Protein glycosylation. Structural and functional aspects

Structure and functional significance of the carbohydrates of the LH/CG receptor

The LH/CG appears to contain 1-4 bi- or multi-antennary complex-type N-linked oligosaccharide side chains, which appear to locate apart from the hormone-binding regions. The exact sites to which the N-linked chains are attached remain to be delineated. The carbohydrates of the mature membrane-inserted receptor do not contribute to either specific high-affinity ligand-binding or signal transduction of the receptor. Thus, the polypeptide core of the receptor is responsible for both high affinity binding and dictating the hormone specificity. Moreover, the deglycosylated receptor, once inserted to the plasma membrane in a functionally mature form, retains its functional conformation or permits the conformational change that is required for coupling of the receptor to effector enzymes. Addition of oligosaccharides to the nascent LH/CG receptor but not their subsequent conversion to complex-type ones appears to be required for acquiring the hormone-binding conformation. On the other hand, neither addition of oligosaccharides to the nascent receptor, nor their further maturation are needed for the transport of the receptor to the plasma membrane. Thus, one function of the N-linked oligosaccharides in the LH/CG receptor appears to be to direct the proper folding of the receptor.

Exploring the substrate specificities of alpha-2,6- and alpha-2,3-sialyltransferases using synthetic acceptor analogues

The acceptor specificities of rat liver Gal(beta 1-4)GlcNAc alpha-2,6-sialyltransferase, recombinant full-length human liver Gal(beta 1-4)GlcNAc alpha-2,6-sialyltransferase, and a soluble form of recombinant rat liver Gal(beta 1-3/4)GlcNAc alpha-2,3-sialyltransferase were studied with a panel of analogues of the trisaccharide Gal(beta 1-4)GlcNAc(beta 1-2)Man(alpha 1-O)(CH2)7CH3. These analogues contain structural variants of D-galactose, modified at either C3, C4 or C5 by deoxygenation, fluorination, O-methylation, epimerization, or by the introduction of an amino group. In addition, the enantiomer of D-galactose is included. The alpha-2,6-sialyltransferases tolerated most of the modifications at the galactose residue to some extent, whereas the alpha-2,3-sialyltransferase displayed a narrower specificity. Molecular dynamics simulations were performed in order to correlate enzymatic activity to three-dimensional structure. Ineffective acceptors for rat liver alpha-2,6-sialyltransferase were shown to be inhibitory towards the enzyme; likewise, the alpha-2,3-sialyltransferase was found to be inhibited by all non-substrates. Modified sialyloligosaccharides were obtained on a milligram scale by incubation of effective acceptors with one of each of the three enzymes, and characterized by 500-MHz 1H-NMR spectroscopy.

Post-fertilization changes in the zona pellucida glycoproteins of rat eggs

The zona pellucida (ZP) is the extracellular coat surrounding the mammalian egg. Numerous evidence supports the role of ZP carbohydrate residues as the specific sperm receptors. In this study we used lectins to study different distribution patterns of carbohydrate residues in the rat ZP, and to follow changes at fertilization. ZP were collected from follicular, ovulated, and fertilized eggs, incubated with one of 11 different biotin-labeled lectins, followed by avidin-fluorescein isothiocyanate (FITC) complex, and visualized by epifluorescent microscopy. For electron microscope (EM) histochemistry, eggs were embedded in LR white and ultrathin sections were stained with the complex Ricinus communis lectin (RCA-1)-colloidal gold. Some lectins (RCA-I, Glycine max) bound to the entire ZP while others were restricted to the inner or outer zones [Griffonia simplicifolia, Concanovalia ensiformis, Triticum vulgaris (WGA), succinyl-WGA]. Other lectins (Lens culinaris, Ulex europhaeus) were totally excluded. The RCA-1 binding pattern changed following sperm penetration, from homogeneous in ZP of ovulated eggs (57%) to uneven in ZP of fertilized (71%) or activated (68%) eggs. Our results demonstrate an uneven distribution of different sugar residues in the rat ZP, and a post-fertilization change in the distribution of beta-galactose, which is specifically recognized by RCA-I, presumably correlated with other changes in the ZP that lead to the block to polyspermy.

Synthesis of Hex p-(1-->4)-beta-D-Glc pNAc-(1-->2)-alpha-D-Man p-(1-->O)(CH2)7CH3 probes for exploration of the substrate specificity of glycosyltransferases: Part I, Hex = beta-D-Gal, 4-deoxy-beta-D-Gal, 4-O-methyl-beta-D-Gal, 4-deoxy-4-fluoro-beta-D-Gal, or beta-D-Glc

Five trisaccharide derivatives designed for detailed exploration of the acceptor specificity of glycosyltransferases involved in termination of N-acetyllactosamine-type structures have been synthesized: beta-D-Gal p-(1-->4)-beta-D-Glc pNAc-(1-->2)-alpha-D-Man p-(1-->0)(CH2)7CH3 (1), 4-deoxy-beta-D-Gal p-(1-->4)-beta-D-Glc pNAc-(1-->2)-alpha-D-Man p-(1-->O)(CH2)7CH3 (2), 4-O-methyl-beta-D-Gal p-(1-->4)-beta-D-Glc pNAc-(1-->2)-alpha-D-Man p-(1-->O)(CH2)7CH3 (3), 4-deoxy-4-fluoro-beta-D-Gal p-(1-->4)-beta-D-Glc pNAc-(1-->2)-alpha-D-Man p(1-->O)(CH2)7CH3 (4), and beta-D-Glc p-(1-->4)-beta-D-Glc pNAc-(1-->2)-alpha-D-Man p-(1-->O)(CH2)7CH3 (5). A general disaccharide acceptor octyl 3,4,6-tri-O-benzyl-2-O-(3,6-di-O-benzyl-2-deoxy-2-phthalimido-beta-D -glucopyranosyl)-alpha-D-mannopyranoside was synthesized by condensation of 4-O-acetyl-3,6-di-O-benzyl-2-deoxy-2-phthalimido-alpha, beta-D-glucopyranosyl trichloroacetimidate with octyl 3,4,6-tri-O-benzyl-alpha-D-mannopyranoside, followed by deacetylation. 2,3,4,6-Tetra-O-acetyl-alpha-D-galactopyranosyl trichloroacetimidate and 2,3,4,6-tetra-O-acetyl-alpha-D-glucopyranosyl trichloroacetimidate were used as the glycosyl donors in the syntheses of 1 and 5. The modified galactosyl derivatives required subtle anomeric activation. Suitable donors for 2 turned out to be 2,3,6-tri-O-acetyl-4-deoxy-alpha-D-xylo-hexopyranosyl trichloroacetimidate and ethyl 2,3,6-tri-O-acetyl-4-deoxy-1-thio-alpha, beta-D-xylo-hexopyranoside; for 3, ethyl 2,3,6-tri-O-acetyl-4-O-methyl-1-thio-alpha, beta-D-galactopyranoside; and for 4, 2,3,6-tri-O-acetyl-4-deoxy-4-fluoro-alpha-D-galactopyranosyl trichloroacetimidate. It was concluded that thioglycosides were most appropriate for stereoselective coupling of activated synthons (carrying deoxy or O-methyl groups), whereas trichloroacetimidates gave high yields with deactivated (fluorine-containing) synthons.

Two lectin-like receptors for alpha 1-acid glycoprotein in mouse testis

Three glycoforms of alpha 1-acid glycoprotein (AGP) were biotinylated to examine their binding in mouse testis by light microscopy. The transition from one stage to another in the spermatogenic cycle is marked with an appearance of a receptor for the Concanavalin A (Con A) non-reactive glycoform AGP-A in the cytoplasm of spermatocytes, young spermatids and Sertoli cells. This receptor disappears in the late stages of the spermatids. The Con-A intermediately reactive and the Con-A reactive glycoforms, AGP-B and AGP-C, showed weak reaction in the cytoplasm of spermatocytes, spermatids and Sertoli cells and, at the last stages in the spermatogenic cycle, a very strong reaction in the late elongated spermatids and the apical extensions of Sertoli cells. The interactions are lectin-like as confirmed by inhibition with simple sugars. In addition, the bindings were inhibited by steroid hormones. AGP-A was inhibited by testosterone, oestradiol and progesterone, while AGP-B and AGP-C were inhibited by mannose, GlcNAc, cortisone, aldosterone, oestradiol and progesterone. The receptors and the corresponding AGP glycoforms may be adhesion molecules between Sertoli cells and the spermatogenic cells and may have a function as a regulatory factor for spermatozoa development.

The structural role of sugars in glycoproteins

To date, high resolution X-ray structures of about 30 glycoproteins have been reported that provide some structural information on glycans. Four solution structures of glycoproteins have been described over the past three years. In all four of these cases, it was shown that glycosylation is stabilizing the glycoprotein structures, indicating that this may be a general glycan function.

Primary cultures of cardiac muscle cells as models for investigation of protein glycosylation

Primary cardiac cell cultures of newborn rats containing approximately 50% (by cell number) spontaneously contracting cardiomyocytes were used to study the role of protein N-glycosylation for the binding of dihydropyridine (DHP) to the voltage-dependent L-type calcium channel. This binding is not influenced by the accompanying non-muscle cells. Exposure of the cells up to 6 micrograms/ml of the N-glycosylation inhibitor tunicamycin for a 44 h period resulted in a decrease of the specific DHP binding sites (Bmax) to 46.0 +/- 17.2% of the untreated control. Similar effects were observed after enzymatic deglycosylation using N-glycosidase F (PNGase F). The results suggest that a posttranslational modification of parts of the cardiac L-type Ca+2 channel by N-glycosylation is an important determinant for the binding of Ca+2 antagonists of the DHP-type to the alpha 1 subunit which itself is not glycosylated. The results suggest a participation of N glycosylation in the assembling of the subunits to the functional channel and/or its turnover. However, a possible effect of tunicamycin on the expression of the Ca channel as an alternative mechanism cannot be excluded.

Influence of glycosylation inhibitors on dihydropyridine binding to cardiac cells

In primary cultures of neonatal rat heart cells we found a linear correlation between the number of L-type calcium channel-specific dihydropyridine (DHP) binding sites and spontaneous beating frequency (v). Formation of glycoproteins in tissue culture was suppressed by different inhibitors of N-glycosylation. This inhibition alters to a different extent the binding of the DHP ligand (+)-[methyl-3H]PN 200-110 and v. The most severe but reversible effect occurs at 6 micrograms/ml tunicamycin (Bmax approximately 45% and v approximately 6%, resp., of control), a slight increase in Bmax at 0.1-0.5 mM castanospermine and 0.05-2.5 mM deoxymannojirimycin. The other inhibitors gave no significant alteration of Bmax.

Cloning, characterization, and expression of cDNAs encoding human delta 1-pyrroline-5-carboxylate dehydrogenase

Delta 1-pyrroline-5-carboxylate dehydrogenase (P5CDh; EC 1.5.1.12), a mitochondrial matrix NAD(+)-dependent dehydrogenase, catalyzes the second step of the proline degradation pathway. Deficiency of this enzyme is associated with type II hyperprolinemia (HPII), an autosomal recessive disorder characterized by accumulation of delta 1-pyrroline-5-carboxylate (P5C) and proline. As an initial step in understanding the biochemistry of human P5CDh and molecular basis of HPII, we utilized published peptide sequence data and degenerate primer polymerase chain reaction to clone two full-length human P5CDh cDNAs, differing in length by 1 kilobase pair (kb). Both cDNAs have the identical 1689-base pair open reading frame encoding a protein of 563 residues with a predicted molecular mass of 62 kDa. The long cDNA contains an additional 1-kb insert in the 3'-untranslated region that appears to be an alternatively spliced intron. The conceptual translation of human P5CDh has 89% sequence identity with the published human P5CDh peptide sequences and 42 and 26% identity with Saccharomyces cerevisiae and Escherichia coli P5CDhs, respectively, as well as homology to several other aldehyde dehydrogenases. Both P5CDh cDNA clones detect a single 3.2-kb transcript on Northern blots of multiple human tissues, indicating the long cDNA containing the 3'-untranslated intron represents the predominant transcript. The P5CDh structural gene appears to be single copy with a size of about 20 kb localized to chromosome 1. To confirm the identity of the putative P5CDh cDNAs, we expressed them in a P5CDh-deficient strain of S. cerevisiae. Both conferred measurable P5CDh activity and the ability to grow on proline as a sole nitrogen source.

Chemoselective Removal of Protecting Groups from O-Glycosyl Amino Acid and Peptide (Methoxyethoxy)ethyl Esters Using Lipases and Papain

The selective C-terminal deprotection of O-glycopeptide (methoxyethoxy)ethyl esters is achieved under mild conditions (pH 6.6, 37 degrees C) by enzymatic hydrolysis using papain or lipase M from Mucor javanicus to give building blocks useful for chain-extending glycopeptide synthesis. On the other hand, the selective removal of acetyl protecting groups from the saccharide portion of glycopeptides is accomplished by alternative enzymatic hydrolysis with lipase WG from wheat germ to furnish model substrates for enzymatic glycosyl transfer reactions in order to extend the carbohydrate side chain of these conjugates.

Development of electrophoretic conditions for the characterization of protein glycoforms by capillary electrophoresis-electrospray mass spectrometry

A capillary electrophoresis (CE) method using acidic buffers and capillaries coated with Polybrene, a cationic polymer has been developed for the separation of glycoproteins and glycopeptides. Electrophoretic conditions have been optimized to provide resolution of individual glycoforms observed for different glycoprotein preparations. These conditions were found to be entirely compatible with the operation of electrospray mass spectrometry (ESMS), which facilitated the assignments of possible carbohydrate compositions of glycopeptides arising from digests of glycoproteins. By using operating conditions enhanced the formation of oxonium fragment ions prior to mass spectral analysis, selective identification of glycopeptides was achieved for complex samples such as those from proteolytic digests or chemical cleavages. Examples of applications are presented for ribonuclease B, ovalbumin, horseradish peroxidase, and a lectin from Erithrina corallodendron using both CE-ESMS and CE with ultraviolet detection (CE-UV).

Synthesis, modifications, and turnover of proteins during aging

Slowing down of bulk protein synthesis is one of the most commonly observed biochemical changes during aging. The implications and consequences of slower rates of protein synthesis are manifold, including a decrease in the availability of enzymes for the maintenance, repair, and normal metabolic functioning of the cell, an inefficient removal of inactive, abnormal, and damaged macromolecules in the cell, the inefficiency of the intracellular and intercellular signalling pathways, and a decrease in the production and secretion of hormones, antibodies, neurotransmitters, and the components of the extracellular matrix. Age-related changes in the activity, specificity, and stability of a large number of proteins have been reported. However, the molecular mechanisms responsible for such alterations are still poorly understood. Studies on various components of the protein synthetic machinery have revealed a decline in the efficiency and accuracy of ribosomes, an increase in the levels of rRNA and tRNA, and a decrease in the amounts and activities of elongation factors. Because posttranslational modifications of proteins determine their activity and stability, alterations in the extent and level of various modifications such as phosphorylation, methylation, ADP-ribosylation, oxidation, glycation, and conformational changes during aging are being studied. Changes in the regulation of protein synthesis, posttranslational modifications, and protein turnover are crucial determinants of age-related decline in the maintenance, repair, and survival of the organism.

N-glycans modulate in vivo and in vitro thyroid hormone synthesis. Study at the N-terminal domain of thyroglobulin

Thyroglobulin (Tg) is the substrate for thyroid hormone biosynthesis, which requires tyrosine iodination and iodotyrosine coupling and occurs at the apical membrane of the thyrocytes. Tg glycoconjugates have been shown to play a major role in Tg routing through cellular compartments and recycling after endocytosis. Here we show that glycoconjugates also play a direct role in hormonosynthesis. The N-terminal domain (NTD; Asn1-Met171) of human Tg, which bears the preferential hormonogenic site, brings two N-glycans (Asn57 and Asn91). NTD preparations were purified from Tg with low and mild iodine content in vivo and from poorly iodinated Tg after in vitro iodination and coupling. NTD separated from poorly iodinated Tg was also submitted to iodination and coupling after desialylation and deglycosylation. The various NTD isoforms were analyzed for their N-glycan structures and hormone contents. Our results show that 1) in vivo as well as in vitro unglycosylated isoforms did not synthesize hormones, whereas fully or partially (at Asn91) glycosylated isoforms did; 2) high mannose type structures enhanced the hormone content; and 3) desialylation did not affect in vitro hormone synthesis. Evidence of a direct involvement in hormonosynthesis adds to the role of N-glycans in Tg function and opens the way to new mechanisms for regulation (e.g. TSH modulation of N-glycan) or alteration (e.g. Asn91 mutation) of thyroid hormone synthesis.

Synthetic substrate analogues for UDP-GlcNAc: Man alpha 1-3R beta 1-2-N-acetylglucosaminyltransferase I. Substrate specificity and inhibitors for the enzyme

UDP-GlcNAc:Man alpha 1-3R beta 1-2-N-acetylglucosaminyltransferase I (GlcNAc-T I; EC 2.4.1.101) catalyses the conversion of [Man alpha 1-6(Man alpha 1-3)Man alpha 1-6][Man alpha 1-3]Man beta-O-R to [Man alpha 1-6(Man alpha 1-3)Man alpha 1-6] [GlcNAc beta 1-2Man alpha 1-3]Man beta-O-R (R = 1-4GlcNAc beta 1-4GlcNAc- Asn-X) and thereby controls the conversion of oligomannose to complex and hybrid asparagine-linked glycans (N-glycans). GlcNAc-T I also catalyses the conversion of Man alpha 1-6(Man alpha 1-3)Man beta-O-octyl to Man alpha 1-6(GlcNAc beta 1-2Man alpha 1-3)Man beta-O-octyl. We have therefore tested a series of synthetic analogues of Man"alpha 1-6(Man'alpha 1-3)Man beta-O-octyl as substrates and inhibitors for rat liver GlcNAc-T I. The 2"-deoxy and the 3"-, 4"- and 6"-O-methyl derivatives are all good substrates confirming previous observations that the hydroxyl groups of the Man"alpha 1-6 residue do not play major roles in the binding of substrate to enzyme. In contrasts, all four hydroxyl groups on the Man'alpha 1-3 residue are essential since the corresponding deoxy derivatives either do not bind (2'- and 3'-deoxy) or bind very poorly (4'- and 6'-deoxy) to the enzyme. The 2'- and 3'-O-methyl derivatives also do not bind to the enzyme. However, the 4'-O-methyl derivative is a substrate (KM = 2.6 mM) and the 6'-O-methyl compound is a competitive inhibitor (Ki = 0.76 mM). We have therefore synthesized various 4'- and 6'-O-alkyl derivatives, some with reactive groups attached to an O-pentyl spacer, and tested these compounds as reversible and irreversible inhibitors of GlcNAc-T I. The 6'-O-(5-iodoacetamido-pentyl) compound is a specific time dependent inhibitor of the enzyme. Four other 6'-O-alkyl compounds showed competitive inhibition while the remaining compounds showed little or no binding indicating that the electronic properties of the attached O-pentyl groups influence binding.

Studies of the bound conformations of methyl alpha-lactoside and methyl beta-allolactoside to ricin B chain using transferred NOE experiments in the laboratory and rotating frames, assisted by molecular mechanics and dynamics calculations

The conformation in solution of methyl beta-galactopyranosyl-(1-->4)-alpha-glucopyranoside (methyl alpha-lactoside) and methyl beta-galactopyranosyl-(1-->6)-beta-glucopyranoside (methyl beta-allolactoside) has been studied through NMR spectroscopy and molecular mechanics calculations. NOE measurements both in the laboratory and rotating frames, have been interpreted in terms of an ensemble average distribution of conformers. Molecular mechanics calculations have been performed to estimate the probability distribution of conformers from the steric energy maps. The experimental results indicate that methyl alpha-lactoside spends about 90% of its time in a broad low-energy region close to the global minimum, while methyl beta-allolactoside presents much higher flexibility. The conformational changes that occur when both disaccharides are bound to the ricin B chain in aqueous solution have been studied using transferred NOE experiments at several protein/ligand ratios. The observed data indicate that the protein causes a conformational variation in the torsion angles of methyl alpha-lactoside changing towards smaller angle values (phi/psi approximately -20/-20), although the recognized conformer is still within the lowest energy region. In particular, the torsional changes separate Gal H1 from Glc H3 and Glc H6 protons, with a noticeable decrease in the intensities of the corresponding NOE cross-peaks, which were clearly observed for the free disaccharide. On the other hand, different conformations around the phi, psi, and omega glycosidic bonds of methyl beta-allolactoside are recognized by the lectin. In fact, for the methyl-beta-allolactoside-ricin-B complex, only the NOESY cross-peaks corresponding to the protons of the galactose residue are negative, as expected for a molecule in the slow motion regime. In contrast, the corresponding cross peaks for the glucose residue were about zero, as expected for a molecule whose motion is practically independent of the protein. However, for the methyl-alpha-lactoside-ricin-B complex, all the NOESY cross-peaks for both the galactose and glucose moieties were clearly negative. From the NMR experimental point of view, it is demonstrated that the comparison of longitudinal and transversal transferred NOEs allows one to clearly differentiate direct enhancements from spin diffusion effects, which are of major concern when analysing NOE spectra of macromolecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and cDNA cloning of a human UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase

A UDP-GalNAc:polypeptide N-acetylgalactosaminyl-transferase (GalNAc-transferase) from human placenta was purified to apparent homogeneity using a synthetic acceptor peptide as affinity ligand. The purified GalNAc-transferase migrated as a single band with an approximate molecular weight of 52,000 by reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Based on a partial amino acid sequence, the cDNA encoding the transferase was cloned and sequenced from a cDNA library of a human cancer cell line. The cDNA sequence has a 571-amino acid coding region indicating a protein of 64.7 kDa with a type II domain structure. The deduced protein sequence showed significant similarity to a recently cloned bovine polypeptide GalNAc-transferase (Homa, F.L., Hollanders, T., Lehman, D.J., Thomsen, D.R., and Elhammer, A.P. (1993) J. Biol. Chem. 268, 12609-12616). A polymerase chain reaction construct was expressed in insect cells using a baculovirus vector. Northern analysis of eight human tissues differed clearly from that of the bovine GalNAc-transferase. Polymerase chain reaction cloning and sequencing of the human version of the bovine transferase are presented, and 98% similarity at the amino acid level was found. The data suggest that the purified human GalNAc-transferase is a novel member of a family of polypeptide GalNAc-transferases, and a nomenclature GalNAc-T1 and GalNAc-T2 is introduced to distinguish the members.

UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase. Identification and separation of two distinct transferase activities

Using a defined acceptor substrate peptide as an affinity chromatography ligand we have developed a purification scheme for a unique human polypeptide, UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (GalNAc-transferase) (White, T., Bennett, E.P., Takio, K., Sørensen, T., Bonding, N., and Clausen, H. (1995) J. Biol. Chem. 270, 24156-24165). Here we report detailed studies of the acceptor substrate specificity of GalNAc-transferase purified by this scheme as well as the Gal-NAc-transferase activity, which, upon repeated affinity chromatography, evaded purification by this affinity ligand. Using a panel of acceptor peptides, a qualitative difference in specificity between these separated transferase activities in four rat organs and two human organs also revealed qualitative differences in specificity. The results support the existence of multiple Gal-NAc-transferase activities and suggest that these are differentially expressed in different organs. As the number of GalNAc-transferases existing is unknown, as is the specificity of the until now cloned and expressed GalNAc-transferases (T1 and T2), it is as yet impossible to relate the results obtained to specific enzyme proteins. The identification of acceptor peptides that can be used to discriminate GalNAc-transferase activities is an important step toward understanding the molecular basis of GalNAc O-linked glycosylation in cells and organs and in pathological conditions.

An investigation into the role of N-glycosylation in the functional expression of a recombinant heteromeric NMDA receptor

The effect of N-glycosylation on the assembly of N-methyl-D-aspartate (NMDA) heteromeric cloned receptors was studied. Thus human embryonic kidney (HEK) 293 cells were cotransfected with N-methyl-D-aspartate R1 (NR1) and N-methyl-D-aspartate R2A (NR2A) clones and the cells grown post-transfection in the presence of tunicamycin (TM). TM treatment resulted in a decrease of the NR1 subunit with M(r) 117 000 with a concomitant increase in a M(r) 97 000 immunoreactive species previously identified as the non-N-glycosylated NR1 subunit. In parallel, TM caused a dose-dependent inhibition of [3H]MK801 binding to the expressed receptor which was a result of an approximate four-fold reduction in the Dissociation Constant (KD) but with no change in the number of binding sites (Bmax). NMDA receptor cell surface expression was unchanged following TM treatment but it did result in a decrease in the percentage cell death post-transfection compared to control samples. The removal of TM from the cell culture media resulted in a return to the control KD value for [3H]MK801 binding and partial reglycosylation of newly synthesized NR1 subunit. These results demonstrate that N-glycosylation is requisite for the efficient expression of functional NR1/NR2A receptors. Furthermore, they suggest that N-glycosylation may be important for the correct formation of the channel domain of the NR1/NR2A receptor.

Production and properties of three pectinolytic activities produced by Aspergillus niger in submerged and solid-state fermentation

Three extracellular pectinases were produced by Aspergillus niger CH4 by submerged and solid-state fermentation, and their physicochemical and kinetic properties were studied. The highest productivities of endo- and exo-pectinase and pectin lyase were obtained with solid-state fermentation. The kinetic and physicochemical properties of these enzymes were influenced by the type of culture method used. All activities were very different in terms of pH and temperature optima, stability at different pH and temperature values and affinity for the substrate (Km values). In solid-state fermentation, all pectinase activities were more stable at extreme pH and temperature values but the Km values of endo-pectinase and pectin lyase were higher with respect to those activities obtained by the submerged-culture technique. The pectin lyase activity obtained by the submerged-culture technique showed substrate inhibition but the enzyme obtained by solid-state fermentation did not. Electrophoresis, using sodium dodecyl sulphate/polyacrylamide gel with enzymatic extracts obtained for both culture methods, showed the same number of protein bands but some differences were found in their electrophoretic position. The results obtained in this work suggest that the culture method (submerged or solid-state) may be responsible for inducing changes in some of the pectinolytic enzymes produced by A. niger.

Enamel matrix protein turnover during amelogenesis: basic biochemical properties of short-lived sulfated enamel proteins

The formation and turnover of sulfated enamel proteins was investigated by SDS-PAGE, fluorography, and TCA-precipitations using freeze-dried incisors of rats injected intravenously with 35S-sulfate (35SO4) and processed at various intervals from 1.6 minutes to 4 hours thereafter. Some rats were injected first with 35SO4 followed 5 minutes later by 0.3 mg of cycloheximide. This was done to terminate protein translation and allow events related to extracellular processing and degradation of the sulfated enamel proteins to be visualized more distinctly. Other rats were injected with cycloheximide followed at 0 minutes (simultaneous injection) to 30 minutes later by 35SO4. This was done to characterize the time required for proteins to travel from endoplasmic reticulum to Golgi apparatus, where they became sulfated. The results indicated that enamel organ cells (ameloblasts) rapidly incorporated 35SO4 into a major approximately 65 kDa protein that was secreted into the enamel within 6-7.5 minutes. This parent protein appeared to be processed extracellularly within 15 minutes into major approximately 49 kDa and approximately 25 kDa fragments which themselves had apparent half-lives of about 1 and 2 hours, respectively. There were also many minor sulfated fragments varying in molecular weight (Mr) from approximately 13-42 kDa, which appeared to originate from extracellular processing and/or degradation of the parent approximately 65 kDa sulfated enamel protein or its major approximately 49 kDa and approximately 25 kDa fragments. Experiments with glycosidases further suggested that the majority of sulfate groups were attached to sugars N-linked by asparagine to the core of the approximately 65 kDa sulfated enamel protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

Extra-nuclear location of histones in activated human peripheral blood lymphocytes and cultured T-cells

Dextrin-2-sulphate (D2S) is a sulphated polysaccharide which inhibits human immunodeficiency virus type 1 infection of T-cells by binding to the cell surface. During our investigations of the nature of this interaction, a cell membrane fraction was prepared by ultracentrifugation from the T-cell line, HPB-ALL. Separation of membrane proteins by sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis and analysis for binding proteins using ligand blotting showed that 3H-D2S bound, in a saturable and displaceable manner, to two regions corresponding to molecular weights of 14,000-18,000 and 28,000-32,000. The N-terminal sequences of two of the major protein components in the 14,000-18,000 region were consistent with those of histones H2B and H3. The presence of histone H2B in the cell membrane preparation was confirmed by immunoblotting and enzyme-linked immunosorbent assay using a specific antibody. Histone standards were used to determine the level of each histone in the cell membrane fraction. In addition, the binding of 3H-D2S to purified histone standards was quantified. These results show that all of the binding of 3H-D2S to proteins in the 14,000-18,000 region of the cell membrane preparation can be attributed to the histones present. In contrast to HPB-ALL cells, a cell membrane fraction from freshly isolated human peripheral blood lymphocytes contained very low levels of histones. However, after culture with phytohaemagglutinin for 3 days the cell membrane fraction contained greatly increased levels of histones. To exclude the possibility of contamination of the cell membrane preparation with histones derived from the nucleus, cell membranes were also prepared using an affinity-based method using polyethyleneimine-cellulose. Immunoblotting of adsorbed plasma membranes showed the presence of histone H2B. SDS-polyacrylamide gels stained for protein also indicated that the preparation contained histones H1, H2A, H3 and H4. In further experiments whole cells were used to avoid contamination from nuclear proteins. Lactoperoxidase mediated 125I labelling, a method specific for radiolabelling cell surface proteins, confirmed the presence of histones H2B, H3 and H4 on the surface of HPB-ALL cells. Also, incubation of HPB-ALL cells or phytohaemagglutinin-activated peripheral blood lymphocytes with D2S caused displacement of histones from the cell surface into the supernatant without altering cell viability. In addition, immunocytochemistry of freshly isolated peripheral blood lymphocytes showed that histone H2B was located predominantly in the nucleus. However, in phytohaemagglutinin-activated peripheral blood lymphocytes immunoreactive material was also prominent in the endoplasmic reticulum and on the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of N-glycosylation sites and functional role of the carbohydrate units of GLAST-1, a cloned rat brain L-glutamate/L-aspartate transporter

The L-glutamate transporter GLAST-1 belongs to the newly discovered family of Na(+)-dependent, high-affinity glutamate transporters, which are involved in the regulation of synaptic excitatory neurotransmitter concentration in mammalian brain. The members of this family have a similar topological organisation with at least six transmembrane helices (TMHs) and two putative N-glycosylation sites located in the extracellular loop connecting TMH 3 and TMH 4. Besides these two conserved N-glycosylation motifs at Asn206 and Asn216, GLAST-1 possesses an additional one at Asn35. The putative N-glycosylation consensus motifs (Asn-Xaa-Ser/Thr) were deleted by replacement of Asn206 and/or Asn216 by Thr using site-directed mutagenesis (mutants N206T, N216T and N206,216T). The cDNAs encoding wild-type GLAST-1 and the three glycosylation-defective transport proteins were expressed in the Xenopus laevis oocyte system. Immunoprecipitation of the [35S]methionine-labeled and glycopeptidase-F-treated transporter molecules indicates that GLAST-1 is glycosylated at Asn206 and Asn216, whereas Asn35 remains unglycosylated. To assess a possible functional role of the two glycosylation sites wild-type and glycosylation-deficient GLAST-1 were expressed in Xenopus oocytes and characterized functionally by using the whole-cell voltage-clamp technique. The results prove that N-glycosylation has no impact on the transport activity of GLAST-1.

Fluorophore-assisted carbohydrate electrophoresis technology and applications

Carbohydrates, in particular the complex carbohydrates conjugated to proteins and lipids, have important functions in a variety of biological systems. Their isolation and structural determination--prerequisites for elucidation of their biological functions--have been technical challenges for many decades. Almost all available chromatographic and electrophoretic methods as well as NMR and MS have been applied to carbohydrate analysis but none has proved satisfactory in terms of simplicity, sensitivity, reproducibility, cost and requirement for materials. Recently, a technique called fluorophore-assisted carbohydrate electrophoresis was developed which is very promising. It separates fluorescently-labeled carbohydrates on polyacrylamide gels and uses a charge-coupled device camera to detect and quantitate the products. This review describes the principles of the method and its applications to several aspects of research on carbohydrate-containing biological biomolecules.

Structural conservation of ion conduction pathways in K channels and glutamate receptors

Single channel recordings demonstrate that ion channels switch stochastically between an open and a closed pore conformation. In search of a structural explanation for this universal open/close behavior, we have uncovered a striking degree of amino acid homology across the pore-forming regions of voltage-gated K channels and glutamate receptors. This suggested that the pores of these otherwise unrelated classes of channels could be structurally conserved. Strong experimental evidence supports a hairpin structure for the pore-forming region of K channels. Consequently, we hypothesized the existence of a similar structure for the pore of glutamate receptors. In ligand-gated channels, the pore is formed by M2, the second of four putative transmembrane segments. A hairpin structure for M2 would affect the subsequent membrane topology, inverting the proposed orientation of the next segments, M3. We have tested this idea for the NR1 subunit of the N-methyl-D-aspartate receptor. Mutations that affected the glycosylation pattern of the NR1 subunit localize both extremes of the M3-M4 linker to the extracellular space. Whole cell currents and apparent agonist affinities were not affected by these mutations. Therefore it can be assumed that they represent the native transmembrane topology. The extracellular assignment of the M3-M4 linker challenged the current topology model by inverting M3. Taken together, the amino acid homology and the new topology suggest that the pore-forming M2 segment of glutamate receptors does not transverse the membrane but, rather, forms a hairpin structure, similar to that found in K channels.

Expression and characterization of the zeta 1 subunit of the N-methyl-D-aspartate (NMDA) receptor channel in a baculovirus system

Using a baculovirus expression vector system, the zeta 1 subunit of the mouse N-methyl-D-aspartate (NMDA) receptor channel was expressed in Spodoptera frugiperda insect cells. The peptide corresponding to the C-terminus of the zeta 1 subunit was synthesized by using the multiple antigen peptide (MAP) system, and an antibody to the synthetic peptide was produced. Immunoblotting using the newly developed antibody revealed the major 122-kDa and the minor 104-kDa protein bands. The effect of tunicamycin on the immunoblots and [35S]methionine/[35S]cysteine metabolic radiolabeling suggested that the two bands corresponded to glycosylated and non-N-glycosylated forms, respectively. Membranes prepared from insect cells infected with the recombinant virus had the binding activity of antagonist ligand 5,7-[3-3H]dichlorokynurenate (DCKA) of a glycine recognition domain of the receptor. Both immunofluorescence labeling and the [3H]DCKA binding assays also showed a greater level of expression (Bmax = 51 pmol/mg protein) in the insect cells. The ligand binding characteristics of the receptors expressed in insect cells suggested that the single zeta 1 subunit protein has glycine antagonist binding properties comparable to those of the native NMDA receptor channels. The lack of DCKA-binding activity of the non-N-glycosylated NMDA receptor expressed in the presence of tunicamycin suggested that N-linked oligosaccharide is essentially required for expression of a functional receptor in insect cells. This is the first report describing the importance of N-glycosylation for the acquisition of ligand binding to NMDA receptor channel subunit protein.

Localization of N-glycosylation sites and functional role of the carbohydrate units of GLAST-1, a cloned rat brain L-glutamate/L-aspartate transporter

The L-glutamate transporter GLAST-1 belongs to the newly discovered family of Na(+)-dependent, high-affinity glutamate transporters, which are involved in the regulation of synaptic excitatory neurotransmitter concentration in mammalian brain. The members of this family have a similar topological organisation with at least six transmembrane helices (TMHs) and two putative N-glycosylation sites located in the extracellular loop connecting TMH 3 and TMH 4. Besides these two conserved N-glycosylation motifs at Asn206 and Asn216, GLAST-1 possesses an additional one at Asn35. The putative N-glycosylation consensus motifs (Asn-Xaa-Ser/Thr) were deleted by replacement of Asn206 and/or Asn216 by Thr using site-directed mutagenesis (mutants N206T, N216T and N206,216T). The cDNAs encoding wild-type GLAST-1 and the three glycosylation-defective transport proteins were expressed in the Xenopus laevis oocyte system. Immunoprecipitation of the [35S]methionine-labeled and glycopeptidase-F-treated transporter molecules indicates that GLAST-1 is glycosylated at Asn206 and Asn216, whereas Asn35 remains unglycosylated. To assess a possible functional role of the two glycosylation sites wild-type and glycosylation-deficient GLAST-1 were expressed in Xenopus oocytes and characterized functionally by using the whole-cell voltage-clamp technique. The results prove that N-glycosylation has no impact on the transport activity of GLAST-1.

Glycosylation of human truncated Fc epsilon RI alpha chain is necessary for efficient folding in the endoplasmic reticulum

The high affinity immunoglobulin E (IgE) receptor is an alpha beta gamma 2 tetrameric complex. The truncated extracellular segment (alpha t) of the heavily glycosylated alpha chain is sufficient for high affinity binding of IgE. Here we have expressed various alpha t mutants in eukaryotic and prokaryotic cells to analyze the role of glycosylation in the folding, stability, and secretion of alpha t. All seven N-linked glycosylation sites in alpha t are glycosylated and their mutations have an additive effect on the folding and secretion of alpha t. Mutation of the seven N-glycosylation sites (delta 1-7 alpha t) induces misfolding and retention of alpha t in the endoplasmic reticulum. Similarly, tunicamycin treatment reduces substantially the folding efficiency of wild-type alpha t. In contrast, no difference in folding efficiency is detected between wild-type alpha t and delta 1-7 alpha t expressed in Escherichia coli. In addition, maturation of N-linked oligosaccharides and addition of O-linked carbohydrates are not required for either the transport or the IgE-binding function of alpha t. Furthermore, complete enzymatic deglycosylation does not affect the stability and the IgE-binding capacity of alpha t. Therefore, glycosylation is not intrinsically necessary for proper folding of alpha t but is required for folding in the endoplasmic reticulum. Our data are compatible with the concept that specific interactions between N-linked oligosaccharides and the folding machinery of the endoplasmic reticulum are necessary for efficient folding of alpha t in eukaryotic cells.

Interferon-gamma receptor extracellular domain-IgG fusion protein produced in Chinese hamster ovary cells as mixture of glycoforms

Glycosylation of proteins fulfills important functions and because of its diversity contributes to apparent protein heterogeneity. We investigated the heterogeneity of a fusion protein comprising the extracellular domain of the human interferon-gamma (IFN-gamma) receptor and parts of the human IgG3 constant region, a potential IFN-gamma antagonist. The protein was produced in Chinese hamster ovary (CHO) cells and was secreted into the culture medium as an 175 kD glycoprotein. Glycosylation represented approximately one-third of the apparent molecular mass of the fusion protein, consisted of N- and O-linked carbohydrate moieties, and included sialic acid residues as part of both N- and O-linked oligosaccharides. Fusion protein forms with different apparent molecular masses and charges were separated by ion-exchange chromatography. Preparative electrofocusing revealed a wide spectrum of glycoforms. Glycosylation of the fusion protein and of soluble IFN-gamma receptors, comprising the extracellular domain of the native sequence, expressed in insect and CHO cells did not interfere with affinity of ligand binding.

Estrous cycle-related alterations in the expression of glycoconjugates and lectins in the mouse endometrium shown histochemically

Alterations of the hormonal status may influence diverse cell features relevant to intra- and intercellular communication. We studied histochemically the expression of glycans and endogenous sugar-binding proteins (endolectins) in the mouse endometrium during the estrous cycle. The avidin-biotin-peroxidase complex (ABC) technique was used on paraffin sections with a panel of biotinylated lectins and neoglycoproteins. Stage-specific changes were observed with Galanthus nivalis agglutinin (GNA), Ulex europaeus-I agglutinin (UEA-1), Sambucus nigra agglutinin (SNA), Ricinus communis agglutinin (RCA-I) and Glycine max agglutinin (SBA) in both the luminal and the glandular epithelium, but to a lesser extent in the latter. The stromal constituents also exhibited modifications in lectin-binding profiles in cyclic mice. In addition to the analysis of cell glycans endogenous carbohydrate-binding sites were studied using synthetic probes, biotinylated neoglycoproteins. These tools made it possible to conclude that hormonal status can affect the expression of endolectins. We have attempted to correlate the glycohistochemical findings with recently published data obtained biochemically. The results indicate that phase-specific alterations in lectin-binding glycoproteins and endolectins occur in the endometrium of mice associated with their short reproductive cycle.

Preparation of deglycosylated egg white avidin

A simple procedure for the preparation of deglycosylated avidin is described. Commercially obtained avidin was treated with a mixed microbial culture. The cells were capable of growing on the oligosaccharide residues, but generally ignored the polypeptide portion of the egg white glycoprotein. The resultant deglycosylated avidin retained its biotin-binding characteristics. The major bacterial strain (strain BECH080), responsible for the deglycosylation, was isolated. On the basis of elementary biochemical tests, fatty acid, and phenotypic analyses, the isolate was identified as a strain of Flavobacterium meningosepticum. The primary enzymatic activity that caused the removal of the oligosaccharide residues of avidin appeared to be similar to endoglycosidase F.

Cloning and sequence analysis of GmII, a Drosophila melanogaster homologue of the cDNA encoding murine Golgi alpha-mannosidase II

Using the murine cDNA that encodes Golgi alpha-mannosidase II (GlcNAc transferase I-dependent alpha 1,3[alpha 1,6] mannosidase; EC 3.2.1.114) as a probe to screen a cDNA library made from Drosophila melanogaster (Dm) embryos, we have isolated GmII, the Dm sequence homologue. The 3926-bp cDNA has an open reading frame of 3327 bp and predicts a polypeptide of approx. 127 kDa, a mass similar to that of the murine protein. The deduced mouse and Dm amino acid (aa) sequences share extensive similarity across their entire lengths and are both type-II transmembrane (TM) proteins with short cytoplasmic tails, single TM domains and large hydrophilic C-terminal domains. A region of approx. 200 aa, within the C-terminal domain, has considerable similarity to a corresponding region from several other alpha-mannosidases. GmII has been localized to a single site (85D14-18) on the right arm of chromosome 3.

Synthesis of the glycosyl amino acids N alpha-Fmoc-Ser[Ac4-beta-D-Galp-(1-->3)-Ac2-alpha-D-GalN3p]-OPfp and N alpha-Fmoc-Thr[Ac4-beta-D-Galp-(1-->3)-Ac2-alpha-D-GalN3p]-OPfp and the application in the solid-phase peptide synthesis of multiply glycosylated mucin peptides with Tn and T antigenic structures

Two new glycosyl amino acids N alpha-Fmoc-Ser[Ac4-beta-D-Galp-(1-->3)-Ac2-alpha-D-GalN3p]-+ ++OPfp and N alpha-Fmoc-Thr[Ac4-beta-D-Galp-(1-->3)-Ac2-alpha-D-GalN3p]-+ ++OPfp were synthesized. Glycosylation of N alpha-Fmoc-Ser-OPfp or N alpha-Fmoc-Thr-OPfp with protected beta-D-Gal-(1-->3)-D-GalN3 donors afforded the glycosyl amino acids containing an activated C-terminus which could be utilized directly for solid-phase glycopeptide synthesis. The transformation of the 2-azido group into the acetamido derivative was achieved quantitatively at the end of the synthesis by treatment of the polymer-bound glycopeptide with thioacetic acid. The versatility of this strategy was demonstrated by the assembly of eight triply glycosylated mucin peptides which were synthesized simultaneously by multiple column techniques. The glycopeptides were prepared in order to investigate the substrate specificity of a galactosyltransferase.

Major organ-specific glycoproteins in isolated brain and kidney membranes identified as Na,K-ATPase subunits by combined glycan-, lectin-, and immunoblotting

In the present work combined glycan-, lectin-, and immunoblotting of isolated brain and kidney membranes shows that the alpha and beta subunits of Na,K-ATPase are the most abundant glycoproteins. Further, Datura stramonium and Galanthus nivalis agglutinins recognize the Na,K-ATPase subunits in a mutually exclusive manner in membranes from human, rabbit and rat brain or human, rabbit, rat, pig and dog kidney indicating the presence of species-independent organ-typical glycoforms. The glycosylation status is not related to the ouabain-sensitivity. Taken together, the data reveals organ-specific glycoforms of Na,K-ATPase which might have roles for organ identification and recognition.

Analysis of the N-linked oligosaccharides of human C1s using electrospray ionisation mass spectrometry

Information on the structures of the oligosaccharides linked to Asn residues 159 and 391 of the human complement protease C1s was obtained using mass spectrometric and monosaccharide analyses. Asn159 is linked to a complex-type biantennary, bisialylated oligosaccharide NeuAc2 Gal2 GlcNAc4 Man3 (molecular mass = 2206 +/- 1). Asn391 is occupied by either a biantennary, bisialylated oligosaccharide, or a triantennary, trisialylated species NeuAc3 Gal3 GlcNAc5 Man3 (molecular mass = 2861 +/- 1), or a fucosylated triatennary, trisialylated species NeuAc3 Gal3 GlcNAc5 Man3 Fuc1 (molecular mass = 3007 +/- 1), in relative proportions of approximately 1:1:1. The carbohydrate heterogeneity at Asn391 gives rise to three major types of C1s molecules of molecular masses 79,318 +/- 8 (A), 79,971 +/- 8 (B), and 80,131 +/- 8 (C), with an average mass of 79,807 +/- 8. A minor modification, yielding an extra mass of 132 +/- 2, is also detected within positions 1-153.

Differential response of the epidermal growth factor receptor tyrosine kinase activity to several plant and mammalian lectins

Biosignalling via lectins may involve modulation of protein kinase activities. This aspect of the biological action of mammalian and plant lectins has been investigated for their effect on the activity of the isolated epidermal growth factor receptor (EGFR). The constitutive tyrosine kinase activity of the epidermal growth factor receptor from rat liver, isolated by calmodulin-affinity chromatography, was activated by concanvalin A (ConA), and wheat germ agglutinin (WGA) to a similar extent as the measured enhancement induced by EGF. In contrast, two mannose-specific lectins, the mannan-binding protein (MBP) and serum amyloid P component (SAP), isolated from human serum, have inhibitory effects, both in the absence and presence of EGF. The differential effects of these lectins were tested using as phosphorylatable substrates a co-polymer of glutamic acid-tyrosine, as well as calmodulin. However, two galactoside-specific lectins, the laminin-binding beta-galactoside-binding 14 kDa lectin, isolated from bovine heart (14K-BHL), and the alpha/beta-galactoside-binding lectin, isolated from mistletoe (Viscum album L.) leaves (VAA), do not inhibit the EGFR tyrosine kinase activity. The sugar dependence of the lectin-mediated action was studied by inhibition assays. Mannose and a mannose-containing neoglycoprotein prevent the activating effect of ConA, and N-acetyl-D-glucosamine partially prevents the activation produced by WGA. However, mannose and mannose-containing neoglycoprotein were ineffective to reduce the inhibitory effect of MBP or SAP.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical and enzymatic synthesis of glycopeptides

Progress recently made in the synthesis of biologically relevant N- and O-glycopeptides is illustrated by examples. In this context, developments in the preparation of complex saccharide side chains and in the subsequent coupling to peptide portions is described. Special emphasis is given to the synthesis of Lewis antigen-type structures. Furthermore, modern methods in solid phase peptide syntheses utilizing glycosylated building blocks are presented. Recent advances in glycopeptide syntheses employing enzymatic methods in deprotection steps as well as in peptide/saccharide chain elongation are reported.