Function and structure of Drosophila glycans

Insights into the salivary N-glycome of Lutzomyia longipalpis, vector of visceral leishmaniasis

During Leishmania transmission sand flies inoculate parasites and saliva into the skin of vertebrates. Saliva has anti-haemostatic and anti-inflammatory activities that evolved to facilitate bloodfeeding, but also modulate the host's immune responses. Sand fly salivary proteins have been extensively studied, but the nature and biological roles of protein-linked glycans remain overlooked. Here, we characterised the profile of N-glycans from the salivary glycoproteins of Lutzomyia longipalpis, vector of visceral leishmaniasis in the Americas. In silico predictions suggest half of Lu. longipalpis salivary proteins may be N-glycosylated. SDS-PAGE coupled to LC-MS analysis of sand fly saliva, before and after enzymatic deglycosylation, revealed several candidate glycoproteins. To determine the diversity of N-glycan structures in sand fly saliva, enzymatically released sugars were fluorescently tagged and analysed by HPLC, combined with highly sensitive LC-MS/MS, MALDI-TOF-MS, and exoglycosidase treatments. We found that the N-glycan composition of Lu. longipalpis saliva mostly consists of oligomannose sugars, with Man5GlcNAc2 being the most abundant, and a few hybrid-type species. Interestingly, some glycans appear modified with a group of 144 Da, whose identity has yet to be confirmed. Our work presents the first detailed structural analysis of sand fly salivary glycans.

Structures and functions of invertebrate glycosylation

Glycosylation refers to the covalent attachment of sugar residues to a protein or lipid, and the biological importance of this modification has been widely recognized. While glycosylation in mammals is being extensively investigated, lower level animals such as invertebrates have not been adequately interrogated for their glycosylation. The rich diversity of invertebrate species, the increased database of sequenced invertebrate genomes and the time and cost efficiency of raising and experimenting on these species have enabled a handful of the species to become excellent model organisms, which have been successfully used as tools for probing various biologically interesting problems. Investigation on invertebrate glycosylation, especially on model organisms, not only expands the structural and functional knowledgebase, but also can facilitate deeper understanding on the biological functions of glycosylation in higher organisms. Here, we reviewed the research advances in invertebrate glycosylation, including N- and O-glycosylation, glycosphingolipids and glycosaminoglycans. The aspects of glycan biosynthesis, structures and functions are discussed, with a focus on the model organisms Drosophila and Caenorhabditis. Analytical strategies for the glycans and glycoconjugates are also summarized.

Mactosylceramide prevents glial cell overgrowth by inhibiting insulin and fibroblast growth factor receptor signaling

Receptor tyrosine kinase (RTK) signaling controls key aspects of cellular differentiation, proliferation, survival, metabolism, and migration. Deregulated RTK signaling also underlies many cancers. Glycosphingolipids (GSL) are essential elements of the plasma membrane. By affecting clustering and activity of membrane receptors, GSL modulate signal transduction, including that mediated by the RTK. GSL are abundant in the nervous system, and glial development in Drosophila is emerging as a useful model for studying how GSL modulate RTK signaling. Drosophila has a simple GSL biosynthetic pathway, in which the mannosyltransferase Egghead controls conversion of glucosylceramide (GlcCer) to mactosylceramide (MacCer). Lack of elongated GSL in egghead (egh) mutants causes overgrowth of subperineurial glia (SPG), largely due to aberrant activation of phosphatidylinositol 3-kinase (PI3K). However, to what extent this effect involves changes in upstream signaling events is unresolved. We show here that glial overgrowth in egh is strongly linked to increased activation of Insulin and fibroblast growth factor receptors (FGFR). Glial hypertrophy is phenocopied when overexpressing gain-of-function mutants of the Drosophila insulin receptor (InR) and the FGFR homolog Heartless (Htl) in wild type SPG, and is suppressed by inhibiting Htl and InR activity in egh. Knockdown of GlcCer synthase in the SPG fails to suppress glial overgrowth in egh nerves, and slightly promotes overgrowth in wild type, suggesting that RTK hyperactivation is caused by absence of MacCer and not by GlcCer accumulation. We conclude that an early product in GSL biosynthesis, MacCer, prevents inappropriate activation of insulin and fibroblast growth factor receptors in Drosophila glia.

Synaptic roles for phosphomannomutase type 2 in a new Drosophila congenital disorder of glycosylation disease model

Congenital disorders of glycosylation (CDGs) constitute a rapidly growing family of human diseases resulting from heritable mutations in genes driving the production and modification of glycoproteins. The resulting symptomatic hypoglycosylation causes multisystemic defects that include severe neurological impairments, revealing a particularly critical requirement for tightly regulated glycosylation in the nervous system. The most common CDG, CDG-Ia (PMM2-CDG), arises from phosphomannomutase type 2 (PMM2) mutations. Here, we report the generation and characterization of the first Drosophila CDG-Ia model. CRISPR-generated pmm2-null Drosophila mutants display severely disrupted glycosylation and early lethality, whereas RNAi-targeted knockdown of neuronal PMM2 results in a strong shift in the abundance of pauci-mannose glycan, progressive incoordination and later lethality, closely paralleling human CDG-Ia symptoms of shortened lifespan, movement impairments and defective neural development. Analyses of the well-characterized Drosophila neuromuscular junction (NMJ) reveal synaptic glycosylation loss accompanied by defects in both structural architecture and functional neurotransmission. NMJ synaptogenesis is driven by intercellular signals that traverse an extracellular synaptomatrix and are co-regulated by glycosylation and matrix metalloproteinases (MMPs). Specifically, trans-synaptic signaling by the Wnt protein Wingless (Wg) depends on the heparan sulfate proteoglycan (HSPG) co-receptor Dally-like protein (Dlp), which is regulated by synaptic MMP activity. Loss of synaptic MMP2, Wg ligand, Dlp co-receptor and downstream trans-synaptic signaling occurs with PMM2 knockdown. Taken together, this Drosophila CDG disease model provides a new avenue for the dissection of cellular and molecular mechanisms underlying neurological impairments and is a means by which to discover and test novel therapeutic treatment strategies.

Drosophila Collection: This work generates a new Drosophila congenital disorder of glycosylation model for the most common disease category, caused by phosphomannomutase-2 mutation, and reveals a synaptic mechanism underlying associated neurological impairments.

Cholesterol-responsive metabolic proteins are required for larval development in Caenorhabditis elegans

Caenorhabditis elegans, a cholesterol auxotroph, showed defects in larval development upon cholesterol starvation (CS) in a previous study. To identify cholesterol-responsive proteins likely responsible for the larval arrest upon CS, a comparative proteomic analysis was performed between C. elegans grown in normal medium supplemented with cholesterol (CN) and those grown in medium not supplemented with cholesterol (cholesterol starvation, CS). Our analysis revealed significant change (more than 2.2-fold, p < 0.05) in nine proteins upon CS. Six proteins were down-regulated [CE01270 (EEF-1A.1), CE08852 (SAMS-1), CE11068 (PMT-2), CE09015 (ACDH-1), CE12564 (R07H5.8), and CE09655 (RLA-0)], and three proteins were up-regulated [CE29645 (LEC-1), CE16576 (LEC-5), and CE01431 (NEX-1)]. RNAi phenotypes of two of the down-regulated genes, R07H5.8 (adenosine kinase) and rla-0 (ribosomal protein), in CN were similar to that of larval arrest in CS, and RNAi of a down-regulated gene, R07H5.8, in CS further enhanced the effects of CS, suggesting that down-regulation of these genes is likely responsible for the larval arrest in CS. All three up-regulated genes contain putative DAF-16 binding sites and mRNA levels of these three genes were all decreased in daf-16 mutants in CN, suggesting that DAF-16 activates expression of these genes.

Mucin-type proteins produced in the Trichoplusia ni and Spodoptera frugiperda insect cell lines carry novel O-glycans with phosphocholine and sulfate substitutions

The O-glycans of a recombinant mucin-type protein expressed in insect cell lines derived from Trichoplusia ni (Hi-5) and Spodoptera frugiperda (Sf9) were characterized. The P-selectin glycoprotein ligand-1/mouse IgG2b (PSGL-1/mIgG2b) fusion protein carrying 106 potential O-glycosylation sites and 6 potential N-glycosylation sites was expressed and purified from the Hi-5 and Sf9 cell culture medium using affinity chromatography and gel filtration. Liquid chromatography mass spectrometry (LC-MS) of O-glycans released from PSGL-1/mIgG2b revealed a large repertoire of structurally diverse glycans, which is in contrast to previous reports of only simple glycans. O-Glycans containing hexuronic acid (HexA, here glucuronic acid and galacturonic acid) were found to be prevalent. Also sulfate (Hi-5 and Sf9) and phosphocholine (PC; Sf9) O-glycan substitutions were detected. Western blotting confirmed the presence of O-linked PC on PSGL-1/mIG2b produced in Sf9 cells. To our knowledge, this is the first structural characterization of PC-substituted O-glycans in any species. The MS analyses revealed that Sf9 oligosaccharides consisted of short oligosaccharides (<6 residues) low in hexose (Hex) and with terminating N-acetylhexosamine (HexNAc) units, whereas Hi-5 produced a family of large O-glycans with (HexNAc-HexA-Hex) repeats and sulfate substitution on terminal residues. In both cell lines, the core N-acetylgalactosamine was preferentially non-branched, but small amounts of O-glycan cores with single fucose or hexose branches were found.

The N's and O's of Drosophila glycoprotein glycobiology

The past 25 years have seen significant advances in understanding the diversity and functions of glycoprotein glycans in Drosophila melanogaster. Genetic screens have captured mutations that reveal important biological activities modulated by glycans, including protein folding and trafficking, as well as cell signaling, tissue morphogenesis, fertility, and viability. Many of these glycan functions have parallels in vertebrate development and disease, providing increasing opportunities to dissect pathologic mechanisms using Drosophila genetics. Advances in the sensitivity of structural analytic techniques have allowed the glycan profiles of wild-type and mutant tissues to be assessed, revealing novel glycan structures that may be functionally analogous to vertebrate glycans. This review describes a selected set of recent advances in understanding the functions of N-linked and O-linked (non-glycosaminoglycan) glycoprotein glycans in Drosophila with emphasis on their relatedness to vertebrate organisms.

Neurofibromatosis-like phenotype in Drosophila caused by lack of glucosylceramide extension

Glycosphingolipids (GSLs) are of fundamental importance in the nervous system. However, the molecular details associated with GSL function are largely unknown, in part because of the complexity of GSL biosynthesis in vertebrates. In Drosophila, only one major GSL biosynthetic pathway exists, controlled by the glycosyltransferase Egghead (Egh). Here we discovered that loss of Egh causes overgrowth of peripheral nerves and attraction of immune cells to the nerves. This phenotype is reminiscent of the human disorder neurofibromatosis type 1, which is characterized by disfiguring nerve sheath tumors with mast cell infiltration, increased cancer risk, and learning deficits. Neurofibromatosis type 1 is due to a reduction of the tumor suppressor neurofibromin, a negative regulator of the small GTPase Ras. Enhanced Ras signaling promotes glial growth through activation of phosphatidylinositol 3-kinase (PI3K) and its downstream kinase Akt. We find that overgrowth of peripheral nerves in egh mutants is suppressed by down-regulation of the PI3K signaling pathway by expression of either dominant-negative PI3K, the tumor suppressor PTEN, or the transcription factor FOXO in the subperineurial glia. These results show that loss of the glycosyltransferase Egh affects membrane signaling and activation of PI3K signaling in glia of the peripheral nervous system, and suggest that glycosyltransferases may suppress proliferation.

Roles of sphingolipids in Drosophila development and disease

The last 10 years have seen a rebirth of interest in lipid biology in the fields of Drosophila development and neurobiology, and sphingolipids have emerged as controlling many processes that have not previously been studied from the viewpoint of lipid biochemistry. Mutations in sphingolipid regulatory enzymes have been pinpointed as affecting cell survival and growth in tissues ranging from muscle to retina. Specification of cell types are also influenced by sphingolipid regulatory pathways, as genetic interactions of glycosphingolipid biosynthetic enzymes with many well-known signaling receptors such as Notch and epidermal growth factor receptor reveal. Furthermore, studies in flies are now uncovering unexpected roles of sphingolipids in controlling lipid storage and response to nutrient availability. The sophisticated genetics of Drosophila is particularly well suited to uncover the roles of sphingolipid regulatory enzymes in development and metabolism, especially in light of conserved pathways that are present in both flies and mammals. The challenges that remain in the field of sphingolipid biology in Drosophila are to combine traditional developmental genetics with more analytical biochemical and biophysical methods, to quantify and localize the responses of these lipids to genetic and metabolic perturbations.

The glycomics of glycan glucuronylation in Drosophila melanogaster

As glycan characterization methods increase in sensitivity, new opportunities arise to undertake glycomic analyses on limiting amounts of material. Developing systems present special challenges since the amount of available tissue can restrict deep glycan characterization. We have optimized mass spectrometric methods with the goal of obtaining full glycan profiles from small amounts of tissue derived from organisms of particular interest. A major target of our efforts has been the Drosophila embryo, allowing us to leverage the tools already developed in this organism to meld glycomics, genomics, and molecular genetics. Our analysis of the N-linked, O-linked (non-GAG), and glycosphingolipid (GSL) glycans of the Drosophila embryo have identified expected and unexpected glycan structures. We have verified previous findings regarding the predominance of high-Man and pauci-Man N-linked glycans, but have also detected minor families of sialylated and glucuronylated N-linked structures. Glucuronic acid (GlcA) also presents itself as an abundant modification of O-linked and GSL glycans. We describe critical advancements in our methodology and present the broad range of contexts in which GlcA is found in the Drosophila embryo.

Glycobiology on the fly: developmental and mechanistic insights from Drosophila

Drosophila melanogaster offers many unique advantages for deciphering the complexities of glycan biosynthesis and function. The completion of the Drosophila genome sequencing project as well as the comprehensive catalogue of existing mutations and phenotypes have lead to a prolific database where many of the genes involved in glycan synthesis, assembly, modification, and recognition have been identified and characterized. Recent biochemical and molecular studies have elucidated the structure of the glycans present in Drosophila. Powerful genetic approaches have uncovered a number of critical biological roles for glycans during development that impact on our understanding of their function during mammalian development. Here, we summarize key recent findings and provide evidence for the usefulness of this model organism in unraveling the complexities of glycobiology across many species.

Sialylation in protostomes: a perspective from Drosophila genetics and biochemistry

Numerous studies have revealed important functions for sialylation in both prokaryotes and higher animals. However, the genetic and biochemical potential for sialylation in Drosophila has only been confirmed recently. Recent studies suggest significant similarities between the sialylation pathways of vertebrates and insects and provide evidence for their common evolutionary origin. These new data support the hypothesis that sialylation in insects is a specialized and developmentally regulated process which likely plays a prominent role in the nervous system. Yet several key issues remain to be addressed in Drosophila, including the initiation of sialic acid de novo biosynthesis and understanding the structure and function of sialylated glycoconjugates. This review discusses our current knowledge of the Drosophila sialylation pathway, as compared to the pathway in bacteria and vertebrates. We arrive at the conclusion that Drosophila is emerging as a useful model organism that is poised to shed new light on the function of sialylation not only in protostomes, but also in a larger evolutionary context.

Glucuronic acid can extend O-linked core 1 glycans, but it contributes only weakly to the negative surface charge of Drosophila melanogaster Schneider-2 cells

Previous studies of the mucin-type O-glycome of the fruit fly Drosophila melanogaster have revealed a restricted pattern of neutral core-type glycans corresponding to the Tn-(GalNAcalpha) and the T-antigen (Galbeta1-3GalNAcalpha). In particular, no extension of the core 1 glycan with acidic sugars, like sialic acid, was detected. Here we report on the identification of an acidic O-linked trisaccharide expressed on secreted endogenous and recombinant glycoproteins of the embryonal hemocyte-like Drosophila Schneider-2 (S2) cell line. The glycan is composed of glucuronic acid, galactose and N-acetylgalactosamine and its structure was determined as GlcA1-3Gal1-3GalNAc. The O-linked trisaccharide resembles the peripheral structures of acidic D. melanogaster glycosphingolipids. Glucuronic acid may substitute for sialic acid in this organism, however its expression on the S2 cell surface may only marginally contribute to the negative surface charge as revealed by free-flow cell electrophoresis prior to and after beta-glucuronidase treatment of the cells.

Gating of the shaker potassium channel is modulated differentially by N-glycosylation and sialic acids

N-linked glycans, including sialic acids, are integral components of ion channel complexes. To determine if N-linked sugars can modulate a rapidly inactivating K+ channel, the glycosylated Drosophila melanogaster Shaker K+ channel (ShB) and the N-glycosylation-deficient mutant (ShNQ), were studied under conditions of full and reduced sialylation. Through an apparent electrostatic mechanism, full sialylation induced uniform and significant hyperpolarizing shifts in all measured voltage-dependent ShB gating parameters compared to those measured under conditions of reduced sialylation. Steady-state gating of ShNQ was unaffected by changes in sialylation and was nearly identical to that observed for ShB under conditions of reduced sialylation, indicating that N-linked sialic acids were wholly responsible for the observed effects of sialic acid on ShB gating. Interestingly, the rates of transition among channel states and the voltage-independent rates of activation and inactivation were significantly slower for ShNQ compared to ShB. Both effects were independent of sialylation, indicating that N-linked sugars other than sialic acids alter ShB gating kinetics but have little to no effect on the steady-state distribution of channels among states. The effect of sialic acids on channel gating, particularly inactivation gating, and the impact of other N-linked sugars on channel gating kinetics are unique to the ShB isoform. Thus, ShB gating is modulated by two complementary but distinct sugar-dependent mechanisms, (1) an N-linked sialic acid-dependent surface charge effect and (2) a sialic acid-independent effect that is consistent with N-linked sugars affecting the stability of ShB among its functional states.

A serial lectin approach to the mucin-typeO-glycoproteome ofDrosophila melanogaster S2 cells

Identification of mucin-type O-glycosylated proteins with known functions in model organisms like Drosophila could provide keys to elucidate functions of the O-glycan moiety and proteomic analyses of O-glycoproteins in higher eukaryotes remain a challenge due to structural heterogeneity and a lack of efficient tools for their specific isolation. Here we report a strategy to evaluate the O-glycosylation potential of the embryonal hemocyte-like Drosophila Schneider 2 (S2) cell line by expression of recombinant glycosylation probes derived from tandem repeats of the human mucin MUC1 or of the Drosophila salivary gland protein Sgs1. We obtained evidence that mucin-type O-glycosylation in S2 cells grown under serum-free conditions is restricted to the Tn-antigen (GalNAcalpha-Ser/Thr) and the T-antigen (Galbeta1-3GalNAcalpha-Ser/Thr) and this structural homogeneity enables unique glycoproteomic strategies. We present a label-free strategy for the isolation, profiling and analysis of O-glycosylated proteins consisting of serial lectin affinity capture, 2-DE-based glycoprotein analysis by O-glycan specific mAbs and protein identification by MALDI-MS. Protein identity and O-glycosylation was confirmed by ESI-MS/MS with detection of diagnostic sugar oxonium-ion fragments. Using this strategy, we established 2-D reference maps and identified 21 secreted and intracellular mucin-type O-glycoproteins. Our results show that Drosophila S2 cells express O-glycoproteins involved in a wide range of biological functions including proteins of the extracellular matrix (Laminin gamma-chain, Peroxidasin and Glutactin), pathogen recognition proteins (Gnbp1), stress response proteins (Glycoprotein 93), secreted proteases (Matrix-metalloprotease 1 and various trypsin-like serine proteases), protease inhibitors (Serpin 27 A) and proteins of unknown function.

Glycosaminoglycans in Hydra magnipapillata (Hydrozoa, Cnidaria): demonstration of chondroitin in the developing nematocyst, the sting organelle, and structural characterization of glycosaminoglycans

The hydrozoan is the simplest organism whose movements are governed by the neuromuscular system, and its de novo morphogenesis can be easily induced by the removal of body parts. These features make the hydrozoan an excellent model for studying the regeneration of tissues in vivo, especially in the nervous system. Although glycosaminoglycans (GAGs) and proteoglycans (PGs) have been implicated in the signaling functions of various growth factors and play critical roles in the development of the central nervous system, the isolation and characterization of GAGs from hydrozoans have never been reported. Here, we characterized GAGs of Hydra magnipapillata. Immunostaining using anti-GAG antibodies showed chondroitin or chondroitin sulfate (CS) in the developing nematocyst, which is a sting organelle specific to cnidarians. The CS-PGs might furnish an environment for assembling nematocyst components, and might themselves be components of nematocysts. Therefore, GAGs were isolated from Hydra and their structural features were examined. A considerable amount of CS, three orders of magnitude less heparan sulfate (HS), but no hyaluronan were found, as in Caenorhabditis elegans. Analysis of the disaccharide composition of HS revealed glucosamine 2-N-sulfation, glucosamine 6-O-sulfation, and uronate 2-O-sulfation. CS contains not only nonsulfated and 4-O-sulfated N-acetylgalactosamine (GalNAc) but also 6-O-sulfated GalNAc. The average molecular size of CS and HS was 110 and 10 kDa, respectively. It has also been established here that CS chains are synthesized on the core protein through the ubiquitous linkage region tetrasaccharide, suggesting that indispensable functions of the linkage region in the synthesis of GAGs have been conserved during evolution.

Dynamic developmental elaboration of N-linked glycan complexity in the Drosophila melanogaster embryo

The structural diversity of glycoprotein N-linked oligosaccharides is determined by the expression and regulation of glycosyltransferase activities and by the availability of the appropriate acceptor/donor substrates. Cells in different tissues and in different developmental stages utilize these control points to manifest unique glycan expression patterns in response to their surroundings. The activity of a Toll-like receptor, called Tollo/Toll-8, induces a pattern of incompletely defined, but neural specific, glycan expression in the Drosophila embryo. Understanding the full extent of the changes in glycan expression that result from altered Tollo/Toll-8 signaling requires characterization of the complete N-linked glycan profile of both wild-type and mutant embryos. N-Linked glycans harvested from wild-type or mutant embryos were subjected to direct structural analysis by analytic and preparative high pressure liquid chromatography, by multidimensional mass spectrometry, and by exoglycosidase digestion, revealing a predominance of high mannose and paucimannose glycans. Di-, mono-, and nonfucosylated forms of hybrid, complex biantennary, and triantennary glycans account for 12% of the total wild-type glycan profile. Two sialylated glycans bearing N-acetylneuraminic acid were detected, the first direct demonstration of this modification in Drosophila. Glycan profiles change during normal development consistent with increasing alpha-mannosidase II and core fucosyl-transferase enzyme activities, and with decreasing activity of the Fused lobes processing hexosaminidase. In tollo/toll-8 mutants, a dramatic, expected loss of difucosylated glycans is accompanied by unexpected decreases in monofucosylated and nonfucosylated hybrid glycans and increases in some nonfucosylated paucimannose and biantennary glycans. Therefore, tollo/toll-8 signaling influences flux through several processing steps that affect the maturation of N-linked glycans.

Is it possible to develop pan-arthropod vaccines?

Hematophagous arthropods that transmit the etiological agents of arthropod-borne diseases have become the focus of anti-vector vaccines, targeted mainly at components of their saliva and midgut. These efforts have been directed mostly towards developing species-specific vaccines. An alternative is to target cross-reactive epitopes that have been preserved during evolution of the arthropods. The N- and O-linked glycans that are attached to arthropod glycoproteins are one of the potential targets of this pan-arthropod vaccine approach. Here, we discuss how genetically modified Drosophila melanogaster cells can be used to synthesize and to deliver these arthropod glycans to vertebrate hosts.

Highly sensitive multistage mass spectrometry enables small-scale analysis of protein glycosylation from two-dimensional polyacrylamide gels

Structural characterization of glycoproteins remains among the most challenging areas of glycomics due to the requirement of large quantities of samples and laborious biochemical steps involved in the analytical procedure. Here we report the structural characterization of glycoproteins separated on a 2-D gel by using a MALDI-QIT-TOF MS where QIT is quadrupole IT. The combination of MALDI-ion source and QIT appears to generate a unique tendency to cause fragmentation of glycopeptides without collision-induced dissociation. The majority of such fragmentations observed in our study result from the cleavage of sugar linkages, but not of peptide-peptide or peptide-sugar linkages. This unique feature allows us to perform pseudo-MS3 analysis of a fragmented glycopeptide. A small gel spot of a glycoprotein in the abundance range of low picomoles was enough for the mass spectrometer to analyze fragmentation pathway of the sugar linkage and peptide backbone. In this study, we demonstrate direct determination of glycosylation sites and N-linked glycan-sequences of the tryptic glycopeptides of Drosophila glycoproteins. Glycopeptides with various MWs up to approximately 4000 Da were suitable for structural analysis, including its attachment site and the amino acid sequence, of the glycopeptide through multistage mass spectrometric analysis.

Expression of a functional Drosophila melanogaster CMP-sialic acid synthetase. Differential localization of the Drosophila and human enzymes

CMP-N-acetylneuraminic acid is a critical metabolite in the generation of glycoconjugates that play a role in development and other physiological processes. Whereas pathways for its generation are firmly established in vertebrates, the presence and function of the relevant synthetic enzyme in insects and other protostomes is unknown. In this study, we characterize the first functional CMP-sialic acid synthase (DmCSAS) from any protostome lineage expressed from a D. melanogaster cDNA clone. Homologous genes were subsequently identified in other insect species. The gene is developmentally regulated, with expression first appearing at 12-24 h of embryogenesis, low expression through larval and pupal stages, and greatly enriched expression in the adult head, suggesting a possible role in the central nervous system. Activity of the enzyme was verified by an increase in in vitro and in vivo CMP-N-acetylneuraminic acid levels when expressed in a heterologous host. Unlike all known vertebrate CMP-sialic acid synthetase (CSAS) proteins that localize to the nucleus, the D. melanogaster CSAS protein was targeted to the Golgi compartment when expressed in both heterologous mammalian and insect cell lines. Replacement of the N-terminal leader sequence of DmCSAS with the human CSAS N-terminal sequence resulted in the redirection of the chimeric CSAS protein to the nucleus but with a concomitant loss of enzymatic activity. The localization of CSAS orthologs to different intracellular organelles represents, to our knowledge, the first example of differential protein targeting of orthologs in eukaryotes and reveals how the sialylation pathway diverged during the evolution of protostomes and deuterostomes.

Characterization of N-linked oligosaccharides in chorion peroxidase of Aedes aegypti mosquito

A peroxidase is present in the chorion of Aedes aegypti eggs and catalyzes chorion protein cross-linking during chorion hardening, which is critical for egg survival in the environment. The unique chorion peroxidase (CPO) is a glycoprotein. This study deals with the N-glycosylation site, structures, and profile of CPO-associated oligosaccharides using mass spectrometric techniques and enzymatic digestion. CPO was isolated from chorion by solubilization and several chromatographic methods. Mono-saccharide composition was analyzed by HPLC with fluorescent detection. Our data revealed that carbohydrate (D-mannose, N-acetyl D-glucosamine, D-arabinose, N-acetyl D-galactosamine, and L-fucose) accounted for 2.24% of the CPO molecular weight. A single N-glycosylation site (Asn328-Cys- Thr) was identified by tryptic peptide mapping and de novo sequencing of native and PNGase A-deglycosylated CPO using matrix-assisted laser/desorption/ionization time-of-flight mass spectrometry (MALDI/TOF/MS) and liquid chromatography/tandem mass spectrometry (LC/MS/MS). The Asn328 was proven to be a major fully glycosylated site. Potential tryptic glycopeptides and profile were first assessed by MALDI/TOF/MS and then by precursor ion scanning during LC/MS/MS. The structures of N-linked oligosaccharides were elucidated from the MS/MS spectra of glycopeptides and exoglycosidase sequencing of PNGase A-released oligosaccharides. These CPO-associated oligosaccharides had dominant Man3GlcNAc2 and Man3 (Fuc) GlcNAc2 and high mannose-type structures (Man(4-8)GlcNAc2). The truncated structures, Man2GlcNAc2 and Man2 (Fuc) GlcNAc2, were also identified. Comparison of CPO activity and Stokes radius between native and deglycosylated CPO suggests that the N-linked oligosaccharides influence the enzyme activity by stabilizing its folded state.

Glycosphingolipid structural analysis and glycosphingolipidomics

Sphingosines, or sphingoids, are a family of naturally occurring long-chain hydrocarbon derivatives sharing a common 1,3-dihydroxy-2-amino-backbone motif. The majority of sphingolipids, as their derivatives are collectively known, can be found in cell membranes in the form of amphiphilic conjugates, each composed of a polar head group attached to an N-acylated sphingoid, or ceramide. Glycosphingolipids (GSLs), which are the glycosides of either ceramide or myo-inositol-(1-O)-phosphoryl-(O-1)-ceramide, are a structurally and functionally diverse sphingolipid subclass; GSLs are ubiquitously distributed among all eukaryotic species and are found in some bacteria. Since GSLs are secondary metabolites, direct and comprehensive analysis (metabolomics) must be considered an essential complement to genomic and proteomic approaches for establishing the structural repertoire within an organism and deducing its possible functional roles. The glycosphingolipidome clearly comprises an important and extensive subset of both the glycome and the lipidome, but the complexities of GSL structure, biosynthesis, and function form the outlines of a considerable analytical problem, especially since their structural diversity confers by extension an enormous variability with respect to physicochemical properties. This chapter covers selected developments and applications of techniques in mass spectrometric (MS) that have contributed to GSL structural analysis and glycosphingolipidomics since 1990. Sections are included on basic characteristics of ionization and fragmentation of permethylated GSLs and of lithium-adducted nonderivatized GSLs under positive-ion electrospray ionization mass spectrometry (ESI-MS) and collision-induced mass spectrometry (CID-MS) conditions; on the analysis of sulfatides, mainly using negative-ion techniques; and on selected applications of ESI-MS and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to emerging GSL structural, functional, and analytical issues. The latter section includes a particular focus on evolving techniques for analysis of gangliosides, GSLs containing sialic acid, as well as on characterizations of GSLs from selected nonmammalian eukaryotes, such as dipterans, nematodes, cestodes, and fungi. Additional sections focus on the issue of whether it is better to leave GSLs intact or remove the ceramide; on development and uses of thin-layer chromatography (TLC) blotting and TLC-MS techniques; and on emerging issues of high-throughput analysis, including the use of flow injection, liquid chromatography mass spectrometry (LC-MS), and capillary electrophoresis mass spectrometry (CE-MS).

Insect galectins: roles in immunity and development

As evidenced by the reviews in this special issue of Glycoconjugate Journal, much research is focused on determining functions for mammalian galectins. However, the identification of precise functions for mammalian galectins may be complicated by redundancy in tissue expression and in target cell recognition of the many mammalian galectins. Therefore, lower organisms may be useful in deciphering precise functions for galectins. Unfortunately, some genetically manipulable model systems such as Caenorhabditis elegans may have more galectins than mammals. Recently, galectins were identified in two well-studied insect systems, Drosophila melanogaster and Anopheles gambiae. In addition to the powerful genetic manipulation available in these insect models, there is a sophisticated understanding of many biological processes in these organisms that can be directly compared and applied to mammalian systems. Understanding the roles of galectins in insects may provide insight into precise functions of galectins in mammals.

Role of glycosylation in development

Researchers have long predicted that complex carbohydrates on cell surfaces would play important roles in developmental processes because of the observation that specific carbohydrate structures appear in specific spatial and temporal patterns throughout development. The astounding number and complexity of carbohydrate structures on cell surfaces added support to the concept that glycoconjugates would function in cellular communication during development. Although the structural complexity inherent in glycoconjugates has slowed advances in our understanding of their functions, the complete sequencing of the genomes of organisms classically used in developmental studies (e.g., mice, Drosophila melanogaster, and Caenorhabditis elegans) has led to demonstration of essential functions for a number of glycoconjugates in developmental processes. Here we present a review of recent studies analyzing function of a variety of glycoconjugates (O-fucose, O-mannose, N-glycans, mucin-type O-glycans, proteoglycans, glycosphingolipids), focusing on lessons learned from human disease and genetic studies in mice, D. melanogaster, and C. elegans.

Evidence for alternative splicing and developmental regulation of the Drosophila melanogaster Mgat2 (N-acetylglucosaminyltransferase II) gene

We present a molecular study of the Drosophila melanogaster Mgat2 gene that codes for the N-acetylglucosaminyltransferase II. Isolation and analysis of two cDNA clones indicated that the gene is alternatively spliced, generating two transcripts of 3.3 and 2.7kb. Developmental specificity was observed between the two alternative transcripts during the major developmental stages of D. melanogaster. The deduced amino acid sequences of the two proteins show significant homology to the equivalent mammalian proteins, especially in the carboxyterminal region. In situ hybridizations in embryos and embryonic imaginal discs showed that the gene is expressed mainly but not exclusively in the brain.

Identification and characterization of a Drosophila melanogaster ortholog of human beta1,4-galactosyltransferase VII

Drosophila melanogaster is widely considered to be an attractive model organism for studying the functions of the carbohydrate moieties of glycoconjugates produced by higher eukaryotes. However, the pathways of glycoconjugate biosynthesis are not as well defined in insects as they are in higher eukaryotes. One way to address this problem is to identify genes in the Drosophila genome that might encode relevant functions, express them, and determine the functions of the gene products by direct biochemical assays. In this study, we used this approach to identify a putative Drosophila beta4-galactosyltransferase gene and determine the enzymatic activity of its product. Biochemical assays demonstrated that this gene product could transfer galactose from UDP-galactose to a beta-xylosyl acceptor, but not to other acceptors in vitro. The apparent K(m) values for the donor and acceptor substrates indicated that this gene product is a functional galactosyltransferase. Additional assays showed that the enzyme is activated by manganese, has a slightly acidic pH optimum, and is localized in the insect cell Golgi apparatus. These results showed that Drosophila encodes an ortholog of human beta4-galactosyltransferase-VII, also known as galactosyltransferase I, which participates in proteoglycan biosynthesis by transferring the first galactose to xylose in the linkage tetrasaccharide of glycosaminoglycan side chains.

Induction of neuron-specific glycosylation by Tollo/Toll-8, a Drosophila Toll-like receptor expressed in non-neural cells

Specific glycan expression is an essential characteristic of developing tissues. Our molecular characterization of a mutation that abolishes neural-specific glycosylation in the Drosophila embryo demonstrates that cellular interactions influence glycan expression. The HRP epitope is an N-linked oligosaccharide expressed on a subset of neuronal glycoproteins. Embryos homozygous for the TM3 balancer chromosome lack neural HRP-epitope expression. Genetic and molecular mapping of the relevant locus reveals that Tollo/Toll-8, a member of the Toll-like receptor family, is altered on the TM3 chromosome. In wild-type embryos, Tollo/Toll-8 is expressed by ectodermal cells that surround differentiating neurons and precedes HRP-epitope appearance. Re-introduction of Tollo/Toll-8 into null embryos rescues neural-specific glycan expression. Thus, loss of an ectodermal cell surface protein alters glycosylation in juxtaposed differentiating neurons. The portfolio of expressed oligosaccharides in a cell reflects its identity and also influences its interactions with other cells and with pathogens. Therefore, the ability to induce specific glycan expression complements the previously identified developmental and innate immune functions of Toll-like receptors.

Drosophila egghead encodes a beta 1,4-mannosyltransferase predicted to form the immediate precursor glycosphingolipid substrate for brainiac

The neurogenic Drosophila genes brainiac and egghead are essential for epithelial development in the embryo and in oogenesis. Analysis of egghead and brainiac mutants has led to the suggestion that the two genes function in a common signaling pathway. Recently, brainiac was shown to encode a UDP-N-acetylglucosamine:beta Man beta 1,3-N-acetylglucosaminyltransferase (beta 3GlcNAc-transferase) tentatively assigned a key role in biosynthesis of arthroseries glycosphingolipids and forming the trihexosylceramide, GlcNAc beta 1-3Man beta 1-4Glc beta 1-1Cer. In the present study we demonstrate that egghead encodes a Golgi-located GDP-mannose:beta Glc beta 1,4-mannosyltransferase tentatively assigned a biosynthetic role to form the precursor arthroseries glycosphingolipid substrate for Brainiac, Man beta 1-4Glc beta 1-1Cer. Egghead is unique among eukaryotic glycosyltransferase genes in that homologous genes are limited to invertebrates, which correlates with the exclusive existence of arthroseries glycolipids in invertebrates. We propose that brainiac and egghead function in a common biosynthetic pathway and that inactivating mutations in either lead to sufficiently early termination of glycolipid biosynthesis to inactivate essential functions mediated by glycosphingolipids.

The Drosophila gene brainiac encodes a glycosyltransferase putatively involved in glycosphingolipid synthesis

The Drosophila genes fringe and brainiac exhibit sequence similarities to glycosyltransferases. Drosophila and mammalian fringe homologs encode UDP-N-acetylglucosamine:fucose-O-Ser beta1,3-N-acetylglucosaminyltransferases that modulate the function of Notch family receptors. The biological function of brainiac is less well understood. brainiac is a member of a large homologous mammalian beta3-glycosyltransferase family with diverse functions. Eleven distinct mammalian homologs have been demonstrated to encode functional enzymes forming beta1-3 glycosidic linkages with different UDP donor sugars and acceptor sugars. The putative mammalian homologs with highest sequence similarity to brainiac encode UDP-N-acetylglucosamine:beta1,3-N-acetylglucosaminyltransferases (beta3GlcNAc-transferases), and in the present study we show that brainiac also encodes a beta3GlcNAc-transferase that uses beta-linked mannose as well as beta-linked galactose as acceptor sugars. The inner disaccharide core structures of glycosphingolipids in mammals (Galbeta1-4Glcbeta1-Cer) and insects (Manbeta1-4Glcbeta1-Cer) are different. Both disaccharide glycolipids served as substrates for brainiac, but glycolipids of insect cells have so far only been found to be based on the GlcNAcbeta1-3Manbeta1-4Glcbeta1-Cer core structure. Infection of High Five(TM) cells with baculovirus containing full coding brainiac cDNA markedly increased the ratio of GlcNAcbeta1-3Manbeta1-4Glcbeta1-Cer glycolipids compared with Galbeta1-4Manbeta1-4Glcbeta1-Cer found in wild type cells. We suggest that brainiac exerts its biological functions by regulating biosynthesis of glycosphingolipids.

Determination of the glycosaminoglycan-protein linkage region oligosaccharide structures of proteoglycans from Drosophila melanogaster and Caenorhabditis elegans

Caenorhabditis elegans and Drosophila melanogaster are relevant models for studying the roles of glycosaminoglycans (GAG) during the development of multicellular organisms. The genome projects of these organisms have revealed the existence of multiple genes related to GAG-synthesizing enzymes. Although the putative genes encoding the enzymes that synthesize the GAG-protein linkage region have also been identified, there is no direct evidence that the GAG chains bind covalently to core proteins. This study aimed to clarify whether GAG chains in these organisms are linked to core proteins through the conventional linkage region tetrasaccharide sequence found in vertebrates and whether modifications by phosphorylation and sulfation reported for vertebrates are present also in invertebrates. The linkage region oligosaccharides were isolated from C. elegans chondroitin in addition to D. melanogaster heparan and chondroitin sulfate after digestion with the respective bacterial eliminases and were then derivatized with a fluorophore 2-aminobenzamide. Their structures were characterized by gel filtration and anion-exchange high performance liquid chromatography in conjunction with enzymatic digestion and matrix-assisted laser desorption ionization time-of-flight spectrometry, which demonstrated a uniform linkage tetrasaccharide structure of -GlcUA-Gal-Gal-Xyl- or -GlcUA-Gal-Gal-Xyl(2-O-phosphate)- for C. elegans chondroitin and D. melanogaster CS, respectively. In contrast, the unmodified and phosphorylated counterparts were demonstrated in heparan sulfate of adult flies at a molar ratio of 73:27, and in that of the immortalized D. melanogaster S2 cell line at a molar ratio of 7:93, which suggests that the linkage region in the fruit fly first becomes phosphorylated uniformly on the Xyl residue and then dephosphorylated. It has been established here that GAG chains in both C. elegans and D. melanogaster are synthesized on the core protein through the ubiquitous linkage region tetrasaccharide sequence, suggesting that indispensable functions of the linkage region in the GAG synthesis have been well conserved during evolution.

An endogenous Drosophila receptor for glycans bearing alpha 1,3-linked core fucose residues

The genome of Drosophila melanogaster encodes several proteins that are predicted to contain Ca(2+)-dependent, C-type carbohydrate-recognition domains. The CG2958 gene encodes a protein containing 359 amino acid residues. Analysis of the CG2958 sequence suggests that it consists of an N-terminal domain found in other Drosophila proteins, a middle segment that is unique, and a C-terminal C-type carbohydrate-recognition domain. Expression studies show that the full-length protein is a tetramer formed by noncovalent association of disulfide-linked dimers that are linked through cysteine residues in the N-terminal domain. The expressed protein binds to immobilized yeast invertase through the C-terminal carbohydrate-recognition domain. Competition binding studies using monosaccharides demonstrate that CG2958 interacts specifically with fucose and mannose. Fucose binds approximately 5-fold better than mannose. Blotting studies reveal that the best glycoprotein ligands are those that contain N-linked glycans bearing alpha1,3-linked fucose residues. Binding is enhanced by the additional presence of alpha1,6-linked fucose. It has previously been proposed that labeling of the Drosophila neural system by anti-horseradish peroxidase antibodies is a result of the presence of difucosylated N-linked glycans. CG2958 is a potential endogenous receptor for such neural-specific carbohydrate epitopes.

Characterization of a novel Drosophila melanogaster galectin. Expression in developing immune, neural, and muscle tissues

We have cloned and characterized the first galectin to be identified in Drosophila melanogaster. The amino acid sequence of Drosophila galectin showed striking sequence similarity to invertebrate and vertebrate galectins and contained amino acids that are crucial for binding beta-galactoside sugars. Confirming its identity as a galectin family member, the Drosophila galectin bound beta-galactoside sugars. Structurally, the Drosophila galectin was a tandem repeat galectin containing two carbohydrate recognition domains connected by a unique peptide link. This divalent structure suggests that like mammalian galectins, Drosophila galectin may mediate cell-cell communication or facilitate cross-linking of receptors to trigger signal transduction events. The Drosophila galectin was very abundant in embryonic, larval, and adult Drosophila. During embryogenesis, Drosophila galectin had a unique and specific tissue distribution. Drosophila galectin expression was concentrated in somatic and visceral musculature and in the central nervous system. Similar to other insect lectins, Drosophila galectin may function in both embryogenesis and in host defense. Drosophila galectin was expressed by hemocytes, circulating phagocytic cells, suggesting a role for Drosophila galectin in the innate immune system.

Composition of Drosophila melanogaster proteome involved in fucosylated glycan metabolism

The whole genome approach enables the characterization of all components of any given biological pathway. Moreover, it can help to uncover all the metabolic routes for any molecule. Here we have used the genome of Drosophila melanogaster to search for enzymes involved in the metabolism of fucosylated glycans. Our results suggest that in the fruit fly GDP-fucose, the donor for fucosyltransferase reactions, is formed exclusively via the de novo pathway from GDP-mannose through enzymatic reactions catalyzed by GDP-D-mannose 4,6-dehydratase (GMD) and GDP-4-keto-6-deoxy-D-mannose 3,5-epimerase/4-reductase (GMER, also known as FX in man). The Drosophila genome does not have orthologs for the salvage pathway enzymes, i.e. fucokinase and GDP-fucose pyrophosphorylase synthesizing GDP-fucose from fucose. In addition we identified two novel fucosyltransferases predicted to catalyze alpha1,3- and alpha1,6-specific linkages to the GlcNAc residues on glycans. No genes with the capacity to encode alpha1,2-specific fucosyltransferases were found. We also identified two novel genes coding for O-fucosyltransferases and a gene responsible for a fucosidase enzyme in the Drosophila genome. Finally, using the Drosophila CG4435 gene, we identified two novel human genes putatively coding for fucosyltransferases. This work can serve as a basis for further whole-genome approaches in mapping all possible glycosylation pathways and as a basic analysis leading to subsequent experimental studies to verify the predictions made in this work.

Human and Drosophila UDP-galactose transporters transport UDP-N-acetylgalactosamine in addition to UDP-galactose

A putative Drosophila nucleotide sugar transporter was characterized and shown to be the Drosophila homologue of the human UDP-Gal transporter (hUGT). When the Drosophila melanogaster UDP-Gal transporter (DmUGT) was expressed in mammalian cells, the transporter protein was localized in the Golgi membranes and complemented the UDP-Gal transport deficiency of Lec8 cells but not the CMP-Sia transport deficiency of Lec2 cells. DmUGT and hUGT were expressed in Saccharomyces cerevisiae cells in functionally active forms. Using microsomal vesicles isolated from Saccharomyces cerevisiae expressing these transporters, we unexpectedly found that both hUGT and DmUGT could transport UDP-GalNAc as well as UDP-Gal. When amino-acid residues that are conserved among human, murine, fission yeast and Drosophila UGTs, but are distinct from corresponding ones conserved among CMP-Sia transporters (CSTs), were substituted by those found in CST, the mutant transporters were still active in transporting UDP-Gal. One of these mutants in which Asn47 was substituted by Ala showed aberrant intracellular distribution with concomitant destabilization of the protein product. However, this mutation was suppressed by an Ile51 to Thr second-site mutation. Both residues were localized within the first transmembrane helix, suggesting that the structure of the helix contributes to the stabilization and substrate recognition of the UGT molecule.

Gliolectin-mediated carbohydrate binding at theDrosophilamidline ensures the fidelity of axon pathfinding

Gliolectin is a carbohydrate-binding protein (lectin) that mediates cell adhesion in vitro and is expressed by midline glial cells in the Drosophila melanogaster embryo. Gliolectin expression is maximal during early pathfinding of commissural axons across the midline (stages 12-13), a process that requires extensive signaling and cell-cell interactions between the midline glia and extending axons. Deletion of the gliolectin locus disrupts the formation of commissural pathways and also delays the completion of longitudinal pathfinding. The disruption in commissure formation is accompanied by reduced axon-glial contact, such that extending axons grow on other axons and form a tightly fasciculated bundle that arches over the midline. By contrast, pioneering commissural axons normally cross the midline as a distributed array of fibers that interdigitate among the midline glia, maximizing contact and, therefor, communication between axon and glia. Restoration of Gliolectin protein expression in the midline glia rescues the observed pathfinding defects of null mutants in a dose-dependent manner. Hypomorphic alleles generated by ethylmethanesulfonate mutagenesis exhibit a similar phenotype in combination with a deletion and these defects are also rescued by transgenic expression of Gliolectin protein. The observed phenotypes indicate that carbohydrate-lectin interactions at the Drosophila midline provide the necessary surface contact to capture extending axons, thereby ensuring that combinatorial codes of positive and negative growth signals are interpreted appropriately.

Genetic model organisms in the study of N-glycans

Recently the genomic sequences of three multicellular eukaryotes, Caenorhabditis elegans, Drosophila melanogaster and Arabidopsis thaliana, have been elucidated. A number of cDNAs encoding glycosyltransferases demonstrated to have a role in N-linked glycosylation have already been cloned from these organisms, e.g., GlcNAc transferases and alpha 1,3-fucosyltransferases. However, many more homologues of glycosyltransferases and other glycan modifying enzymes have been predicted by analysis of the genome sequences, but the predictions of full length open reading frames appear to be particularly poor in Caenorhabditis. The use of these organisms as models in glycobiology may be hampered since they all have N-linked glycosylation repertoires unlike those of mammals. Arabidopsis and Drosophila have glycosylation similar to that of other plants or insects, while our new data from MALDI-TOF analysis of PNGase A-released neutral N-glycans of Caenorhabditis indicate that there exists a range of pauci- and oligomannosidic structures, with up to four fucose residues and up to two O-methyl groups. With all these three 'genetic model organisms', however, much more work is required for a full understanding of their glycobiology.

Identification of core alpha 1,3-fucosylated glycans and cloning of the requisite fucosyltransferase cDNA from Drosophila melanogaster. Potential basis of the neural anti-horseadish peroxidase epitope

For many years, polyclonal antibodies raised against the plant glycoprotein horseradish peroxidase have been used to specifically stain the neural and male reproductive tissue of Drosophila melanogaster. This epitope is considered to be of carbohydrate origin, but no glycan structure from Drosophila has yet been isolated that could account for this cross-reactivity. Here we report that N-glycan core alpha1,3-linked fucose is, as judged by preabsorption experiments, indispensable for recognition of Drosophila embryonic nervous system by anti-horseradish peroxidase antibody. Further, we describe the identification by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry and high performance liquid chromatography of two Drosophila N-glycans that, as already detected in other insects, carry both alpha1,3- and alpha1,6-linked fucose residues on the proximal core GlcNAc. Moreover, we have isolated three cDNAs encoding alpha1,3-fucosyltransferase homologues from Drosophila. One of the cDNAs, when transformed into Pichia pastoris, was found to direct expression of core alpha1,3-fucosyltransferase activity. This recombinant enzyme preferred as substrate a biantennary core alpha1,6-fucosylated N-glycan carrying two non-reducing N-acetylglucosamine residues (GnGnF6; Km 11 microm) over the same structure lacking a core fucose residue (GnGn; Km 46 microm). The Drosophila core alpha1,3-fucosyltransferase enzyme was also shown to be able to fucosylate N-glycan structures of human transferrin in vitro, this modification correlating with the acquisition of binding to anti-horseradish peroxidase antibody.

Lectin-like proteins in model organisms: implications for evolution of carbohydrate-binding activity

Classes of intracellular lectins that recognize core-type structures and mediate intracellular glycoprotein trafficking are present in vertebrates, model invertebrates such as Caenorhabditis elegans and Drosophila melanogaster, plants, and yeasts. Lectins that recognize more complex structures at the cell surface, such as C-type lectins and galectins, are also found in invertebrate organisms as well as vertebrates, but the functions of these proteins have evolved differently in different animal lineages.

Congenital disorders of glycosylation: glycosylation defects in man and biological models for their study

Several inherited disorders affecting the biosynthetic pathways of N-glycans have been discovered during the past years. This review summarizes the current knowledge in this rapidly expanding field and covers the molecular bases of these disorders as well as their phenotypical consequences.