole anterior and oblique images (15 minutes and 2 and 4 hours after injection) with correlative pertechnetate thyroid images. Ninety-seven patients underwent dual- or triple-phase Tc-99m MIBI imaging and correlative pertechnetate thyroid imaging before surgery. Two nuclear medicine physicians blinded to the surgical findings interpreted all available images and various Tc-99m MIBI image combinations at 15 minutes alone; 15 minutes and 2 hours, 15 minutes and 4 hours; and 15 minutes and 2 and 4 hours each with and without correlative pertechnetate thyroid imaging.

RESULTS

Ninety parathyroid adenomas were detected in 86 patients. The optimal results were achieved with 15-minute and 2- and 4-hour Tc-99m-MIBI images, with correlative thyroid scans resulting in a sensitivity rate of 88%. Fifteen-minute and 2-hour Tc-99m-MIBI images and correlative thyroid scans and 15-minute and 4-hour Tc-99m MIBI images and correlative thyroid scans produced similar results (sensitivity rate, 86% and 83%, respectively; P = not significant). Compared with all Tc-99m MIBI image combinations alone, the addition of the routine correlative thyroid scan significantly improved sensitivity and also improved reporter confidence in 45% of studies.

CONCLUSIONS

Of the pinhole techniques compared, 15-minute and 2-hour Tc-99m MIBI images with correlative thyroid scanning may be the preferred imaging protocol, because this yields results similar to imaging for as long as 4 hours after injection in a shorter, more logistically acceptable imaging time.

The yeast cyclins Pc16p and Pc17p are involved in the control of glycogen storage by the cyclin-dependent protein kinase Pho85p

Pho85p is a yeast cyclin-dependent protein kinase (Cdk) that can interact with 10 cyclins (Pcls) to form multiple protein kinases. The functions of most of the Pcls, including Pc16p and Pc17p, are poorly defined. We report here that Pc16p and Pc17p are involved in the metabolism of the branched storage polysaccharide glycogen under certain conditions and deletion of PCL6 and PCL7 restores glycogen accumulation to a snf1 pcl8 pcl10 triple mutant, paradoxically activating both glycogen synthase and phosphorylase. Pho85p thus affects glycogen accumulation through multiple Cdks composed of different cyclin partners.

Antagonistic controls of autophagy and glycogen accumulation by Snf1p, the yeast homolog of AMP-activated protein kinase, and the cyclin-dependent kinase Pho85p

In the yeast Saccharomyces cerevisiae, glycogen is accumulated as a carbohydrate reserve when cells are deprived of nutrients. Yeast mutated in SNF1, a gene encoding a protein kinase required for glucose derepression, has diminished glycogen accumulation and concomitant inactivation of glycogen synthase. Restoration of synthesis in an snf1 strain results only in transient glycogen accumulation, implying the existence of other SNF1-dependent controls of glycogen storage. A genetic screen revealed that two genes involved in autophagy, APG1 and APG13, may be regulated by SNF1. Increased autophagic activity was observed in wild-type cells entering the stationary phase, but this induction was impaired in an snf1 strain. Mutants defective for autophagy were able to synthesize glycogen upon approaching the stationary phase, but were unable to maintain their glycogen stores, because subsequent synthesis was impaired and degradation by phosphorylase, Gph1p, was enhanced. Thus, deletion of GPH1 partially reversed the loss of glycogen accumulation in autophagy mutants. Loss of the vacuolar glucosidase, SGA1, also protected glycogen stores, but only very late in the stationary phase. Gph1p and Sga1p may therefore degrade physically distinct pools of glycogen. Pho85p is a cyclin-dependent protein kinase that antagonizes SNF1 control of glycogen synthesis. Induction of autophagy in pho85 mutants entering the stationary phase was exaggerated compared to the level in wild-type cells, but was blocked in apg1 pho85 mutants. We propose that Snf1p and Pho85p are, respectively, positive and negative regulators of autophagy, probably via Apg1 and/or Apg13. Defective glycogen storage in snf1 cells can be attributed to both defective synthesis upon entry into stationary phase and impaired maintenance of glycogen levels caused by the lack of autophagy.

Surprises of genetic engineering: a possible model of polyglucosan body disease

BACKGROUND

The authors previously reported the generation of a knockout mouse model of Pompe disease caused by the inherited deficiency of lysosomal acid alpha-glucosidase (GAA). The disorder in the knockout mice (GAA-/-) resembles the human disease closely, except that the clinical symptoms develop late relative to the lifespan of the animals. In an attempt to accelerate the course of the disease in the knockouts, the authors increased the level of cytoplasmic glycogen by overexpressing glycogen synthase (GSase) or GlutI glucose transporter.

METHODS

GAA-/- mice were crossed to transgenic mice overexpressing GSase or GlutI in skeletal muscle.

RESULTS

Both transgenics on a GAA knockout background (GS/GAA-/- and GlutI/GAA-/-) developed a severe muscle wasting disorder with an early age at onset. This finding, however, is not the major focus of the study. Unexpectedly, the mice bearing the GSase transgene, but not those bearing the GlutI transgene, accumulated structurally abnormal polysaccharide (polyglucosan) similar to that observed in patients with Lafora disease, glycogenosis type IV, and glycogenosis type VII. Ultrastructurally, the periodic acid-Schiff (PAS)-positive polysaccharide inclusions were composed of short, amorphous, irregular branching filaments indistinguishable from classic polyglucosan bodies. The authors show here that increased level of GSase in the presence of normal glycogen branching enzyme (GBE) activity leads to polyglucosan accumulation. The authors have further shown that inactivation of lysosomal acid alpha-glucosidase in the knockout mice does not contribute to the process of polyglucosan formation.

CONCLUSIONS

An imbalance between GSase and GBE activities is proposed as the mechanism involved in the production of polyglucosan bodies. The authors may have inadvertently created a "muscle polyglucosan disease" by simulating the mechanism for polyglucosan formation.

The value of oblique pinhole images in pre-operative localisation with 99mTc-MIBI for primary hyperparathyroidism

Technetium-99m labelled 2-methoxyisobutylisonitrile (MIBI) has been extensively utilised for preoperative localisation of parathyroid adenomas. Imaging techniques have varied widely, with many centres not performing routine oblique images; thus this study aimed to examine the value of routine oblique pinhole imaging. Ninety-two patients underwent pre-operative 99mTc-MIBI imaging including early and delayed anterior oblique pinhole images in addition to standard anterior pinhole images and a thyroid study prior to surgery for primary hyperparathyroidism. These studies were reviewed blindly comparing anterior and oblique images and anterior images only in relation to surgical findings. Of the 92 patients, 83 were found to have 86 parathyroid adenomas or parathyroid adenoma/hyperplasia at surgery. When compared to anterior images only, oblique views improved overall sensitivity from 76% to 88% (P<0.05), correctly localised 11 more adenomas than anterior images alone (13%) and improved the confidence of interpretation in 17 patients (20%). In conclusion, routine oblique pinhole views result in greater sensitivity and reporter confidence in pre-operative parathyroid localisation with 99mTc-MIBI.

Casein kinase I associates with members of the centaurin-alpha family of phosphatidylinositol 3,4,5-trisphosphate-binding proteins

Mammalian casein kinases I (CKI) belong to a family of serine/threonine protein kinases involved in diverse cellular processes including cell cycle progression, membrane trafficking, circadian rhythms, and Wnt signaling. Here we show that CKIalpha co-purifies with centaurin-alpha(1) in brain and that they interact in vitro and form a complex in cells. In addition, we show that the association is direct and occurs through the kinase domain of CKI within a loop comprising residues 217-233. These residues are well conserved in all members of the CKI family, and we show that centaurin-alpha(1) associates in vitro with all mammalian CKI isoforms. To date, CKIalpha represents the first protein partner identified for centaurin-alpha(1). However, our data suggest that centaurin-alpha(1) is not a substrate for CKIalpha and has no effect on CKIalpha activity. Centaurin-alpha(1) has been identified as a phosphatidylinositol 3,4,5-trisphosphate-binding protein. Centaurin-alpha(1) contains a cysteine-rich domain that is shared by members of a newly identified family of ADP-ribosylation factor guanosine trisphosphatase-activating proteins. These proteins are involved in membrane trafficking and actin cytoskeleton rearrangement, thus supporting a role for CKIalpha in these biological events.

Characterization of Gac1p, a regulatory subunit of protein phosphatase type I involved in glycogen accumulation in Saccharomyces cerevisiae

GAC1 and GLC7 encode regulatory and catalytic subunits, respectively, of a type 1 phosphatase (PP1) in Saccharomyces cerevisiae that controls glycogen synthesis by regulating the phosphorylation state of glycogen synthase (Gsy2p). To investigate the role of Gac1p in this process, a set of GAC1 deletions were tested for their ability to complement a gac1 null mutation and to associate with Glc7p and with Gsy2p. The N-terminal 93 amino acids of Gaclp are necessary and sufficient for the interaction with Glc7p, whereas a region spanning residues 130-502 is required for Gsy2p binding. Both domains are required for full activity in vivo, although the Glc7p-binding domain retains some residual activity and can alter the phosphorylase a phosphatase activity of Glc7p in vitro. Further mutational analysis showed that Val71 and Phe73 of Gaclp are necessary for binding to Glc7p, while Asn356 and Tyr357 of Gaclp are necessary for binding to Gsy2p. These results suggest that Gac1p targets PPI to its substrate Gsy2p and that Gac1p may alter the catalytic activity of PP . Our data also show that overexpression of Gac1p affects glucose repression and ion homeostasis, two additional targets of GLC7, suggesting that multiple regulatory subunits compete for Glc7p binding in vivo.

Do rodents have a gene encoding glycogenin-2, the liver isoform of the self-glucosylating initiator of glycogen synthesis?

The discovery of a second human gene, GYG2, encoding a liver-specific isoform of glycogenin, the self-glucosylating initiator of glycogen biosynthesis, raised the possibility for differential controls of this protein in liver and muscle. The new protein, glycogenin-2, had several properties similar biochemically to the muscle isoform, glycogenin-1, but unlike glycogenin-1, stable expression in fibroblasts led to a significant overaccumulation of glycogen. Ensuing attempts to generate reagents suitable for use with rodents, to examine the physiological regulation of glycogenin-2 by nutritional and hormonal factors, have been unsuccessful. Proof of a negative is difficult but the weight of the evidence is beginning to mitigate against the existence of a second glycogenin gene in rodents leading us to hypothesize that the presence of the GYG2 gene is limited to primates.

Regulation of glycogen synthase. Identification of residues involved in regulation by the allosteric ligand glucose-6-P and by phosphorylation

The major yeast glycogen synthase, Gsy2p, is inactivated by phosphorylation and activated by the allosteric ligand glucose-6-P. From studies of recombinant proteins, the control can be accommodated by a three-state model, in which unphosphorylated enzyme has intermediate activity (state II). Glucose-6-P increased V(max)/K(m) by about 2-fold (state III), whereas phosphorylation by the cyclin-dependent protein kinase Pcl10p/Pho85p decreased V(max)/K(m) by approximately 30-fold (state I). In the presence of glucose-6-P, state III is achieved regardless of phosphorylation state. The enzyme forms complexes in solution with the yeast glycogenin Glg2p, but this interaction appears not to affect control either by glucose-6-P binding or by phosphorylation. Scanning mutagenesis was applied to identify residues potentially involved in ligand binding. Of 22 mutant enzymes analyzed, seven were essentially inactive. Five mutant proteins were altered in their activation by glucose-6-P, and two were completely unaffected by the hexose phosphate. One of these, R586A/R588A/R591A (all three of the indicated Arg residues mutated to Ala), had wild-type activity and was normally inactivated by phosphorylation. A second mutant, R579A/R580A/R582A, had somewhat reduced V(max), but its activity was not greatly reduced by phosphorylation. The Arg residues in these two mutants are restricted to a highly conserved, 13-residue segment of Gsy2p that we propose to be important for glucose-6-P binding and/or the ability of the enzyme to undergo transitions between activity states.

Glycogen synthase sensitivity to insulin and glucose-6-phosphate is mediated by both NH2- and COOH-terminal phosphorylation sites

In skeletal muscle, insulin activates glycogen synthase by reducing phosphorylation at both NH2- and COOH-terminal sites of the enzyme and by elevating the levels of glucose-6-phosphate, an allosteric activator of glycogen synthase. To study the mechanism of regulation of glycogen synthase by insulin and glucose-6-phosphate, we generated stable Rat-1 fibroblast clones expressing rabbit muscle glycogen synthase with Ser-->Ala substitutions at key phosphorylation sites. We found that 1) elimination of the phosphorylation of either NH2- or COOH-terminal sites did not abolish insulin stimulation of glycogen synthase; 2) mutations at both Ser-7 and Ser-640 were necessary to bypass insulin activation; 3) mutation at Ser-7, coupled with the disruption of the motif for recognition by glycogen synthase kinase-3 (GSK-3), did not eliminate the insulin effect; and 4) mutation of either Ser-7 or Ser-640 increased the sensitivity of glycogen synthase to glucose 6-phosphate >10-fold. We conclude that Ser-7 and Ser-640 are both involved in mediating the response of glycogen synthase to insulin and activation by glucose 6-phosphate. In Rat-1 fibroblasts, GSK-3 action is not essential for glycogen synthase activation by insulin, and GSK-3-independent mechanisms also operate.

Control of glycogen synthesis is shared between glucose transport and glycogen synthase in skeletal muscle fibers

The effects of transgenic overexpression of glycogen synthase in different types of fast-twitch muscle fibers were investigated in individual fibers from the anterior tibialis muscle. Glycogen synthase was severalfold higher in all transgenic fibers, although the extent of overexpression was twofold greater in type IIB fibers. Effects of the transgene on increasing glycogen and phosphorylase and on decreasing UDP-glucose were also more pronounced in type IIB fibers. However, in any grouping of fibers having equivalent malate dehydrogenase activity (an index of oxidative potential), glycogen was higher in the transgenic fibers. Thus increasing synthase is sufficient to enhance glycogen accumulation in all types of fast-twitch fibers. Effects on glucose transport and glycogen synthesis were investigated in experiments in which diaphragm, extensor digitorum longus (EDL), and soleus muscles were incubated in vitro. Transport was not increased by the transgene in any of the muscles. The transgene increased basal [(14)C]glucose into glycogen by 2.5-fold in the EDL, which is composed primarily of IIB fibers. The transgene also enhanced insulin-stimulated glycogen synthesis in the diaphragm and soleus muscles, which are composed of oxidative fiber types. We conclude that increasing glycogen synthase activity increases the rate of glycogen synthesis in both oxidative and glycolytic fibers, implying that the control of glycogen accumulation by insulin in skeletal muscle is distributed between the glucose transport and glycogen synthase steps.

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.

Substrate targeting of the yeast cyclin-dependent kinase Pho85p by the cyclin Pcl10p

In Saccharomyces cerevisiae, PHO85 encodes a cyclin-dependent protein kinase (Cdk) catalytic subunit with multiple regulatory roles thought to be specified by association with different cyclin partners (Pcls). Pcl10p is one of four Pcls with little sequence similarity to cyclins involved in cell cycle control. It has been implicated in specifying the phosphorylation of glycogen synthase (Gsy2p). We report that recombinant Pho85p and Pcl10p produced in Escherichia coli reconstitute an active Gsy2p kinase in vitro. Gsy2p phosphorylation required Pcl10p, occurred at physiologically relevant sites, and resulted in inactivation of Gsy2p. The activity of the reconstituted enzyme was even greater than Pho85p-Pcl10p isolated from yeast, and we conclude that, unlike many Cdks, Pho85p does not require phosphorylation for activity. Pcl10p formed complexes with Gsy2p, as judged by (i) gel filtration of recombinant Pcl10p and Gsy2p, (ii) coimmunoprecipitation from yeast cell lysates, and (iii) enzyme kinetic behavior consistent with Pcl10p binding the substrate. Synthetic peptides modeled on the sequences of known Pho85p sites were poor substrates with high K(m) values, and we propose that Pcl10p-Gsy2p interaction is important for substrate selection. Gel filtration of yeast cell lysates demonstrated that most Pho85p was present as a monomer, although a portion coeluted in high-molecular-weight fractions with Pcl10p and Gsy2p. Overexpression of Pcl10p sequestered most of the Pho85p into association with Pcl10p. We suggest a model for Pho85p function in the cell whereby cyclins like Pcl10p recruit Pho85p from a pool of monomers, both activating the kinase and targeting it to substrate.

Novel aspects of the regulation of glycogen storage

The storage polysaccharide glycogen is widely distributed in nature, from bacteria to mammals. Study of its regulated accumulation has resulted in the discovery or elaboration of several important biochemical principles. Many aspects of the control of glycogen storage still remain poorly understood and glycogen metabolism continues to provide interesting models of more general relevance.

Self-glucosylation of glycogenin, the initiator of glycogen biosynthesis, involves an inter-subunit reaction

Glycogenin is a dimeric self-glucosylating protein involved in the initiation phase of glycogen biosynthesis. As an enzyme, glycogenin has the unusual property of transferring glucose residues from UDP-glucose to itself, forming an alpha-1,4-glycan of around 10 residues attached to Tyr194. Whether this self-glucosylation reaction is inter- or intramolecular has been debated. We used site-directed mutagenesis of recombinant rabbit muscle glycogenin-1 to address this question. Mutation of highly conserved Lys85 to Gln generated a glycogenin mutant (K85Q) that had only 1-2% of the self-glucosylating activity of wild-type enzyme. Consistent with previous work, mutation of Tyr194 to Phe in a GST-fusion protein yielded a mutant, Y194F, that was catalytically active but incapable of self-glucosylation. The Y194F mutant was able to glucosylate the K85Q mutant. However, there was an initial lag in the self-glucosylation reaction that was abolished by preincubation of the two mutant proteins. The interaction between glycogenin subunits was relatively weak, with a dissociation constant inferred from kinetic experiments of around 2 microM. We propose a model for the glucosylation of K85Q by Y194F in which mixing of the proteins is followed by rate-limiting formation of a species containing both subunit types. The results provide the most direct evidence to date that the self-glucosylation of glycogenin involves an inter-subunit reaction.

Characterization of human glycogenin-2, a self-glucosylating initiator of liver glycogen metabolism

Glycogenin-2 is a recently described self-glucosylating protein potentially involved in the initiation of glycogen biosynthesis (Mu, J., Skurat, A. V., and Roach, P. J. (1997) J. Biol. Chem. 272, 27589-27597). In human liver extracts, most of the glycogenin-2 was only detectable after treatment with alpha-amylase. Similarly, purifed high Mr glycogen was only detected after release by alpha-amylase treatment. Based on analysis by polymerase chain reaction, the predominant isoform in liver was glycogenin-2beta. Glycogenin-2 was found in Ewing's sarcoma RD-ES cells where, however, it was not associated with high Mr carbohydrate. Both human liver and human RD-ES cell extracts also contained glycogenin-1. Glycogenin-1 and glycogenin-2 interact with one another, based on in vitro interactions and co-immunoprecipitation from liver and cell extracts. Mutation of Tyr-196 in glycogenin-2 to a Phe residue abolished the ability of glycogenin-2 to self-glucosylate but not to interact with glycogenin-1. Stable overexpression of glycogenin-2alpha in Rat-1 fibroblast cells resulted in a 5-fold increase in the level of glycogen present in the low speed pellet but little change in the low speed supernatant. This result is important since it indicates that the level of glycogenin-2 can determine glycogen accumulation and hence has the potential to control glycogen synthesis.