The effects of the site-directed removal of N-glycosylation sites from beta-1,4-N-acetylgalactosaminyltransferase on its function

N-Glycan on the Non-Consensus N-X-C Glycosylation Site Impacts Activity, Stability, and Localization of the Sda Synthase B4GALNT2

The Sd^a carbohydrate epitope and its biosynthetic B4GALNT2 enzyme are expressed in the healthy colon and down-regulated to variable extents in colon cancer. The human B4GALNT2 gene drives the expression of a long and a short protein isoform (LF-B4GALNT2 and SF-B4GALNT2) sharing identical transmembrane and luminal domains. Both isoforms are trans-Golgi proteins and the LF-B4GALNT2 also localizes to post-Golgi vesicles thanks to its extended cytoplasmic tail. Control mechanisms underpinning Sd^a and B4GALNT2 expression in the gastrointestinal tract are complex and not fully understood. This study reveals the existence of two unusual N-glycosylation sites in B4GALNT2 luminal domain. The first atypical N-X-C site is evolutionarily conserved and occupied by a complex-type N-glycan. We explored the influence of this N-glycan using site-directed mutagenesis and showed that each mutant had a slightly decreased expression level, impaired stability, and reduced enzyme activity. Furthermore, we observed that the mutant SF-B4GALNT2 was partially mislocalized in the endoplasmic reticulum, whereas the mutant LF-B4GALNT2 was still localized in the Golgi and post-Golgi vesicles. Lastly, we showed that the formation of homodimers was drastically impaired in the two mutated isoforms. An AlphaFold2 model of the LF-B4GALNT2 dimer with an N-glycan on each monomer corroborated these findings and suggested that N-glycosylation of each B4GALNT2 isoform controlled their biological activity.

Effect of N-glycosylation on secretion, stability, and biological activity of recombinant human interleukin-3 (hIL-3) in Pichia pastoris

Human interleukin-3 (hIL-3) is a clinically important cytokine used to treat hematological malignancies, bone marrow transplantation, cytopenias, and immunological disorders. The cloning of hIL-3 gene was previously reported by our group, where its expression was optimized under methanol-inducible AOX1 promoter having N-terminal α mating factor signal sequence from Saccharomyces cerevisiae. This study investigated the role of glycosylation pattern on its molecular stability, secretion efficiency, and biological activity using the mutagenesis approach. The two N-linked glycosylation positions at N15th (Asn15) and N70th (Asn70) were sequentially mutated to generate three recombinant hIL-3 variants, i.e., N15A, N70A, and N15/70A. Asparagine at these positions was replaced with non-polar alanine amino acid (Ala, A). The alteration of N-linked glycosylation sites was disadvantageous to its efficient secretion in Pichia pastoris, where a 52.32%, 36.48%, 71.41% lower production was observed in N15A, N70A, and N15/70A mutants, respectively, as compared to native control. The fully glycosylated native hIL-3 protein showed higher thermal stability over its deglycosylated counterparts. The biological activity of native, N15A, N70A, and N15/70A hIL-3 protein was evaluated, where N15/70A mutant showed slightly higher proliferation efficacy than other combinations.

Genetic modifications of metallothionein enhance the tolerance and bioaccumulation of heavy metals in Escherichia coli

Metallothioneins (MTs) are low molecular weight cysteine-rich proteins that bind to metals. Owing to their high cysteine (Cys) content, MTs are effective mediators of heavy metal detoxification. To enhance the heavy metal binding ability of MT from the freshwater crab Sinopotamon henanense (ShMT), sequence-based multiple sequence alignment (MSA) and structure-based molecular docking simulation (MDS) were conducted in order to identify amino acid residues that could be mutated to bolster such metal-binding activity. Site-directed mutagenesis was then used to modify the primary structure of ShMT, and the recombinant proteins were further enhanced using the SUMO fusion expression system to yield SUMO-ShMT1, SUMO-ShMT2, and SUMO-ShMT3 harboring one-, two-, and three- point mutations, respectively. The resultant modified proteins were primarily expressed in a soluble form and exhibited the ability to readily bind to heavy metals. Importantly, these modified proteins exhibited significantly enhanced heavy metal binding capacities, and they improved Cd²⁺, Cu²⁺ and Zn²⁺ tolerance and bioaccumulation in Escherichia coli (E. coli) in a manner dependent upon the number of introduced point mutations (SUMO-ShMT3 > SUMO-ShMT2 > SUMO-ShMT1 > SUMO-ShMT > control). Indeed, E. coli cells harboring the pET28a-SUMO-ShMT3 expression vector exhibited maximal Cd²⁺, Cu²⁺, and Zn²⁺ bioaccumulation that was increased by 1.86 ± 0.02-, 1.71 ± 0.03-, and 2.13 ± 0.02-fold relative to that in E. coli harboring the pET28a-SUMO-ShMT vector. The present study offers a basis for the preparation of genetically engineered bacteria that are better able to bioaccumulate and tolerate heavy metals, thus providing a foundation for biological heavy metal water pollution treatment.

N-glycosylation of the human β1,4-galactosyltransferase 4 is crucial for its activity and Golgi localization

β1,4-galactosyltransferase 4 (B4GalT4) is one of seven B4GalTs that belong to CAZy glycosyltransferase family 7 and transfer galactose to growing sugar moieties of proteins, glycolipids, glycosaminoglycans as well as single sugar for lactose synthesis. Herein, we identify two asparagine-linked glycosylation sites in B4GalT4. We found that mutation of one site (Asn220) had greater impact on enzymatic activity while another (Asn335) on Golgi localization and presence of N-glycans at both sites is required for production of stable and enzymatically active protein and its secretion. Additionally, we confirm B4GalT4 involvement in synthesis of keratan sulfate (KS) by generating A375 B4GalT4 knock-out cell lines that show drastic decrease in the amount of KS proteoglycans and no significant structural changes in N- and O-glycans. We show that KS decrease in A375 cells deficient in B4GalT4 activity can be rescued by overproduction of either partially or fully glycosylated B4GalT4 but not with N-glycan-depleted B4GalT4 version.

N-Glycans on EGF domain-specific O-GlcNAc transferase (EOGT) facilitate EOGT maturation and peripheral endoplasmic reticulum localization

Epidermal growth factor (EGF) domain-specific O-GlcNAc transferase (EOGT) is an endoplasmic reticulum (ER)-resident protein that modifies EGF repeats of Notch receptors and thereby regulates Delta-like ligand-mediated Notch signaling. Several EOGT mutations that may affect putative N-glycosylation consensus sites are recorded in the cancer database, but the presence and function of N-glycans in EOGT have not yet been characterized. Here, we identified N-glycosylation sites in mouse EOGT and elucidated their molecular functions. Three predicted N-glycosylation consensus sequences on EOGT are highly conserved among mammalian species. Within these sites, we found that Asn-263 and Asn-354, but not Asn-493, are modified with N-glycans. Lectin blotting, endoglycosidase H digestion, and MS analysis revealed that both residues are modified with oligomannose N-glycans. Loss of an individual N-glycan on EOGT did not affect its endoplasmic reticulum (ER) localization, enzyme activity, and ability to O-GlcNAcylate Notch1 in HEK293T cells. However, simultaneous substitution of both N-glycosylation sites affected both EOGT maturation and expression levels without an apparent change in enzymatic activity, suggesting that N-glycosylation at a single site is sufficient for EOGT maturation and expression. Accordingly, a decrease in O-GlcNAc stoichiometry was observed in Notch1 co-expressed with an N263Q/N354Q variant compared with WT EOGT. Moreover, the N263Q/N354Q variant exhibited altered subcellular distribution within the ER in HEK293T cells, indicating that N-glycosylation of EOGT is required for its ER localization at the cell periphery. These results suggest critical roles of N-glycans in sustaining O-GlcNAc transferase function both by maintaining EOGT levels and by ensuring its proper subcellular localization in the ER.

How glycosylation affects glycosylation: the role of N-glycans in glycosyltransferase activity

N-glycosylation is one of the most important posttranslational modifications of proteins. It plays important roles in the biogenesis and functions of proteins by influencing their folding, intracellular localization, stability and solubility. N-glycans are synthesized by glycosyltransferases, a complex group of ubiquitous enzymes that occur in most kingdoms of life. A growing body of evidence shows that N-glycans may influence processing and functions of glycosyltransferases, including their secretion, stability and substrate/acceptor affinity. Changes in these properties may have a profound impact on glycosyltransferase activity. Indeed, some glycosyltransferases have to be glycosylated themselves for full activity. N-glycans and glycosyltransferases play roles in the pathogenesis of many diseases (including cancers), so studies on glycosyltransferases may contribute to the development of new therapy methods and novel glycoengineered enzymes with improved properties. In this review, we focus on the role of N-glycosylation in the activity of glycosyltransferases and attempt to summarize all available data about this phenomenon.

Protein Quality Control in the Endoplasmic Reticulum

The site of protein folding and maturation for the majority of proteins that are secreted, localized to the plasma membrane or targeted to endomembrane compartments is the endoplasmic reticulum (ER). It is essential that proteins targeted to the ER are properly folded in order to carry out their function, as well as maintain protein homeostasis, as accumulation of misfolded proteins could lead to the formation of cytotoxic aggregates. Because protein folding is an error-prone process, the ER contains protein quality control networks that act to optimize proper folding and trafficking of client proteins. If a protein is unable to reach its native state, it is targeted for ER retention and subsequent degradation. The protein quality control networks of the ER that oversee this evaluation or interrogation process that decides the fate of maturing nascent chains is comprised of three general types of families: the classical chaperones, the carbohydrate-dependent system, and the thiol-dependent system. The cooperative action of these families promotes protein quality control and protein homeostasis in the ER. This review will describe the families of the ER protein quality control network and discuss the functions of individual members.

Loss of Enzyme Activity in Mutated B4GALNT1 Gene Products in Patients with Hereditary Spastic Paraplegia Results in Relatively Mild Neurological Disorders: Similarity with Phenotypes of B4galnt1 Knockout Mice

B4GALNT1 is an enzyme essential for the synthesis of complex gangliosides, whose absence leads to progressive neurodegeneration with aging in mice. Recently, eleven cases of hereditary spastic paraplegia with mutation in the coding region of B4GALNT1 were reported. However, changes in the enzymatic activity of their products have never been studied. We have constructed expression vectors for individual mutant cDNAs, and examined their activities by cell-free in vitro enzyme assays, and flow cytometry of cells transfected with their expression vectors. Among them, almost all mutant genes showed the complete loss of B4GALNT1 activity in both the in vitro enzyme assays and flow cytometry. Two mutants exceptionally showed weak activity. One of them, M4, had a mutation at amino acid 228 with a premature termination codon. Interestingly, the intensity of fluorescence of GM2 measured by flow cytometry was equivalent between the WT and M4 mutant, although the positive cell population was relatively small in M4. Western immunoblotting of cell lysates from transfectants with cDNA plasmids revealed 67-kDa bands except those containing premature termination codons or frame-shift mutation. Taken together with the clinical findings of patients, loss of enzyme activity may be responsible for the clinical features of hereditary spastic paraplegia, whereas the intensity of neurological disorders was relatively milder than expected. These clinical features of patients including those with male hypogonadism are very similar to the abnormal phenotypes detected in B4galnt1-deficient mice.

Critical role of evolutionarily conserved glycosylation at Asn211 in the intracellular trafficking and activity of sialyltransferase ST3Gal-II

ST3Gal-II is largely responsible for ganglioside terminal α2,3-sialylation in mammals. We demonstrated that ST3Gal-II mainly distributes in proximal Golgi compartments and that the inhibition of N-glycosylation and oligosaccharide trimming is critical for its enzymatic activity and intracellular distribution.

Improving the thermostability of Escherichia coli phytase, appA, by enhancement of glycosylation

A codon-optimized Escherichia coli appA phytase gene was synthesized and expressed in Pichia pastoris. Two residue substitutions (Q258N, Q349N) were sequentially introduced to enhance its glycosylation activity. Secretion of appA-Q258N/Q349N was approx. 0.3 mg ml(-1) and enzyme activity reached 1,030 U ml(-1). Purified appA-Q258N/Q349N had a specific activity of 3,137 U mg(-1) with an MW of approx. 53 kDa. Compared with appA-WT, appA-Q258N/Q349N showed over 40 % enhancement in thermostability (85 °C for 10 min) and 4-5 °C increases in the melting temperatures (Tm). The Km and Kcat of appA-Q258N/Q349N were 0.43 mM and 3,058 s(-1), respectively, which are similar with that of appA-WT. The mutant appA-Q258N/Q349N obtained in this study could be used for the large-scale commercial production of phytase.

Trans-activity of plasma membrane-associated ganglioside sialyltransferase in mammalian cells

Gangliosides are acidic glycosphingolipids that contain sialic acid residues and are expressed in nearly all vertebrate cells. They are synthesized at the Golgi complex by a combination of glycosyltransferase activities followed by vesicular delivery to the plasma membrane, where they participate in a variety of physiological as well as pathological processes. Recently, a number of enzymes of ganglioside anabolism and catabolism have been shown to be associated with the plasma membrane. In particular, it was observed that CMP-NeuAc:GM3 sialyltransferase (Sial-T2) is able to sialylate GM3 at the plasma membrane (cis-catalytic activity). In this work, we demonstrated that plasma membrane-integrated ecto-Sial-T2 also displays a trans-catalytic activity at the cell surface of epithelial and melanoma cells. By using a highly sensitive enzyme-linked immunosorbent assay combined with confocal fluorescence microscopy, we observed that ecto-Sial-T2 was able to sialylate hydrophobically or covalently immobilized GM3 onto a solid surface. More interestingly, we observed that ecto-Sial-T2 was able to sialylate GM3 exposed on the membrane of neighboring cells by using both the exogenous and endogenous donor substrate (CMP-N-acetylneuraminic acid) available at the extracellular milieu. In addition, the trans-activity of ecto-Sial-T2 was considerably reduced when the expression of the acceptor substrate was inhibited by using a specific inhibitor of biosynthesis of glycolipids, indicating the lipidic nature of the acceptor. Our findings provide the first direct evidence that an ecto-sialyltransferase is able to trans-sialylate substrates exposed in the plasma membrane from mammalian cells, which represents a novel insight into the molecular events that regulate the local glycosphingolipid composition.

Cellular and molecular biology of glycosphingolipid glycosylation

Brain tissue is characterized by its high glycosphingolipid content, particularly those containing sialic acid (gangliosides). As a result of this observation, brain tissue was a focus for studies leading to the characterization of the enzymes participating in ganglioside biosynthesis, and their participation in driving the compositional changes that occur in glycolipid expression during brain development. Later on, this focus shifted to the study of cellular aspects of the synthesis, which lead to the identification of the site of synthesis in the neuronal soma and their axonal transport toward the periphery. In this review article, we will focus in subcellular aspects of the biosynthesis of glycosphingolipid oligosaccharides, particularly the mechanisms underlying the trafficking of glycosphingolipid glycosyltransferases from the endoplasmic reticulum to the Golgi, those that promote their retention in the Golgi and those that participate in their topological organization as part of the complex membrane bound machinery for the synthesis of glycosphingolipids.

N-linked glycosylation of G. mellonella juvenile hormone binding protein - comparison of recombinant mutants expressed in P. pastoris cells with native protein

Juvenile hormone (JH) regulates insect growth and development. JH present in the hemolymph is bound to juvenile hormone binding protein (hJHBP) which protects JH from degradation. In G. mellonella, this protein is glycosylated only at one (Asn(94)) of the two potential N-linked glycosylation sites (Asn(4) and Asn(94)). To investigate the function of glycosylation, each of the two potential glycosylation sites in the rJHBP molecule was examined by site-directed mutagenesis. MS analysis revealed that rJHBP overexpressed in the P. pastoris system may appear in a non-glycosylated as well as in a glycosylated form at both sites. We found that mutation at position Asn(94) reduces the level of protein secretion whereas mutation at the Asn(4) site has no effect on protein secretion. Purified rJHBP and its mutated forms (N4W and N94A) have the same JH binding activities similar to that of hJHBP. However, both mutants devoid of the carbohydrate chain are more susceptible to thermal inactivation. It is concluded that glycosylation of JHBP molecule is important for its thermal stability and secretion although it is not required for JH binding activity.

Biochemical characterization and in vitro digestibility assay of Eupenicillium parvum (BCC17694) phytase expressed in Pichia pastoris

A mature phytase cDNA, encoding 441 amino acids, from Eupenicillium parvum (BCC17694) was cloned into a Pichia pastoris expression vector, pPICZ alpha A, and was successfully expressed as active extracellular glycosylated protein. The recombinant phytase contained the active site RHGXRXP and HD sequence motifs, a large alpha/beta domain and a small alpha-domain that are typical of histidine acid phosphatase. Glycosylation was found to be important for enzyme activity which is most active at 50 degrees C and pH 5.5. The recombinant phytase displayed broad substrate specificity toward p-nitrophenyl phosphate, sodium-, calcium-, and potassium-phytate. The enzyme lost its activity after incubating at 50 degrees C for 5 min and is 50% inhibited by 5mM Cu(2+). However, the enzyme exhibits broad pH stability from 2.5 to 8.0 and is resistant to pepsin. In vitro digestibility test suggested that BCC17694 phytase is at least as effective as another recombinant phytase (r-A170) which is comparable to Natuphos, a commercial phytase, in releasing phosphate from corn-based animal feed, suggesting that BCC17694 phytase is suitable for use as phytase supplement in the animal diet.

Effects of N-glycosylation on the activity and localization of GlcNAc-6-sulfotransferase 1

N-Acetylglucosamine-6-sulfotransferase-1 (GlcNAc6ST-1) is a Golgi-resident glycoprotein that is responsible for sulfation of the l-selectin ligand on endothelial cells. Here, we report the sites at which GlcNAc6ST-1 is modified with N-linked glycans and the effects that each glycan has on enzyme activity, specificity, and localization. We determined that glycans are added at three of four potential N-linked glycosylation sites: N196, N410, and N428. The N428 glycan is required for the production of sulfated cell surface glycans: cells expressing a mutant enzyme lacking this glycan were unable to sulfate the sialyl Lewis X tetrasaccharide or a putative extended core 1 O-linked glycan. The N196 and N410 glycans differentially affect sulfation of two different substrates: cells that express an enzyme lacking the N410 glycan are able to sulfate the sialyl Lewis X substrate, but produce reduced levels of a sulfated peripheral lymph node addressin epitope and cells that express an enzyme lacking the N196 glycan are able to produce a sulfated peripheral lymph node addressin epitope, but are impaired in their ability to sulfate sialyl Lewis X. The glycans' effects on enzyme activity may be mediated, in part, by changes in enzyme localization. While most mutants that lacked glycans localized normally within the Golgi, the N428A mutant and a mutant lacking all glycans were also found to localize ectopically. Altered trafficking of mutants may be associated with the mechanisms by which misglycosylated enzyme is degraded.

N-linked oligosaccharides are required to produce and stabilize the active form of chondroitin 4-sulphotransferase-1

C4ST-1 (chondroitin 4-sulphotransferase-1) transfers sulphate to position 4 of N-acetylgalactosamine in chondroitin. We showed previously that purified C4ST-1 from the culture medium of rat chondrosarcoma cells was a glycoprotein containing approx. 35% N-linked oligosaccharides. In the present paper, we investigated the functional role of the N-linked oligosaccharides attached to C4ST-1. We found that (i) treatment of recombinant C4ST-1 with peptide N-glycosidase F caused a marked decrease in activity, (ii) production of the active form of C4ST-1 by COS-7 cells transfected with cDNA of C4ST-1 was inhibited by tunicamycin, (iii) deletion of the N-glycosylation site located at the C-terminal region of C4ST-1 abolished activity, (iv) attachment of a single N-glycan at the C-terminal region supported production of the active form of C4ST-1, but the resulting recombinant enzyme was much more unstable at 37 degrees C than the control recombinant protein, and (v) truncation of C-terminal region up to the N-glycosylation site at the C-terminal region resulted in total loss of activity. These observations strongly suggest that N-linked oligosaccharides attached to C4ST-1 contribute to the production and stability of the active form of C4ST-1. In addition, the N-linked oligosaccharide at the C-terminal region appears to affect the glycosylation pattern of recombinant C4ST; a broad protein band of the wildtype protein resulting from microheterogeneity of N-linked oligosaccharides disappeared and four discrete protein bands with different numbers of N-linked oligosaccharides appeared when the N-linked oligosaccharide at the C-terminal region was deleted.

Role of glycosylation in conformational stability, activity, macromolecular interaction and immunogenicity of recombinant human factor VIII

Factor VIII (FVIII) is a multi-domain glycoprotein that is an essential cofactor in the blood coagulation cascade. Its deficiency or dysfunction causes hemophilia A, a bleeding disorder. Replacement using exogenous recombinant human factor VIII (rFVIII) is the first line of therapy for hemophilia A. The role of glycosylation on the activity, stability, protein-lipid interaction, and immunogenicity of FVIII is not known. In order to investigate the role of glycosylation, a deglycosylated form of FVIII was generated by enzymatic cleavage of carbohydrate chains. The biochemical properties of fully glycosylated and completely deglycosylated forms of rFVIII (degly rFVIII) were compared using enzyme-linked immunosorbent assay, size exclusion chromatography, and clotting activity studies. The biological activity of degly FVIII decreased in comparison to the fully glycosylated protein. The ability of degly rFVIII to interact with phosphatidylserine containing membranes was partly impaired. Data suggested that glycosylation significantly influences the stability and the biologically relevant macromolecular interactions of FVIII. The effect of glycosylation on immunogenicity was investigated in a murine model of hemophilia A. Studies showed that deletion of glycosylation did not increase immunogenicity.

The effect of individual N-glycans on enzyme activity

In a series of investigations, N-glycosylation has proven to be a key determinant of enzyme secretion, activity, binding affinity and substrate specificity, enabling a protein to fine-tune its activity. In the majority of cases elimination of all putative N-glycosylation sites of an enzyme results in significantly reduced protein secretion levels, while removal of individual N-glycosylation sites often leads to the expression of active enzymes showing markedly reduced catalytic activity, with the decreased activity often commensurate with the number of glycosylation sites available, and the fully deglycosylated enzymes showing only minimal activity relative to their glycosylated counterparts. On the other hand, several cases have also recently emerged where deglycosylation of an enzyme results in significantly increased catalytic activity, binding affinity and altered substrate specificity, highlighting the very unique and diverse roles that individual N-glycans play in regulating enzyme function.

Glycosylation of glycolipids in the Golgi complex

Gangliosides are a family of glycolipids characterized by containing a variable number of sialic acid residues. Nearly, all animal cells contain at least some class of ganglioside in their membranes, but membranes from the CNS are characterized by their high content of these lipids. The synthesis of the oligosaccharide moiety of glycolipids is carried out in the Golgi complex. In this study, I will discuss the cellular and molecular basis of the organization of the glycosylating machinery in the Golgi complex, with particular attention to the mutual relationships, sub-Golgi localization, and intracellular trafficking of glycolipid glycosyltransferases, and to their relationships with the corresponding glycolipid acceptors and sugar nucleotide donors. I will also discuss how the organization of the glycosylating machinery in the Golgi may adapt to events controlling glycolipid expression.

Role of the linker region in the expression of Rhizopus oryzae glucoamylase

Background

Rhizopus oryzae glucoamylase (RoGA) consists of three domains: an amino (N)-terminal raw starch-binding domain (SBD), a glycosylated linker domain, and a carboxy (C)-terminal catalytic domain. The 36-amino-acid linker region (residues 132–167) connects the two functional domains, but its structural and functional roles are unclear.

Results

To characterize the linker sequences of RoGA and its involvement in protein expression, a number of RoGA variants containing deletions and mutations were constructed and expressed in Saccharomyces cerevisiae. Deletion analyses demonstrate that the linker region, especially within residues 161 to 167, is required for protein expression. In addition, site-directed mutagenesis and deglycosylation studies reveal that the linker region of RoGA contains both N- and O-linked carbohydrate moieties, and the N-linked oligosaccharides play a major role in the formation of active enzyme. Although the linker segment itself appears to have no ordered secondary structural conformation, the flexible region indeed contributes to the stabilization of functional N- and C-terminal domains.

Conclusion

Our data provide direct evidence that the length, composition, and glycosylation of the interdomain linker play a central role in the structure and function of RoGA.

Substitution of the N-glycan function in glycosyltransferases by specific amino acids: ST3Gal-V as a model enzyme

The sialyltranferase ST3Gal-V transfers a sialic acid to lactosylceramide. We investigated the role of each of the N-glycans modifying mouse ST3Gal-V (mST3Gal-V) by measuring the in vitro enzyme activity of Chinese hamster ovary (CHO) cells transfected with ST3Gal-V cDNA or its mutants. By examining mutants of mST3Gal-V, in which each asparagine was replaced with glutamine (N180Q, N224Q, N334Q), we determined that all three sites are N-glycosylated and that each N-glycan is required for enzyme activity. Despite their importance, N-glycosylation sites in ST3Gal-V are not conserved among species. Therefore, we considered whether the function in the activity that is performed in mST3Gal-V by the N-glycan could be substituted for by specific amino acid residues selected from the ST3Gal-V of other species or from related sialyltransferases (ST3Gal-I, -II, -III, and -IV), placed at or near the glycosylation sites. To this end, we constructed a series of interspecies mutants for mST3Gal-V, specifically, mST3Gal-V-H177D-N180S (medaka or tetraodon type), mST3Gal-V-N224K (human type), and mST3Gal-V-T336Q (zebrafish type). The ST3Gal-V activity of these mutants was quite similar to that of the wild-type enzyme. Thus, we have demonstrated here that the N-glycans on mST3Gal-V are required for activity but can be substituted for specific amino acid residues placed at or near the glycosylation sites. We named this method SUNGA (substitution of N-glycan functions in glycosyltransferases by specific amino acids). Furthermore, we verified that the ST3Gal-V mutant created using the SUNGA method maintains its high activity when expressed in Escherichia coli thereby establishing the usefulness of the SUNGA method in exploring the function of N-glycans in vivo.

Expression of Escherichia coli AppA2 phytase in four yeast systems

To develop an effective fermentation system for producing Escherichia coliphytase AppA2, we expressed the enzyme in three inducible yeast systems: Saccharomyces cerevisiae (pYES2), Schizosaccharomyces pombe (pDS472a), and Pichia pastoris (pPICZ alphaA), and one constitutive system: P. pastoris (pGAPZalphaA). All four systems produced an extracellular functional AppA2 phytase with apparent molecular masses ranging from 51.5 to 56 kDa. During 8-day batch fermentation in shaking flasks, the inducible Pichia system produced the highest activity (272 units ml(-1) medium), whereas the Schizo. pombe system produced the lowest activity (2.8 units ml(-1)). The AppA2 phytase expressed in Schizo. pombe had 60-75% lower K(m)for sodium phytate and 28% higher heat-stability at 65 degrees C than that expressed in other three systems. However, all four recombinant AppA2 phytases had pH optimum at 3.5 and temperature optimum at 55 degrees C and similar efficacy in hydrolyzing phytate-phosphate from soybean meal.

The STT3a subunit isoform of the Arabidopsis oligosaccharyltransferase controls adaptive responses to salt/osmotic stress

Arabidopsis stt3a-1 and stt3a-2 mutations cause NaCl/osmotic sensitivity that is characterized by reduced cell division in the root meristem. Sequence comparison of the STT3a gene identified a yeast ortholog, STT3, which encodes an essential subunit of the oligosaccharyltransferase complex that is involved in protein N-glycosylation. NaCl induces the unfolded protein response in the endoplasmic reticulum (ER) and cell cycle arrest in root tip cells of stt3a seedlings, as determined by expression profiling of ER stress-responsive chaperone (BiP-GUS) and cell division (CycB1;1-GUS) genes, respectively. Together, these results indicate that plant salt stress adaptation involves ER stress signal regulation of cell cycle progression. Interestingly, a mutation (stt3b-1) in another Arabidopsis STT3 isogene (STT3b) does not cause NaCl sensitivity. However, the stt3a-1 stt3b-1 double mutation is gametophytic lethal. Apparently, STT3a and STT3b have overlapping and essential functions in plant growth and developmental processes, but the pivotal and specific protein glycosylation that is a necessary for recovery from the unfolded protein response and for cell cycle progression during salt/osmotic stress recovery is associated uniquely with the function of the STT3a isoform.

N-glycosylation of recombinant human fucosyltransferase III is required for its in vivo folding in mammalian and insect cells

Human alpha3/4fucosyltransferase (FT3) catalyses the synthesis of fucosylated glycoconjugates involved in cell-cell interactions. FT3 has two potential N-glycosylation sites at Asn(154) and Asn(185). Soluble secretory forms of the enzyme (SFT3) and mutant forms with the first, second and both glycosylation sites (SFT3DN1, SFT3DN2, SFT3DN) mutated have been expressed in baby hamster kidney (BHK) and Spodoptera frugiperda (Sf9) cells. Deletion of the first or both sites caused total enzyme inactivation. Deletion of the second site caused 99% and 75% decrease of secretory enzyme expression in BHK and Sf9 cells, respectively. Sf9 cells produced 1 mg/l SFT3 and 0.3 mg/l SFT3DN2; these values were 175- and 3750-fold higher, respectively, than those observed for BHK cells. A significant amount of protein was accumulated intracellularly in Sf9 cells which for SFT3 was active and for SFT3DN2 was inactive, indicating the importance of the glycans from the second glycosylation site for protein folding. The corresponding full-length forms FT3, FT3DN1 and FT3DN2 associated with calnexin as observed by immunoprecipitation studies, which indicated the possible role of this chaperon in the folding of glycosylated glycosyltransferases.

Beta1,4-N-acetylgalactosaminyltransferase--GM2/GD2 synthase: a key enzyme to control the synthesis of brain-enriched complex gangliosides

Beta1,4-N-acetylgalactosaminyltransferase (GM2/GD2 synthase) is a key enzyme which catalyzes the conversion of GM3, GD3 and lactosylceramide (LacCer) to GM2, GD2 and asialo-GM2 (GA2), respectively. This step is critical for the synthesis of all complex gangliosides enriched in the nervous system of vertebrates. Following the cloning of cDNAs encoding GM2/GD2 synthase by an expression cloning approach, substantial evidence for the roles of complex gangliosides have been obtained. Above all, knock-out mice lacking all complex gangliosides revealed important roles of complex gangliosides in vivo, i.e., in the maintenance and repair of nervous tissues, in the intact differentiation of spermatocytes via the transport of testosterone, and in the regulation of interleukin-2 receptor complex. Molecular mechanisms for these functions of complex gangliosides in vivo remain to be clarified.

N-glycosylation is required for full enzymic activity of the murine galactosylceramide sulphotransferase

3- O -Sulphogalactosylceramide (sulphatide) is a major lipid component of myelin membranes, and is required for proper myelin formation. Sulphatide is synthesized in the Golgi apparatus by galactosylceramide sulphotransferase (CST; EC 2.8.2.11). Murine and human CSTs contain two putative N-glycosylation sites (Asn-66 and Asn-312). The second site is conserved among all galactose 3-O-sulphotransferases cloned to date. In order to study the functional relevance of N-glycosylation, we generated epitope-tagged CST and soluble Protein A-CST fusion proteins lacking both N-glycosylation sites, separately or in combination. Our results show that both sites are glycosylated when CST is expressed in Chinese hamster ovary (CHO) or COS cells. Moreover, transfecting CST mutants lacking both N-glycosylation sites, or only Asn-312, reduced significantly the amount of sulphatide synthesized, whereas substituting Asn-66 with a glutamine residue did not. In contrast, activity in vitro was reduced by approx. 50% in the Asn-66-->Gln (N66Q) mutant, and was almost undetectable in N312Q and N66/312Q transfectants. Furthermore, soluble Protein A-CST expressed in the presence of tunicamycin was almost inactive, and accumulated in transfected cells. Expression of fully active CST in a CHO-glycosylation mutant lacking N-acetylglucosaminyltransferase I demonstrated that condensation of the N-linked pentamannosyl-core structure is sufficient to form a fully active enzyme.

Understanding the stepwise synthesis of glycolipids

Glycolipid expression is highly regulated during development and differeniation. The control relies mainly on transcriptional modulation of key glycosyltransferases acting at the branching points of the pathway of biosynthesis. Transferases are Golgi residents that depend on N-glycosylation and oligosaccharide processing for proper folding in the endoplasmic reticulum. The N-terminal domain bears information for their transport to the Golgi, retention in the organelle and differential concentration in sub-Golgi compartments. In the Golgi, some transferases associate forming functional multienzyme complexes. It is envisaged that the machinery for synthesis in the Golgi complex, and its dynamics, constitute a potential target for fine tuning of the control of glycolipid expression according to cell demands.

Role of loop structures of neuropsin in the activity of serine protease and regulated secretion

Neuropsin involved in neural plasticity in adult mouse brain is a member of the S1 (clan SA) family of serine proteases and forms characteristic surface loops surrounding the substrate-binding site (Kishi, T., Kato, M., Shimizu, T., Kato, K., Matsumoto, K., Yoshida, S., Shiosaka, S., and Hakoshima, T. (1999) J. Biol. Chem. 274, 4220-4224). Little, however, is known about the roles of these loops. Thus, the present study investigated whether surface loop structures of neuropsin were essential for the generation of enzymatic activity and/or secretion of the enzyme via a regulated secretory pathway. The loops include those stabilized by six disulfide bonds or a loop C (Gly(69)-Glu(80)) and an N-glycosylated kallikrein loop (His(91)-Ile(103)) not containing a site linked by a disulfide bond. First, among the six disulfide bonds, only SS1 in loop E (Gly(142)-Leu(155)) and SS6 in loop G (Ser(185)-Gly(197)) were necessary for the catalytic efficiency of neuropsin. Second, disruptions of loop C and the N-linked oligosaccharide chain on the kallikrein loop affected the catalytic efficiency and P2 specificity, respectively. Alternatively, disruptions of loop C and the kallikrein loop enhanced the regulated secretion, whereas there was no one disruption that inhibited the secretion, indicating that there was no critical loop required for the regulated secretion among loops surrounding the substrate-binding site.

The role of protein glycosylation in the control of cellular N-sialyltransferase activity

Protein glycosylation, which is a key post-translational event, is catalysed by the glycosyltransferase family of enzymes. There is an increasing body of evidence to suggest that these enzymes may themselves be glycosylated, possibly as an autocatalytic event. Using a novel in vitro system, we have investigated the role of enzyme glycosylation in sialyltransferase catalytic activity. The enzyme activity is glycosylation dependent, with the penultimate galactose residue on complex N-linked oligosaccharides playing a pivotal role. These results serve to underline the complexity of the glycosylation process.

Engineering N-glycosylation mutations in IL-12 enhances sustained cytotoxic T lymphocyte responses for DNA immunization

Interleukin-12 (IL-12), consisting of p40 and p35 subunits, produces both p70 heterodimer and free p40. p70 is essential for the induction of T-helper 1 (Th1) and cytotoxic T-cell (CTL) immunity, whereas p40 inhibits p70-mediated function. Here, we found that mutations introduced into N-glycosylation sites (N220 of murine p40 and N222 of human p40) reduced secretion of p40 but not p70. Co-immunization of N220 mutant mIL-12 gene with hepatitis C virus (HCV) E2 DNA significantly enhanced long-term E2-specific CD8+ T-cell response and protection against tumor challenge compared with that of wild type. Our results indicate that the ratio of p70 to p40 is important for generating sustained long-term cell-mediated immunity. Thus, the mutant IL-12 could be utilized for the development of DNA vaccines as an adjuvant for the generation of long-term memory T-cell responses.

Delineation of the minimal catalytic domain of human Galbeta1-3GalNAc alpha2,3-sialyltransferase (hST3Gal I)

The CMP-Neu5Ac:Galbeta1-3GalNAc alpha2,3-sialyltransferase (ST3Gal I, EC 2.4.99.4) is a Golgi membrane-bound type II glycoprotein that catalyses the transfer of sialic acid residues to Galbeta1-3GalNAc disaccharide structures found on O-glycans and glycolipids. In order to gain further insight into the structure/function of this sialyltransferase, we studied protein expression, N-glycan processing and enzymatic activity upon transient expression in the COS-7 cell line of various constructs deleted in the N-terminal portion of the protein sequence. The expressed soluble polypeptides were detected within the cell and in the cell culture media using a specific hST3Gal I monoclonal antibody. The soluble forms of the protein consisting of amino acids 26-340 (hST3-Delta25) and 57-340 (hST3-Delta56) were efficiently secreted and active. In contrast, further deletion of the N-terminal region leading to hST3-Delta76 and hST3-Delta105 gave also rise to various polypeptides that were not active within the transfected cells and not secreted in the cell culture media. The kinetic parameters of the active secreted forms were determined and shown to be in close agreement with those of the recombinant enzyme already described (H. Kitagawa, J.C. Paulson, J. Biol. Chem. 269 (1994)). In addition, the present study demonstrates that the recombinant hST3Gal I polypeptides transiently expressed in COS-7 cells are glycosylated with complex and high mannose type glycans on each of the five potential N-glycosylation sites.

The impact of N-glycosylation on the functions of polysialyltransferases

Poly-alpha-2,8-sialic acid (polysialic acid) is a post-translational modification of the neural cell adhesion molecule (NCAM) and an important regulator of neuronal cell-cell interactions. The synthesis of polysialic acid depends on the two polysialyltransferases ST8SiaII and ST8SiaIV. Understanding the catalytic mechanisms of the polysialyltransferases is critical toward the aim of influencing physiological and pathophysiological functions mediated by polysialic acid. We recently demonstrated that polysialyltransferases are bifunctional enzymes exhibiting auto- and NCAM polysialylation activity. Autopolysialylation occurs on N-glycans of the enzymes, and glycosylation variants lacking sialic acid and galactose were found to be inactive for both auto- and NCAM polysialylation. In the present study, we have analyzed the number and functional importance of N-linked oligosaccharides present on polysialyltransferases. We demonstrate that autopolysialylation depends on specific N-glycans attached to Asn(74) in ST8SiaIV and Asn(89) and Asn(219) in ST8SiaII. Deletion of polysialic acid acceptor sites by site-directed mutagenesis rendered the polysialyltransferases inactive in vitro and in vivo. The inactivity of autopolysialylation-negative polysialyltransferases in vivo was not caused by the absence or default targeting of the enzymes. The data presented in this study clearly show that active polysialyltransferases are competent to perform autopolysialylation and provide strong evidence for a tight functional link between the two catalytic functions.

The effects of the site-directed removal of N-glycosylation from cationic peanut peroxidase on its function

Peanut peroxidase has been diffracted. The location of its heme and calcium moieties have been shown and their role demonstrated. However, the structure and role of its glycans is only now being elucidated. The role of three N-linked complex glycans on cationic peroxidase (cPrx) of peanut (Arachis hypogaea L cv. Valencia), as expressed by prxPNC1 in transgenic tobacco, was analyzed by site-directed replacement of each of the three glycosylation sites, N-60, N-144, and N-185 with Q, individually. The mutant prxPNC1 cDNAs with a 3' histidine-tag were expressed in transgenic tobacco. The effect on the catalytic ability, thermal stability, and unfolding properties of the mutant peroxidases, isolated from the medium of transgenic tobacco cell suspension cultures were compared with those of the wild cPrx from peanut. It was found that the ablation of the glycans at N-60 and N-144 influences the full expression of the cPrx catalytic ability. The glycan at N-185 is important for the thermostability, as is the removal of the carbohydrate chain at N-185, resulting in rapid enzymatic decrease at temperatures of 50 degrees C. All three glycans appeared to influence the folding of the protein.

Disulfide bonds of GM2 synthase homodimers. Antiparallel orientation of the catalytic domains

GM2 synthase is a homodimer in which the subunits are joined by lumenal domain disulfide bond(s). To define the disulfide bond pattern of this enzyme, we analyzed a soluble form by chemical fragmentation, enzymatic digestion, and mass spectrometry and a full-length form by site-directed mutagenesis. All Cys residues of the lumenal domain of GM2 synthase are disulfide bonded with Cys(429) and Cys(476) forming a disulfide-bonded pair while Cys(80) and Cys(82) are disulfide bonded in combination with Cys(412) and Cys(529). Partial reduction to produce monomers converted Cys(80) and Cys(82) to free thiols while the Cys(429) to Cys(476) disulfide remained intact. CNBr cleavage at amino acid 330 produced a monomer-sized band under nonreducing conditions which was converted upon reduction to a 40-kDa fragment and a 24-kDa myc-positive fragment. Double mutation of Cys(80) and Cys(82) to Ser produced monomers but not dimers. In summary these results demonstrate that Cys(429) and Cys(476) form an intrasubunit disulfide while the intersubunit disulfides formed by both Cys(80) and Cys(82) with Cys(412) and Cys(529) are responsible for formation of the homodimer. This disulfide bond arrangement results in an antiparallel orientation of the catalytic domains of the GM2 synthase homodimer.

Glycosylation of the N-terminal potential N-glycosylation sites in the human alpha1,3-fucosyltransferase V and -VI (hFucTV and -VI)

Human alpha1,3-fucosyltransferase V and -VI (hFucTV and -VI) each contain four potential N-glycosylation sites (hFucTV: Asn60, Asn105, Asn167 and Asn198 and hFucTVI: Asn46, Asn91, Asn153 and Asn184). Glycosylation of the two N-terminal potential N-glycosylation sites (hFucTV: Asn60, Asn105 and hFucTVI: Asn46 and Asn91) have never been studied in detail. In the present study, we have analysed the glycosylation of these potential N-glycosylation sites. Initially, we compared the molecular mass of hFucTV and -VI expressed in COS-7 cells treated with tunicamycin with the mass of the proteins in untreated cells. The difference in molecular mass between the proteins in treated and untreated cells corresponded to the presence of at least three N-linked glycans. We then made a series of mutants, in which the asparagine residues in the N-terminal potential N-glycosylation sites were replaced by glutamine. Western blotting analyses demonstrated that both sites in hFucTV were glycosylated, whereas in hFucTVI only one of the sites (Asn91) was glycosylated. All the single mutants and the hFucTVI N46Q/N91Q double mutant exhibited enzyme activities that did not differ considerably from the wt activities. However, the enzyme activity of the hFucTV N60Q/N105Q double mutant was reduced to approximately 40% of the wt activity. In addition, castanospermine treatment diminished the enzyme activity and hence trimming of the N-linked glycans are required for expression of full enzyme activity of both hFucTV and -VI. The present study demonstrates that both of the N-terminal potential N-glycosylation sites in hFucTV and one of the sites in hFucTVI are glycosylated. Individually, their glycosylation does not contribute considerably to expression of enzyme activity. However, elimination of both sites in hFucTV reduces the enzyme activity.

Site-directed mutagenesis improves catalytic efficiency and thermostability of Escherichia coli pH 2.5 acid phosphatase/phytase expressed in Pichia pastoris

Escherichia coli pH 2.5 acid phosphatase gene (appA) and three mutants were expressed in Pichia pastoris to assess the effect of strategic mutations or deletion on the enzyme (EcAP) biochemical properties. Mutants A131N/ V134N/D207N/S211N, C200N/D207N/S211N, and A131N/ V134N/C200N/D207N/S211N had four, two, and four additional potential N-glycosylation sites, respectively. Extracellular phytase and acid phosphatase activities were produced by these mutants and the intact enzyme r-AppA. The N-glycosylation level was higher in mutants A131N/V134N/D207N/S211N (48%) and A131N/V134N/ C200N/D207N/S211N (89%) than that in r-AppA (14%). Despite no enhancement of glycosylation, mutant C200N/ D207N/S211N was different from r-AppA in the following properties. First, it was more active at pH 3.5-5.5. Second, it retained more (P < 0.01) phytase activity than that of r-AppA. Third, its specific activity of phytase was 54% higher. Lastly, its apparent catalytic efficiency kcat/Km for either p-nitrophenyl phosphate (5.8 x 10(5) vs 2.0 x 10(5) min(-1) M(-1)) or sodium phytate (6.9 x 10(6) vs 1.1 x 10(6) min(-1) M(-1)) was improved by factors of 1.9- and 5.3-fold, respectively. Based on the recently published E. coli phytase crystal structure, substitution of C200N in mutant C200N/D207N/S211N seems to eliminate the disulfide bond between the G helix and the GH loop in the alpha-domain of the protein. This change may modulate the domain flexibility and thereby the catalytic efficiency and thermostability of the enzyme.

Effect of N-glycosylation on turnover and subcellular distribution of N-acetylgalactosaminyltransferase I and sialyltransferase II in neuroblastoma cells

Gangliosides are sialylated glycosphingolipids whose biosynthesis is catalyzed by a series of endoplasmic reticulum (ER)- and Golgi-resident glycosyltransferases. Protein expression, processing, and subcellular localization of the key regulatory enzymes for ganglioside biosynthesis, sialyltransferase II (ST-II) and N-acetylgalactosaminyltransferase I (GalNAcT), were analyzed upon transient expression of the two enzymes in the neuroblastoma cell lines NG108-15 and F-11. The enzymes were endowed with a C-terminal epitope tag peptide (FLAG) for immunostaining and immunoaffinity purification using a FLAG-specific antibody. Mature ST-II-FLAG and GalNAcT-FLAG were expressed as N-glycoproteins with noncomplex oligosaccharides. ST-II-FLAG was distributed to the Golgi apparatus, whereas GalNAcT-FLAG was found in the ER and Golgi. Inhibition of early N-glycoprotein processing with castanospermine resulted in a distribution of ST-II-FLAG to the ER, whereas that of GalNAcT-FLAG remained unaltered. In contrast to GalNAcT, the activity of ST-II and the amount of immunostained enzyme were reduced concomitantly by 75% upon incubation with castanospermine. This was due to a fourfold increased turnover of ST-II-FLAG, which was not found with GalNAcT-FLAG. The ER retention and increased turnover of ST-II-FLAG were most likely due to its inability to bind to calnexin upon inhibition of early N-glycoprotein processing. Calnexin binding was not observed for GalNAcT-FLAG, indicating a differential effect of N-glycosylation on the turnover and subcellular localization of the two glycosyltransferases.

GM1 synthase depends on N-glycosylation for enzyme activity and trafficking to the Golgi complex

Glycosyltransferase cDNAs contain a variable number of potential N-glycosylation sites. Here we examined the occupancy and relevance for the activity and intracellular trafficking of the only potential N-glycosylation site of the mouse beta1,3galactosyltransferase (Gal-T2 or GA1/GM1/GD1b synthase) in Gal-T2 cDNA transfected CHO-K1 cells. Transfected cells synthesize a Golgi located active enzyme of 43 kDa whose N-glycan was metabolically labeled from [3H]mannose and was Endo-H sensitive. Inhibition of N-glycosylation by Tunicamycin or by point mutation of the N-glycosylation site resulted in the synthesis of a polypeptide of 40 kDa which lacked enzyme activity and was concentrated in the endoplasmic reticulum (ER). Inhibition of ER glucosidases by Castanospermine impaired the exit of a form of Gal-T2 having reduced enzyme activity from the ER. The N-terminal Gal-T2 domain (aa 1-52) was able to direct and to retain the green fluorescence protein in the Golgi complex. Taken together, these results indicate that Gal-T2 depends on N-glycosylation for its activity and for proper trafficking to, but not its retention in, the Golgi complex.

Influence of N-glycosylation and N-glycan trimming on the activity and intracellular traffic of GD3 synthase

GD3 synthase (ST8Sia I) transfers a sialic acid in alpha-2-->8 linkage to the sialic acid moiety of GM3 to form the ganglioside GD3. The cDNAs of GD3 synthases predict several putative N-glycosylation sites. In this work we have examined the occupancy of these sites in a chicken GD3 synthase and how they affect its activity and intracellular traffic. COS-7 cells were transfected with an influenza virus hemagglutinin (HA) epitope-tagged form of GD3 synthase (GD3 synthase-HA). Cells acquired GD3 synthase activity, cell surface GD3 immunoexpression, and GD3 synthase-HA immunoreactivity in the Golgi complex. In Western blots, a main GD3 synthase-HA band of 47 kDa was detected, which was radioactive upon metabolic labeling with [2-3H] mannose. Tunicamycin prevented the incorporation of [2-3H]mannose into GD3 synthase-HA, blocked the enzyme activity, and promoted a reduction of the enzyme molecular mass of 6-7 kDa. Timed deglycosylation with N-glycosidase F showed that all three potential N-glycosylation sites of GD3 synthase-HA were glycosylated. The deglycosylated forms were enzymatically more unstable than the native form. Tunicamycin treatment of cells led to retention of GD3 synthase-HA immunoreactivity in the endoplasmic reticulum (ER). Castanospermine and deoxynojirimycin, inhibitors of the ER-processing enzymes alpha-glucosidases I and II, also prevented the exit from the ER but did not essentially affect the enzyme specific activity. 1-Deoxymannojirimycin and swainsonine, inhibitors of mannosidases, did not affect either the enzyme activity or the Golgi localization. Results indicate that (a) N-glycosylation is necessary for GD3 synthase to attain and to maintain a catalytically active folding, and for exiting the ER; and (b) N-glycan trimming in the ER, while not required for enzyme activity, is necessary for proper trafficking of GD3 synthase to the Golgi complex.

Expression of the Schwanniomyces occidentalis SWA2 amylase in Saccharomyces cerevisiae: role of N-glycosylation on activity, stability and secretion

The role of N-linked glycosylation on the biological activity of Schwanniomyces occidentalis SWA2 alpha-amylase, as expressed in Saccharomyces cerevisiae, was analysed by site-directed mutagenesis of the two potential N-glycosylation sites, Asn-134 and Asn-229. These residues were replaced by Ala or Gly individually or in various combinations and the effects on the activity, secretion and thermal stability of the enzyme were studied. Any Asn-229 substitution caused a drastic decrease in activity levels of the extracellular enzyme. In contrast, substitutions of Asn-134 had little or no effect. The use of antibodies showed that alpha-amylase was secreted in all the mutants tested, although those containing substitutions at Asn-229 seemed to have a lower rate of synthesis and/or higher degradation than the wild-type strain. alpha-Amylases with substitution at Asn-229 had a 2 kDa lower molecular mass than the wild-type protein, as did the wild-type protein itself after treatment with endoglycosidase F. These findings indicate that Asn-229 is the single glycosylated residue in SWA2. Thermostability analysis of both purified wild-type (T50=50 degrees C, where T50 is the temperature resulting in 50% loss of activity) and mutant enzymes indicated that removal of carbohydrate from the 229 position results in a decrease of approx. 3 degrees C in the T50 of the enzyme. The Gly-229 mutation does not change the apparent affinity of the enzyme for starch (Km) but decreases to 1/22 its apparent catalytic efficiency (kcat/Km). These results therefore indicate that glycosylation at the 229 position has an important role in the extracellular activity levels, kinetics and stability of the Sw. occidentalis SWA2 alpha-amylase in both its wild-type and mutant forms.

Cloning, characterization and developmental expression of alpha2,8 sialyltransferase (GD3 synthase, ST8Sia I) gene in chick brain and retina

GD3 and GM2 synthases act on ganglioside GM3 at the branching point of the pathway of synthesis of gangliosides in which the "a", "b" and "c" families are produced. The relative activities of these enzymes are important for regulating the ganglioside composition of a given tissue. In the present work, we report the cloning and characterization of a chick GD3 synthase cDNA. The cloned cDNA directed the synthesis of a functionally active enzyme in transiently transfected CHO-K1 cells and was highly homologous to mammalian GD3 synthases. In Northern blot experiments the cDNA detected a single specific GD3 synthase mRNA of about 9.0 kb both in the chicken brain and retina. The abundance of the specific mRNA transcript declined steadily from E7-E9 to very low values around PN2. The levels of enzyme activities measured at the same developmental stages roughly followed the changes of specific mRNA levels in both tissues. In situ hybridization of embryonic neural retina cells in culture showed that both glial- and neuron-like cells expressed the specific GD3 synthase mRNA, although with different intensities. Results indicate that transcription and/or stability of the specific GD3 synthase mRNA constitute a level of control of the expression of GD3 synthase and indirectly of the ganglioside composition in the developing chicken central nervous system (CNS).

Expression of beta 1-4 N-acetylgalactosaminyltransferase gene in the developing rat brain and retina: mRNA, protein immunoreactivity and enzyme activity

The developmental pattern of expression of the UDP-GalNAc:GM3 N-acetylgalactosaminyltransferase (GalNAc-T) gene was examined in the rat brain and retina. A GalNAc-T cDNA cloned from a rat olfactory bulb cDNA library was used as a probe for Northern blot and in situ hybridization experiments and a rabbit polyclonal antibody to rat GalNAc-T peptide was used for Western blot analysis. In Northern blot experiments, a single approximately 3 kb transcript was detected both in brain and retina. In brain, the abundance of this transcript increased from E15 to PN1-5 and then declined while, in retina, it increased steadily from PN1 to PN13-24. The developmental trends of GalNAc-T mRNA expression, GalNAc-T immunoreactive protein and GalNAc-T activity were comparable in brain. In retina, however, GalNAc-T activity and GalNAc-T peptide immunoreactivity followed developmental patterns that were similar between them and different from that of the specific mRNA. Results suggest that post-transcriptional controls of the GalNAc-T gene expression operate in the rat CNS, which are particularly evident in retina. The expression of the GalNAc-T gene in glial and neuronal cells was examined in rat retina cell cultures by in situ hybridization. The GalNAc-T mRNA was abundant in GM1+/GD3+ neurons and almost absent in the flat, GM1-/GD3+ Müller glia-derived cells.

Domain-specific N-glycosylation of the membrane glycoprotein dipeptidylpeptidase IV (CD26) influences its subcellular trafficking, biological stability, enzyme activity and protein folding

Dipeptidyl peptidase IV (DPPIV, CD26) is an N-glycosylated type II plasma membrane protein. The primary structure of rat wild-type DPPIV contains eight potential N-glycosylation sites. To investigate the role of N-glycosylation in the function of DPPIV, three of its asparagine residues were separately converted to glutamine by site-directed mutagenesis. The resulting N-glycosylation mutants of rat DPPIV were studied in stable transfected Chinese hamster ovary cells. All three N-glycosylation mutants of DPPIV showed a reduced half-life, as well as differing degrees of inhibition of the processing of their N-glycans. Mutation of the first (Asn83-->Gln) or eighth (Asn686-->Gln) N-glycosylation site had only a small effect on its enzymatic activity, cell-surface expression and dimer formation, whereas the mutation of the sixth N-glycosylation site (Asn319-->Gln) abolished the enzymatic activity, eliminated cell-surface expression and prevented the dimerization of the DPPIV protein. The mutant [Gln319]DPPIV is retained in the cytoplasm and its degradation was drastically increased. Our data suggest that the N-glycosylation at Asn319 is involved in protein trafficking and correct protein folding.