戸谷 希一郎

During N-glycosylation of proteins, significant amounts of free unconjugated glycans are also generated in the lumen of the endoplasmic reticulum (ER). These ER-derived free glycans are translocated into the cytosol by a putative transporter on the ER membrane for further processing. However, the molecular nature of the transporter remains to be determined. Here, we report the establishment of a novel assay method for free oligosaccharide transport from the ER lumen using chemically synthesized fluorescence-labeled N-glycan derivatives. In this method, fluorescence-labeled glycan substrates were encapsulated inside mouse liver microsomes, followed by incubation with the cytosol and a fluorescence-quenching agent (anti-fluorophore antibody). The rate of substrate efflux was then monitored in real time by the decrease in the fluorescence intensity. The present data clearly demonstrated that the oligosaccharide transport activity under the current assay conditions was both ATP and cytosol dependent. The transporter activity was also found to be glycan structure specific because free glucosylated glycans were unable to be transported out of the microsomes. This new assay method will be a useful tool for identifying the transporter protein on the ER membrane.

Glucosidase II (G-II) is a glycoprotein-processing enzyme that successively cleaves two alpha 1,3-linked glucose residues from N-linked oligosaccharides in the endoplasmic reticulum. G-II is a heterodimer whose alpha-subunit contains a glycosidase active site, but the function(s) of the beta-subunit remain poorly defined. We report here an in vivo enzymatic analysis using gene disruptants lacking either the G-II alpha- or beta-subunit in the filamentous fungus Aspergillus oryzae. Using synthetic oligosaccharides as probes, G-II activity of the membranous fraction of the gene disruptants was investigated. The fraction lacking the beta-subunit retained hydrolytic activity toward p-nitrophenyl alpha-d-glucopyranoside but was inactive toward both Glc(2)Man(9)GlcNAc(2) and Glc(1)Man(9)GlcNAc(2). When the fraction containing the beta-subunit was added to the one including the alpha-subunit, the glucosidase activity was restored. These results suggested that the beta-subunit confers the substrate specificity toward di- and monoglucosylated glycans on the glucose-trimming activity of the alpha-subunit.

The folding of glycoproteins is primarily mediated by a quality control system in the ER, in which UDP-Glc:glycoprotein glucosyltransferase (UGGT) serves as a "folding sensor". In this system, client glycoproteins are delivered to UGGT after the trimming of their innermost glucose residue by glucosidase II, which releases them from the lectin chaperones calnexin (CNX) and calreticulin (CRT). UGGT is inactive against folded proteins, allowing them to proceed to the Golgi apparatus for further processing to complex- or hybrid-type glycoforms. On the other hand, this enzyme efficiently glucosylates incompletely folded glycoproteins to monoglucosylated structures, providing them with an opportunity to interact with CNX/CRT. In order to clarify the mode of this enzyme's substrate recognition, we conducted a structure-activity relationship study using a series of synthetic probes. The inhibitory activities of various glycans suggest that UGGT has a strong affinity for the core pentasaccharide (Man(3)GlcNAc(2)) of high-mannose-type glycans. Our comparison of the reactivity of acceptors that have been modified by various aglycons supports the hypothesis that UGGT recognizes the hydrophobic region of client glycoproteins. Moreover, we discovered fluorescently labeled substrates that will be valuable for highly sensitive detection of UGGT activity.

It has been shown that free oligosaccharides derived from N-linked glycans accumulate in the cytosol of animal cells. Most of the glycans have only a single GIcNAc at their reducing termini (Gn1 glycans), whereas the original N-glycans retain N,N'-diacetylchitobiose at their reducing termini (Gn2 glycans). Under the conditions of high-performance liquid chromatography (HPLC) mapping established for pyridylamine (PA)-labeled Gn2 N-glycans, Gn1 glycans are not well retained on reversed-phase HPLC, making simultaneous analysis of GO I and Gn2 glycans problematic. We introduced a dual gradient (i.e., pH and butanol gradient) for the separation of Gn1 and Gn2 glycans in a single reversed-phase HPLC. Determination of elution time for various standard Gn2 high-mannose-type glycans, as well as Gn1 glycans found in the cytosol of animal cells, showed that elution of Gn1 and Gn2 glycans Could be separated. Sufficient separation for most Of the Structural isomers could be achieved for Gill and Gn2 glycans. This HPLC, therefore, is a powerful method for identification of the structures of PA-labeled glycans, especially Gn1-type glycans, isolated from the cytosol of animal cells. (C) 2008 Elsevier Inc. All rights reserved.

The Jacalin-related lectin (JRL) family comprises galactose-binding-type (gJRLs) and mannose-binding-type (mJRLs) lectins. Although the documented occurrence of gJRLs is confined to the family Moraceae, mJRLs are widespread in the plant kingdom. A detailed comparison of sugar-binding specificity was made by frontal affinity chromatography to corroborate the structure-function relationships of the extended mJRL subfamily. Eight mJRLs covering a broad taxonomic range were used: Artocarpin from Artocarpus integrifolia (jackfruit, Moraceae), BanLec from Musa acuminata (banana, Musaceae), Calsepa from Calystegia sepium (hedge bindweed, Convolvulaceae), CCA from Castanea crenata (Japanese chestnut, Fagaceae), Conarva from Convolvulus arvensis (bindweed, Convolvulaceae), CRLL from Cycas revoluta (King Sago palm tree, Cycadaceae), Heltuba from Helianthus tuberosus (Jerusalem artichoke, Asteraceae) and MornigaM from Morus nigra (black mulberry, Moraceae). The result using 103 pyridylaminated glycans clearly divided the mJRLs into two major groups, each of which was further divided into two subgroups based on the preference for high-mannose-type N-glycans. This criterion also applied to the binding preference for complex-type N-glycans. Notably, the result of cluster analysis of the amino acid sequences clearly corresponded to the above specificity classification. Thus, marked correlation between the sugar-binding specificity of mJRLs and their phylogeny should shed light on the functional significance of JRLs.

Combined deficiency of factors V and VIII (F5F8D) is a bleeding disorder caused by mutations in LMAN1 or MCFD2. LMAN1 encodes ERGIC-53, a cargo receptor with an L-type lectin domain, and MCFD2 is a EF-hand-containing protein. We prepared a biotinylated, soluble form of ERGIC-53, which we labeled with R-phycoerythrin conjugated streptavidin. By flow cytometry, sERGIC-53-SA bound to HeLaS3 cells in the presence of calcium but only after preincubation with MCFD2. Treating the cells with endo H or incubating them with high mannose-type oligosaccharides, especially M-8B, abrogated sERGIC-53-SA binding. Surface plasmon resonance experiments demonstrated that MCFD2 specifically bound to sERGIC-53 and 2 MCFD2 mutants found in F5F8D patients had a K-a that was 3 or 4 orders of magnitude lower for sERGIC-53 than for wild-type MCFD2. The K-a of sERGIC-53 and MCFD2 was measured at several pH values and calcium concentrations, and we found that at a calcium concentration less than 0.2 mM, this interaction became significantly weaker. These results demonstrate that the binding of ERGIC-53 to sugar is enhanced by its interaction with MCFD2, and defects in this interaction in F5F8D patients may be the cause for reduced secretion of factors V and VIII.

yIntracellular environments are highly crowded due to the presence of various biomacromolecules. In this study, we estimated the property of the endoplasmic reticulum glucosidase II (G-II) under macromolecular crowding conditions. A crowded milieu that contains bovine serum albumin greatly enhanced the second trimming step (cleavage 2), which deglucosylates Glc(1)Man(9)GIcNAc(2), but not the first trimming step (cleavage 1), which removes the terminal glucose residue from GIC(2)Man(9)GIcNAC(2). A similar effect was obtained with ribonuclease A and high molecular weight polyethylene glycol 20 000. An analysis of CD spectra suggested that G-II enhanced its cleavage 2 activity through conformational change. We also investigated the effects of molecular crowding on other N-linked glycan-processing enzymes, UDP-Glc: glycoprotein glucosyltransfe rase and 1,2-alpha-mannosidase. Our results indicate that the kinetics of glycan processing under crowded conditions may be quite different from those measured in dilute buffers.

VIP36-like protein (VIPL) was identified as an endoplasmic reticulum (ER) resident protein with homology to VIP36, a cargo receptor involved in the transport of glycoproteins within cells. Although VIPL is structurally similar to VIP36, VIPL is thought not to be a lectin, because its sugar-binding activity has not been detected in several experiments. Here, recombinant soluble VIPL proteins (sVIPL) were expressed in Escherichia coli, biotinylated with biotin ligase and oligomerized with R-phycoerythrin (PE)-labeled streptavidin (SA). As measured with flow cytometry, PE-labeled sVIPL-SA bound to deoxymannojirimycin (DMJ)-or kifunensine (KIF)- but not to swainsonine (SW)-treated HeLaS3 cells in the presence of calcium. A surface plasmon resonance analysis showed that the avidity of sVIPL was enhanced after it formed a complex with SA. The binding of PE-labeled sVIPL-SA was abrogated by endo beta-N-acetylglucosaminidase H treatment of the DMJ- or KIF-treated cells. Competition with several high-mannose-type N-glycans inhibited VIPL binding, and indicated that VIPL recognizes the Man alpha 1-2Man alpha 1-2Man sequence. Glucosylation of the outer mannose residue of this portion decreased the binding. Although the biochemical characteristics of VIPL are similar to those of VIP36, the sugar-binding activity of VIPL was stronger at neutral pH, corresponding to the pH in the lumen of the ER, than under acidic conditions.

Mannan-binding protein (MBP) is a C-type mammalian lectin specific for mannose and N-acetylglucosamine. MBP is mainly synthesized in the liver and occurs naturally in two forms, serum MBP (S-MBP) and intracellular MBP (I-MBP). S-MBP activates complement in association with MBP-associated serine proteases via the lectin pathway. Despite our previous study (Mori, K., Kawasaki, T., and Yamashina, I. (1984) Arch. Biochem. Biophys. 232, 223 -233), the subcellular localization of I-MBP and its functional implication have not been clarified yet. Here, as an extension of our previous studies, we have demonstrated that the expression of human MBP cDNA reproduces native MBP differentiation of S-MBP and I-MBP in human hepatoma cells. I-MBP shows distinct accumulation in cytoplasmic granules, and is predominantly localized in the endoplasmic reticulum (ER) and involved in COPII vesicle-mediated ER-to-Golgi transport. However, the subcellular localization of either a mutant (C236S/ C244S) I-MBP, which lacks carbohydrate-binding activity, or the wild-type I-MBP in tunicamycin-treated cells shows an equally diffuse cytoplasmic distribution, suggesting that the unique accumulation of I-MBP in the ER and COPII vesicles is mediated by an N-glycan-lectin interaction. Furthermore, the binding of I-MBP with glycoprotein intermediates occurs in the ER, which is carbohydrate-and pH-dependent, and is affected by glucose-trimmed high-mannose-type oligosaccharides. These results strongly indicate that I-MBP may function as a cargo transport lectin facilitating ER-to-Golgi traffic in glycoprotein quality control.

High mannose-type glycan-lectin interactions play important roles especially in quality control of glycoproteins. VIP36 is a receptor with homology to plant leguminous lectins in its luminal region. The luminal region of VIP36 with a C-terminal biotinylation-tag (sVIP36) was expressed in Escherichia coli and oligomerized with R-phycoerythrin (PE)-labelled streptavidin. Flow cytometric analysis revealed that PE-labelled sVIP36-SA complex (sVIP36-SA) bound to deoxymannojirimycin (DMJ)- and kifunensine (KIF)-treated HeLaS3 cells. The binding of sVIP36-SA to HeLaS3 cells treated with DMJ or KIF was abolished by endo-p-N-acetylglucosaminidase H treatment of the cells. Furthermore, the binding of sVIP36-SA to the cells was inhibited by high mannose-type glycans especially Man(7-9) GIcNAC(2), indicating that the binding of sVIP36-SA to cell surfaces was mediated by high mannose-type glycans. Although VIP36 has the lower affinity for ligands than typical homologous plant lectins, we were able to monitor the sugarbinding activity of VIP36 using less than 100ng of the sVIP36-SA. This method is highly sensitive and suitable for detecting interactions between lectins and sugar chains of low affinity.

High mannose-type glycan-lectin interactions play important roles especially in quality control of glycoproteins. VIP36 is a receptor with homology to plant leguminous lectins in its luminal region. The luminal region of VIP36 with a C-terminal biotinylation-tag (sVIP36) was expressed in Escherichia coli and oligomerized with R-phycoerythrin (PE)-labelled streptavidin. Flow cytometric analysis revealed that PE-labelled sVIP36-SA complex (sVIP36-SA) bound to deoxymannojirimycin (DMJ)- and kifunensine (KIF)-treated HeLaS3 cells. The binding of sVIP36-SA to HeLaS3 cells treated with DMJ or KIF was abolished by endo-p-N-acetylglucosaminidase H treatment of the cells. Furthermore, the binding of sVIP36-SA to the cells was inhibited by high mannose-type glycans especially Man(7-9) GIcNAC(2), indicating that the binding of sVIP36-SA to cell surfaces was mediated by high mannose-type glycans. Although VIP36 has the lower affinity for ligands than typical homologous plant lectins, we were able to monitor the sugarbinding activity of VIP36 using less than 100ng of the sVIP36-SA. This method is highly sensitive and suitable for detecting interactions between lectins and sugar chains of low affinity.

Glucosidase II (Glc'ase II) is a glycan-processing enzyme that trims two alpha 1,3-linked Glc residues in succession from the glycoprotein oligosaccharide Glc2Man9GlcNAc2 to give Glc1Man9GlcNAc2 and Man9GlcNAc2 in the endoplasmic reticulum (ER). Monoglucosylated glycans, such as Glc1Man9GlcNAc2, 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 (Man alpha 1,2Man alpha 1,6Man-) of high man-nose-type glycans.

Systematic synthesis of high-mannose-type sugar chains of asparagine-linked glycoproteins is described. To construct the target sugar chains, we employed the convergent route, using three oligosaccharide components, the common hexasaccharide, branched tri-, tetra-and pentasaccharides, and mono-, di-, and triglucosyl fragments. Construction of the beta-mannoside linkage was performed using p-methoxybenzyl-assisted intramolecular aglycon delivery. The hexasaccharide fragment was coupled with the branched mannooligosaccharide donors such as M5, M4B, WC, and M3 to give undecasaccharide (M9), decasaccharide (M8B and M8C), and nonasaccharide (M17), respectively. Incorporation of mono- di-, and triglucosyl fragments toward them gave tetradecasaccharide (G3M9), tridecasaccharide (G2M9), dodecasaccharide (G1M9), undecasaccharide (G1M8B and G1M8C), and decasaccharide (G1M7), respectively. (c) 2006 Elsevier Ltd. All rights reserved.

Various high-mannose-type glycan modifications of dihydrofolate reductase (DHFR) were achieved by ligand-based approach using glycan-methotrexate (MTX) conjugates as tight binding glycan bearing ligands for DHFR. The resulting glycan-MTX conjugates and the corresponding artificial glycoproteins could be useful as oligosaccharide- and glycoprotein-probes to perform quantitative analysis of glycan recognizing protein such as lectins, glycosyltransferases or glycosidases. Moreover, artificial glycoproteins having two different high-mannose-type glycans were developed for the first time by a combination of two different types of glycan modification strategies. (c) 2006 Elsevier Ltd. All rights reserved.

Peptide: N-glycanase ( PNGase) is the deglycosylating enzyme, which releases N-linked glycan chains from N-linked glyco peptides and glycoproteins. Recent studies have revealed that the cytoplasmic PNGase is involved in the degradation of misfolded/unassembled glycoproteins. This enzyme has a Cys, His, and Asp catalytic triad, which is required for its enzymatic activity and can be inhibited by "free" N-linked glycans. These observations prompted us to investigate the possible use of haloacetamidyl derivatives of N-glycans as potent inhibitors and labeling reagents of this enzyme. Using a cytoplasmic PNGase from budding yeast (Png1), Man(9)GlcNAc(2)-iodoacetoamide was shown to be a strong inhibitor of this enzyme. The inhibition was found to be through covalent binding of the carbohydrate to a single Cys residue on Png1, and the binding was highly selective. The mutant enzyme in which Cys(191) of the catalytic triad was changed to Ala did not bind to the carbohydrate probe, suggesting that the catalytic Cys is the binding site for this compound. Precise determination of the carbohydrate attachment site by mass spectrometry clearly identified Cys191 as the site of covalent attachment. Molecular modeling of N, N'-diacetylchitobiose (chitobiose) binding to the protein suggests that the carbohydrate binding site is distinct from but adjacent to that of Z-VAD-fmk, a peptide-based inhibitor of this enzyme. These results suggest that cytoplasmic PNGase has a separate binding site for chitobiose and other carbohydrates, and haloacetamide derivatives can irreversibly inhibit that catalytic Cys in a highly specific manner.

High-mannose-type oligosaccharides, which are co-translationally introduced to nascent polypeptides, play important roles in protein quality control. This process is very complex, involving a number of lectins, chaperones and glycan processing enzymes. For example, calnexin (CNX) and calreticulin (CRT) are molecular chaperons that recognize mono-glucosylated form of high-mannose-type glycans. UDP-Glucose : glycoprotein glucosyltransferase (UGGT) only glucosylates high-mannose-type glycans of unfolded glycoproteins. Fbs 1 is a part of ubiquitin ligase that recognizes sugar chains. Although recent studies have clarified properties of these proteins, most of them used oligosaccharides derived from natural source, which contain structural heterogeneity. In order to gain a precise insight about protein quality control, we comprehensively synthesized high-mannose-type glycans associated with protein quality control system. Additionally, during our investigations of artificial glycoproteins having homogeneous oligosaccharides, a novel non-peptidic substrate for UGGT was discovered. Using these synthetic oligosaccharide probes, we quantitatively evaluated the activity of CRT, Fbs 1 and UGGT.

High-mannose-cype oligosaccharides, which are cotranslationally introduced to nascent polypeptides, play important roles in glycoprotein quality control. This process is highly complex, involving a number of lectins, chaperones, and glycan-processing enzymes. For example, calnexin and calreticulin (CRT) are molecular chaperones that recognize monoglucosylated forms of high-mannose-type glycans. UDP-glucose:glycoprotein glucosyltransferase (UGGT) only glucosylates high-mannose-type glycans attached to partially folded proteins. Fbs1 is a component of ubiquitin ligase that recognizes sugar chains. Although recent studies have clarified the properties of these proteins, most of them used oligosaccharides derived from natural sources, which contain structural heterogeneity. In order to gain a more precise understanding, we started our program to comprehensively synthesize high-mannose-type glycans associated with a protein quality control system. Additionally, investigation of artificial glycoproteins led us to the discovery of the first nonpeptidic substrate of UGGT. These synthetic oligosaccharide probes have allowed us to conduct quantitative evaluations of the activity and specificity of CRT, Fbs1, and UGGT. (c) 2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

Calnexin (CNX) and its soluable homologue calreticulin (CRT) are lectin-like molecular chaperones that help newly synthesized glycoproteins to fold correctly in the rough endoplasmic reticulum (ER). To investigate the mechanism of glycoprotein-quality control, we have synthesized structurally defined high mannose-type oligosaccharides related to this system. The paper describes the synthesis of the non-natural undecsaccharide 2 and heptasaccharide 16, designed as potential inhibitors of the ER quality-control system. Each possesses the key tetrasaccharide element (Glc(1)Man(3)) critical for the CNX/CRT binding, while lacking the pentamannosyl branch required for glucosidase II recognition. These oligosaccharides were evaluated for their ability to bind CRT by isothermal titration colorimetry (ITC). As expected, each of them had a significant affinity towards CRT In addition, these compounds were shown to be resistant to glucosidose II digestion. Their activities in blocking the chaperone function of CRT wet I e next measured by using malate dehydrogenose (MDH) as a substrate. Their inhibitory effects were shown to correlate well with their CRT-binding affinities, both being critically dependent upon the presence of the terminal glucose (Glc) residue.

High-mannose-type oligosaccharicles have been shown to play important roles in protein quality control. Several intracellular proteins, such as lectins, chaperones and glycan-processing enzymes, are involved in this process. These include calnexin/calreticulin, UDP-glucose:glycoprotein glucosyltransferase (UGGT), cargo receptors (such as VIP36 and ERGIC-53), mannosidase-like proteins (e.g. EDEM and Htm1p) and ubiquitin ligase (Fbs). They are thought to recognize high-mannose-type glycans with subtly different structures, although the precise specificities are yet to be clarified. In order to gain a clear understanding of these protei n-carbohydrate interactions, comprehensive synthesis of high-mannose-type glycans was conducted. In addition, two approaches to the synthesis of artificial glycoproteins with homogeneous oligosaccharides were investigated. Furthermore, a novel substrate of UGGT was discovered.

Lectins belonging to the jacalin-related lectin family are distributed widely in the plant kingdom. Recently, two mannose-specific lectins having tandem repeat-type structures were discovered in Castanea crenata (angiosperm) and Cycas revoluta (gymnosperm). The occurrence of such similar molecules in taxonomically less related plants suggests their importance in the plant body. To obtain clues to understand their physiological roles, we performed detailed analysis of their sugar-binding specificity. For this purpose, we compared the dissociation constants (K-d) of Castanea crenata agglutinin (CCA) and Cycas revoluta leaf lectin (CRLL) by using 102 pyridylaminated and 13 p-nitrophenyl oligosaccharides with a recently developed automated system for frontal affinity chromatography. As a result, we found that the basic carbohydrate-binding properties of CCA and CRLL were similar, but differed in their preference for larger N-linked glycans (e.g. Man7-9 glycans). While the affinity of CCA decreased with an increase in the number of extended alpha 1-2 mannose residues, CRLL could recognize these Man7-9 glycans with much enhanced affinity. Notably, both lectins also preserved considerable affinity for mono-antennary, complex type N-linked glycans, though the specificity was much broader for CCA. The information obtained here should be helpful for understanding their functions in vivo as well as for development of useful probes for animal cells. This is the first systematic approach to elucidate the fine specificities of plant lectins by means of high-throughput, automated frontal affinity chromatography.

Triglucosylated high-mannose-type tetradeca saccharide (Glc(3)Man(9)GlcNAc(2)), the oligosaccharide part of the donor substrate of oligosaccharyl transferase (OST) complex, and diglucosylated tridecasaccharide (Glc(2)Man(9)GlcNAc(2)) were synthesized. These oligosaccharides were assembled in a convergent and stereoselective manner. Undecasaccharide 5 was employed as the common intermediate, and coupling with trisaccharide (4) and disaccharide (3) donor afforded fully protected tetradeca-(17) and tridecasaccharide (16), respectively. These oligosaccharides were deprotected to give Glc(3)Man(9)GlcNAc(2) and Glc(2)Man(9)GlcNAc(2), respectively. (c) 2005 Elsevier Ltd. All rights reserved.

Fbs1 is a recently discovered F-box protein that was proposed to recognize high-mannose-type asparagine-linked glycoprotein sugar chains. To reveal the specificity of Fbs1, Manα 1→ 6Manβ 1→ 4GlcNAc(2), Manα 1→ 3Marβ→ 4GlcNAc(2), and Manα 1→ 3(Manα 1→ 6)Manβ 1→ 4GlcNAc(2) were synthesized and their affinities for Fbs1 were evaluated in comparison with previously synthesized Man(9)GlcNAc(2) and Man(8)GlcNAc(2). These analyses revealed that Man(3)GlcNAc(2) had the strongest affinity and the chitobiose and α 1→ 6 linked Man residue are necessary for Fbs1 to recognize a sugar.

Oligosaccharide parts of glycoproteins play important roles in a variety of biological events. In particular, the functions of asparagine (Asn)-linked oligosaccharides in ER glycoprotein quality control are attracting recent attention (Figure 1). Natural glycoproteins usually exist as a mixture of various glycoforms that differ in the structures of oligosaccharides. In order to understand oligosaccharide functions in ER glycoprotein quality control system precisely, molecular probes and glycoprotein probes having homogeneous and structurally defined oligosaccharide are desired. We wish to report the synthesis of novel ER-related N-glycan-methotrexate (MTX) conjugates (1)-(5) as functional oligosaccharide probes (Figure 2). MTX is known to be a strong inhibitor of dihydrofolate reductase (DHFR) (K_D<1nM) and its specific absorption at 304 nm may be useful as detectable tag. The tight binding between N-glycan-MTX and DHFR could be applicable to create N-glycan-grafted DHFR as an artificial glycoprotein molecular probe. Moreover, this approach was applied to create first bifunctional artificial glycoprotein (9a-c) by connecting G1M9-DHFR (8) and N-glycan-MTX (2)-(4) (Figure 3). To correlate the functions of glycan chains with their structures in glycoprotein quality control system, interaction analyses between 2-5 and UDP-Glc: glycoprotein glucosyl transferase (UGGT), which play an important role as a folding sensor by specifically glucosylating Man_9GlcNAc_2-glycans in incompletely folded glycoprotein in ER protein quality control system, were also examined. The unique substrate specificity of UGGT (Table 1), which accepts neither glycan chain (Man_9GlcNAc_2) nor corresponding short glycopeptides as substrates, have hampered the creation of small molecule oligosaccharide probes. We found that M9MTX (2) is first small molecule oligosaccharide probe being recognized by UGGT. Furthermore, the first chemical characterization of glucosylated product of UGGT reaction using HPLC, MALDI-TOF MS and NMR was achieved (Figure 4). Oligosaccharide specificities of UGGT reactions became apparent that M8(B)MTX (3) and M8(C)MTX (4) show 60% UGGT activities compared with M9MTX (2), though M8(B)GN1MTX (5) is completely unreacted.

Highly diastereoselective carbon-carbon bond-forming reactions achieved on a variety of sugar templates were found. For example, methyl 6-deoxy-2,3-di-O-(t-butyldimethylsilyl)-alpha-D-glucopyranoside can serve as a versatile and effective chiral auxiliary. Namely, the 1,4-addition reactions to its 4-O-crotonyl ester of some carbon nucleophiles or alkyl radicals proceeded highly diastereoselectively to afford the corresponding adducts. The alkylation of the enolate generated from its 4-O-propionyl ester afforded the alpha-alkylated product with a useful level of stereoselectivity. Furthermore, the Diels-Alder reaction of its 4-O-acryloyl ester with cyclopentadiene afforded an endo-adduct stereoselectively. In the case of the 1,3-dipolar cycloaddition of the acryloyl ester, again the cycloadduct was obtained highly stereoselectively. Some other types of sugar templates prepared from methyl alpha-D-glucopyranoside, alpha-D-mannopyranoside and alpha-D-galactopyranoside were explored to verify their ability to function as chiral templates. In some cases, these templates afforded complementary results in the sense of diastereoselectivity. Methodologies for the removal of the sugar template from the products were developed.

A novel type of artificial glycoprotein was developed, by using dihydrofolate reductase (DHFR) and methotrexate (MTX) as a protein-ligand pair. Various oligosaccharides linked to MTX were shown to bind tightly with DHFR and afforded oligosaccharide-grafted protein, which could be isolated easily by lectin beads. (C) 2004 Elsevier Ltd. All rights reserved.

1,4-Addition of the enolate generated from methyl 6-deoxy-2,3-di-O-(t-butyldimethylsilyl)-4-O-propionyl-alpha-D-gluco- pyranoside to methyl crotonate provided a single anti-adduct with exceptionally high stereoselectivity. From this adduct, (-)-lasiol, an acyclic monoterpene alcohol isolated from the males of Lasius meridionalis ants, was synthesized concisely.

The 1,4-addition of magnesium divinylcuprate prepared from vinylmagnesium bromide and cuprous bromide to some 4-O-crotonyl derivatives of methyl α-D-glucopyranoside proceeds with a high level of diastereochemical induction, providing the adduct in good-to-excellent yields. Other organocuprates also serve as effective carbon nucleophiles for the 1,4-addition. Removal of the carbohydrate moiety from each adduct afforded a variety of β-C-substituted butanoic esters in remarkable enantiomeric excess. The 1,4-addition of the same cuprate to some methyl α-D-manno- or α-D-galactopyranosidic substrates in which a crotonyl group was incorporated, each at 3-OH, was also investigated. The reverse π-facial attack of the cuprate was observed when some D-manno-type substrates were subjected to 1,4-addition conditions similar to those used for the D-gluco-type substrates. Furthermore, some D-galacto-type substrates provided 1,4-adducts with higher diastereoselectivities.

The Diels-Alder reactions of some carbohydrate derivatives, as chiral acrylic esters, with cyclopentadiene proceed highly diastereoselectively to provide the adducts carrying a norbornene carboxylate. By reductive removal of the carbohydrate templates from the adducts, both 2S and 2R-enriched 5-norbornene-2-methanol are obtained.

The Diels-Alder reactions of a variety of hexopyranosides carrying an acrylic ester with cyclopentadiene were examined. Some acrylic esters provided the cycloaddition products carrying a norbornene carboxylate with a high level of diastereoselectivity. Plausible mechanisms are presented for the cases of a 40-acryloyl-6-deoxy-alpha -D-glucopyranosidic and 2-O-acryloyl-alpha -D-glucopyranosidic substrates. By reductive removal of the carbohydrate templates from the adducts, either 2S or 2R-enriched 5-norbornene-2-methanol were obtained.

The Diels-Alder reactions of a variety of hexopyranosides carrying an acrylic ester with cyclopentadiene were examined. Some acrylic esters provided the cycloaddition products carrying a norbornene carboxylate with a high level of diastereoselectivity. Plausible mechanisms are presented for the cases of a 40-acryloyl-6-deoxy-alpha -D-glucopyranosidic and 2-O-acryloyl-alpha -D-glucopyranosidic substrates. By reductive removal of the carbohydrate templates from the adducts, either 2S or 2R-enriched 5-norbornene-2-methanol were obtained.

A highly functionalized cyclopentene carboxylic acid derivative, as an advanced synthetic intermediate of viridenomycin, was synthesized. The synthesis commenced with the previously reported highly functionalized tetrahydrofuran derivative prepared from D-glucose. The stereochemical confirmation of the present final compound was conducted by comparison with a compound synthesized very recently by the Meyers group. (C) 2000 Elsevier Science Ltd. All rights reserved.

The reaction of (Z)-3-deoxy-3-C-[(hydroxymethyl)methylene]-1,2:5,6-di-O-isopropylidene-alpha-D-ribo-hexofuranose, prepared from D-glucose, with 1,1-dimethoxycyclohexane in the presence of propanoic acid at 135 degrees C, then at 200 degrees C, provided two Claisen rearrangement products, namely (2R,3R,4S,5S)-2,3-(isopropylidene)dioxy-5-[(1R)-1,2-(isopropylidene)dioxyethyl]-4-[(1S)- and (1R)- 2-oxocyclohexyl]-4-vinyltetrahydrofuran in a ratio of 3.3:1. L-Selectride (R) reduction of the major product gave the corresponding (S)-cyclohexanol exclusively. In contrast, the Claisen rearrangement of the aforementioned allylic alcohol with 3,3-dimethoxycyclohexene proceeded with complete stereoselectivity to provide the corresponding 4-[(1 S)-2-oxocyclohex-3-enyl]-4-vinyltetrahydrofuran exclusively. The 1,4-conjugate additions to the thus formed cyclohexenone derivative with dimethyl and divinylcuprates proceeded with complete pi-facial selection to provide the 3-methylated and 3-vinylated cyclohexanone derivatives, both in high yields. (C) 1998 Elsevier Science Ltd. All rights reserved.