Membrane Glycoproteins

Synthesis, Processing, and Function of N-Glycans in N-Glycoproteins

Many membrane-resident and secreted proteins, including growth factors and their receptors are N-glycosylated. The initial N-glycan structure is synthesized in the endoplasmic reticulum (ER) as a branched structure on a lipid anchor (dolicholpyrophosphate) and then co-translationally, "en bloc" transferred and linked via N-acetylglucosamine to asparagine within a specific N-glycosylation acceptor sequence of the nascent recipient protein. In the ER and then the Golgi apparatus, the N-linked glycan structure is modified by hydrolytic removal of sugar residues ("trimming") followed by re-glycosylation with additional sugar residues ("processing") such as galactose, fucose or sialic acid to form complex N-glycoproteins. While the sequence of the reactions leading to biosynthesis, "en bloc" transfer and processing of N-glycans is well investigated, it is still not completely understood how N-glycans affect the biological fate and function of N-glycoproteins. This review will discuss the biology of N-glycoprotein synthesis, processing and function with specific reference to the physiology and pathophysiology of the immune and nervous system, as well as infectious diseases such as Covid-19.

Protein C-Mannosylation and C-Mannosyl Tryptophan in Chemical Biology and Medicine

C-Mannosylation is a post-translational modification of proteins in the endoplasmic reticulum. Monomeric α-mannose is attached to specific Trp residues at the first Trp in the Trp-x-x-Trp/Cys (W-x-x-W/C) motif of substrate proteins, by the action of C-mannosyltransferases, DPY19-related gene products. The acceptor substrate proteins are included in the thrombospondin type I repeat (TSR) superfamily, cytokine receptor type I family, and others. Previous studies demonstrated that C-mannosylation plays critical roles in the folding, sorting, and/or secretion of substrate proteins. A C-mannosylation-defective gene mutation was identified in humans as the disease-associated variant affecting a C-mannosylation motif of W-x-x-W of ADAMTSL1, which suggests the involvement of defects in protein C-mannosylation in human diseases such as developmental glaucoma, myopia, and/or retinal defects. On the other hand, monomeric C-mannosyl Trp (C-Man-Trp), a deduced degradation product of C-mannosylated proteins, occurs in cells and extracellular fluids. Several studies showed that the level of C-Man-Trp is upregulated in blood of patients with renal dysfunction, suggesting that the metabolism of C-Man-Trp may be involved in human kidney diseases. Together, protein C-mannosylation is considered to play important roles in the biosynthesis and functions of substrate proteins, and the altered regulation of protein C-manosylation may be involved in the pathophysiology of human diseases. In this review, we consider the biochemical and biomedical knowledge of protein C-mannosylation and C-Man-Trp, and introduce recent studies concerning their significance in biology and medicine.

Synthesis, Processing, and Function of N-glycans in N-glycoproteins

Many membrane-resident and secrected proteins, including growth factors and their receptors, are N-glycosylated. The initial N-glycan structure is synthesized in the endoplasmic reticulum (ER) as a branched structure on a lipid anchor (dolichol pyrophosphate) and then co-translationally, "en bloc" transferred and linked via N-acetylglucosamine to asparagine within a specific N-glycosylation acceptor sequence of the nascent recipient protein. In the ER and then the Golgi apparatus, the N-linked glycan structure is modified by hydrolytic removal of sugar residues ("trimming") followed by re-glycosylation with additional sugar residues ("processing") such as galactose, fucose, or sialic acid to form complex N-glycoproteins. While the sequence of the reactions leading to biosynthesis, "en bloc" transfer and processing of N-glycans is well investigated, it is still not completely understood how N-glycans affect the biological fate and function of N-glycoproteins. This review discusses the biology of N-glycoprotein synthesis, processing, and function with specific reference to the physiology and pathophysiology of the nervous system.

Inhibitors of glycosyl-phosphatidylinositol anchor biosynthesis

Glycosyl-phosphatidylinositol (GPI) is a complex glycolipid structure that acts as a membrane anchor for many cell-surface proteins of eukaryotes. GPI-anchored proteins are particularly abundant in protozoa such as Trypanosoma brucei, Leishmania major, Plasmodium falciparum and Toxoplasma gondii, and represent the major carbohydrate modification of many cell-surface parasite proteins. Although the GPI core glycan is conserved in all organisms, many differences in additional modifications to GPI structures and biosynthetic pathways have been reported. Therefore, the characteristics of GPI biosynthesis are currently being explored for the development of parasite-specific inhibitors. In vitro and in vivo studies using sugars and substrate analogues as well as natural compounds have shown that it is possible to interfere with GPI biosynthesis at different steps in a species-specific manner. Here we review the recent and promising progress in the field of GPI inhibition.

The accumulation of Man(6)GlcNAc(2)-PP-dolichol in the Saccharomyces cerevisiae Deltaalg9 mutant reveals a regulatory role for the Alg3p alpha1,3-Man middle-arm addition in downstream oligosaccharide-lipid and glycoprotein glycan processing

N-Glycans in nearly all eukaryotes are derived by transfer of a precursor Glc(3)Man(9)GlcNAc(2) from dolichol (Dol) to consensus Asn residues in nascent proteins in the endoplasmic reticulum. The Saccharomyces cerevisiae alg (asparagine-linked glycosylation) mutants fail to synthesize oligosaccharide-lipid properly, and the alg9 mutant, accumulates Man(6)GlcNAc(2)-PP-Dol. High-field (1)H NMR and methylation analyses of Man(6)GlcNAc(2) released with peptide-N-glycosidase F from invertase secreted by Deltaalg9 yeast showed its structure to be Manalpha1,2Manalpha1,2Manalpha1, 3(Manalpha1,3Manalpha1,6)-Manbeta1,4GlcNAcbeta1, 4GlcNAcalpha/beta, confirming the addition of the alpha1,3-linked Man to Man(5)GlcNAc(2)-PP-Dol prior to the addition of the final upper-arm alpha1,6-linked Man. This Man(6)GlcNAc(2) is the endoglycosidase H-sensitive product of the Alg3p step. The Deltaalg9 Hex(7-10)GlcNAc(2) elongation intermediates were released from invertase and similarly analyzed. When compared with alg3 sec18 and wild-type core mannans, Deltaalg9 N-glycans reveal a regulatory role for the Alg3p-dependent alpha1,3-linked Man in subsequent oligosaccharide-lipid and glycoprotein glycan maturation. The presence of this Man appears to provide structural information potentiating the downstream action of the endoplasmic reticulum glucosyltransferases Alg6p, Alg8p and Alg10p, glucosidases Gls1p and Gls2p, and the Golgi Och1p outerchain alpha1,6-Man branch-initiating mannosyltransferase.

Protein C-mannosylation is enzyme-catalysed and uses dolichyl-phosphate-mannose as a precursor

C-mannosylation of Trp-7 in human ribonuclease 2 (RNase 2) is a novel kind of protein glycosylation that differs fundamentally from N- and O-glycosylation in the protein-sugar linkage. Previously, we established that the specificity determinant of the acceptor substrate (RNase 2) consists of the sequence -x-x-W, where the first Trp becomes C-mannosylated. Here we investigated the reaction with respect to the mannosyl donor and the involvement of a glycosyltransferase. C-mannosylation of Trp-7 was reduced 10-fold in CHO (Chinese hamster ovary) Lec15 cells, which are deficient in dolichyl-phosphate-mannose (Dol-P-Man) synthase activity, compared with wild-type cells. This was not a result of a decrease in C-mannosyltransferase activity. Rat liver microsomes were used to C-mannosylate the N-terminal dodecapeptide from RNase 2 in vitro, with Dol-P-Man as the donor. This microsomal transferase activity was destroyed by heat and protease treatment, and displayed the same acceptor substrate specificity as the in vivo reaction studied previously. The C-C linkage between the indole and the mannosyl moiety was demonstrated by tandem electrospray mass spectrometry analysis of the product. GDP-Man, in the presence of Dol-P, functioned as a precursor in vitro with membranes from wild-type but not CHO Lec15 cells. In contrast, with Dol-P-Man both membrane preparations were equally active. It is concluded that a microsomal transferase catalyses C-mannosylation of Trp-7, and that the minimal biosynthetic pathway can be defined as: Man -> -> GDP-Man -> Dol-P-Man -> (C2-Man-)Trp.

The VRG4 gene is required for GDP-mannose transport into the lumen of the Golgi in the yeast, Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, glycoproteins and sphingolipids are modified in the Golgi by the addition of mannose residues. The critical mannosyl donor for these reactions is the nucleotide sugar, GDP-mannose, whose transport into the Golgi from the cytoplasm is required for mannosylation. This transport reaction has been well characterized, but the nucleotide sugar transporter has yet to be identified in yeast. VRG4 is an essential gene whose product is required for a number of Golgi-specific functions, including glycosylation and the organization of the endomembrane system. Here, data are presented that demonstrate that the primary role of Vrg4p is in the transport of GDP-mannose into the Golgi. The vrg4 mutation causes a general impairment in mannosylation, affecting N-linked and O-linked glycoprotein modifications as well as the mannosylation of sphingolipids. By using an in vitro assay, vrg4 mutants were shown to be specifically defective in the transport of GDP-mannose into Golgi vesicles. The Vrg4 protein localizes to the Golgi complex in a pattern that suggests a wide distribution throughout the Golgi. Vrg4p displays homology to other putative nucleotide sugar transporters, suggesting that the VRG4 gene encodes a Golgi GDP-mannose transporter. As Vrg4p is essential, these results suggest that a complete lack of mannosylation of glycoproteins in the Golgi leads to inviability. Alternatively, the essential function of Vrg4p in yeast involves its effect on sphingolipids, which would imply a critical role for mannosylinositol phosphorylceramides or mannosyl diphosphoinositol ceramides on growth and viability.

Thyrotropin controls dolichol-linked sugar pools and oligosaccharyltransferase activity in thyroid cells

We previously showed that thyroglobulin (Tg) glycosylation is enhanced 1.5-fold under thyrotropin (TSH) stimulation, corresponding to an increased number of oligosaccharide chains per molecule of Tg. Now the steps involving dolichol components and oligosaccharyltransferase activity have been studied. Porcine thyroid cells were cultured on porous bottom filters with or without TSH and incubated with [14C]mevalonate. Under TSH regulation, the level of the whole of dolichol components was increased 1.25-fold without modifying their distribution. Dolichol, and free and monosaccharide-linked dolichyl-phosphate, represented respectively 40% and 45% of total dolichol components while dolichyl-pyrophosphate-oligosaccharide represented 3% only. A marked enhancement (4.2-fold) of oligosaccharyltransferase activity occurred in stimulated cells, which could correspond to the addition of the two TSH effects: stimulation of Tg synthesis (3-fold) and of Tg glycosylation (1.5-fold). The amount of lipid carriers appeared to be insufficiently increased but no component is a limiting step, suggesting that the turnover of dolichol derivatives may be increased under TSH control through their use by higher amounts of Tg.

N-linked glycoprotein biosynthesis in the developing mouse embryo

We have developed microenzymic assays that have, for the first time, enabled analysis of several enzymes in the pathway for N-linked glycoprotein biosynthesis in pre- and peri-implantation mouse embryos. The in vitro activities of the glycosyl transferases responsible for the formation of N-acetylglucosaminylpyrophosphoryldolichol,N, N'-diacetyl-chitobiosylpyrophosphoryldolichol, mannosylphosphoryldolichol, and glucosylphosphoryldolichol were found to decrease after fertilization before increasing significantly at the blastocyst stage, a stage that was also found to be highly sensitive to the glycosylation inhibitor, tunicamycin. The observed elevation in the activities of these enzymes in blastocysts still occurred when ebbryos were cultured in alpha-amanitin, indicating that de novo mRNA synthesis is unnecessary for the observed increase in their activities. Thus, an elevated capacity for N-glycosylation exists at the blastocyst stage, a time when dramatic increases in cell-cell interactions are known to occur.

Regulation of dolichyl phosphate-mediated protein glycosylation: estrogen effects on glucosyl transfers in oviduct membranes

Regulation of Glc transfer from UDP-Glc via Glc-P-Dolichol to form Glc3-Man9-oligosaccharide-lipid has been studied during estrogen-induced chick oviduct differentiation. The process was studied as two distinct reactions: transfer of Glc from UDP-Glc to Dol-P, forming Glc-P-Dol; and transfer of Glc from Glc-P-Dol to Man9-OL (oligosaccharide-lipid), forming a series of glucosylated oligosaccharide-lipids. Kinetic analysis of [14C]Glc transfer from UDP-[14C]Glc to endogenous Dol-P shows that Dol-P is limiting in membrane preparations and that, concomitant with estrogen-induced differentiation, there is an increase in Dol-P available for Glc transfers. There is also greater glucosyl transferase activity present in membranes from mature hens and estrogenized chicks than in membranes from immature chicks. In order to study the second phase of glucosylation, transfer to the oligosaccharide, it was necessary to develop an assay in which membranes could be reacted with exogenously added substrates, [14C]Glc-P-Dol and [3H]Man9-OL. This reaction is dependent on detergent (0.02% NP-40 was used) and is stimulated by EDTA. The apparent Km for Glc-P-Dol was about 1.5 microM. A series of double-labeled oligosaccharides having sizes consistent with Glc1-, Glc2-, and Glc3-Man9-OL were formed. Chemical and enzymatic analysis of [14C]Glc oligosaccharides formed by incubation with the exogenous substrates, or by incubation with UDP-[14C]Glc and endogenous acceptors, indicated that the glucosylated oligosaccharides were similar. Assays of membranes from estrogenized chicks, mature hens, and hormone-withdrawn chicks showed increased glucosyl transferase activity upon hormone treatment. Similar assays in the absence of exogenous Man9-OL indicated that hormone treatment was also accompanied by increased levels of endogenous oligosaccharide-lipid acceptors.

Inhibition of synthesis of herpesvirus (HSV-1) glycoproteins and endogenous fusion by beta-hydroxynorvaline in BHK-21 cells

Treatment of HSV-infected BHK-21 cells with 5-10 mM of beta-hydroxynorvaline (Hnv), an analog of threonine which blocked attachment of oligosaccharides at the Asn-X-Thr sites, markedly inhibited the synthesis of all viral glycoproteins as well as the major capsid protein. However, the synthesis of host-specific dolichol-linked oligosaccharides was not significantly affected by Hnv. Treatment of cells with 10 mM reduced the yield of virus greater than 95% and completely blocked endogenous fusion. Inhibition of Hnv could be reversed by simultaneous addition of threonine to the culture medium. It is likely that the incorporation of Hnv into HSV polypeptides at Asn-X-Thr (in place of Thr) sites blocked transfer of N-linked oligosaccharides.

Glucose requirement for induction by sodium butyrate of the glycoprotein hormone alpha subunit in HeLa cells

Butyric acid produces multiple effects on mammalian cells in culture, including alterations in morphology, depression of growth rate, increased histone acetylation, and modified production of various proteins and enzymes. The latter effect is exemplified by the induction in HeLa cells of the glycoprotein hormone alpha subunit by millimolar concentrations of the fatty acid. This report demonstrates that increased subunit accumulation in response to sodium butyrate is strikingly dependent on the presence of glucose (or mannose) in the growth medium. In contrast, basal levels of subunit synthesis are only marginally affected when the culture medium is supplemented with one of a variety of hexoses. An increase in the accumulation of HeLa alpha does not occur in medium containing pyruvate as the energy source, and sustained induction requires the simultaneous and continued presence of both glucose and butyrate. The effects of butyrate on HeLa cell morphology and subunit induction can be separated, since the latter is glucose-dependent while the former is not. Failure of butyrate to induce alpha in medium containing pyruvate does not result from restricted subunit secretion, since the levels of intracellular alpha are not increased disproportionately relative to those in the medium. The hexoses which support induction of HeLa alpha (glucose greater than or equal to mannose greater than galactose greater than fructose) are identical to those which have been shown previously to stimulate the glucosylation of lipid-linked oligosaccharides and enhance the synthesis of certain glycoproteins. Labeling of various glycosylation intermediates with [3H]mannose indicates that in glucose medium there is a decrease in the level of radioactivity associated with both dolicholpyrophosphoryl oligosaccharide and cellular glycoproteins and a concomitant increase in the fraction of label recovered in secreted glycoproteins. Butyrate also causes a decrease in [3H]mannose-labeled cellular glycoproteins and an increase in tritiated extracellular glycoproteins, particularly in glucose medium. Likewise, glucose stimulates the incorporation of [3H]glucosamine into immunoprecipitable alpha subunit relative to the bulk of HeLa-secreted glycoproteins, and this is further enhanced by butyrate. However, as demonstrated by lectin chromatography of conditioned media, a nonglycosylated subunit does not accumulate in pyruvate medium, either in the absence or presence of butyrate.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of solubilized brain fucosyltransferase activity by phospholipids

Phospholipids interact on Triton X-100 solubilized GDP-fucose: asialofetuin fucosyltransferase (EC 2.4.1.68) isolated from sheep brain. This enzymatic activity is modulated by charged phospholipids. In particular, phosphatidic acid and analogues markedly inhibit the transfer of fucose from GDP-[14C]fucose. Kinetic studies show that phosphatidic acid interacts as a mixed inhibitor: the velocity and affinity of fucosyltransferase for the GDP-fucose and asialofetuin substrates are strongly decreased. However, this inhibitory effect is not related to stereospecificity, and the different parameters involved in the enzymatic reaction of glycosylation are not modified. The nature of fatty acids and chemical bond (ester or ether) occurring in the carbohydrate chain does not modify the behaviour of phosphatidic acid with respect to fucosyltransferase activity. Further, the physical state of phosphatidic acid (gel phase or liquid crystalline phase) has no influence. However, as the inhibition is closely pH-dependent, these data suggest that phosphatidic acid might directly interact with the active site of the enzyme and induce a conformational change.

Relationship between oligosaccharide-lipid synthesis and protein synthesis in mouse LM cells

Previous studies from several laboratories have reported that inhibition of protein synthesis results in a concomitant reduction in synthesis of oligosaccharide-diphosphoryldolichol. We have investigated this phenomenon in LM cells grown in defined culture medium. The results of this study indicate that incubation of LM cell with cycloheximide at a concentration sufficient to totally arrest polypeptide synthesis, results in a rapid reduction in the synthesis of [3H]mannose-labeled or [3H]glucosamine-labeled oligosaccharide-lipid within 2 min. Cycloheximide treatment had only a slight inhibitory effect on synthesis of GDP-Man. Furthermore, during the first 5 min after the addition of cycloheximide to LM cells, when [3H]mannose incorporation into oligosaccharide-lipid was maximally reduced, the specific activity of the GDP-Man pool was identical to that observed in control cells. Labeling in vivo of the lipid-linked saccharide precursors of oligosaccharide-lipid revealed that cycloheximide had virtually no effect on the synthesis of Man-P-Dol, GlcNAc-PP-Dol, GlcNAc2-PP-Dol, and beta-Man-(GlcNAc)2-PP-Dol (Dol = dolichol). In agreement with this finding, results of experiments in vitro using microsomes prepared from LM cells indicate that cycloheximide did not directly inhibit the enzymes responsible for the synthesis of these lipid-linked saccharide precursors. Supplementation of mouse LM cells by preincubation for 1 h with dolichyl phosphate (5 micrograms/ml) resulted in a 300% stimulation of oligosaccharide-lipid synthesis when compared to non-supplemented cells. However, dolichyl phosphate supplementation of cycloheximide-treated cells failed to restore oligosaccharide-lipid synthesis to the level observed in control cells. In control cells pre-labeled oligosaccharide-lipid turned over rapidly and, as expected, cycloheximide addition to the chase medium significantly retarded this turnover process. However, preincubation of cells with exogenous dolichyl phosphate had little or no effect on the turnover of oligosaccharide-lipid in control or cycloheximide-treated cells. These findings argue against dolichyl phosphate deficiency as the primary cause of reduced oligosaccharide-lipid synthesis when protein synthesis is blocked. The implications of these results, and an alternative hypothesis to explain the effect of inhibition of protein synthesis on oligosaccharide-lipid synthesis based on elevation of intracellular GTP levels, are discussed.

The biosynthesis of crustacean chitin. Isolation and characterization of polyprenol-linked intermediates from brine shrimp microsomes

The biosynthesis of crustacean chitin appears to involve the participation of a lipid-linked intermediate. A microsomal preparation from larval stages of the brine shrimp Artemia salina was found to catalyze the glycosylation of exogenous [3H]dolichol phosphate, yielding a product which was insoluble in chloroform:methanol (2:1) but soluble in chloroform:methanol:water (10:10:3). Artemia microsomes catalyze the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine to a lipid acceptor. After extraction of labeled lipids with either chloroform:methanol (2:1) or chloroform:methanol:water (10:10:3), labeled compounds could be purified by ion-exchange chromatography on DEAE-Sephacel. Mild acid hydrolysis of 3H-N-acetylglucosamine labeled material soluble in chloroform:methanol:water (10:10:3) yielded a series of oligosaccharides ranging from 2 to about 8 glycosyl units in size. The larger components were shown to be sensitive to chitinase digestion but resistant to treatment with alpha-mannosidase. Such 3H-N-acetylglucosamine containing compounds, prepared by both in vivo and in vitro procedures, appear to be chitin oligosaccharides. Brine shrimp microsomes also catalyze the transfer of mannose from GDP-mannose to a lipid acceptor. Mild acid hydrolysis of mannosyl lipids soluble in chloroform:methanol:water (10:10:3) yielded oligosaccharides which were sensitive to alpha-mannosidase digestion and resistant to treatment with endochitinase. The results suggest 3H-N-acetylglucosamine-labeled oligosaccharide-lipids are distinct from the mannose-labeled fraction and may participate in the formation of an endogenous primer for chitin synthesis after their transfer to a protein acceptor.

Effect of phospholipids on the regulation of a soluble galactosyltransferase in aortic wall

A galactosyltransferase activity is located in the cell-sap of aortic intima-media cells. This enzymatic system calatyzes [14C]galactose transfer from UDP-[14C]galactose into endogenous and exogenous proteinic acceptors. Labelled products are isolated from the proteinic fraction obtained in 20% trichloroacetic acid pellet or from organic solvent extractions. Maximal [14C]galactose incorporation occurs at pH 7.8 in Tris-HCl buffer in the presence of 0.1 mM MnCl2 at 30 degrees C. The enzymatic activity is modified by phospholipids, particularly by phosphatidic acid and lysophosphatidylcholine, which behave as mixed inhibitors, while L-alpha-phosphatidylserine interacts as a competitive inhibitor. The effect of phospholipids is not stereospecific but appeared to be closely related to their polar headgroups, especially the acidic headgroups of phosphatidylcholine and phosphatidic acid. The chain length and the unsaturation degree of fatty acids involved in phospholipid structures are not a main factor of regulation. The lysophosphatidylcholine effect could be explained by its solubilization properties, as non-ionic detergents interact in the same way with galactosyltransferase activity. Exogenous phospholipids probably interact with the enzymatic environment by their own molecular arrangement and so could exert a control on galactosyltransferase activity or lead to a conformation change of this enzyme.

Glycosylation of myelin glycoproteins in peripheral nerve via lipid intermediates

Membrane preparations from chick peripheral nervous system (PNS) catalyzed the transfer of [3H]glucose from UDP-[3H]glucose into glucosylphosphoryl dolichol. The initial rate of glucosylphophoryl dolichol formation in a non-myelin membrane fraction from actively myelinating chick PNS was 11 fold higher than that from adult. Exogenous dolichyl monophosphate stimulated glucosylphosphoryl dolichol synthesis in both fractions. The higher level of glucosylphosphoryl dolichol synthesis corresponded to the onset of myelination in chick PNS. Exogenous dolichyl monophosphate also stimulated the labeling of glucosylated oligosaccharide lipids and glycoproteins in the fraction. On SDS polyacrylamide gel electrophoresis, the relative mobility of the major and minor radioactive glycoprotein corresponded with that of the P0 and PASII glycoprotein in PNS myelin, respectively. The results suggest that myelin glycoproteins in PNS are glycosylated via lipid intermediates.