The initiation of glycogen synthesis

A Micro-Scale Analytical Method for Determining Glycogen Turnover by NMR and FTMS

Glycogen is a readily deployed intracellular energy storage macromolecule composed of branched chains of glucose anchored to the protein glycogenin. Although glycogen primarily occurs in the liver and muscle, it is found in most tissues, and its metabolism has been shown to be important in cancers and immune cells. Robust analysis of glycogen turnover requires stable isotope tracing plus a reliable means of quantifying total and labeled glycogen derived from precursors such as 13C6-glucose. Current methods for analyzing glycogen are time- and sample-consuming, at best semi-quantitative, and unable to measure stable isotope enrichment. Here we describe a microscale method for quantifying both intact and acid-hydrolyzed glycogen by ultra-high-resolution Fourier transform mass spectrometric (UHR-FTMS) and/or NMR analysis in stable isotope resolved metabolomics (SIRM) studies. Polar metabolites, including intact glycogen and their 13C positional isotopomer distributions, are first measured in crude biological extracts by high resolution NMR, followed by rapid and efficient acid hydrolysis to glucose under N2 in a focused beam microwave reactor, with subsequent analysis by UHR-FTMS and/or NMR. We optimized the microwave digestion time, temperature, and oxygen purging in terms of recovery versus degradation and found 10 min at 110−115 °C to give >90% recovery. The method was applied to track the fate of 13C6-glucose in primary human lung BEAS-2B cells, human macrophages, murine liver and patient-derived tumor xenograft (PDTX) in vivo, and the fate of 2H7-glucose in ex vivo lung organotypic tissue cultures of a lung cancer patient. We measured the incorporation of 13C6-glucose into glycogen and its metabolic intermediates, UDP-Glucose and glucose-1-phosphate, to demonstrate the utility of the method in tracing glycogen turnover in cells and tissues. The method offers a quantitative, sensitive, and convenient means to analyze glycogen turnover in mg amounts of complex biological materials.

From Prokaryotes to Eukaryotes: Insights Into the Molecular Structure of Glycogen Particles

Glycogen is a highly-branched polysaccharide that is widely distributed across the three life domains. It has versatile functions in physiological activities such as energy reserve, osmotic regulation, blood glucose homeostasis, and pH maintenance. Recent research also confirms that glycogen plays important roles in longevity and cognition. Intrinsically, glycogen function is determined by its structure that has been intensively studied for many years. The recent association of glycogen α-particle fragility with diabetic conditions further strengthens the importance of glycogen structure in its function. By using improved glycogen extraction procedures and a series of advanced analytical techniques, the fine molecular structure of glycogen particles in human beings and several model organisms such as Escherichia coli, Caenorhabditis elegans, Mus musculus, and Rat rattus have been characterized. However, there are still many unknowns about the assembly mechanisms of glycogen particles, the dynamic changes of glycogen structures, and the composition of glycogen associated proteins (glycogen proteome). In this review, we explored the recent progresses in glycogen studies with a focus on the structure of glycogen particles, which may not only provide insights into glycogen functions, but also facilitate the discovery of novel drug targets for the treatment of diabetes mellitus.

A Review of Starch Biosynthesis in Relation to the Building Block-Backbone Model

Starch is a water-insoluble polymer of glucose synthesized as discrete granules inside the stroma of plastids in plant cells. Starch reserves provide a source of carbohydrate for immediate growth and development, and act as long term carbon stores in endosperms and seed tissues for growth of the next generation, making starch of huge agricultural importance. The starch granule has a highly complex hierarchical structure arising from the combined actions of a large array of enzymes as well as physicochemical self-assembly mechanisms. Understanding the precise nature of granule architecture, and how both biological and abiotic factors determine this structure is of both fundamental and practical importance. This review outlines current knowledge of granule architecture and the starch biosynthesis pathway in relation to the building block-backbone model of starch structure. We highlight the gaps in our knowledge in relation to our understanding of the structure and synthesis of starch, and argue that the building block-backbone model takes accurate account of both structural and biochemical data.

From the seminal discovery of proteoglycogen and glycogenin to emerging knowledge and research on glycogen biology

Although the discovery of glycogen in the liver, attributed to Claude Bernard, happened more than 160 years ago, the mechanism involved in the initiation of glucose polymerization remained unknown. The discovery of glycogenin at the core of glycogen's structure and the initiation of its glucopolymerization is among one of the most exciting and relatively recent findings in Biochemistry. This review focuses on the initial steps leading to the seminal discoveries of proteoglycogen and glycogenin at the beginning of the 1980s, which paved the way for subsequent foundational breakthroughs that propelled forward this new research field. We also explore the current, as well as potential, impact this research field is having on human health and disease from the perspective of glycogen storage diseases. Important new questions arising from recent studies, their links to basic mechanisms involved in the de novo glycogen biogenesis, and the pervading presence of glycogenin across the evolutionary scale, fueled by high throughput -omics technologies, are also addressed.

Lafora disease - from pathogenesis to treatment strategies

Lafora disease is a severe, autosomal recessive, progressive myoclonus epilepsy. The disease usually manifests in previously healthy adolescents, and death commonly occurs within 10 years of symptom onset. Lafora disease is caused by loss-of-function mutations in EPM2A or NHLRC1, which encode laforin and malin, respectively. The absence of either protein results in poorly branched, hyperphosphorylated glycogen, which precipitates, aggregates and accumulates into Lafora bodies. Evidence from Lafora disease genetic mouse models indicates that these intracellular inclusions are a principal driver of neurodegeneration and neurological disease. The integration of current knowledge on the function of laforin-malin as an interacting complex suggests that laforin recruits malin to parts of glycogen molecules where overly long glucose chains are forming, so as to counteract further chain extension. In the absence of either laforin or malin function, long glucose chains in specific glycogen molecules extrude water, form double helices and drive precipitation of those molecules, which over time accumulate into Lafora bodies. In this article, we review the genetic, clinical, pathological and molecular aspects of Lafora disease. We also discuss traditional antiseizure treatments for this condition, as well as exciting therapeutic advances based on the downregulation of brain glycogen synthesis and disease gene replacement.

Identification and Functional Characterization of the Glycogen Synthesis Related Gene Glycogenin in Pacific Oysters (Crassostrea gigas)

High glycogen levels in the Pacific oyster (Crassostrea gigas) contribute to its flavor, quality, and hardiness. Glycogenin (CgGN) is the priming glucosyltransferase that initiates glycogen biosynthesis. We characterized the full sequence and function of C. gigas CgGN. Three CgGN isoforms (CgGN-α, β, and γ) containing alternative exon regions were isolated. CgGN expression varied seasonally in the adductor muscle and gonadal area and was the highest in the adductor muscle. Autoglycosylation of CgGN can interact with glycogen synthase (CgGS) to complete glycogen synthesis. Subcellular localization analysis showed that CgGN isoforms and CgGS were located in the cytoplasm. Additionally, a site-directed mutagenesis experiment revealed that the Tyr200Phe and Tyr202Phe mutations could affect CgGN autoglycosylation. This is the first study of glycogenin function in marine bivalves. These findings will improve our understanding of glycogen synthesis and accumulation mechanisms in mollusks. The data are potentially useful for breeding high-glycogen oysters.

Lack of Glycogenin Causes Glycogen Accumulation and Muscle Function Impairment

Glycogenin is considered essential for glycogen synthesis, as it acts as a primer for the initiation of the polysaccharide chain. Against expectations, glycogenin-deficient mice (Gyg KO) accumulate high amounts of glycogen in striated muscle. Furthermore, this glycogen contains no covalently bound protein, thereby demonstrating that a protein primer is not strictly necessary for the synthesis of the polysaccharide in vivo. Strikingly, in spite of the higher glycogen content, Gyg KO mice showed lower resting energy expenditure and less resistance than control animals when subjected to endurance exercise. These observations can be attributed to a switch of oxidative myofibers toward glycolytic metabolism. Mice overexpressing glycogen synthase in the muscle showed similar alterations, thus indicating that this switch is caused by the excess of glycogen. These results may explain the muscular defects of GSD XV patients, who lack glycogenin-1 and show high glycogen accumulation in muscle.

Glycogen metabolism has a key role in the cancer microenvironment and provides new targets for cancer therapy

Metabolic reprogramming is a hallmark of cancer cells and contributes to their adaption within the tumour microenvironment and resistance to anticancer therapies. Recently, glycogen metabolism has become a recognised feature of cancer cells since it is upregulated in many tumour types, suggesting that it is an important aspect of cancer cell pathophysiology. Here, we provide an overview of glycogen metabolism and its regulation, with a focus on its role in metabolic reprogramming of cancer cells under stress conditions such as hypoxia, glucose deprivation and anticancer treatment. The various methods to detect glycogen in tumours in vivo as well as pharmacological modulators of glycogen metabolism are also reviewed. Finally, we discuss the therapeutic value of targeting glycogen metabolism as a strategy for combinational approaches in cancer treatment.

Getting a handle on glycogen synthase - Its interaction with glycogenin

Glycogen is a polymer of glucose that serves as a major energy reserve in eukaryotes. It is synthesized through the cooperative action of glycogen synthase (GS), glycogenin (GN) and glycogen branching enzyme. GN initiates the first enzymatic step in the glycogen synthesis process by self glucosylation of a short 8-12 glucose residue primer. After interacting with GN, GS then extends this sugar primer to form glycogen particles of different sizes. We discuss recent developments in the structural biology characterization of GS and GN enzymes, which have contributed to a better understanding of how the two proteins interact and how they collaborate to synthesize glycogen particles.

Why the linkage of glycogen to glycogenin was so difficult to determine

Glycogenin is the self-glucosylating enzyme that primes mammalian and yeast glycogen synthesis. It proved to be the long-suspected, covalently bound protein component of glycogen. One of the most difficult aspects in elucidating the role of glycogenin was to learn the nature of its covalent bond to glycogen.

Identification and cloning of a submergence-induced gene OsGGT (glycogenin glucosyltransferase) from rice (Oryza sativa L.) by suppression subtractive hybridization

A submergence-induced gene, OsGGT, was cloned from 7-day submerged rice (Oryza sativa L. plants, FR13A (a submergence-tolerant cultivar, Indica), using suppression subtractive hybridization and both 5'- and 3'-rapid amplification of cDNA ends (RACE). The full-length OsGGT cDNA contains 1,273 bp with an open reading frame of 1,140 bp (17-1,156) that encodes 379 amino acids. Its deduced amino acid sequence is homologous with glycogenin glucosyltransferase. We found that the OsGGT gene is located in the 17,970-20,077 bp region of genome fragment AAAA01002475.1 of the Indica cultivar and in the 53,293-51,186 bp region of genome fragment AC037426.12 of chromosome 10 of the Japanica cultivar. A time-course study showed that OsGGT-gene expression increased in FR13A during submergence but decreased in IR42 (submergence-intolerant cultivar, Indica). The expression of the OsGGT gene in FR13A was induced by salicylic acid and benzyladenine. The accumulation of OsGGT mRNA in FR13A also increased in response to ethylene, gibberellin, abscisic acid, drought and salt treatment, but methyl jasmonate treatment and cold stress had no effect on expression. These results suggest that the OsGGT gene could be related to submergence stress and associated with a general defensive response to various environmental stresses.

Reserve carbohydrates metabolism in the yeast Saccharomyces cerevisiae

Glycogen and trehalose are the two glucose stores of yeast cells. The large variations in the cell content of these two compounds in response to different environmental changes indicate that their metabolism is controlled by complex regulatory systems. In this review we present information on the regulation of the activity of the enzymes implicated in the pathways of synthesis and degradation of glycogen and trehalose as well as on the transcriptional control of the genes encoding them. cAMP and the protein kinases Snf1 and Pho85 appear as major actors in this regulation. From a metabolic point of view, glucose-6-phosphate seems the major effector in the net synthesis of glycogen and trehalose. We discuss also the implication of the recently elucidated TOR-dependent nutrient signalling pathway in the control of the yeast glucose stores and its integration in growth and cell division. The unexpected roles of glycogen and trehalose found in the control of glycolytic flux, stress responses and energy stores for the budding process, demonstrate that their presence confers survival and reproductive advantages to the cell. The findings discussed provide for the first time a teleonomic value for the presence of two different glucose stores in the yeast cell.

Characterization of the human glycogenin-1 gene: identification of a muscle-specific regulatory domain

The de-novo synthesis of glycogen is now known to involve a novel class of self-glucosylating protein primers. In mammalian skeletal muscle, glycogenin-1 is the protein responsible for this initiation step. Northern blot analysis revealed that glycogenin-1 gene transcription is differentially regulated in the C2C12 mouse muscle cell line. To define the regulatory elements that control expression of the glycogenin-1 gene, we have cloned and characterized the genomic structure of the human glycogenin-1 gene and its promoter region. This gene consists of seven exons and six introns, and spans over 13kb. Transcription of human glycogenin-1 is initiated at two major sites, 80 and 86bp upstream from the initiation of translation codon. Nucleotide sequence analysis of 2.1kb of the 5'-flanking region revealed the proximal promoter contains both a TATA box and two putative Sp1 binding sites located in a CpG island. There are numerous binding sites for developmental and cell-type-specific transcription factors, including AP-1, AP-2, GATA, and several potential Oct 1 binding domains. There are also nine consensus E-boxes that bind the basic helix-loop-helix family of muscle-specific transcription factors. The transcriptional activity of the glycogenin-1 gene was investigated by transient transfection of the 5'-flanking region in HepG2 cells and C2C12 myoblasts and myotubes. These results permitted the definition of a minimal 232bp promoter fragment that is responsible for basal level transcription in a cell-type-independent manner. Furthermore, we have identified a regulatory region located between -2076 and -1736 of the 5'-flanking region of the human glycogenin-1 gene that allows myotube-specific expression in C2C12 cells.

Assessment of glycogen turnover in aerobic, ischemic, and reperfused working rat hearts

Glycogen and its turnover are important components of myocardial glucose metabolism that significantly impact on postischemic recovery. We developed a method to measure glycogen turnover (rates of glycogen synthesis and degradation) in isolated working rat hearts using [3H]- and [14C]glucose. In aerobic hearts perfused with 11 mM glucose, 1.2 mM palmitate, and 100 microU/ml insulin, rates of glycogen synthesis and degradation were 1.24 +/- 0.3 and 0.53 +/- 0. 25 micromol. min-1. g dry wt-1, respectively. Low-flow ischemia (0.5 ml/min, 60 min) elicited a marked glycogenolysis; rates of glycogen synthesis and degradation were 0.54 +/- 0.16 and 2.12 +/- 0.14 micromol. min-1. g dry wt-1, respectively. During reperfusion (30 min), mechanical function recovered to 20% of preischemic values. Rates of synthesis and degradation were 1.66 +/- 0.16 and 1.55 +/- 0. 21 micromol. min-1. g dry wt-1, respectively, and glycogen content remained unchanged (25 +/- 3 micromol/g dry wt). The assessment of glycogen metabolism needs to take into account the simultaneous synthesis and degradation of glycogen. With this approach, a substantial turnover of glycogen was detectable not only during aerobic conditions but also during ischemia as well as reperfusion.

Glycosomes--the organelles of glycogen metabolism

This article reviews the data concerning the electron microscopical interpretation of glycogen. It demonstrates that glycogen in the cell is associated with the enzymes involved in its metabolism and that the glycogen-protein complex forms morphologically distinct cell organelles called glycosomes. Glycogen can be visualized in the electron microscope (EM) by histochemical procedures, or by negative staining, but it does not react with heavy metals such as uranium and lead. The protein component of glycosomes, stainable by heavy metals, appears in EM as 20-30 nm granules. While biochemical findings have long indicated the association of glycogen and protein in the cell, morphological interpretation traditionally defined the protein component of glycosomes as particles of glycogen. Accordingly, the term alpha or beta particles, introduced to define particles of glycogen, became subsequently applied to the protein component visible in sections stained by heavy metals. The history of microscopic research reveals the conditions which led to such interpretation. Morphological analysis of the reaction of glycosomes to the acids shows that glycosomes deposited free in the cytosol (lyoglycosomes) are acid labile, whereas the others (desmoglycosomes), intimately associated with different cellular structures, are acid-resistant. These 2 groups correspond to lyo- and desmoglycogen distinguished in early biochemical studies on the basis of their different resistance to the cold trichloroacetic acid. The theory of glycosomes provides a new paradigm which clarifies numerous unexplained data in the microscopic literature on glycogen, and opens a vast field for the research on the cellular metabolism of glycogen, with the use of modern molecular and cellular biology techniques.

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.

Inhibition of glycogenin-catalyzed glucosyl and xylosyl transfer by cytidine 5'-diphosphate and related compounds

The self-glucosylation of beef kidney glycogenin was inhibited by the following pyrimidine nucleotides and nucleotide sugars, listed in order of decreasing effectiveness: CDP-glucose, CDP, UDP-xylose, UDP-N-acetylglucosamine, UDP-galactose, UDP, CTP, CDP-choline, UDP-glucuronic acid, beta-S-UDP-glucose, and CMP. In contrast, the purine nucleotide sugars, ADP-glucose and GDP-glucose, were essentially ineffective, as was the pyrimidine nucleoside, cytidine. UDP-Xylose may be utilized by glycogenin as an alternative sugar donor instead of UDP-glucose (Rodén, L., Ananth, S., Campbell, P., Manzella, S., and Meezan, E. (1994) J. Biol. Chem. 269, 11509-11513) and therefore presumably inhibited the glucosyl transfer reaction by being a competitive substrate. Like glucosyl transfer, xylosyl incorporation into glycogenin was also inhibited effectively by CDP. On the other hand, UDP-xylose:proteoglycan core protein xylosyltransferase (EC 2.4.2.26) was not affected by CDP, nor was it inhibited by UDP-glucose. Addition of CDP or UDP-glucose to reaction mixtures containing both enzymes therefore made it possible to assay xylosyltransferase EC 2.4.2.26 reliably without the extensive product characterization that is otherwise necessary. The CDP effect on glycogenin further allowed the development of an improved procedure for the purification of this enzyme, in which specific elution of an affinity matrix (UDP-glucuronic acid-agarose) was carried out with CDP as the eluant.

Catalytic activities of glycogenin additional to autocatalytic self-glucosylation

Glycogenin is the autocatalytic, self-glucosylating protein that initiates glycogen synthesis in muscle and other tissues. We have sequenced the cDNA for rabbit muscle glycogenin and expressed and purified the protein in high yield as well as two mutant proteins in which Phe or Thr replaces Tyr-194, the site of glucosylation. While the wild-type protein can self-glucosylate, the mutants cannot, but all three utilize alternative acceptors by intermolecular glucose transfer for which the mutants have altered specificity. Tyr-194 is therefore not essential for the catalytic activity of glycogenin. All three proteins also hydrolyze UDP-glucose to glucose at rates comparable with the rate of self-glucosylation. The hydrolysis is competitive with glucose transfer to p-nitrophenyl alpha-maltoside. Self-glucosylation, glucosylation of other acceptors, and hydrolysis all appear to be catalyzed by the same active center. In the absence of peptidase inhibitors, the homogenous recombinant proteins of M(r) 37,000 break down to equally active species having M(r) 32,000. The kinetics of self-glucosylation catalyzed by the wild-type enzyme suggest that the reaction could be intermolecular rather than, as previously reported, intramolecular. The wild-type recombinant enzyme and native muscle glycogenin, which is phosphorylated, are inhibited quite differently by ATP at physiological concentration.

New and specific nucleoside diphosphate glucose substrates for glycogenin

Glycogenin, the autocatalytic, self-glucosylating primer for glycogen synthesis by glycogen synthase, is presumed, in vivo, to use UDP-glucose as the source of the glucose residues it adds to itself. When we tested its ability to utilize other nucleoside diphosphate glucoses, it emerged that purine nucleotides are not utilized but two pyrimidine nucleotides are used, in addition to UDP-glucose. These are CDP-glucose and TDP-glucose. CDP-glucose is utilized at 70% of the rate of UDP-glucose. While there is no evidence that CDP-glucose is a natural substrate for glycogenin, it has the advantage over UDP-glucose in that it can be used specifically to detect and assay glycogenin in the presence of glycogen synthase because CDP-glucose, unlike UDP-glucose, is not a substrate for the synthase.

Properties of carbohydrate-free recombinant glycogenin expressed in an Escherichia coli mutant lacking UDP-glucose pyrophosphorylase activity

Glycogenin, the self-glucosylating primer for glycogen synthesis, is expressed in wild-type E. coli as a recombinant protein in an already partly glucosylated form, owing to the presence of its substrate, UDP-glucose. By using an E. coli mutant strain lacking in UDP-glucose pyrophosphorylase activity, we have succeeded in expressing carbohydrate-free glycogenin (apo-glycogenin) in good yield. When provided with UDPxylose, it autocatalytically adds 1 xylose residue. With UDP-glucose, an average of 8 glucose residues are added. However, release of the self-synthesized maltosaccharide chains with isoamylase reveals them to be a mixture. Chains as long as 11 glucose residues (maltoundecaose) are present. The ability of recombinant apo-glycogenin to self-glucosylate is further proof that a separate enzyme is not needed for the addition of the first glucose residue to Tyr-194 of the protein.

Pre-eclampsia is associated with an increase in trophoblast glycogen content and glycogen synthase activity, similar to that found in hydatidiform moles

Pre-eclampsia is a placental disorder, but until now, biochemical details of dysfunction have been lacking. During an analysis of the oligosaccharide content of syncytiotrophoblast microvesicles purified from the placental chorionic villi of 10 primigravid women with proteinuric pre-eclampsia, we found an excess of glycogen breakdown products. Further investigation revealed a 10-fold increase in glycogen content (223 +/- 117 micrograms glycogen/mg protein), when compared with controls matched for gestational age at delivery (23 +/- 18 micrograms glycogen/mg protein) (P < 0.01). This was confirmed by examination of electron micrographs of chorionic villous tissue stained for glycogen. The increase in glycogen content was associated with 16 times more glycogen synthase (1,323 +/- 1,013 relative to 83 +/- 96 pmol glucose/mg protein per min) (P < 0.001), and a threefold increase in glycogen phosphorylase activity (2,280 +/- 1,360 relative to 700 +/- 540 pmol glucose/mg protein per min; P < 0.05). Similar changes in glycogen metabolism were found in trophoblast microvesicles derived from hydatidiform moles. Glycogen accumulation in villous syncytiotrophoblast may be a metabolic marker of immaturity of this cell which is unable to divide. The implications of these findings with regard to the pathogenesis of pre-eclampsia are discussed.

The discovery of glycogenin and the priming mechanism for glycogen biogenesis

The biogenesis of glycogen in skeletal muscle requires a priming mechanism that has recently been elucidated. The first step is catalysed by a protein tyrosine glucosyltransferase and involves the formation of a novel glycosidic linkage, namely the covalent attachment of glucose to a single tyrosine residue (Tyr194) on a priming protein, termed glycogenin. The next stage is the extension of the glucan chain from Tyr194 and involves the sequential addition of up to seven further glucosyl residues. This reaction is brought about autocatalytically by glycogenin itself, which is a Mn2+/Mg(2+)-dependent UDP-Glc-requiring glucosyltransferase. The glucan primer is elongated by glycogen synthase, but only when glycogenin and glycogen synthase are complexed together. Glycogen synthase dissociates from glycogenin during the synthesis of a glycogen molecule, enabling glycogen molecules to reach their maximum theoretical size. Each mature glycogen beta particle in muscle contains one molecule of glycogenin attached covalently, and an average one glycogen synthase catalytic subunit bound non-covalently. As evidence accumulates that a priming protein may be a fundamental property of polysaccharide synthesis in general, the molecular details of mammalian glycogen biogenesis may serve as a useful model for other systems.

Parafusin, an exocytic-sensitive phosphoprotein, is the primary acceptor for the glucosylphosphotransferase in Paramecium tetraurelia and rat liver

Parafusin, the major protein in Paramecium tetraurelia to undergo dephosphorylation in response to secretory stimuli, appears to be the primary acceptor for the glucosylphosphotransferase in this species based on five independent criteria: identical molecular size of 63 kD; identical isoelectric points in the phosphorylated state of pH 5.8 and 6.2; identical behavior in reverse-phase chromatography; immunological cross-reactivity with an affinity-purified anti-parafusin antibody; the presence of a phosphorylated sugar after acid hydrolysis. It appears likely that the dephosphorylation observed with secretion reflects the removal of alpha Glc-1-P from parafusin's oligosaccharides and is consistent, therefore, with a regulatory role for this cytoplasmic glycosylation event. The glucosylphosphotransferase acceptor in rat liver is also immunoprecipitated by the anti-parafusin antibody and is very similar in physical characteristics to the paramecium protein. This conservation suggests a role for parafusin in mammalian exocytosis as well, at a step common to both the regulated and constitutive secretory pathways.

Glycogenesis in the amphibian retina: in vitro conversion of [2-3H]mannose to [3H]glucose and subsequent incorporation into glycogen

We previously demonstrated by light and electron microscopic autoradiography that Xenopus retinas incubated with [3H]mannose exhibit tunicamycin-insensitive radiolabeling of glycogen storage compartments, especially cone parabaloids. In the present study, we utilized biochemical methods to evaluate the identity of the material presumed to be [3H]glycogen in Xenopus retinas obtained from eyecups incubated under similar conditions. A crude glycogen-containing fraction was isolated, solubilized with 8 M urea, and purified by Sepharose CL-4B column chromatography. The retinal glycogen was hydrolyzed either chemically or with specific amylolytic enzymes, followed by Sephacryl S-200 column chromatography and HPLC of the hydrolysis products. Under the conditions employed, [3H]glycogen represented at least 10% of the total radiolabeled macromolecules. Hydrolysis of the [3H]glycogen released all of the radiolabel in the form of [3H]glucose, not [3H]mannose, which indicated that direct incorporation of [3H]mannose into glycogen had not occurred. [3H]Glucose was distributed throughout the glycogen molecule, not just in the outer tiers, which indicated that de novo glycogenesis had occurred. Furthermore, enzymatic isomerization of the glycogen-derived [3H]glucose with glucose isomerase yielded fructose with retention of tritium. This demonstrated that positions other than the C-2 carbon of glucose were radiolabeled. Analysis of the medium after several hours of incubation revealed the presence of 3H2O as the major radiolabeled compound. These results support the conclusion that the in vitro incorporation of [2-3H]mannose into retinal glycogen involves initial catabolism of the radiolabeled substrate and subsequent reincorporation of the label via gluconeogenesis into precursors utilized for de novo glycogenesis.

D-[3H]galactose incorporation into glycogen in retinal cone cells

Previous studies have shown that bovine retinas incubated with [3H]galactose incorporated it, unmodified, into large molecules. Light and electron microscope autoradiography showed a significant proportion of the label to be in cone inner segments, and pulse-chase studies showed it was subsequently transported to the synaptic pedicles. In this report, evidence is presented to show that the galactose-labelled macromolecules are resistant to hydrolysis by proteolytic enzymes, testicular hyaluronidase, chondroitinase ABC, beta-glucosidase and beta-glucuronidase, but are readily degraded by alpha-amylase and beta-galactosidase, and to a lesser extent by beta-amylase. Treatment with alpha-amylase also leads to specific removal of radioactivity from cone inner segments and pedicles, as judged by light-microscopic autoradiography. These studies appear to indicate that the cone-specific galactose label is in glycogen or glycogen-like molecules.

Glycogen synthase turnover and phosphorylation in rat H4IIE hepatoma cells

Glycogen synthase was isolated from rat H4IIE hepatoma cells by the use of specific antibodies. Immunoprecipitates from cells grown in the presence of [35S]methionine contained two 35S-labeled polypeptides, designated GS1 and GS2, separable by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Labeling of both species was half-maximal after 3 h and remained constant up to 48 h. When cells were incubated with [32P]-phosphate, 32P was incorporated into both species with similar kinetics, half-maximal labeling occurring after 2-3 h. The steady-state ratio 32P/35S was significantly higher for the lower mobility GS2 polypeptide. Pulse-chase experiments showed that the two subunits followed similar kinetics with respect to 35S-labeling. However, the turnover of 32P on the GS2 subunit was significantly faster (t1/2 approximately 30 min) than that on the GS1 subunit (t1/2 approximately 2 h). We suggest that the two polypeptides represent different phosphorylation states of the glycogen synthase subunit and are rapidly interconverted.

Structural and functional studies on rabbit liver glycogenin

Glycogenin, the protein primer required for the biogenesis of muscle glycogen, has been isolated from rabbit liver glycogen. The protein comprised 0.0025% of liver glycogen by mass, 200-fold lower than the glycogenin content of muscle glycogen. Structural analyses, including determination of the amino acid sequence surrounding the glucosylated-tyrosine residue, showed identity with muscle glycogenin. Catalytically active liver glycogenin was partially purified and, like the skeletal muscle protein, catalysed an intramolecular, Mn2+- and UDP-Glc-dependent autoglucosylation reaction, forming a primer on which glycogen synthase could act. The results demonstrate that hepatic and muscle glycogenins are almost certainly identical proteins and that liver and skeletal muscle share a common mechanism for the biogenesis of glycogen molecules. The results also indicate that there is about one glycogenin molecule/liver glycogen alpha particle.

Characterization of the proteoglycogen fraction non-extractable from retina by trichloroacetic acid

The trichloroacetic acid-insoluble 1,4-alpha-glucan fraction from bovine retina was purified and characterized. It is a proteoglycogen fraction containing a 42 kDa protein moiety similar in size to the protein moiety of the trichloroacetic acid-soluble proteoglycogen fraction. The apparent weight-average Mr of acid-insoluble and acid-soluble proteoglycogens are 4.7 x 10(5) and 7.0 x 10(5) respectively. The present results support suggestions from earlier studies indicating that acid-insoluble proteoglycogen is the precursor of the acid-soluble form.

A 42,000-Da protein in rabbit tissues and in a glycogen synthase preparation cross-reacts with antibodies to glycogenin

Rabbit skeletal muscle glycogen previously has been shown to be covalently bound to a 40,000-Da protein ("glycogenin") via a novel glucosyl-tyrosine linkage [I.R. Rodriguez and W.J. Whelan (1985) Biochem. Biophys. Res. Commun. 132, 829-836]. Antibodies raised against rabbit skeletal muscle glycogenin cross-react with a similar protein present in rabbit heart and liver glycogens, as well as with a 42,000-Da "acceptor protein" present in high-speed supernatants of rabbit muscle, heart, retina, and liver. This 42,000-Da protein incorporates [U-14C]Glc when an ammonium sulfate fraction prepared from the tissue supernatants is incubated with UDP-[U-14C]Glc. The [U-14C]Glc incorporated can be removed quantitatively by treatment with amylolytic enzymes, indicating that the [U-14C]Glc incorporation represents elongation of a preexisting glucan attached to the acceptor protein. Furthermore, a commercial preparation of rabbit skeletal muscle glycogen synthase contains this 42,000-Da protein. We propose that the 42,000-Da protein represents the free form of glycogenin in tissues, with its covalently attached glucan chain(s) providing a "primed" elongation site for glycogen synthesis.

Identification of the 38-kDa subunit of rabbit skeletal muscle glycogen synthase as glycogenin

Glycogen synthase from rabbit skeletal muscle has been shown to be a complex of two types of subunit which have apparent molecular masses of 86 kDa and 38 kDa and are present in a 1:1 molar ratio. The 38-kDa component was separated from the 86-kDa catalytic subunit by gel filtration in the presence of 2 M LiBr, and a number of chymotryptic peptides were sequenced. This demonstrated that the 38-kDa subunit was glycogenin, the protein that is bound covalently to glycogen and believed to be the 'primer' involved in the initiation of de novo glycogen synthesis.