Purification and characterization of an α-1,2-mannosidase involved in processing asparagine-linked oligosaccharides

Sequential processing of mannose-containing glycans by two α-mannosidases from Solitalea canadensis

Two putative α-mannosidase genes isolated from the rather unexplored soil bacterium Solitalea canadensis were cloned and biochemically characterised. Both recombinant enzymes were highly selective in releasing α-linked mannose but no other sugars. The α-mannosidases were designated Sca2/3Man2693 and Sca6Man4191, and showed the following biochemical properties: the temperature optimum for both enzymes was 37 °C, and their pH optima lay at 5.0 and 5.5, respectively. The activity of Sca2/3Man2693 was found to be dependent on Ca(2+) ions, whereas Cu(2+) and Zn(2+) ions almost completely inhibited both α-mannosidases. Specificity screens with various substrates revealed that Sca2/3Man2693 could release both α1-2- and α1-3-linked mannose, whereas Sca6Man4191 only released α1-6-linked mannose. The combined enzymatic action of both recombinant α-mannosidases allowed the sequential degradation of high-mannose-type N-glycans. The facile expression and purification procedures in combination with strict substrate specificities make α-mannosidases from S. canadensis promising candidates for bioanalytical applications.

Recombinant Aspergillus β-galactosidases as a robust glycomic and biotechnological tool

Galactosidases are widespread enzymes that are used for manifold applications, including production of prebiotics, biosynthesis of different transgalactosylated products, improving lactose tolerance and in various analytical approaches. The nature of these applications often require galactosidases to be present in a purified form with clearly defined properties, including precisely determined substrate specificities, low sensitivity to inhibitors, and high efficiency and stability under distinct conditions. In this study, we present the recombinant expression and purification of two previously uncharacterized β-galactosidases from Aspergillus nidulans as well as one β-galactosidase from Aspergillus niger. All enzymes were active toward p-nitrophenyl-β-d-galactopyranoside as substrate and displayed similar temperature and pH optima. The purified recombinant galactosidases digested various complex substrates containing terminal galactose β-1,4 linked to either N-acetylglucosamine or fucose, such as N-glycans derived from bovine fibrin and Caenorhabditis elegans. In our comparative study of the recombinant galactosidases with the commercially available galactosidase from Aspergillus oryzae, all enzymes also displayed various degrees of activity toward complex oligosaccharides containing β-1,3-linked terminal galactose residues. All recombinant enzymes were found to be robust in the presence of various organic solvents, temperature variations, and freeze/thaw cycles and were also tested for their ability to synthesize galactooligosaccharides. Furthermore, the use of fermentors considerably increased the yield of recombinant galactosidases. Taken together, we demonstrate that purified recombinant galactosidases from A. niger and from A. nidulans are suitable for various glycobiological and biotechnological applications.

The class I α1,2-mannosidases of Caenorhabditis elegans

During the biosynthesis of N-glycans in multicellular eukaryotes, glycans with the compositions Man(5)GlcNAc(2-3) are key intermediates. However, to reach this 'decision point', these N-glycans are first processed from Glc(3)Man(9)GlcNAc(2) through to Man(5)GlcNAc(2) by a number of glycosidases, whereby up to four α1-2-linked mannose residues are removed by class I mannosidases (glycohydrolase family 47). Whereas in the yeast Saccharomyces cerevisiae there are maximally three members of this protein family, in higher organisms there are multiple class I mannosidases residing in the endoplasmic reticulum and Golgi apparatus. The genome of the model nematode Caenorhabditis elegans encodes seven members of this protein family, whereby four are predicted to be classical processing mannosidases and three are related proteins with roles in quality control. In this study, cDNAs encoding the four predicted mannosidases were cloned and expressed in Pichia pastoris and the activity of these enzymes, designated MANS-1, MANS-2, MANS-3 and MANS-4, was verified. The first two can, dependent on the incubation time, remove three to four residues from Man(9)GlcNAc(2), whereas the action of the other two results in the appearance of the B isomer of Man(8)GlcNAc(2); together the complementary activities of these enzymes result in processing to Man(5)GlcNAc(2). With these data, another gap is closed in our understanding of the N-glycan biosynthesis pathway of the nematode worm.

Characterization of α-mannosidase from Erythrina indica seeds and influence of endogenous lectin on its activity

alpha-mannosidase from Erythrina indica seeds is a Zn(2+) dependent glycoprotein with 8.6% carbohydrate. The enzyme has a temperature optimum of 50 degrees C and energy of activation calculated from Arrhenius plot was found to be 23 kJ mol(-1). N-terminal sequence up to five amino acid residues was found to be DTQEN (Asp, Thr, Gln, Glu, and Asn). In chemical modification studies treatment of the enzyme with NBS led to total loss of enzyme activity and modification of a single tryptophan residue led to inactivation. Fluorescence studies over a pH range of 3-8 have shown tryptophan residue to be in highly hydrophobic environment and pH change did not bring about any appreciable change in its environment. Far-UV CD spectrum indicated predominance of alpha-helical structure in the enzyme. alpha-Mannosidase from E indica exhibits immunological identity with alpha-mannosidase from Canavalia ensiformis but not with the same enzyme from Glycine max and Cicer arietinum. Incubation of E. indica seed lectin with alpha-mannosidase resulted in 35% increase in its activity, while no such activation was observed for acid phosphatase from E. indica. Lectin induced activation of alpha-mannosidase could be completely abolished in presence of lactose, a sugar specific for lectin.

Expression of the Streptococcus pneumoniae type 3 synthase in Escherichia coli. Assembly of type 3 polysaccharide on a lipid primer

Synthesis of the type 3 capsular polysaccharide of Streptococcus pneumoniae is catalyzed by the membrane-localized type 3 synthase, which utilizes UDP-Glc and UDP-GlcUA to form high molecular mass [3-beta-d-GlcUA-(1-->4)-beta-d-Glc-(1-->](n). Expression of the synthase in Escherichia coli resulted in synthesis of a 40-kDa protein that was reactive with antibody directed against the C terminus of the synthase and was the same size as the native enzyme. Membranes isolated from E. coli contained active synthase, as demonstrated by the ability to incorporate Glc and GlcUA into a high molecular mass polymer that could be degraded by type 3 polysaccharide-specific depolymerase. As in S. pneumoniae, the membrane-bound synthase from E. coli catalyzed a rapid release of enzyme-bound polysaccharide when incubated with either UDP-Glc or UDP-GlcUA alone. The recombinant enzyme expressed in E. coli was capable of releasing all of the polysaccharide from the enzyme, although the chains remained associated with the membrane. The recombinant enzyme was also able to reinitiate polysaccharide synthesis following polymer release by utilizing a lipid primer present in the membranes. At low concentrations of UDP-Glc and UDP-GlcUA (1 microm in the presence of Mg(2+) and 0.2 microm in Mn(2+)), novel glycolipids composed of repeating disaccharides with linkages consistent with type 3 polysaccharide were synthesized. As the concentration of the UDP-sugars was increased, there was a marked transition from glycolipid to polymer formation. At UDP-sugar concentrations of either 5 microm (with Mg(2+)) or 1.5 microm (with Mn(2+)), 80% of the incorporated sugar was in polymer form, and the size of the polymer increased dramatically as the concentration of UDP-sugars was increased. These results suggest a cooperative interaction between the UDP-precursor-binding site(s) and the nascent polysaccharide-binding site, resulting in a non-processive addition of sugars at the lower UDP-sugar concentrations and a processive reaction as the substrate concentrations increase.

Cloning and expression of a specific human alpha 1,2-mannosidase that trims Man9GlcNAc2 to Man8GlcNAc2 isomer B during N-glycan biosynthesis

We report the isolation of a novel human cDNA encoding a type II membrane protein of 79.5 kDa with amino acid sequence similarity to Class I alpha 1,2-mannosidases. The catalytic domain of the enzyme was expressed as a secreted protein in Pichia pastoris. The recombinant enzyme removes a single mannose residue from Man9GlcNAc and [1H]-NMR analysis indicates that the only product is Man8GlcNAc isomer B, the form lacking the middle-arm terminal alpha 1,2-mannose. Calcium is required for enzyme activity and both 1-deoxymannojirimycin and kifunensine inhibit the human alpha 1,2-mannosidase. The properties and specificity of this human alpha 1,2-mannosidase are identical to the endoplasmic reticulum alpha 1,2-mannosidase from Saccharomyces cerevisiae and differ from those of previously cloned Golgi alpha 1,2-mannosidases that remove up to four mannose residues from Man9GlcNAc2 during N-glycan maturation. Northern blot analysis showed that all human tissues examined express variable amounts of a 3 kb transcript. This highly specific alpha 1,2-mannosidase is likely to be involved in glycoprotein quality control since there is increasing evidence that trimming of Man9GlcNAc2 to Man8GlcNAc2 isomer B in yeast cells is important to target misfolded glycoproteins for degradation.

Substrate specificities of recombinant murine Golgi alpha1, 2-mannosidases IA and IB and comparison with endoplasmic reticulum and Golgi processing alpha1,2-mannosidases

The catalytic domains of murine Golgi alpha1,2-mannosidases IA and IB that are involved in N-glycan processing were expressed as secreted proteins in P.pastoris . Recombinant mannosidases IA and IB both required divalent cations for activity, were inhibited by deoxymannojirimycin and kifunensine, and exhibited similar catalytic constants using Manalpha1,2Manalpha-O-CH3as substrate. Mannosidase IA was purified as a 50 kDa catalytically active soluble fragment and shown to be an inverting glycosidase. Recombinant mannosidases IA and IB were used to cleave Man9GlcNAc and the isomers produced were identified by high performance liquid chromatography and proton-nuclear magnetic resonance spectroscopy. Man9GlcNAc was rapidly cleaved by both enzymes to Man6GlcNAc, followed by a much slower conversion to Man5GlcNAc. The same isomers of Man7GlcNAc and Man6GlcNAc were produced by both enzymes but different isomers of Man8GlcNAc were formed. When Man8GlcNAc (Man8B isomer) was used as substrate, rapid conversion to Man5GlcNAc was observed, and the same oligosaccharide isomer intermediates were formed by both enzymes. These results combined with proton-nuclear magnetic resonance spectroscopy data demonstrate that it is the terminal alpha1, 2-mannose residue missing in the Man8B isomer that is cleaved from Man9GlcNAc at a much slower rate. When rat liver endoplasmic reticulum membrane extracts were incubated with Man9GlcNAc2, Man8GlcNAc2was the major product and Man8B was the major isomer. In contrast, rat liver Golgi membranes rapidly cleaved Man9GlcNAc2to Man6GlcNAc2and more slowly to Man5GlcNAc2. In this case all three isomers of Man8GlcNAc2were formed as intermediates, but a distinctive isomer, Man8A, was predominant. Antiserum to recombinant mannosidase IA immunoprecipitated an enzyme from Golgi extracts with the same specificity as recombinant mannosidase IA. These immunodepleted membranes were enriched in a Man9GlcNAc2to Man8GlcNAc2-cleaving activity forming predominantly the Man8B isomer. These results suggest that mannosidases IA and IB in Golgi membranes prefer the Man8B isomer generated by a complementary mannosidase that removes a single mannose from Man9GlcNAc2.

Molecular cloning, chromosomal mapping and tissue-specific expression of a novel human alpha1,2-mannosidase gene involved in N-glycan maturation

Class I alpha1,2-mannosidases play an essential role in the elaboration of complex and hybrid N -glycans in mammalian cells. Using degenerate primers based on amino acid sequences conserved in all members of this enzyme family for RT-PCR, two distinct PCR products were obtained from placenta and lymphocyte cDNAs. One of these was related to the previously cloned human and murine alpha1, 2-mannosidase IA whereas the other was very similar to murine alpha1, 2-mannosidase IB. Northern blot analysis of human tissues with these two alpha1,2-mannosidase probes revealed very different patterns of tissue-specific expression. Similar tissue-specific expression of alpha1,2-mannosidase IA and IB was also observed on Northern blots of adult mouse tissues. A human placenta cDNA library was screened and PCR of brain, placenta, and lymphocyte cDNAs was performed in order to isolate the human alpha1,2-mannosidase IB cDNA. This cDNA encodes a type II membrane protein of 73 kDa that is 94% identical in amino acid sequence to the murine alpha1,2-mannosidase IB (Herscovics et al., 1994, J. Biol. Chem., 269, 9864-9871). A truncated soluble form of the human alpha1,2-mannosidase IB lacking its N -terminal transmembrane domain was expressed as a secreted protein in Pichia pastoris . The recombinant enzyme was incubated with [3H]Man9GlcNAc and [3H]Man8GlcNAc (isomer B), and high performance liquid chromatography analysis of the products showed that [3H]Man9GlcNAc was readily converted to [3H]Man6GlcNAc and much more slowly to [3H]Man5GlcNAc, whereas [3H]Man8GlcNAc was rapidly trimmed to [3H]Man5GlcNAc. The human alpha1,2-mannosidase IB gene was isolated from a P1 human genomic library and shown to be at least 60 kb in size and to contain at least 13 exons. The gene was localized by fluorescence in situ hybridization to human chromosome 1p13, a region that undergoes many aberrations in various types of human cancers. These results show that there are at least two Class I alpha1,2-mannosidases in the human and murine genomes with very distinct transcriptional regulation in different tissues.

Role of the cysteine residues in the alpha1,2-mannosidase involved in N-glycan biosynthesis in Saccharomyces cerevisiae. The conserved Cys340 and Cys385 residues form an essential disulfide bond

The Saccharomyces cerevisiae alpha1,2-mannosidase, which removes one specific mannose residue from Man9GlcNAc2 to form Man8GlcNAc2, is a member of a family of alpha1,2-mannosidases with similar amino acid sequences. The yeast alpha1,2-mannosidase contains five cysteine residues, three of which are conserved. Recombinant yeast alpha1, 2-mannosidase, produced as the soluble catalytic domain, was shown to contain two disulfide bonds and one free thiol group using 2-nitro-5-thiosulfobenzoate and 5,5'-dithiobis(2-nitrobenzoate), respectively. Cys485 contains the free thiol group, as demonstrated by sequencing of labeled peptides following modification with [3H]ICH2COOH and by high performance liquid chromatography/mass spectrometry tryptic peptide mapping. A Cys340-Cys385 disulfide was demonstrated by sequencing a purified peptide containing this disulfide and by tryptic peptide mapping. Cys468 and Cys471 were not labeled with [3H]ICH2COOH and a peptide containing these two residues was identified in the tryptic peptide map, showing that Cys468 and Cys471 form the second disulfide bond. The alpha1, 2-mannosidase loses its activity in the presence of dithiothreitol with first order kinetics, suggesting that at least one disulfide bond is essential for activity. Mutagenesis of each cysteine residue to serine showed that Cys340 and Cys385 are essential for production of recombinant enzyme, whereas Cys468, Cys471, and Cys485 are not required for production and enzyme activity. These results indicate that the sensitivity to dithiothreitol is due to reduction of the Cys340-Cys385 disulfide. Since Cys340 and Cys385 are conserved residues, it is likely that this disulfide bond is important to maintain the correct structure in the other members of the alpha1, 2-mannosidase family.

Two naturally occurring mouse alpha-1,2-mannosidase IB cDNA clones differ in three point mutations. Mutation of Phe592 to Ser592 is sufficient to abolish enzyme activity

In mammalian cells, alpha-1,2-mannosidases play an essential role in the early steps of N-linked oligosaccharide maturation. We previously reported (Herscovics, A., Schneikert, J., Athanassiadis, A., and Moremen, K. W. (1994) J. Biol. Chem. 269, 9864-9871) the isolation of mouse alpha-mannosidase IB cDNA clones from a Balb/c 3T3 cDNA library. Clone 4 encodes a type II membrane protein of 641 amino acids with a cytoplasmic tail of 35 amino acids, followed by a transmembrane domain and a large C-terminal catalytic domain, whereas clone 16 encodes only the last 471 amino acids. Their overlapping sequences (from amino acid 152) are identical, except for three point mutations that result in three amino acid differences in the catalytic domain of the enzyme (Thr411, Leu468, and Ser592 in clone 4 to Met411, Phe468, and Phe592 in clone 16, respectively). Both sequences could be amplified by polymerase chain reaction using templates of cDNAs derived from colon and brain of CD1 mice and from L cells derived from the C3H/An mouse, indicating that both are natural isoforms found in two inbred and one outbred mouse strains. When expressed in COS7 cells as a secreted protein A fusion protein, the catalytic domain of clone 16 displays alpha-1,2-mannosidase activity using [3H]mannose-labeled Man9GlcNAc as substrate, but the corresponding region of clone 4 is poorly secreted under identical conditions. The contribution of each point mutation to this differential secretion and enzyme activity of the two fusion proteins was assessed by testing the six recombinants corresponding to all the possible sequence permutations. Mutation of Phe592 to Ser592, as found in clone 4, is sufficient to abolish alpha-1,2-mannosidase activity, whereas mutation of Met411 to Thr411 or of Phe468 to Leu468 affects secretion with relatively little effect on enzyme activity. Phe592 is part of a highly conserved region that seems important for enzyme activity of class 1 alpha-1,2-mannosidases.

Molecular cloning and primary structure of Man9-mannosidase from human kidney

Man9-mannosidase, a processing enzyme found in the endoplasmic reticulum (ER), catalyses the removal of three distinct mannose residues from peptide-bound Man9-GlcNAc2 oligosaccharides producing a single Man6 isomer [Bause, E., Breuer, W., Schweden, J., Roesser, R. & Geyer, R. (1992) Eur. J. Biochem. 208, 451-457]. We have isolated four Man9-mannosidase-specific clones from a human kidney cDNA library and used these to construct a full-length cDNA of 3250 base pairs. A single open reading frame of 1875 nucleotides encodes a protein of approximately 71 kDa, consistent with data from immunological studies. Analysis of the coding sequence predicts that Man9-mannosidase is a type II transmembrane protein consisting of a short cytoplasmic polypeptide tail, a single transmembrane domain acting as a non-cleavable signal sequence and a large luminal catalytic domain. This domain architecture closely resembles that of other ER and Golgi-located processing enzymes, pointing to common structural motifs involved in membrane insertion and topology. The protein sequence of the Man9-mannosidase contains three potential N-glycosylation sites of which only one site is used. The amino acid sequence of several peptide regions, including a calcium-binding consensus sequence, bears striking similarities to an ER alpha-1,2-mannosidase from yeast, whereas, by contrast, no sequence similarity was detectable with rat liver ER alpha-mannosidase and Golgi alpha-mannosidase II. This finding may indicate that the mammalian alpha-mannosidases, which differ significantly in their substrate specificity, are coded for by evolutionarily unrelated genes, providing an attractive means of regulation and fine-tuning oligosaccharide processing, not only at the enzymic but also at the transcriptional level.

Beta-glucosidase activity towards a bile acid glucoside in human liver

Human liver contains hydrolytic activity toward 3 beta-glucosido-chenodeoxycholic acid. This beta-glucosidase activity, localized predominantly in the microsomal fraction, was optimally active in the presence of divalent metal ions close to pH 5.0 and was inhibited by EDTA. Kinetic parameters and other catalytic properties of hydrolytic activity towards 3 beta-glucosido-chenodeoxycholic acid from human liver microsomes are described.

Effect of substrate structure on the activity of Man9-mannosidase from pig liver involved in N-linked oligosaccharide processing

Man9-mannosidase, an alpha 1,2-specific enzyme located in the endoplasmic reticulum and involved in N-linked-oligosaccharide processing, has been isolated from crude pig-liver microsomes and its substrate specificity studied using a variety of free and peptide-bound high-mannose oligosaccharide derivatives. The purified enzyme displays no activity towards synthetic alpha-mannosides, but removes three alpha 1,2-mannose residues from the natural Man9-(GlcNAc)2 substrate (M9). The alpha 1,2-mannosidic linkage remaining in the M6 intermediate is cleaved about 40-fold more slowly. Similar kinetics of hydrolysis were determined with Man9-(GlcNAc)2 N-glycosidically attached to the hexapeptide Tyr-Asn-Lys-Thr-Ser-Val (GP-M9), indicating that the specificity of the enzyme is not influenced by the peptide moiety of the substrate. The alpha 1,2-mannose residue which is largely resistant to hydrolysis, was found to be attached in both the M6 and GP-M6 intermediate to the alpha 1,3-mannose of the peripheral alpha 1,3/alpha 1,6-branch of the glycan chain. Studies with glycopeptides varying in the size and branching pattern of the sugar chains, revealed that the relative rates at which the various alpha 1,2-mannosidic linkages were cleaved, differed depending on their structural complexity. This suggests that distinct sugar residues in the aglycon moiety may be functional in substrate recognition and binding. Reduction or removal of the terminal GlcNAc residue of the chitobiose unit in M9 increased the hydrolytic susceptibility of the fourth (previously resistant) alpha 1,2-mannosidic linkage significantly. We conclude from this observation that, in addition to peripheral mannose residues, the intact chitobiose core represents a structural element affecting Man9-mannosidase specificity. A possible biological role of the enzyme during N-linked-oligosaccharide processing is discussed.

In vitro hydrolysis of oligomannose-type sugar chains by an alpha-1,2-mannosidase from microsomes of developing castor bean cotyledons

An alpha-1,2-mannosidase involved in the processing of N-linked oligosaccharides was prepared from the microsomal fraction of developing castor bean cotyledons. The processing alpha-mannosidase was solubilized with 1.0% Triton X-100 and purified by ion-exchange chromatography followed by two gel filtration steps. The enzyme obtained could convert Man9GlcNAc2-PA to Man5GlcNAc2-PA, but this enzyme was inactive with Man5GlcNAc2-PA, Man4GlcNAc2-PA, and p-nitrophenyl-alpha-D-mannopyranoside. The enzyme was optimally active between pH 5.5-6.0. The processing mannosidase was inhibited by deoxymannojirimycin, EDTA, and Tris ions but not by swainsonine. Structural analyses of the mannose-trimming intermediates produced by the alpha-mannosidase revealed that specific intermediates were formed during conversion of Man9GlcNAc2-PA to Man5GlcNAc2-PA.

Use of exoglycosidases from Mercenaria mercenaria (hard shelled clam) as a tool for structural studies of glycosphingolipids and glycoproteins

The hepatopancreatic extract of M. mercenaria (hard shelled clam) was found to be a rich source for at least 16 different glycosidases. These glycosidases were successfully employed for the degradation of oligosaccharides, glycolipids, and glycoproteins at analytical as well as preparative levels. The identified glycosidases differ considerably in their stability profiles with respect to time and temperature of storage and presence of glycerol. However, most of the enzymes show higher activity at pH 4.5 than at pH 7.0, and could be bound on a DEAE CL-6B Sepharose anion-exchange column suggesting similar charge characteristics on the protein surface. A Gal beta 1, 3R linkage-specific beta-galactosidase activity has also been detected in the glycosidase-enriched fraction and has been utilized to obtain quantitative conversion of the ganglioside GM1 to GM2 on a preparative scale. The glycosidase-rich extract does not have detectable protease activity at the pH of optimal glycosidase activity (pH 4.5) and, hence, can be safely used for specific hydrolysis of carbohydrate moieties of glycoproteins and glycopeptides. This is the first report to characterize a repertoire of glycosidases from an inexpensive, dependable and convenient source that can be easily employed for compositional studies involving glycoconjugates.

Purification and characterization of a novel broad-specificity (alpha 1----2, alpha 1----3 and alpha 1----6) mannosidase from rat liver

We have identified a mannosidase in rat liver that releases alpha 1----2, alpha 1----3 and alpha 1----6 linked manose residues from oligosaccharide substrates, MannGlcNAc where n = 4-9. The end product of the reaction is Man alpha 1----3[Man alpha 1----6]Man beta 1----4GlcNAc. The mannosidase has been purified to homogeneity from a rat liver microsomal fraction, after solubilization into the aqueous phase of Triton X-114, by anion-exchange, hydrophobic and hydroxyapatite chromatography followed by chromatofocusing. The purified enzyme is a dimer of a 110-kDa subunit, has a pH optimum between 6.1 and 6.5 and a Km of 65 microM and 110 microM for the Man5GlcNAc-oligosaccharide or Man9GlcNAc-oligosaccharide substrates, respectively. Enzyme activity is inhibited by EDTA, by Zn2+ and Cu2+, and to lesser extent by Fe2+ and is stabilized by Co2+. The pattern of release of mannose residues from a Man6GlcNAc substrate shows an ordered hydrolysis of the alpha 1----2 linked residue followed by hydrolysis of alpha 1----3 and alpha 1----6 linked residues. The purified enzyme shows no activity against p-nitrophenyl-alpha-mannoside nor the hybrid GlcNAc Man5GlcNAc oligosaccharide. The enzyme activity is inhibited by swainsonine and 1-deoxymannojirimycin at concentrations 50-500-fold higher than required for complete inhibition of Golgi-mannosidase II and mannosidase I, respectively. The data indicate strongly that the enzyme has novel activity and is distinct from previously described mannosidases.

Environmental effects on protein glycosylation

Cultured mammalian cells are being used to produce proteins for therapeutic and diagnostic use because of their ability to perform complex post-translational modifications, including glycosylation. The oligosaccharide moieties can play an important role in defining several biological properties of glycoproteins, including clearance rate, immunogenicity, and biological specific activity. There is a growing interest in defining the factors that influence glycosylation, including the cell culture environment. In this review we organize the published data from in vitro cell culture and tissue culture studies that demonstrate direct effects of the culture environment on N-linked glycosylation.