Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:2.4.99.6 (sialyltransferase)
1,546 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The cytoplasmic droplet of epididymal spermatozoa is a small localized outpouching of cytoplasm of the tail of unknown significance. EM revealed flattened saccular elements as the near exclusive membranous component of the droplet. Light and electron microscopic immunolabeling for Golgi/TGN markers showed these saccules to be reactive for antibodies to TGN38, protein affinity-purified alpha 2,6 sialyltransferase, and anti-human beta 1,4 galactosyltransferase. The saccules were isolated by subcellular fractionation and antibodies raised against this fraction immunolabeled the saccules of the droplet in situ as well as the Golgi region of somatic epithelial cells lining the epididymis. The isolated droplet fraction was enriched in galactosyltransferase and sialyltransferase activities, and endogenous glycosylation assays identified the modification of several endogenous glycopeptides. EM lectin staining in situ demonstrated galactose and N-acetyl galactosamine constituents in the saccules. Endocytic studies with cationic and anionic ferritin as well as HRP failed to identify the saccules as components of the endocytic apparatus. Epididymal spermatozoa were devoid of markers for the ER as well as the Golgi-associated coatamer protein beta-COP. It is therefore unlikely that the saccular elements of the droplet participate in vesicular protein transport. However, the identification of Golgi/TGN glycosylating activities in the saccules may be related to plasma membrane modifications which occur during epididymal sperm maturation.
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PMID:The cytoplasmic droplet of rat epididymal spermatozoa contains saccular elements with Golgi characteristics. 822 42

The B lymphocyte cell surface receptor CD22 is an adhesion molecule that can mediate binding to several leukocyte subsets. The first CD22 ligand to be identified was the receptor-linked phosphotyrosine phosphatase CD45, but several lines of evidence suggest that CD22 may interact with multiple counter receptors on adjacent lymphocytes. In the present work, we show that in addition to CD45, a soluble CD22-immunoglobulin fusion protein (CD22Rg) recognizes several other distinct lymphocyte sialoglycoproteins. CD22-mediated adhesion is dependent upon the presence of sialic acids on ligands. CD22Rg is observed to bind specifically to a 115-kDa sialoglycoprotein in COS cells transfected with an alpha-2,6-sialyltransferase cDNA, but not in COS cells transfected with unrelated cDNA clones, indicating that at least some CD22-mediated interactions require presentation of sialic acid in an alpha-2,6 linkage by CD22 ligands. In all cases, truncation of the side chain of sialic acids by mild periodate oxidation abolishes recognition by CD22Rg. Direct binding of CD22Rg to lymphoid cells also requires sialic acids and their side chains. Taken together, these observations indicate that CD22 is a sialic acid-binding lectin and may define a novel functional subset of immunoglobulin superfamily adhesion molecules.
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PMID:CD22, a B cell-specific immunoglobulin superfamily member, is a sialic acid-binding lectin. 846 34

CD22 beta is a glycoprotein found on the surface of B cells during restricted stages of development. It is believed to play a role in cell-cell interactions and B cell activation. The accompanying paper (Sgroi, D., Varki, A., Braesch-Andersen, S., and Stamenkovic, I. (1993) J. Biol. Chem. 268, 7011-7018) shows that CD22 beta recognizes multiple glycoproteins on the surfaces of T and B cells and that sialylation of these ligands is essential for binding. To identify the structure(s) of the sialylated oligosaccharide(s) recognized by CD22 beta, [3H]glucosamine-labeled glycoproteins were purified from Daudi cells by adsorption onto a CD22 beta recombinant immunoglobulin (CD22 beta Rg) chimera attached to protein A-Sepharose (PAS), and the N-linked oligosaccharides were released by peptide N-glycosidase F. These released oligosaccharides failed to bind to CD22 beta Rg-PAS under the conditions used initially to adsorb the glycoproteins, but their elution from a column of CD22 beta Rg-PAS was significantly retarded. Populations of oligosaccharides with different affinities could be identified by their order of elution. Specific sialidases were used to determine the content of alpha-2,3- and alpha-2,6-linked sialic acid in these different populations and their contribution to binding. Multiantennary oligosaccharides with one alpha-2,6-linked residue bound marginally, and those with two or more bound more tightly. alpha-2,3-Linked sialic acid residues were without effect. Binding did not require divalent cations and was abrogated by mild periodate oxidation of the outer side chain of sialic acid. No marked differences in size or fucose content were found between the populations of high and low affinity oligosaccharides. However, the low affinity population could be partially converted into higher affinity by treatment with beta-galactoside alpha-2,6 sialyltransferase and CMP-sialic acid. Thus, CD22 beta is a mammalian lectin that can recognize specific N-linked oligosaccharide structures containing alpha-2,6-linked sialic acids.
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PMID:Natural ligands of the B cell adhesion molecule CD22 beta carry N-linked oligosaccharides with alpha-2,6-linked sialic acids that are required for recognition. 846 35

CD22 is an immunoglobulin superfamily B lymphocyte-specific adhesion receptor and a member of the recently identified I-type class of lectins. Recent work has shown that CD22 specifically recognizes sialic acid linked alpha2,6 to terminal N-linked oligosaccharides on selected cell surface glycoproteins. CD22-ligand interaction is regulated by the activity of a beta-galactoside alpha2, 6-sialyltransferase that can inactivate CD22-mediated binding by sialylating the CD22 receptor itself. These observations suggest that N-linked glycosylation sites on the CD22 molecule may play a role in the regulation of CD22-mediated adhesion. In this work we have performed site-specific mutagenesis of potential N-linked glycosylation sites on CD22 in an effort to determine whether they might be involved in ligand recognition. We show that mutation of a single potential N-linked glycosylation site in the first immunoglobulin domain of CD22 completely abrogates ligand recognition. Interestingly, this site is characterized by the sequence NCT, where the cysteine is thought to be involved in an intrachain disulfide bond. Site-directed mutagenesis of similar NC(T/S) motifs in the first or second Ig domains of the I-type lectins myelin-associated glycoprotein, and sialoadhesin did not disrupt their ability to mediate sialic acid binding. In contrast, mutation of a NCS motif in the first Ig domain of the I-type lectin CD33 unmasked its sialic acid binding activity. These observations suggest that a single N-linked glycosylation site located at a similar position in the CD22 and CD33 glycoproteins is critical for regulating ligand recognition by both receptors.
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PMID:A single N-linked glycosylation site is implicated in the regulation of ligand recognition by the I-type lectins CD22 and CD33. 870 38

Mammalian spermatozoa must undergo maturational changes between the events of mating and fertilization. These biochemical and functional alterations, collectively termed capacitation, take place as spermatozoa traverse the female reproductive tract. The preparatory biochemical changes include removal, modification, and reorganization of sperm surface molecules. Although details of all the changes are not known, lectin binding studies have provided evidence suggesting that carbohydrate moieties of sperm surface glycoproteins are modified during capacitation. In an attempt to gain insight into the potential modifications of sperm plasma membrane glycoproteins, we quantified glycoprotein-modifying enzyme activities in the uterine and oviductal fluid of the hamster during the 4 days of the estrous cycle. These enzymes are known to modify existing glycoproteins, either by adding sugar residues (glycosyltransferases) or by removing terminal sugar residues (glycosidases). Data from these studies showed that 1) levels of all glycosyltransferase activities assayed (sialyltransferase, fucosyltransferase, galactosyltransferase, and N-acetylglucosaminyltransferase) were negligible in the uterine fluid at the onset of ovulation (Day 1) but sharply increased preceding ovulation (Day 4); 2) levels of the four glycosyltransferase activities assayed were higher in the oviductal fluid at the onset of ovulation (Day 1) and then gradually decreased through the remainder of the estrous cycle (Day 2 to Day 4); 3) levels of all glycohydrolase activities (acidic alpha-D-mannosidase, beta-D-galactosidase, beta-D-glucuronidase, beta-D-glucosaminidase, and alpha-L-fucosidase) and protein in the uterine and oviductal fluids did not vary widely during the 4 days of the cycle. These results demonstrate a temporal surge of glycosyltransferase activities in the genital tract fluids of the hamster. The temporal changes in the glycoprotein-modifying enzymes may have an effect on the glycosylation of sperm plasma membrane and zona pellucida glycoproteins at the site of fertilization or may alter the surface glycoproteins of the fertilized egg in the uterus prior to implantation.
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PMID:Temporal surge of glycosyltransferase activities in the genital tract of the hamster during the estrous cycle. 872 23

The expression of CMP-NeuAc: Gal beta 1,4GlcNAc alpha 2,6 sialyltransferase (alpha 2,6-ST) [EC 2.4.99.1] and glycoproteins bearing alpha 2,6-linked sialic acids were examined in primary human brain tumours and cell lines. 79% (19/24) of the meningiomas expressed alpha 2,6-ST mRNA, 42% (10/24) of which showed very high expression. alpha 2,6-ST mRNA expression was undetectable in normal brain tissue. In contrast, only 1/13 of the gliomas examined expressed detectable alpha 2,6-ST mRNA. Metastases to the brain did not express measurable amounts of alpha 2,6-ST mRNA. Less expression was found in malignant (i.e. anaplastic) compared to benign (i.e. meningothelial) meningiomas. Two-dimensional SDS-PAGE of glioma and meningioma proteins, followed by Sambucus nigra lectin staining, revealed the presence of a glycoprotein bearing alpha 2,6-linked sialic acids, M(r) = 53 kDa and a pI = 7.0 (MEN-1) that appeared in all seven of the meningiomas examined, but was expressed at barely detectable levels, if at all, in seven out of the seven glioblastomas examined. Thus, decreased alpha 2,6-ST expression may play a role in the aggressive nature of anaplastic meningiomas, but appears to be virtually absent in all tumours of glial origin.
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PMID:The expression of CMP-NeuAc: Gal beta 1,4GlcNAc alpha 2,6 sialyltransferase [EC 2.4.99.1] and glycoproteins bearing alpha 2,6-linked sialic acids in human brain tumours. 874 63

When treated with retinoic acid in vivo, C6 glioma cells show an enhancement of CMP-Neu5Ac:Gal beta 1-3 GalNAc-R alpha-2,3 sialyltransferase activity. A 300 kDa glycoprotein was detected by lectin affinoblotting in retinoic acid-treated C6 cells which stained weakly or not at all in control cells. Comparative studies with different lectins demonstrated that this glycoprotein contains alpha 2,3 Neu5Ac Gal-GalNAc O-glycan moieties. Cultures in the presence of an inhibitor of O-glycan synthesis (N-acetylgalactosaminide alpha-O-benzyl) demonstrated that enhancement of staining of the 300 kDa glycoprotein was not due to the increase of the alpha 2,3 sialytransferase but to the de novo synthesis of the polypeptide chain of this glycoprotein.
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PMID:Study of O-sialylation of glycoproteins in C6 glioma cells treated with retinoic acid. 878 91

CMP-NeuAc: Gal beta 1,4GlcNAc alpha 2,6 sialyltransferase (alpha 2,6-ST) [EC 2.4.99.1] is developmentally regulated, shows a high degree of tissue specificity, and appears to play a role in oncogenic transformation and metastasis. In the present study, we have performed the first detailed analysis of the expression of alpha 2,6-ST and alpha 2,6-linked sialoglycoconjugates in human brain tumors. We used a polyclonal, monospecific anti-rat alpha 2,6-ST antibody and the alpha 2,6-linked sialic acid-specific lectin, Sambucus nigra agglutinin (SNA) for histochemical studies, and a human alpha 2,6-ST-specific cDNA probe for Northern analysis. Meningiomas, chordomas and craniopharyngiomas frequently expressed alpha 2,6-ST and alpha 2,6-linked sialoglycoconjugates. Among the different meningioma subtypes, meningothelial meningiomas stained more strongly with both anti-alpha 2,6-ST antibody and SNA than the fibroblastic and anaplastic meningiomas. On the other hand, all tumors of glial origin and medulloblastomas were virtually devoid of either alpha 2,6-ST or alpha 2,6-linked sialoglycoconjugate expression. Moreover, very weak to negligible expression of both alpha 2,6-ST and alpha 2,6-linked sialoglycoconjugates was observed in brain metastases. In conclusion, alpha 2,6-ST and alpha 2,6-linked sialoglycoconjugate expression is associated with non-neuroectodermal epithelial-like tumors.
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PMID:The expression of Gal beta 1,4GlcNAc alpha 2,6 sialyltransferase and alpha 2,6-linked sialoglycoconjugates in human brain tumors. 883 41

The binding sites of the antibody to cytosolic sialidase on the rat and monkey photoreceptor cells were examined immunohistochemically using the avidin-biotinylated peroxidase method. In the rat photoreceptor cells, the antibody bound diffusely to the inner segment and the outer nuclear layers which are composed chiefly of rod cells. In the monkey photoreceptor cells, the antibody bound to the rod inner segments which were clearly distinct, morphologically, from the cone inner segments. The antibody also bound to the rod cell bodies in the outer nuclear layer. These binding patterns show that the antibody bound preferentially to rod photoreceptor cells. This observation is consistent with previous lectin histochemical findings that sialoglycans are preferentially present on the surfaces of rod photoreceptors, and in the rod-associated interphotoreceptor matrix. Sialidase in rod inner segments may function by balancing with sialyltransferase, also preferentially expressed in rod inner segments, to form sialyl residues on the termini of sugar chains in the rod-associated sialoglycoconjugates.
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PMID:Immunohistochemical localization of cytosolic sialidase in photoreceptor cells. 887 86

In human colon carcinoma, increased amounts of sialic acids have been found and correlated with tumor progression. Further, the degree of O-acetylation of sialic acid residues in normal mucosa is higher than in colon carcinoma. Thus, tumor-associated sialylated antigens may be constitutively expressed in O-acetylated form in normal mucosa unreactive with the respective monoclonal antibodies. We have earlier demonstrated a colon carcinoma-associated expression of alpha 2,6-linked sialic acid residues with the Sambucus nigra agglutinin (SNA). We report now that de-acetylation of normal and transitional colonic mucosa, in contrast to sialyl-Tn antigen, does not result in SNA binding. Further, the alpha 2,6-linked sialic acid recognized by SNA is distinct from that of sialyl-Tn antigen. This is confirmed by Northern blotting detecting transcripts for alpha 2,6 sialyltransferase of N-glycoproteins and measurement of activity for this sialyltransferase. Blot analysis by SNA of colon carcinoma cells revealed few reactive glycoproteins. Quantitative differences in lectin labeling and sialyltransferase activity were found in HCT116 colon carcinoma cell sub-lines. Our data suggest that SNA binding in human colon carcinoma is due to de novo expression of a specific sialic acid present on selected glycoproteins.
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PMID:Colon carcinoma glycoproteins carrying alpha 2,6-linked sialic acid reactive with Sambucus nigra agglutinin are not constitutively expressed in normal human colon mucosa and are distinct from sialyl-Tn antigen. 905 58


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