The C-terminal N-glycosylation sites of the human alpha1,3/4-fucosyltransferase III, -V, and -VI (hFucTIII, -V, adn -VI) are necessary for the expression of full enzyme activity

The alpha1,3/4-fucosyltransferases are involved in the synthesis of fucosylated cell surface glycoconjugates. Human alpha1,3/4-fucosyltransferase III, -V, and -VI (hFucTIII, -V, and -VI) contain two conserved C-terminal N-glycosylation sites (hFucTIII: Asn154 and Asn185; hFucTV: Asn167 and Asn198; and hFucTVI: Asn153 and Asn184). In the present study, we have analyzed the functional role of these potential N-glycosylation sites, laying the main emphasis on the sites in hFucTIII. Tunicamycin treatment completely abolished hFucTIII enzyme activity while castanospermine treatment diminished hFucTIII enzyme activity to approximately 40% of the activity of the native enzyme. To further analyze the role of the conserved N-glycosylation sites in hFucTIII, -V, and -VI, we made a series of mutant genomic DNAs in which the asparagine residues in the potential C-terminal N-glycosylation sites were replaced by glutamine. Subsequently, the hFucTIII, -V, and -VI wild type and the mutants were expressed in COS-7 cells. All the mutants exhibited lower enzyme activity than the wild type and elimination of individual sites had different effects on the activity. The mutations did not affect the protein level of the mutants in the cells, but reduced the molecular mass as predicted. Kinetic analysis of hFucTIII revealed that lack of glycosylation at Asn185 did not change the Km values for the oligosaccharide acceptor and the nucleotide sugar donor. The present study demonstrates that hFucTIII, -V, and -VI require N-glycosylation at the two conserved C-terminal N-glycosylation sites for expression of full enzyme activity.     

The effects of N-glycosylation sites and the N-terminal region on the biological function of beta1,3-N-acetylglucosaminyltransferase 2 and its secretion

Human beta1,3-N-acetylglucosaminyltransferase 2 (beta3GnT2) is thought to be an enzyme that extends the polylactosamine acceptor chains, but its function and structure analysis are unknown. To obtain insight into the structure of beta3GnT2, the effects of N-glycosylation on its biological function were evaluated using the addition of inhibitors, site-directed mutagenesis of potential N-glycosylation sites, and deletion of its N-terminal region using a fusion protein with GFP(uv) in a baculovirus expression system. Four of five potential N-glycosylation sites were found to be occupied, and their biological function and secretion were inhibited with the treatment of N-glycosylation inhibitor, tunicamycin. The N-glycosylation at Asn219 was necessary for the beta3GnT activity; moreover, N-glycosylation at Asn127 and Asn219 was critical for efficient protein secretion. When Ser221 was replaced with Thr, fusion protein was expressed as a single band, indicating that the double band of the expressed fusion protein was due to the heterogeneity of the glycosylation at Asn219. The truncated protein consisting of amino acids 82-397 (GFP(uv)-beta3GnT2Delta83), which lacked both one N-glycosylation site at Asn79 and the stem region of glycosyltransferase, was expressed as only a small form and showed no beta3GnT activity. These results suggest that the N-glycosylation site at Asn219, which is conserved throughout the beta1,3-glycosyltransferase family, is indispensable not only with regard to its biological function, but also to its secretion. The N-terminal region, which belongs to a stem region of glycosyltransferase, might also be important to the active protein structure.     

Glycosylation of the N-terminal potential N- glycosylation sites in the human α1,3- fucosyltransferase V and -VI (hFucTV and -VI)

Human 1,3-fucosyltransferase V and -VI (hFucTV and -VI) each contain four potential N-glycosylation sites (hFucTV: Asn60, Asn105, Asn167 and Asn198 and hFucTVI: Asn46, Asn91, Asn153 and Asn184). Glycosylation of the two N-terminal potential N-glycosylation sites (hFucTV: Asn60, Asn105 and hFucTVI: Asn46 and Asn91) have never been studied in detail. In the present study, we have analysed the glycosylation of these potential N-glycosylation sites. Initially, we compared the molecular mass of hFucTV and -VI expressed in COS-7 cells treated with tunicamycin with the mass of the proteins in untreated cells. The difference in molecular mass between the proteins in treated and untreated cells corresponded to the presence of at least three N-linked glycans. We then made a series of mutants, in which the asparagine residues in the N-terminal potential N-glycosylation sites were replaced by glutamine. Western blotting analyses demonstrated that both sites in hFucTV were glycosylated, whereas in hFucTVI only one of the sites (Asn91) was glycosylated. All the single mutants and the hFucTVI N46Q/N91Q double mutant exhibited enzyme activities that did not differ considerably from the wt activities. However, the enzyme activity of the hFucTV N60Q/N105Q double mutant was reduced to approximately 40% of the wt activity. In addition, castanospermine treatment diminished the enzyme activity and hence trimming of the N-linked glycans are required for expression of full enzyme activity of both hFucTV and -VI. The present study demonstrates that both of the N-terminal potential N-glycosylation sites in hFucTV and one of the sites in hFucTVI are glycosylated. Individually, their glycosylation does not contribute considerably to expression of enzyme activity. However, elimination of both sites in hFucTV reduces the enzyme activity.     

Functional analysis of the C-terminal region of recombinant human thrombopoietin. C-terminal region of thrombopoietin is a "shuttle" peptide to help secretion

Thrombopoietin (TPO) is a cytokine that primarily stimulates megakaryocytopoiesis and thrombopoiesis. TPO has a unique C-terminal tail peptide of about 160 amino acids that consists mostly of hydrophilic residues and contains six N-linked sugar chains. In order to investigate the biological function of the C-terminal domain, two series of mutations were performed. One is systematic truncation from the C terminus. Another is elimination of N-glycosylation sites in the C-terminal domain by Asn to Gln mutations. After the mutant proteins were expressed by mammalian cells, it was found that the elimination of the N-linked sugar sites did not affect the biological activity, whereas truncation of the C-terminal domain resulted in elevation of in vitro activity up to 4-fold. The C-terminal peptide itself was found to inhibit the in vitro activity. Moreover, both the C-terminal truncation and the elimination of the N-glycosylation sites decreased the secretion level progressively down to (1)/(10) that of wild type, and the amount of the mutant left in the cell increased. The N-glycosylation in the C-terminal region was found to be important for secretion of TPO. Among six N-glycosylation sites in the C-terminal region, two locations, Asn-213 and Asn-234, were found to be critical for secretion, and two other locations, Asn-319 and Asn-327, did not affect the secretion.     

N-Glycosylation is requisite for the enzyme activity and Golgi retention of N-acetylglucosaminyltransferase III

UDP-N-acetylglucosamine:ß-D-mannoside ß-1,4N-acetylglucosaminyltransferaseIII (GnT-III, EC 2.4.1.144 [EC] ) is a glycoprotein involved in thebiosynthesis of N-linked oligosaccharides. Rat GnT-III containsthree potential Nglycosylation sites, which have been predictedto be Asn243, Asn261, and Asn399. To study the roles of Nglycosylationin the GnT-III function, rat GnT-III was expressed in COS-1cells under tunicamycin or castanospermine treatment. The tunicamycin-treatedGnT-III, which was not N-glycosylated, had almost no activity.The castanospermine-treated GnT-III was not localized in theGolgi, but glucosylation did not affect its activity. To clarifythe role of individual N-glycosylations, we obtained a seriesof mutant cDNAs in which some or all of the potential glycosylationsites were eliminated by site-directed mutagenesis, and expressedthem in COS-1 cells. All the mutants exhibited lower enzymeactivity than the wild-type, but deglycosylation at individualsites had different effects on the enzyme activity. The deglycosylationat Asn243 or Asn261 was more effective on the activity thanthat at Asn399. The enzyme activity decreased as the numberof glycosylation sites decreased. The null glycosylation mutanthad no activity, corresponding to the case of tunicainycin-treatedwild-type GnT-III. Kinetic analysis revealed that the deglycosylationat Asn243 or Asn.261 resulted in slightly lower affinity forthe donor substrate, but the other mutation did not significantlychange the K_m value for either the donor or acceptor. None ofthe mutant GnT-IIIs showed perinuclear localization or Golgiretention, that was observed for the wild-type protein. Thisis the first demonstration that the glycosyltransferase localizedin the Golgi apparatus requires N-glycosylation for its activityand retention. N-acetylglucosaminyltransferase III N-glycosylation Golgi apparatus glycoprotein protein folding     

Secretion of heparanase protein is regulated by glycosylation in human tumor cell lines

The endo-beta-d-glucuronidase, heparanase, is capable of specifically degrading heparan sulfate, and this activity is associated with the metastatic potential of tumor cells. The predicted amino acid sequence of heparanase includes six putative N-glycosylation sites; however, the precise biochemical role of glycosylated heparanase remains unknown. In this study, we examined the link between glycosylation and the function of heparanase in human tumor cell lines. Heparanase protein was glycosylated at six Asn residues in human tumor cell lines. Treatment with a glycosylation inhibitor demonstrated that glycosylation was not required for the activity of heparanase. However, glycosylation affected the kinetics of endoplasmic reticulum-to-Golgi transport and of secretion of the enzyme.     

Studies on the N-glycosylation of the subunits of oligosaccharyl transferase in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, oligosaccharyl transferase (OT) consists of nine different subunits. Three of the essential gene products, Ost1p, Wbp1p, and Stt3p, are N-linked glycoproteins. To study the function of the N-glycosylation of these proteins, we prepared single or multiple N-glycosylation site mutations in each of them. We established that the four potential N-glycosylation sites in Ost1p and the two potential N-glycosylation sites in Wbp1p were occupied in the mature proteins. Interestingly, none of the N-glycosylation sites in these two proteins was conserved, and no defect in growth or OT activity was observed when the N-glycosylation sites were mutated to block N-glycosylation in either subunit. However, in the third glycosylated subunit, Stt3p, there are two adjacent potential N-glycosylation sites (N(535)NTWN(539)NT) that, in contrast to the other subunits, are highly conserved in eukaryotic organisms. Mass spectrometric analysis of a tryptic digest of Stt3p showed that the peptide containing the two adjacent N-glycosylation sites was N-glycosylated at one site. Furthermore, the glycan chain identified as Man(8)GlcNAc(2) is found linked only to Asn(539). Mutation experiments were carried out at these two sites. Four single amino acid mutations blocking either N-glycosylation site (N535Q, T537A, N539Q, and T541A) resulted in strains that were either lethal or extremely temperature sensitive. However, other mutations in the two N-glycosylation sites N(535)NTWN(539)NT (N536Q, T537S, N540Q, and T541S), did not exhibit growth defects. Based on these studies, we conclude that N-glycosylation of Stt3p at Asn(539) is essential for its function in the OT complex.     

Site-directed removal of N-glycosylation sites in human gastric lipase.

Human gastric lipase (HGL) is a highly glycosylated protein, as glycan chains account for about 15% of the molecular mass of the native HGL. Four potential N-glycosylation consensus sites (Asn15, 80, 252 and 308) can be identified from the HGL amino acid sequence. We studied the functional role of the individual N-linked oligosaccharide chains by removing one by one all the N-glycosylation sites, via Ala residue replacement by site-directed mutagenesis of Ser and Thr residues from the consensus sequences Asn-X-Ser/Thr. Mutagenized cDNA constructs were heterologously expressed in the baculovirus/insect cell system. Removal of oligosaccharides either at Asn15, 80 or 252 was found to have no significant influence on the enzymatic activity measured in vitro. However, the absence of glycosylation at Asn308, as well as a total deglycosylation, reduced the specific enzymatic activity of recombinant HGL (r-HGL), measured on short- and long-chain triglycerides, to about 50% of normal values. Furthermore, biosynthesis and secretion of r-HGL markedly dropped when all four potential glycosylation sites were mutated. The kinetics of the interfacial adsorption of r-HGL and the completely deglycosylated r-HGL (four-site mutant) were found to be identical when recording the changes with time of the surface pressure either at the air-water interface or in the presence of an egg phosphatidylcholine (PtdCho) monomolecular film spread at various initial surface pressures. This indicates that both recombinant HGLs are identical, as far as recognition of phospholipid film and adsorption on PtdCho are concerned. The N-glycosylation of HGL may contribute to the enzyme stability in the stomach, as under acidic conditions the degradation by pepsin of the unglycosylated r-HGL is increased.     

Posttranslational modifications of the lutropin receptor: mass spectrometric analysis

Our present knowledge of the lutropin (LH/hCG) receptor structure derives from deductions made from its amino acid sequence as established by studying the cDNA. To obtain direct experimental information, luteinizing hormone (LH) receptor expressed in L cells was immunopurified in sufficient amounts to warrant analysis by mass spectrometry and microsequencing. The mature receptor, complexed to human chorionic gonadotropin (hCG), was purified by using monoclonal antibodies recognizing the hormone, whereas the mannose-rich non-hormone-binding precursor was purified by use of antireceptor antibodies. Determination of the N-terminus showed that (2)/(3) of protein molecules started at Thr24 whereas (1)/(3) started at Ala28. All these molecules bound hCG, suggesting that the most N-terminal region of the receptor does not participate in hormone binding. Six N-glycosylation sites have been predicted from the amino acid sequence. One of them (Asn299) was found to be nonglycosylated in both the precursor and the mature protein. The most heavily glycosylated residue was Asn291, followed by Asn195 and Asn99. These three sites accounted for 82% and 97% of carbohydrate moieties in the mature receptor and in the mannose-rich precursor, respectively. The presence of some receptor molecules nonglycosylated at sites 99, 174, and 195 in hormone-receptor complexes dismisses a direct role of these glycosylation sites in hormone binding or in the correct folding of the protein. The mature carbohydrate chains were homogeneous at position 174, 195, and 313 (absence of Golgi mannosidase II activity at positions 174 and 313, absence of GlcNAc tranferases III and IV activity at position 195). Heterologous carbohydrates were present at sites 99 and 291. The latter, which is highly variable in carbohydrate chains, is unlikely to participate in a direct interaction with hormone. Site 313 thus remains as the main candidate for a role in hormone binding.     

cDNA cloning, expression, and characterization of an arylphorin-like hexameric storage protein, AgeHex2, from the mulberry longicorn beetle, Apriona germari

An arylphorin-like hexameric storage protein, AgeHex2, cDNA was cloned from the mulberry longicorn beetle, Apriona germari (Coleoptera, Cerambycidae), larval cDNA library. The complete cDNA sequence of AgeHex2 is comprised of 2,088 bp encoding 696 amino acid residues. The AgeHex2 had four potential N-glycosylation sites. The AgeHex2 contained the highly conserved two larval storage protein signature motifs. The deduced protein sequence of AgeHex2 showed high homology with A. germari hexamerin1 (51% amino acid identity), Tenebrio molitor hexamerin2 (49% amino acid identity), T. molitor early-staged encapsulation inducing protein (43% amino acid identity), and Leptinotarsa decemlineata diapause protein1 (43% amino acid identity). Phylogenetic analysis further confirmed the AgeHex2 is more closely related to coleopteran hexamerins than to the other insect storage proteins. Northern blot analysis confirmed that the AgeHex2 showed fat body-specific expression. The cDNA encoding AgeHex2 was expressed as a 75-kDa protein in the baculovirus-infected insect cells. Furthermore, N-glycosylation of the recombinant AgeHex2 was revealed by tunicamycin to the recombinant virus-infected Sf9 cells, demonstrating that the AgeHex2 is N-glycosylated. Western blot analysis using the polyclonal antiserum against recombinant AgeHex2 indicated that the AgeHex2 corresponds to a 75-kDa storage protein present in the A. germari larval hemolymph.     

Cloning and expression of two genes encoding auxin-binding proteins from tobacco

Two genes encoding the auxin-binding protein (ABP1) of tobacco (Nicotiana tabacum L.), both of which possess the characteristics of a luminal protein of the endoplasmic reticulum (ER), were isolated and sequenced. These genes were composed of at least five exons and four introns. The two coding exons showed 95% sequence homology and coded for two precursor proteins of 187 amino acid residues with molecular masses of 21 256 and 21 453 Da. The deduced amino acid sequences were 93% identical and both possessed an amino-terminal signal peptide, a hydrophilic mature protein region with two potential N-glycosylation sites and a carboxyl-terminal sorting signal, KDEL, for the ER. Restriction mapping of the cDNAs encoding tobacco ABP1, previously purified by amplification of tobacco cDNA libraries by polymerase chain reaction (PCR) using specific primers common to both genes, indicated that both genes were expressed, although one was expressed at a higher level than the other. Genomic Southern blot hybridization showed no other homologous genes except for these two in the tobacco genome. The apparent molecular mass of the mature form of tobacco ABP1 was revealed to be 25 kDa by SDS polyacrylamide gel electrophoresis using affinity-purified anti (tobacco ABP1) antibodies raised against a fusion protein with maltose-binding protein. Expression of the recombinant ABP1 gene in transgenic tobacco resulted in accumulation of the 25 kDa protein. A single point mutation of an amino acid residue at either of the two potential N-glycosylation sites resulted in a decrease in the apparent molecular mass and produced a 22 kDa protein. Mutations at both sites resulted in the formation of a 19.3 kDa protein, suggesting that tobacco ABP1 is glycosylated at two asparagine residues.     

On the post-translational modifications at the C-terminal domain of the major cysteine proteinase (cruzipain) from Trypanosoma cruzi

Cruzipain, the major cysteine proteinase from Trypanosoma cruzi, has a 130 amino acid-long C-terminal domain, which, although microheterogeneous in SDS-PAGE, has a single N-terminal amino acid sequence. Most of the Thr residues present at the beginning of this sequence are modified; the nature of this modification is still unknown, but O-glycosylation and phosphorylation seem both to be absent. The only potential site for N-glycosylation (Asn 254) is glycosylated in vivo. Most of the eight Cys residues are involved in disulfide bridges. The results are consistent with cruzipain being made of two well-defined domains, a catalytic one with high homology to cathepsin L, and a C-terminal domain, linked to the former by a 'hinge' corresponding to the Pro- and Thr-rich region at its N-terminus.     

N-glycosylation at Asn residues 554 and 566 of E-cadherin affects cell cycle progression through extracellular signal-regulated protein kinase signaling pathway

E-cadherin, which has a widely acknowledged role in mediating calcium-dependent cell-cell adhesion between epithelial cells, also functions as a tumor suppressor. The ectodomain of human E-cadherin contains four potential Nglycosylation sites at Asn residues 554, 566, 618, and 633. We investigated the role of E-cadherin N-glycosylation in cell cycle progression by site-directed mutagenesis. We showed previously that all four potential N-glycosylation sites of E-cadherin w ere N-gly cosylated in human breast carcinoma MDA-MB-435 cells. Removal of N-glycan at Asn633 dramatically affected E-cadherin stability. In this study we showed that E-cadherin mutant missing N-glycans at AsnS54, Asn566 and Asn618 failed to induce cell cycle arrest in Gt phase and to suppress cell proliferation in comparison with wild-type E-cadherin. Moreover, N-glycans at Asn554 and Asn566, but not at Asn618, seemed to be indispensable for E-cadherin-mediated suppression of cell cycle progression. Removal of N-glycans at either Asn554 or Asn566 of E-cadherin was accompanied with the activation of the extracellular signal-regulated protein kinase signaling pathway. After treatment with PD98059, an inhibitor of the extracellular signal-regulated protein kinase signaling pathway, wild-type E-cadherin transfected MDA-MB-435 and E-cadherin N-glycosylation-deficient mutant transfected MDA-MB-435 cells had equivalent numbers of cells in G1 phase. These rmdings implied that N-glycosylation might be crucial for E-cadherin-mediated suppression of cell cycle progression.     

N-glycosylation influences the latency and catalytic properties of mammalian purple acid phosphatase

Purple acid phosphatase (PAP), also known as tartrate-resistant acid phosphatase or uteroferrin, contains two potential consensus N-glycosylation sites at Asn(97) and Asn(128). In this study, endogenous rat bone PAP was found to possess similar N-glycan structures as rat recombinant PAP heterologously expressed in baculovirus-infected Sf9 insect cells. PAP from Sf9 cells was shown to contain two N-linked oligosaccharides, whereas PAP expressed by mammalian CHO-K1 cells was less extensively glycosylated. The extent of N-glycosylation affected the catalytic properties of the enzyme, as N97Q and N128Q mutants, containing a single oligosaccharide chain, exhibited a lower substrate affinity and catalytic activity compared to those of the fully glycosylated PAP in the native, monomeric state. The differences in substrate affinity and catalytic activity were abolished and partially restored, respectively, by proteolytic cleavage in the loop domain, indicating that the extent of N-glycosylation influences the interaction of the repressive loop domain with catalytically important residues.     

Structural characterization and independent folding of a chimeric glycoprotein comprising granulocyte-macrophage colony stimulating factor and erythropoietin sequences

MEN 11300 is a hybrid glycoprotein of 297 amino acids obtained by fusion of the cDNA encoding GM-CSF with the cDNA encoding EPO followed by transfection of the hybrid gene into CHO cells. The oligonucleotide construct incorporated a spacing sequence between the two individual cDNAs which encodes eight amino acids constituting a linker peptide intended to separate the GM-CSF and EPO moieties. The recombinant MEN 11300 protein was submitted to a detailed structural characterization including the verification of the entire amino acid sequence, the assignment of the disulfide bridges pattern, the identification of the glycosylation sites and the definition of the glycosidic moiety, including site-specificity. Partial processing of the C-terminal Arg residue and the occurrence of N-glycosylation sites at Asn27, Asn155, Asn169, Asn214 were established. Moreover, O-glycosylation at Ser257 and at the N-terminal region was also detected. A large heterogeneity was observed in the N-glycans due to the presence of differently sialylated and fucosylated branched complex type oligosaccharides whereas O-linked glycans were constituted by GalGalNAc chains with a different number of sialic acids. The disulfide bridges pattern was established by direct FABMS analysis of the proteolytic digests or by ESMS analysis of HPLC purified fractions. Pairing of the eight cysteine residues resulted in Cys54-Cys96, Cys88-Cys121, Cys138-Cys292, and Cys160-Cys164. This S-S bridges pattern is identical to that occurring in the individual natural GM-CSF and EPO, thus showing that the two protein moieties in MEN 11300 can independently acquire their native three-dimensional structure.      

Structural characterization of the oligosaccharides of a human monoclonal anti-lipopolysaccharide immunoglobulin M

The oligosaccharide side chains of a human anti-lipopolysaccharide IgM produced by a human-human-mouse heterohybridoma were analyzed at each of its five conserved N-glycosylation sites. This antibody also has a potential sixth N-glycosylation site in the variable region of its heavy chain which is not glycosylated. The oligosaccharides were released by digestion with various endo- and exoglycosidases and analyzed by matrix-assisted laser desorption/ionization-time of flight mass spectrometry and fluorophore-assisted carbohydrate electrophoresis. The antibody has various complex- and hybrid-type oligosaccharide structures at Asn 171, various sialylated complex-type oligosaccharides at Asn 332 and 395, and high-mannose-type oligosaccharides at Asn 402 and 563. Of note is the presence in this human IgM of oligosaccharides containing N-glycolylneuraminic acid and N-acetylneuraminic acid in the ratio of 98:2 as determined using anion- exchange chromatography. Furthermore, we observed oligosaccharide structures containing Gal alpha (1,3)Gal that have not been reported as components of human glycoproteins.      

Site-specific N-glycan characterization of human complement factor H

Human complement factor H (CFH) is a plasma glycoprotein involved in the regulation of the alternative pathway of the complement system. A deficiency in CFH is a cause of severe pathologies like atypical haemolytic uraemic syndrome (aHUS). CFH is a 155-kDa glycoprotein containing nine potential N-glycosylation sites. In the current study, we present a quantitative glycosylation analysis of CFH using capillary electrophoresis and a complete site-specific N-glycan characterization using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESIMS/MS). A 17.9-kDa mass decrease, observed after glycosidase treatment, indicated that N-glycosylation is the major post-translational modification of CFH. This mass difference is consistent with CFH glycosylation by diantennary disialylated glycans of 2204 Da on eight sites. CFH was not sensitive to endoglycosidase H (Endo H) deglycosylation, indicating the absence of hybrid and oligomannose structures. Quantitative analysis showed that CFH is mainly glycosylated by complex, diantennary disialylated, non-fucosylated glycans. Disialylated fucosylated and monosialylated non-fucosylated oligosaccharides were also identified. MS analysis allowed complete characterization of the protein backbone, verification of the glycosylation sites and site-specific N-glycan identification. The absence of glycosylation at Asn199 of the NGSP sequence of CFH is shown. Asn511, Asn700, Asn784, Asn804, Asn864, Asn893, Asn1011 and Asn1077 are glycosylated essentially by diantennary disialylated structures with a relative distribution varying between 45% for Asn804 and 75% for Asn864. Diantennary monosialylated glycans and triantennary trisialylated fucosylated and non-fucosylated structures have also been identified. Interestingly, the sialylation level along with the amount of triantennary structures decreases from the N- to the C-terminal side of the protein.     

Positions of disulfide bonds and N-glycosylation site in juvenile hormone binding protein

The juvenile hormone binding protein (JHBP) from Galleria mellonella hemolymph is a glycoprotein composed of 225 amino acid residues. It contains four Cys residues forming two disulfide bridges. In this study, the topography of the disulfide bonds as well as the site of glycan attachment in the JHBP molecule from G. mellonella was determined, using electrospray mass spectrometry. The MS analysis was performed on tryptic digests of JHBP. Our results show that the disulfide bridges link Cys10 and Cys17, and Cys151 and Cys195. Of the two potential N-glycosylation sites in JHBP, Asn4, and Asn94, only Asn94 is glycosylated. This site of glycosylation is also found in the fully biologically active recombinant JHBP expressed in the yeast Pichia pastoris.     

The role of N-linked glycosylation in determining the surface expression,G protein interaction and effector coupling of the alpha (alpha) isoform of the human thromboxane A(2) receptor

In humans, thromboxane (TX) A(2) signals through two TXA(2) receptor (TP) isoforms, termed TPalpha and TPbeta, that diverge exclusively within the carboxyl terminal cytoplasmic domains. The amino terminal extracellular region of the TPs contains two highly conserved Asn (N)-linked glycosylation sites at Asn(4) and Asn(16). While it has been established that impairment of N-glycosylation of TPalpha significantly affects ligand binding/intracellular signalling, previous studies did not ascertain whether N-linked glycosylation was critical for ligand binding per se or whether it was required for the intracellular trafficking and the functional expression of TPalpha on the plasma membrane (PM). In the current study, we investigated the role of N-linked glycosylation in determining the functional expression of TPalpha, by assessment of its ligand binding, G protein coupling and intracellular signalling properties, correlating it with the level of antigenic TPalpha protein expressed on the PM and/or retained intracellularly. From our data, we conclude that N-glycosylation of either Asn(4) or Asn(16) is required and sufficient for expression of functionally active TPalpha on the PM while the fully non-glycosylated TPalpha(N4,N16-Q4,Q16) is almost completely retained within the endoplasmic reticulum (ER) and remains functionally inactive, failing to associate with its coupling G protein Galpha(q) and, in turn, failing to mediate phospholipase (PL) Cbeta activation.     

Role of the putative N-glycosylation and PKC-phosphorylation sites of the human sodium-dependent multivitamin transporter (hSMVT) in function and regulation

The sodium-dependent multivitamin transporter (SMVT) is a major biotin transporter in a variety of tissues including the small intestine. The human SMVT (hSMVT) polypeptide is predicted to have four N-glycosylation sites and two putative PKC phosphorylation sites but their role in the function and regulation of the protein is not known and was examined in this investigation. Our results showed that the hSMVT protein is glycosylated and that this glycosylation is important for its function. Studies utilizing site-directed mutagenesis revealed that the N-glycosylation sites at positions Asn(138) and Asn(489) are important for the function of hSMVT and that mutating these sites significantly reduces the V(max) of the biotin uptake process. Mutating the putative PKC phosphorylation site Thr(286) of hSMVT led to a significant decrease in the PMA-induced inhibition in biotin uptake. The latter effect was not mediated via changes in the level of expression of the hSMVT protein and mRNA or in its level of expression at the cell membrane. These findings demonstrate that the hSMVT protein is glycosylated, and that glycosylation is important for its function. Furthermore, the study shows a role for the putative PKC-phosphorylation site Thr(286) of hSMVT in the PKC-mediated regulation of biotin uptake.