Large-scale expression of recombinant sialyltransferases and comparison of their kinetic properties with native enzymes

Probing the CMP-Sialic Acid Donor Specificity of Two Human β-d-Galactoside Sialyltransferases (ST3Gal I and ST6Gal I) Selectively Acting on O- and N-Glycosylproteins

Sialylation of glycoproteins and glycolipids is catalyzed by sialyltransferases in the Golgi of mammalian cells, whereby sialic acid residues are added at the nonreducing ends of oligosaccharides. Because sialylated glycans play critical roles in a number of human physio-pathological processes, the past two decades have witnessed the development of modified sialic acid derivatives for a better understanding of sialic acid biology and for the development of new therapeutic targets. However, nothing is known about how individual mammalian sialyltransferases tolerate and behave towards these unnatural CMP-sialic acid donors. In this study, we devised several approaches to investigate the donor specificity of the human β-d-galactoside sialyltransferases ST6Gal I and ST3Gal I by using two CMP-sialic acids: CMP-Neu5Ac, and CMP-Neu5N-(4pentynoyl)neuraminic acid (CMP-SiaNAl), an unnatural CMP-sialic acid donor with an extended and functionalized N-acyl moiety.

A systematic analysis of acceptor specificity and reaction kinetics of five human α(2,3)sialyltransferases: Product inhibition studies illustrate reaction mechanism for ST3Gal-I

Sialyltransferases (STs) catalyze the addition of sialic acids to the non-reducing ends of glycoproteins and glycolipids. In this work, we examined the acceptor specificity of five human α(2,3)sialyltransferases, namely ST3Gal -I, -II, -III, -IV and -VI. KM values for each of these enzymes is presented using radioactivity for acceptors containing Type-I (Galβ1,3GlcNAc), Type-II (Galβ1,4GlcNAc), Type-III (Galβ1,3GalNAc) and Core-2 (Galβ1,3(GlcNAcβ1,6)GalNAc) reactive groups. Several variants of acceptors inhibited ST3Gal activity emphasizing structural role of acceptor in enzyme-catalyzed reactions. In some cases, mass spectrometry was performed for structural verification. The results demonstrate human ST3Gal-I catalysis towards Type-III and Core-2 acceptors with KM = 5-50 μM and high VMax values. The KM for ST3Gal-I and ST3Gal-II was 100 and 30-fold lower, respectively, for Type-III compared to Type-I acceptors. Variants of Type-I and Type-II structures characterized ST3Gal-III, -IV and -VI for their catalytic specificity. This manuscript also estimates KM for human ST3Gal-VI using Type-I and Type-II substrates. Together, these findings built a platform for designing inhibitors of STs having therapeutic potential.

Fluorescent mimetics of CMP-Neu5Ac are highly potent, cell-permeable polarization probes of eukaryotic and bacterial sialyltransferases and inhibit cellular sialylation

Oligosaccharides of the glycolipids and glycoproteins at the outer membranes of human cells carry terminal neuraminic acids, which are responsible for recognition events and adhesion of cells, bacteria, and virus particles. The synthesis of neuraminic acid containing glycosides is accomplished by intracellular sialyl transferases. Therefore, the chemical manipulation of cellular sialylation could be very important to interfere with cancer development, inflammations, and infections. The development and applications of the first nanomolar fluorescent inhibitors of sialyl transferases are described herein. The obtained carbohydrate-nucleotide mimetics were found to bind all four commercially available and tested eukaryotic and bacterial sialyl transferases in a fluorescence polarization assay. Moreover, it was observed that the anionic mimetics intruded rapidly and efficiently into cells in vesicles and translocated to cellular organelles surrounding the nucleus of CHO cells. The new compounds inhibit cellular sialylation in two cell lines and open new perspectives for investigations of cellular sialylation.

A new assay for determining ganglioside sialyltransferase activities lactosylceramide-2,3-sialyltransferase (SAT I) and monosialylganglioside-2,3-sialyltransferase (SAT IV)

A new assay for the determination of lactosylceramide-2,3-sialyltransferase (SAT I, EC 2.4.99.9) and monosialoganglioside sialyltransferase (SAT IV, EC 2.4.99.2) is described. The assay utilised the commercially available fluorophore labelled sphingolipids, boron dipyrromethene difluoride (BODIPY) lactosylceramide (LacCer), and BODIPY-monosialotetrahexosylganglioside (GM1) as the acceptor substrates, for SAT I and SAT IV, respectively. HPLC coupled with fluorescence detection was used to analyse product formation. The analysis was performed in a quick and automated fashion. The assay showed good linearity for both BODIPY sphingolipids with a quantitative detection limit of 0.05 pmol. The high sensitivity enabled the detection of SAT I and SAT IV activities as low as 0.001 μU, at least 200 fold lower than that of most radiometric assays. This new assay was applied to the screening of SAT I and SAT IV activities in ovine and bovine organs (liver, heart, kidney, and spleen). The results provided evidence that young animals, such as calves, start to produce ganglioside sialyltransferases as early as 7 days after parturition and that levels change during maturation. Among the organs tested from a bovine source, spleen had the highest specific ganglioside sialyltransferase activity. Due to the organ size, the greatest total ganglioside sialyltransferase activities (SAT I and SAT IV) were detected in the liver of both bovine and ovine origin.

α2-3- and α2-6- N-linked sialic acids allow efficient interaction of Newcastle Disease Virus with target cells

Receptor recognition and binding is the first step in the viral cycle. It has been established that Newcastle Disease Virus (NDV) interacts with sialylated molecules such as gangliosides and glycoproteins at the cell surface. Nevertheless, the specific receptor(s) that mediate virus entry are not well known. We have analysed the role of the sialic acid linkage in the early steps of the viral infection cycle. Pretreatment of ELL-0 cells with both α2,3 and α2,6 specific sialidases led to the inhibition of NDV binding, fusion and infectivity, which were restored after α2,3(N)- and α2,6(N)-sialyltransferase incubation. Moreover, α2,6(N)-sialyltransferases also restored NDV activities in α2-6-linked sialic acid deficient cells. Competition with α2-6 sialic acid-binding lectins led to a reduction in the three NDV activities (binding, fusion and infectivity) suggesting a role for α2-6- linked sialic acid in NDV entry. We conclude that both α2-3- and α2-6- linked sialic acid containing glycoconjugates may be used for NDV infection. NDV was able to efficiently bind, fuse and infect the ganglioside-deficient cell line GM95 to a similar extent to that of its parental MEB4, suggesting that gangliosides are not essential for NDV binding, fusion and infectivity. Nevertheless, the fact that the interaction of NDV with cells deficient in N-glycoprotein expression such as Lec1 was less efficient prompted us to conclude that NDV requires N-linked glycoproteins for efficient attachment and entry into the host cell.

Synthesis of sialoglycopolypeptide for potentially blocking influenza virus infection using a rat alpha2,6-sialyltransferase expressed in BmNPV bacmid-injected silkworm larvae

Background

Sialic acid is a deoxy uronic acid with a skeleton of nine carbons which is mostly found on cell surface in animals. This sialic acid on cell surface performs various biological functions by acting as a receptor for microorganisms, viruses, toxins, and hormones; by masking receptors; and by regulating the immune system. In order to synthesize an artificial sialoglycoprotein, we developed a large-scale production of rat α2,6-sialyltransferase (ST6Gal1). The ST6Gal1 was expressed in fifth instar silkworm larval hemolymph using recombinant both cysteine protease- and chitinase-deficient Bombyx mori nucleopolyhedrovirus (BmNPV-CP^--Chi^-) bacmid. The expressed ST6Gal1 was purified, characterized and used for sialylation of asialoglycopolypeptide. We tested the inhibitory effect of the synthesized α2,6-sialoglycopolypeptide on hemagglutination by Sambucus nigra (SNA) lectin.

Results

FLAG-tagged recombinant ST6Gal1 was expressed efficiently and purified by precipitation with ammonium sulphate followed by affinity chromatography on an anti-FLAG M2 column, generating 2.2 mg purified fusion protein from only 11 silkworm larvae, with a recovery yield of 64%. The purified ST6Gal1 was characterized and its N-glycan patterns were found to be approximately paucimannosidic type by HPLC mapping method. Fluorescently-labelled N-acetyllactosamine (LacNAc) glycoside containing dansyl group was synthesized chemo-enzymatically as high-sensitivity acceptor substrate for ST6Gal1. The acceptor substrate specificity of the enzyme was similar to that of rat liver ST6Gal1. The fluorescent glycoside is useful as a substrate for a highly sensitive picomole assay of ST6Gal1. Asialoglycopolypeptide was regioselectively and quantitatively sialylated by catalytic reaction at the terminal Gal residue to obtain α2,6-sialoglycopolypeptide using ST6Gal1. The α2,6-sialoglycopolypeptide selectively inhibited hemagglutination induced by Sambucus nigra (SNA) lectin, showing about 780-fold higher affinity than the control fetuin. Asialoglycopolypeptide and γ-polyglutamic acid did not affect SNA lectin-mediated hemagglutination.

Conclusion

The recombinant ST6Gal1 from a silkworm expression system is useful for the sialylation of asialoglycopeptide. The sialylated glycoprotein is a valuable tool for investigating the molecular mechanisms of biological and physiological events, such as cell-cell recognition and viral entry during infection.

Recapitulation of IVIG anti-inflammatory activity with a recombinant IgG Fc

It is well established that high doses of monomeric immunoglobulin G (IgG) purified from pooled human plasma [intravenous immunoglobulin (IVIG)] confer anti-inflammatory activity in a variety of autoimmune settings. However, exactly how those effects are mediated is not clear because of the heterogeneity of IVIG. Recent studies have demonstrated that the anti-inflammatory activity of IgG is completely dependent on sialylation of the N-linked glycan of the IgG Fc fragment. Here we determine the precise glycan requirements for this anti-inflammatory activity, allowing us to engineer an appropriate IgG1 Fc fragment, and thus generate a fully recombinant, sialylated IgG1 Fc with greatly enhanced potency. This therapeutic molecule precisely defines the biologically active component of IVIG and helps guide development of an IVIG replacement with improved activity and availability.

Direct correlation between sialic acid binding and infection of cells by two human polyomaviruses (JC virus and BK virus)

For the human polyomaviruses JC virus (JCV) and BK virus (BKV), the first step to a successful infection involves binding to sialic acid moieties located on the surfaces of host cells. By stripping and then reconstituting specific sialic acid linkages on host cells, we show that JCV uses both alpha(2,3)-linked and alpha(2,6)-linked sialic acids on N-linked glycoproteins to infect cells. For both JCV and BKV, the sialic acid linkages required for cell surface binding directly correlate with the linkages required for infection. In addition to sialic acid linkage data, these data suggest that the third sugar from the carbohydrate chain terminus is important for virus binding and infection.

Glycan microarrays for screening sialyltransferase specificities

Here we demonstrate that glycan microarrays can be used for high-throughput acceptor specificity screening of various recombinant sialyltransferases. Cytidine-5'-monophospho-N-acetylneuraminic acid (CMP-Neu5Ac) was biotinylated at position 9 of N-acetylneuraminic acid (Neu5Ac) by chemoenzymatic synthesis generating CMP-9Biot-Neu5Ac. The activated sugar nucleotide was used as donor substrate for various mammalian sialyltranferases which transferred biotinylated sialic acids simultaneously onto glycan acceptors immobilized onto a microarray glass slide. Biotinylated glycans detected with fluorescein-streptavidin conjugate to generate a specificity profile for each enzyme both confirming previously known specificities and reveal additional specificity information. Human alpha2,6sialyltransferase-I (hST6Gal-I) also sialylates chitobiose structures (GlcNAcbeta1-4GlcNAc)(n) including N-glycans, rat alpha2,3sialyltransferase (rST3Gal-III) tolerates fucosylated acceptors such as Lewis(a), human alpha2,3sialyltransferase-IV (hST3Gal-IV) broadly sialylates oligosaccharides of types 1-4 and porcine alpha2,3sialyltransferase-I (pST3Gal-I) sialylates ganglio-oligosaccharides and core 2 O-glycans in our array system. Several of these sialyltransferases perform a substitution reaction and exchange a sialylated acceptor with a biotinylated sialic acid but are restricted to the most specific acceptor substrates. Thus, this method allows for a rapid generation of enzyme specificity information and can be used towards synthesis of new carbohydrate compounds and expand the glycan array compound library.

An N-linked glycoprotein with alpha(2,3)-linked sialic acid is a receptor for BK virus

BK virus (BKV) is a common human polyomavirus infecting >80% of the population worldwide. Infection with BKV is asymptomatic, but reactivation in renal transplant recipients can lead to polyomavirus-associated nephropathy. In this report, we show that enzymatic removal of alpha(2,3)-linked sialic acid from cells inhibited BKV infection. Reconstitution of asialo cells with alpha(2,3)-specific sialyltransferase restored susceptibility to infection. Inhibition of N-linked glycosylation with tunicamycin reduced infection, but inhibition of O-linked glycosylation did not. An O-linked-specific alpha(2,3)-sialyltransferase was unable to restore infection in asialo cells. Taken together, these data indicate that an N-linked glycoprotein containing alpha(2,3)-linked sialic acid is a critical component of the cellular receptor for BKV.

Chemoenzymatic synthesis of 2-azidoethyl-ganglio-oligosaccharides GD3, GT3, GM2, GD2, GT2, GM1, and GD1a

We have synthesized several ganglio-oligosaccharide structures using glycosyltransferases from Campylobacter jejuni. The enzymes, alpha-(2-->3/8)-sialyltransferase (Cst-II), beta-(1-->4)-N-acetylgalactosaminyltransferase (CgtA), and beta-(1-->3)-galactosyltransferase (CgtB), were produced in large-scale fermentation from Escherichia coli and further characterized based on their acceptor specificities. 2-Azidoethyl-glycosides corresponding to the oligosaccharides of GD3 (alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->3)-beta-D-Galp-(1-->4)-beta-D-Glcp-), GT3 (alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->3)-beta-D-Galp-(1-->4)-beta-D-Glcp-), GM2 (beta-D-GalpNAc-(1-->4)-[alpha-D-Neup5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-Glcp-), GD2 (beta-D-GalpNAc-(1-->4)-[alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-Glcp-), GT2 (beta-D-GalpNAc-(1-->4)-[alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->8)-alpha-D-Neup5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-Glcp-), and GM1 (beta-D-Galp-(1-->3)-beta-D-GalpNAc-(1-->4)-[alpha-D-Neup5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-Glcp-) were synthesized in high yields (gram-scale). In addition, a mammalian alpha-(2-->3)-sialyltransferase (ST3Gal I) was used to sialylate GM1 and generate GD1a (alpha-D-Neup5Ac-(2-->3)-beta-D-Galp-(1-->3)-beta-D-GalpNAc-(1-->4)-[alpha-D-Neup5Ac-(2-->3)]-beta-D-Galp-(1-->4)-beta-D-Glcp-) oligosaccharide. We also cloned and expressed a rat UDP-N-acetylglucosamine-4'epimerase (GalNAcE) in E. coli AD202 cells for cost saving in situ conversion of less expensive UDP-GlcNAc to UDP-GalNAc.

Combinatorial Synthesis of MUC1 Glycopeptides: Polymer Blotting Facilitates Chemical and Enzymatic Synthesis of Highly Complicated Mucin Glycopeptides

The chemoselective polymer blotting method allows for rapid and efficient synthesis of glycopeptides based on a "catch and release" strategy between solid-phase and water-soluble polymer supports. We have developed a heterobifunctional linker sensitive to glutamic acid specific protease (BLase). The general procedure consists of five steps, namely (i) the solid-phase synthesis of glycopeptide containing BLase sensitive linker, (ii) subsequent deprotections and the release of the glycopeptide from the resin, (iii) chemoselective blotting of the glycopeptide intermediates in the presence of water-soluble polymers with oxylamino functional groups, (iv) sugar elongations using glycosyltransferases, and (v) the release of target glycopeptides from the polymer platform by selective BLase promoted hydrolysis. The combined use of the solid-phase chemical syntheses of peptides and the enzymatic syntheses of carbohydrates on water-soluble polymers would greatly contribute to the production of complicated glycopeptide libraries, thereby enhancing applicative research. We report here a high-throughput synthetic system for the various types of MUC1 glycopeptides exhibiting a variety of sugar moieties. It is our belief that this concept will become part of the entrenched repertoire for the synthesis of biologically important glycopeptides on the basis of glycosyltransferase reactions in automated and combinatorial syntheses.

A mathematical model of N-linked glycosylation

Metabolic engineering of N-linked oligosaccharide biosynthesis to produce novel glycoforms or glycoform distributions of a recombinant glycoprotein can potentially lead to an improved therapeutic performance of the glycoprotein product. A mathematical model for the initial stages of this process, up to the first galactosylation of an oligosaccharide, was previously developed by Umana and Bailey (1997) (UB1997). Building on this work, an extended model is developed to include further galactosylation, fucosylation, extension of antennae by N-acetyllactosamine repeats, and sialylation. This allows many more structural features to be predicted. A number of simplifying assumptions are also relaxed to incorporate more variables for the control of glycoforms. The full model generates 7565 oligosaccharide structures in a network of 22,871 reactions. Methods for solving the model for the complete product distribution and adjusting the parameters to match experimental data are also developed. A basal set of kinetic parameters for the enzyme-catalyzed reactions acting on free oligosaccharide substrates is obtained from the previous model and existing literature. Enzyme activities are adjusted to match experimental glycoform distributions for Chinese Hamster Ovary (CHO). The model is then used to predict the effect of increasing expression of a target glycoprotein on the product glycoform distribution and evaluate appropriate metabolic engineering strategies to return the glycoform profile to its original distribution pattern. This model may find significant utility in the future to predict glycosylation patterns and direct glycoengineering projects to optimize glycoform distributions.

Interactions of influenza A virus with sialic acids present on porcine surfactant protein D

Pigs can be infected with both human and avian influenza A virus (IAV) strains and are therefore considered to be important intermediates in the emergence of new IAV strains due to mixing of viral genes derived from human, avian, or porcine influenza viruses. These reassortant strains may have potential to cause pandemic influenza outbreaks in humans. The innate immune response against IAV plays a significant role in containment of IAV in the airways. We studied the interactions of IAV with porcine surfactant protein D (pSP-D), an important component of this first line defense system. Hemagglutination inhibition analysis shows that the distinct interactions of pSP-D with IAV mediated by the N-linked carbohydrate moiety in the carbohydrate recognition domain of pSP-D depend on the terminal sialic acids (SAs) present on this carbohydrate. Analysis by both lectin staining and by cleavage with linkage-specific sialidases shows that the carbohydrate of pSP-D is exclusively sialylated with alpha(2,6)-linked SAs, in contrast to surfactant protein A, which contains both alpha(2,3)- and alpha(2,6)-linked SAs on its N-linked carbohydrate. Enzymatic modification of the SA-linkages present on pSP-D demonstrates that the type of SA-linkage is important for its hemagglutination-inhibitory activity, and correlates with receptor-binding specificity of the IAV strains. The SAs present on pSP-D appear especially important for interactions with poorly glycosylated IAV strains. It remains to be elucidated to what extent the unique sialylation profile of pSP-D is involved in host range control of IAV in pigs, and whether it facilitates adaptation of avian or human IAV strains that can contribute to the production of reassortant strains in pigs.

High-level expression of human glycosyltransferases in insect cells as biochemically active form

cDNAs, encoding human beta1,4-galactosyltransferase (hGalT I, EC 2.4.1.22), human Galbeta1,3(4)-GlcNAc alpha2,3-sialyltransferase (hST3GalIII, EC 2.4.99), and human Galbeta1,4-GlcNAc alpha2,6-sialyltransferase (hST6Gal I, EC 2.4.99.1), were cloned from human cell lines. In order to express these glycosyltransferases as secreted form in insect cells, cDNAs were inserted into a novel baculovirus transfer vector equipped with the mouse IgM signal peptide and IgG binding domain of the Staphylococcus aureus protein A as an N-terminal fusion partner. About 14 mg hGalT I, 8 mg hST3GalIII, and 6.4 mg hST6Gal I were purified from 1 liter of recombinant baculovirus infected insect cell culture media. The specific activities of recombinant hGalT I and hST6Gal I were determined as 0.65 and 1.6 U/mg protein, respectively. These results indicated that the recombinant hGalT I and hST6Gal I retained enzyme activities at similar level to those of the authentic one although they were fused with the IgG binding domain at the N-terminus. Taken together, the mouse IgM signal peptide and IgG binding domain of the protein A could be efficiently used as an N-terminus fusion partner for the over-expression of heterologous proteins in insect cells.

Recombinant (2-->3)-alpha-sialyltransferase immobilized on nickel-agarose for preparative synthesis of sialyl Lewis(x) and Lewis(a) precursor oligosaccharides

The specificity of recombinant (2-->3)-alpha-sialyltransferase (ST3Gal-III), expressed in baculovirus-infected insect cells, has been determined with various oligosaccharide acceptors and sugar-nucleotide donors using a fluorescence based assay. Recombinant ST3Gal-III tagged with a polyhistidine tail was immobilized on Ni(2+)-NTA-Agarose as an active enzyme for use in the synthesis of three sialylated oligosaccharides: (i) the divalent molecule [alpha-Neu5Ac-(2-->3)-D-Galp-(1-->4)-beta-D-GlcpNAc-O-CH(2)](2)-C-(CH(2)OBn)(2) (12); (ii) the dansylated derivative, alpha-Neu5Ac-(2-->3)-D-Galp-(1-->3)-beta-D-GlcpNAc-O-(CH(2))(6)-NH-dansyl and; (iii) the tetrasacharide alpha-Neu5Ac-(2-->3)-beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-O-CH(3). Compound 12 was itself prepared from the divalent N-acetyllactosamine molecule built on pentaerythritol by a chemo-enzymatic route.

Identification and functional expression of a second human beta-galactoside alpha2,6-sialyltransferase, ST6Gal II

BLAST analysis of the human and mouse genome sequence databases using the sequence of the human CMP-sialic acid:beta-galactoside alpha-2,6-sialyltransferase cDNA (hST6Gal I, EC2.4.99.1) as a probe allowed us to identify a putative sialyltransferase gene on chromosome 2. The sequence of the corresponding cDNA was also found as an expressed sequence tag of human brain. This gene contained a 1590 bp open reading frame divided in five exons and the deduced amino-acid sequence didn't correspond to any sialyltransferase already known in other species. Multiple sequence alignment and subsequent phylogenic analysis showed that this new enzyme belonged to the ST6Gal subfamily and shared 48% identity with hST6Gal-I. Consequently, we named this new sialyltransferase ST6Gal II. A construction in pFlag vector transfected in COS-7 cells gave raise to a soluble active form of ST6Gal II. Enzymatic assays indicate that the best acceptor substrate of ST6Gal II was the free disaccharide Galbeta1-4GlcNAc structure whereas ST6Gal I preferred Galbeta1-4GlcNAc-R disaccharide sequence linked to a protein. The alpha2,6-linkage was confirmed by the increase of Sambucus nigra agglutinin-lectin binding to the cell surface of CHO transfected with the cDNA encoding ST6Gal II and by specific sialidases treatment. In addition, the ST6Gal II gene showed a very tissue specific pattern of expression because it was found essentially in brain whereas ST6Gal I gene is ubiquitously expressed.

Conserved cysteines in the sialyltransferase sialylmotifs form an essential disulfide bond

The sialyltransferase gene family is comprised of 16 cloned enzymes. All members contain two conserved protein domains, termed the S- and L-sialylmotifs, that participate in substrate binding. Of only six invariant amino acids, two are cysteines, with one found in each sialylmotif. Although the recombinant soluble form of ST6Gal I has six cysteines, quantitative analysis indicated the presence of only one disulfide linkage, and thiol reducing agents dithiothreitol and beta-mercaptoethanol inactivated the enzyme. Analysis of site-directed mutants showed that alanine or serine mutants of invariant Cys(181) or Cys(332) exhibit no detectable activity, either by direct assay or by staining of the transfected cells with Sambucus nigra agglutinin, which recognizes the product NeuAcalpha2,6Galbeta1,4GlcNAc on glycoproteins. In contrast, alanine mutations of charged residues adjacent to either cysteine showed little or no effect on enzyme activity. Immunofluorescence microscopy showed that although the wild type sialyltransferase is properly localized in the Golgi apparatus, the inactive cysteine mutants are retained in the endoplasmic reticulum. The results suggest that the invariant cysteine residues in the L- and S-sialylmotifs participate in the formation of an intradisulfide linkage that is essential for proper conformation and activity of ST6Gal I.

Synthesis and characterization of an anomeric sulfur analogue of CMP-sialic acid

alpha-2,3-Sialyltransferase catalyzes the transfer of sialic acid from CMP-sialic acid (1) to a lactose acceptor. An analogue of 1 was synthesized in which the anomeric oxygen atom was replaced with a sulfur atom (1S). The key step in the synthesis of 1S was a tetrazole-promoted coupling of a cytidine-5'-phosphoramidite with a glycosyl thiol of a protected sialic acid. Compounds 1 and 1S were characterized for their activity in a sialyl transfer assay. The rate of solvolysis in aqueous buffer of analogue 1S was 50-fold slower than that of 1. Analogue 1S was found to be substrate for alpha-2,3-sialyltransferase. The K(m) of 1S was just 3-fold higher than that of 1, while the k(cat) of 1S was 2 orders of magnitude lower compared to 1.

Expression of alpha2,8/2,9-polysialyltransferase from Escherichia coli K92. Characterization of the enzyme and its reaction products

The capsular polysaccharide of Escherichia coli K92 contains alternating -8-NeuAcalpha2- and -9-NeuAcalpha2- linkages. The enzyme catalyzing this polymerizing reaction has been cloned from the genomic DNA of E. coli K92. The 1.2-kilobase polymerase chain reaction fragment was subcloned in pRSET vector and the protein was expressed in the BL21(DE3) strain of E. coli with a hexameric histidine at its N-terminal end. The enzyme was isolated in the supernatant after lysis of the cells and fractionated by ultracentrifugation. Western blotting using anti-histidine antibody showed the presence of a band that migrated at about 47.5 kDa on both reducing and nonreducing SDS-polyacrylamide gel electrophoresis, indicating a monomeric enzyme. Among the carbohydrate acceptors tested, N-acetylneuraminic acid and the gangliosides G(D3) and G(Q1b) were preferred substrates. The cell-free enzyme reaction products obtained were characterized by NMR and mass spectrometry, which indicated the presence of both alpha2,9- and alpha2,8-linked polysialyl structure. The K92 neuS gene was used to transform the K1 strain of E. coli, the capsule of which contains only -8-NeuAcalpha2- linkages. Analysis of the polysaccharides isolated from these transformed cells is consistent with the presence of both -8-NeuAcalpha2- and -9-NeuAcalpha2- linkages. Our results suggest that the neuS gene product of E. coli K92 catalyzes the synthesis of polysialic acid with alpha2,9- and alpha2,8-linkages in vitro and in vivo.

The yeast expression system for recombinant glycosyltransferases

Glycosyltransferases are increasingly being used for in vitro synthesis of oligosaccharides. Since these enzymes are difficult to purify from natural sources, expression systems for soluble forms of the recombinant enzymes have been developed. This review focuses on the current state of development of yeast expression systems. Two yeast species have mainly been used, i.e. Saccharomyces cerevisiae and Pichia pastoris. Safety and ease of fermentation are well recognized for S. cerevisiae as a biotechnological expression system; however, even soluble forms of recombinant glycosyltransferases are not secreted. In some cases, hyperglycosylation may occur. P. pastoris, by contrast, secrete soluble orthoglycosylated forms to the supernatant where they can be recovered in a highly purified form. The review also covers some basic features of yeast fermentation and describes in some detail those glycosyltransferases that have successfully been expressed in yeasts. These include beta1,4galactosyltransferase, alpha2,6sialyltransferase, alpha2,3sialyltransferase, alpha1,3fucosyltransferase III and VI and alpha1,2mannosyltransferase. Current efforts in introducing glycosylation systems of higher eukaryotes into yeasts are briefly addressed.

Glycan engineering of proteins with whole living yeast cells expressing rat liver alpha2,3-sialytransferase in the porous cell wall

The N-glycans of recombinant proteins produced via the secretory pathway of cultured mammalian cells are often undersialylated, and insect cells lack sialytransferases. Undersialylated glycoproteins are rapidly cleared from the circulation, compromising the effect of pharmaceuticals. We show that incubation with living Saccharomyces cerevisiae cells expressing the catalytic ectodomain of rat liver alpha2,3-sialyltransferase (ST3Ne) in the porous cell wall resulted in sialylation of glycoproteins. The Km values of the yeast enzyme for several substrates were similar to those of recombinant ST3Ne from insect cells and of authentic ST3N. The yeast strain provides an inexpensive self-perpetuating source of ST3N activity for glycan engineering of recombinant proteins.

Remodeling of glycoprotein and carbohydrate antigens

Glycosylation is the most extensive of all post-translational modifications in proteins. It has important functions in their secretion, antigenicity and metabolic clearance through structural polymorphism. In recent years, advances in recombinant DNA technology allowed the production of recombinant therapeutic proteins, among which glycosylated proteins displayed differences compared to their native counterparts, including antigenic carbohydrates. In this review, we discuss the potential use of cloned glycosyltransferases in remodeling recombinant glycoprotein antigens as well as in synthesizing tumor-associated carbohydrate antigens.

Characterization of a recombinant Neisseria meningitidis alpha-2,3-sialyltransferase and its acceptor specificity

The structure and specificity of the recombinant alpha-2,3-sialyltransferase from Neisseria meninigitidis are reported. This enzyme showed an unusual acceptor specificity in that it could use alpha-terminal and beta-terminal Gal residues as acceptors. In addition (beta1-->4)-linked and (beta1-->3)-linked terminal Gal served as acceptors. These properties distinguish the bacterial enzyme from the more widely investigated mammalian equivalents. The protein was expressed as a membrane-associated protein in Escherichia coli at a level of 750 U/l (approximately 250 mg/l). The protein could be extracted with buffers containing 0.2% Triton X-100 and purified to homogeneity using immobilized-metal-affinity chromatography. Electrospray-ionization mass spectrometry of peptides obtained by cleavage with cyanogen bromide and trypsin confirmed over 95% of the deduced amino acid sequence. When used for enzymatic synthesis in coupled reactions with recombinant CMP-Neu5Ac synthetase, the alpha-2,3-sialyltransferase could sialylate fluorescent derivatives of N-acetyllactosamine with N-acetylneuraminic acid, N-propionylneuraminic acid and N-glycoloylneuraminic acid.

High-level expression and purification of a recombinant human alpha-1, 3-fucosyltransferase in baculovirus-infected insect cells

A human alpha-1,3-fucosyltransferase (Fuc-TVII) was expressed by recombinant baculovirus-infected insect Sf9 cells as a secretory fusion protein. The fusion protein consisted of the human granulocyte colony-stimulating factor signal peptide followed by an IgG-binding domain of protein A, a Fuc-TVI-derived peptide, and the putative catalytic domain of Fuc-TVII. The signal peptide was correctly cleaved and the recombinant Fuc-TVII was secreted into the culture medium at a concentration of 10 micrograms/ml. The recombinant Fuc-TVII could be highly purified in a single-step purification procedure, i.e., IgG-Sepharose column chromatography. The enzymatic properties of the Sf9-produced Fuc-TVII were compared with the properties of that expressed by a human B-cell line, Namalwa KJM-1, transfected with an episomal plasmid carrying the fusion Fuc-TVII cDNA. Both recombinant proteins showed alpha-1,3-fucosyltransferase activity toward a type II oligosaccharide with a terminal alpha-2,3-linked sialic acid among various acceptors. The apparent Km values of Sf9-produced Fuc-TVII for GDP-fucose and its acceptor substrate were slightly lower than those of the Fuc-TVII produced by Namalwa KJM-1 cells. Sf9-produced Fuc-TVII has N-linked carbohydrate chains whose molecular weights are lower than those linked to Namalwa KJM-1-produced Fuc-TVII. This difference in carbohydrate structure hardly affects the thermal stability of Fuc-TVII. The baculovirus expression system is available for high-level expression of stable and enzymatically active secretory Fuc-TVII.

Cloning and expression of cDNA for a human Gal(beta1-3)GalNAc alpha2,3-sialyltransferase from the CEM T-cell line

Complementary DNA encoding a human Gal(beta1-3)GalNAc alpha2,3-sialyltransferase type II (hST3Gal II) was cloned from a CEM T-cell cDNA library using a 23-base oligonucleotide probe. The sequence of this probe was established on the basis of a slightly divergent sialylmotif L that was obtained by polymerase chain reaction with degenerate oligonucleotide primers based on the conserved sialylmotif L of mammalian Gal(beta1-3)GalNAc alpha2,3-sialyltransferases. It was thus confirmed that a short oligonucleotide probe may be sensitive and highly specific. The nucleotide and amino acid sequences of hST3Gal II show, respectively, 56.3% and 49.3% similarity to hST3Gal I [Kitagawa, H. & Paulson, J. C. (1994) J. Biol. Chem. 269, 17872-17878] and 88.1% and 93.7% similarity to murine ST3Gal II [Lee, Y. C., Kojima, N., Wada, E., Kurosawa, N., Nakaoka, T., Hamamoto, T. & Tsuji, S. (1994) J. Biol. Chem. 269, 10028-10033]. hST3Gal II mRNA was highly expressed in heart, liver, skeletal muscle and various lymphoid tissues but not in brain and kidney. A soluble form of hST3Gal II expressed in COS-7 cells was tested in vitro for substrate specificity and kinetic properties. Asialofetuin and asialo-bovine submaxillary mucin appeared better substrates for hST3Gal II than for its murine counterpart as previously reported [Kojima, N., Lee, Y.-C., Hamamoto, T., Kurosawa, N. & Tsuji, S. (1994) Biochemistry 33, 5772-5776]. In previous studies, we have shown hyposialylation of O-glycans attached to two major lymphocyte CD43 and CD45 cell surface molecules in human-immunodeficiency-virus-1(HIV-1)-infected T-cell lines. Since comparable levels of hST3Gal I and hST3Gal II mRNA and enzymatic activity were observed in parental and HIV-1-infected CEM T-cell lysates, the sialylation defect associated with HIV infection of this cell line is probably due to a mechanism different from a simple altered catalytic activity of these sialyltransferases.

Aberrant O-glycosylation in the collagenous domain of pro alpha2(I) procollagen subunits synthesized by chemically transformed hamster fibroblasts

Chemically transformed Syrian hamster embryo fibroblasts (NQT-SHE) do not synthesize the pro alpha1(I) subunit of type I collagen, but they secrete two forms of the pro alpha2(I) subunit (N33 and N50) with abnormal post-translational modifications localized in the alpha2CB3,5 cyanogen bromide peptide of the collagenous domain (B. Peterkofsky and W. Prather (1992) J. Biol. Chem. 267 5388-5395). Isoelectric focusing and treatment of the modified chains with glycosidases and biotinylated Jacalin lectin identified the modifications as Gal beta1,3-GalNAc-O-Ser/Thr with or without a terminal sialic acid in an alpha2,6 linkage. Unhydroxylated N33 alpha-chains also reacted with Jacalin, confirming that the abnormal modification was O-glycosylation and not hyperhydroxylation of proline or lysine. Cells were treated with benzyl GalNAc, a competitive inhibitor of galactosyl transferase that prevents addition of Gal to GalNAc-O-Ser/Thr and thus blocks elongation of O-glycosyl chains. Treated cells secreted pro alpha2(I) chains containing GalNAc-O-Ser/Thr but no galactose or sialic acid, which suggested that Gal addition takes place before sialylation. Treatment of NQT-SHE cells with monensin and brefeldin A inhibited secretion and led to intracellular accumulation of pro alpha2(I) chains that contained only GalNAc. Therefore, it appears that GalNAc addition to pro alpha2(I) chains in NQT-SHE cells occurs in the cis-Golgi, while sialic acid and galactose are added in the trans-Golgi network. The pro alpha2(I) chains produced by NQT-SHE cells most likely are modified because they are in the denatured state, and thus potential O-glycosylation sites become available that would not be exposed in normal triple helical procollagen.

Glycobiotechnology: enzymes for the synthesis of nucleotide sugars

Complex carbohydrates, as constituting part of glycoconjugates such as glycoproteins, glycolipids, hormones, antibiotics and other secondary metabolites, play an active role in inter- and intracellular communication. The aim of "glycobiotechnology" as an upcoming interdisciplinary research field is to develop highly efficient synthesis strategies, including in vivo and in vitro approaches, in order to bring such complex molecules into analytical and therapeutic studies. The enzymatic synthesis of glycosidic bonds by Leloir-glycosyltransferases is an efficient strategy for obtaining saccharides with absolute stereo- and regioselectivity in high yields and under mild conditions. There are, however, two obstacles hindering the realization of this process on a biotechnological scale, namely the production of recombinant Leloir-glycosyltransferases and the availability of enzymes for the synthesis of nucleotide sugars (the glycosyltransferase donor substrates). The present review surveys some synthetic targets which have attracted the interest of glycobiologists as well as recombinant expression systems which give Leloir-glycosyltransferase activities in the mU and U range. The main part summarizes publications concerned with the complex pathways of primary and secondary nucleotide sugars and the availability and use of these enzymes for synthesis applications. In this context, a survey of our work will demonstrate how enzymes from different sources and pathways can be combined for the synthesis of nucleotide deoxysugars and oligosaccharides.

Cloning of the lipooligosaccharide alpha-2,3-sialyltransferase from the bacterial pathogens Neisseria meningitidis and Neisseria gonorrhoeae

The genes encoding the alpha-2,3-sialyltransferases involved in lipooligosaccharide biosynthesis from Neisseria meningitidis and Neisseria gonorrhoeae have been cloned and expressed in Escherichia coli. A high sensitivity enzyme assay using a synthetic fluorescent glycosyltransferase acceptor and capillary electrophoresis was used to screen a genomic library of N. meningitidis MC58 L3 in a "divide and conquer" strategy. The gene, denoted lst, was found on a 2. 0-kilobase fragment of DNA, and its sequence was determined and then used to design probes to amplify and subsequently clone the corresponding lst genes from N. meningitidis 406Y L3, N. meningitidis M982B L7, and N. gonorrhoeae F62. Functional sialyltransferase was produced from the genes derived from both L3 N. meningitidis strains and the N. gonorrhoeae F62. However, the N. meningitidis M982B L7 gene contained a frameshift mutation that renders it inactive. The expression of the lst gene was easily detected using the enzyme assay, and the protein expression could be detected when an immunodetection tag was added to the COOH-terminal end of the protein. Using the synthetic acceptor N-acetyllactosamine-aminophenyl-(6-(5-(fluorescein-carboxamido)-hexan oic acid amide), the alpha-2,3 specificity of the enzyme was confirmed by NMR examination of the reaction product. The enzyme could also use synthetic acceptors with lactose or galactose as the saccharide portion. This study is the first example of the cloning, expression, and examination of alpha-2,3-sialyltransferase activity from a bacterial source.

Molecular cloning of a developmentally regulated N-acetylgalactosamine alpha2,6-sialyltransferase specific for sialylated glycoconjugates

A cDNA encoding a novel sialyltransferase has been isolated employing the polymerase chain reaction using degenerate primers to conserved regions of the sialylmotif that is present in all eukaryotic members of the sialyltransferase gene family examined to date. The cDNA sequence revealed an open reading frame coding for 305 amino acids, making it the shortest sialyltransferase cloned to date. This open reading frame predicts all the characteristic structural features of other sialyltransferases including a type II membrane protein topology and both sialylmotifs, one centrally located and the second in the carboxyl-terminal portion of the cDNA. When compared with all other sialyltransferase cDNAs, the predicted amino acid sequence displays the lowest homology in the sialyltransferase gene family. Northern analysis shows this sialyltransferase to be developmentally regulated in brain with expression persisting through adulthood in spleen, kidney, and lung. Stable transfection of the full-length cDNA in the human kidney carcinoma cell line 293 produced an active sialyltransferase with marked specificity for the sialoside, Neu5Ac-alpha2,3Gal-beta1,3GalNAc and glycoconjugates carrying the same sequence such as G(M1b) and fetuin. The disialylated tetrasaccharide formed by reacting the sialyltransferase with the aforementioned sialoside was analyzed by one- and two-dimensional 1H and 13C NMR spectroscopy and was shown to be the Neu5Ac-alpha2,3Gal-beta1,3(Neu5Ac-alpha2,6)GalNAc sialoside. This indicates that the enzyme is a GalNAc alpha-2,6-sialyltransferase. Since two other ST6GalNAc sialyltransferase cDNAs have been isolated, this sialyltransferase has been designated ST6GalNAc III. Of these three, ST6GalNAc III displays the most restricted acceptor specificity and is the only sialyltransferase cloned to date capable of forming the developmentally regulated ganglioside G(D1alpha) from G(M1b).