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Inhibition of Mucin-Type O-Glycosylation through …
The GalNAc -Ser/Thr linkage has been considered a hallmark of mucins where it occurs in clusters. However, this linkage has also been found in a wide variety of other proteins (). No primary amino acid consensus sequence has emerged for mucin-type O-linked glycosylation. In general, glycosylation of Thr is preferred over Ser () and the linkage is found in clusters of Ser/Thr residues with a turn near Pro and at a distance from charged amino acids ().
The genome contains sequences similar to a large number of mammalian genes implicated in the assembly, processing and modification of a variety of glycans (; see ). Although only a limited number of these genes have been functionally characterized, the degree of homology suggests that possesses a repertoire of enzymes for the synthesis of a variety of complex carbohydrates. The occurrence of N-glycans [reviewed in ], GalNAc-O-Ser/Thr glycans (), glycosaminoglycans (; ), glycolipids (; ) and chitin () has been verified in in structural or biochemical studies. Although N- and O-glycans have common features with vertebrate glycans in terms of their core glycan biosynthesis, their terminal structures show significant differences. Among them, glycans lack sialic acid () and contain unusual fucose additions, O-methylated fucose and mannose and phosphorylcholine substitutions (; ). In recent years, spectacular progress has been made in developing and refining tools to define the structures of glycans. Our understanding of the role of glycoconjugates in the development of to date is largely due to the identification of mutations in genes involved in their biosynthesis.
Mucin-type O-glycans form one of the most abundant and ..
Proteoglycans consist of a core protein and one or more covalently attached glycosaminoglycan (GAG) chains. GAG chains are attached to serine residues in core proteins by a common “tetrasaccharide linkage” region. Structural studies have confirmed that synthesizes this ubiquitous linker region (). GAGs are linear polysaccharides, composed of repeating disaccharide units consisting of an amino sugar (GlcNAc or GalNAc) and an uronic acid (GlcA and IdoA; see ). Different classes of GAGs are defined by the composition of the disaccharides: heparan sulfate (HS) has a repeating disaccharide of GlcNAc-1,4-GlcA-1,4, whereas chondroitin and chondroitin sulfate (CS) have GalNAc-1,4-GlcA1,3. Monosaccharides within the GAG polymers can be modified in several ways: GlcNAc N-deacetylation and N-sulfation, uronic acid epimerization and O-sulfation at different positions. These modifications result in an enormous molecular complexity that increases the capacity of GAGs to interact with proteins through varied arrangements of sulfated sugar residues (). produces heparan sulfate containing all modifications previously described in other organisms (; ). In contrast, chondroitin appears not to be secondarily modified in ().
Figure 2. Model of the function of the SQV proteins in the biosynthesis of heparan and chondroitin chains. (UDP-glucose dehydrogenase) synthesizes UDP-glucuronic acid () and catalyzes the synthesis of UDP-xylose by decarboxylation of UDP-glucuronic acid (). transports UDP-glucuronic acid, UDP-galactose and UDP-N-acetylgalactosamine into the Golgi lumen (). and encode the xylosyltransferase, galactosyltransferase I, galactosyltransferase II and glucuronyltransferase I, respectively, involved in the synthesis of the common “core tetrasaccharide” linkage region (; ; ). is required for the polymerization of chondroitin chains (; ). Reprinted by permission from Macmillan Publishers Ltd: Nature 423, 439–443, copyright (2003).
Synthesis of mucin-type O -glycan probes as …
Defects in heparan sulfate synthesis or specific modifications disrupt neuroblast migrations. C-5 epimerized and 6-O-sulfated heparan sulfate chains attached to the cell surface proteoglycans syndecan () and glypican () are required for the activity of , the anosmin ortholog, in embryonic neuroblast migration (). These analyses underscore the importance of molecular diversity encoded by HS chains and argue that each individual modification provides information required for axon patterning and cell migration.
Metabolism1.0 Global and overview maps1.1 Carbohydrate metabolism1.2 Energy metabolism1.3 Lipid metabolism1.4 Nucleotide metabolism1.5 Amino acid metabolism1.6 Metabolism of other amino acids1.7 Glycan biosynthesis and metabolism1.8 Metabolism of cofactors and vitamins1.9 Metabolism of terpenoids and polyketides1.10 Biosynthesis of other secondary metabolites1.11 Xenobiotics biodegradation and metabolism1.12 Chemical structure transformation maps
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Synthesis of mucin-type O-glycan probes as aminopropyl glycosides
In individual studies, some proteins have been demonstrated to contain N-linked oligosaccharides. , a member of the /Notch family of receptor proteins essential for distal tip cell (DTC) control of germline proliferation () is a glycoprotein ().
Identification of Galectin-3 and Mucin-Type O-Glycans …
Recently developed methods have allowed the identification of 304 proteins containing N-glycans and the simultaneous determination of the glycosylation sites (; ; ). These studies employed lectin affinity chromatography to isolate glycopeptides generated by tryptic digestion of protein fractions, followed by a variety of mass spectrometry approaches. All but four of the N-glycosylation sites identified correspond to the conventional Asn-X-Ser/Thr. Three peptides contain Asn-X-Cys (; ). The identified N-glycoproteins comprise soluble and hydrophobic proteins, and many of them are extracellular matrix components which have been implicated in cell adhesion or are components of basement membranes. Among them, , (the two integrin subunits), (the integrin subunit), , , , (the four laminin subunit gene products), (nidogen) and (dystroglycan) contain at least one N-linked glycan [see ; for complete lists].
Identification of Galectin-3 and Mucin-Type O-Glycans in ..
The rather strict consensus peptide sequence Asn-X-Ser/Thr for N-glycosylation has been supported by numerous structural, mutagenic and approaches (). Although this sequence occurs frequently in proteins, N-linked glycosylation does not occur at every potential glycosylation site, most probably due to conformational factors (). Thus, it is not possible to predict with certainty if a protein is in fact N-glycosylated. Microheterogeneity in N-glycan chains can also occur among different molecules of the same protein ().
Type O-Glycans in Breast Cancer and Its Metastasis ..
The large amounts of O-glycans purified in this study suggest that expresses an abundance of highly O-glycosylated proteins. Studies on the parasitic nematode have shown that it secretes a family of highly O-glycosylated mucin type glycoproteins (; ; ). In , encodes a mucin-type glycoprotein (). Mutations in 3 result in larval death and vacuole formation anterior to the pharyngeal bulb, suggesting malfunction or deformation of the excretory/secretory apparatus. Biochemical characterization of a L1-specific surface antigen suggested that it contains O-linked carbohydrates (). This antigen is not detected on the surface of L1 animals carrying mutations in the gene, which encodes a Golgi UDP-Gal/UDP-GlcNAc transporter implicated in glycosylation of surface components (). Loss of activity affects both N-linked and O-linked glycosylation ().
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