Characterization of a Thyroid Sulfotransferase Responsible for the 3-O-Sulfation of Terminal β-D-Galactosyl Residues in N-Linked Carbohydrate Units

Highly efficient chemoenzymatic synthesis of beta1-4-linked galactosides with promiscuous bacterial beta1-4-galactosyltransferases

Two bacterial beta1-4-galactosyltransferases, NmLgtB and Hp1-4GalT, exhibit promiscuous and complementary acceptor substrate specificity. They have been used in an efficient one-pot multienzyme system to synthesize LacNAc, lactose, and their derivatives including those containing negatively charged 6-O-sulfated GlcNAc and C2-substituted GlcNAc or Glc, from monosaccharide derivatives and inexpensive Glc-1-P.

Sulfonation and molecular action

The sulfonation of endogenous molecules is a pervasive biological phenomenon that is not always easily understood, and although it is increasingly recognized as a function of fundamental importance, there remain areas in which significant cognizance is still lacking or at most minimal. This is particularly true in the field of endocrinology, in which the sulfoconjugation of hormones is a widespread occurrence that is only partially, if at all, appreciated. In the realm of steroid/sterol sulfoconjugation, the discovery of a novel gene that utilizes an alternative exon 1 to encode for two sulfotransferase isoforms, one of which sulfonates cholesterol and the other pregnenolone, has been an important advance. This is significant because cholesterol sulfate plays a crucial role in physiological systems such as keratinocyte differentiation and development of the skin barrier, and pregnenolone sulfate is now acknowledged as an important neurosteroid. The sulfonation of thyroglobulin and thyroid hormones has been extensively investigated and, although this transformation is better understood, there remain areas of incomplete comprehension. The sulfonation of catecholamines is a prevalent modification that has been extensively studied but, unfortunately, remains poorly understood. The sulfonation of pituitary glycoprotein hormones, especially LH and TSH, does not affect binding to their cognate receptors; however, sulfonation does play an important role in their plasma clearance, which indirectly has a significant effect on biological activity. On the other hand, the sulfonation of distinct neuroendocrine peptides does have a profound influence on receptor binding and, thus, a direct effect on biological activity. The sulfonation of specific extracellular structures plays an essential role in the binding and signaling of a large family of extracellular growth factors. In summary, sulfonation is a ubiquitous posttranslational modification of hormones and extracellular components that can lead to dramatic structural changes in affected molecules, the biological significance of which is now beginning to be appreciated.

A remodeling system of the 3'-sulfo-Lewis a and 3'-sulfo-Lewis x epitopes

It has been reported that the chemically synthesized 3'-sulfo-Le(a) and 3'-sulfo-Le(x) epitopes have a high potential as a ligand for selectins. To elucidate the physiological functions of 3'-sulfated Lewis epitopes, a remodeling system was developed using a combination of a betaGal-3-O-sulfotransferase GP3ST, hitherto known alpha1,3/1,4-fucosyltransferases (FucT-III, IV, V, VI, VII, and IX) and arylsulfatase A. The pyridylaminated (PA) lacto-N-tetraose (Galbeta1-3GlcNAcbeta1-3Galbeta1-4Glc) was first converted to 3'-sulfolacto-N-fucopentaose II (sulfo-3Galbeta1-3(Fucalpha1-4)GlcNAcbeta1-3Galbeta1-4Glc)-PA by sequential reactions with GP3ST and FucT-III. The 3'-sulfolacto-N-fucopentaose III (sulfo-3Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Glc)-PA was then synthesized from lacto-N-neotetraose (Galbeta1-4GlcNAcbeta1-3Galbeta1-4Glc)-PA by GP3ST and FucT-III, -IV, -V, -VI, -VII, or -IX in a similar manner. The substrate specificity for the 3'-sulfated acceptor of the alpha1,3-fucosyltransferases was considerably different from that for the non-substituted and 3'-sialylated varieties. When the GP3ST gene was introduced into A549 and Chinese hamster ovary cells expressing FucT-III, they began to express 3'-sulfo-Le(a) and 3'-sulfo-Le(x) epitopes, respectively, suggesting that GP3ST is responsible for their biosynthesis in vivo. The expression of the 3'-sialyl-Le(x) epitope on Chinese hamster ovary cells was attenuated by the introduction of GP3ST gene, indicating that GP3ST and alpha2,3-sialyltransferase compete for the common Galbeta1-4GlcNAc-R oligosaccharides. Last, arylsulfatase A, which is a lysosomal hydrolase that catalyzes the desulfation of 3-O-sulfogalactosyl residues in glycolipids, was found to hydrolyze the sulfate ester bond on the 3'-sulfo-Le(x) (type 2 chain) but not that on the 3'-sulfo-Le(a) (type 1 chain). The present remodeling system might be of potential use as a tool for the study of the physiological roles of 3'-sulfated Lewis epitopes, including interaction with selectins.

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.

A Novel Human Gal-3-O-Sulfotransferase

Based on sequence homology with the previously cloned human cerebroside sulfotransferase (CST) cDNA, a novel sulfotransferase was cloned by screening a human fetal brain cDNA library. The novel sulfotransferase gene was present on human chromosome 11q13; the location was different from human CST and from that of the recently cloned human beta-Gal 3'-sulfotransferase (GP3ST). The isolated cDNA contained an open reading frame that encoded a predicted protein of 431 amino acid residues with type II transmembrane topology. The amino acid sequence showed 33% identity with that of human CST and 38% with that of human GP3ST. The recombinant enzyme expressed in Chinese hamster ovary cells catalyzed transfer of sulfate to position 3 of non-reducing beta-galactosyl residues in Galbeta1-4GlcNAc. Type 2 chains served as good acceptors, whereas type 1 chains served as poor acceptors, and intermediate activity was found toward Galbeta1-3GalNAc. Therefore, the substrate specificity was different from that of GP3ST. CST activity was not detected in the newly cloned enzyme. Northern blotting analysis showed that the sulfotransferase mRNA was strongly expressed in the thyroid and moderately expressed in the brain, heart, kidney, and spinal cord. Co-transfection of the enzyme cDNA and fucosyltransferase III into COS-7 cells resulted in expression of (SO(4)-3)Galbeta1-4(Fucalpha1-3)GlcNAc and a small amount of (SO(4)-3)Galbeta1-3(Fucalpha1-4)GlcNAc. These results indicated that the newly cloned enzyme is a novel Gal-3-O-sulfotransferase and is involved in biosynthesis of the (SO(4)-3)Galbeta1-4(Fucalpha1-3)GlcNAc structure.

Molecular cloning and expression of a novel human beta-Gal-3-O-sulfotransferase that acts preferentially on N-acetyllactosamine in N- and O-glycans

A novel cDNA-encoding galactose 3-O-sulfotransferase was cloned by screening the expressed sequence tag data base using the previously cloned cDNA encoding a galactosyl ceramide 3-O-sulfotransferase, which we term Gal3ST-1. The newly isolated cDNA encodes a novel 3-O-sulfotransferase, termed Gal3ST-3, that acts exclusively on N-acetyllactosamine present in N-glycans and core2-branched O-glycans. These conclusions were confirmed by analyzing CD43 chimeric proteins in Chinese hamster ovary cells expressing core2 beta1,6-N-acetylglucosaminyltransferase. The acceptor specificity of Gal3ST-3 contrasts with that of the recently cloned galactose 3-O-sulfotransferase (Honke, K., Tsuda, M., Koyota, S., Wada, Y., Iida-Tanaka, N., Ishizuka, I., Nakayama, J., and Taniguchi, N. (2001) J. Biol. Chem. 276, 267-274), which we term Gal3ST-2 in the present study because the latter enzyme can also act on core1 O-glycan and type 1 oligosaccharides, Galbeta1-->3GlcNAc. Moreover, Gal3ST-3 but not Gal3ST-2 can act on Galbeta1-->4(sulfo-->6)GlcNAc, indicating that disulfated sulfo-->3Galbeta1-->4(sulfo-->6) GlcNAc-->R may be formed by Gal3ST-3 in combination with GlcNAc 6-O-sulfotransferase. Although both Gal3ST-2 and Gal3ST-3 do not act on galactosyl ceramide, Gal3ST-3 is only moderately more homologous to Gal3ST-2 (40.1%) than to Gal3ST-1 (38.0%) at the amino acid level. Northern blot analysis demonstrated that transcripts for Gal3ST-3 are predominantly expressed in the brain, kidney, and thyroid where the presence of 3'-sulfation of N-acetyllactosamine has been reported. These results indicate that the newly cloned Gal3ST-3 plays a critical role in 3'-sulfation of N-acetyllactosamine in both O- and N-glycans.

Molecular cloning and characterization of a human beta-Gal-3'-sulfotransferase that acts on both type 1 and type 2 (Gal beta 1-3/1-4GlcNAc-R) oligosaccharides

A novel sulfotransferase gene (designated GP3ST) was identified on human chromosome 2q37.3 based on its similarity to the cerebroside 3'-sulfotransferase (CST) cDNA (Honke, K., Tsuda, M., Hirahara, Y., Ishii, A., Makita, A., and Wada, Y. (1997) J. Biol. Chem. 272, 4864-4868). A full-length cDNA was obtained by reverse transcription-polymerase chain reaction and 5'- and 3'-rapid amplification of cDNA ends analyses of human colon mRNA. The isolated cDNA clone predicts that the protein is a type II transmembrane protein composed of 398 amino acid residues. The amino acid sequence indicates 33% identity to the human CST sequence. A recombinant protein that is expressed in COS-1 cells showed no CST activity, but did show sulfotransferase activities toward oligosaccharides containing nonreducing beta-galactosides such as N-acetyllactosamine, lactose, lacto-N-tetraose (Lc4), lacto-N-neotetraose (nLc4), and Gal beta 1-3GalNAc alpha-benzyl (O-glycan core 1 oligosaccharide). To characterize the cloned sulfotransferase, a sulfotransferase assay method was developed that uses pyridylaminated (PA) Lc4 and nLc4 as enzyme substrates. The enzyme product using PA-Lc4 as an acceptor was identified as HSO(3)-3Gal beta 1-3GlcNAc beta 1-3Gal beta 1- 4Glc-PA by two-dimensional (1)H NMR. Kinetics studies suggested that GP3ST is able to act on both type 1 (Gal beta 1-3GlcNAc-R) and type 2 (Gal beta 1-4GlcNAc-R) chains with a similar efficiency. In situ hybridization demonstrated that the GP3ST gene is expressed in epithelial cells lining the lower to middle layer of the crypts in colonic mucosa, hepatocytes surrounding the central vein of the liver, extravillous cytotrophoblasts in the basal plate and septum of the placenta, renal tubules of the kidney, and neuronal cells of the cerebral cortex. The results of this study indicate the existence of a novel beta-Gal-3'-sulfotransferase gene family.

Sulfation of the N-linked oligosaccharides of influenza virus hemagglutinin: temporal relationships and localization of sulfotransferases

The occurrence of sulfate substituents on several positions of glycoprotein N-linked oligosaccharides prompted us to determine the subcellular localization and temporal relationships of the addition of these anionic groups employing as a model system the hemagglutinin (HA) produced by influenza virus-infected Madin-Darby canine kidney (MDCK) cells. It became apparent from a study of the HA glycoprotein in subcellular fractions resolved by Nycodenz gradient centrifugation following pulse-chase radiolabeling that sulfation of the complex N-linked oligosaccharides occurs only after they have been processed to an endo-beta-N-acetylglucosaminidase-resistant state and have reached the medial/trans Golgi and the trans Golgi network (TGN), with the former carrying out most of the sulfation activity. Hydrazine/nitrous acid/NaBH(4) treatment of the HA from the subcellular fractions indicated that C-3 of the galactose as well as C-6 of the N-acetylglucosamine residues of the N-acetyllactosamine chains became sulfated in these post ER fractions, as did the C-6 of the outer N-acetylglucosamine of the di-N-acetylchitobiose core. Consistent with the specificities of the stepwise assembly of the N-acetyllactosamine branches, we observed that the 3'-phosphoadenosine 5'-phosphosulfate (PAPS):GlcNAc-6-O-sulfotransferase migrated in the gradient to a medial/trans Golgi position while in contrast the PAPS:Gal-3-O-sulfotransferase was found in both Golgi and TGN locations. In accordance with the concept that beta-galactosylation must precede the sulfation catalyzed by the latter enzyme, we observed the presence of UDP-Gal:GlcNAc galactosyltransferase in both these sites in the MDCK cells. The presence of the Gal-3-O-sulfotransferase in the TGN is particularly important in the influenza virus-infected cells, as it makes possible the addition of terminal anionic groups after removal of the sialic acid residues by the viral neuraminidase.

Intracellular CFTR: localization and function

Intracellular CFTR: Localization and Function. Physiol. Rev. 79, Suppl.: S175-S191, 1999. - There is considerable evidence that CFTR can function as a chloride-selective anion channel. Moreover, this function has been localized to the apical membrane of chloride secretory epithelial cells. However, because cystic fibrosis transmembrane conductance regulator (CFTR) is an integral membrane protein, it will also be present, to some degree, in a variety of other membrane compartments (including endoplasmic reticulum, Golgi stacks, endosomes, and lysosomes). An incomplete understanding of the molecular mechanisms by which alterations in an apical membrane chloride conductance could give rise to the various clinical manifestations of cystic fibrosis has prompted the suggestion that CFTR may also play a role in the normal function of certain intracellular compartments. A variety of intracellular functions have been attributed to CFTR, including regulation of membrane vesicle trafficking and fusion, acidification of organelles, and transport of small anions. This paper aims to review the evidence for localization of CFTR in intracellular organelles and the potential physiological consequences of that localization.

Cystic fibrosis transmembrane conductance regulator-associated ATP and adenosine 3'-phosphate 5'-phosphosulfate channels in endoplasmic reticulum and plasma membranes

Cystic fibrosis (CF) is characterized by abnormal regulation of epithelial ion and fluid transport due to mutations in the CF transmembrane conductance regulator (CFTR), an apical membrane-localized Cl- channel, that usually prevent it from exiting the endoplasmic reticulum. Defective or absent CFTR in the epithelium is believed to disrupt fluid balance in human airways and thereby contribute to chronic respiratory inflammation. Patch-clamp of the plasma membrane and outer membrane of the nuclear envelope of nuclei isolated from CFTR-expressing Chinese hamster ovary cells revealed that CFTR is associated with a regulated ATP channel in both membrane compartments. CFTR expression was also shown to be associated with permeability to another adenine nucleotide, adenosine 3'-phosphate 5'-phosphosulfate, the universal sulfate donor in cells. These results may provide a link between the ion channel function of CFTR and abnormal glycoprotein processing observed in CF.

Zona pellucida glycoprotein mZP3 bioactivity is not dependent on the extent of glycosylation of its polypeptide or on sulfation and sialylation of its oligosaccharides

During fertilization in mice, free-swimming sperm bind to mZP3, one of three egg zona pellucida glycoproteins. Sperm recognize and bind to specific serine/threonine-linked (O-linked) oligosaccharides located at the mZP3 combining site for sperm. Shortly after binding to mZP3, sperm undergo the acrosome reaction, a form of cellular exocytosis. Here, we examined the influence of extent of glycosylation, sulfation, and sialylation of mZP3 (M(r) approximately 65,000-100,000) on its bioactivity; i.e. its ability to inhibit binding of sperm to eggs and to induce the acrosome reaction in vitro. Low (av. M(r) approximately 70,000), medium (av. M(r) approximately 82,000), and high (av. M(r) approximately 94,000) M(r) fractions of mZP3 were purified and shown to vary in extent of asparagine-linked (N-linked) glycosylation. All three size-fractions exhibited bioactivity, suggesting that the ability of mZP3 to inhibit binding of sperm to eggs is not related to the extent of glycosylation of its polypeptide (M(r) approximately 44,000). Digestion of mZP3 by neuraminidase decreased its average M(r) from approximately 83,000 to approximately 77,000 and increased its average pI from approximately 4.7 to approximately 6.0, but did not significantly affect mZP3 bioactivity. Terminal sialic acid largely accounts for the glycoprotein's acidic nature, but is not an essential element of the mZP3 combining site for sperm. Experiments with stably transfected embryonal carcinoma (EC) cells that secrete bioactive EC-mZP3 revealed that, of the sulfate present, approximately 70–75% was located on N-linked and approximately 25–30% on O-linked oligosaccharides. EC-mZP3 devoid of sulfate inhibited binding of sperm to eggs and induced the acrosome reaction to the same extent as sulfated EC-mZP3. These results suggest that sulfation of EC-mZP3 oligosaccharides is not essential for bioactivity. Overall, these findings contrast with those reported for certain other glycoproteins involved in cellular adhesion that require sulfate and/or sialic acid for bioactivity.

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.

Characterization of a sulfotransferase from human airways responsible for the 3-O-sulfation of terminal galactose in N-acetyllactosamine-containing mucin carbohydrate chains

A galactose 3-O-sulfotransferase activity able to transfer a sulfate group from adenosine 3'-phosphate 5'-phosphosulfate to methyl galactosides or terminal N-acetyllactosamine-containing carbohydrate chains from human respiratory mucins was characterized in microsomal fractions prepared from human respiratory mucosa. The reaction products, methyl alpha- or beta-galactose 3-sulfate and three oligosaccharide alditols containing the sequence HSO3-3Gal beta 1-4GlcNAc beta 1-6GalNAc-itol were identified by high performance anion-exchange chromatography. Using methyl beta-galactoside as a substrate, the optimum activity was obtained with 0.1% Triton X-100, 30 mM NaF, 20 mM Mn2+, and 10 mM AMP in a 30 mM 2-(N-morpholino)ethanesulfonic acid buffer at pH 6.1. The apparent Km for methyl beta-galactoside and for adenosine 3'-phosphate 5'-phosphosulfate were observed at 0.69 x 10(-3) M and at 4 x 10(-6) M respectively. This sulfotransferase is different from that responsible for sulfatide synthesis.

Mucin-type glycoproteins

Considerable advances have been made in recent years in our understanding of the biochemistry of mucin-type glycoproteins. This class of compounds is characterized mainly by a high level of O-linked oligosaccharides. Initially, the glycoproteins were solely known as the major constituents of mucus. Recent studies have shown that mucins from the gastrointestinal tract, lungs, salivary glands, sweat glands, breast, and tumor cells are structurally related to high-molecular-weight glycoproteins, which are produced by epithelial cells as membrane proteins. During mucin synthesis, an orchestrated sequence of events results in giant molecules of Mr 4 to 6 x 10(6), which are stored in mucous granules until secretion. Once secreted, mucin forms a barrier, not only to protect the delicate epithelial cells against the extracellular environment, but also to select substances for binding and uptake by these epithelia. This review is designed to critically examine relations between structure and function of the different compounds categorized as mucin glycoproteins.