Role Of Glycans Research Articles

Viral glycosyltransferases

During the last decades, advances in cellular biology highlighted the crucial roles of glycans in numerous important biological processes, raising the concept of glycomics, which is currently considered to be complementary to genomics, transcriptomics and proteomics.Viruses are forced parasites, depending on and interfering with the host cell machinery. Studies of the last decade revealed that viruses have acquired the capability to interfere with the glycome at their own benefit. The study of the glycans resulting from viral infection and their functions is called « Glycovirology ». One of the most fascinating aspects of glycovirology is the study of how viruses affect the glycome. Viruses reach that goal either by affecting the expression of host glycosyltransferases, or by expressing their own glycosyltransferases. Up to now, viral glycosyltransferases have been reported in one herpesvirus and several poxviruses, baculoviruses, phycodnaviruses and bacteriophages. In this review, viral glycosyltransferases will be described exhaustively and their established or putative functions will be discussed. The description of those enzymes illustrates several fundamental aspects of virology.

Apical Secretion and Sialylation of Soluble Dipeptidyl Peptidase IV Are Two Related Events

The role of glycans in the apical targeting of proteins in epithelial cells remains a debated question. We have expressed the mouse soluble dipeptidyl peptidase IV (DPP IV ectodomain) in kidney (MDCK) and in intestinal (Caco-2) epithelial cell lines, as a model to study the role of glycosylation in apical targeting. The mouse DPP IV ectodomain was secreted mainly into the apical medium by MDCK cells. Exposure of MDCK cells to GalNac-α-O-benzyl, a drug previously described as an inhibitor of mucin O-glycosylation, produced a protein with a lower molecular weight. In addition this treatment resulted in a decreased apical secretion and an increased basolateral secretion of mouse DPP IV ectodomain. When expressed in Caco-2 cells, the mouse DPP IV ectodomain was secreted mainly into the basolateral medium. However, BGN was still able to decrease the amount of apically secreted protein and to increase its basolateral secretion. Neuraminidase digestion showed that the most stricking effect of BGN was a blockade of DPP IV sialylation in both MDCK and Caco-2 cells. These results indicate that a specific glycosylation step, namely, sialylation, plays a key role in the control of the apical targeting of a secreted DPP IV both in MDCK and Caco-2 cells.

Upregulation of interleukin-10 and inhibition of alloantigen responses by transferrin and transferrin-derived glycans.

Previous studies have shown that critically timed administration of transferrin (Tf) facilitates induction of immunologic unresponsiveness. Here, we determined in mixed leukocyte culture (MLC) and in concanavalin A (ConA)-driven cultures the effect of exogenous Tf and Tf-derived glycans (Tf-Gly) on lymphocyte proliferation. In cultures of human blood lymphocytes, Tf inhibited selectively alloantigen-driven proliferation in MLC, but not ConA-stimulated lymphocyte proliferation. Deglycosylation of Tf abrogated the inhibitory effect of Tf on alloantigen-induced lymphocyte proliferation, and, consistent with a role for glycans, an effect qualitatively and quantitatively similar to Tf was exerted by purified Tf-Gly. Glycans isolated from other proteins, for example, immunoglobulin G (IgG) or fibrinogen, failed to inhibit alloantigen-induced proliferation selectively. Rather, they suppressed lymphocyte proliferation in a non-specific manner. Determination of cytokines in MLC supernatant showed a downregulation of interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), IL-2, and IL-12 (p40), along with an upregulation of IL-10, a pattern entirely consistent with the observed effects of Tf and Tf-Gly on alloantigen-induced lymphocyte proliferation. The effect of Tf on MLC was directly IL-10-dependent. IL-10 levels were inversely correlated with lymphocyte proliferation and CD86 expression. Neutralization of IL-10 by anti-IL-10 monoclonal antibody (mAb) blocked the effect of Tf. The MLC-modulating effect of Tf (or Tf-Gly) was not dependent upon the Tf receptor CD71 but appeared to be mediated by a Gly-responsive receptor. These data suggest a role of Tf, and, in particular, Tf-Gly, in allo-interactions that is independent from the role of Tf in iron metabolism, and appears to involve co-stimulatory signals.

Structure, Function and Synthesis of Glycoconjugates

coupled cells. Interestingly, electrical events had also been detected by other workers in the very early stages of the interaction between the Nod factor glycolipid and the root hair cell of legumes. The role of glycans in regulatory events was also discussed in a very different context by Professor Jeff Schell (MPI Cologne). He described data to suggest that the activity of the classical plant growth regulators, auxins and cytokinins, was modulated by their conjugation to sugars. Conjugation to a glycan affects both their biological activity and hydrophilic properties. Thus, addition and removal of the sugars by the relevant transferases and hydrolases could thereby represent a new mechanism of regulating the effects of the plant growth regulators at a particular place and time within the plant. The covalent attachment of glycans to a growth regulator was also described by Dr Dan Klessig (Rutgers) in the context of salicylic acid. This has recently been recognized as an important signalling molecule in plants: it is a weak acid able to cross hydrophobic barriers. In a manner similar to the auxins and cytokinins, the combined action of the /3-glucosidase and glucosyl transferase, can be used to regulate levels of free SA in the extraand intracellular compartments of the cell.

Expression of mutant patatin protein in transgenic tobacco plants: role of glycans and intracellular location.

The influence of N-glycosylation and subcellular compartmentation on various characteristics of a vacuolar glycoprotein is described. One member of the patatin gene family was investigated as a model system. Different glycosylation mutants obtained by destroying the consensus site Asn-X-Ser/Thr by oligonucleotide-directed mutagenesis were expressed in leaves of transgenic tobacco plants under the control of a light-inducible promoter. The various patatin glycomutants retained their properties in comparison with the wild-type protein with respect to protein stability, subcellular compartmentation, enzymatic activity, and various physicochemical properties studied showing the N-glycosylation not to be essential for any of these characteristics. To test the importance of the cotranslational transport and the subcellular (vacuolar) location for the properties of the patatin protein, another mutant was constructed in which the signal peptide was deleted, leading to its synthesis and accumulation in the cytosol. Biochemical analysis of this protein in comparison with its vacuolar form again revealed no significant differences with respect to its enzymatic activity or its stability in normal vegetative cells. During seed development, however, the cytoplasmic form was more stable than the vacuolar form, indicating the appearance of proteases specific for the protein bodies of developing seeds.

Expression of Mutant Patatin Protein in Transgenic Tobacco Plants: Role of Glycans and Intracellular Location

The influence of N-glycosylation and subcellular compartmentation on various characteristics of a vacuolar glycoprotein is described. One member of the patatin gene family was investigated as a model system. Different glycosylation mutants obtained by destroying the consensus site Asn-X-Ser/Thr by oligonucleotide-directed mutagenesis were expressed in leaves of transgenic tobacco plants under the control of a light-inducible promoter. The various patatin glycomutants retained their properties in comparison with the wild-type protein with respect to protein stability, subcellular compartmentation, enzymatic activity, and various physicochemical properties studied showing the N-glycosylation not to be essential for any of these characteristics. To test the importance of the cotranslational transport and the subcellular (vacuolar) location for the properties of the patatin protein, another mutant was constructed in which the signal peptide was deleted, leading to its synthesis and accumulation in the cytosol. Biochemical analysis of this protein in comparison with its vacuolar form again revealed no significant differences with respect to its enzymatic activity or its stability in normal vegetative cells. During seed development, however, the cytoplasmic form was more stable than the vacuolar form, indicating the appearance of proteases specific for the protein bodies of developing seeds.

In vitro Mutated Phytohemagglutinin Genes Expressed in Tobacco Seeds: Role of Glycans in Protein Targeting and Stability

In vitro Mutated Phytohemagglutinin Genes Expressed in Tobacco Seeds: Role of Glycans in Protein Targeting and Stability