Mouse lymphoma cell lines resistant to pea lectin are defective in fucose metabolism

Rhamnose-Containing Compounds: Biosynthesis and Applications

Rhamnose-associated molecules are attracting attention because they are present in bacteria but not mammals, making them potentially useful as antibacterial agents. Additionally, they are also valuable for tumor immunotherapy. Thus, studies on the functions and biosynthetic pathways of rhamnose-containing compounds are in progress. In this paper, studies on the biosynthetic pathways of three rhamnose donors, i.e., deoxythymidinediphosphate-L-rhamnose (dTDP-Rha), uridine diphosphate-rhamnose (UDP-Rha), and guanosine diphosphate rhamnose (GDP-Rha), are firstly reviewed, together with the functions and crystal structures of those associated enzymes. Among them, dTDP-Rha is the most common rhamnose donor, and four enzymes, including glucose-1-phosphate thymidylyltransferase RmlA, dTDP-Glc-4,6-dehydratase RmlB, dTDP-4-keto-6-deoxy-Glc-3,5-epimerase RmlC, and dTDP-4-keto-Rha reductase RmlD, are involved in its biosynthesis. Secondly, several known rhamnosyltransferases from Geobacillus stearothermophilus, Saccharopolyspora spinosa, Mycobacterium tuberculosis, Pseudomonas aeruginosa, and Streptococcus pneumoniae are discussed. In these studies, however, the functions of rhamnosyltransferases were verified by employing gene knockout and radiolabeled substrates, which were almost impossible to obtain and characterize the products of enzymatic reactions. Finally, the application of rhamnose-containing compounds in disease treatments is briefly described.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

Flavoenzyme CrmK-mediated substrate recycling in caerulomycin biosynthesis

Biochemical and structural investigations into the flavoenzyme CrmK reveal a substrate recycling/salvaging mechanism in caerulomycin biosynthesis.

Substrate salvage or recycling is common and important for primary metabolism in cells but is rare in secondary metabolism. Herein we report flavoenzyme CrmK-mediated shunt product recycling in the biosynthesis of caerulomycin A (CRM A 1), a 2,2′-bipyridine-containing natural product that is under development as a potent novel immunosuppressive agent. We demonstrated that the alcohol oxidase CrmK, belonging to the family of bicovalent FAD-binding flavoproteins, catalyzed the conversion of an alcohol into a carboxylate via an aldehyde. The CrmK-mediated reactions were not en route to 1 biosynthesis but played an unexpectedly important role by recycling shunt products back to the main pathway of 1. Crystal structures and site-directed mutagenesis studies uncovered key residues for FAD-binding, substrate binding and catalytic activities, enabling the proposal for the CrmK catalytic mechanism. This study provides the first biochemical and structural evidence for flavoenzyme-mediated substrate recycling in secondary metabolism.

Cell lines expressing mutant FX proteins to generate proteins with reduced rate of fucosylation: WO2010/141478

BACKGROUND

The application (WO 2010/141478) is in the field of glycobiology and deals with the synthesis of recombinant proteins with a reduced rate of fucosylation.

OBJECTIVE

It aims at generating mammalian cell lines that express mutant forms of GDP-4-keto-deoxy-mannose-3,5,-epimerase-4-reductase (FX) and cells that conditionally fucosylate proteins.

METHODS

Mutant forms of FX were synthesized, and mammalian cell lines genetically engineered to express mutant FX proteins and the protein of interest. Cell lines that conditionally fucosylate proteins were generated from a mutant FX form that has reduced ability to fucosylate glycoproteins at 37°C and not at 34°C.

RESULTS

Cells genetically engineered to express mutant forms of FX protein show reduced ability to fucosylate proteins, particularly antibodies, with rates as low as 5 - 0.5% fucosylation, compared to cells expressing wild-type FX.

CONCLUSION

Cells genetically engineered to express the mutant forms of FX protein provide a means to synthesize and express proteins with a reduced rate of fucosylation. The paradigm may be used to synthesize antibodies that mediate antibody-dependent cell-mediated cytotoxicity more efficiently. The application uses mammalian cell lines, genetically engineered to express mutant FX proteins, to synthesize and produce proteins with a reduced rate of fucosylation. The application claims the conditional control of protein fucosylation by FX mutant proteins.

Modulation of GDP-fucose level for generating proteins with reduced rate of fucosylation (WO2010141855)

BACKGROUND

The application (WO2010141855) is in the field of glycobiology, and involves the control of the rate of fucosylation of proteins by exogenous factors.

OBJECTIVE

It aims at controlling the rate of protein fucosylation with inhibitors (drugs or nucleic acid antagonists) of enzymes involved in the synthesis of GDP-fucose.

METHODS

Mammalian cell lines were cultured in the presence of inhibitors, for example, siRNA. The rates of GDP-fucose in cells and during protein fucosylation were characterized.

RESULTS

The level of protein fucosylation decreases rapidly in response to a decrease in GDP-fucose level.

CONCLUSION

The relationship between the rate of fucosylation of proteins and the level of GDP-fucose in a cell is non-linear. Reduction in the rate of protein fucosylation can be achieved with a minimal reduction of the level of GDP-fucose in cells. The paradigm may be used to synthesize proteins and antibodies, with a reduced rate of fucosylation. The application claims that the use of drugs or nucleic acid antagonists that inhibit the enzymes involved in GDP-fucose biosynthesis optimizes the level of GDP-fucose present in cells, and reduces the rate of fucosylation of glycoproteins.

Functional expression of L-fucokinase/guanosine 5'-diphosphate-L-fucose pyrophosphorylase from Bacteroides fragilis in Saccharomyces cerevisiae for the production of nucleotide sugars from exogenous monosaccharides

The biosynthesis of glycoconjugates requires the relevant glycosyltransferases and nucleotide sugars that can act as donors. Given the biological importance of posttranslational glycosylation, a facile, robust and cost-effective strategy for the synthesis of nucleotide sugars is highly desirable. In this study, we demonstrate the synthesis of nucleotide sugars from corresponding monosaccharides in a highly efficient manner via metabolic engineering, using an enzymatic approach. This method exploits l-fucokinase/guanosine 5'-diphosphate (GDP)-l-fucose (L-Fuc) pyrophosphorylase (FKP), a bifunctional enzyme isolated from Bacteroides fragilis 9343, which converts l-Fuc into GDP-L-Fuc via an L-Fuc-1-phosphate intermediate. Because L-Fuc and d-arabinose (D-Ara) are structurally similar, it is assumed that the biosynthesis of GDP-D-Ara in a recombinant Saccharomyces cerevisiae strain harboring the FKP gene can occur through a mechanism akin to that of GDP-L-Fuc via the salvage pathway. Thus, we reasoned that by exogenously supplying different monosaccharides structurally related to L-Fuc, it should be possible to produce the corresponding nucleotide sugars with this recombinant yeast strain, regardless of internal acquisition of nucleotide sugars through expression of additive enzymes in the de novo pathway.

Establishment of a GDP-mannose 4,6-dehydratase (GMD) knockout host cell line: a new strategy for generating completely non-fucosylated recombinant therapeutics

Currently, removal of core fucose from the Fc oligosaccharides of therapeutic antibodies is widely recognized as being of great importance for the effector function of antibody-dependent cellular cytotoxicity, and alpha-1,6-fucosyltransferase (FUT8) knockout cells have been generated as an ideal host cell line for manufacturing such therapeutics. Here, we attempted to identify genes other than FUT8 that could be targeted for the manufacture of non-fucosylated therapeutics. Loss-of-function analyses using siRNAs against three key genes involved in oligosaccharide fucosylation in Chinese hamster ovary (CHO) cells revealed that there was a positive correlation between the Fc oligosaccharide fucosylation and the mRNA expression through the origin in the cases of both GDP-fucose 4,6-dehydratase (GMD) and FUT8, but not for the GDP-fucose transporter, suggesting that there is no functional redundancy in GMD and FUT8. GMD knockout CHO/DG44 cells were successfully established, and were confirmed to be devoid of intracellular GDP-fucose and to produce completely non-fucosylated antibodies. GMD knockout cells recovered their fucosylation capability through the salvage pathway upon addition of l-fucose into the culture medium, and exhibited equable morphology, growth kinetics and recombinant protein productivity, demonstrating that loss of oligosaccharide fucosylation has no impact on these cellular phenotypes. Our results demonstrate that GMD knockout is a new strategy applicable to the manufacture of non-fucosylated therapeutic antibodies, and completely O-fucose-negative therapeutics as well.

Reconstitution in vitro of the GDP-fucose biosynthetic pathways of Caenorhabditis elegans and Drosophila melanogaster

The deoxyhexose sugar fucose has an important fine-tuning role in regulating the functions of glycoconjugates in disease and development in mammals. The two genetic model organisms Caenorhabditis elegans and Drosophila melanogaster also express a range of fucosylated glycans, and the nematode particularly has a number of novel forms. For the synthesis of such glycans, the formation of GDP-fucose, which is generated from GDP-mannose in three steps catalysed by two enzymes, is required. By homology we have identified and cloned cDNAs encoding these two proteins, GDP-mannose dehydratase (GMD; EC 4.2.1.47) and GDP-keto-6-deoxymannose 3,5-epimerase/4-reductase (GER or FX protein; EC 1.1.1.271), from both Caenorhabditis and Drosophila. Whereas the nematode has two genes encoding forms of GMD (gmd-1 and gmd-2) and one GER-encoding gene (ger-1), the insect has, like mammalian species, only one homologue of each (gmd and gmer). This compares to the presence of two forms of both enzymes in Arabidopsis thaliana. All corresponding cDNAs from Caenorhabditis and Drosophila, as well as the previously uncharacterized Arabidopsis GER2, were separately expressed, and the encoded proteins found to have the predicted activity. The biochemical characterization of these enzymes is complementary to strategies aimed at manipulating the expression of fucosylated glycans in these organisms.

Cloning and expression of murine enzymes involved in the salvage pathway of GDP-L-fucose

In the salvage pathway of GDP-L-fucose, free cytosolic fucose is phosphorylated by L-fucokinase to form L-fucose-L-phosphate, which is then further converted to GDP-L-fucose in the reaction catalyzed by GDP-L-fucose pyrophosphorylase. We report here the cloning and expression of murine L-fucokinase and GDP-L-fucose pyrophosphorylase. Murine L-fucokinase is expressed as two transcripts of 3057 and 3270 base pairs, encoding proteins of 1019 and 1090 amino acids with predicted molecular masses of 111 kDa and 120 kDa respectively. Only the longer splice variant of L-fucokinase was enzymatically active when expressed in COS-7 cells. Murine GDP-L-fucose pyrophosphorylase has an open reading frame of 1773 base pairs encoding a protein of 591 amino acids with a predicted molecular mass of 65.5 kDa. GDP-L-fucose, the reaction product of GDP-L-pyrophosphorylase, was identified by HPLC and MALDI-TOF MS analysis. The tissue distribution of murine L-fucokinase and GDP-L-fucose pyrophosphorylase was investigated by quantitative real time PCR, which revealed high expression of L-fucokinase and GDP-L-fucose pyrophosphorylase in various tissues. The wide expression of both enzymes can also be observed from the large amount of data collected from a number of expressed sequence tag libraries, which indicate that not only the de novo pathway alone, but also the salvage pathway, could have a significant role in the synthesis of GDP-L-fucose in the cytosol.

Fucose: biosynthesis and biological function in mammals

Fucose is a deoxyhexose that is present in a wide variety of organisms. In mammals, fucose-containing glycans have important roles in blood transfusion reactions, selectin-mediated leukocyte-endothelial adhesion, host-microbe interactions, and numerous ontogenic events, including signaling events by the Notch receptor family. Alterations in the expression of fucosylated oligosaccharides have also been observed in several pathological processes, including cancer and atherosclerosis. Fucose deficiency is accompanied by a complex set of phenotypes both in humans with leukocyte adhesion deficiency type II (LAD II; also known as congenital disorder of glycosylation type IIc) and in a recently generated strain of mice with a conditional defect in fucosylated glycan expression. Fucosylated glycans are constructed by fucosyltransferases, which require the substrate GDP-fucose. Two pathways for the synthesis of GDP-fucose operate in mammalian cells, the GDP-mannose-dependent de novo pathway and the free fucose-dependent salvage pathway. In this review, we focus on the biological functions of mammalian fucosylated glycans and the biosynthetic processes leading to formation of the fucosylated glycan precursor GDP-fucose.

Conditional control of selectin ligand expression and global fucosylation events in mice with a targeted mutation at the FX locus

Glycoprotein fucosylation enables fringe-dependent modulation of signal transduction by Notch transmembrane receptors, contributes to selectin-dependent leukocyte trafficking, and is faulty in leukocyte adhesion deficiency (LAD) type II, also known as congenital disorder of glycosylation (CDG)-IIc, a rare human disorder characterized by psychomotor defects, developmental abnormalities, and leukocyte adhesion defects. We report here that mice with an induced null mutation in the FX locus, which encodes an enzyme in the de novo pathway for GDP-fucose synthesis, exhibit a virtually complete deficiency of cellular fucosylation, and variable frequency of intrauterine demise determined by parental FX genotype. Live-born FX(-/-) mice exhibit postnatal failure to thrive that is suppressed with a fucose-supplemented diet. FX(-/-) adults suffer from an extreme neutrophilia, myeloproliferation, and absence of leukocyte selectin ligand expression reminiscent of LAD-II/CDG-IIc. Contingent restoration of leukocyte and endothelial selectin ligand expression, general cellular fucosylation, and normal postnatal physiology is achieved by modulating dietary fucose to supply a salvage pathway for GDP-fucose synthesis. Conditional control of fucosylation in FX(-/-) mice identifies cellular fucosylation events as essential concomitants to fertility, early growth and development, and leukocyte adhesion.

Congenital disorders of glycosylation: glycosylation defects in man and biological models for their study

Several inherited disorders affecting the biosynthetic pathways of N-glycans have been discovered during the past years. This review summarizes the current knowledge in this rapidly expanding field and covers the molecular bases of these disorders as well as their phenotypical consequences.

Leukocyte adhesion deficiency type II

Leukocyte adhesion deficiency type II (LAD II) is a rare disorder characterized by recurrent infections, persistent leukocytosis, and severe mental and growth retardation. LAD II neutrophils are deficient in expression of selectin ligand activity, and exhibit a correspondingly diminished ability to roll on endothelium and to traffic to inflammatory sites in vivo. LAD II patients exhibit a deficiency in the expression of cell surface fucosylated glycan structures that include the H and Lewis blood group determinants and the sialyl Lewis x epitope, yet the corresponding fucosyltransferase activities responsible for synthesis of these structures are expressed at normal levels. The molecular defect in LAD II has been localized to the pathway that synthesizes GDP-fucose from GDP-mannose. However, the two known component enzymes in this GDP-fucose biosynthetic pathway are normal in sequence and in expression levels in LAD II cells. The genetic lesion in LAD II that accounts for the generalized fucosylation defect in LAD II patients remains to be determined.

Structural and enzymatic characterization of human recombinant GDP-D-mannose-4,6-dehydratase

GDP-D-mannose-4,6-dehydratase (GMD) is the key enzyme in the 'de novo' pathway of GDP-L-fucose biosynthesis. The reported cDNA sequences for human GMD predict three forms of different length, whose 'in vivo' occurrence and molecular properties are completely undefined. Here, we report the expression in Escherichia coli and the properties of each native recombinant GMD form. Only the 42 kDa long GMD (L-GMD) and the 40.2 kDa (M-GMD) forms were recovered as soluble functional proteins, while the 38.7 kDa form, short GMD (S-GMD), lacking an N-terminal domain critical for dinucleotide binding, was inactive and formed a precipitate. Both L-GMD and M-GMD are homodimers and contain 1 mol of tightly bound NADP+. Their kinetic properties (Km, Kcat) are apparently identical and both forms are non-competitively feedback-inhibited by GDP-L-fucose to a similar extent. M-GMD is the predominant enzyme form expressed in several human cell lines. These data seem to suggest that modulation of the 'de novo' pathway of GDP-L-fucose biosynthesis involves mechanisms other than differential 'in vivo' expression of GMD forms.

The metabolism of 6-deoxyhexoses in bacterial and animal cells

L-fucose and L-rhamnose are two 6-deoxyhexoses naturally occurring in several complex carbohydrates. In prokaryotes both of them are found in polysaccharides of the cell wall, while in animals only L-fucose has been described, which mainly participates to the structure of glycoconjugates, either in the cell membrane or secreted in biological fluids, such as ABH blood groups and Lewis system antigens. L-fucose and L-rhamnose are synthesized by two de novo biosynthetic pathways starting from GDP-D-mannose and dTDP-D-glucose, respectively, which share several common features. The first step for both pathways is a dehydration reaction catalyzed by specific nucleotide-sugar dehydratases. This leads to the formation of unstable 4-keto-6-deoxy intermediates, which undergo a subsequent epimerization reaction responsible for the change from D- to L-conformation, and then a NADPH-dependent reduction of the 4-keto group, with the consequent formation of either GDP-L-fucose or dTDP-L-rhamnose. These compounds are then the substrates of specific glycosyltransferases which are responsible for insertion of either L-fucose or L-rhamnose in the corresponding glycoconjugates. The enzyme involved in the first step of GDP-L-fucose biosynthesis in E. coli, i.e., GDP-D-mannose 4,6 dehydratase, has been recently expressed as recombinant protein and characterized in our laboratory. We have also cloned and fully characterized a human protein, formerly named FX, and an E. coli protein, WcaG, which display both the epimerase and the reductase activities, thus indicating that only two enzymes are required for GDP-L-fucose production. Fucosylated complex glycoconjugates at the cell surface can then be recognized by specific counter-receptors in interacting cells, these mechanisms initiating important processes including inflammation and metastasis. The second pathway starting from dTDP-D-glucose leads to the synthesis of antibiotic glycosides or, alternatively, to the production of dTDP-L-rhamnose. While several sets of data are available on the first enzyme of the pathway, i.e., dTDP-D-glucose dehydratase, the enzymes involved in the following steps still need to be identified and characterized.

The MUR1 gene of Arabidopsis thaliana encodes an isoform of GDP-D-mannose-4,6-dehydratase, catalyzing the first step in the de novo synthesis of GDP-L-fucose

GDP-L-fucose is the activated nucleotide sugar form of L-fucose, which is a constituent of many structural polysaccharides and glycoproteins in various organisms. The de novo synthesis of GDP-L-fucose from GDP-D-mannose encompasses three catalytic steps, a 4,6-dehydration, a 3,5-epimerization, and a 4-reduction. The mur1 mutant of Arabidopsis is deficient in L-fucose in the shoot and is rescued by growth in the presence of exogenously supplied L-fucose. Biochemical assays of the de novo pathway for the synthesis of GDP-L-fucose indicated that mur1 was blocked in the first nucleotide sugar interconversion step, a GDP-D-mannose-4,6-dehydratase. An expressed sequence tag was identified that showed significant sequence similarity to proposed bacterial GDP-D-mannose-4,6-dehydratases and was tightly linked to the mur1 locus. A full-length clone was isolated from a cDNA library, and its coding region was expressed in Escherichia coli. The recombinant protein exhibited GDP-D-mannose-4,6-dehydratase activity in vitro and was able to complement mur1 extracts in vitro to complete the pathway for the synthesis of GDP-L-fucose. All seven mur1 alleles investigated showed single point mutations in the coding region for the 4,6-dehydratase, confirming that it represents the MUR1 gene.

Synthesis of GDP-L-fucose by the human FX protein

FX is a homodimeric NADP(H)-binding protein of 68 kDa, first identified in human erythrocytes, from which it was purified to homogeneity. Its function has been unrecognized despite partial structural and genetic characterization. Recently, on the basis of partial amino acid sequence, it proved to be the human homolog of the murine protein P35B, a tumor rejection antigen. In order to address the biochemical role of FX, its primary structure was completed by cDNA sequencing. This sequence revealed a significant homology with many proteins from different organisms. Specifically, FX showed a remarkable similarity with a putative Escherichia coli protein, named Yefb, whose gene maps in a region of E. coli chromosome coding for enzymes involved in synthesis and utilization of GDP-D-mannose. Accordingly, a possible role of FX in this metabolism was investigated. The data obtained indicate FX as the enzyme responsible for the last step of the major metabolic pathway resulting in GDP-L-fucose synthesis from GDP-D-mannose in procaryotic and eucaryotic cells. Specifically, purified FX apparently catalyzes a combined epimerase and NADPH-dependent reductase reaction, converting GDP-4-keto-6-D-deoxymannose to GDP-L-fucose. This is the substrate of several fucosyltranferases involved in the correct expression of many glyconjugates, including blood groups and developmental antigens.

A method for [3H]mannose labeling of Asn-linked oligosaccharides on recombinant glycoproteins synthesized in Xenopus oocytes

We have developed an efficient method for labeling the Asn-linked oligosaccharides of recombinant glycoproteins synthesized in Xenopus laevis oocytes. By coinjecting GDP-[3,4-(3)H]mannose with mRNA for human cathepsin D, it was possible to incorporate as much as 1800 cpm per oocyte into each of the two Asn-linked oligosaccharides of this glycoprotein. Overall, about 50% of the microinjected GDP-[3,4-(3)H]mannose was incorporated into Asn-linked oligosaccharides, a 10-fold greater value than that obtained when [2-(3)H]mannose was microinjected. Less than 10% of the injected GDP-[3,4-(3)H]mannose was metabolized to water or converted to amino acids. This technique should facilitate studies of Asn-linked oligosaccharide biosynthesis, processing, and structure in recombinant proteins synthesized in Xenopus oocytes.

Partial characterization of fucosylated cell surface glycoproteins of cultured RPE

The major high molecular weight, fucose containing, cell surface glycoproteins of cultured rat retinal pigment epithelial (RPE) cells were partially characterized. One dimensional peptide mapping by the Cleveland method showed that the polypeptide chains of these proteins were not highly related in structure. Incorporation of 3H-mannose into these glycoproteins was equivalent for normal and dystrophic (RCS rdy-p+) RPE. Furthermore, treatment of the glycoproteins from either normal or dystrophic RPE with Endo-beta-N-acetylglucosaminidase H (Endo H) did not cause a shift in their Mr's, as determined by SDS PAGE. These results suggest that the high Mr glycoproteins do not contain a large quantity of unprocessed, mannose containing core type N-linked oligosaccharides in either normal or dystrophic RPE. Digestion of the 3H-fucose labeled glycoproteins with Peptide N-glycosidase F (PNGase F) demonstrated that at least 90% of the 3H-fucose incorporated into these glycoproteins is in N-linked oligosaccharides. Endo-beta-N-acetylglucosaminidase F (Endo F) treatment showed that at least 75-80% of the 3H-fucose is located in more terminal positions (distal to the fucose that is found in alpha 1,6 linkage to the asparagine-linked N-acetylglucosamine residue) in N-linked carbohydrate. Overall, these results support the hypothesis that if the dystrophic RPE possesses a defect in glycoprotein processing, then this defect affects terminal processing of oligosaccharides and addition of terminally located fucose residues. A homologous group of high Mr, fucosylated glycoproteins was found in plasma membranes from cultured monkey RPE, suggesting atht they may be common to other species.

A cloned human cDNA determines expression of a mouse stage-specific embryonic antigen and the Lewis blood group alpha(1,3/1,4)fucosyltransferase

The stage-specific embryonic antigen SSEA-1 is a cell-surface oligosaccharide molecule expressed with temporal precision during the murine preimplantation period and implicated in adhesive events involving the process of compaction. We used a mammalian transient expression system to isolate a cloned human cDNA that determines expression of the SSEA-1 molecule. The cDNA sequence predicts a type II transmembrane protein with a domain structure similar to mammalian glycosyltransferases, but without primary sequence similarity to these enzymes. The carboxy-terminal domain of this protein was shown to be catalytically active as a fucosyltransferase when expressed in COS-1 cells as a portion of a secreted protein A fusion peptide. The enzyme is an exceptional glycosyltransferase in that it can use both type I and type II oligosaccharides as acceptor substrates to generate subterminal Fuc alpha(1,4)- and Fuc alpha(1,3)-linkages, respectively, in a manner analogous to the human Lewis blood group fucosyltransferase. Southern blot analysis shows that the cDNA corresponds to sequences syntenic to the Lewis locus on chromosome 19. These results indicate that this cDNA is the product of the human Lewis blood group locus, provide genetic confirmation of the hypothesis that this enzyme can catalyze two distinct transglycosylation reactions, and outline an approach to the isolation of other sequences that determine expression of developmentally regulated oligosaccharide antigens.

The spore coat of a fucosylation mutant in Dictyostelium discoideum

Strain HL250 of Dictyostelium discoideum cannot convert GDP-mannose to GDP-fucose, resulting in an inability to fucosylate protein. This affects a group of proteins which are normally fucosylated intracellularly and then secreted via prespore vesicles to become part of the outer lamina of the spore coat. We have found that strain HL250 nevertheless accumulates typical amounts of these proteins, stores them normally in prespore vesicles, and secretes them normally to become a part of the spore coat. However, affected proteins are proteolyzed after germination, the spore coat is more accessible to penetration by a macromolecular probe, and germination is inefficient in older spores. These findings can be explained by a dependence of the integrity of the outer layer of the spore coat on protein-linked fucose.

Characterization of gp39, a B-lymphocyte associated differentiation antigen which is also present on granulocytes and macrophages

The biosynthesis and biochemical characteristics of the 39,000 cell surface glycoprotein detected by Mab 41H.16 were investigated. Experiments utilizing tunicamycin, endoglycosidase H, endoglycosidase F and N-glycosidase F indicate that the mature molecule expressed at the cell surface is composed largely of N-linked oligosaccharides of both the complex and high mannose types. When synthesized in the presence of tunicamycin, the molecule appeared on the cell surface with a Mr of 32,000. Digestion with both endoglycosidase H and endoglycosidase F yielded a single band of Mr 37,000. Parallel experiments with N-glycosidase F revealed species of approx. 35,000 and 32,000. Synthesis in the presence of monensin yielded a 37,500 product. [3H]Glucosamine and [3H]mannose were incorporated into the molecule but no evidence for fucose incorporation could be found. Microheterogeneity of gp39 with respect to Mr and oligosaccharide structure was demonstrated by biosynthetic labelling and lectin chromatography. Biosynthetic pulse-chase labelling showed that the de novo synthesis of the 39,000 molecule occurs without detectable precursor formation. Results of temperature-dependent phase separation experiments were consistent with gp39 being an integral membrane protein. Two-dimensional electrophoresis showed heterogeneity of the isoelectric points associated with the N-linked oligosaccharides. Galactose oxidase/NaB[3H]4 labelling showed that a terminal sialic acid protects a galactose residue. All results are consistent with the conclusion that the gp39 molecule is an integral membrane glycoprotein composed of heterogeneous N-linked oligosaccharides of both the complex and high mannose types.

A quantitative method for GDP-L-Fuc:N-acetyl-beta-D-glucosaminide alpha 1----6fucosyltransferase activity with lectin affinity chromatography

A quantitative method for the activity of GDP-L-Fuc:N-acetyl-beta-D-glucosaminide alpha 1----6fucosyltransferase has been developed using a well-characterized substrate to which other fucosyltransferases fail to transfer and lentil lectin-Sepharose, which will bind this substrate only after fucosylation of the asparagine-linked N-acetylglucosamine. The enzyme was extracted from human skin fibroblasts and incubated with GDP-[14C]fucose and a specific substrate, asialo-agalactotransferrin glycopeptide. The product of the enzyme reaction, [14C]fucose alpha 1----6 to the asparagine-linked N-acetylglucosamine of the substrate, bound to lentil lectin-Sepharose and was eluted with 0.4 M methyl alpha-D mannopyranoside. The method was shown to be specific after characterization of the lentil lectin-bound glycopeptides by enzyme degradation and affinity chromatography. Quantitation of the method was shown by several parameters, including the linearity of product formed with respect to time, GDP-[14C]fucose concentration and enzyme concentration.

Effects of glucose starvation and puromycin treatment on lipid-linked oligosaccharide precursors and biosynthetic enzymes in Chinese hamster ovary cells in vivo and in vitro

Previous studies from several laboratories have demonstrated that glucose-starved Chinese hamster ovary (CHO) cells and other cells in culture switch from synthesis of the normal Glc3Man9GlcNAc2-P-P-Dol to Man5-GlcNAc2-P-P-Dol. In this study we have investigated this phenomenon in CHO cells in vitro and in vivo in order to determine the possible site of this block. Our results demonstrate that enzymatic activities responsible for Man9GlcNA2 synthesis in vitro are normal in glucose-starved cells. In vivo, however, the pool of GDP-[3H]Man is severely depleted, while [3H]mannose incorporation into lipid-linked and protein-bound Man5GlcNAc2 is increased. This result suggests that the available GDP-Man in starved cells is utilized to synthesize Man5GlcNAc2 preferentially, resulting in a reduction of Dol-P-Man and Man6-Man9 GlcNAc2 synthesis in vivo in glucose-starved cells. Conditions which prevent the depletion of GDP-[3H]Man in glucose-starved cells, such as puromycin or cycloheximide treatment, result in normal synthesis of Man9GlcNAc2 by glucose-starved cells. An unexpected finding in the course of this study is that puromycin or cycloheximide treatment of cells, which is known to inhibit lipid-linked oligosaccharide synthesis in glucose-fed cells, has no such inhibitory effect on glucose-starved cells.

Glycophorins of human erythroleukemic K562 cells

Glycophorins related to alpha glycophorin, of the human erythrocyte membrane, were isolated from human erythroleukemic K562 cells. The glycophorins were purified using sodium dodecyl sulfate (SDS)/trichloroacetic acid fractionation and Folch and hot phenol extractions. 0.1-0.2 micrograms was obtained/10(8) cells, or approximately a 15% yield. SDS-gel electrophoresis revealed a pattern similar to erythrocyte alpha glycophorin except for the slower mobility of the glycophorin monomer. Two populations of K562 glycophorins, present in nearly equivalent amounts, were distinguished by their binding to Lens culinaris lectin agarose. The two populations exhibited similar gel electrophoretic patterns except for the presence of delta-like glycophorin exclusively in the population that did not bind to L. culinaris lectin. Immunoblotting revealed a lack of reaction of the major alpha and delta-like glycophorin bands in all K562 glycophorins with M or N erythrocyte glycophorin-specific monoclonal antibodies. Only minor species of intermediate electrophoretic mobility in glycophorins not binding to L. culinaris showed a reaction with these antibodies. Both populations of glycophorins incorporated radiolabeled glucosamine, mannose, and fucose and contained O-glycosidically linked tri- and tetrasaccharides, present in a ratio of approximately 1:1 indicating a significant degree of hyposialylation when compared to erythrocyte alpha glycophorin. No precursor/product relationship was demonstrated between the major forms of two populations. K562 cell surface labeling with lactoperoxidase revealed that only the glycophorins that exhibited binding to L. culinaris were accessible to iodination and could be the only species expressed at the cell surface.

Blood group-related antigens as markers of malignant potential and heterogeneity in human carcinomas

The expression of BGR-Ags is often aberrant in human carcinomas. The observation that BGR-Ag expression in human bladder carcinomas correlates with prognosis for patients with these tumors is especially interesting in light of the numerous reports of correlations between cell surface glycosylation and malignant phenotype in experimental animal tumors. Many observations suggest how this relation might be mediated. It seems reasonable to anticipate that the study of the BGR-Ags and their expression in carcinoma may emerge from its current predominantly descriptive phase and become an important part of the investigation of human tumor biology.

Two Chinese hamster ovary glycosylation mutants affected in the conversion of GDP-mannose to GDP-fucose

A biochemical basis for the pea and lentil lectin resistance of two Chinese hamster ovary (CHO) cell mutants, Lec13 and Lec13A, was investigated. Studies of the G glycopeptides of vesicular stomatitis virus grown in the mutants indicated that Lec13 cells essentially lack the ability to add fucose to complex carbohydrates while Lec13A cells synthesize significant proportions of fucosylated, complex moieties. However, both mutants were known to be reverted to lectin sensitivity by growth in L-fucose, making them similar to the mouse lymphoma mutant, PLR1.3, which is defective in the conversion of GDP-mannose to GPD-fucose [M. L. Reitman, I. S. Trowbridge, and S. Kornfeld (1980) J. Biol. Chem. 255, 9900-9906]. Optimal conditions for the production of GDP-fucose from GDP-mannose by CHO cytosol were found to occur at pH 8 in the presence of 7.5 microM GDP-mannose, 15 mM Mg2+, 0.2 mM NAD+, 0.2 mM NADPH, 10 mM niacinamide, 5 mM ATP, and 50 mM Tris-HCl. Under these conditions, Lec13 cytosol produced no detectable GDP-fucose nor GDP-sugar intermediates while Lec13A cytosol produced significant quantities of both. Mixing experiments with Lec13 cytosol identified the first enzyme of the conversion pathway (GDP-mannose 4,6-dehydratase, EC 4.2.1.47) as the site of the block. In addition to being markedly reduced, the Lec13A 4,6-dehydratase activity was relatively insensitive to changes in pH in comparison to the activity in parental cytosol, suggesting that Lec13A cells might possess a structurally altered GDP-mannose 4,6-dehydratase enzyme.

Characteristics of two wheat germ agglutinin-resistant variants of B16 mouse melanoma cells with reduced tumorigenicity

Two variants of B16 mouse melanoma cells, selected for their resistance to toxic levels of wheat germ agglutinin isolectin 1 (WGA-1) in serum-free medium, showed by chromosome analysis that they are still mouse cell-lines, continue to produce melanin, and are less tumorigenic in mice than the parent B16 cells. The variants showed a marked decrease in cell agglutination with the wheat germ lectin and a slight increase in cell agglutination with concanavalin A. The binding of 125I-labeled wheat germ agglutinin to the two variant lines was likewise decreased over a 10(3)-fold range of lectin concentrations. Terminal sialyl residues were critical in WGA-1 binding to the wild-type cells. The binding data indicated a decrease in high-affinity binding as well as a decrease in the total number of binding sites in the variants. Polyacrylamide gel electrophoresis, followed by affinity staining with 125I-wheat germ agglutinin, showed alterations in the wheat germ agglutinin-binding glycoproteins in the variants compared to those of the parent cell line. However, lactoperoxidase-catalyzed iodination revealed a similar cell-surface protein pattern among the three cell lines. Radioactive glycoproteins secreted or shed by the three cell lines grown in the presence of [3H]glucosamine in serum-free medium were fractionated on the basis of their interaction with WGA-Sepharose (2 mg/mL). The WGA-bound glycoproteins from the two variants had molecular weights of 92,000, 56,000, and 42,000. None of these components was detected in the parent cell-line. A major WGA-binding glycoprotein, which accounted for 37% of the total [3H]glucosamine incorporated, was isolated from the spent medium of the parent mouse melanoma cell-line. This glycoprotein was apparently absent in the WGA-1-resistant variants.

Reversible defects in O-linked glycosylation and LDL receptor expression in a UDP-Gal/UDP-GalNAc 4-epimerase deficient mutant

We previously isolated an unusual hamster cell mutant (ldlD) that does not express LDL receptor activity unless it is cocultivated with other cells or grown in high concentrations of serum. We now show that ldlD cells are deficient in the enzyme UDP-galactose and UDP-N-acetylgalactosamine (GalNAc) 4-epimerase. When ldlD cells are grown in glucose-based media, they cannot synthesize enough UDP-galactose and UDP-GalNAc to allow normal synthesis of glycolipids and glycoproteins. The 4-epimerase deficiency accounts for all glycosylation defects previously observed in ldlD cells, including production of abnormal LDL receptors. All abnormal phenotypes of ldlD cells can be fully corrected by exogenous galactose and GalNAc. The separate effects of these sugars on LDL receptor activity suggest that O-linked carbohydrate chains are crucial for receptor stability. ldlD cells may be useful for structural and functional studies of many proteins, proteoglycans, and glycolipids containing galactose or GalNAc.

Lectin-resistant CHO cells: selection of four new pea lectin-resistant phenotypes

The cytotoxic plant lectins from P. sativum have been used to select new lectin-resistant mutants from Chinese hamster ovary (CHO) cells. Two novel phenotypes that behave recessively but fall into the same complementation group have been termed Lec13 and Lec13A. Both of these mutant types are phenotypically reverted to pea lectin sensitivity following growth in L-fucose. In contrast, two other unique phenotypes behave dominantly in somatic cell hybrids and maintain their pea lectin-resistance in the presence of L-fucose. They have been termed LEC14 and LEC18, respectively. The lectin-resistance and complementation properties of the four new mutant types suggest that they define three different glycosylation genes of the CHO genome.

Effect of monensin on myoblast fusion

Monensin, at a concentration of 0.5-10 microM, completely (100%) and reversibly inhibits fusion of embryonic chick myoblasts in vitro. At the same time, monensin administration leads to a marked accumulation of glycopeptides inside the cells and a decrease of those secreted into the medium. Chromatography of the intracellularly retained glycopeptides on Con A-Sepharose shows that the increase is most pronounced in the high-mannose fraction. Mild proteolysis of cells labeled with [2-3H]mannose releases less radioactivity from the surface of monensin-treated than from control cells, although the amount of total radioactivity is almost four times higher than in the control cells. Since it has now been established that monensin interferes with the intracellular transport of newly synthesized glycoproteins it is assumed that its inhibitory effect is the result of the inability of glycoprotein(s) essential for myoblast fusion to reach the cell surface.

Structures of N-glycans of a ricin-resistant mutant of baby hamster kidney cells. Synthesis of high-mannose and hybrid N-glycans

The asparagine-linked glycopeptides (N-glycans) of a ricin-resistant mutant of baby hamster kidney (BHK) cells, RicR21, have been isolated and fractionated from a Pronase digest of disrupted cells by concanavalin A (Con A)-Sepharose chromatography, ion-exchange chromatography, and lentil lectin chromatography. The structures of all the major N-glycans have been determined by 500-MHz H NMR spectroscopy. RicR21 synthesizes only hybrid and high-mannose N-glycans. All the hybrid structures contain only three mannose residues. The major hybrid glycopeptide has the following structure: (Formula: see text). There is also about 15% of the nonfucosylated species present. Only a small amount (less than or equal to 5%) of the asialo hybrid is produced. Branched hybrid N-glycans are also present in RicR21 cells, containing two complex antenna linked beta 1----2 and beta 1----4 to the Man alpha 1----3 arm; about 70% of this species is core fucosylated. Man6GlcNAc2 glycopeptide is the most abundant (about 70%) of the high-mannose N-glycans. These studies account for the very poor ricin binding property of this mutant, as the sialic acid residues of the major hybrid N-glycan are exclusively linked alpha 2----3 to galactose and ricin is unable to bind to alpha 2----3-substituted galactosyl residues [Baenziger, J. U., & Fiete, D. (1979) J. Biol. Chem. 254, 9795-9799].

Inhibitors of glycoprotein biosynthesis

Altogether 30 different sugar analogues have been tested in a cell free system from rat liver or, in part, in freshly prepared hepatocytes. It is our aim to find suitable compounds which are able either to interfere with the metabolization of L-fucose, galactose and N-acetylmannosamine or, alternatively, to block the attachment of these sugars to the nascent oligosaccharide chain. 1-Methylfucoside inhibits the fucokinase by a competitive mode (Ki = 1.1 mmol/l). Both the fucokinase and fucose-1-phosphate pyrophosphorylase activity are impaired by Clobenoside, a chloro-containing glucofuranoside (Ki values between 5 to 10 mmol/l). In hepatocytes this inhibition leads to a drastic reduction of fucoprotein biosynthesis and secretion. 1-Methylenegalactose proved to be a promising competitive inhibitor of the galactokinase (Ki = 4.1 mmol/l), while the efficacy of 2-deoxy-2-fluoro-galactose and 6-deoxy-6-fluoro-galactose is less pronounced. Part of these sugar analogues could become a suitable tool in order to elucidate the biological significance of terminal and subterminal sugars.

An assay for GDP-D-mannose-4,6-dehydratase

The mechanism of GDP-D-mannose-4,6-dehydratase action with respect to loss of the C5 hydrogen has been established using GDP-D-[5-3H]-mannose as a substrate. This observation has been incorporated into a rapid assay for the enzyme based on the equilibration of 3H with the aqueous medium.

Study of the conversion of GDP-mannose into GDP-fucose in Nereids: a biochemical marker of oocyte maturation

Homogenates of Perinereis cultrifera oocytes were found to transform GDP-D-mannose into another sugar nucleotide. Ultraviolet absorption spectra, chromatographic behaviour, gas-liquid chromatography coupled to mass spectrometry analysis revealed that GDP-D-mannose had been converted into GDP-L-fucose. This conversion is a multi-step reaction as proved by the involvement of two intermediates identified as GDP-4-oxo-6-deoxy-D-mannose and GDP-4-oxo-6-deoxy-L-galactose, this latter being reduced by NADPH to give GDP-L-fucose. It is shown that the enzymatic activities responsible for the conversion of GDP-D-mannose into GDP-L-fucose is recovered only in oocytes and is not present in the other coelomic cells (i.e. coelomocytes). More interesting is the fact that maximum activity is recovered at a well defined stage of the hormone-controlled oogenesis. Thus, this enzymatic system appears as a biochemical marker of the oocyte maturation in P. cultrifera.

Modification of the intramolecular turnover of terminal carbohydrates of dipeptidylaminopeptidase IV isolated from rat-liver plasma membrane during liver regeneration

An intramolecular turnover of the terminal carbohydrates L-fucose, N-acetylneuraminic acid and D-galactose is a characteristic property of several liver plasma membrane glycoproteins, first demonstrated for dipeptidylaminopeptidase IV (EC 3.4.14.5., DPP IV). The core carbohydrates D-mannose and N-acetyl-D-glucosamine turn over like the polypeptide chain. The ratio of apparent half-lives of L-fucose and L-methionine of DPP IV is shifted from 0.17 in normal liver to 0.60 in regenerating liver. The ratio of half-lives of N-acetylneuraminic acid and L-methionine is only slightly changed from 0.43 in normal liver to 0.61 in regenerating liver. The ratio of apparent half-lives of D-mannose and L-methionine amounts to 0.80 in normal liver and 0.71 after partial hepatectomy. From this a drastic reduction of the intramolecular turnover of L-fucose on plasma membrane DPP IV in regenerating liver can be derived. The intramolecular N-acetylneuraminic acid turnover is affected to only a minor extent. D-Mannose turns over like the polypeptide in both normal and regenerating liver. The intramolecular L-fucose turnover may be involved in membrane glycoprotein recycling, which presumably is altered in regenerating liver. Additionally, L-fucose could regulate the rate of degradation of DPP IV, since core-fucosylated glycoproteins appear to be resistant to mammalian endo-N-acetylglucosaminidase.

Autoradiographic detection and characterization of a Chinese hamster ovary cell mutant deficient in fucoproteins

Autoradiography of colony replicas immobilized on filter paper was used to isolate a Chinese hamster ovary cell line deficient in incorporation of radiolabeled fucose into a trichloroacetic acid-insoluble fraction. This cell line, called 62.1, has the same growth rate at 37 degrees C as wild-type cells, but incorporates five times less fucose into acid-insoluble radioactivity. Chemical analysis of fucose bound to macromolecules also showed a fivefold reduction in the mutant. The fucoproteins of the mutant cell line differ qualitatively from those of wild-type cells as visualized by SDS gel electrophoresis fluorography; no differences were detected between total proteins as visualized by coomassie blue staining. The macromolecular sialic acid content of the mutant was somewhat higher than the wild type (20%). Studies of the synthesis of the glycoprotein of vesicular stomatitis virus in mutant and wild-type cells showed that the mutant is unable to synthesize complex-type N-linked oligosaccharides. Enzyme assays show that ths defect in the mutant is due to reduction in UDP-N-acetylglucosamine-glycoprotein N-acetyl-glucosaminyltransferase, a key enzyme in the assembly of complex glycopeptides. Hybridization studies have shown that mutant 62.1 has common mutations belonging to the same complementation group as mutant PhaR1-1. This latter mutant was previously isolated using lectin resistance by Stanley et al. (1975) and was also deficient in the above N-acetyl-glucosaminyltransferase.