A conserved disulphide bond in sialyltransferases

Computational analysis of the structure, glycosylation and CMP binding of human ST3GAL sialyltransferases

Sialyltransferases (STs) are the fundamental enzymes which are related to many biological processes such as cell signalling, cellular recognition, cell-cell and host-pathogen interactions and metastasis of cancer. All STs catalyse the terminal sialic acid addition from CMP donor to the glycan units. ST3GAL family is one of the most important STs and divided into the six subfamily in mouse and humans which are ST3Gal I, ST3Gal II, ST3Gal III, ST3Gal IV, ST3Gal V, and ST3Gal VI. The members of the ST3GAL family transfer sialic acid to the terminal galactose residues of glycochains through an α2,3-linkage. There are many reports on the ST3GAL function in mammals but, there is a paucity of information about structure of human ST3GAL family. Herein, we investigated the structure, glycosylation and CMP binding site of human ST3GAL family using computational methods. We found for the first time N-glycosylation positions in ST3Gal IV and VI, mucin type glycosylation in ST3Gal III and O-GlcNAcylation in ST3Gal V and their relation with sialylmotifs. In addition, we predicted CMP binding positions of human ST3GAL enzyme family on three-dimensional structure using molecular docking and first demonstrated the sialylmotifs relation with the CMP binding positions in ST3Gal III-VI subfamilies.

Novel Zebrafish Mono-α2,8-sialyltransferase (ST8Sia VIII): An Evolutionary Perspective of α2,8-Sialylation

The mammalian mono-α2,8-sialyltransferase ST8Sia VI has been shown to catalyze the transfer of a unique sialic acid residues onto core 1 O-glycans leading to the formation of di-sialylated O-glycosylproteins and to a lesser extent to diSia motifs onto glycolipids like GD1a. Previous studies also reported the identification of an orthologue of the ST8SIA6 gene in the zebrafish genome. Trying to get insights into the biosynthesis and function of the oligo-sialylated glycoproteins during zebrafish development, we cloned and studied this fish α2,8-sialyltransferase homologue. In situ hybridization experiments demonstrate that expression of this gene is always detectable during zebrafish development both in the central nervous system and in non-neuronal tissues. Intriguingly, using biochemical approaches and the newly developed in vitro MicroPlate Sialyltransferase Assay (MPSA), we found that the zebrafish recombinant enzyme does not synthetize diSia motifs on glycoproteins or glycolipids as the human homologue does. Using comparative genomics and molecular phylogeny approaches, we show in this work that the human ST8Sia VI orthologue has disappeared in the ray-finned fish and that the homologue described in fish correspond to a new subfamily of α2,8-sialyltransferase named ST8Sia VIII that was not maintained in Chondrichtyes and Sarcopterygii.

Effects of amino acid substitutions in the sialylmotifs on molecular expression and enzymatic activities of α2,8-sialyltransferases ST8Sia-I and ST8Sia-VI

Mouse sialyltransferases are grouped into four families according to the type of carbohydrate linkage they synthesize: β-galactoside α2,3-sialyltransferases (ST3Gal-I-VI), β-galactoside α2,6-sialyltransferases (ST6Gal-I and ST6Gal-II), N-acetylgalactosamine α2,6-sialyltransferases (ST6GalNAc-I-VI) and α2,8-sialyltransferases (ST8Sia-I-VI). These sialyltransferases feature a type II transmembrane topology and contain highly conserved motifs termed sialylmotifs L, S, III and VS. Sialylmotifs L and S are involved in substrate binding, whereas sialylmotifs III and VS are involved in catalytic activity. In addition to the conventional sialylmotifs, family and subfamily specific sequence motifs have been proposed. In this study, we analyzed the properties and functions of sialylmotifs in characterizing the enzymatic activity of mouse ST8Sia-I and ST8Sia-VI, both of which are α2,8-sialyltransferases involved in the synthesis of either ganglioside GD3 or disialic acid structures on O-glycans, respectively. The ST8Sia-VI-based chimeric enzymes, whose sialylmotif L sequences were replaced with those of ST8Sia-I and ST8Sia-IV (polysialic acid synthetase), were still active toward O-glycans. However, ST8Sia-VI-based chimeric enzymes lost expression or activity when their sialylmotif L sequences were replaced with those of ST3Gal-I and ST6GalNAc-II, suggesting the existence of an ST8Sia family specific motif in the sialylmotif L. The ST8Sia-I- and ST8Sia-VI-based chimeric enzymes lost enzymatic activity when their sialylmotif S sequences were interchanged. Amino acid substitutions in the sialylmotif S of ST8Sia-I and ST8Sia-VI also affected the enzymatic activity in many cases, indicating the crucial and functional importance of the sialylmotif S in substrate binding, which determines the substrate specificity of sialyltransferase.

Disulphide linkage in mouse ST6Gal-I: determination of linkage positions and mutant analysis

All cloned sialyltransferases from vertebrates are classified into four subfamilies and are characterized as having type II transmembrane topology. The catalytic domain has highly conserved motifs known as sialylmotifs. Besides sialylmotifs, each family has several unique conserved cysteine (Cys) residues mainly in the catalytic domain. The number and loci of conserved amino acids, however, differ with each subfamily, suggesting that the conserved Cys-residues and/or disulphide linkages they make may contribute to linkage specificity. Using Matrix Assisted Laser Desorption/Ionization-Time of Flight Mass Spectrometry (MALDI-TOF)-mass spectrometry, the present study performed disulphide linkage analysis on soluble mouse ST6Gal-I, which has six Cys-residues. Results confirmed that there were no free Cys-residues, and all six residues contributed to disulphide linkage formation, C(139)-C(403), C(181)-C(332) and C(350)-C(361). Study of single amino acid-substituted mutants revealed that the disulphide linkage C(181)-C(332) was necessary for molecular expression of the enzyme, and that the disulphide linkage C(350)-C(361) was necessary for enzyme activity. The remaining disulphide linkage C(139)-C(403) was not necessary for enzyme expression or for activity, including substrate specificity. Crystallographic study of pig ST3Gal I has recently been reported. Interestingly, the loci of disulphide linkages in ST6Gal-I differ from those in ST3Gal I, suggesting that the linkage specificity of sialyltransferase may results from significant structural differences, including the loci of disulphide linkages.

Primer on genes encoding enzymes in sialic acid metabolism in mammals

Sialic acid, a nine-carbon sugar acid usually is present in the non-reducing terminal position of free oligosaccharides and glycoconjugates. Sialylated conjugates in mammals perform important roles in cellular recognition, signaling, host-pathogen interaction and neuronal development. Metabolism of sialylated conjugates involves a complex pathway consisting of enzymes distributed among the different compartments in the cell. These enzymes are encoded by 32 genes diversely distributed throughout the mammalian genome. Genetic variants in some of these genes are associated with embryonic lethality and abnormal phenotypes in mice and neuromuscular diseases, carcinomas and immune-mediated diseases in humans. In humans, the CMP-NeuAc-hydroxylase (CMAH) enzyme is inactivated due to a deletion mutation in the encoded enzyme. This lack of Neu5Gc phenotype makes humans unique among mammals. This review focuses on genes encoding enzymes in sialic acid metabolism pathways in mammalian cells with special emphasis on the human, mouse and cow.

MALE GAMETOPHYTE DEFECTIVE 2, encoding a sialyltransferase-like protein, is required for normal pollen germination and pollen tube growth in Arabidopsis

Sialyltransferases (SiaTs) exist widely in vertebrates and play important roles in a variety of biological processes. In plants, several genes have also been identified to encode the proteins that share homology with the vertebrate SiaTs. However, very little is known about their functions in plants. Here we report the identification and characterization of a novel Arabidopsis gene, MALE GAMETOPHYTE DEFECTIVE 2 (MGP2) that encodes a sialyltransferase-like protein. MGP2 was expressed in all tissues including pollen grains and pollen tubes. The MGP2 protein was targeted to Golgi apparatus. Knockout of MGP2 significantly inhibited the pollen germination and retarded pollen tube growth in vitro and in vivo, but did not affect female gametophytic functions. These results suggest that the sialyltransferase-like protein MGP2 is important for normal pollen germination and pollen tube growth, giving a novel insight into the biological roles of the sialyltransferase-like proteins in plants.

Identification of sequences in the polysialyltransferases ST8Sia II and ST8Sia IV that are required for the protein-specific polysialylation of the neural cell adhesion molecule, NCAM

The polysialyltransferases ST8Sia II and ST8Sia IV polysialylate the glycans of a small subset of mammalian proteins. Their most abundant substrate is the neural cell adhesion molecule (NCAM). An acidic surface patch and a novel alpha-helix in the first fibronectin type III repeat of NCAM are required for the polysialylation of N-glycans on the adjacent immunoglobulin domain. Inspection of ST8Sia IV sequences revealed two conserved polybasic regions that might interact with the NCAM acidic patch or the growing polysialic acid chain. One is the previously identified polysialyltransferase domain (Nakata, D., Zhang, L., and Troy, F. A. (2006) Glycoconj. J. 23, 423-436). The second is a 35-amino acid polybasic region that contains seven basic residues and is equidistant from the large sialyl motif in both polysialyltransferases. We replaced these basic residues to evaluate their role in enzyme autopolysialylation and NCAM-specific polysialylation. We found that replacement of Arg(276)/Arg(277) or Arg(265) in the polysialyltransferase domain of ST8Sia IV decreased both NCAM polysialylation and autopolysialylation in parallel, suggesting that these residues are important for catalytic activity. In contrast, replacing Arg(82)/Arg(93) in ST8Sia IV with alanine substantially decreased NCAM-specific polysialylation while only partially impacting autopolysialylation, suggesting that these residues may be particularly important for NCAM polysialylation. Two conserved negatively charged residues, Glu(92) and Asp(94), surround Arg(93). Replacement of these residues with alanine largely inactivated ST8Sia IV, whereas reversing these residues enhanced enzyme autopolysialylation but significantly reduced NCAM polysialylation. In sum, we have identified selected amino acids in this conserved polysialyltransferase polybasic region that are critical for the protein-specific polysialylation of NCAM.

Evolutionary history of the alpha2,8-sialyltransferase (ST8Sia) gene family: tandem duplications in early deuterostomes explain most of the diversity found in the vertebrate ST8Sia genes

BACKGROUND

The animal sialyltransferases, which catalyze the transfer of sialic acid to the glycan moiety of glycoconjugates, are subdivided into four families: ST3Gal, ST6Gal, ST6GalNAc and ST8Sia, based on acceptor sugar specificity and glycosidic linkage formed. Despite low overall sequence identity between each sialyltransferase family, all sialyltransferases share four conserved peptide motifs (L, S, III and VS) that serve as hallmarks for the identification of the sialyltransferases. Currently, twenty subfamilies have been described in mammals and birds. Examples of the four sialyltransferase families have also been found in invertebrates. Focusing on the ST8Sia family, we investigated the origin of the three groups of alpha2,8-sialyltransferases demonstrated in vertebrates to carry out poly-, oligo- and mono-alpha2,8-sialylation.

RESULTS

We identified in the genome of invertebrate deuterostomes, orthologs to the common ancestor for each of the three vertebrate ST8Sia groups and a set of novel genes named ST8Sia EX, not found in vertebrates. All these ST8Sia sequences share a new conserved family-motif, named "C-term" that is involved in protein folding, via an intramolecular disulfide bridge. Interestingly, sequences from Branchiostoma floridae orthologous to the common ancestor of polysialyltransferases possess a polysialyltransferase domain (PSTD) and those orthologous to the common ancestor of oligosialyltransferases possess a new ST8Sia III-specific motif similar to the PSTD. In osteichthyans, we have identified two new subfamilies. In addition, we describe the expression profile of ST8Sia genes in Danio rerio.

CONCLUSION

Polysialylation appeared early in the deuterostome lineage. The recent release of several deuterostome genome databases and paralogons combined with synteny analysis allowed us to obtain insight into events at the gene level that led to the diversification of the ST8Sia genes, with their corresponding enzymatic activities, in both invertebrates and vertebrates. The initial expansion and subsequent divergence of the ST8Sia genes resulted as a consequence of a series of ancient duplications and translocations in the invertebrate genome long before the emergence of vertebrates. A second subset of ST8sia genes in the vertebrate genome arose from whole genome duplication (WGD) R1 and R2. Subsequent selective ST8Sia gene loss is responsible for the characteristic ST8Sia gene expression pattern observed today in individual species.

Sialylation in protostomes: a perspective from Drosophila genetics and biochemistry

Numerous studies have revealed important functions for sialylation in both prokaryotes and higher animals. However, the genetic and biochemical potential for sialylation in Drosophila has only been confirmed recently. Recent studies suggest significant similarities between the sialylation pathways of vertebrates and insects and provide evidence for their common evolutionary origin. These new data support the hypothesis that sialylation in insects is a specialized and developmentally regulated process which likely plays a prominent role in the nervous system. Yet several key issues remain to be addressed in Drosophila, including the initiation of sialic acid de novo biosynthesis and understanding the structure and function of sialylated glycoconjugates. This review discusses our current knowledge of the Drosophila sialylation pathway, as compared to the pathway in bacteria and vertebrates. We arrive at the conclusion that Drosophila is emerging as a useful model organism that is poised to shed new light on the function of sialylation not only in protostomes, but also in a larger evolutionary context.

Molecular evolution and expression of zebrafish St8SiaIII, an alpha-2,8-sialyltransferase involved in myotome development

Enzymes of the St8Sia family, a subgroup of the glycosyltransferases, mediate the transfer of sialic acid to glycoproteins or glycolipids. Here, we describe the cloning of the zebrafish St8SiaIII gene and study its developmental activity. A conserved synteny relationship among vertebrate chromosome regions containing St8SiaIII loci underscores an ancient duplication of this gene in the teleost fish lineage and a specific secondary loss of one paralog in the zebrafish. The single zebrafish St8SiaIII enzyme, which is expected to function as an oligosialyltransferase, lacks maternal activity, is weakly expressed during nervous system development, and shows a highly dynamic expression pattern in somites and somite-derived structures. Morpholino knock-down of St8SiaIII leads to anomalous somite morphologies, including defects in segment boundary formation and myotendious-junction integrity. These phenotypes hint for a basic activity of zebrafish St8SiaIII during segmentation and somite formation, providing novel evidence for a non-neuronal function of sialyltransferases during vertebrate development.

Multifunctionality of Campylobacter jejuni sialyltransferase CstII: characterization of GD3/GT3 oligosaccharide synthase, GD3 oligosaccharide sialidase, and trans-sialidase activities

CstII from bacterium Campylobacter jejuni strain OH4384 has been previously characterized as a bifunctional sialyltransferase having both alpha2,3-sialyltransferase (GM3 oligosaccharide synthase) and alpha2,8-sialyltransferase (GD3 oligosaccharide synthase) activities which catalyze the transfer of N-acetylneuraminic acid (Neu5Ac) from cytidine 5'-monophosphate (CMP)-Neu5Ac to C-3' of the galactose in lactose and to C-8 of the Neu5Ac in 3'-sialyllactose, respectively (Gilbert M, Karwaski MF, Bernatchez S, Young NM, Taboada E, Michniewicz J, Cunningham AM, Wakarchuk WW. 2002. The genetic bases for the variation in the lipo-oligosaccharide of the mucosal pathogen, Campylobacter jejuni. Biosynthesis of sialylated ganglioside mimics in the core oligosaccharide. J Biol Chem. 277:327-337). We report here the characterization of a truncated CstII mutant (CstIIDelta32(I53S)) cloned from a synthetic gene whose codons are optimized for an Escherichia coli expression system. In addition to the alpha2,3- and alpha2,8-sialyltransferase activities reported before for the synthesis of GM3- and GD3-type oligosaccharides, respectively, the CstIIDelta32(I53S) has alpha2,8-sialyltransferase (GT3 oligosaccharide synthase) activity for the synthesis of GT3 oligosaccharide. It also has alpha2,8-sialidase (GD3 oligosaccharide sialidase) activity that catalyzes the specific cleavage of the alpha2,8-sialyl linkage of GD3-type oligosaccharides and alpha2,8-trans-sialidase (GD3 oligosaccharide trans-sialidase) activity that catalyzes the transfer of a sialic acid from a GD3 oligosaccharide to a different GM3 oligosaccharide (3'-sialyllactoside). The donor substrate specificity study of the CstIIDelta32(I53S) GD3 oligosaccharide synthase activity indicates that the enzyme is flexible in using different CMP-activated sialic acids and their analogs for the synthesis of GD3 oligosaccharides containing natural and nonnatural modifications at the terminal sialic acid.

Cloning and transcriptional regulation of genes responsible for synthesis of gangliosides

Ganglioside synthases are glycosyltransferases involved in the biosynthesis of glycoconjugates. A number of ganglioside synthase genes have been cloned and characterized. They are classified into different families of glycosyltransferases based on similarities of their amino acid sequences. Tissue-specific expression of these genes has been analyzed by hybridization using cDNA fragments. Enzymatic characterization with the expressed recombinant enzymes showed these enzymes differ in their donor and acceptor substrate specificities and other biochemical parameters. In vitro enzymatic analysis also showed that one linkage can be synthesized by multiple enzymes and one enzyme may be responsible for synthesis of multiple gangliosides. Following the cloning of the ganglioside synthase genes, the promoters of the key synthase genes in the ganglioside biosynthetic pathway have been cloned and analyzed. All of the promoters are TATA-less, lacking a CCAAT box but containing GC-rich boxes, characteristic of the house-keeping genes, although transcription of ganglioside synthase genes is subject to complex developmental and tissue-specific regulation. A set of cis-acting elements and transcription factors, including Sp1, AP2, and CREB, function in the proximal promoters. Negative-regulatory regions have also been defined in most of the promoters. We present here an overview of these genes and their transcriptional regulation.

Reversible sialylation: synthesis of cytidine 5'-monophospho-N-acetylneuraminic acid from cytidine 5'-monophosphate with alpha2,3-sialyl O-glycan-, glycolipid-, and macromolecule-based donors yields diverse sialylated products

Sialyltransferases transfer sialic acid from cytidine 5'-monophospho-N-acetylneuraminic acid (CMP-NeuAc) to an acceptor molecule. Trans-sialidases of parasites transfer alpha2,3-linked sialic acid from one molecule to another without the involvement of CMP-NeuAc. Here we report another type of sialylation, termed reverse sialylation, catalyzed by mammalian sialyltransferase ST3Gal-II. This enzyme synthesizes CMP-NeuAc by transferring NeuAc from the NeuAcalpha2,3Galbeta1,3GalNAcalpha unit of O-glycans, 3-sialyl globo unit of glycolipids, and sialylated macromolecules to 5'-CMP. CMP-NeuAc produced in situ is utilized by the same enzyme to sialylate other O-glycans and by other sialyltransferases such as ST6Gal-I and ST6GalNAc-I, forming alpha2,6-sialylated compounds. ST3Gal-II also catalyzed the conversion of 5'-uridine monophosphate (UMP) to UMP-NeuAc, which was found to be an inactive sialyl donor. Reverse sialylation proceeded without the need for free sialic acid, divalent metal ions, or energy. Direct sialylation with CMP-NeuAc as well as the formation of CMP-NeuAc from 5'-CMP had a wide optimum range (pH 5.2-7.2 and 4.8-6.4, respectively), whereas the entire reaction comprising in situ production of CMP-NeuAc and sialylation of acceptor had a sharp optimum at pH 5.6 (activity level 50% at pH 5.2 and 6.8, 25% at pH 4.8 and 7.2). Several properties distinguish forward/conventional versus reverse sialylation: (i) sodium citrate inhibited forward sialylation but not reverse sialylation; (ii) 5'-CDP, a potent forward sialyltransferase inhibitor, did not inhibit the conversion of 5'-CMP to CMP-NeuAc; and (iii) the mucin core 2 compound 3-O-sulfoGalbeta1,4GlcNAcbeta1,6(Galbeta1,3)GalNAcalpha-O-benzyl, an efficient acceptor for ST3Gal-II, inhibited the conversion of 5'-CMP to CMP-NeuAc. A significant level of reverse sialylation activity is noted in human prostate cancer cell lines LNCaP and PC3. Overall, the study demonstrates that the sialyltransferase reaction is readily reversible in the case of ST3Gal-II and can be exploited for the enzymatic synthesis of diverse sialyl products.

Molecular basis for polysialylation: a novel polybasic polysialyltransferase domain (PSTD) of 32 amino acids unique to the alpha 2,8-polysialyltransferases is essential for polysialylation

To determine the molecular basis of eukaryotic polysialylation, the function of a structurally unique polybasic motif of 32 amino acids (pI approximately 12) in the polysialyltransferases (polySTs), ST8Sia II (STX and ST8Sia IV (PST) was investigated. This motif, designated the "polysialyltransferase domain" (PSTD), is immediately upstream of the sialylmotif S (SM-S). PolyST activity was lost in COS-1 mutants in which the entire PSTD in ST8Sia IV was deleted, or in mutants in which 10 and 15 amino acids in either the N- or C- terminus of PSTD were deleted. Site-directed mutagenesis showed that Ile(275), Lys(276) and Arg(277) in the C-terminus of PSTD in ST8Sia IV, which is contiguous with the N-terminus of sialylmotif-S, were essential for polysialylation. Arg(252) in the N-terminus segment of the PSTD was also required, as was the overall positive charge. Thus, multiple domains in the polySTs can influence their activity. Immunofluorescent microscopy showed that the mutated proteins were folded correctly, based on their Golgi localization. The structural distinctness of the conserved PSTD in the polySTs, and its absence in the mono- oligoSTs, suggests that it is a "polymerization domain" that distinguishes a polyST from a monosialyltransferases. We postulate that the electrostatic interaction between the polybasic PSTD and the polyanionic polySia chains may function to tether nascent polySia chains to the enzyme, thus facilitating the processive addition of new Sia residues to the non-reducing end of the growing chain. In accord with this hypothesis, the polyanion heparin was shown to inhibit recombinant human ST8Sia II and ST8Sia IV at 10 microM.

Biochemical characterization of a Neisseria meningitidis polysialyltransferase reveals novel functional motifs in bacterial sialyltransferases

The extracellular polysaccharide capsule is an essential virulence factor of Neisseria meningitidis, a leading cause of severe bacterial meningitis and sepsis. Serogroup B strains, the primary disease causing isolates in Europe and America, are encapsulated in α-2,8 polysialic acid (polySia). The capsular polymer is synthesized from activated sialic acid by action of a membrane-associated polysialyltransferase (NmB-polyST). Here we present a comprehensive characterization of NmB-polyST. Different from earlier studies, we show that membrane association is not essential for enzyme functionality. Recombinant NmB-polyST was expressed, purified and shown to synthesize long polySia chains in a non-processive manner in vitro. Subsequent structure–function analyses of NmB-polyST based on refined sequence alignments allowed the identification of two functional motifs in bacterial sialyltransferases. Both (D/E-D/E-G and HP motif) are highly conserved among different sialyltransferase families with otherwise little or no sequence identity. Their functional importance for enzyme catalysis and CMP-Neu5Ac binding was demonstrated by mutational analysis of NmB-polyST and is emphasized by structural data available for the Pasteurella multocida sialyltransferase PmST1. Together our data are the first description of conserved functional elements in the highly diverse families of bacterial (poly)sialyltransferases and thus provide an advanced basis for understanding structure–function relations and for phylogenetic sorting of these important enzymes.

Determination of glycosylation sites and disulfide bond structures using LC/ESI-MS/MS analysis

Significant progress has been made in discovering and cloning a host of proteins, including a range of glycoproteins. The availability of their predicted amino acid sequences provides useful information, including potential N-linked glycosylation sites. However, only a limited number of protein structures have been solved, and very little is known about the structures of membrane proteins. One of the important structural elements of a protein is its disulfide bonds. These covalent bonds place conformational constraints on the overall protein structure, and thus, their identification provides important structural information. A second important posttranslational modification found in proteins is N-linked glycosylation. Although potential sites of N-linked glycosylation can be predicted from a protein's primary sequence based on the presence of N-X-S/T sequences, not all of the predicted sites will be glycosylated. Therefore, N-linked glycosylation sites must be located by structural analysis. We have developed a simple and sensitive method for determining the presence of free cysteine (Cys) residues and disulfide-bonded Cys residues, as well as the N-linked glycosylation sites in glycoproteins by liquid chromatography/electrospray ionization-tandem mass spectrometry (LC/ESI-MS/MS) in combination with protein database searching using the programs Sequest and Mascot. The details of our method are described in this chapter.

Analysis of the specificity of sialyltransferases toward mucin core 2, globo, and related structures. identification of the sialylation sequence and the effects of sulfate, fucose, methyl, and fluoro substituents of the carbohydrate chain in the biosynthesis of selectin and siglec ligands, and novel sialylation by cloned alpha2,3(O)sialyltransferase

Sialic acids are key determinants in many carbohydrates involved in biological recognition. We studied the acceptor specificities of three cloned sialyltransferases (STs) [alpha2,3(N)ST, alpha2,3(O)ST, and alpha2,6(N)ST] and another alpha2,3(O)ST present in prostate cancer cell LNCaP toward mucin core 2 tetrasaccharide [Galbeta1,4GlcNAcbeta1,6(Galbeta1,3)GalNAcalpha-O-Bn] and Globo [Galbeta1,3GalNAcbeta1,3Galalpha-O-Me] structures containing sialyl, fucosyl, sulfo, methyl, or fluoro substituents by identifying the products by electrospray ionization tandem mass spectral analysis and other biochemical methods. The Globo precursor was an efficient acceptor for both alpha2,3(N)ST and alpha2,3(O)ST, whereas only alpha2,3(O)ST used its deoxy analogue (d-Fucbeta1,3GalNAcbeta1,3-Gal-alpha-O-Me); 2-O-MeGalbeta1,3GlcNAc and 4-OMeGalbeta1,4GlcNAc were specific acceptors for alpha2,3(N)ST. Other major findings of this study include: (i) alpha2,3 sialylation of beta1,3Gal in mucin core 2 can proceed even after alpha1,3 fucosylation of beta1,6-linked LacNAc. (ii) Sialylation of beta1,3Gal must precede the sialylation of beta1,4Gal for favorable biosynthesis of mucin core 2 compounds. (iii) alpha2,3 sialylation of the 6-O-sulfoLacNAc moiety in mucin core 2 (e.g., GlyCAM-1) is facilitated when beta1,3Gal has already been alpha2,3 sialylated. (iv) alpha2,6(N)ST was absolutely specific for the beta1,4Gal in mucin core 2. Either alpha1,3 fucosylation or 6-O-sulfation of the GlcNAc moiety reduced the activity. Sialylation of beta1,3Gal in addition to 6-O-sulfation of GlcNAc moiety abolished the activity. (v) Prior alpha2,3 sialylation or 3-O-sulfation of beta1,3Gal would not affect alpha2,6 sialylation of Galbeta1,4GlcNAc of mucin core 2. (vi) A 3- or 4-fluoro substituent in beta1,4Gal resulted in poor acceptors for the cloned alpha2,6(N)ST and alpha2,3(N)ST, whereas 4-fluoro- or 4-OMe-Galbeta1,3GalNAcalpha was a good acceptor for cloned alpha2,3(O)ST. (vii) 4-O-Methylation of beta1,4Gal abolished the acceptor ability toward alpha2,6(N)ST but increased the acceptor efficiency considerably toward alpha2,3(N)ST. (viii) Just like LNCaPalpha1,2-FT and Gal-3-O-sulfotransferase T2, the cloned alpha2,3(N)ST which modifies terminal Gal in Galbeta1,4GlcNAc also efficiently utilizes the terminal beta1,3Gal in the Globo backbone. Utilization of C-3 blocked compounds such as 3-O-sulfo-Galbeta1,3GalNAcbeta1,3Galalpha-OMe as acceptors by cloned alpha2,3(O)ST and analyses of the resulting products by lectin chromatography and mass spectrometry indicate that alpha2,3(O)ST is capable of attaching NeuAc to another position in C-3-substituted beta1,3Gal.

Analysis of Sialyltransferase-Like Proteins from Oryza sativa

Sialic acids are widely distributed among living creatures, from bacteria to mammals, but it has been commonly accepted that they do not exist in plants. However, with the progress of genome analyses, putative gene homologs of animal sialyltransferases have been detected in the genome of some plants. In this study, we cloned three genes from Oryza sativa (Japanese rice) that encode sialyltransferase-like proteins, designated OsSTLP1, 2, and 3, and analyzed the enzymatic activity of the proteins. OsSTLP1, 2, and 3 consist of 393, 396, and 384 amino acids, respectively, and each contains sequences similar to the sialyl motifs that are highly conserved among animal sialyltransferases. The recombinant soluble forms of OsSTLPs produced by COS-7 cells were analyzed for sialyltransferase-like activity. OsSTLP1 exhibited such activity toward the oligosaccharide Galbeta1,4GlcNAc and such glycoproteins as asialofetuin, alpha1-acid glycoprotein, and asialo-alpha1-acid glycoprotein; OsSTLP3 exhibited similar activity toward asialofetuin; and OsSTLP2 exhibited no sialyltransferase-like activity. The sialic acid transferred by OsSTLP1 or 3 was linked to galactose of Galbeta1,4GlcNAc through alpha2,6-linkage. This is the first report of plant proteins having sialyltransferase-like activity.

Probing the substrate specificity of four different sialyltransferases using synthetic beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-(1-->O) (CH(2))7CH3 analogues general activating effect of replacing N-acetylglucosamine by N-propionylglucosamine

The acceptor specificities of ST3Gal III, ST3Gal IV, ST6Gal I and ST6Gal II were investigated using a panel of beta-D-Galp-(1-->4)-beta-D-GlcpNAc-(1-->2)-alpha-D-Manp-(1-->O)(CH(2))(7)CH(3) analogues. Modifications introduced at either C2, C3, C4, C5, or C6 of terminal D-Gal, as well as N-propionylation instead of N-acetylation of subterminal D-GlcN were tested for their influence on the alpha-2,3- and alpha-2,6-sialyltransferase acceptor activities. Both ST3Gal enzymes displayed the same narrow acceptor specificity, and only accept reduction of the Gal C2 hydroxyl function. The ST6Gal enzymes, however, do not have the same acceptor specificity. ST6Gal II seems less tolerant towards modifications at Gal C3 and C4 than ST6Gal I, and prefers beta-D-GalpNAc-(1-->4)-beta-D-GlcpNAc (LacdiNAc) as an acceptor substrate, as shown by replacing the Gal C2 hydroxyl group with an N-acetyl function. Finally, a particularly striking feature of all tested sialyltransferases is the activating effect of replacing the N-acetyl function of subterminal GlcNAc by an N-propionyl function.

Identification of linkage-specific sequence motifs in sialyltransferases

Eukaryotic sialyltransferases (SiaTs) comprise a superfamily of enzymes catalyzing the transfer of sialic acid (Sia) from a common donor substrate to various acceptor substrates in different linkages. These enzymes have been classified as ST3Gal, ST6Gal, ST6GalNAc, and ST8Sia families based on linkage- and acceptor monosaccharide-specificities and sequence similarities. It was recognized early on that SiaTs contain certain well-conserved motifs, and these were denoted as L (large)-, S (small)-, and VS (very small)-motifs; recently, a fourth motif, denoted as motif III, was identified. These four motifs are common to all the SiaTs, irrespective of the linkage- and acceptor saccharide-specificities. In this study, the sequences of the various families have been analyzed, and sequence motifs that are unique to the various families have been identified. These unique motifs are expected to contribute to the characteristic linkage- and acceptor saccharide-specificities of the family members. One of the linkage specific motifs is contiguous to L-motif. Members of ST3Gal and ST8Sia families share significant sequence similarities; in contrast, the ST6Gal family is distinct from the ST6GalNAc family. The latter consists of two subfamilies, one comprising ST6GalNAc I and ST6GalNAc II, and the other comprising ST6GalNAc III, ST6GalNAc IV, ST6GalNAc V, and ST6GalNAc VI. Each of these subfamilies has characteristic sequence motifs not present in the other subfamily.

The animal sialyltransferases and sialyltransferase-related genes: a phylogenetic approach

The animal sialyltransferases are Golgi type II transmembrane glycosyltransferases. Twenty distinct sialyltransferases have been identified in both human and murine genomes. These enzymes catalyze transfer of sialic acid from CMP-Neu5Ac to the glycan moiety of glycoconjugates. Despite low overall identities, they share four conserved peptide motifs [L (large), S (small), motif III, and motif VS (very small)] that are hallmarks for sialyltransferase identification. We have identified 155 new putative genes in 25 animal species, and we have exploited two lines of evidence: (1) sequence comparisons and (2) exon-intron organization of the genes. An ortholog to the ancestor present before the split of ST6Gal I and II subfamilies was detected in arthropods. An ortholog to the ancestor present before the split of ST6GalNAc III, IV, V, and VI subfamilies was detected in sea urchin. An ortholog to the ancestor present before the split of ST3Gal I and II subfamilies was detected in ciona, and an ortholog to the ancestor of all the ST8Sia was detected in amphioxus. Therefore, single examples of the four families (ST3Gal, ST6Gal, ST6GalNAc, and ST8Sia) have appeared in invertebrates, earlier than previously thought, whereas the four families were all detected in bony fishes, amphibians, birds, and mammals. As previously hypothesized, sequence similarities among sialyltransferases suggest a common genetic origin, by successive duplications of an ancestral gene, followed by divergent evolution. Finally, we propose predictions on these invertebrates sialyltransferase-related activities that have not previously been demonstrated and that will ultimately need to be substantiated by protein expression and enzymatic activity assays.

Molecular Dissection of the ST8Sia IV Polysialyltransferase

Polysialic acid, a homopolymer of alpha2,8-linked sialic acid expressed on the neural cell adhesion molecule (NCAM), is thought to play critical roles in neural development. Two highly homologous polysialyltransferases, ST8Sia II and ST8Sia IV, which belong to the sialyltransferase gene family, synthesize polysialic acid on NCAM. By contrast, ST8Sia III, which is moderately homologous to ST8Sia II and ST8Sia IV, adds oligosialic acid to itself but very inefficiently to NCAM. Here, we report domains of polysialyltransferases required for NCAM recognition and polysialylation by generating chimeric enzymes between ST8Sia IV and ST8Sia III or ST8Sia II. We first determined the catalytic domain of ST8Sia IV by deletion mutants. To identify domains responsible for NCAM polysialylation, different segments of the ST8Sia IV catalytic domain, identified by the deletion experiments, were replaced with corresponding segments of ST8Sia II and ST8Sia III. We found that larger polysialic acid was formed on the enzymes themselves (autopolysialylation) when chimeric enzymes contained the carboxyl-terminal region of ST8Sia IV. However, chimeric enzymes that contain only the carboxyl-terminal segment of ST8Sia IV and the amino-terminal segment of ST8Sia III showed very weak activity toward NCAM, even though they had strong activity in polysialylating themselves. In fact, chimeric enzymes containing the amino-terminal portion of ST8Sia IV fused to downstream sequences of ST8Sia III inhibited NCAM polysialylation in vitro, although they did not polysialylate NCAM. These results suggest that in polysialyltransferases the NCAM recognition domain is distinct from the polysialylation domain and that some chimeric enzymes may act as a dominant negative enzyme for NCAM polysialylation.

Structure-Function Analysis of the Human Sialyltransferase ST3Gal I

All eukaryotic sialyltransferases have in common the presence in their catalytic domain of several conserved peptide regions (sialylmotifs L, S, and VS). Functional analysis of sialylmotifs L and S previously demonstrated their involvement in the binding of donor and acceptor substrates. The region comprised between the sialylmotifs S and VS contains a stretch of four highly conserved residues, with the following consensus sequence (H/y)Y(Y/F/W/h)(E/D/q/g). (Capital letters and lowercase letters indicate a strong or low occurrence of the amino acid, respectively.) The functional importance of these residues and of the conserved residues of motif VS (HX(4)E) was assessed using as a template the human ST3Gal I. Mutational analysis showed that residues His(299) and Tyr(300) of the new motif, and His(316) of the VS motif, are essential for activity since their substitution by alanine yielded inactive enzymes. Our results suggest that the invariant Tyr residue (Tyr(300)) plays an important conformational role mainly attributable to the aromatic ring. In contrast, the mutants W301F, E302Q, and E321Q retained significant enzyme activity (25-80% of the wild type). Kinetic analyses and CDP binding assays showed that none of the mutants tested had any significant effect in nucleotide donor binding. Instead the mutant proteins were affected in their binding to the acceptor and/or demonstrated lower catalytic efficiency. Although the human ST3Gal I has four N-glycan attachment sites in its catalytic domain that are potentially glycosylated, none of them was shown to be necessary for enzyme activity. However, N-glycosylation appears to contribute to the proper folding and trafficking of the enzyme.

Comparison of glycosyltransferase families using the profile hidden Markov model

In order to investigate the relationship between glycosyltransferase families and the motif for them, we classified 47 glycosyltransferase families in the CAZy database into four superfamilies, GTS-A, -B, -C, and -D, using a profile Hidden Markov Model method. On the basis of the classification and the similarity between GTS-A and nucleotidylyltransferase family catalyzing the synthesis of nucleotide-sugar, we proposed that ancient oligosaccharide might have been synthesized by the origin of GTS-B whereas the origin of GTS-A might be the gene encoding for synthesis of nucleotide-sugar as the donor and have evolved to glycosyltransferases to catalyze the synthesis of divergent carbohydrates. We also suggested that the divergent evolution of each superfamily in the corresponding subcellular component has increased the complexities of eukaryotic carbohydrate structure.

Recent development in the design of sialyltransferase inhibitors

Sialylation at the non-reducing end of glycoconjugates is an important biological process in cellular recognitions, tumor metastases, and immune responses, which are mediated by a family of enzymes known as sialyltransferases. Inhibition of sialyltransferases may prove useful in elucidating the biological functions of sialylation and may have therapeutic applications. This review summarizes the recent development in this field with particular focus on the strategies used for the design of carbohydrate mimetics and the structure-activity relationships of substrate-based sialyltransferase inhibitors.

Characterization of the second type of human beta-galactoside alpha 2,6-sialyltransferase (ST6Gal II), which sialylates Galbeta 1,4GlcNAc structures on oligosaccharides preferentially. Genomic analysis of human sialyltransferase genes

A novel member of the human beta-galactoside alpha2,6-sialyltransferase (ST6Gal) family, designated ST6Gal II, was identified by BLAST analysis of expressed sequence tags and genomic sequences. The sequence of ST6Gal II encoded a protein of 529 amino acids, and it showed 48.9% amino acid sequence identity with human ST6Gal I. Recombinant ST6Gal II exhibited alpha2,6-sialyltransferase activity toward oligosaccharides that have the Galbeta1,4GlcNAc sequence at the nonreducing end of their carbohydrate groups, but it exhibited relatively low and no activities toward some glycoproteins and glycolipids, respectively. It is concluded that ST6Gal II is an oligosaccharide-specific enzyme compared with ST6Gal I, which exhibits broad substrate specificities, and is mainly involved in the synthesis of sialyloligosaccharides. The expression of the ST6Gal II gene was significantly detected by reverse transcription PCR in small intestine, colon, and fetal brain, whereas the ST6Gal I gene was ubiquitously expressed, and its expression levels were much higher than those of the ST6Gal II gene. The ST6Gal I gene was also expressed in all tumors examined, but no expression was observed for the ST6Gal II gene in these tumors. The ST6Gal II gene is located on chromosome 2 (2q11.2-q12.1), and it spans over 85 kb of human genomic DNA consisting of at least eight exons and shares a similar genomic structure with the ST6Gal I gene. In this paper, we have shown that ST6Gal I, which has been known as the sole member of the ST6Gal family, also has the counterpart enzyme (ST6Gal II) like other sialyltransferases.

Molecular cloning and expression of a sixth type of alpha 2,8-sialyltransferase (ST8Sia VI) that sialylates O-glycans

A novel member of the mouse alpha2,8-sialyltransferase (ST8Sia) family, designated ST8Sia VI, was identified by BLAST analysis of expressed sequence tags. The sequence of ST8Sia VI encodes a protein of 398 amino acids and shows 42.0 and 38.3% amino acid sequence identities to mouse alpha2,8-sialyltransferases ST8Sia I (GD3 synthase) and ST8Sia V (GD1c, GT1a, GQ1b, and GT3 synthases), respectively. The recombinant soluble form of ST8Sia VI expressed in COS-7 cells exhibited alpha2,8-sialyltransferase activity toward both glycolipids and glycoproteins that have the NeuAcalpha2,3(6)Gal sequence at the nonreducing end of their carbohydrate groups. This enzyme formed NeuAcalpha2,8NeuAc structures, but not oligosialic or polysialic acid structures. Analysis of the fetuin sialylated by ST8Sia VI indicated that ST8Sia VI prefers O-glycans to N-glycans as acceptor substrates. Substrate specificities and kinetic properties also showed that ST8Sia VI prefers O-glycans to glycolipids as acceptor substrates. ST8Sia VI also exhibited activity toward oligosaccharides such as sialyllactose and sialyllactosamine, and the structure of the minimal acceptor substrate for ST8Sia VI was determined as the NeuAcalpha2,3(6)Gal sequence. The expression of the ST8Sia VI gene was ubiquitous, and the highest expression was observed in kidney, with three major transcripts of 8.2, 3.8, and 2.7 kb. This is the first report of a mammalian alpha2,8-sialyltransferase that sialylates O-glycans preferentially.

Efficient chemoenzymatic synthesis of O-linked sialyl oligosaccharides

The tumor associated Tn (GalNAcalpha(1-1)-Thr/Ser)- and T (Galbeta(1-3)-GalNAcalpha(1-1)Thr/Ser)-antigens and their sialylated derivatives are present on the surface of many cancer cells. Preparative synthesis of these sialylated T- and Tn-structures has been achieved mainly from a chemical synthetic approach due to the lack of the required glycosyltransferases. We demonstrate a flexible and efficient chemoenzymatic approach for using recombinant sialyltransferases including a chicken GalNAcalpha2,6-sialyltransferase (chST6GalNAc I) and a porcine Galbeta(1-3)GalNAcalpha-2,3-sialyltransferase (pST3Gal I). Using these enzymes, the common O-linked sialosides Neu5Acalpha(2-6)GalNAcalpha(1-1)Thr, Galbeta(1-3)[Neu5Acalpha(2-6)]GalNAcalpha(1-1)Thr, Neu5Acalpha(2-3)Galbeta(1-3)GalNAcalpha(1-1)Thr, and Neu5Acalpha(2-3)Galbeta(1-3)[Neu5Acalpha(2-6)]GalNAcalpha(1-1)Thr were readily prepared at preparative scale. The chST6GalNAc I was found to require at least one amino acid (Thr/Ser) for optimal activity, and is thus an ideal catalyst for synthesis of synthetic glycopeptides and glycoconjugates with O-linked glycans.

Delineation of the minimal catalytic domain of human Galbeta1-3GalNAc alpha2,3-sialyltransferase (hST3Gal I)

The CMP-Neu5Ac:Galbeta1-3GalNAc alpha2,3-sialyltransferase (ST3Gal I, EC 2.4.99.4) is a Golgi membrane-bound type II glycoprotein that catalyses the transfer of sialic acid residues to Galbeta1-3GalNAc disaccharide structures found on O-glycans and glycolipids. In order to gain further insight into the structure/function of this sialyltransferase, we studied protein expression, N-glycan processing and enzymatic activity upon transient expression in the COS-7 cell line of various constructs deleted in the N-terminal portion of the protein sequence. The expressed soluble polypeptides were detected within the cell and in the cell culture media using a specific hST3Gal I monoclonal antibody. The soluble forms of the protein consisting of amino acids 26-340 (hST3-Delta25) and 57-340 (hST3-Delta56) were efficiently secreted and active. In contrast, further deletion of the N-terminal region leading to hST3-Delta76 and hST3-Delta105 gave also rise to various polypeptides that were not active within the transfected cells and not secreted in the cell culture media. The kinetic parameters of the active secreted forms were determined and shown to be in close agreement with those of the recombinant enzyme already described (H. Kitagawa, J.C. Paulson, J. Biol. Chem. 269 (1994)). In addition, the present study demonstrates that the recombinant hST3Gal I polypeptides transiently expressed in COS-7 cells are glycosylated with complex and high mannose type glycans on each of the five potential N-glycosylation sites.

Location and mechanism of alpha 2,6-sialyltransferase dimer formation. Role of cysteine residues in enzyme dimerization, localization, activity, and processing

A significant proportion of the alpha2,6-sialyltransferase of protein Asn-linked glycosylation (ST6Gal I) forms disulfide-bonded dimers that exhibit decreased activity, but retain the ability to bind asialoglycoprotein substrates. Here, we have investigated the subcellular location and mechanism of ST6Gal I dimer formation, as well as the role of Cys residues in the enzyme's trafficking, localization, and catalytic activity. Pulse-chase analysis demonstrated that the ST6Gal I disulfide-bonded dimer forms in the endoplasmic reticulum. Mutagenesis experiments showed that Cys-24 in the transmembrane region is required for dimerization, while catalytic domain Cys residues are required for trafficking and catalytic activity. Replacement of Cys-181 and Cys-332 generated proteins that are largely retained in the endoplasmic reticulum and minimally active or inactive, respectively. Replacement of Cys-350 or Cys-361 inactivated the enzyme without compromising its localization or processing, suggesting that these amino acids are part of the enzyme's active site. Replacement of Cys-139 or Cys-403 generated proteins that are catalytically active and appear to be more stably localized in the Golgi, since they exhibited decreased cleavage and secretion. The Cys-139 mutant also exhibited increased dimer formation suggesting that ST6Gal I dimers may be critical in the oligomerization process involved in stable ST6Gal I Golgi localization.

Differential biosynthesis of polysialic or disialic acid Structure by ST8Sia II and ST8Sia IV

ST8Sia II (STX) and ST8Sia IV (PST) are polysialic acid (polySia) synthases that catalyze polySia formation of neural cell adhesion molecule (NCAM) in vivo and in vitro. It still remains unclear how these structurally similar enzymes act differently in vivo. In the present study, we performed the enzymatic characterization of ST8Sia II and IV; both ST8Sia II and IV have pH optima of 5.8-6.1 and have no requirement of metal ions. Because the pH dependence of ST8Sia II and IV enzyme activities and the pK profile of His residues are similar, we hypothesized that a histidine residue would be involved in their catalytic activity. There is a conserved His residue (cf. His(348) in ST8Sia II and His(331) in ST8Sia IV, respectively) within the sialyl motif VS in all sialyltransferase genes cloned to date. Mutant ST8Sia II and IV enzymes in which this His residue was changed to Lys showed no detectable enzyme activity, even though they were folded correctly and could bind to CDP-hexanolamine, suggesting the importance of the His residue for their catalytic activity. Next, the degrees of polymerization of polySia in NCAM catalyzed by ST8Sia II and IV were compared. ST8Sia IV catalyzed larger polySia formation of NCAM than ST8Sia II. We also analyzed the (auto)polysialylated enzymes themselves. Interestingly, when ST8Sia II or IV itself was sialylated under conditions for polysialylation, the disialylated compound was the major product, even though polysialylated compounds were also observed. These results suggested that both ST8Sia II and IV catalyze polySia synthesis toward preferred acceptor substrates such as NCAM, whereas they mainly catalyze disialylation, similarly to ST8Sia III, toward unfavorable substrates such as enzyme themselves.

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.

Unique disulfide bond structures found in ST8Sia IV polysialyltransferase are required for its activity

NCAM polysialylation plays a critical role in neuronal development and regeneration. Polysialylation of the neural cell adhesion molecule (NCAM) is catalyzed by two polysialyltransferases, ST8Sia II (STX) and ST8Sia IV (PST), which contain sialylmotifs L and S conserved in all members of the sialyltransferases. The members of the ST8Sia gene family, including ST8Sia II and ST8Sia IV are unique in having three cysteines in sialylmotif L, one cysteine in sialylmotif S, and one cysteine at the COOH terminus. However, structural information, including how disulfide bonds are formed, has not been determined for any of the sialyltransferases. To obtain insight into the structure/function of ST8Sia IV, we expressed human ST8Sia IV in insect cells, Trichoplusia ni, and found that the enzyme produced in the insect cells catalyzes NCAM polysialylation, although it cannot polysialylate itself ("autopolysialylation"). We also found that ST8Sia IV does not form a dimer through disulfide bonds. By using the same enzyme preparation and performing mass spectrometric analysis, we found that the first cysteine in sialylmotif L and the cysteine in sialylmotif S form a disulfide bridge, whereas the second cysteine in sialylmotif L and the cysteine at the COOH terminus form a second disulfide bridge. Site-directed mutagenesis demonstrated that mutation at cysteine residues involved in the disulfide bridges completely inactivated the enzyme. Moreover, changes in the position of the COOH-terminal cysteine abolished its activity. By contrast, the addition of green fluorescence protein at the COOH terminus of ST8Sia IV did not render the enzyme inactive. These results combined indicate that the sterical structure formed by intramolecular disulfide bonds, which bring the sialylmotifs and the COOH terminus within close proximity, is critical for the catalytic activity of ST8Sia IV.

Enzymatic and Chemical Approaches for the Synthesis of Sialyl Glycoconjugates

In conclusion, either enzymatic or chemical approaches have their unique features and unavoidable disadvantages. Enzyme-catalyzed sialylations provide the desired sialo-glycosidic linkages in the two enzyme reactions (CMP-NeuAc synthetase and sialyltransferase) with exclusive stereoselectivity and high yield as long as the required sialyltransferase is available. High substrate specificity of the two enzymes is a limitation so that many unnatural glycoconjugates cannot be prepared enzymatically. As for chemical glycosylations of sialic acids, it is possible to introduce any modification in sialyl donor and acceptor, in addition to create special sugar linkages. Nevertheless, reducing the number of reaction steps (for preparing both donors and acceptors of glycosylation), and enhancing stereoselectivity, as well as reaction yield are still problems to be overcome.

Disulfide bonds of GM2 synthase homodimers. Antiparallel orientation of the catalytic domains

GM2 synthase is a homodimer in which the subunits are joined by lumenal domain disulfide bond(s). To define the disulfide bond pattern of this enzyme, we analyzed a soluble form by chemical fragmentation, enzymatic digestion, and mass spectrometry and a full-length form by site-directed mutagenesis. All Cys residues of the lumenal domain of GM2 synthase are disulfide bonded with Cys(429) and Cys(476) forming a disulfide-bonded pair while Cys(80) and Cys(82) are disulfide bonded in combination with Cys(412) and Cys(529). Partial reduction to produce monomers converted Cys(80) and Cys(82) to free thiols while the Cys(429) to Cys(476) disulfide remained intact. CNBr cleavage at amino acid 330 produced a monomer-sized band under nonreducing conditions which was converted upon reduction to a 40-kDa fragment and a 24-kDa myc-positive fragment. Double mutation of Cys(80) and Cys(82) to Ser produced monomers but not dimers. In summary these results demonstrate that Cys(429) and Cys(476) form an intrasubunit disulfide while the intersubunit disulfides formed by both Cys(80) and Cys(82) with Cys(412) and Cys(529) are responsible for formation of the homodimer. This disulfide bond arrangement results in an antiparallel orientation of the catalytic domains of the GM2 synthase homodimer.

Molecular cloning and functional expression of human ST6GalNAc II. Molecular expression in various human cultured cells

A cDNA clone encoding a human Galbeta1-3GalNAc alpha2, 6-sialyltransferase (designated hST6GalNAc II) was identified employing the PCR with degenerated primers to the sialylmotifs, followed by BLAST analysis of databanks. This sialyltransferase sequence is similar to that of previously cloned ST6GalNAc II (chicken and mouse) and shows the sialylmotifs that are present in all eukaryotic members of the sialyltransferase gene family. The predicted amino acid sequence encodes a putative type II transmembrane protein as found for other eukaryotic sialyltransferases and shows significant similarity to chicken (56. 8% identity) and mouse (74.6% identity) enzymes. Expression of a secreted form of hST6GalNAc II in COS-7 cells showed that the gene product had Galbeta1-3GalNAc (sialyl to GalNAc) alpha2, 6-sialyltransferase activity. In vitro analysis of substrate specificity revealed that the enzyme required a peptide aglycone fraction to be active and used both Galbeta1-3GalNAc and Neu5Acalpha2-3Galbeta1-3GalNAc as acceptor substrates. Northern analysis revealed a restricted expression pattern of two hST6GalNAc II transcripts, a 2.0 kb mRNA found mainly in skeletal muscle, heart and kidney and a 1.8 kb mRNA found in placenta, lung and leukocytes. No transcriptional expression was detected in brain, thymus or spleen. Transcriptional expression of the ST6GalNAc II gene was followed in various human cell lines and found to be expressed in almost all cell types with notable exceptions for several myeloid and lymphoid cell lines.

A novel viral alpha2,3-sialyltransferase (v-ST3Gal I): transfer of sialic acid to fucosylated acceptors

The substrate specificity of an alpha2,3-sialyltransferase (v-ST3Gal I) obtained from myxoma virus infected RK13 cells has been determined. Like mammalian sialyltransferase enzymes, the viral enzyme contains the characteristic L- and S-sialyl motif sequences in its catalytic domain. Analysis of the deduced amino acid sequences of cloned sialyltransferases suggests that v-ST3Gal I is closely related to mammalian ST3Gal IV. v-ST3Gal I catalyzes the transfer of sialic acid from CMP-NeuAc to Type I (Galbeta1-3GlcNAcbeta) II (Galbeta1-4GlcNAcbeta) and III (Galbeta1-3GalNAcbeta) acceptors. In addition, the viral enzyme also transfers sialic acid to the fucosylated acceptors Lewis(x) and Lewis(a). This substrate specificity is unlike any sialyltransferases described to date, though it is most comparable with those of mammalian ST3Gal IV enzymes. The products from reactions with fucosylated acceptors were characterized by capillary zone electrophoresis, (1)H-NMR spectroscopy and mass spectrometry. They were shown to be 2,3-sialylated Lewis(x) and 2,3-sialylated Lewis(a), respectively.

Identification and characterization of large galactosyltransferase gene families: galactosyltransferases for all functions

Enzymatic glycosylation of proteins and lipids is an abundant and important biological process. A great diversity of oligosaccharide structures and types of glycoconjugates is found in nature, and these are synthesized by a large number of glycosyltransferases. Glycosyltransferases have high donor and acceptor substrate specificities and are in general limited to catalysis of one unique glycosidic linkage. Emerging evidence indicates that formation of many glycosidic linkages is covered by large homologous glycosyltransferase gene families, and that the existence of multiple enzyme isoforms provides a degree of redundancy as well as a higher level of regulation of the glycoforms synthesized. Here, we discuss recent cloning strategies enabling the identification of these large glycosyltransferase gene families and exemplify the implication this has for our understanding of regulation of glycosylation by discussing two galactosyltransferase gene families.

Structure/function studies of glycosyltransferases

Glycosyltransferases are the enzymes that synthesize oligosaccharides, polysaccharides and glycoconjugates. The analysis of the wealth of sequences that are now available in databases allowed the determination of conserved peptide motifs for each class of enzyme. Recent experimental data demonstrated their importance in donor and acceptor substrate binding and in catalysis. Fold-recognition studies provided the first models of the catalytic domains of some of these enzymes, while the first successes in glycosyltransferase crystallography are opening new routes in structural glycobiology.

Molecular cloning, expression and exon/intron organization of the bovine beta-galactoside alpha2,6-sialyltransferase gene

In this study, we report the first isolation and characterization of a bovine sialyltransferase gene. Bovine cDNAs prepared from different tissues contain an open-reading frame encoding a 405 amino acid sequence showing 83%, 75%, and 60% identity with human, murine, and chicken ST6Gal I (beta-galactoside alpha2,6-sialyltransferase) sequences, respectively. When transfected into COS-7 cells, a recombinant enzyme was obtained which catalyzed the in vitro alpha2, 6-sialylation of LacNAc (NeuAcalpha2-6Galbeta1-4GlcNAc) and LacdiNAc (NeuAcalpha2-6GalNAcbeta1-4GlcNAc) acceptor substrates. The K (m) values were 2.8 and 6.9 mM, respectively. Different relative efficiencies (Vmax/Km) for the two precursors (36 for LacNAc and 4.3 for LacdiNAc) were observed. Bovine ST6Gal I gene consists of four 5'-untranslated exons E(-2) to E(1), and five coding exons from E(2) to E(6). This later carries a 3'-untranslated region of 2. 7 kb. Gene sequence spans at least 80 kb of genomic DNA. Two processed pseudogenes have been identified. They are 94.3 and 95.6% similar to the bovine cDNA, respectively. Three families of mRNA isoforms were isolated. They differed by their 5'-untranslated regions and could be generated by three tissue-specific promoters. Family 1 is made up of exons E(-2) and E(1) to E(6), family 2 of exons E(-1) to E(6), and family 3 of exons E(1) to E(6). Tissular distribution of transcript families appears noticeably different than those described in human and rat.

Molecular cloning and functional expression of two members of mouse NeuAcalpha2,3Galbeta1,3GalNAc GalNAcalpha2,6-sialyltransferase family, ST6GalNAc III and IV

Two cDNA clones encoding NeuAcalpha2,3Galbeta1,3GalNAc GalNAcalpha2, 6-sialyltransferase have been isolated from mouse brain cDNA libraries. One of the cDNA clones is a homologue of previously reported rat ST6GalNAc III according to the amino acid sequence identity (94.4%) and the substrate specificity of the expressed recombinant enzyme, while the other cDNA clone includes an open reading frame coding for 302 amino acids. The deduced amino acid sequence is not identical to those of other cloned mouse sialyltransferases, although it shows the highest sequence similarity with mouse ST6GalNAc III (43.0%). The expressed soluble recombinant enzyme exhibited activity toward NeuAcalpha2, 3Galbeta1, 3GalNAc, fetuin, and GM1b, while no significant activity was detected toward Galbeta1,3GalNAc or asialofetuin, or the other glycoprotein substrates tested. The sialidase sensitivity of the 14C-sialylated residue of fetuin, which was sialylated by this enzyme with CMP-[14C]NeuAc, was the same as that of ST6GalNAc III. These results indicate that the expressed enzyme is a new type of GalNAcalpha2,6-sialyltransferase, which requires sialic acid residues linked to Galbeta1,3GalNAc residues for its activity; therefore, we designated it mouse ST6GalNAc IV. Although the substrate specificity of this enzyme is similar to that of ST6GalNAc III, ST6GalNAc IV prefers O-glycans to glycolipids. Glycolipids, however, are better substrates for ST6GalNAc III.

Genomic organization of the murine polysialyltransferase gene ST8SiaIV (PST-1)

Polysialic acid (PSA) is an important regulator of cellular interactions. Two enzymes (ST8SiaII and ST8SiaIV) are capable of synthesizing PSA. In the present study, the gene encoding the murine ST8SiaIV (PST-1) has been isolated and characterized. In contrast to the ST8SiaII (STX) gene which contains six exons and spans about 80 kb, the ST8SiaIV gene comprises only five exons spanning over at least 55 kb. However, alignment of the two genes revealed that exon-intron boundaries of exons 2-5 of ST8SiaIV and exons 3-6 of ST8SiaII are located at identical sites. Differences are restricted to the 5'-region encoded by one exon in the case of ST8SiaIV, whereas the corresponding region of ST8SiaII is interrupted by a very long intron. 5'-RACE analysis of the ST8SiaIV transcript using mRNA from AtT20 cells identified two transcription start sites at positions -324 and -204 relative to the translation start codon. The promoter region of ST8SiaIV lacks TATA- and CAAT-like sequences and is enriched in G+C (60%). The promoter contains putative Sp1, AP-1, AP-2, and PEA3 binding sites, as well as a purine- and a pyrimidine-rich region. Luciferase reporter gene assays demonstrated that the region between nucleotides -443 and -162 is sufficient to direct gene expression. The induction of luciferase activity was 30- and 10-fold in the PSA-positive AtT20 and CHO cells, but only 5- and 7-fold in the PSA-negative NIH-3T3 cells and in a PSA-negative subline of AtT20. Thus, although decreased in activity in PSA-negative cell lines, the basal promoter is not sufficient for the strong cell-type and tissue specific regulation of the ST8SiaIV gene, suggesting regulatory elements in the more upstream 5'-region.

Expression cloning and functional characterization of human cDNA for ganglioside GM3 synthase

Ganglioside GM3 is a major glycosphingolipid in the plasma membrane and is widely distributed in vertebrates. We describe here the isolation of a human cDNA whose protein product is responsible for the synthesis of GM3. The cloned cDNA spanned 2,359 base pairs, with an open reading frame encoding a protein of 362 amino acids with a predicted molecular mass of 41.7 kDa. The deduced primary structure shows features characteristic of the sialyltransferase family, including a type II transmembrane topology and the sialylmotifs L at the center and S at the C-terminal region. An amino acid substitution from aspartic acid to histidine was demonstrated at a position invariant in sialylmotif L of all the other sialyltransferases so far cloned. The best acceptor substrate for the gene product was lactosylceramide, and cells transfected with the cloned cDNA clearly exhibited de novo synthesis of GM3, with a measurable decrease in the precursor lactosylceramide. Despite the ubiquitous distribution of ganglioside GM3 in human tissues, a major 2.4-kilobase transcript of the gene was found in a tissue-specific manner, with predominant expression in brain, skeletal muscle, and testis, and very low expression in liver.

Polysialic acid: three-dimensional structure, biosynthesis and function

Polysialic acid is a unique cell surface polysaccharide found in the capsule of neuroinvasive bacteria and as a highly regulated post-translational modification of the neural cell adhesion molecule. Recent progress has been achieved in research on both the physicochemical properties of polysialic acid and the biosynthetic pathways leading to polysialic acid expression in bacteria and mammals.