Vesicular-integral membrane protein, VIP36, recognizes high-mannose type glycans containing alpha1-->2 mannosyl residues in MDCK cells.

The 36 kDa vesicular-integral membrane protein, VIP36, has been originally isolated from MDCK cells as a component of glycolipid-enriched detergent-insoluble complexes containing apical marker proteins, and its luminal domain shows homology to leguminous plant lectins and ERGIC-53. As the first step to identify the functional role of VIP36, the carbohydrate binding specificity of VIP36 was investigated using a fusion protein of glutathione- S -transferase and luminal domain of VIP36 (Vip36). It was found that VIP36 recognizes high-mannose type glycans containing alpha1-->2 Man residues and alpha-amino substituted asparagine. The binding of Vip36 to high-mannose type glycans was independent of Ca(2+)and theoptimal condition was pH 6.0 at 37 degrees C. The concentration at which half inhibition of the binding by Man(7-9).GlcNAc(2). N Ac. Asn occurred was 1.0 x 10(-9)M. The association constant between Man(7-9).GlcNAc(2)in porcine thyroglobulin and immobilized Vip36 was 2.1 x 10(8)M(-1)as determined by means of a biosensor based on surface plasmon resonance. These results indicate that VIP36 functions as an intracellular lectin recognizing glycoproteins which possess high-mannose type glycans, (Manalpha1-->2)(2-4).Man(5). GlcNAc(2).     

Multiple modes of binding enhance the affinity of DC-SIGN for high mannose N-linked glycans found on viral glycoproteins

The dendritic cell surface receptor DC-SIGN and the closely related endothelial cell receptor DC-SIGNR specifically recognize high mannose N-linked carbohydrates on viral pathogens. Previous studies have shown that these receptors bind the outer trimannose branch Manalpha1-3[Manalpha1-6]Manalpha present in high mannose structures. Although the trimannoside binds to DC-SIGN or DC-SIGNR more strongly than mannose, additional affinity enhancements are observed in the presence of one or more Manalpha1-2Manalpha moieties on the nonreducing termini of oligomannose structures. The molecular basis of this enhancement has been investigated by determining crystal structures of DC-SIGN bound to a synthetic six-mannose fragment of a high mannose N-linked oligosaccharide, Manalpha1-2Manalpha1-3[Manalpha1-2Manalpha1-6]Manalpha1-6Man and to the disaccharide Manalpha1-2Man. The structures reveal mixtures of two binding modes in each case. Each mode features typical C-type lectin binding at the principal Ca2+-binding site by one mannose residue. In addition, other sugar residues form contacts unique to each binding mode. These results suggest that the affinity enhancement displayed toward oligosaccharides decorated with the Manalpha1-2Manalpha structure is due in part to multiple binding modes at the primary Ca2+ site, which provide both additional contacts and a statistical (entropic) enhancement of binding.     

Substrate specificity analysis of endoplasmic reticulum glucosidase II using synthetic high mannose-type glycans

Glucosidase II (Glc'ase II) is a glycan-processing enzyme that trims two alpha1,3-linked Glc residues in succession from the glycoprotein oligosaccharide Glc2Man9GlcNAc2 to give Glc1Man9GlcNAc2 and Man9GlcNAc2 in the endoplasmic reticulum (ER). Monoglucosylated glycans, such as Glc1-Man9GlcNAc2, generated by this process play a key role in glycoprotein quality control in the ER, because they are primary ligands for the lectin chaperones calnexin (CNX) and calreticulin (CRT). A precise analysis of the substrate specificity of Glc'ase II is expected to further our understanding of the molecular basis to glycoprotein quality control, because Glc'ase II potentially competes with CNX/CRT for the same glycans, Glc1Man7-9GlcNAc2. In this study, a quantitative analysis of the specificity of Glc'ase II using a series of structurally defined synthetic glycans was carried out. In the presence of CRT, Glc'ase II-mediated trimming from Glc2Man9GlcNAc2 stopped at Glc1Man9GlcNAc2, supporting the notion that the glycan structure delivered to the CNX/CRT cycle is Glc1Man9GlcNAc2. Unexpectedly, our experiments showed that Glc1Man8(B)GlcNAc2 had nearly the same reactivity as Glc1Man9GlcNAc2, which was markedly greater than that of its positional isomer Glc1Man8(C)GlcNAc2. An analysis with glycoprotein-like probes revealed the stepwise formation of Glc1Man9GlcNAc2 and Man9GlcNAc2 from Glc2Man9GlcNAc2, even in the presence of CRT. It was also shown that Glc1Man8(B)GlcNAc2 had even greater reactivity than Glc1Man9GlcNAc2 at the glycoprotein level. Moreover, inhibitory activities by nonglucosylated glycans suggested that Glc'ase II recognized the C arm (Manalpha1, 2Manalpha1, 6Man-) of high mannose-type glycans.     

Isolation of a mutant Arabidopsis plant that lacks N-acetyl glucosaminyl transferase I and is unable to synthesize Golgi-modified complex N-linked glycans.

The complex asparagine-linked glycans of plant glycoproteins, characterized by the presence of beta 1-->2 xylose and alpha 1-->3 fucose residues, are derived from typical mannose9(N-acetylglucosamine)2 (Man9GlcNAc2) N-linked glycans through the activity of a series of glycosidases and glycosyl transferases in the Golgi apparatus. By screening leaf extracts with an antiserum against complex glycans, we isolated a mutant of Arabidopsis thaliana that is blocked in the conversion of high-manne to complex glycans. In callus tissues derived from the mutant plants, all glycans bind to concanavalin A. These glycans can be released by treatment with endoglycosidase H, and the majority has the same size as Man5GlcNAc1 glycans. In the presence of deoxymannojirimycin, an inhibitor of mannosidase I, the mutant cells synthesize Man9GlcNAc2 and Man8GlcNAc2 glycans, suggesting that the biochemical lesion in the mutant is not in the biosynthesis of high-mannose glycans in the endoplasmic reticulum but in their modification in the Golgi. Direct enzyme assays of cell extracts show that the mutant cells lack N-acetyl glucosaminyl transferase I, the first enzyme in the pathway of complex glycan biosynthesis. The mutant plants are able to complete their development normally under several environmental conditions, suggesting that complex glycans are not essential for normal developmental processes. By crossing the complex-glycan-deficient strain of A. thaliana with a transgenic strain that expresses the glycoprotein phytohemagglutinin, we obtained a unique strain that synthesizes phytohemagglutinin with two high-mannose glycans, instead of one high-mannose and one complex glycan.     

Co(II)-regulated substrate specificity of cytosolic alpha-mannosidase

Cytosolic neutral alpha-mannosidase is a putative catabolic enzyme that produces cytosolic free oligomannosides. Activation of the enzyme by Co(II) treatment has been reported using pyridylamino derivatives of Man(5)GlcNAc and Man(5)GlcNAc2, and p-nitrophenyl alpha-mannoside as substrates, with the Co(II)-treated enzyme releasing four alpha-mannose residues from Man(9)GlcNAc to give Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3)Manbeta1-4GlcNAc as an end product. When Man(9)GlcNAc, which is considered to be the actual substrate in the cytosol, was used as a substrate, we found that even before treatment with Co(II) the enzyme was able to cleave a single Manalpha1-2 residue from Man(9)GlcNAc to give Manalpha1-6(Manalpha1-2Manalpha1-3)Manalpha1-6(Manalpha1-2Manalpha1-2Manalpha1-3)Manbeta1-4GlcNAc as the end product. The K(m) value of the Co(II)-treated enzyme for Man(9)GlcNAc was found to be 37 microM, which is one-twelfth that of the non-treated enzyme, while the values were V(max) values were almost the same, indicating that the affinity of the substrate is higher with Co(II). These results indicate that Co(II) regulates the substrate specificity of the enzyme.     

Biochemical and functional properties of the full-length cation-dependent mannose 6-phosphate receptor expressed in Pichia pastoris

A glycosylation-deficient, full-length cation-dependent mannose 6-phosphate receptor (CD-MPR) containing a yeast signal sequence was expressed in Pichia pastoris using the constitutive promoter of the PGAP gene. The membrane-bound receptor was solubilized using detergents and purified by pentamannosyl phosphate-agarose affinity chromatography. Equilibrium binding studies identified a binding affinity of 2 nM for the lysosomal enzyme, beta-glucuronidase. To probe the linkage specificity of the recombinant CD-MPR, inhibition binding studies were conducted using non-phosphorylated oligomannoses which demonstrated that Manalpha1,2Man exhibits a 4-fold higher inhibition than Manalpha1,3Man and Manalpha1,6Man. The receptor was capable of associating into oligomeric forms and enzymatic deglycosylation revealed the presence of high-mannose sugars at the single potential N-glycosylation site. Mass spectrometric analysis revealed that the receptor was palmitoylated at the two potential cysteines in its cytoplasmic domain. In conclusion, the full-length CD-MPR produced in P. pastoris is structurally and functionally suitable for crystallization studies.     

Structural analysis of free N-glycans occurring in soybean seedlings and dry seeds

The structures of unconjugated or free N-glycans in stems of soybean seedlings and dry seeds have been identified. The free N-glycans were extracted from the stems of seedlings or defatted dry seeds. After desalting by two kinds of ion-exchange chromatography and a gel filtration, the free N-glycans were coupled with 2-aminopyridine. The resulting fluorescence-labeled (PA-) N-glycans were purified by gel filtration, Con A affinity chromatography, reverse-phase HPLC, and size-fractionation HPLC. The structures of the PA-sugar chains purified were analyzed by the combination of two-dimensional sugar chain mapping, jack bean alpha-mannosidase digestion, alpha-1,2-mannosidase digestions, partial acetolysis, and ESI-MS/MS. The free N-glycan structures found showed that two categories of free N-glycans occur in the stems of soybean seedlings. One is a high-mannose type structure having one GlcNAc residue at the reducing end (Man 9 approximately 5 GlcNAc1, 93%), that would be derived by endo-GM (Kimura, Y. et al., Biochim. Biophys. Acta, 1381, 27-36 (1998)). The other small component is a xylose-containing type one having two GlcNAc residues at the reducing end (Man3Xyl1GlcNAc2, 7%), which would be derived by PNGase-GM (Kimura, Y. and Ohno, A., Biosci. Biotechnol. Biochem., 62, 412-418 (1998)). The detailed structural analysis of free glycans showed that high-mannose type free N-glycans (Man 9 approximately 5 GlcNAc1) in the soybean seedlings have a common core structural unit; Manalpha1-6(Man1-3)Manalpha1-6(Manalpha1-3)Ma nbeta1-4GlcNAc. Comparing the amount of free N-glycans in the seedling stems and dry seeds, the amount in the stems of seedlings was much higher than that in the dry seeds; approximately 700 pmol per one stem, 8 pmol in one dry seed. This fact suggested that free N-glycans in soybean seedlings could be produced by two kinds of N-glycan releasing enzymes during germination or seedling-development.     

Crocus sativus lectin recognizes Man3GlcNAc in the N-glycan core structure

Crocus sativus lectin (CSL) is one of the truly mannose-specific plant lectins that has a unique binding specificity that sets it apart from others. We studied sugar-binding specificity of CSL in detail by a solution phase method (fluorescence polarization) and three solid phase methods (flow injection, surface plasmon resonance, and microtiter plate), using a number of different glycopeptides and oligosaccharides. CSL binds the branched mannotriose structure in the N-glycan core. Substitution of the terminal Man in the Manalpha(1-3)Man branch with GlcNAc drastically decreases binding affinity much more than masking of the terminal Man in the Manalpha(1-6)Man branch. Most interestingly, the beta-Man-linked GlcNAc in N-glycan core structure contributes greatly to the binding. The effect of this GlcNAc is so strong that it can substantially offset the negative effect of substitution on the nonreducing terminal Man residues. On the other hand, the GlcNAc that is usually attached to Asn in N-glycans and the l-Fuc linked at the 6-position of the GlcNAc are irrelevant to the binding. A bisecting GlcNAc neither contributes to nor interferes with the binding. This unique binding specificity of CSL offers many possibilities of its use in analytical and preparative applications.     

Identification of an N-acetylglucosaminyltransferase in Dictyostelium discoideum that transfers an "intersecting" N-acetylglucosamine residue to high mannose oligosaccharides

Glycoproteins synthesized by the cellular slime mold Dictyostelium discoideum have been shown to contain asparagine-linked high-mannose oligosaccharides which have an N-acetylglucosamine group in a novel intersecting position (attached beta 1-4 to the mannose linked alpha 1-6 to the core mannose). We have used crude membrane preparations from vegetative D. discoideum (strain M4) to characterize the enzyme activity responsible for catalyzing the transfer of GlcNAc to the intersecting position of high-mannose oligosaccharides. UDP-GlcNAc:oligosaccharide beta-N-acetylglucosaminyltransferase activity in these preparations attaches GlcNAc to the mannose residue-linked alpha 1-6 to the beta-linked core mannose of the following Man9GlcNAc oligosaccharide as shown by the arrow. (formula; see text) It will also attach GlcNAc to the same intersecting position and/or to the bisecting position (beta-linked core mannose) of the following Man5GlcNAc oligosaccharide. (formula; see text) An analysis of the pH profiles, effects of heat denaturation, and substrate inhibitions on the addition of GlcNAc to either the intersecting or bisecting position of this Man5GlcNAc oligosaccharide indicates that a single enzyme activity is responsible for transferring GlcNAc to both positions. Various oligosaccharides were assayed to determine the substrate specificity of the transferase activity. These data indicate that both the mannose-attached alpha 1-3 and the mannose-attached alpha 1-6 to the mannose receiving the GlcNAc play a critical role in substrate suitability; absence of the alpha 1-6 mannose results in at least a 90% decrease in activity, while absence of the alpha 1-3 mannose results in a completely inactive substrate. This suggests that the minimal substrate is the disaccharide Man alpha 1-3Man.     

Structural characterization of NETNES, a novel glycoconjugate in Trypanosoma cruzi epimastigotes

The unicellular stercorarian protozoan parasite Trypanosoma cruzi is the etiological agent of Chagas' disease. The epimastigote form of the parasite is covered in a dense coat of glycoinositol phospholipids and short glycosylphosphatidylinositol (GPI)-anchored mucinlike molecules. Here, we describe the purification and structural characterization of NETNES, a relatively minor but unusually complex glycoprotein that coexists with these major surface components. The mature glycoprotein is only 13 amino acids in length, with the sequence AQENETNESGSID, and exists in two forms with either four or five post-translational modifications. These are either one or two asparagine-linked oligomannose glycans, two linear alpha-mannose glycans linked to serine residues via phosphodiester linkages, and a GPI membrane anchor attached to the C-terminal aspartic acid residue. The variety and density of post-translational modifications on an unusually small peptide core make NETNES a unique type of glycoprotein. The N-glycans are predominantly Manalpha1-6(Manalpha1-3) Manalpha1-6(Manalpha1-3)Manbeta1-4GlcNAcbeta1-4GlcNAcbeta1-Asn; the phosphate-linked glycans are a mixture of (Manalpha1-2)0-3Man1-P-Ser; and the GPI anchor has the structure Manalpha1-2(ethanolamine phosphate)Manalpha1-2Manalpha1-6Manalpha1-4(2-aminoethylphosphonate-6)GlcNalpha1-6-myo-inositol-1-P-3(sn-1-O-(C16:0)alkyl-2-O-(C16:0)acylglycerol). Four putative NETNES genes were found in the T. cruzi genome data base. These genes are predicted to encode 65-amino acid proteins with cleavable 26-amino acid N-terminal signal peptides and 26-amino acid C-terminal GPI addition signal peptides.     

Use of HDEL-tagged Trichoderma reesei mannosyl oligosaccharide 1,2-alpha-D-mannosidase for N-glycan engineering in Pichia pastoris

Therapeutic glycoprotein production in the widely used expression host Pichia pastoris is hampered by the differences in the protein-linked carbohydrate biosynthesis between this yeast and the target organisms such as man. A significant step towards the generation of human-compatible N-glycans in this organism is the conversion of the yeast-type high-mannose glycans to mammalian-type high-mannose and/or complex glycans. In this perspective, we have co-expressed an endoplasmic reticulum-targeted Trichoderma reesei 1,2-alpha-D-mannosidase with two glycoproteins: influenza virus haemagglutinin and Trypanosoma cruzi trans-sialidase. Analysis of the N-glycans of the two purified proteins showed a >85% decrease in the number of alpha-1,2-linked mannose residues. Moreover, the human-type high-mannose oligosaccharide Man(5)GlcNAc(2) was the major N-glycan of the glyco-engineered trans-sialidase, indicating that N-glycan engineering can be effectively accomplished in P. pastoris.     

Purification to homogeneity and properties of mannosidase II from mung bean seedlings

Mannosidase II was purified from mung bean seedlings to apparent homogeneity by using a combination of techniques including DEAE-cellulose and hydroxyapatite chromatography, gel filtration, lectin affinity chromatography, and preparative gel electrophoresis. The release of radioactive mannose from GlcNAc[3H]Man5GlcNAc was linear with time and protein concentration with the purified protein, did not show any metal ion requirement, and had a pH optimum of 6.0. The purified enzyme showed a single band on SDS gels that migrated with the Mr 125K standard. The enzyme was very active on GlcNAcMan5GlcNAc but had no activity toward Man5GlcNAc, Man9GlcNAc, Glc3Man9GlcNAc, or other high-mannose oligosaccharides. It did show slight activity toward Man3GlcNAc. The first product of the reaction of enzyme with GlcNAcMan5GlcNAc, i.e., GlcNAcMan4GlcNAc, was isolated by gel filtration and subjected to digestion with endoglucosaminidase H to determine which mannose residue had been removed. This GlcNAcMan4GlcNAc was about 60% susceptible to Endo H indicating that the mannosidase II preferred to remove the alpha 1,6-linked mannose first, but 40% of the time removed the alpha 1,3-linked mannose first. The final product of the reaction, GlcNAcMan3GlcNAc, was characterized by gel filtration and various enzymatic digestions. Mannosidase II was very strongly inhibited by swainsonine and less strongly by 1,4-dideoxy-1,4-imino-D-mannitol. It was not inhibited by deoxymannojirimycin.     

Spectroscopic studies on the interaction of pradimicin BMY-28864 with mannose derivatives

Pradimicin BMY-28864 (Pm) is an antibiotic effective against yeasts and fungi, and is known to bind mannose in the presence of Ca2+. We examined spectroscopically the mode of interactions among Pm, Ca2+, and glycosides of mannose and mannose oligosaccharides (Manalpha1-OMe, Manalpha1-2Manalpha1-OMe, Manalpha1-3Manalpha1-OMe, Manalpha1- 4Manalpha1-OMe, Manalpha1-6Manalpha1-OMe, Manalpha1-6(Manalpha1- 3)Manalpha1-OMe, and Man9GlcNAc2-Asn, a high mannose type N-linked oligosaccharide). All the mannosides interacted with Pm in the presence of Ca2+ and caused absorbance changes. The absorbance changes occurred nonlinearly with respect to the carbohydrate concentration and do not follow a simple binding isotherm equation, suggesting a unique multistep interaction mode. The concentrations that induced half the maximum absorbance change were approximately 10 mM for the mono- and di- mannosides and around 1.5 mM for the trimannoside and Man9GlcNAc2-Asn. Methyl alpha-D-glucopyranoside, methyl alpha-D-galactopyranoside, lactose, and myo-inositol did not affect the absorbance of Pm up to 50 mM. Ca2+ alone also influenced the absorbance of Pm. The absorbance between 200 and 700 nm decreased hypochromically when Ca2+ was added. The concentration that gave half the maximum absorbance decrease caused by Ca2+was around 15 microM. Our results suggest that two Pm molecules bind one C a2+, and each Pm binds two mannosyl residues.      

Processing of N-linked oligosaccharides from precursor- to mature-form herpes simplex virus type 1 glycoprotein gC.

Immature and mature forms of glycoprotein gC were purified by immunoadsorbent from herpes simplex virus type 1-infected BHK cells labeled with [3H]mannose for a 20-min pulse or for 11 h followed by a 3-h chase. The nature of N-asparagine-linked oligosaccharides carried by the immature form, pgC (molecular weight = 92,000), and the mature gC (molecular weight = 120,000) has been investigated. All pronase-digested glycopeptides of pgC were susceptible to endo-beta-N-acetylglucosaminidase H treatment; thus they have a high-mannose structure. Using thin-layer chromatography to separate endo-beta-N-acetylglucosaminidase H-cleaved oligosaccharides, polymannosyl chains of different sizes, ranging from Man9GlcNAc to Man5GlcNAc, were separated. The major components were Man8GlcNAc and Man7GlcNAc, suggesting that pgC labeled in a 20-min pulse represents the form of glycoprotein already routed to the Golgi apparatus. Analysis of glycopeptides of mature gC showed that the majority (95%) of N-linked glycans were converted to complex-type glycans. Ion-exchange chromatography and affinity chromatography on concanavalin A-Sepharose and leucoagglutinin-agarose revealed that diantennary and triantennary glycans predominated, whereas tetrantennary chains were not present. Parts of the di- and triantennary chains were not fully sialylated. The high heterogeneity of complex-type chains found in mature gC may be related to the high number of N-glycosylation sites of the glycoprotein as predicted by DNA sequencing studies (Frink et al., J. Virol. 45:634-647, 1983).     

Site-specific glycosylation analysis of the bovine lysosomal alpha-mannosidase

Lysosomal alpha-mannosidase is a broad specificity exoglycosidase involved in the ordered degradation of glycoproteins. The bovine enzyme is used as an important model for understanding the inborn lysosomal storage disorder alpha-mannosidosis. This enzyme of about 1,000 amino acids consists of five peptide chains, namely a- to e-peptides and contains eight N-glycosylation sites. The N(497) glycosylation site of the c-peptide chain is evolutionary conserved among LAMANs and is very important for the maintenance of the lysosomal stability of the enzyme. In this work, relying on an approach based on mass spectrometric techniques in combination with exoglycosidase digestions and chemical derivatizations, we will report the detailed structures of the N-glycans and their distribution within six of the eight N-glycosylation sites of the bovine glycoprotein. The analysis of the PNGase F-released glycans from the bovine LAMAN revealed that the major structures fall into three classes, namely high-mannose-type (Fuc(0-1)Glc(0-1)Man(4-9)GlcNAc(2)), hybrid-type (Gal(0-1)Man(4-5)GlcNAc(4)), and complex-type (Fuc(0-1)Gal(0-2)Man(3)GlcNAc(3-5)) N-glycans, with core fucosylation and bisecting GlcNAc. To investigate the exact structure of the N-glycans at each glycosylation site, the peptide chains of the bovine LAMAN were separated using SDS-PAGE and in-gel deglycosylation. These experiments revealed that the N(497) and N(930) sites, from the c- and e-peptides, contain only high-mannose-type glycans Glc(0-1)Man(5-9)GlcNAc(2), including the evolutionary conserved Glc(1)Man(9)GlcNAc(2) glycan, and Fuc(0-1)Man(3-5)GlcNAc(2), respectively. Therefore, to determine the microheterogeneity within the remaining glycosylation sites, the glycoprotein was reduced, carboxymethylated, and digested with trypsin. The tryptic fragments were then subjected to concanavalin A (Con A) affinity chromatography, and the material bound by Con A-Sepharose was purified using reverse-phase high-performance liquid chromatography (HPLC). The tandem mass spectrometry (ESI-MS/MS) and the MALDI analysis of the PNGase F-digested glycopeptides indicated that (1) N(692) and N(766) sites from the d-peptide chain both bear glycans consisting of high-mannose (Fuc(0-1)Man(3-7)GlcNAc(2)), hybrid (Fuc(0-1) Gal(0-1)Man(4-5)GlcNAc(4)), and complex (Fuc(0-1)Gal(0-2)Man(3)GlcNAc(4-5)) structures; and (2) the N(367) site, from the b-peptide chain, is glycosylated only with high-mannose structures (Fuc(0-1)Man(3-5)GlcNAc(2)). Taking into consideration the data obtained from the analysis of either the in-gel-released glycans from the abc- and c-peptides or the tryptic glycopeptide containing the N(367) site, the N(133) site, from the a-peptide, was shown to be glycosylated with truncated and high-mannose-type (Fuc(0-1)Man(4-5)GlcNAc(2)), complex-type (Fuc(0-1)Gal(0-1)Man(3)GlcNAc(5)), and hybrid-type (Fuc(0-1)Gal(0-1)Man(5)GlcNAc(4)) glycans.     

Carbohydrate-based interactions of oviductal sperm reservoir formation-studies in the pig.

Competitive inhibition of sperm to explants of the oviductal epithelium was used to study the complementary receptor system that may be involved in the establishment of the oviductal sperm reservoir in the pig. Sperm binding to the oviductal explants is expressed as Binding Index (BI = sperm cells/0.01 mm(2)). From a set of glycoproteins with known oligosaccharide structures, only asialofetuin and ovalbumin showed inhibitory activity, indicating that ovalbumin may block high affinity binding sites (IC(50) congruent with 1.3 microM) and asialofetuin low affinity sites (IC(50) congruent with 18 microM) of the complementary receptor systems, whereas fetuin carrying terminal sialic acid has no effect. Ovalbumin glycopeptides were isolated by Con A affinity chromatography and reverse-phase HPLC following tryptic digestion. Glycopeptides and enzymatically released glycans were analyzed by MS, and were shown to represent preferentially the two high mannose type glycans (Man)(5)(GlcNAc)(2) and (Man)(6)(GlcNAc)(2), and as a minor component the hybrid type glycan (Hex)(4)(GlcNAc)(5). Glycopeptides (84% inhibition) and glycans (81% inhibition) significantly reduced sperm-oviduct binding at a concentration of 3 microM, whereas the deglycosylated peptides showed no inhibitory activity. Mannopentaose (IC(50) congruent with 0.8 microM) representing the oligomannose residue of the high mannose glycans of ovalbumin was as effective as ovalbumin. These data indicate that the carbohydrate-based mechanisms underlying the formation of the oviductal sperm reservoir in the pig is the result of the concerted action of at least the high-affinity binding sites for oligomannose or nonreducing terminal mannose residues and low-affinity binding of galactose.     

Free N-glycans already occur at an early stage of seed development

As a part of our studies to elucidate the physiological significance of free N-glycans in differentiating or growing plant cells, we first demonstrate that two kinds of free N-glycans already occur at an early stage of seed development. In this report, we used the developing Ginkgo biloba seeds as a model plant, since we have already revealed a functional feature of the Ginkgo endo-beta-N-acetylglucosaminidase and structural features of N-glycans linked to storage glycoproteins in the developing seeds [Kimura, Y. et al. (1998) Biosci. Biotechnol. Biochem. 62, 253-261; Kimura, Y. and Matsuo, S. (2000) Biosci. Biotechnol. Biochem. 64, 562-568]. The structures of free N-glycans, which were determined by a combination of ESI-MS, sequential a-mannosidase digestions, partial acetolysis, and two dimensional sugar chain map, fell into two categories. One dominant species is a high-mannose type structure having one GlcNAc residue at the reducing end (Man(9-5)GlcNAc(1)). The concentration of this type of free glycan (as the pyridylaminated derivatives) is about 2.2 nmol in 1 g fresh weight. The detailed structural analysis revealed that the high-mannose type structures have a common core unit; Manalpha1-6(Man1-3)Manalpha1-6(Manalpha1-3)Ma nbeta1-4GlcNAc. The other minor species of free N-glycans is the plant complex type structure having an N-acetylchitobiose unit at the reducing end (Man(3)Xyl(1)Fuc(1)GlcNAc(2)). The concentration of this type of free glycan (as the pyridylaminated derivative) was about 75 pmol in 1 g fresh weight.     

Structural studies of two ovalbumin glycopeptides in relation to the endo-beta-N-acetylglucosaminidase specificity.

Heterogeneities of the two ovalbumin glycopeptides, (Man)5(GlcNAc)2Asn and (Man)6(GlcNAc)2Asn, were revealed by borate paper electrophoresis of oligosaccharide alcohols obtained from the glycopeptides by endo-beta-N-acetylglucosaminidase H digestion and NaB3H4 reduction. The structures of the major components of the oligosaccharides were determined by the combination of methylation analysis, acetolysis, and alpha-mannosidase digestion. Based on the results, the whole structures of the major components of (Man)5(GlcNAc)2Asn and (Man)6(GlcNAc)2Asn were elucidated as Manalpha1 leads to 6[Manalpha1 leads to 3]-Manalpha1 leads to 6[Manalpha1 leads to 3[Manbeta1 leads to 4GlcNAcbeta1 leads to 4GlcNAc leads to Asn and Manalpha1 leads to 6[Manalpha1 leads to 3]Manalpha1 leads to 6[Manalpha1 leads to 2Manalpha1 leads to 3]Manbeta1 leads to 4GlcNAcbeta1 leads to GlcNAc leads to Asn, respectively. Since endo-beta-N-acetylglucosamini dase D hydrolyzes (Man)5(GlcNAc)2Asn but not (Man)6(GlcNAc)2Asn, the presence of the unsubstituted alpha-mannosyl residue linked at the C-3 position of the terminal mannose of Manbeta1 leads to 4GlcNAcbeta1 leads to 4 GlcNAcAsn core must be essential for the action of the enzyme.     

Isolation and purification of a neutral alpha(1,2)-mannosidase from Trypanosoma cruzi

Trypanosoma cruzi is an obligatory intracellular protozoan parasite that causes Chagas' disease in humans. Although a fair amount is known about the biochemistry of certain trypanosomes, very little is known about the enzymic complement of synthesis and processing of glycoproteins and/or functions of the subcellular organelles in this parasite. There have been very few reports on the presence of acid and neutral hydrolases in Trypanosoma cruzi. Here we report the first purification and characterization of a neutral mannosidase from the epimastigote stage of Trypanosoma cruzi. The neutral mannosidase was purified nearly 800-fold with an 8% recovery to apparent homogeneity from a CHAPS extract of epimastigotes by the following procedures: (1) metal affinity chromatography on Co+2-Sepharose, (2) anion exchange, and (3) hydroxylapatite. The purified enzyme has a native molecular weight of 150-160 kDa and is apparently composed of two subunits of 76 kDa. The purified enzyme exhibits a broad pH profile with a maximum at pH 5.9-6.3. It is inhibited by swainsonine (Ki, 0.1 microM), D-mannono-delta-lactam (Ki, 20 microM), kifunensine (Ki, 60 microM) but not significantly by deoxymannojirimycin. The enzyme is activated by Co2+and Ni2+and strongly inhibited by EDTA and Fe2+.The purified enzyme is active against p-nitrophenyl alpha-D-mannoside (km = 87 microM). High-mannose Man9GlcNAc substrate was hydrolyzed by the purified enzyme to Man7GlcNAc at pH 6.1. The purified enzyme does not show activity against alpha1,3- or alpha1,6-linked mannose residues. Antibodies against the recently purified lysosomal alpha-mannosidase from T.cruzi did not react with the neutral mannosidase reported here.     

Comparative study of lysosomal and cytosolic catabolisms of oligomannosidic-type glycans

In order to study the substrate specificities of the enzymes implicated in the catabolism of oligomannosidic-type glycans, the oligosaccharides Man9GlcNAc and Man5GlcNAc were incubated with rat liver lysosomal and cytosolic alpha-D-mannosidases and the hydrolysis products were characterized by 400 MHz 1H-NMR spectroscopy. Although they both occur in an ordered way, the two catabolic pathways are quite different. The lysomal pathway is realized in two stages: the first leads from Man9GlcNAc to Man5GlcNAc by preferential cleavage of the four alpha-1,2-linked mannose residues, and the second, Zn(2+)-dependent, leads from Man5GlcNAc to Man (beta 1-4) GlcN Ac by hydrolysis of alpha-1, 3- and alpha-1,6-linked residues. On the contrary, the cytosolic pattern leads by a pathway quite different to a unique hexasaccharide Man5GlcNAc which has, curiously, the same structure as one of the polyprenolic intermediates occurring in the cytosol during the biosynthesis of N-glycosylprotein glycans: Man (alpha 1-2) Man (alpha 1-2) Man (alpha 1-3) [Man (alpha 1-6)] Man (beta 1-4) GlcN Ac (beta 1-4) GlcNAc alpha 1-P-P-Dol.