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

A fungal protein organizes both glycogen and cell wall glucans

Glycogen is a glucose storage molecule composed of branched α-1,4-glucan chains, best known as an energy reserve that can be broken down to fuel central metabolism. Because fungal cells have a specialized need for glucose in building cell wall glucans, we investigated whether glycogen is used for this process. For these studies, we focused on the pathogenic yeast Cryptococcus neoformans, which causes ~150,000 deaths per year worldwide. We identified two proteins that influence formation of both glycogen and the cell wall: glycogenin (Glg1), which initiates glycogen synthesis, and a protein that we call Glucan organizing enzyme 1 (Goe1). We found that cells missing Glg1 lack α-1,4-glucan in their walls, indicating that this material is derived from glycogen. Without Goe1, glycogen rosettes are mislocalized and β-1,3-glucan in the cell wall is reduced. Altogether, our results provide mechanisms for a close association between glycogen and cell wall.

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.

Brain Glycogen Structure and Its Associated Proteins: Past, Present and Future

This chapter reviews the history of glycogen-related research and discusses in detail the structure, regulation, chemical properties and subcellular distribution of glycogen and its associated proteins, with particular focus on these aspects in brain tissue.

The dynamic life of the glycogen granule

Glycogen, the primary storage form of glucose, is a rapid and accessible form of energy that can be supplied to tissues on demand. Each glycogen granule, or "glycosome," is considered an independent metabolic unit composed of a highly branched polysaccharide and various proteins involved in its metabolism. In this Minireview, we review the literature to follow the dynamic life of a glycogen granule in a multicompartmentalized system, i.e. the cell, and how and where glycogen granules appear and the factors governing its degradation. A better understanding of the importance of cellular compartmentalization as a regulator of glycogen metabolism is needed to unravel its role in brain energetics.

Regulation of glycogen synthase from mammalian skeletal muscle--a unifying view of allosteric and covalent regulation

It is widely accepted that insufficient insulin-stimulated activation of muscle glycogen synthesis is one of the major components of non-insulin-dependent (type 2) diabetes mellitus. Glycogen synthase, a key enzyme in muscle glycogen synthesis, is extensively regulated, both allosterically (by glucose-6-phosphate, ATP, and others) and covalently (by phosphorylation). Although glycogen synthase has been a topic of intense study for more than 50 years, its kinetic characterization has been confounded by its large number of phosphorylation states. Questions remain regarding the function of glycogen synthase regulation and the relative importance of allosteric and covalent modification in fulfilling this function. In this review, we consider both earlier kinetic studies and more recent site-directed mutagenesis and crystal structure studies in a detailed qualitative discussion of the effects of regulation on the kinetics of glycogen synthase. We propose that both allosteric and covalent modification of glycogen synthase may be described by a Monod-Wyman-Changeux model in terms of apparent changes to L, the equilibrium constant for transition between the T and R conformers. As, with the exception of L, all parameters of this model are independent of the glycogen synthase phosphorylation state, the need to determine kinetic parameters for all possible states is eliminated; only the relationship between a particular state and L must be established. We conclude by suggesting that renewed efforts to characterize the relationship between phosphorylation and the kinetics of glycogen synthase are essential in order to obtain a better quantitative understanding of the function of glycogen synthesis regulation. The model we propose may prove useful in this regard.

Sulfatase inhibitors: a patent review

INTRODUCTION

Steroid sulfatase (STS) converts sulfated hormones to free hormones of importance in hormone-dependent diseases such as breast cancer and endometriosis. Carbohydrate sulfatases degrade complex carbohydrates as part of normal cellular turnover; certain lysosomal storage disorders (LSDs) involve defective processing of sulfated glycosaminoglycans by mutant sulfatases.

AREAS COVERED

Aryl sulfamates have been developed as STS inhibitors, and STX64 and PGL2001 are under evaluation in Phase I and II clinical trials for treatment of endometrial and metastatic breast and prostate cancers and endometriosis. Dual-acting compounds have emerged that are aromatase inhibitors (AIs), selective estrogen receptor antagonists, or inhibitors of microtubule polymerization. Sulfamidase inhibitors as pharmacological chaperones to assist maturation of folding-defective mutants for the treatment of Sanfilippo type A disease are under investigation. Coverage: The patent literature after the mid-1990s.

EXPERT OPINION

The failure of STX64 in a Phase II monotherapy clinical trial should not dissuade further investigations in multidrug regimens, particularly in combination with AIs. The recent development of dual-acting compounds may enhance the potential for success in the clinic. Further investigations into aryl sulfamates are required to clarify the molecular mechanism of action; additionally, new reversible sulfatase inhibition concepts are needed for the development of pharmacological chaperones for sulfatase LSDs.

Molecular pathogenesis of a new glycogenosis caused by a glycogenin-1 mutation

Glycogenin-1 initiates the glycogen synthesis in skeletal muscle by the autocatalytic formation of a short oligosaccharide at tyrosine 195. Glycogenin-1 catalyzes both the glucose-O-tyrosine linkage and the α1,4 glucosidic bonds linking the glucose molecules in the oligosaccharide. We recently described a patient with glycogen depletion in skeletal muscle as a result of a non-functional glycogenin-1. The patient carried a Thr83Met substitution in glycogenin-1. In this study we have investigated the importance of threonine 83 for the catalytic activity of glycogenin-1. Non-glucosylated glycogenin-1 constructs, with various amino acid substitutions in position 83 and 195, were expressed in a cell-free expression system and autoglucosylated in vitro. The autoglucosylation was analyzed by gel-shift on western blot, incorporation of radiolabeled UDP-(14)C-glucose and nano-liquid chromatography with tandem mass spectrometry (LC/MS/MS). We demonstrate that glycogenin-1 with the Thr83Met substitution is unable to form the glucose-O-tyrosine linkage at tyrosine 195 unless co-expressed with the catalytically active Tyr195Phe glycogenin-1. Our results explain the glycogen depletion in the patient expressing only Thr83Met glycogenin-1 and why heterozygous carriers without clinical symptoms show a small proportion of unglucosylated glycogenin-1.

Mechanisms of monomeric and dimeric glycogenin autoglucosylation

Initiation of glucose polymerization by glycogenin autoglucosylation at Tyr-194 is required to prime de novo biosynthesis of glycogen. It has been proposed that the synthesis of the primer proceeds by intersubunit glucosylation of dimeric glycogenin, even though it has not been demonstrated that this mechanism is responsible for the described polymerization extent of 12 glucoses produced by the dimer. We reported previously the intramonomer glucosylation capability of glycogenin without determining the extent of autoglucopolymerization. Here, we show that the maximum specific autoglucosylation extent (MSAE) produced by the non-glucosylated glycogenin monomer is 13.3 ± 1.9 glucose units, similar to the 12.5 ± 1.4 glucose units measured for the dimer. The mechanism and capacity of the dimeric enzyme to carry out full glucopolymerization were also evaluated by construction of heterodimers able to glucosylate exclusively by intrasubunit or intersubunit reaction mechanisms. The MSAE of non-glucosylated glycogenin produced by dimer intrasubunit glucosylation was 16% of that produced by the monomer. However, partially glucosylated glycogenin was able to almost complete its autoglucosylation by the dimer intrasubunit mechanism. The MSAE produced by heterodimer intersubunit glucosylation was 60% of that produced by the wild-type dimer. We conclude that both intrasubunit and intersubunit reaction mechanisms are necessary for the dimeric enzyme to acquire maximum autoglucosylation. The full glucopolymerization capacity of monomeric glycogenin indicates that the enzyme is able to synthesize the glycogen primer without the need for prior dimerization.

Functional states of homooligomers: insights from the evolution of glycosyltransferases

Glycosylation is an important aspect of epigenetic regulation. Glycosyltransferase is a key enzyme in the biosynthesis of glycans, which glycosylates more than half of all proteins in eukaryotes and is involved in a wide range of biological processes. It has been suggested previously that homooligomerization in glycosyltransferases and other proteins might be crucial for their function. In this study, we explore functional homooligomeric states of glycosyltransferases in various organisms, trace their evolution, and perform comparative analyses to find structural features that can mediate or disrupt the formation of different homooligomers. First, we make a structure-based classification of the diverse superfamily of glycosyltransferases and confirm that the majority of the structures are indeed clustered into the GT-A or GT-B folds. We find that homooligomeric glycosyltransferases appear to be as ancient as monomeric glycosyltransferases and go back in evolution to the last universal common ancestor (LUCA). Moreover, we show that interface residues have significant bias to be gapped out or unaligned in the monomers, implying that they might represent features crucial for oligomer formation. Structural analysis of these features reveals that the majority of them represent loops, terminal regions, and helices, indicating that these secondary-structure elements mediate the formation of glycosyltransferases' homooligomers and directly contribute to the specific binding. We also observe relatively short protein regions that disrupt the homodimer interactions, although such cases are rare. These results suggest that relatively small structural changes in the nonconserved regions may contribute to the formation of different functional oligomeric states and might be important in regulation of enzyme activity through homooligomerization.

Glycogenin-1 deficiency and inactivated priming of glycogen synthesis

Glycogen, which serves as a major energy reserve in cells, is a large, branched polymer of glucose molecules. We describe a patient who had muscle weakness, associated with the depletion of glycogen in skeletal muscle, and cardiac arrhythmia, associated with the accumulation of abnormal storage material in the heart. The skeletal muscle showed a marked predominance of slow-twitch, oxidative muscle fibers and mitochondrial proliferation. Western blotting showed the presence of unglucosylated glycogenin-1 in the muscle and heart. Sequencing of the glycogenin-1 gene, GYG1, revealed a nonsense mutation in one allele and a missense mutation, Thr83Met, in the other. The missense mutation resulted in inactivation of the autoglucosylation of glycogenin-1 that is necessary for the priming of glycogen synthesis in muscle.

Direct detection of glycogenin reaction products during glycogen initiation

Glycogenin initiates glycogen synthesis in an autocatalytic reaction in which individual glucose residues are covalently linked to Tyrosine 194 in order to form a short priming chain of glucose residues that is a substrate for glycogen synthase which, combined with the branching enzyme, catalyzes the bulk synthesis of glycogen. We sought to develop a new enzymatic assay to better characterize both the chemical and enzymatic characteristics of this unusual reaction. By directly detecting the reaction products using electrospray mass spectrometry this procedure permits both the visualization of the intact individual reaction species produced as a function of time and quantitation of the levels of each of species. The quantitation of the reaction agrees well with previous measurements of both catalytic rate and the change in rate as a function of average glucosylation. The results from this assay provide new insight into the mechanism by which glycogenin catalyzes the initiation reaction.

Increases in glycogenin and glycogenin mRNA accompany glycogen resynthesis in human skeletal muscle

Glycogenin is the self-glycosylating protein primer that initiates glycogen granule formation. To examine the role of this protein during glycogen resynthesis, eight male subjects exercised to exhaustion on a cycle ergometer at 75% Vo2 max followed by five 30-s sprints at maximal capacity to further deplete glycogen stores. During recovery, carbohydrate (75 g/h) was supplied to promote rapid glycogen repletion, and muscle biopsies were obtained from the vastus lateralis at 0, 30, 120, and 300 min postexercise. At time 0, no free (deglycosylated) glycogenin was detected in muscle, indicating that all glycogenin was complexed to carbohydrate. Glycogenin activity, a measure of the glycosylating ability of the protein, increased at 30 min and remained elevated for the remainder of the study. Quantitative RT-PCR showed elevated glycogenin mRNA at 120 min followed by increases in protein levels at 300 min. Glycogenin specific activity (glycogenin activity/relative protein content) was also elevated at 120 min. Proglycogen increased at all time points, with the highest rate of resynthesis occurring between 0 and 30 min. In comparison, macroglycogen levels did not significantly increase until 300 min postexercise. Together, these results show that, during recovery from prolonged exhaustive exercise, glycogenin mRNA and protein content and activity increase in muscle. This may facilitate rapid glycogen resynthesis by providing the glycogenin backbone of proglycogen, the major component of glycogen synthesized in early recovery.

Requirements for catalysis in mammalian glycogenin

Glycogenin is a glycosyltransferase that functions as the autocatalytic initiator for the synthesis of glycogen in eukaryotic organisms. Prior structural work identified the determinants responsible for the recognition and binding of UDP-glucose and the catalytic manganese ion and implicated two aspartic acid residues in the reaction mechanism for self-glucosylation. We examined the effects of substituting asparagine and serine for the aspartic acid residues at positions 159 and 162. We also examined whether the truncation of the protein at residue 270 (delta270) was compatible with its structural integrity and its functional role as the initiator for glycogen synthesis. The truncated form of the enzyme was indistinguishable from the wild-type enzyme by all measures of activity and could support glycogen accumulation in a glycogenin-deficient yeast strain. Substitution of aspartate 159 by either serine or asparagine eliminated self-glucosylation and reduced trans-glucosylation activity by at least 260-fold but only reduced UDP-glucose hydrolytic activity by 4-14-fold. Substitution of aspartate 162 by either serine or asparagine eliminated self-glucosylation activity and reduced UDP-glucose hydrolytic activity by at least 190-fold. The trans-glucosylation of maltose was reduced to undetectable levels in the asparagine 162 mutant, whereas the serine 162 enzyme showed only an 18-30-fold reduction in its ability to trans-glucosylate maltose. These data support a role for aspartate 162 in the chemical step for the glucosyltransferase reaction and a role for aspartate 159 in binding and activating the acceptor molecule.

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.

GNN is a self-glucosylating protein involved in the initiation step of glycogen biosynthesis in Neurospora crassa

The initiation of glycogen synthesis requires the protein glycogenin, which incorporates glucose residues through a self-glucosylation reaction, and then acts as substrate for chain elongation by glycogen synthase and branching enzyme. Numerous sequences of glycogenin-like proteins are available in the databases but the enzymes from mammalian skeletal muscle and from Saccharomyces cerevisiae are the best characterized. We report the isolation of a cDNA from the fungus Neurospora crassa, which encodes a protein, GNN, which has properties characteristic of glycogenin. The protein is one of the largest glycogenins but shares several conserved domains common to other family members. Recombinant GNN produced in Escherichia coli was able to incorporate glucose in a self-glucosylation reaction, to trans-glucosylate exogenous substrates, and to act as substrate for chain elongation by glycogen synthase. Recombinant protein was sensitive to C-terminal proteolysis, leading to stable species of around 31kDa, which maintained all functional properties. The role of GNN as an initiator of glycogen metabolism was confirmed by its ability to complement the glycogen deficiency of a S. cerevisiae strain (glg1 glg2) lacking glycogenin and unable to accumulate glycogen. Disruption of the gnn gene of N. crassa by repeat induced point mutation (RIP) resulted in a strain that was unable to synthesize glycogen, even though the glycogen synthase activity was unchanged. Northern blot analysis showed that the gnn gene was induced during vegetative growth and was repressed upon carbon starvation.

Yeast glycogenin (Glg2p) produced in Escherichia coli is simultaneously glucosylated at two vicinal tyrosine residues but results in a reduced bacterial glycogen accumulation

Saccharomyces cerevisiae possesses two glycogenin isoforms (designated as Glg1p and Glg2p) that both contain a conserved tyrosine residue, Tyr232. However, Glg2p possesses an additional tyrosine residue, Tyr230 and therefore two potential autoglucosylation sites. Glucosylation of Glg2p was studied using both matrix-assisted laser desorption ionization and electrospray quadrupole time of flight mass spectrometry. Glg2p, carrying a C-terminal (His6) tag, was produced in Escherichia coli and purified. By tryptic digestion and reversed phase chromatography a peptide (residues 219-246 of the complete Glg2p sequence) was isolated that contained 4-25 glucosyl residues. Following incubation of Glg2p with UDPglucose, more than 36 glucosyl residues were covalently bound to this peptide. Using a combination of cyanogen bromide cleavage of the protein backbone, enzymatic hydrolysis of glycosidic bonds and reversed phase chromatography, mono- and diglucosylated peptides having the sequence PNYGYQSSPAM were generated. MS/MS spectra revealed that glucosyl residues were attached to both Tyr232 and Tyr230 within the same peptide. The formation of the highly glucosylated eukaryotic Glg2p did not favour the bacterial glycogen accumulation. Under various experimental conditions Glg2p-producing cells accumulated approximately 30% less glycogen than a control transformed with a Glg2p lacking plasmid. The size distribution of the glycogen and extractable activities of several glycogen-related enzymes were essentially unchanged. As revealed by high performance anion exchange chromatography, the intracellular maltooligosaccharide pattern of the bacterial cells expressing the functional eukaryotic transgene was significantly altered. Thus, the eukaryotic glycogenin appears to be incompatible with the bacterial initiation of glycogen biosynthesis.

The cellular fate of glucose and its relevance in type 2 diabetes

Type 2 diabetes is a complex disorder with diminished insulin secretion and insulin action contributing to the hyperglycemia and wide range of metabolic defects that underlie the disease. The contribution of glucose metabolic pathways per se in the pathogenesis of the disease remains unclear. The cellular fate of glucose begins with glucose transport and phosphorylation. Subsequent pathways of glucose utilization include aerobic and anaerobic glycolysis, glycogen formation, and conversion to other intermediates in the hexose phosphate or hexosamine biosynthesis pathways. Abnormalities in each pathway may occur in diabetic subjects; however, it is unclear whether perturbations in these may lead to diabetes or are a consequence of the multiple metabolic abnormalities found in the disease. This review is focused on the cellular fate of glucose and relevance to human type 2 diabetes.

Structure-function analysis of GNIP, the glycogenin-interacting protein

Glycogenin is a self-glucosylating protein that initiates glycogen biosynthesis. We recently identified a family of proteins, GNIPs, that interact with glycogenin and stimulate its self-glucosylating activity [J. Biol. Chem. 277 (2002) 19331]. The GNIP gene (also called TRIM7) encodes at least four distinct isoforms of GNIP, three of which (GNIP1, GNIP2, and GNIP3) have in common a COOH-terminal B30.2 domain and predicted coiled-coil regions. Based on Western blot analysis, the GNIP1 protein is widely distributed in tissues. From analysis of a series of deletion mutants of GNIP2 using the yeast two-hybrid system, the B30.2 domain was found to be responsible for the interaction with glycogenin. A truncated form of recombinant GNIP2, lacking the NH2-terminal coiled-coil region, was cross-linked to glycogenin by glutaraldehyde treatment, supporting the idea that the B30.2 domain was sufficient for the interaction. In the course of this study, GNIP2 was also found to interact with itself, via the coiled-coil domain. Heterologous interactions between GNIP1 and GNIP2 were also detected. Since glycogenin is also a dimer, higher order multimeric complexes between glycogenin and GNIPs would be possible.

Crystal structure of the autocatalytic initiator of glycogen biosynthesis, glycogenin

Glycogen is an important storage reserve of glucose present in many organisms, from bacteria to humans. Its biosynthesis is initiated by a specialized protein, glycogenin, which has the unusual property of transferring glucose from UDP-glucose to form an oligosaccharide covalently attached to itself at Tyr194. Glycogen synthase and the branching enzyme complete the synthesis of the polysaccharide. The structure of glycogenin was solved in two different crystal forms. Tetragonal crystals contained a pentamer of dimers in the asymmetric unit arranged in an improper non-crystallographic 10-fold relationship, and orthorhombic crystals contained a monomer in the asymmetric unit that is arranged about a 2-fold crystallographic axis to form a dimer. The structure was first solved to 3.4 A using the tetragonal crystal form and a three-wavelength Se-Met multi-wavelength anomalous diffraction (MAD) experiment. Subsequently, an apo-enzyme structure and a complex between glycogenin and UDP-glucose/Mn2+ were solved by molecular replacement to 1.9 A using the orthorhombic crystal form. Glycogenin contains a conserved DxD motif and an N-terminal beta-alpha-beta Rossmann-like fold that are common to the nucleotide-binding domains of most glycosyltransferases. Although sequence identity amongst glycosyltransferases is minimal, the overall folds are similar. In all of these enzymes, the DxD motif is essential for coordination of the catalytic divalent cation, most commonly Mn2+. We propose a mechanism in which the Mn2+ that associates with the UDP-glucose molecule functions as a Lewis acid to stabilize the leaving group UDP and to facilitate the transfer of the glucose moiety to an intermediate nucleophilic acceptor in the enzyme active site, most likely Asp162. Following transient transfer to Asp162, the glucose moiety is then delivered to the final acceptor, either directly to Tyr194 or to glucose residues already attached to Tyr194. The positioning of the bound UDP-glucose far from Tyr194 in the glycogenin structure raises questions as to the mechanism for the attachment of the first glucose residues. Possibly the initial glucosylation is via inter-dimeric catalysis with an intra-molecular mechanism employed later in oligosaccharide synthesis.

GNIP, a novel protein that binds and activates glycogenin, the self-glucosylating initiator of glycogen biosynthesis

Glycogenin is a self-glucosylating protein involved in the initiation of glycogen biosynthesis. Self-glucosylation leads to the formation of an oligosaccharide chain, which, when long enough, supports the action of glycogen synthase to elongate it and form a mature glycogen molecule. To identify possible regulators of glycogenin, the yeast two-hybrid strategy was employed. By using rabbit skeletal muscle glycogenin as a bait, cDNAs encoding three different proteins were isolated from the human skeletal muscle cDNA library. Two of the cDNAs encoded glycogenin and glycogen synthase, respectively, proteins known to be interactors. The third cDNA encoded a polypeptide of unknown function and was designated GNIP (glycogenin interacting protein). Northern blot analysis revealed that GNIP mRNA is highly expressed in skeletal muscle. The gene for GNIP generates at least four isoforms by alternative splicing. The largest isoform GNIP1 contains, from NH(2)- to COOH-terminal, a RING finger, a B box, a putative coiled-coil region, and a B30.2-like motif. The previously identified protein TRIM7 (tripartite motif containing protein 7) is also derived from the GNIP gene and is composed of the RING finger, B box, and coiled-coil regions. The GNIP2 and GNIP3 isoforms consist of the coiled-coil region and B30.2-like domain. Physical interaction between GNIP2 and glycogenin was confirmed by co-immunoprecipitation, and in addition GNIP2 was shown to stimulate glycogenin self-glucosylation 3-4-fold. GNIPs may represent a novel participant in the initiation of glycogen synthesis.

Protein glycosylation: nature, distribution, enzymatic formation, and disease implications of glycopeptide bonds

Formation of the sugar-amino acid linkage is a crucial event in the biosynthesis of the carbohydrate units of glycoproteins. It sets into motion a complex series of posttranslational enzymatic steps that lead to the formation of a host of protein-bound oligosaccharides with diverse biological functions. These reactions occur throughout the entire phylogenetic spectrum, ranging from archaea and eubacteria to eukaryotes. It is the aim of this review to describe the glycopeptide linkages that have been found to date and specify their presence on well-characterized glycoproteins. A survey is also made of the enzymes involved in the formation of the various glycopeptide bonds as well as the site of their intracellular action and their affinity for particular peptide domains is evaluated. This examination indicates that 13 different monosaccharides and 8 amino acids are involved in glycoprotein linkages leading to a total of at least 41 bonds, if the anomeric configurations, the phosphoglycosyl linkages, as well as the GPI (glycophosphatidylinositol) phosphoethanolamine bridge are also considered. These bonds represent the products of N- and O-glycosylation, C-mannosylation, phosphoglycation, and glypiation. Currently at least 16 enzymes involved in their formation have been identified and in many cases cloned. Their intracellular site of action varies and includes the endoplasmic reticulum, Golgi apparatus, cytosol, and nucleus. With the exception of the Asn-linked carbohydrate and the GPI anchor, which are transferred to the polypeptide en bloc, the sugar-amino acid linkages are formed by the enzymatic transfer of an activated monosaccharide directly to the protein. This review also deals briefly with glycosidases, which are involved in physiologically important cleavages of glycopeptide bonds in higher organisms, and with a number of human disease states in which defects in enzymatic transfer of saccharides to protein have been implicated.

Complex glycosylation of Skp1 in Dictyostelium: implications for the modification of other eukaryotic cytoplasmic and nuclear proteins

Recently, complex O-glycosylation of the cytoplasmic/nuclear protein Skp1 has been characterized in the eukaryotic microorganism Dictyostelium. Skp1's glycosylation is mediated by the sequential action of a prolyl hydroxylase and five conventional sugar nucleotide-dependent glycosyltransferase activities that reside in the cytoplasm rather than the secretory compartment. The Skp1-HyPro GlcNAcTransferase, which adds the first sugar, appears to be related to a lineage of enzymes that originated in the prokaryotic cytoplasm and initiates mucin-type O-linked glycosylation in the lumen of the eukaryotic Golgi apparatus. GlcNAc is extended by a bifunctional glycosyltransferase that mediates the ordered addition of beta1,3-linked Gal and alpha1,2-linked Fuc. The architecture of this enzyme resembles that of certain two-domain prokaryotic glycosyltransferases. The catalytic domains are related to those of a large family of prokaryotic and eukaryotic, cytoplasmic, membrane-bound, inverting glycosyltransferases that modify glycolipids and polysaccharides prior to their translocation across membranes toward the secretory pathway or the cell exterior. The existence of these enzymes in the eukaryotic cytoplasm away from membranes and their ability to modify protein acceptors expose a new set of cytoplasmic and nuclear proteins to potential prolyl hydroxylation and complex O-linked glycosylation.

The catalytic site of the pectin biosynthetic enzyme alpha-1,4-galacturonosyltransferase is located in the lumen of the Golgi

Alpha-1,4-galacturonosyltransferase (GalAT) is an enzyme required for the biosynthesis of the plant cell wall pectic polysaccharide homogalacturonan (HGA). GalAT activity in homogenates from pea (Pisum sativum L. var. Alaska) stem internodes co-localized in linear and discontinuous sucrose gradients with latent UDPase activity, an enzyme marker specific for Golgi membranes. GalAT activity was separated from antimycin A-insensitive NADH:cytochrome c reductase and cytochrome c oxidase activities, enzyme markers for the endoplasmic reticulum and the mitochondria, respectively. GalAT and latent UDPase activities were separated from the majority (80%) of callose synthase activity, a marker for the plasma membrane, suggesting that little or no GalAT is present in the plasma membrane. GalAT activities in proteinase K-treated and untreated Golgi vesicles were similar, whereas no GalAT activity was detected after treating Golgi vesicles with proteinase K in the presence of Triton X-100. These results demonstrate that the catalytic site of GalAT resides within the lumen of the Golgi. The products generated by Golgi-localized GalAT were converted by endopolygalacturonase treatment to mono- and di-galacturonic acid, thereby showing that GalAT synthesizes 1-->4-linked alpha-D-galacturonan. Our data provide the first enzymatic evidence that a glycosyltransferase involved in HGA synthesis is present in the Golgi apparatus. Together with prior results of in vivo labeling and immunocytochemical studies, these results show that pectin biosynthesis occurs in the Golgi. A model for the biosynthesis of the pectic polysaccharide HGA is proposed.

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.

Characterization of mouse glycogenin-1 cDNA and promoter region

Glycogenin-1 is an autocatalytic, self-glucosylating protein which acts as the primer for glycogen synthesis in mammalian skeletal muscle. In this study, we have cloned the glycogenin-1 cDNA from mouse skeletal muscle. Mouse glycogenin-1 has a predicted molecular mass of 37¿ omitted¿399 Da, and the deduced amino acid sequence exhibited 87% homology with human glycogenin-1. Northern blot analysis specifically detected mouse glycogenin-1 transcript in skeletal muscle and heart, and to a lesser extent in kidney, lung and brain. 5'-RACE analysis revealed the major transcription start site to be localized 47 bp upstream of the initiation of translation codon. Sequence analysis of approximately 2 kb of the 5'-flanking region revealed potentially important regions of homology between the mouse and human glycogenin-1 promoters. Several conserved but putative elements, including a TATA box, Sp1 site, and a cyclic AMP responsive element, were observed proximal to the transcription start site. Significantly, Northern blot analysis revealed dibutyryl-cAMP treatment of cultured mouse C2C12 myotubes markedly reduced the levels of glycogenin-1 mRNA in a dose-dependent manner.