Two step single primer mediated polymerase chain reaction. Application to cloning of putative mouse, beta-galactoside alpha 2,6-sialyltransferase cDNA

Sialylation is involved in cell fate decision during development, reprogramming and cancer progression

Sialylation, or the covalent addition of sialic acid to the terminal end of glycoproteins, is a biologically important modification that is involved in embryonic development, neurodevelopment, reprogramming, oncogenesis and immune responses. In this review, we have given a comprehensive overview of the current literature on the involvement of sialylation in cell fate decision during development, reprogramming and cancer progression. Sialylation is essential for early embryonic development and the deletion of UDP-GlcNAc 2-epimerase, a rate-limiting enzyme in sialic acid biosynthesis, is embryonically lethal. Furthermore, the sialyltransferase ST6GAL1 is required for somatic cell reprogramming, and its downregulation is associated with decreased reprogramming efficiency. In addition, sialylation levels and patterns are altered during cancer progression, indicating the potential of sialylated molecules as cancer biomarkers. Taken together, the current evidences demonstrate that sialylation is involved in crucial cell fate decision.

The role of milk sialyllactose in intestinal bacterial colonization

Milk oligosaccharides influence the composition of intestinal microbiota and thereby mucosal inflammation. Some of the major milk oligosaccharides are α2,3-sialyllactose (3SL) and α2,6-sialyllactose, which are mainly produced by the sialyltransferases ST3GAL4 and ST6GAL1, respectively. Recently, we showed that mice fed milk deficient in 3SL were more resistant to dextran sulfate sodium-induced colitis. By contrast, the exposure to milk containing or deficient in 3SL had no impact on the development of mucosal leukocyte populations. Milk 3SL mainly affected the colonization of the intestine by clostridial cluster IV bacteria.

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.

The second bovine beta-galactoside-alpha2,6-sialyltransferase (ST6Gal II): genomic organization and stimulation of its in vitro expression by IL-6 in bovine mammary epithelial cells

We have cloned a cDNA sequence encoding the second bovine beta-galactoside-alpha2,6-sialyltransferase whose sequence shares more than 75% of identity with hST6Gal II cDNA coding sequence. The bovine gene, located on BTA 11, spans over 50 kbp with five exons (E1-E5) containing the 1488 bp open reading frame and a 5'-untranslated exon (E0). The gene expression pattern reveals a specific tissue distribution (brain, lungs, spleen, salivary, and mammary glands) compared to ST6Gal I which is ubiquitously expressed. We identified for bovine ST6Gal II three kinds of transcripts which differ by their 5'-untranslated regions. Among them, two transcripts are brain specific whereas the third one is found in all of the tissues expressing the gene. Two pFlag-bST6Gal II vector constructions were separately transfected in COS-1 cells in order to express either membrane-bound or soluble active forms of ST6Gal II. Enzymatic assays with these two forms indicated that the enzyme used the LacdiNAc structure (GalNAcbeta1,4GlcNAc) as a better acceptor substrate than the Type II (Galbeta1-4GlcNAc) disaccharide. Moreover, the enzyme's efficiency is improved when the acceptor substrate is provided as a free oligosaccharide rather than as a protein-bound oligosaccharide. In order to investigate the potential role of ST6Gal II during the acute phase of inflammation, we used primary cultures of bovine mammary epithelial cells which were stimulated with pro-inflammatory cytokines. It appears that the ST6Gal II gene was upregulated in cells stimulated by IL-6. This result suggested that alpha2,6-sialylation mediated by this gene could contribute to organism's response to infections.

The cytoplasmic tail of GM3 synthase defines its subcellular localization, stability, and in vivo activity

GM3 synthase (SAT-I) is the primary glycosyltransferase responsible for the biosynthesis of ganglio-series gangliosides. In this study, we identify three isoforms of mouse SAT-I proteins, named M1-SAT-I, M2-SAT-I, and M3-SAT-I, which possess distinct lengths in their NH(2)-terminal cytoplasmic tails. These isoforms are produced by leaky scanning from mRNA variants of mSAT-Ia and mSAT-Ib. M2-SAT-I and M3-SAT-I were found to be localized in the Golgi apparatus, as expected, whereas M1-SAT-I was exclusively found in the endoplasmic reticulum (ER). Specific multiple arginines (R) arranged in an R-based motif, RRXXXXR necessary for ER targeting, were found in the cytoplasmic tail of M1-SAT-I, and in vivo GM3 biosynthesis by M1-SAT-I was very low because of restricted transport to the Golgi apparatus. In addition, M1-SAT-I and M3-SAT-I had a long half-life relative to M2-SAT-I. This is the first report demonstrating the presence of an ER-targeting R-based motif in the cytoplasmic tail of a protein in the mammalian glycosyltransferase family of enzymes. The system, which produces SAT-I isoforms having distinct characteristics, is likely to be of critical importance for the regulation of GM3 biosynthesis under various pathological and physiological conditions.

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.

Region-specific ontogeny of alpha-2,6-sialyltransferase during normal and cortisone-induced maturation in mouse intestine

Regional differences in the ontogeny of mouse intestinal alpha-2,6-sialyltransferase activities (alpha-2,6-ST) and the influence of cortisone acetate (CA) on this expression were determined. High ST activity and alpha-2,6-ST mRNA levels were detected in immature small and large intestine, with activity increasing distally from the duodenum. As the mice matured, ST activity (predominantly alpha-2,6-ST) in the small intestine decreased rapidly to adult levels by the fourth postnatal week. CA precociously accelerated this region-specific ontogenic decline. A similar decline of ST mRNA levels reflected ST activity in the small, but not the large, intestine. Small intestinal sialyl alpha-2,6-linked glycoconjugates displayed similar developmental and CA induced-precocious declines when probed using Sambucus nigra agglutinin (SNA) lectin. SNA labeling demonstrated age-dependent diminished sialyl alpha2,6 glycoconjugate expression in goblet cells in the small (but not large) intestine, but no such regional specificity was apparent in microvillus membrane. This suggests differential regulation of sialyl alpha-2,6 glycoconjugates in absorptive vs. globlet cells. These age-dependent and region-specific differences in sialyl alpha-2,6 glycoconjugates may be mediated in part by altered alpha-2,6-ST gene expression regulated by trophic factors such as glucocorticoids.

The sialyl-alpha2,6-lactosaminyl-structure: biosynthesis and functional role

Sialylation represents one of the most frequently occurring terminations of the oligosaccharide chains of glycoproteins and glycolipids. Sialic acid is commonly found alpha2,3- or alpha2,6-linked to galactose (Gal), alpha2,6-linked to N-acetylgalactosamine (GalNAc) or alpha2,8-linked to another sialic acid. The biosynthesis of the various linkages is mediated by the different members of the sialyltransferase family. The addition of sialic acid in alpha2,6-linkage to the galactose residue of lactosamine (type 2 chains) is catalyzed by beta-galactoside alpha2,6-sialyltransferase (ST6Gal.I). Although expressed by a single gene, this enzyme shows a complex pattern of regulation which allows its tissue- and stage-specific modulation. The cognate oligosaccharide structure, NeuAcalpha2,6Galbeta1,4GlcNAc, is widely distributed among tissues and is involved in biological processes such as the regulation of the immune response and the progression of colon cancer. This review summarizes the current knowledge on the biochemistry of ST6Gal.I and on the functional role of the sialyl-alpha2,6-lactosaminyl structure.

Molecular cloning of brain-specific GD1alpha synthase (ST6GalNAc V) containing CAG/Glutamine repeats

A novel member of the mouse CMP-NeuAc: beta-N-acetylgalactosaminide alpha2,6-sialyltransferase (ST6GalNAc) subfamily, designated ST6GalNAc V, was identified by BLAST analysis of expressed sequence tags. The sequence of the longest cDNA clone of ST6GalNAc V encoded a type II membrane protein with 8 amino acids comprising the cytoplasmic domain, 21 amino acids comprising the transmembrane region, and 306 amino acids comprising the catalytic domain. The predicted amino acid sequence showed homology to the previously cloned ST6GalNAc III and IV, with common amino acid sequences in sialyl motifs L and S among these three enzymes. Eleven CAG repeats were found in the stem region. A fusion protein with protein A and extracts from L cells transfected with ST6GalNAc V in a expression vector showed enzyme activity of alpha2,6-sialyltransferase almost exclusively for GM1b, but not toward glycoproteins. Sialidase treatment and thin layer chromatography immunostaining revealed that the product was GD1alpha. Northern blotting revealed that three transcripts of the gene were expressed specifically in brain tissues. It is concluded that this enzyme is involved in the synthesis of GD1alpha in the nervous tissues, and the CAG repeats may have implications in neurodegenerative diseases.

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.

Up-regulation of the alpha2,6-sialyltransferase messenger ribonucleic acid increases glycoconjugates containing alpha2, 6-linked sialic acid residues in granulosa cells during follicular atresia of porcine ovaries

The sugar chains in cellular glycoconjugates have many biological functions. Extensive morphological development and remodeling occur in the ovary of female animals. This caused us to study glycobiological characteristics of ovarian cells, particularly granulosa cells that undergo apoptosis during follicular atresia. The lectin Sambucus sieboldiana agglutinin (SSA) specific for Siaalpha2,6Gal/GalNAc showed positive staining for granulosa cells only in atretic follicles of porcine ovaries by lectin histochemistry. Lectin blot analysis for SSA demonstrated specific glycoproteins only in atretic follicles. Furthermore, we performed analysis of backbone structures of SSA-positive glycans carried by granulosa cell glycoproteins increased during atresia by glycosidase treatment. Most of these structures were Siaalpha2,6Galbeta1,4GlcNAc on complex-type N-glycans, suggesting that only ST6Gal I of four distinct alpha2,6-sialyltransferases catalyzes alpha2,6-sialic acid transfer in most of the increased glycoproteins of granulosa cells during follicular atresia. Reverse transcription-polymerase chain reaction analysis demonstrated that the expression of ST6Gal I mRNA was up-regulated in granulosa cells during atresia. These results suggested that the alteration of glycoconjugates by ST6Gal I in granulosa cells during atresia is involved in some processes of ovarian follicular atresia and granulosa cell apoptosis.

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.

Reduction of the major swine xenoantigen, the alpha-galactosyl epitope by transfection of the alpha2,3-sialyltransferase gene

alpha2,3-Sialyltransferase represents a putative enzyme that reduces the Galalpha1-3Gal beta1-4GlcNAc-R (the alpha-galactosyl epitope) by intracellular competition with alpha1,3-galactosyltransferase for a common acceptor substrate. This study demonstrates that the overexpression of the alpha2,3-sialyltransferase gene suppresses the antigenicity of swine endothelial cells to human natural antibodies by 77% relative to control cells and by 30% relative to cells transfected with alpha1,2-fucosyltransferase, and in addition, it reduces the complement-mediated cell lysis by 75% compared with control cells and by 22% compared with cells transfected with alpha1, 2-fucosyltransferase. The mechanism by which the alpha-galactosyl epitope was reduced was also studied. Suppression of alpha1, 3-galactosyltransferase activity by 30-63% was observed in the transfectants with alpha2,3-sialyltransferase, and mRNA expression of the alpha1,3-galactosyltransferase gene was reduced as well. The data suggest that the alpha2,3-sialyltransferase effectively reduced the alpha-galactosyl epitope as well as or better than the alpha1, 2-fucosyltransferase did and that the reduction of the alpha-galactosyl epitope is due not only to substrate competition but also to an overall reduction of endogenous alpha1, 3-galactosyltransferase enzyme activity.

Mutation of the sialyltransferase S-sialylmotif alters the kinetics of the donor and acceptor substrates

Protein sequence analysis of the cloned sialyltransferase gene family has revealed the presence of two conserved protein motifs in the middle of the lumenal catalytic domain, termed L-sialylmotif and S-sialylmotif. In our previous study (Datta, A. K., and Paulson, J. C. (1995) J. Biol. Chem. 270, 1497-1500) the larger L-sialylmotif of ST6Gal I was analyzed by site-directed mutagenesis, which provided evidence that it participates in the binding of the CMP-NeuAc, a common donor substrate for all the sialyltransferases. However, none of the mutants tested in this motif had any significant effect on their binding affinities toward the acceptor substrate asialo alpha1-acid glycoprotein. In this study, we have investigated the role of the S-sialylmotif of the same enzyme ST6Gal I. In total, nine mutants have been constructed by changing the conserved amino acids of this motif to mostly alanine by site-directed mutagenesis. Kinetic analysis for the mutants which retained sialyltransferase activity showed that the mutations in the S-sialylmotif caused a change of Km values for both the donor and the acceptor substrates. Our results indicated that this motif participates in the binding of both the substrates. A sequence homology search also supported this finding, which showed that the downstream amino acid sequence of the S-sialylmotif is conserved for each subgroup of this enzyme family, indicating its association with the acceptor substrate.

Protein N-glycosylation: molecular genetics and functional significance

Protein N-glycosylation is a metabolic process that has been highly conserved in evolution. In all eukaryotes, N-glycosylation is obligatory for viability. It functions by modifying appropriate asparagine residues of proteins with oligosaccharide structures, thus influencing their properties and bioactivities. N-glycoprotein biosynthesis involves a multitude of enzymes, glycosyltransferases, and glycosidases, encoded by distinct genes. The majority of these enzymes are transmembrane proteins that function in the endoplasmic reticulum and Golgi apparatus in an ordered and well-orchestrated manner. The complexity of N-glycosylation is augmented by the fact that different asparagine residues within the same polypeptide may be modified with different oligosaccharide structures, and various proteins are distinguished from one another by the characteristics of their carbohydrate moieties. Furthermore, biological consequences of derivatization of proteins with N-glycans range from subtle to significant. In the past, all these features of N-glycosylation have posed a formidable challenge to an elucidation of the physiological role for this modification. Recent advances in molecular genetics, combined with the availability of diverse in vivo experimental systems ranging from yeast to transgenic mice, have expedited the identification, isolation, and characterization of N-glycosylation genes. As a result, rather unexpected information regarding relationships between N-glycosylation and other cellular functions--including secretion, cytoskeletal organization, proliferation, and apoptosis--has emerged. Concurrently, increased understanding of molecular details of N-glycosylation has facilitated the alignment between N-glycosylation deficiencies and human diseases, and has highlighted the possibility of using N-glycan expression on cells as potential determinants of disease and its progression. Recent studies suggest correlations between N-glycosylation capacities of cells and drug sensitivities, as well as susceptibility to infection. Therefore, knowledge of the regulatory features of N-glycosylation may prove useful in the design of novel therapeutics. While facing the demanding task of defining properties, functions, and regulation of the numerous, as yet uncharacterized, N-glycosylation genes, glycobiologists of the 21st century offer exciting possibilities for new approaches to disease diagnosis, prevention, and cure.

Murine hepatic beta-galactoside alpha 2,6-sialyltransferase gene expression involves usage of a novel upstream exon region

ST6Gal I (beta-galactoside alpha 2,6-sialyltransferase, SiaT-1, ST6N, EC 2.4.99.1) mediates the attachment of the alpha 2,6-sialyl linkage common on N-linked glycans. Previous work suggests substantial inter-species conservation in SIAT1, the gene encoding ST6Gal I. In human and in rat, hepatic-specific SIAT1 transcription is initiated at Exon I. Here we report a surprising departure in the structural organization of the murine ST6Gal I gene. By a combination of primer extension analysis, 5'-RACE analysis, and analysis of genomic sequences, we show that the murine hepatic ST6Gal I mRNA contains a novel region 5' of Exon I. This novel sequence is encoded on a discrete upstream exon, Exon H. In contrast to human and rat hepatic ST6Gal I, the murine mRNA is transcriptionally initiated at the start of Exon H. Differential mRNA blot analysis indicates that transcripts containing Exon H sequences are preferentially expressed in liver.

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

Values of Km were determined for three purified sialyltransferases and the corresponding recombinant enzymes. The enzymes were Gal beta 1-4GlcNAc alpha 2-6 sialyltransferase and Gal beta 1-3(4)GlcNAc alpha 2-3 sialyltransferase from rat liver; these enzymes are responsible for the attachment of sialic acid to N-linked oligosaccharide chains; and the Gal beta 1-3GalNAc alpha 2-3 sialyltransferase from porcine submaxillary gland that is responsible for the attachment of sialic acid to O-linked glycoproteins and glycolipids. A procedure for the large scale expression of active sialyltransferases from recombinant baculovirus-infected insect cells is described. For the liver enzymes values of Km were determined using rat and human asialo alpha 1 acid glycoprotein and N-acetyllactosamine as variable substrates; lacto-N-tetraose was also used with the Gal beta 1-3(4)GlcNAc alpha 2-3 sialyltransferases. Antifreeze glycoprotein was used as the macromolecular acceptor for the porcine enzyme. Values for Km were also determined using CMP-NeuAc as the variable substrate.

Molecular cloning and expression of chick Gal beta 1,3GalNAc alpha 2,3-sialyltransferase

A cDNA clone encoding chick Gal beta 1,3GalNAc alpha 2,3-sialyltransferase (ST3Gal I) was isolated from a chick embryo brain cDNA library. The cDNA sequence included an open reading frame coding for 342 amino acids, and the deduced amino acid sequence showed 64% identity with that of the mouse enzyme. Northern blot analysis of chick embryos revealed that the ST3Gal I gene was expressed in early embryonic stages. The identity of the enzyme was confirmed by construction of a recombinant sialyltransferase in which the N-terminal part including the cytoplasmic tail and signal anchor domain was replaced with an immunoglobulin signal peptide sequence. This enzyme expressed in COS-7 cells exhibited transferase activity similar to that of mouse ST3Gal I.

The sialyltransferase "sialylmotif" participates in binding the donor substrate CMP-NeuAc

All members of the sialyltransferase gene family cloned to date contain a conserved region, the "sialylmotif," consisting of 48-49 amino acids in the center of the coding sequence. To investigate the function of this motif, mutant constructs of the Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase were designed by site-directed mutagenesis, replacing 11 individual conserved amino acids with alanine. Each of the mutants was expressed in COS-1 cells, and eight of these retained sialyltransferase activity, allowing comparison of their enzymatic properties with that of the wild type enzyme. Kinetic analysis showed that six of eight mutants had a 3-12-fold higher Km for the donor substrate CMP-NeuAc relative to the wild type enzyme, while the Km values for the acceptor substrate were within 0.5-1.2-fold of the wild type for all eight mutants evaluated. The Ki of the donor substrate analog CDP was also evaluated for the recombinant sialyltransferase with the Val to Ala mutation at residue 220, which produced a 6-fold increase in Km of CMP-NeuAc. A corresponding increase in Ki of 3.4-fold was observed for CDP, indicating a decreased affinity for the cytidine nucleotide. Taken together, these results suggest that the conserved sialylmotif in the sialyltransferase gene family participates in the binding of the common donor substrate, CMP-NeuAc.

Molecular cloning and expression of chick embryo Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase. Comparison with the mammalian enzyme

DNA clones encoding beta-galactoside alpha 2,6-sialyltransferase have been isolated from chick embryonic cDNA libraries using sequence information obtained from the conserved amino acid sequence of the previously cloned enzymes. The cDNA sequence revealed an open-reading frame coding for 413 amino acids, and the deduced amino acid sequence showed 57.6% identity with the sequence of rat liver Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase. The primary structure of this enzyme suggested a putative domain structure, similar to structures found in other glycosyltransferases, consisting of a short N-terminal cytoplasmic domain, a signal-membrane anchor domain, a proteolytically sensitive stem region and a large C-terminal active domain. The identity of this enzyme was confirmed by construction of a recombinant sialyltransferase in which the N-terminus part including the cytoplasmic tail, signal anchor domain and stem region was replaced with an immunoglobulin signal peptide sequence. The expression of this recombinant protein in COS-7 cells resulted in secretion of a catalytically active and soluble form of the enzyme into the medium. The expressed enzyme exhibited activity only towards the disaccharide moiety of Gal beta 1,4GlcNAc in glycoproteins.