Purification and characterization of the GalNAc-4-sulfotransferase responsible for sulfation of GalNAc beta 1,4GlcNAc-bearing oligosaccharides

Structural basis of oligosaccharide processing by glycosaminoglycan sulfotransferases

Heparan sulfate (HS) is a sulfated polysaccharide that plays a key role in morphogenesis, physiology and pathogenesis. The biosynthesis of HS takes place in the Golgi apparatus by a group of enzymes that polymerize, epimerize and sulfate the sugar chain. This biosynthetic process introduces varying degrees of sulfate substitution, which are tightly regulated and directly dictate binding specificity to different cytokines, morphogens and growth factors. Here, we report the use of molecular dynamics simulations to investigate the dynamics of substrate recognition of two glycosaminoglycan (GAG) sulfotransferases, N-deacetylase-N-sulfotransferase and 2-O-sulfotransferase to the HS chain during the biosynthetic process. We performed multiple simulations of the binding of the sulfotransferase domains to both the HS oligosaccharide substrate and sulfate donor, 3'-phosphoadenosine-5'-phosphosulfate. Analysis of extended simulations provide detailed and useful insights into the atomic interactions that are at work during oligosaccharide processing. The fast information matching method was used to detect the enzyme global dynamics and to predict the pairwise contact of residues responsible for GAG-enzyme binding and unbinding. The correlation between HS displacement and the location of the modified GAG chain were calculated, indicating a possible route for HS and heparin during sulfotransferase processing. Our data also show sulfotransferases contain a conserved interspaced positively charged amino acid residues that form a patch which controls the protein-GAG binding equilibrium. Together, our findings provide further understanding on the fine-tuned complex mechanism of GAG biosynthesis. Our findings can also be extrapolated to other systems for calculating rates of protein-GAG binding.

Isolation and sequencing of swine carbonic anhydrase VI, an enzyme expressed in the swine kidney

Background

Carbonic anhydrase VI (CA-VI) is produced by the salivary gland and is secreted into the saliva. Although CA-VI is found in the epithelial cells of distal straight tubule of swine kidneys, the exact function of CA-VI in the kidneys remains unclear.

Results

CA-VI was located in the epithelial cells of distal straight tubule of swine kidneys.

A full-length cDNA clone of CA-VI was generated from the swine parotid gland by reverse transcription polymerase chain reaction, using degenerate primers designed based on conserved regions of the same locus in human and bovine tissues.

The cDNA sequence was 1348 base pairs long and was predicted to encode a 317 amino acid polypeptide with a putative signal peptide of 17 amino acids. The deduced amino acid sequence of mature CA-VI was most similar (77.4%) to that of human CA-VI. CA-VI expression was confirmed in both normal and nephritic kidneys, as well as parotid. As the primers used in this study spanned two exons, the influence of genomic DNA was not detected. The expression of CA-VI was demonstrated in both normal and nephritic kidneys, and mRNA of CA-VI in the normal kidneys which was the normalised to an endogenous β–actin was 0.098 ± 0.047, while it was significantly lower in the diseased kidneys (0.012 ± 0.007). The level of CA-VI mRNA in normal kidneys was 19-fold lower than that of the parotid gland (1.887).

Conclusions

The localisation of CA-VI indicates that it may play a specialised role in the kidney.

Molecular basis for protein-specific transfer of N-acetylgalactosamine to N-linked glycans by the glycosyltransferases β1,4-N-acetylgalactosaminyl transferase 3 (β4GalNAc-T3) and β4GalNAc-T4

Two closely related β1,4-N-acetylgalactosaminyltransferases, β4GalNAc-T3 and β4GalNAc-T4, are thought to account for the protein-specific addition of β1,4-linked GalNAc to Asn-linked oligosaccharides on a number of glycoproteins including the glycoprotein hormone luteinizing hormone and carbonic anhydrase-6 (CA6). We have utilized soluble, secreted forms of β4GalNAc-T3 and β4GalNAc-T4 to define the basis for protein-specific GalNAc transfer in vitro to chimeric substrates consisting of Gaussia luciferase followed by a glycoprotein substrate. Transfer of GalNAc by β4GalNAc-T3 and β4GalNAc-T4 to terminal GlcNAc is divalent cation-dependent. Transfer of GalNAc to glycoprotein acceptors that contain a peptide recognition determinant is maximal between 0.5 and 1.0 mM MnCl(2); however, transfer is increasingly inhibited by concentrations of MnCl(2) above 1 mM and by anion concentrations above 15 mM. In contrast, transfer of GalNAc to the simple sugar acceptor N-acetylglucosamine-β-p-nitrophenol (GlcNAcβ-pNP) is not inhibited by concentrations of MnCl(2) or anions that would inhibit transfer to glycoprotein acceptors by >90%. This finding indicates that interaction with the peptide recognition determinant in the substrate is sensitive to the anion concentration. β4GalNAc-T3 and β4GalNAc-T4 have similar but distinct specificities, resulting in a 42-fold difference in the IC(50) for transfer of GalNAc to chimeric glycoprotein substrates by agalacto human chorionic gonadotropin, comprising 29 nM for β4GalNAc-T3 and 1.2 μM for β4GalNAc-T4. Our in vitro analysis indicates that enzymatic recognition of the peptide determinant and the oligosaccharide acceptor are independent events.

Differential expression and enzymatic properties of GalNAc-4-sulfotransferase-1 and GalNAc-4-sulfotransferase-2

We have cloned two GalNAc-4-sulfotransferases, GalNAc-4-ST1 and GalNAc-4-ST2, that transfer sulfate to terminal beta1,4-linked GalNAc. In conjunction with the action of protein-specific beta1,4GalNAc-transferases, GalNAc-4-ST1 and GalNAc-4-ST2 account for the presence of terminal beta1,4-linked GalNAc-4-SO(4) on glycoproteins such as lutropin, thyrotropin (TSH), proopiomelanocortin (POMC), carbonic anhydratase-VI (CA-VI), and tenascin-R. GalNAc-4-ST1 and GalNAc-4-ST2 can be distinguished by their differing specificity for oligosaccharide acceptors and temperature lability. The differences in properties have been used to show that the levels of GalNAc-4-ST1 and GalNAc-4-ST2 activity are proportionate to the levels of their respective transcripts. Furthermore, we have found that both transcript and activity levels of GalNAc-4-ST1 and GalNAc-4-ST2 vary widely among different tissues indicating that the regulation of their expression differs. Differences in specificity and the regulation of expression may account for existence of two GalNAc-4-sulfotransferases in vivo. The highest levels of both GalNAc-4-ST1 and GalNAc-4-ST2 transcripts are present in the pituitary of the mouse with multiple cell types that produce glycoproteins terminating with GalNAc-4-SO(4). Genetic ablation of both GalNAc-4-ST1 and GalNAc-4-ST2 may be necessary to alter the pattern and/or extent of sulfate addition to terminal beta1,4GalNAc in tissues such as pituitary.

Glycosylation-related Gene Expression in Prion Diseases

Several lines of evidence indicate that some glycoconjugates are efficient effectors of the cellular prion protein (PrP(C)) conversion into its pathogenic (PrP(Sc)) isoform. To assess how glycoconjugate glycan moieties participate in the biogenesis of PrP(Sc), an exhaustive comparative analysis of the expression of about 200 glycosylation-related genes was performed on prion-infected or not, hypothalamus-derived GT1 cells by hybridization of DNA microarrays, semiquantitative RT-PCR, and biochemical assays. A significant up- (30-fold) and down- (17-fold) regulation of the expression of the ChGn1 and Chst8 genes, respectively, was observed in prion-infected cells. ChGn1 and Chst8 are involved in the initiation of the synthesis of chondroitin sulfate and in the 4-O-sulfation of non-reducing N-acetylgalactosamine residues, respectively. A possible role for a hyposulfated chondroitin in PrP(Sc) accumulation was evidenced at the protein level and by determination of chondroitin and heparan sulfate amounts. Treatment of Sc-GT1 cells with a heparan mimetic (HM2602) induced an important reduction of the amount of PrP(Sc), associated with a total reversion of the transcription pattern of the N-acetylgalactosamine-4-O-sulfotransferase 8. It suggests a link between the genetic control of 4-O-sulfation and PrP(Sc) accumulation.

Nucleotide sequence and expression of a cDNA encoding canine carbonic anhydrase VI (CA-VI)

A full-length cDNA clone of a canine carbonic anhydrase VI (CA-VI) was generated from the canine parotid gland by using a reverse transcription-polymerase chain reaction (RT-PCR) technique with degenerate primers designed from conserved regions of the same locus in humans and bovines employing RACE (rapid amplification of cDNA ends) techniques. The cDNA sequence was 1351 base pairs (bp) long and was predicted to encode a 320-amino-acid polypeptide containing a putative signal peptide of 17 amino acids. The deduced amino acid sequence of mature CA-VI showed the highest similarity of 74% to that of human CA-VI. RT-PCR analysis with primers specific to the canine CA-VI demonstrated strong signals in the major salivary glands and weak signals in the minor salivary glands and esophagus of a healthy dog. No CA-VI mRNA was detected in the pancreas, liver or the digestive tract except the esophagus.

Enhanced 3-O-sulfation of galactose in Asn-linked glycans and Maackia amurensis lectin binding in a new Chinese hamster ovary cell line

We report the characterization of two Chinese hamster ovary cell lines that produce large amounts of sulfated N-linked oligosaccharides. Clones 26 and 489 were derived by stable transfection of the glycosaminoglycan-deficient cell mutant pgsA-745 with a cDNA library prepared from wild-type cells. Peptide:N-glycanase F released nearly all of the sulfate label, indicating that sulfation had occurred selectively on the Asn-linked glycans. Hydrazinolysis followed by nitrous acid treatment at pH 4 and borohydride reduction yielded reduced sulfated disaccharides that comigrated with standard Gal3SO4beta1-4anhydromannitol. The disaccharides were resistant to periodate oxidation but became sensitive after the sulfate group was removed by methanolysis, indicating that the sulfate was located at C3 of the galactose residues. Maackia amurensis lectin bound to the sulfated glycopeptides on the cell surface and in free form, even after sialidase treatment. This finding indicates that the lectin requires only a charged group at C3 of the galactose unit and not an intact sialic acid. Growth of cells with chlorate restored sialidase sensitivity to lectin binding, indicating that sulfation and sialylation occurred largely at the same sites. The enhanced sulfation was due to elevated sulfotransferase activity that catalyzed transfer of sulfate from phosphoadenosine-5'-phosphosulfate to Galbeta1-4(3)GlcNAcbeta-O-naphthalenemethanol.

Increasing the intra-Golgi pH of cultured LS174T goblet-differentiated cells mimics the decreased mucin sulfation and increased Thomsen-Friedenreich antigen (Gal beta1-3GalNac alpha-) expression seen in colon cancer

Mucins in ulcerative colitis and colon cancer share common properties of reduced sulfation and increased oncofetal carbohydrate antigen expression. It has previously been shown that there is no simple correlation between these changes and the activity of the relevant glycosyl-, sialyl-, and sulfo-transferases. We examined mucin sulfation and expression of oncofetal Thomsen-Friedenreich (TF) antigen (galactosyl beta1-3N-acetylgalactosamine alpha-) in the goblet cell-differentiated human colon cancer cell line LS174T following treatment with bafilomycin A(1, )which raises intra-Golgi pH, or monensin, which disrupts medial-trans Golgi transport. Cells were dual-labeled with sodium [(35)S]-sulfate and D-[6-(3)H(N)]-glucosamine hydrochloride, or labeled with L-[U-(14)C]-threonine alone. Mucin was purified using Sepharose CL-4B gel filtration. Mucin sulfo-Lewis(a) and TF antigen expression were assessed using the F2 anti-sulfo-Lewis(a) monoclonal antibody and peanut agglutinin binding respectively. Bafilomycin (0.01 microM; 48 h) reduced total mucin sulfation, expressed relative to incorporation of glucosamine, to 0.50 +/- 0.04 d.p.m. [(35)S]-sulfate per d.p.m. [(3)H]-glucosamine compared to control, 0.84 +/- 0.05 (p < 0.001, n = 16). This was accompanied by 50.3 +/- 8.0% increased expression of TF antigen (p < 0.01) and 50.1 +/- 5.5% decreased expression of sulfo-Lewis(a) (p < 0.01). The reduced sulfate:glucosamine ratio was largely due to increased incorporation of glucosamine into newly synthesized mucin rather than reduction in total sulfate incorporation. In contrast, monensin only reduced total mucin glycosylation at concentrations > 0.1 microM and had no significant effect on mucin sulfation or TF expression. Intra-Golgi alkalinization affects mucin glycosylation, resulting in decreased mucin sulfation and increased expression of TF antigen, changes that mimic those seen in cancerous and premalignant human colonic epithelium.

Molecular cloning and expression of an N-acetylgalactosamine-4-O-sulfotransferase that transfers sulfate to terminal and non-terminal beta 1,4-linked N-acetylgalactosamine

We have identified and characterized an N-acetylgalactosamine-4-O-sulfotransferase designated GalNAc-4-ST2 (GenBank(TM) accession number ) based on its homology to HNK-1 sulfotransferase (HNK-1 ST). The cDNA predicts an open reading frame encoding a type II membrane protein of 443 amino acids with a 12-amino acid cytoplasmic domain, a 23-amino acid transmembrane domain, and a 408-amino acid luminal domain containing four potential N-linked glycosylation sites. GalNAc-4-ST2 displays a high degree of amino acid sequence identity with GalNAc-4-ST1 (46%), HNK-1 ST (23%), chondroitin 4-O-sulfotransferase-1 (C4ST-1) (27%), and chondroitin 4-O-sulfotransferase-2 (C4ST-2) (24%). GalNAc-4-ST2 transfers sulfate to the C-4 hydroxyl of terminal beta1,4-linked GalNAc in the sequence GalNAc-beta1,4GlcNAcbeta-R found on N-linked oligosaccharides and nonterminal beta1,4-linked GalNAc in chondroitin and dermatan. The translated region of GalNAc-4-ST2 is encoded by five exons located on human chromosome 18q11.2. Northern blot analysis reveals a 2.1-kilobase transcript. GalNAc-4-ST2 message is most highly expressed in trachea and to a lesser extent in heart, liver, pancreas, salivary gland, and testis. The I.M.A.G.E. cDNA clone 49547 contains a putative GalNAc-4-ST2 splice form with an open reading frame encoding a protein of 358 amino acids that lacks the transmembrane domain and the stem region. This form of GalNAc-4-ST2 is not retained by transfected cells and is active against chondroitin but not terminal beta1,4-linked GalNAc. Thus, as with GalNAc-4-ST1, sequences N-terminal to the catalytic domain contribute to the specificity of GalNAc-4-ST2 toward terminal beta1,4-linked GalNAc.

Molecular cloning and characterization of GalNAc 4-sulfotransferase expressed in human pituitary gland

We have previously cloned chondroitin-4-sulfotransferase (C4ST) cDNA from mouse brain. In this paper, we report cloning and characterization of GalNAc 4-sulfotransferase (GalNAc4ST), which transfers sulfate to position 4 of the nonreducing terminal GalNAc residue. The obtained cDNA contains a single open reading frame that predicts a type II transmembrane protein composed of 424 amino acid residues. Identity of the amino acid sequence between GalNAc4ST and human C4ST was 30%. When the cDNA was transfected in COS-7 cells, sulfotransferase activity toward carbonic anhydrase VI was overexpressed but no sulfotransferase activity toward chondroitin or desulfated dermatan sulfate was increased over the control. Sulfation of carbonic anhydrase VI by the recombinant GalNAc4ST occurred at position 4 of the GalNAc residue of N-linked oligosaccharides. The recombinant GalNAc4ST transferred sulfate to position 4 of GalNAc residue of p-nitrophenyl GalNAc, indicating that this sulfotransferase transfers sulfate to position 4 at the nonreducing terminal GalNAc residue. Dot blot analysis showed that the message of GalNAc4ST was expressed strongly in the human pituitary, suggesting that the cloned GalNAc4ST may be involved in the synthesis of the nonreducing terminal GalNAc 4-sulfate residues found in the N-linked oligosaccharides of pituitary glycoprotein hormones.

Molecular cloning and expression of the pituitary glycoprotein hormone N-acetylgalactosamine-4-O-sulfotransferase

N-Linked oligosaccharides terminating with the sequence SO(4)-4-GalNAcbeta1,4GlcNAcbeta1,2Manalpha are present on the pituitary hormones lutropin (LH), thyrotropin, and pro-opiomelanocortin. The sulfated structures on LH are essential for expression of its biologic function in vivo. We have cloned the N-acetylgalactosamine-4-sulfotransferase (GalNAc-4-ST1, GenBank(TM) accession number ), which mediates sulfate addition to the N-linked oligosaccharides on LH and other pituitary glycoproteins with terminal (beta1,4-linked GalNAc based on its homology to HNK-1 sulfotransferase (HNK-1 ST). GalNAc-4-ST1 displays 23% identity to HNK-1 ST and 28% to chondroitin 4-sulfotransferase 1 (C4ST-1) and 26% to chondroitin 4-sulfotransferase 2 (C4ST-2). The cDNA predicts a type II transmembrane protein of 424 amino acids with four potential N-linked glycosylation sites and a single membrane-spanning domain. GalNAc-4-ST1 has putative 5'-phosphosulfonate and 3'-phosphate binding sites. Three more carboxyl-terminal regions of unknown function also show a high degree of identity with HNK-1 ST, C4ST-1, and C4ST-2. The membrane-bound form of GalNAc-4-ST1 transfers sulfate to GalNAcbeta1, 4GlcNAcbeta-R but not to chondroitin, whereas truncated forms of GalNAc-4-ST1 that are released into the medium transfer sulfate to both GalNAcbeta1,4GlcNAcbeta-R and chondroitin. The first 118 amino acids of GalNAc-4-ST1 appear to contribute to both its activity and specificity for terminal beta1,4-linked GalNAc. GalNAc-4-ST1 also efficiently transfers sulfate to N-linked oligosaccharides on native LH and other glycoproteins terminating with beta1,4-linked GalNAc. A single transcript of 2.4 kilobases is most highly expressed in the pituitary and other regions of the central nervous system. The GalNAc-4-ST1 gene is located on human chromosome 19q13.1.

Isolation and characterization from porcine serum of a soluble sulfotransferase responsible for 6-O-sulfation of the galactose residue in 2'-fucosyllactose: implications in the synthesis of the ligand for L-selectin

A soluble sulfotransferase from porcine serum which catalyzes the transfer of sulfate from adenosine 3'-phosphate 5'-phosphosulphate (PAPS) to 2'-fucosyllactose (2'-FL) was purified 36,333-fold using a combination of conventional and affinity chromatographic steps. The purified enzyme preparation after non-denaturing discontinuous-PAGE exhibited a molecular mass of about 80 kDa by reducing SDS-PAGE. However, when a partially purified enzyme preparation was subjected to gel filtration on Sephacryl S-300, the enzyme activity eluted in the void volume, which indicated that the native enzyme existed as an oligomer. The purified enzyme showed Km values of 9.15 microM for PAPS and 15.38 mM for 2'-FL at the optimum pH value of 7.4. The substrate specificity of the purified enzyme was evaluated with various sugars that are structurally similar to sialyl LewisX (sLeX). Results indicated that 3'-sialyllactose and lactose were efficient acceptors of sulfation, whereas 6'-sialyllactose and 6'-sialyllactosamine were poor substrates for this sulfotransferase. Further, the reaction product analysis revealed that the sulfate substitution, when using 2'-FL as the substrate, was at the C-6 position of the galactose residue. Coincidentally, a similar enzyme activity was also found in porcine lymphoid tissues such as, lymph nodes (peripheral and mesenteric) and spleen. Collectively, these findings suggest that this enzyme might be involved in the synthesis of the ligand for L-selectin.

Purification and characterization of chondroitin 4-sulfotransferase from the culture medium of a rat chondrosarcoma cell line

Chondroitin 4-sulfotransferase, which transfers sulfate from 3'-phosphoadenosine 5'-phosphosulfate to position 4 of N-acetylgalactosamine in chondroitin, was purified 1900-fold to apparent homogeneity with 6.1% yield from the serum-free culture medium of rat chondrosarcoma cells by affinity chromatography on heparin-Sepharose CL-6B, Matrex gel red A-agarose, 3',5'-ADP-agarose, and the second heparin-Sepharose CL-6B. SDS-polyacrylamide gel electrophoresis of the purified enzyme showed two protein bands. Molecular masses of these protein were 60 and 64 kDa under reducing conditions and 50 and 54 kDa under nonreducing conditions. Both the protein bands coeluted with chondroitin 4-sulfotransferase activity from Toyopearl HW-55 around the position of 50 kDa, indicating that the active form of chondroitin 4-sulfotransferase is a monomer. Dithiothreitol activated the purified chondroitin 4-sulfotransferase. The purified enzyme transferred sulfate to chondroitin and desulfated dermatan sulfate. Chondroitin sulfate A and chondroitin sulfate C were poor acceptors. Chondroitin sulfate E from squid cartilage, dermatan sulfate, heparan sulfate, and completely desulfated N-resulfated heparin hardly served as acceptors of the sulfotransferase. The transfer of sulfate to the desulfated dermatan sulfate occurred preferentially at position 4 of the N-acetylgalactosamine residues flanked with glucuronic acid residues on both reducing and nonreducing sides.

Carbohydrate sulfotransferases: mediators of extracellular communication

Sulfated carbohydrates mediate diverse extracellular recognition events in both normal and pathological processes. The sulfotransferases that generate specific carbohydrate 'sulfoforms' have recently been recognized as key modulators of these processes and therefore represent potential therapeutic targets.

Identification of an N-acetylglucosamine-6-0-sulfotransferase activity specific to lymphoid tissue: an enzyme with a possible role in lymphocyte homing

BACKGROUND

The leukocyte adhesion molecule L-selection participates in the initial attachment of blood-borne lymphocytes to high endothelial venules (HEVs) during lymphocyte homing to secondary lymphoid organs, and contributes to leukocyte adhesion and extravasation in HEV-like vessels at sites of chronic inflammation. The L-selection ligands on lymph mode HEVs are mucin-like glycoproteins adorned with the unusual sulfated carbohydrate epitope, 6-sulfo sialyl Lewis x. Sulfation of this epitope on the N-acetylglucosamine (GlcNAc) residue confers high-avidity L-selection binding, and is thought to be restricted in the vasculature to sites of sustained lymphocyte recruitment. The GlcNAc-6-0 sulfotransferase that installs the sulfate ester may be a key modulator of lymphocyte recruitment to secondary lymphoid organs and sites of chronic inflammation and is therefore a potential target for anti-inflammatory therapy.

RESULTS

A GlcNAc-6-0-sulfotransferase activity was identified within porcine lymph nodes and characterized using a rapid, sensitive, and quantitative assay. We synthesized two unnatural oligosaccharide substrates, GlcNAc beta 1-->6Gal alpha-R and Gal beta 1-->4GlcNAc beta 1-->6Gal alpha-R, that incorporate structural motifs from the native L-selection ligands into an unnatural C-glycosyl hydrocarbon scaffold. The sulfotransferase incorporated greater than tenfold more sulfate into the disaccharide than the trisaccharide, indicating a requirement for a terminal GlcNAc. Activity across tissues was highly restricted to the HEVs within peripheral lymph node.

CONCLUSIONS

The restricted expression of the GlcNAc-6-0-sulfotransferase activity to lymph node HEVs strongly suggestions a role in the biosynthesis of L-selection ligands. In addition, similar sulfated epitopes are known to be expressed on HEV-like vessels of chronically inflamed tissues; indicating that this sulfotransferase may also contribute to inflammatory lymphocyte recruitment. We identified a concise disaccharide motif, GlcNAc beta 1-->6Gal alpha-R, that preserved both recognition and specificity determinants for the GlcNAc-6-0-sulfotransferase. The absence of activity on the trisaccharide Gal beta 1-->6Gal alpha-R indicates a requirement for a substrate with a terminal GlcNAc residue, suggesting that sulfation precedes further biosynthetic assembly of L-selection ligands.

Sulfomucins in the human body

Mucins are widely distributed in mucous secretion fluids or are associated with plasma membranes. Up to now 9 genes of epithelial mucins have been identified, distributed over five chromosomes. Superposed on the genetic diversity, each type of mucin displays heterogeneity in oligosaccharide composition, including the terminal sugar residues. On top of that there is variation between individuals brought about by blood group antigens. Heterogeneity is further incited by the degree of sulfation. This tremendous structural heterogeneity endows mucin molecules with properties suggestive for a multifunctional role. The major biological function assigned to mucins is still the protection of tissues covered by the mucous gel. Current knowledge on the specific biological functions of the sulfate residues is fragmentary and periphrastic. Glycosylation including sulfation appears to be subject to modification under pathological conditions. There is evidence that sulfation rate-limits bacterial degradation of mucins. Moreover, accumulating data focus towards their involvement in recognition phenomena. Sulfate residues on blood group related structures provoke specific epitopes for selective interaction with microorganisms e.g. Helicobacter pylori. A distinct class of mucins acts as ligands for selectins, crucial in cellular recognition processes like cellular homing of lymphocytes. Whereas in earlier days mucins were only seen as water-binding molecules, protecting the underlying mucosa against harmful agents, the current picture of these molecules is characterized by the selective interaction with their environment, including epithelial-, and endothelial cells and microorganisms, thereby regulating a great number of biological processes. However, the specific role of sulfate remains to be further elucidated.

Secretory carbonic anhydrase isoenzyme (CA VI) in human serum

Carbonic anhydrase VI (CA VI) is a secretory isoenzyme that, by analogy to α-amylase, is produced in the salivary glands and delivered into saliva. To determine whether CA VI is transferred into the circulation and is detectable in human serum, we collected blood samples from four healthy subjects at 3-h intervals throughout a 24-h period and measured concentrations of CA VI by a specific time-resolved immunofluorometric assay. All serum samples contained CA VI, the concentrations being ∼22 times lower in serum than in the corresponding saliva samples. The presence of CA VI in serum was confirmed by Western blotting, which under reducing conditions identified a 42-kDa polypeptide band corresponding to the monomeric CA VI. The described time-resolved immunofluorometric assay for CA VI might be useful to identify or exclude diseases of the salivary glands in the differential diagnosis of patients whose serum amylase concentrations are increased.

Developmental regulation of a pregnancy-specific oligosaccharide structure, NeuAcalpha2,6GalNAcbeta1,4GlcNAc, on select members of the rat placental prolactin family

Successful pregnancy is dependent upon an array of signaling proteins secreted by the trophoblast cells of the placenta. Among these is a group of proteins related to pituitary prolactin, known as the prolactin/growth hormone family. These proteins are expressed at specific times during gestation and synthesized in distinct trophoblast cell types in the rat placenta. We report here that select members of this family, prolactin-like protein (PLP-A), PLP-B, PLP-C, decidual/trophoblast PRP, and placental lactogen I variant, only which are expressed in the spongiotrophoblast, late in rat placental development bear Asn-linked oligosaccharides terminating with NeuAcalpha2,6GalNAcbeta1,4GlcNAcbeta-R. This reflects the concurrent expression of these prolactin/growth hormone family members with the peptide-specific beta1,4GalNAc-transferase and an alpha2,6-sialyltransferase, which can add sialic acid to terminal beta1,4-linked GalNAc. We have determined that at least one of the prolactin-like proteins, PLP-A, is recognized by the protein-specific GalNAc-transferase. The presence of NeuAcalpha2, 6GalNAcbeta1,4GlcNAcbeta-R on only a limited number of glycoproteins synthesized by the spongiotrophoblasts between mid gestation and birth reflects the need for both the GalNAc-transferase and the peptide recognition determinant for efficient addition of GalNAc. Thus, expression of the GalNAc-transferase and specific members of the prolactin/growth hormone family is developmentally regulated in the rat placenta, suggesting a physiological role for the terminal NeuAcalpha2,6GalNAcbeta1,4GlcNAcbeta-R sequence on Asn-linked oligosaccharides of these proteins.

Characterization of the major core structures of the alpha2-->8-linked polysialic acid-containing glycan chains present in neural cell adhesion molecule in embryonic chick brains

To gain more insight into the possible functional significance of the core glycan chain(s) on which polysialylation takes place in polysialic acid (poly-Sia)-containing glycoproteins, the structure of the core glycans in the embryonic form of chick brain neural cell adhesion molecule (N-CAM) were examined using chemical and instrumental techniques. The following new structural features, which had not been reported by the early pioneering study by Finne (Finne, J. (1982) J. Biol. Chem. 257, 11966-11970), were revealed (Structure I). (i) Two distinct types of multiantennary N-linked glycans, i.e. tri- and tetra-antennary structures, are present; (ii) an alpha1-->6-linked fucosyl residue is attached to the proximal GlcNAc residue of the di-N-acetylchitobiosyl unit; (iii) that the action of GlcNAc-transferase V, which catalyzes the attachment of the beta-(1-->6)-linked GlcNAc residue on the (1-->6)-alpha-linked mannose (Man) arm, appears to be essential for polysialylation to occur on the core glycan chain is suggested by the fact that the Man residue alpha1-->6-linked to the beta-linked Man residue is invariably 2,6-di-O-substituted by the GlcNAc residue; (iv) both type 1 (Galbeta1-->3GlcNAc) and type 2 (Galbeta1-->4 GlcNAc) sequences are present in the peripheral portion of the core glycan structure. An extended form of the type 2 chain, i.e. Galbeta1-->4GlcNAcbeta1-->3Galbeta1-->4GlcNAc, is also expressed on the (1-->3)- and (1-->6)-alpha-linked Man arms; (v) on average about 1.4 mol of sulfate is attached to the type 2 N-acetyllactosamine chain(s), where in the extended form the sulfate group is probably substituted at the O-3 position of the outmost GlcNAc residue, i.e. Galbeta1-->4(HSO3-->3)GlcNAcbeta1-->3Galbeta1--> 4GlcNAcbeta1-->Man. It is possible that the unusual structural features identified in this study might play a role in the initiation of polysialylation and our data should facilitate future research regarding the signals that control polysialylation.

Expression cloning of Forssman glycolipid synthetase: a novel member of the histo-blood group ABO gene family

A phenotypic cloning approach was used to isolate a canine cDNA encoding Forssman glycolipid synthetase (FS; UDP-GalNAc:globoside alpha-1,3-N-acetylgalactosaminyltransferase; EC 2.4.1.88). The deduced amino acid sequence of FS demonstrates extensive identity to three previously cloned glycosyltransferases, including the enzymes responsible for synthesis of histo-blood group A and B antigens. These three enzymes, like FS, catalyze the addition of either N-acetylgalactosamine (GalNAc) or galactose (Gal) in alpha-1,3-linkage to their respective substrates. Despite the high degree of sequence similarity among the transferases, we demonstrate that the FS cDNA encodes an enzyme capable of synthesizing Forssman glycolipid, and demonstrates no GalNAc or Gal transferase activity when closely related substrates are examined. Thus, the FS cDNA is a novel member of the histo-blood group ABO gene family that encodes glycosyltransferases with related but distinct substrate specificity. Cloning of the FS cDNA will allow a detailed dissection of the roles Forssman glycolipid plays in cellular differentiation, development, and malignant transformation.