Protein glycosylation. Structural and functional aspects

Expression of heterologous proteins in Pichia pastoris: a useful experimental tool in protein engineering and production

The use of the methylotrophic yeast, Pichia pastoris, as a cellular host for the expression of recombinant proteins has become increasing popular in recent times. P. pastoris is easier to genetically manipulate and culture than mammalian cells and can be grown to high cell densities. Equally important, P. pastoris is also a eukaryote, and thereby provides the potential for producing soluble, correctly folded recombinant proteins that have undergone all the post-translational modifications required for functionality. Additionally, linearized foreign DNA can be inserted in high efficiency via homologous recombination procedures to generate stable cell lines whilst expression vectors can be readily prepared that allow multiple copies of the target protein, multimeric proteins with different subunit structures, or alternatively the target protein and its cognate binding partners, to be expressed. A further benefit of the P. pastoris system is that strong promoters are available to drive the expression of a foreign gene(s) of interest, thus enabling production of large amounts of the target protein(s) with relative technical ease and at a lower cost than most other eukaryotic systems. The purpose of this review is to summarize important developments and features of this expression system and, in particular, to examine from an experimental perspective the genetic engineering, protein chemical and molecular design considerations that have to be taken into account for the successful expression of the target recombinant protein. Included in these considerations are the influences of P. pastoris strain selection; the choice of expression vectors and promoters; procedures for the transformation and integration of the vectors into the P. pastoris genome; the consequences of rare codon usage and truncated transcripts; and techniques employed to achieve multi-copy integration numbers. The impact of the alcohol oxidase (AOX) pathways in terms of the mut+ and mut(s) phenotypes, intracellular expression and folding pathways is examined. The roles of pre-pro signal sequences such as the alpha mating factor (alpha-MF) and the Glu-Ala repeats at the kex2p cleavage site on the processing of the protein translate(s) have also been considered. Protocols for the generation of protein variants and mutants for screening for orphan cognate binding partners and the use of experimental platforms addressing the molecular recognition behaviour of recombinant proteins such as the extracellular domains of transmembrane receptors with their physiological ligands are also described. Finally, the palindromic patterns of glycosylation that can occur with these expression systems, in terms of the role and location of the sequon in the primary structure, the number of mannose units and the types of oligosaccharides incorporated as Asn- or O-linkages and their impact on the thermostability and immunogenicity of the recombinant protein are considered. Procedures to prevent glycosylation through manipulation of cell culture conditions or via enzymatic and site-directed mutagenesis methods are also discussed.

The role of N-glycosylation in the stability, trafficking and GABA-uptake of GABA-transporter 1. Terminal N-glycans facilitate efficient GABA-uptake activity of the GABA transporter

Neurotransmitter transporters play a major role in achieving low concentrations of their respective transmitter in the synaptic cleft. The GABA transporter GAT1 belongs to the family of Na(+)- and Cl(-)-coupled transport proteins which possess 12 putative transmembrane domains and three N-glycosylation sites in the extracellular loop between transmembrane domain 3 and 4. To study the significance of N-glycosylation, green fluorescence protein (GFP)-tagged wild type GAT1 (NNN) and N-glycosylation defective mutants (DDQ, DGN, DDN and DDG) were expressed in CHO cells. Compared with the wild type, all N-glycosylation mutants showed strongly reduced protein stability and trafficking to the plasma membrane, which however were not affected by 1-deoxymannojirimycin (dMM). This indicates that N-glycosylation, but not terminal trimming of the N-glycans is involved in the attainment of a correctly folded and stable conformation of GAT1. All N-glycosylation mutants were expressed on the plasma membrane, but they displayed markedly reduced GABA-uptake activity. Also, inhibition of oligosaccharide processing by dMM led to reduction of this activity. Further experiments showed that both N-glycosylation mutations and dMM reduced the V(max) value, while not increasing the K(m) value for GABA uptake. Electrical measurements revealed that the reduced transport activity can be partially attributed to a reduced apparent affinity for extracellular Na+ and slowed kinetics of the transport cycle. This indicates that N-glycans, in particular their terminal trimming, are important for the GABA-uptake activity of GAT1. They play a regulatory role in the GABA translocation by affecting the affinity and the reaction steps associated with the sodium ion binding.

Comprehensive assessment of N-glycans derived from a murine monoclonal antibody: a case for multimethodological approach

Highly efficient separation techniques, laser-induced fluorescence (LIF) detection, and different mass-spectrometric (MS) measurements were combined in a multimethodological scheme to perform a comprehensive structural characterization of N-linked oligosaccharides in a murine monoclonal antibody (immunoglobulin G (IgG(kappa))). Monosaccharide compositional analysis was carried out through a capillary electrophoresis (CE)-LIF method, in which the chemically and enzymatically released sugars were fluorescently labeled. This analysis provides a preliminary assessment of certain structures, being followed by CE-LIF and matrix-assisted laser desorption/ionization (MALDI)-MS profiling of the intact glycan structures. Linkages and monosaccharide residues were confirmed by MALDI-MS in conjunction with exoglycosidase digestion. MALDI-MS and CE data were effectively combined to reveal the overall structural diversity of both acidic and neutral glycans. Finally, the sites of glycosylation and site occupancies were deduced through the measurements performed with microcolumn liquid chromatography coupled via electrospray to a quadrupole/time-of-flight instrument.

Towards understanding the interaction between oligosaccharides and water molecules

Complex carbohydrates are implicated in many important biological processes, and have a strong interaction with water. This close interplay with molecular water through multiple hydroxyls may be an integral part of their emergent structure and dynamics, as selected during evolution. Using molecular dynamics simulations with explicit water the interactions at the linkages within a variety of oligosaccharides are investigated and contrasted, in order to establish correlations between linkage orientation, sugar epimerization, and water interaction. In particular, interactions at alpha linkages, and between mannose and glucose residues, that are common in oligosaccharides are considered. Sugars joined by alpha linkages at the 2-, 3-, and 6-position were found to interact via a combination of weak hydrogen-bonds and water-bridges, which is dependent on the epimerization state of the sugars. Due to their three-dimensional structure, they are also likely to interact with noncontiguous sugar residues in an oligosaccharide, which can lead to ordered structures through the exclusion of water. On the other hand, beta linkages (to 3- and 4-position) maintain strong hydrogen-bonds, have a limited ability to be involved in water-bridges, and predominantly interact with the directly attached sugars. Therefore, sequences of alpha-linked sugars form compact, branched structures that have conformational flexibility, and beta linkages form extended, relatively rigid structures, suitable for structural molecules, and at the termini of protein bound oligosaccharides. These results provide further tentative ties between chemical structure, water interactions, and the emergent form and function of specific sugars and linkages in oligosaccharides.

The role of electron capture dissociation in biomolecular analysis

The introduction of electron capture dissociation (ECD) to electrospray (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) constitutes a significant advance in the structural analysis of biomolecules. The fundamental features and benefits of ECD are discussed in this review. ECD is currently unique to FT-ICR MS and the fundamentals of that technique are outlined. The advantages and complementarity of ECD in relation to other tandem mass spectrometry (MS/MS) techniques, such as infrared multiphoton dissociation (IRMPD) and sustained off-resonance collision-induced dissociation (SORI-CID), are discussed. The instrumental considerations associated with implementation of ECD, including activated ion techniques and coupling to on-line separation techniques, are covered, as are the allied processes electronic excitation dissociation (EED), electron detachment dissociation (EDD), and hot electron capture (HECD). A major theme of this review is the role of ECD in proteomics, particularly for characterization of post-translational modifications (phosphorylation, glycosylation, carboxyglutamic acid, sulfation, acylation, and methionine oxidation) and the top-down approach to protein identification. The application of ECD to the analysis of polymers, peptide nucleic acids, and oligonucleotides is also discussed.

Effect of Glycosylation on the Catalytic and Conformational Stability of Homologous α-Amylases

A thermostable alpha-amylase from B. licheniformis (BLA) and a mesophilic amylase from B. amyloliquefaciens (BAA) were covalently coupled to oxidized synthetic sucrose polymers (OSP400 and OSP70) and polyglutaraldehyde (PGA) by reductive alkylation to study the effect of neoglycosylation on the activity, kinetic and thermodynamic stability. The catalytic efficiency of the modified enzymes was comparable to that of the native enzyme. Covalent coupling decreased the rate of inactivation at all the temperatures studied, both in the presence and absence of added Ca2+. The stability of the native enzyme was found to increase upon modification as observed from the increase in tl/2 in the absence of Ca2+ ions by about 1.5 - 13.7 times (at 85 degrees C) in the case of BLA and 5.7 - 8.4 times (at 50 degrees C) for BAA. The highest stability was observed for OSP400 modified enzyme with Delta Cm and Delta Tm values of 0.63 M and 7.92 degrees C for BLA and 0.85 M and 5.3 degrees C for BAA, respectively. The order of stability was OSP400 > OSP70 > PGA > Native for both BLA and BAA. The stability of the modified amylases obtained from the present study were superior compared to most of the single and double mutants obtained by site-directed mutagenesis that were constructed so as to enhance the intrinsic stability of these enzymes.

Different adhesion and migration properties of human HCV29 non-malignant urothelial and T24 bladder cancer cells: role of glycosylation

In tumour cells, alterations in cellular glycosylation may play a key role in their metastatic behaviour. This study used cell lines having very different behaviour in vivo: HCV29 non-malignant transitional epithelium and T24 bladder transitional cell carcinoma. These differences in behaviour might be due in part to differences in cellular glycosylation patterns. Glycan chain analysis of their glycoproteins was performed with the use of specific lectins. The functional role of carbohydrates was studied by treating these cells with swainsonine, an inhibitor of Golgi alpha-mannosidase II, and in vitro adhesion and migration assays. The adhesion of swainsonine-treated HCV29 and T24 cells was increased on fibronectin and type IV collagen by 1.5- and 2-fold, respectively, whereas adhesion on laminin was virtually unchanged after swainsonine-treatment in HCV29 cells and was increased in T24 cells. Swainsonine treatment reduced the rate of T24 cell migration by 20%. We concluded that beta1-6 branched tri- and tetraantennary complex-type glycans have an important function in adhesion and migration in the studied cell lines. These data support the view that oligosaccharides are involved in several steps of the metastatic process.

Versatile approach for the synthesis of novel seven-membered iminocyclitols via ring-closing metathesis dihydroxylation reaction

Seven-membered iminocyclitols with diverse diastereomers were prepared starting with d- and l-serines and employing ring-closing olefin metathesis and dihydroxylation reaction sequence. The iminocyclitols were assayed for glycosidase inhibition and compound 20 was found to be a competitive inhibitor for beta-glucosidase with Ki 26.3 microM.

Structural characterization of 2-aminobenzamide-derivatized oligosaccharides using a matrix-assisted laser desorption/ionization two-stage time-of-flight tandem mass spectrometer

Oligosaccharides were derivatized by reductive amination using 2-aminobenzamide (2-AB) and analyzed by matrix-assisted laser desorption/ionization two-stage time-of-flight (MALDI-TOF/TOF) tandem mass spectrometry (MS/MS) in the positive ion mode. The major signals were obtained under these conditions from the [M+Na]+ ions for all 2-AB-derivatized oligosaccharides. A systematic study was conducted on a series of 2-AB-derivatized oligosaccharides to allow rationalization of the fragmentation processes. The MALDI-TOF/TOF-MS/MS spectra of the [M+Na]+ ions of 2-AB-derivatized oligosaccharides were dominated by glycosidic cleavages. These fragments originating both from the reducing and the non-reducing ends of the oligosaccharide yield information on sequence and branching. Moreover, the MALDI-TOF/TOF-MS/MS spectra were also characterized by abundant cross-ring fragments which are very informative on the linkages of the monosaccharide residues constituting these oligosaccharides. MALDI-TOF/TOF-MS/MS analysis of 2-AB-derivatized oligosaccharides, by providing structural information at the low-picomole level, appears to be a powerful tool for carbohydrate structural analysis.

Glycopeptide and glycoprotein synthesis involving unprotected carbohydrate building blocks

This review summarizes the chemical and chemoenzymatic synthesis of glycopeptides and glycoproteins using unprotected carbohydrates as key intermediates. The synthetic methods covered herein include the convergent synthesis of glycopeptides by chemoselective ligation of peptides and free glycans, solution- and solid-phase synthesis of glycopeptides by sequential peptide elongation with unprotected glycosyl amino acids or short glycopeptides as building blocks, and the synthesis of glycopeptides by enzymatic and/or chemical elongation of the free glycans. The use of unprotected carbohydrates in these syntheses can circumvent the final-stage carbohydrate deprotection, lead to highly convergent synthetic designs, and more significantly, take advantage of the commercially available free glycans isolated from nature, which could considerably facilitate the synthesis of complex glycopeptides and glycoproteins.

Glycan side chains on naturally presented MHC class II ligands

The molecular characterization of unknown naturally presented major histocompatibility complex (MHC) class II glycopeptides carrying complex glycans has so far not been achieved, reflecting the different fragmentation characteristics of sugars and peptides in mass spectrometric analysis. Human leukocyte antigen (HLA)-DR-bound peptides were isolated by affinity purification, separated via high performance liquid chromatography and analyzed by matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry. We were able to identify two naturally processed MHC class II ligands, CD53(122-136) and CD53(121-136), carrying complex N-linked glycan side chains by a combination of in-source and collision-induced fragmentation on a quadrupole time-of-flight tandem mass spectrometer.

Neural Cell Adhesion Molecule-associated Polysialic Acid Potentiates α-Amino-3-hydroxy-5-methylisoxazole-4-propionic Acid Receptor Currents

The highly negatively charged polysialic acid (PSA) is a carbohydrate predominantly carried by the neural cell adhesion molecule (NCAM) in mammals. NCAM and, in particular, PSA play important roles in cellular and synaptic plasticity. Here we investigated whether PSA modulates the activity of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtype of glutamate receptors (AMPA-Rs). Single channel recordings of affinity-purified AMPA-Rs reconstituted in lipid bilayers revealed that bacterially derived PSA, called colominic acid, prolonged the open channel time of AMPA-R-mediated currents by severalfold and altered the bursting pattern of the receptor channels but did not modify AMPA-R single channel conductance. This effect was reversible, concentration-dependent, and specific, since monomers of sialic acid and another negatively charged carbohydrate, chondroitin sulfate, did not potentiate single channel AMPA-R currents. Recombinant PSA-NCAM also potentiated currents mediated by reconstituted AMPA-Rs. In pyramidal neurons acutely isolated from the CA1 region of the early postnatal hippocampus, l-glutamate or AMPA (applied in the presence of antagonists blocking voltage-gated Na(+) and K(+) currents and N-methyl-d-aspartate and metabotropic glutamate receptors) induced inward currents, which were significantly increased by co-application of colominic acid. Chondroitin sulfate did not affect AMPA-R-mediated currents in CA1 neurons. The effect of colominic acid was age-dependent, since in pyramidal neurons from adult hippocampus, colominic acid failed to potentiate glutamate responses. Thus, our study demonstrates age-dependent potentiation of AMPA receptors by PSA via a mechanism probably involving direct PSA-AMPA-R interactions. This mechanism might amplify AMPA-R-mediated signaling in immature cells, thereby affecting their development.

The glycosylated cell surface protein Rpf2, containing a resuscitation-promoting factor motif, is involved in intercellular communication of Corynebacterium glutamicum

The genome of Corynebacterium glutamicum ATCC 13032 contains two genes, rpf1 and rpf2, encoding proteins with similarities to the essential resuscitation-promoting factor (Rpf) of Micrococcus luteus. Both the Rpf1 (20.4 kDa) and Rpf2 (40.3 kDa) proteins share the so-called Rpf motif, a highly conserved protein domain of approximately 70 amino acids, which is also present in Rpf-like proteins of other gram-positive bacteria with a high G+C content of the chromosomal DNA. Purification of the C. glutamicum Rpf2 protein from concentrated supernatants, SDS-PAGE and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry identified modified Rpf2 variants with increased or reduced mobility when compared with the calculated size of Rpf2. A Western blot-based enzyme immunoassay demonstrated glycosylation of the Rpf2 variants with higher molecular masses. Galactose and mannose were identified as two components of the oligosaccharide portion of the Rpf2 glycoprotein by capillary gas chromatography coupled to mass spectrometry. The Rpf2 protein was localized on the surface of C. glutamicum with the use of immuno-fluorescence microscopy. C. glutamicum strains with defined deletions in the rpf1 or rpf2 gene or simultaneous deletions in both rpf genes were constructed, indicating that the rpf genes are neither individually nor collectively essential for C. glutamicum. The C. glutamicum rpf double mutant displayed slower growth and a prolonged lag phase after transfer of long-stored cells into fresh medium. The addition of supernatant from exponentially growing cultures of the rpf double mutant, the wild type or C. glutamicum strains with increased expression of the rpf1 or rpf2 gene significantly reduced the lag phase of long-stored wild-type and rpf single mutant strains, but addition of purified His-tagged Rpf1 or Rpf2 did not. In contrast, the lag phase of the C. glutamicum rpf double mutant was not affected upon addition of these culture supernatants.

Antinociceptive structure-activity studies with enkephalin-based opioid glycopeptides

Development of opioid peptides as therapeutic agents has historically been limited due to pharmacokinetic issues including stability and blood-brain barrier (BBB) permeability. Glycosylation of opioid peptides can increase peptide serum stability and BBB penetration. To further define the requirements for optimizing in vivo antinociceptive potency following intravenous administration, we synthesized a series of enkephalin-based glycopeptides using solid phase 9-fluorenylmethyloxy carbamate methods. The compounds differed in the sixth and subsequent amino acid residues (Ser or Thr) and in the attached carbohydrate moiety. In vitro binding and functional smooth muscle bioassays indicated that the addition of mono- or disaccharides did not significantly affect the opioid receptor affinity or agonist activity of the glycopeptides compared with their unglycosylated parent peptides. All of the glycopeptides tested produced potent antinociceptive effects in male ICR mice following intracerebroventricular injection in the 55 degrees C tail-flick test. The calculated A(50) values for the Ser/Thr and monosaccharide combinations were all very similar with values ranging from 0.02 to 0.09 nmol. Selected compounds were administered to mice intravenously and tested for antinociception to indirectly assess serum stability and BBB penetration. All compounds tested produced full antinociceptive effects with calculated A (50) values ranging from 2.2 to 46.4 micromol/kg with the disaccharides having potencies that equaled or exceeded that of morphine on a micromoles per kilogram basis. Substitution of a trisaccharide or bis- and tris-monosaccharides resulted in a decrease in antinociceptive potency. These results provide additional support for the utility of glycosylation to increase central nervous system bioavailability of small peptides and compliment our ongoing stability and blood-brain barrier penetration studies.

The Role of N-Glycosylation in Function and Surface Trafficking of the Human Dopamine Transporter

The present study addressed the role of N-linked glycosylation of the human dopamine transporter (DAT) in its function with the help of mutants, in which canonical N-glycosylation sites have been removed (N181Q, N181Q,N188Q, and N181Q,N188Q,N205Q), expressed in human embryonic kidney-293 cells. Removal of canonical sites produced lower molecular weight species as did enzymatic deglycosylation or blockade of glycosylation, and all three canonical sites were found to carry sugars. Prevention of N-glycosylation reduced both surface and intracellular DAT. Although partially or non-glycosylated DAT was somewhat less represented at the surface, no evidence was found for preferential exclusion of such material from the plasma membrane, indicating that glycosylation is not essential for DAT expression. Non-glycosylated DAT was less stable at the surface as revealed by apparently enhanced endocytosis, consonant with weaker DAT immunofluorescence at the cell surface and stronger presence in cytosol in confocal analysis of the double and triple mutant. Non-glycosylated DAT did not transport dopamine as efficiently as wild-type DAT as judged from the sharp reduction in uptake V(max), and prevention of N-glycosylation enhanced the potency of cocaine-like drugs in inhibiting dopamine uptake into intact cells without changing their affinity for DAT when measured in membrane preparations prepared from these cells. Thus, non-glycosylated DAT at the cell surface displays appreciably reduced catalytic activity and altered inhibitor sensitivity compared with wild type.

Synthesis of NovelS-Neoglycopeptides from Glycosylthiomethyl Derivatives

Reaction of glycosylthiomethyl azides with amino acid and peptide derivatives containing aspartate and glutamate thio acids gave the corresponding glycosylthiomethyl amides in excellent yields. Another type of neoglycopeptides was obtained via reaction of glycosylthiomethyl bromide with cysteine and homocysteine containing peptide derivatives, thus affording the corresponding S-(glycosylthiomethyl) peptides.

Surface glycans of Candida albicans and other pathogenic fungi: physiological roles, clinical uses, and experimental challenges

Although fungi have always been with us as commensals and pathogens, fungal infections have been increasing in frequency over the past few decades. There is a growing body of literature describing the involvement of carbohydrate groups in various aspects of fungal disease. Carbohydrates comprising the cell wall or capsule, or as a component of glycoproteins, are the fungal cell surface entities most likely to be exposed to the surrounding environment. Thus, the fungus-host interaction is likely to involve carbohydrates before DNA, RNA, or even protein. The interaction between fungal and host cells is also complex, and early studies using whole cells or crude cell fractions often produced seemingly conflicting results. What was needed, and what has been developing, is the ability to identify specific glycan structures and determine how they interact with immune system components. Carbohydrate analysis is complicated by the complexity of glycan structures and by the challenges of separating and detecting carbohydrates experimentally. Advances in carbohydrate chemistry have enabled us to move from the foundation of composition analysis to more rapid characterization of specific structures. This, in turn, will lead to a greater understanding of how fungi coexist with their hosts as commensals or exist in conflict as pathogens.

The two envelope membrane glycoproteins of Tomato spotted wilt virus show differences in lectin-binding properties and sensitivities to glycosidases

Tomato spotted wilt virus (TSWV, Genus: Tospovirus, Family: Bunyaviridae) is a major constraint to the production of several different crops of agronomic and horticultural importance worldwide. The amino acid sequence of the two envelope membrane glycoproteins, designated as G(N) (N-terminal) and G(C) (C-terminal), of TSWV contain several tripeptide sequences, Asn-Xaa-Ser/Thr, suggesting that the proteins are N-glycosylated. In this study, the lectin-binding properties of the viral glycoproteins and their sensitivities to glycosidases were examined to obtain information on the nature of potential oligosaccharide moieties present on G(N) and G(C). The viral proteins were separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and probed by affinoblotting using a battery of biotinylated lectins with specificity to different oligosaccharide structures. G(C) showed strong binding with five mannose-binding lectins, four N-acetyllactosamine-binding lectins and one fucose-binding lectin. G(N) was resolved into two molecular masses and only the slow migrating form showed binding, albeit to a lesser extent than G(C), with three of the five mannose-binding lectins. The N-acetyllactosamine- and fucose-specific lectins did not bind to either molecular mass form of G(N). None of the galactose-, N-acetylgalactosamine-, or sialic acid-binding lectins tested showed binding specificity to G(C) or G(N). Treatment of the denatured virions with endoglycosidase H and peptide:N-glycosidase F (PNGase F) resulted in a significant decrease in the binding of G(C) to high mannose- and N-acetyllactosamine-specific lectins. However, no such differences in lectin binding were apparent with G(N). These results indicate the presence of N-linked oligosaccharides of high mannose- and complex-type on G(C) and possibly high mannose-type on G(N). Differences in the extent of binding of the two envelope glycoproteins to different lectins suggest that G(C) is likely to be more heavily N-glycosylated than G(N). No evidence was observed for the presence of O-linked oligosaccharides on G(N) or G(C).

Carbohydrate moieties can induce mediator release: a detailed characterization of two major timothy grass pollen allergens

Specific IgE binding to carbohydrate moieties of glycosylated allergens has been known for years, but the importance of these structures for the elicitation of allergic reactions is still a matter of debate. Because of their conserved carbohydrate structures, especially N-glycans have always been prime candidates for IgE cross-reactivity between allergens from unrelated species. The aim of our study was to determine whether carbohydrate structures on glycoproteins can by themselves elucidate allergic reactions. We characterized in detail the carbohydrate moieties of the major allergens Phl p 1 and Phl p 13 of timothy grass pollen (Phleum pratense L.) by performing tryptic digests followed by HPLC, N-terminal sequencing, sugar analysis, MALDI-TOF- and ESI-ICRFT-MS. Phl p 1 contains one N-glycan with one of the two glycoforms MMXF3 and M0XF3 and a single furanosidic arabinose, which is bound to a hydroxyproline residue in direct vicinity to the N-glycan. This O-glycosylation is probably due to an arabinosylation consensus sequence found in the N-terminal part of Phl p 1 and other group 1 allergens, but displayed no IgE-reactivity. Thus, Phl p 1 is monovalent with respect to its IgE-binding carbohydrate epitopes and showed no mediator release. In contrast, the carbohydrate moiety of Phl p 13, which carries four of the same N-glycans (like Phl p 1), can cross-link IgE-receptors via carbohydrate chains and elicits IL-4 release from basophils.

Structural analysis of permethylated oligosaccharides using electrospray ionization quadrupole time-of-flight tandem mass spectrometry and deutero-reduction

Deutero-reduced permethylated oligosaccharides were analyzed by electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) using a hybrid quadrupole orthogonal acceleration time-of-flight mass spectrometer, fitted with a nanoflow ESI source. Under these ionization conditions such derivatives preferentially form sodiated molecular species in addition to protonated molecular species. Under collision-induced dissociation, protonated and sodiated molecular species yield simple and predictable fragment mass spectra. A systematic study was conducted on a series of deutero-reduced permethylated glycans to allow rationalization of the fragmentation processes. MS/MS spectra were characterized by fragments resulting from the cleavage of glycosidic bonds. These fragments originating from both the reducing and the non-reducing ends of the glycan yield information on sequence and branching. Furthermore, the substituent 3-linked to a HexNAc unit was readily eliminated. Special attention was devoted to a systematic study of fucosylated glycans. The fucosylated deutero-reduced permethylated glycans were submitted to an acidic hydrolysis, releasing specifically the fucosyl residues. The nascent free hydroxyl groups were subsequently CD3-labelled in order to determine the positions initially bearing the fucosyl residues along the oligosaccharide backbone. This methodology was finally applied to characterize a glycan pool enzymatically released from glycoproteins. The present data show that structural elucidation can be achieved at the 50 fmol level.

Demonstration of multiple novel glycoforms of the stem cell survival factor CCg

We have investigated the presence of different glycoforms of cystatin C secreted by adult hippocampal rat-derived stem/progenitor cells (AHPs) into conditioned medium. A glycosylated form of cystatin C (CCg) has been identified previously in conditioned medium from AHPs as an autocrine/paracrine cofactor. Fibroblast growth factor-2 (FGF2) requires cooperation with CCg to support AHP survival at low density in vitro. The purpose of the present study was to investigate further if cystatin C consists of one glycoform or if several different glycoforms are secreted by AHPs in vitro. The presence of the glycoforms was studied using enzymatic deglycosylation in conjunction with gel electrophoresis and Western blotting. The glycoforms of cystatin C were isolated with a combination of gel electrophoresis and electroelution, yielding the intact glycoforms in liquid phase before enzymatic deglycosylation. Our results revealed several novel glycoforms, in contrast to previous publication. The results suggest that N- and O-linked glycans with sialic acid are attached to cystatin C. Furthermore, we have demonstrated that all glycoforms are present in conditioned medium after only 48 hr of culturing and that all nestin-positive AHPs are immunopositive against cystatin C. These findings suggest secretion of the glycoforms by cultured AHPs.

On the structural significance of the linkage region constituents of N-glycoproteins: an X-ray crystallographic investigation using models and analogs

The linkage region constituents, namely, 2-acetamido-2-deoxy-beta-D-glucopyranose and asparagine are conserved in the N-glycoproteins of all the eukaryotes. The present work is aimed at understanding the reasons for the occurrence of GlcNAc and Asn as the linkage region constituents. A total of six sugar amides have been designed as models and analogs of the linkage region and their crystal structures have been solved. This is the first report on the X-ray crystallographic investigation of the effect of systematic changes in the linkage sugar as well as its aglycon moiety on the N-glycosidic torsion, psi(N) (O5-C1-N1-C1(')). This also forms the first report on the crystal structure of a model of L-RhabetaAsn, a variant linkage found in the surface layer glycoprotein of Bacillus stearothermophillus. Among the models and analogs examined, the acetamido derivatives of Man and Xyl, the linkage sugars of O-glycoproteins, show a psi(N) value of -114.5 degrees and -121.2 degrees, respectively, deviating maximum from the value of -89.8 degrees reported for the model compound GlcNAcbetaNHAc. The L-Rha and Gal derivatives also show noticeable deviations. The psi(N) values, -89.5 degrees and -91.0 degrees, of the propionamide derivatives of Glc and GlcNAc (analogs of GlcbetaGln and GlcNAcbetaGln, respectively) agree well with those (-93.8 degrees and -89.8 degrees ) reported for their corresponding acetamide derivatives suggesting Gln could serve as well as Asn as the linkage region amino acid. However, the rotational freedom about the additional C-C bond would lead to altered rigidity of the linkage region. An analysis of packing reveals that the molecular assembly of these compounds is driven by different infinite and finite chains of hydrogen bonds. The double pillaring of hydrogen bonds involving the amide groups at C1 and C2 is seen as a unique packing feature characteristic of beta-1-N-acyl derivatives of GlcNAc. Based on the findings of the present study, it is speculated that the linkage region constituents of the eukaryotic N-glycoproteins appear to fulfill three essential structural requirements: rigidity, planarity, and linearity and these are met by the trisaccharide core and Asn at the linkage region.

Effect of pH, temperature and alcohols on the stability of glycosylated and deglycosylated stem bromelain

The biological significance of the carbohydrate moiety of a glycoprotein has been a matter of much speculation. In the present work, we have chosen stem bromelain from Ananas comosus as a model to investigate the role of glycosylation of proteins. Stem bromelain is a thiol protease which contains a single hetero-oligosaccharide unit per molecule. Here, the deglycosylated form of the enzyme was obtained by periodate oxidation. The differences in the glycosylated and deglycosylated forms of the glycoprotein have been studied at various temperatures and pH values, using probes such as loss of enzyme activity and by the changes in fluorescence and circular dichroism spectra. Deglycosylated bromelain showed decreased enzyme activity and perturbed fluorescence and circular dichroism spectra. In addition to this, a comparative study of their activities in different organic solvents showed a marked decrease in case of deglycosylated form of the enzyme. It is thus concluded that glycosylation contributes towards the functional stability of glycoenzymes.

Synthesis of an S-linked glycopeptide analog derived from human Tamm-Horsfall glycoprotein

Direct base catalyzed S-glycosylation of a cysteine and a homocysteine containing peptide with O-acetyl protected bromides in DMF-water solution furnished two glycopeptide fragments. The two glycopeptide fragments were linked to the target glycopeptide with two S-glycosyl residues mimicking a part of Tamm-Horsfall glycoprotein.

N-linked glycan structures of mouse interferon-β produced by Bombyx mori larvae

The full-length mouse interferon-beta (mIFN-beta) cDNA, including the secretion signal peptide coding region under control of the polyhedrin promoter, was introduced into Bombyx mori nucleopolyhedrovirus (BmNPV). Recombinant mIFN-beta (rmIFN-beta) was accumulated in the haemolymph of infected silkworm larvae. Western blot analysis showed isoforms of rmIFN-beta, suggesting that rmIFN-beta is glycosylated. The glycan structures of purified rmIFN-beta were determined. The N-glycans were liberated by hydrazinolysis and the resulting oligosaccharides were labeled with 2-aminopyridine. The pyridylaminated (PA) glycans were purified by gel filtration, reversed-phase HPLC, and size-fractionation HPLC. The structures of the PA-sugar chains were identified by a combination of two-dimensional PA-sugar chain mapping, MS analysis, and exoglycosidase digestions.

A surface exposed O-linked galactose residue destabilises the structure of a folded helix-loop-helix dimer

A 42-residue glycopeptide Tn-15 and the corresponding reference polypeptide Thr-15 were designed and synthesized to provide a model system for the study of how glycosylation affects the stability of a molten globule-like protein. Tn-15 and Thr-15 fold into hairpin helix-loop-helix motifs that dimerise to form four-helix bundles and the only difference between the sequences is that Tn-15 carries an O-linked N-acetylgalactosamine residue at the side chain of threonine-15 whereas the sequence Thr-15 is unglycosylated. An analysis of the mean residue ellipticities at 222 nm of the two polypeptides and of the alpha-H chemical shift deviations from random coil values showed that glycosylation reduced the helical content of the polypeptides and increased the dissociation constant of the helix-loop-helix dimer to form monomers. The pH dependencies of the helical content of Tn-15 and Thr-15 differed as that of Thr-15 was largely unaffected by pH in the range from pH 4 to pH 10, whereas Tn-15 lost almost half of the helical content at pH 4 upon raising the pH to 10. No single amino acid residue was found to ionize in a way that could explain the observed pH dependence of Tn-15. The temperature dependence of the mean residue ellipticity of Tn-15 revealed a surprising decrease in helicity at 278 K in comparison with that at 293 K, reminiscent of cold denaturation, that was not observed for the reference four-helix bundle Thr-15.

Synthesis of S-linked glycopeptides in aqueous solution

Investigation of direct S-glycosylation of homocysteine and cysteine containing peptides with O-acetyl protected glycosyl halides led under two-phase conditions in the presence of sodium carbonate as base to excellent results. Thus, from glucosyl bromide, galactosyl bromide, lactosyl bromide, sialyl chloride, and N-Troc protected 2-amino-2-deoxyglucosyl bromide S-glycosylated dipeptides 15, 18-21, 23, 24, and 26-29, respectively, were obtained in excellent yields. Alternatively, depending on the solubility of the peptide moiety, mixtures of DMF and water could be employed for successfully carrying out this reaction. Thus, S-glycosylated tripeptides 42-45 could be obtained. Combination of this method with chemical ligation was also successfully carried out.

2-amino-3-phenylpyrazine, a sensitive fluorescence prelabeling reagent for the chromatographic or electrophoretic determination of saccharides

2-Amino-3-phenylpyrazine is found to be a sensitive fluorescence labeling reagent for saccharides with a reducing end. The labeled monosaccharides show strong fluorescence under various pH conditions, and could be analyzed by both HPLC and HPCE techniques. Laser induced fluorescence detection is also applicable. Following derivatization with 2-amino-3-phenylpyrazine, six monosaccharides are separated by an HPCE system within 23 min in the calibration range of 5 or 10 fmol to 5 pmol (injection amount). The within-day and day-to-day precisions of the monosaccharide determinations are 3.83-4.86% (RSD) and 3.37-4.56% (RSD), respectively. This method was successfully applied to the determination of component monosaccharides in a glycoprotein, bovine serum fetuin.

Imparting bone affinity to glycoproteins through the conjugation of bisphosphonates

PURPOSE

To develop a novel means of conjugating bisphosphonates onto the carbohydrate moieties of glycoproteins to enhance protein affinity to bone.

METHODS

1-Amino-1,1-diphosphonate methane (aminoBP) was conjugated onto the carbohydrate moietites of oxidized fetuin by using 4-(maleimidomethyl)cyclohexane-1-carboxyl-hydrazide (MMCCH). Bone affinity of the resulting conjugates was compared to proteins obtained from another means of conjugation, whereby aminoBP was conjugated onto fetuin's lysine moieties by using succinimidyl-4-(N-maleimidomethyl)-cyclohexane-1-carboxylate (SMCC).

RESULTS

The use of the MMCCH resulted in the conjugation of up to seven aminoBPs per molecule of fetuin. These conjugates gave a 2.6-, 2.0-, 30.5-. and 1.84-fold increased affinity for untreated, ashed, demineralized bone and hydroxyapatite, respectively, as compared to conjugates from the SMCC reaction. Both conjugates exhibited a pH-independent, equally slow degradation in adult bovine serum-containing media.

CONCLUSION

The use of the MMCCH chemistry to conjugate aminoBP onto fetuin was feasible. Furthermore, the described processes of conjugation resulted in amino-BP-dependent increase in the glycoprotein's affinity to various bone matrices in a manner that exceeds the affinity produced by the previously established method, which used SMCC.

N-glycosylation sites on the nicotinic ACh receptor subunits regulate receptor channel desensitization and conductance

The present study investigated the effects of N-glycosylation sites on Torpedo acetylcholine (ACh) receptors expressed in Xenopus oocytes by monitoring whole-cell membrane currents and single-channel currents from excised patches. Receptors with the mutant subunit at the asparagine residue on the conserved N-glycosylation site (mbetaN141D, mgammaN141D, or mdeltaN143D) or the serine/threonine residue (mbetaT143A, mgammaS143A, or mdeltaS145A) delayed the rate of current decay as compared with wild-type receptors, and the most striking effect was found with receptors with mbetaT143A or mgammaS143A. For wild-type receptors, the lectin concanavalin A, that binds to glycosylated membrane proteins with high affinity, mimicked this effect. Receptors with mbetaN141D or mdeltaN143D exhibited lower single-channel conductance, but those with mbetaT143A, mgammaS143A, or mdeltaS145A otherwise revealed higher conductance than wild-type receptors. Mean opening time of single-channel currents was little affected by the mutation. N-glycosylation sites, thus, appear to play a role in the regulation of ACh receptor desensitization and ion permeability.

Transferrin microheterogeneity in pregnancies with preeclampsia

BACKGROUND

It has been reported that concentrations of serum transferrin (Tf) and its highly sialylated subfraction increase in normal pregnancy. This study investigated changes in the concentrations of serum transferrin and its subfractions in preeclampsia.

METHODS

The serum concentration of transferrin was determined by a standard turbidimetric assay and microheterogeneous transferrin subgroups (low sialylated, 4-sialo and highly sialylated transferrins) were assessed by crossed immuno-isoelectric focusing.

RESULTS

Compared to normal pregnancy, the concentrations of total, 4-sialo and highly sialylated transferrins decreased by 27%, 16% and 38%, respectively, in severe preeclampsia, while these values did not significantly decrease in mild preeclampsia. The concentration of low sialylated transferrin involving 2-sialo- and 3-sialo-transferrins significantly decreased both in mild and severe preeclampsia, the value in severe preeclampsia was even significantly lower than that in nonpregnant women. The serum concentrations of total and highly sialylated transferrins in preeclampsia were correlated positively with infant birth weights (r=0.587 and r=0.645, respectively).

CONCLUSIONS

The serum concentrations of total and highly sialylated transferrins in severe preeclampsia decrease significantly. This might have a negative impact on intrauterine growth. Additionally, the serum low sialylated transferrin decreases more sensitively in preeclampsia, although the concentration is low even in normal pregnancy.

Characterization of the functional role of the N-glycans in the AMPA receptor ligand-binding domain

The ligand-binding domains of AMPA receptor subunits carry two conserved N-glycosylation sites. In order to gain insight into the functional role of the corresponding N-glycans, we examined how the elimination of glycosylation at these sites (N407 and N414) affects the ligand-binding characteristics, structural stability, cell-surface expression, and channel properties of homomeric GluR-D (GluR4) receptor and its soluble ligand-binding domain (S1S2). GluR-D S1S2 protein expressed as a secreted protein in insect cells was found to be glycosylated at N407 and N414. No major differences in the ligand-binding properties were observed between the 'wild-type' S1S2 and non-glycosylated N407D/N414Q double mutant, or between S1S2 proteins expressed in the presence or absence of tunicamycin, an inhibitor of N-glycosylation. Purified glycosylated and non-glycosylated S1S2 proteins also showed similar thermostabilities as determined by CD spectroscopy. Full-length homomeric GluR-D receptor with N407D/N414Q mutation was expressed on the surface of HEK293 cells like the wild-type GluR-D. In outside-out patches, GluR-D and the N407D/N414Q mutant produced similar rapidly desensitizing current responses to glutamate and AMPA. We therefore report that the two conserved ligand-binding domain glycans do not play any major role in receptor-ligand interactions, do not impart a stabilizing effect on the ligand-binding domain, and are not critical for the formation and surface localization of homomeric GluR-D AMPA receptors in HEK293 cells.

Plasma membrane localization and fusion inhibitory activity of the cowpox virus serpin SPI-3 require a functional signal sequence and the virus encoded hemagglutinin

The cowpox virus (CPV) glycoprotein serpin SPI-3, a functional protease inhibitor, and the viral hemagglutinin (HA) are required to prevent fusion of wt CPV infected cells. SPI-3 and HA from CPV infected cells co-localize to the plasma membrane and are found in extracellular enveloped virus (EEV). We also show that an N-terminal SPI-3 signal sequence, but not glycosylation, is required for membrane localization and fusion inhibition. In the absence of HA (CPVDeltaHA), no SPI-3 is found on the membrane and infected cells fuse. Conversely, HA from both wt CPV and CPVDeltaSPI-3 infections is on the membrane, indicating a requirement of HA for SPI-3 plasma membrane localization. In the absence of HA, secretion of SPI-3 or SPI-3 N-glyc(-) was markedly enhanced, suggesting HA serves to retain SPI-3 on the plasma membrane,thereby preventing cell fusion.

Glycocalyx of lung epithelial cells

Due to their diversity and external location on cell membranes, glycans, as glycocalyx components, are key elements in eukaryotic cell, tissue, and organ homeostasis. Although information on the lung glycocalyx is scarce, this article aims to review, discuss, and summarize what is known about bronchoalveolar glycocalyx composition, mainly the sialic acids. It was deemed relevant, however, to make a brief introductory overview of the cell glycocalyx and its particular development in epithelial cells. After that, follows a summary of the evolution of the knowledge regarding the bronchoalveolar glycocalyx composition throughout the years, particularly its morphological features. Since sialic acids are located terminally on the bronchoalveolar lining cells' glycocalyx and play crucial roles, we focused mainly on the existing lung histochemical and biochemical data of these sugar residues, as well as their evolution throughout lung development. The functions of the lung glycocalyx sialic acids are discussed and interpretations of their roles analyzed, including those related to the negative overall superficial shield provided by these molecules. The increasing presence of these sugar residues throughout postnatal lung development should be regarded as pivotal in the development and maintenance of a dynamic bronchoalveolar architecture, supporting the normal histophysiology of the respiratory system. The case for a profound knowledge of lung glycocalyx--given its potential to provide answers to serious clinical problems--is made with particular reference to cystic fibrosis. Finally, concluding remarks and perspectives for future research in this field are put forth.

Synthesis of sugar arrays in microtiter plate

1,3-Dipolar cycloadditions between azides and alkynes were exploited to attach oligosaccharides to a C(14) hydrocarbon chain that noncovalently binds to the microtiter well surface. Synthesis of sugar arrays was performed on a micromolar scale in situ in the microtiter plate. As a model study, the beta-galactosyllipid 5 was displayed on a 4-micromol scale. Formation of product was confirmed via ESI-MS, and the yield was determined via chemical and biological assays. Several complex carbohydrates (6-16) were also displayed in microtiter plates and successfully screened with various lectins. Moreover, sialyl Lewis x (17) was synthesized via the enzymatic fucosylation of a precursor displayed in the plate. Studies on inhibition of this biotransformation have been carried out, and the IC(50) value found for the known inhibitor 20 was consistent with previous studies in solution.

Structural, quantitative and functional comparison of amyloid P component in sera from patients with systemic lupus erythematosus and healthy donors

Serum amyloid P component (SAP) is a serum protein that has a function as opsonin and is known to bind nuclear material with high affinity. Quantitative and/or qualitative deficiencies in SAP may possibly lead to the impairment of normal homoeostatic mechanisms of tissue turnover. Thus, SAP knockout mice display systemic lupus erythematosus (SLE)-like manifestations such as nephritis and circulating antinuclear antibodies. In the present study, we investigated whether there are changes in the structure, function or serum levels of SAP in serum from SLE patients as compared with those from healthy donors. We found that SAP in SLE sera has the same molecular mass as that of in the sera of normal individuals, when analysed by online immunoaffinity reversed phase mass spectrometry. Also, the serum levels of SAP did not differ significantly between the two groups. Finally, as an estimate of function, SAP from SLE patients appeared to have the same affinity for heparin and nucleosomes as SAP from normal individuals, when analysed by crossed affinity immunoelectrophoresis and enzyme-linked immunosorbent capture assay (ELISA). In conclusion, the data do not support alterations in the levels, structure or function of SAP circulating in SLE patients.

Syntheses of unnatural N-substituted UDP-galactosamines as alternative substrates for N-acetylgalactosaminyl transferases

UDP-GalNAc analogues with slight modifications in the 2-acetamido group of the GalNAc moiety are prepared in order to study their role in the mechanism of the N-acetylgalactosaminyl transferase mediated glycosylation step. The analogues with N-propionyl-, N-butyryl- and N-bromoacetyl-groups were synthesized, utilizing Khorana's morpholidate coupling method starting from D-galactosaminyl-1-phosphate after selective N-acylation of its amino group with the appropriate N-acyloxysuccinimides. Furthermore, in addition to UDP-galactosamine its 2-azido analogue has been efficiently prepared involving a metal catalyzed diazo transfer reaction.

Determination of linkages of linear and branched oligosaccharides using closed-ring chromophore labeling and negative ion trap mass spectrometry

Linear as well as branched oligosaccharides were labeled with p-aminobenzoic ethyl ester (ABEE) using the glycosylamine closed-ring labeling approach and analyzed by negative-ion electrospray ionization mass spectrometry (ESI-MS). Linkage specific fragment ions of ABEE labeled linear oligosaccharides were proposed based on the MS2 and MS3 data for several ABEE labeled linear oligosaccharides with known linkage configurations. Fragmentation at the reducing end was similar to that observed for ABEE disaccharides whereas the fragmentation pattern not involving the reducing end was similar to underivatized disaccharides. Based on these ions, all the linkages of linear oligosaccharides could be unambiguously determined. The fragmentation pattern at the branched sugar was in general not quite the same as the linear one. However, many linkage specific fragment ions were also observed for linkages at the branched sugar. These ions along with the ions proposed for linear oligosaccharides were found to be quite useful for the determination of all the linkages of branched oligosaccharides.

Ca2+ binding to bovine lactoferrin enhances protein stability and influences the release of bacterial lipopolysaccharide

Bovine lactoferrin (bLf) is known to damage the outer membrane of Gram-negative bacteria by binding to bacterial lipopolysaccharide (LPS). We report that LPS is released from bacterial outer membranes also when apo- or metal-saturated Lf is separated from bacterial cells by a dialysis membrane. This process occurs in phosphate-buffered saline with no added Ca2+ and Mg2+ and is hindered by addition of these cations. The effect of bLf is similar to that induced by EDTA and has been ascribed to chelation of Ca2+. In fact, it may be envisaged that Ca2+-binding sites on LPS have different affinities and that bLf can remove those ions that are more weakly bound. Ca2+ binding does not alter Lf iron-binding properties significantly or its UV and CD spectral features but brings about changes in the FT-IR bands due to carboxylate residues. Ca2+ binding is characterized by an apparent dissociation constant of 6 microM and a stoichiometry of 1.55 Ca2+ per Lf molecule; it enhances bLf stability towards chemical and thermal denaturation. The increase in stability takes place in both the apo- and iron-saturated forms but not in the desialilated protein, indicating that the carboxylate groups of the sialic acid residues present on two of the glycan chains are involved in Ca2+ binding.

Preliminary observations on the influence of rheumatoid alpha-1-acid glycoprotein on collagen fibril formation

This study investigates the effect of alpha(1)-acid glycoprotein (AGP) isolated from both normal and rheumatoid plasma on type II collagen fibril formation. Rheumatoid samples were obtained over 2 years from two patients with early arthritis. The glycosylation of each sample was analysed to establish any correlation with fibrillogenesis. Rheumatoid AGP displays increased fucosylation compared to normal AGP. In both patients the fucosylation dipped after 1 year, then rose again over year 2. It is proposed that year 1 corresponds to the acute phase of the disease and the onset of chronic inflammation after this time produces a subsequent increase in fucosylation. Rheumatoid AGP influences type II collagen fibrillogenesis. Native fibrils were produced but with differences in the rate and extent of fibrillogenesis depending on AGP concentration and fucosylation. Low concentrations produced a decrease in fibrillogenesis rate and fibril diameter. High concentrations produced fibrils at a rate and diameter dependent on fucosylation. Highly fucosylated AGP produced narrow fibrils slowly, whereas poorly fucosylated AGP produced thicker fibrils more quickly. We propose that differences in glycosylation (especially fucosylation) of AGP are responsible for differences in collagen fibrillogenesis and this phenomenon may contribute to the exacerbation of cartilage destruction in rheumatoid arthritis.