Carbohydrate structures of a human tissue plasminogen activator variant expressed in recombinant Chinese hamster ovary cells

Identification, characterization, and engineering of glycosylation in thrombolyticsa

Cardiovascular diseases, such as myocardial infarction, ischemic stroke, and pulmonary embolism, are the most common causes of disability and death worldwide. Blood clot hydrolysis by thrombolytic enzymes and thrombectomy are key clinical interventions. The most widely used thrombolytic enzyme is alteplase, which has been used in clinical practice since 1986. Another clinically used thrombolytic protein is tenecteplase, which has modified epitopes and engineered glycosylation sites, suggesting that carbohydrate modification in thrombolytic enzymes is a viable strategy for their improvement. This comprehensive review summarizes current knowledge on computational and experimental identification of glycosylation sites and glycan identity, together with methods used for their reengineering. Practical examples from previous studies focus on modification of glycosylations in thrombolytics, e.g., alteplase, tenecteplase, reteplase, urokinase, saruplase, and desmoteplase. Collected clinical data on these glycoproteins demonstrate the great potential of this engineering strategy. Outstanding combinatorics originating from multiple glycosylation sites and the vast variety of covalently attached glycan species can be addressed by directed evolution or rational design. Directed evolution pipelines would benefit from more efficient cell-free expression and high-throughput screening assays, while rational design must employ structure prediction by machine learning and in silico characterization by supercomputing. Perspectives on challenges and opportunities for improvement of thrombolytic enzymes by engineering and evolution of protein glycosylation are provided.

Construction and characterization of stably transfected BHK-21 cells with human-type sialylation characteristic

The human Golgi enzyme CMP-NeuAc:Gal(beta1-4)GlcNAc-R alpha2,6-sialyltransferase (ST6N) was stably coexpressed with human erythropoietin (EPO) from a BHK-21A cell line. The cell line was characterized with respect to the expression and in vitro activity of the ST6N and the endogenous alpha2,3-sialyltransferase. Detailed structural analysis of the N-linked carbohydrates of the rhuEPO expressed from the new cell line was performed by HPAE-PAD-mapping, MALDI/TOF-MS and methylation analysis after purification of the recombinant protein by immunoaffinity chromatography. This is the first report describing that the human alpha2,6-sialyltransferase is capable of sialylating, apart from Gal(beta1-4)GlcNAc-R, also GalNAc(beta1-4)GlcNAc-R motifs in vivo, which is not the case for the endogenous BHK-cell alpha2,3-sialyltransferase.

N- and O-linked carbohydrates and glycosylation site occupancy in recombinant human granulocyte-macrophage colony-stimulating factor secreted by a Chinese hamster ovary cell line

GM-CSF is one of several naturally occurring glycoproteins that regulate leukocyte production, migration and function. It has been produced in different cell types, with different properties that depend on the production process used. The purpose of this work was to characterize the recombinant human GM-CSF from an engineered Chinese hamster ovary cell line grown in suspension and as adherent culture for the identification of the glycosylation sites and the definition of the glycosidic moiety, including the degree of site occupancy. Both preparations exhibited size heterogeneity in SDS/PAGE with multiple bands containing glycoprotein forms with either two or one N-glycosylation sites occupied. Minor low molecular mass forms completely lacked N-linked oligosaccharides but contained 1-3 O-linked glycans. Twelve differently charged isoforms were detected in isoelectric focusing gels. At least 16 glycoforms, differing in the number of Hex-HexNAc units (Deltam 365 Da), were detected in MALDI-TOF MS spectra of the desialylated GM-CSFs. MALDI-TOF MS and HPAEC-PAD analysis indicated the presence of predominantly tri- and tetraantennary N-linked oligosaccharide chains with and without N-acetyllactosamine repeat units and some 10% of biantennary oligosaccharides, all containing more than 90% proximal alpha1-6-linked fucose. The oligosaccharide patterns of both GM-CSF preparations were found to be very similar. More than 90% of terminal galactose residues of the N-glycans were found alpha2-3 sialylated with NeuNAc (93%) or NeuNGc (7%). Site specific glycosylation was analysed by electrospray ionization MS and it was found that in the mono glycosylated GM-CSF form more than 90% of the Asn37 were occupied by N-glycans. O-glycosylation at the N-terminus of the polypeptide was detected at Ser7 and Ser9 or Thr10, in the predominantly doubly O-glycosylated glycoprotein form. In the triply modified GM-CSF molecules, Ser5 was additionally O-glycosylated. The major difference between both preparations was found in the MALDI spectra of the desialylated glycoproteins, revealing a higher proportion of forms with a single N-glycosylation site occupied in the preparation derived from suspension culture. ESI-MS and MALDI-MS analysis of endoproteolytically cleaved peptides as well as MALDI-TOF MS of the intact glycoprotein demonstrated the N- and C-termini integrity of the GM-CSF preparations.

Process development of a recombinant antibody/interleukin-2 fusion protein expressed in protein-free medium by BHK cells

The production, purification and stability of quality (in terms of integrity and glycosylation) of an antibody/interleukin-2 fusion protein with potential application in tumour-targeted therapy expressed in BHK21 cells are described. Consistency of the product throughout time was determined by analysis of glycosylation of the fusion protein using MALDI-TOF mass spectroscopy and HPAEC-PAD combined with product integrity studies by SDS-PAGE and Western blotting. These investigations showed consistent expression in terms of integrity and of three major oligosaccharide structures of the fusion protein after 62 generations. The data obtained at this stage indicated the suitability of the cell line for production purposes. Different approaches for the production of this protein were subsequently carried out. The relative productivity of the recombinant fusion protein and general performance of the cells in two different protein-free medium (PFM) culture systems, continuous chemostat and continuous perfusion using a Centritech centrifuge as a cell retention device, were studied. The results indicate that the chemostat culture resulted in more stable and controllable nutrient environment, which could indicate better product consistency, in accordance with what has been observed under serum-containing conditions, in relation to the perfusion culture. Finally, product obtained from the chemostat culture was analysed and purified. The purification process was optimised with an increase in the overall yield from 38 to 70% being obtained, a significant improvement with important consequences for the implementation of an industrial-scale culture system. In conclusion, it was possible to produce and purify the recombinant antibody/interleukin-2 fusion protein assuring the quality and stability of the product in terms of integrity and glycosylation. Therefore, a candidate production process was established.

Application of MS and NMR to the structure elucidation of complex sugar moieties of natural products: exemplified by the steroidal saponin from Yucca filamentosa L

An approach, using well characterized procedures, is presented that should be of general applicability for the structural elucidation of complex sugar moieties of natural products. The methods used are exemplified by the structure elucidation of a new gitogenin-based steroidal saponin that has a strong leishmanicidal activity similar to preparations used in clinical practice and has been isolated by bioactivity-guided fractionation of the ethanolic extract of Yucca filamentosa L. leaves. The saponin has been characterized as 3-O-((beta-D-glucopyranosyl-(1-->3)- beta-D-glucopyranosy-(1-->2))(alpha-L-rhamnopyranosyl-(1-->4)-beta-D-glucopyranosyl-(1-->3))-beta-D-glucopyranosyl-(1-->4)-beta-D-galactopyranosyl)-25R,5alpha-spirostan-2alpha,3beta-diol.

Metabolic shifts do not influence the glycosylation patterns of a recombinant fusion protein expressed in BHK cells

BHK-21 cells expressing a human IgG-IL2 fusion protein, with potential application in tumor-targeted therapy, were grown under different nutrient conditions in a continuous system for a time period of 80 days. At very low-glucose (< 0.5 mM) or glutamine (< 0. 2 mM) concentrations, a shift toward an energetically more efficient metabolism was observed. Cell-specific productivity was maintained under metabolically shifted growth conditions and at the same time an almost identical intracellular ATP content, obtained by in vivo (31)P NMR experiments, was observed. No significant differences in the oligosaccharide structures were detected from the IgG-IL2 fusion protein preparations obtained by growing cells under the different metabolic states. By using oligosaccharide mapping and MALDI/TOF-MS, only neutral diantennary oligosaccharides with or without core alpha1-6-linked fucose were detected that carried no, one or two beta1-4-linked galactose. Although the O-linked oligosaccharide structures that are present in the IL2 moiety of the protein were studied with less detail, the data obtained from the hydrazinolysis procedure point to the presence of the classical NeuAcalpha2-3Galbeta1-3GalNAc structure. Here, it is shown that under different defined cellular metabolic states, the quality of a recombinant product in terms of O- and N-linked oligosaccharides is stable, even after a prolonged cultivation period. Moreover, unaffected intracellular ATP levels under the different metabolic states were observed.

Biochemical and functional characterization of the Tn-specific lectin from Salvia sclarea seeds

SSL, the lectin isolated from Salvia sclarea seeds, recognizes the Tn antigen (GalNAcalpha-O-Ser/Thr), a specific marker of many human carcinomas. Two-dimensional electrophoresis, amino-acid and amino-sugar analysis, and MALDI-TOF MS showed that SSL is an acidic (pI 5.5), 60-61-kDa dimeric glycoprotein composed of apparently identical subunits linked by a single disulfide bond. The apparent molecular mass of SSL in solution determined by equilibrium sedimentation analytical ultracentrifugation was 59 +/- 9 kDa. This value did not change in the pH range 2.5-8.5, indicating that SSL does not associate into higher order structures. Tandem mass spectrometry and methylation analysis of N-glycans released from SSL by hydrazinolysis indicated that SSL possesses 2-3 glycosylation sites occupied with the typical plant glycans Manalpha1-6[(Manalpha1-3)(Xylbeta1-2)]Manbeta1-4 -GlcNAcbeta1-4(Fucalp ha1-3)GlcNAc and [(Manalpha1-3/6)(Xylbeta1-2)]Manbeta1-4-GlcNAcbeta1 -4(Fucalpha1-3)Glc NAc. The influence of adjacent Tn structures on the binding of two Tn-specific lectins (SSL and the isolectin B4 from Vicia villosa) and an anti-Tn monoclonal antibody (mAb 83D4) was evaluated using synthetic Tn glycopeptides. The binding of both lectins to the synthetic Tn glycopeptides was independent of the density of Tn structures. On the other hand, mAb 83D4 only reacted with glycopeptides displaying two or three consecutive Tn structures.

Monosaccharide composition analysis of pamiteplase by anion exchange chromatography with pulsed amperometric detection

The monosaccharides (neutral and amino sugars) of palmiteplase (recombinant modified human tissue plasminogen activator) were analyzed by high performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). Since the palmiteplase formulation contains sucrose, it was removed by reverse-phase high-performance liquid chromatography (RP-HPLC) prior to analysis. Acid hydrolysis with TFA was performed at 100 degrees C for 4 h. Fucose, glucosamine, galactose and mannose were detected by HPAEC-PAD analysis after hydrolysis. The linearity range of HPAEC-PAD analysis was 20-200 pmol/ml(-1) (r > 0.999) and the RSD value for repeatability was less than 7%. The recovery of each monosaccharide spiked into samples was more than 90%. The monosaccharide composition of palmiteplase suggests that it has complex-type oligosaccharides lacking in high-mannose-type oligosaccharides.

Structural characterization of the oligosaccharide chains of native and crystallized boar seminal plasma spermadhesin PSP-I and PSP-II glycoforms

The PSP-I/PSP-II heterodimer is the major protein of boar seminal plasma. Both subunits are glycoproteins of the spermadhesin family and each contains a single N-glycosylation site. After enzymatic release of the oligosaccharides from isolated PSP-I and PSP-II, mainly neutral and monosialylated oligosaccharides, and small amounts of disialylated oligosaccharides, were recovered from both proteins. Twenty-two neutral oligosaccharides, 11 monosialylated glycans and three disialylated carbohydrate chains were characterized using mass spectrometric and NMR techniques. PSP-I and PSP-II share the same glycans but differ in their relative molar ratios. Most glycan structures are proximally alpha1-6-fucosylated, diantennary complex-type bearing nonsialylated or alpha2-6-sialylated N-acetyllactosamine or di-N-acetyllactosamine antennae. The majority of nonsialylated N-acetyllactosamine antennae bear terminal alpha1-3-linked Gal residues. In addition, the N-acetylglucosamine residue of nonsialylated N-acetyl and di-N-acetyllactosamine antennae can be modified by an alpha1-3-linked fucose residue. Structures of higher antennarity, as well as structures 3,6-branched at galactose residues, were found in smaller amounts. In one oligosaccharide, N-acetylneuraminic acid is substituted by N-glycolylneuraminic acid. Mass spectrometric analysis of PSP-I and PSP-II glycoforms isolated from crystallized PSP-I/PSP-II heterodimer showed the coexistence of major PSP-I and PSP-II glycoforms in the hexagonal crystals. Oligosaccharides with the NeuNAcalpha2-6GalNAcbeta1-4GlcNAc-R motif block adhesive and activation-related events mediated by CD22, suggesting a possible immunoregulatory activity for PSP-I/PSP-II.

Deletion of new covalently linked cell wall glycoproteins alters the electrophoretic mobility of phosphorylated wall components of Saccharomyces cerevisiae

The incorporation of radioactive orthophosphate into the cell walls of Saccharomyces cerevisiae was studied. 33P-labeled cell walls were extensively extracted with hot sodium dodecyl sulfate (SDS). Of the remaining insoluble radioactivity more than 90% could be released by laminarinase. This radioactive material stayed in the stacking gel during SDS-polyacrylamide gel electrophoresis but entered the separating gel upon treatment with N-glycosidase F, indicating that phosphate was linked directly or indirectly to N-mannosylated glycoproteins. The phosphate was bound to covalently linked cell wall proteins as mannose-6-phosphate, the same type of linkage shown previously for soluble mannoproteins (L. Ballou, L. M. Hernandez, E. Alvarado, and C. E. Ballou, Proc. Natl. Acad. Sci. USA 87:3368-3372, 1990). From the phosphate-labeled glycoprotein fraction released by laminarinase, three cell wall mannoproteins, Ccw12p, Ccw13p and Ccw14p, were isolated and identified by N-terminal sequencing. For Ccw13p (encoded by DAN1 [also called TIR3]) and Ccw12p the association with the cell wall has not been described before; Ccw14p is identical with cell wall protein Icwp (I. Moukadiri, J. Armero, A. Abad, R. Sentandreu, and J. Zueco, J. Bacteriol. 179:2154-2162, 1997). In ccw12, ccw13, or ccw14 single or double mutants neither the amount of radioactive phosphate incorporated into cell wall proteins nor its position in the stacking gel was changed. However, the triple mutant brought about a shift of the 33P-labeled glycoprotein components from the stacking gel into the separating gel. The disruption of CCW12 results in a pronounced sensitivity of the cells to calcofluor white and Congo red. In addition, the ccw12 mutant shows a decrease in mating efficiency and a defect in agglutination.

Differential expression of alpha2-6 sialylated polylactosamine structures by human B and T cells

We found that human peripheral B and T cells differed in the surface expression of alpha2-6 sialylated type 2 chain glycans. In contrast to B cells, T cells expressed only sialoglycans with repeated N-acetyllactosamine (Galss1-4GlcNAc) disaccharides. This finding was based on the specificity of the monoclonal antibodies HB6, HB9 (CD24), HD66 (CDw76), FB21, and CRIS4 (CDw76) with the alpha2-6 sialylated model gangliosides IV6NeuAcnLc4Cer (2-6 SPG), VI6NeuAcnLc6Cer (2-6 SnHC), VIII6NeuAcnLc8Cer (2-6 SnOC), and X6NeuAcnLc10Cer (2-6 SnDC). We found that, in addition to their common requirement of an alpha2-6 bound terminal sialic acid for binding, the antibodies displayed preferences for the length of the carbohydrate backbones. Some of them bound mainly to 2-6 SPG with one N-acetyllactosamine (LacNAc) unit (HB9, HD66); others preferentially to 2-6 SnHC and 2-6 SnOC, with two and three LacNAc units, respectively (HB6 and FB21); and one of them exclusively to very polar alpha2-6 sialylated type 2 chain antigens (CRIS4) such as to 2-6 SnOC and even more polar gangliosides with three and more LacNAc units. These specificities could be correlated with the cellular binding of the antibodies as follows: whereas all antibodies bound to human CD 19 positive peripheral B cells, their reactivity with CD3 positive T cells was either nearly lacking (HD66, HB9), intermediate (about 65%: HB6, FB21) or strongly positive (CRIS4, 95%). Thus, the binding of the antibodies to 2-6 sialylated glycans with multiple lactosamine units appeared to determine their binding to T-cells.

In vivo specificity of human alpha1,3/4-fucosyltransferases III-VII in the biosynthesis of LewisX and Sialyl LewisX motifs on complex-type N-glycans. Coexpression studies from bhk-21 cells together with human beta-trace protein

Each of the five human alpha1,3/4-fucosyltransferases (FT3 to FT7) has been stably expressed in BHK-21 cells together with human beta-trace protein (beta-TP) as a secretory reporter glycoprotein. In order to study their in vivo properties for the transfer of peripheral Fuc onto N-linked complex-type glycans, detailed structural analysis was performed on the purified glycoprotein. All fucosyltransferases were found to peripherally fucosylate 19-52% of the diantennary beta-TP N-glycans, and all enzymes were capable of synthesizing the sialyl LewisX (sLex) motif. However, each enzyme produced its own characteristic ratio of sLex/Lex antennae as follows: FT7 (only sLex), FT3 (14:1), FT5 (3:1), FT6 (1.1:1), and FT4 (1:7). Fucose transfer onto beta-TP N-glycans was low in FT3 cells (11% of total antennae), whereas the values for FT7, FT5, FT4, and FT6 cells were 21, 25, 35, and 47%, respectively. FT3, FT4, FT5, and FT7 transfer preponderantly one Fuc per diantennary N-glycan. FT4 preferentially synthesizes di-Lex on asialo diantennary N-glycans and mono-Lex with monosialo chains. In contrast, FT6 forms mostly alpha1,3-difucosylated chains with no, one, or two NeuAc residues. FT3, FT4, and FT6 were proteolytically cleaved and released into the culture medium in significant amounts, whereas FT7 and FT5 were found to be largely resistant toward proteolysis. Studies on engineered soluble variants of FT6 indicate that these forms do not significantly contribute to the in vivo fucose transfer activity of the enzyme when expressed at activity levels comparable to those obtained for the wild-type Golgi form of FT6 in the recombinant host cells.

In vitro alpha1-3 or alpha1-4 fucosylation of type I and II oligosaccharides with secreted forms of recombinant human fucosyltransferases III and VI

Transgalactosylation of chitobiose and chitotriose employing beta-galactosidase from bovine testes yielded mixtures with beta1-3 linked galactose (type I) and beta1-4 linked galactose (type II) in a final ratio of 1:1 for the tri- and 1:1.4 for the tetrasaccharide. After 24 h incubations of the two purified oligosaccharide mixtures with large amounts (20-fold increase compared with standard conditions) of human alpha1,3/4-fucosyltransferase III (FucT III), the type I tri-/tetrasaccharides were completely converted to the Lewis(a) structure, whereas approximately 10% fucosylation of the type II isomers to the Lewis(x) oligosaccharides was observed in long-term incubations. Employing large amounts of human alpha1,3-fucosyltransferase VI (FucT VI), the type I trisaccharide substrate was exclusively fucosylated at the proximal O-4 substituted N-acetylglucosamine (GlcNAc) (20%) whereas almost all of the type II isomers was converted to the corresponding Lewis(x) product. 45% of the type I tetrasaccharide was fucosylated at the second GlcNAc solely by FucT VI. The type II isomer was almost completely alpha1-3 fucosylated to yield the Lewisx derivative with traces of a structure that contained an additional fucose at the reducing GlcNAc. The results obtained in the present study employing high amounts of enzyme confirmed our previous results that FucT III acts preponderantly as a beta1-4 fucosyltransferase onto GlcNAc in vitro. Human FucT VI attaches fucose exclusively in an alpha1-3 linkage to 4-substituted GlcNAc in vitro and does not modify any 3-substituted GlcNAc to yield Lewis(a) oligosaccharides. With 8-methoxycarbonyloctyl glycoside acceptors used under standard conditions, FucT III acts exclusively on the type I and FucT VI only on the type II derivative. With lacto-N-tetraose, lacto-N-fucopentraose I, or LS-tetrasaccharide as substrates, FucT III modified the 3-substituted GlcNAc and the reducing glucose; FucT VI recognized only lacto-N-neotetraose as a substrate.

Amino acid sequence, glycan structure, and proteolytic processing of the lectin of Vatairea macrocarpa seeds

VML is a galactose-binding lectin isolated from Vatairea macrocarpa seeds. By SDS-polyacrylamide gel electrophoresis, VML is a glycoprotein composed of a major 32-34 kDa double band (alpha-chain) and minor 22 kDa and 13 kDa bands. N-terminal sequencing of electroblotted samples showed that the 22 and 13 kDa bands corresponded to C-(beta) and N-(gamma) terminal fragments of the alpha-chain, respectively. The primary structure of VML displays similarity with other leguminous lectins, particularly with Erythrina variegata, Robinia pseudoacacia and Sophora japonica lectins. VML is N-glycosylated at asparagine residues at positions 111 and 183 with one major glycan structure. Tandem mass spectrometry and methylation analysis indicated the presence of Manalpha1-6[(Manalpha1-3)(Xylbeta1-2)]Manbeta1-4 -GlcNAcbeta1-4(Fucalpha1-3)GlcNAc, a typical plant Nglycan. Equilibrium sedimentation analysis by analytical centrifugation showed that VML had a mass of 122-130 kDa, which did not change within the pH range 2.5-8.5. These data indicated that VML is a pH-independent homotetrameric protein and that a small proportion of the alpha-subunits is cleaved into noncovalently associated N- and C-terminal fragments. Mass spectrometric analysis suggested a mechanism for the proteolytic processing of VML. V. macrocarpa lectin contains a mixture of doubly (28,525 Da) and singly (27,354 Da) glycosylated alpha-chains. Deglycosylation of Asn-111 correlates with proteolytic cleavage of the Asn-114-Lys-115 bond yielding glycosylated gamma (residues 1-114, 12,304 Da) and nonglycosylated beta-(residues 115-239, 14,957 Da) chains. Some beta-chain molecules are further deglycosylated and N-terminally processed yielding products of molecular masses of 13,783 Da and 13,670 Da.

Identification of the human Lewis(a) carbohydrate motif in a secretory peroxidase from a plant cell suspension culture (Vaccinium myrtillus L.)

This paper reports for the first time the presence of the human Lewis(a) type determinant in glycoproteins secreted by plant cells. A single glycopeptide was identified in the tryptic hydrolysis of the peroxidase VMPxC1 from Vaccinium myrtillus L. by HPLC/ESI-MS. The oligosaccharide structures were elucidated by ESI-MS-MS and by methylation analysis before and after removal of fucose by mild acid hydrolysis. The major structure determined is of the biantennary plant complex type containing the outer chain motif Lewis(a) [structure in text]. A corresponding fucosyltransferase activity catalyzing the formation of Lewis(a) type structures in vitro was identified in cellular extracts of the suspension cultures.

Stable expression of the Golgi form and secretory variants of human fucosyltransferase III from BHK-21 cells. Purification and characterization of an engineered truncated form from the culture medium

Stable BHK-21 cell lines were constructed expressing the Golgi membrane-bound form and two secretory forms of the human alpha1, 3/4-fucosyltransferase (amino acids 35-361 and 46-361). It was found that 40% of the enzyme activity synthesized by cells transfected with the Golgi form of the fucosyltransferase was constitutively secreted into the medium. The corresponding enzyme detected by Western blot had an apparent molecular mass similar to those of the truncated secretory forms. The secretory variant (amino acids 46-361) was purified by a single affinity-chromatography step on GDP-Fractogel resin with a 20% final recovery. The purified enzyme had a unique NH2 terminus and contained N-linked endo H sensitive carbohydrate chains at its two glycosylation sites. The fucosyltransferase transferred fucose to the O-4 position of GlcNAc in small oligosaccharides, glycolipids, glycopeptides, and glycoproteins containing the type I Galbeta1-3GlcNAc motif. The acceptor oligosaccharide in bovine asialofetuin was identified as the Man-3 branched triantennary isomer with one Galbeta1-3GlcNAc. The type II motif Galbeta1-4GlcNAc in bi-, tri-, or tetraantennary neutral or alpha2-3/alpha2-6 sialylated oligosaccharides with or without N-acetyllactosamine repeats and in native glycoproteins were not modified. The soluble forms of fucosyltransferase III secreted by stably transfected cells may be used for in vitro synthesis of the Lewisa determinant on carbohydrates and glycoproteins, whereas Lewisx and sialyl-Lewisx structures cannot be synthesized.

Genetic transfer of amylovoran and stewartan synthesis between Erwinia amylovora and Erwinia stewartii

DNA fragments with ams genes of Erwinia amylovora and cps genes of Erwinia stewartii were transferred to exopolysaccharide (EPS)-deficient mutants of the other species. The resulting EPSs were characterized by sensitivity to EPS-dependent bacteriophages, staining with amylovoran-specific fluorescein-isothiocyanate-labelled lectin and chemical techniques, such as determination of the sugar composition and methylation analysis in order to distinguish between amylovoran and stewartan. Degradation by the stewartan-dependent phage phi-K9 was used to detect stewartan production, and staining with a lectin from Abrus precatorius detected amylovoran capsules. The patterns of sugar linkages were determined by methylation analysis. Stewartan contained a significantly higher glucose to galactose ratio than amylovoran and produced a characteristic signal from 6-linked glucose residues. By these criteria, most E. stewartii cps mutants displayed exclusively amylovoran synthesis when complemented with the E. stewartii cps genes produced stewartan. The complementation to an EPS-positive phenotype may require most genes of the ams or the cps operon. An exception was an E. stewartii cpsK mutant that made predominantly stewartan when complemented with the ams cosmid. IR spectra showed that both amylovoran and stewartan were acylated when synthetized in E. amylovora, but not in E. stewartii. The amylovoran-producing E. stewartii merodiploids regained virulence to corn seedlings when mucoidy was restored by the ams cluster, but the stewartan-producing E. amylovora ams-/cps+ strains were weakly virulent on pear slices and avirulent on apple seedlings.

Characterization of changes in the glycosylation pattern of recombinant proteins from BHK-21 cells due to different culture conditions

The N-glycosylation patterns of a genetically engineered human interleukin-2 variant glycoprotein (IL-Mu6), produced by BHK-21 cells from long-term suspension and microcarrier cultures in the presence and absence of fetal calf serum were compared. IL-Mu6 was used as a model protein in studying the effect of different controlled cell culture conditions on the expression of N-glycans in recombinant glycoproteins. IL-Mu6 contains a single amino acid substitution (Glu100<==>Asn) generating a potential N-glycosylation recognition site (Asn100-Xxx-Thr/Ser) in addition to the natural O-glycosylation at position Thr3. Parallel cell cultivations were carried out in two continuously perfused 2.5-liter stirred bioreactors under defined culture conditions. Major differences were found in the glycoprotein products obtained during these different cultivation conditions. Serum-free cultures resulted in a higher level of terminal sialylation and proximal alpha 1-6 fucosylation. The ratio of O- to N-glycans as well as the amount of nonglycosylated product and the antennarity of N-linked carbohydrates in the model protein exhibited major differences depending on the presence or absence of serum, the condition of growth and the cultivation procedure.

Construction of stable BHK-21 cells coexpressing human secretory glycoproteins and human Gal(beta 1-4)GlcNAc-R alpha 2,6-sialyltransferase alpha 2,6-linked NeuAc is preferentially attached to the Gal(beta 1-4)GlcNAc(beta 1-2)Man(alpha 1-3)-branch of diantennary oligosaccharides from secreted recombinant beta-trace protein

The human beta-trace protein has been cloned and has been expressed for the first time in a mammalian host cell line. Stable BHK-21 cell lines exhibiting altered terminal sialylation properties were constructed by cotransfection of cells with the plasmids pMT-beta TP or pAB3-1 which contain the cDNAs encoding the human secretory glycoproteins beta-trace protein or antithrombin III and pABSial containing the human Golgi enzyme CMP-NeuAc:Gal(beta 1-4)GlcNAc-R alpha 2,6-sialyltransferase (ST6N) gene. The beta-trace protein was purified by immunoaffinity chromatography and N-linked oligosaccharides were subjected to carbohydrate structural analysis. The enzymically liberated oligosaccharides were found to consist of 90% of diantennary chains as is the case for natural beta-trace protein from human cerebrospinal fluid. About 90% of the total oligosaccharides were recovered in the monosialo and disialo fractions in a ratio of 1:5. The monosialylated oligosaccharides of beta-trace protein coexpressed with human ST6N were found to contain NeuAc in alpha 2,6- or alpha 2,3-linkage in the same ratio. From 1H-NMR analysis as well as calculations of peak areas obtained by HPLC, 60% of the molecules of the disialo fraction were found to contain NeuAc in both alpha 2,3- and alpha 2,6-linkage to Gal beta(1-4)GlcNAc-R, whereas 40% of the molecules of this fraction contained NeuAc in only alpha 2,3-linkage to Gal(beta 1-4)GlcNAc-R. The alpha 2,6-linked NeuAc was shown to be attached preferentially to the Gal(beta 1-4)GlcNAc(beta 1-2)Man(alpha 1-3) branch of the diantennary structure. Therefore the in vivo specificity of the newly introduced recombinant human ST6N observed in this study supports the previously reported in vitro branch specificity of the bovine colostrum ST6N activity. Furthermore, these studies demonstrate the suitability of genetically engineered mammalian host cell lines with novel glycosylation properties for the production of human-type glycosylated secretory recombinant polypeptides.

Identification and structural characterization of a mannose-6-phosphate containing oligomannosidic N-glycan from human erythropoietin secreted by recombinant BHK-21 cells

A sialidase resistant mono-charged N-glycan was isolated from glycosylation site I (Asn-24) of recombinant human erythropoietin expressed from baby hamster kidney (BHK-21) cells and constituted approximately 2-4% of the oligosaccharide material at this glycosylation site. Mass spectrometry and both 1- and 2-dimensional NMR techniques revealed a high mannose type structure (Man6) with a phospho-diesterbridged N-acetylglucosamine as follows: [formula: see text]

Structure and physical properties of the extracellular polysaccharide PS-P4 produced by Sphingomonas paucimobilis P4 (DSM 6418)

A new strain, Sphingomonas paucimobilis P4 (DSM 6418), was found during a screening programme for exopolysaccharide-producing bacteria. The highly viscous fermentation broth yields a polysaccharide (up to 10 kg/m3), named PS-P4, and shows thixotropic flow behaviour. In the presence of phosphate ions, PS-P4 forms aqueous gels after heating and cooling at alkaline pH. After isolation and purification of the exopolysaccharide, structural analysis by 1D and 2D 1H NMR spectroscopy and mass spectrometry was performed. The deacylated exopolysaccharide has the following repeating trisaccharide structure:-->4)-beta-D-Glcp-(1-->4)-alpha-L-Rhap-(1-->3)-beta-D- Glcp(1-->Additionally, the presence of ester-bound acetic acid, D-glyceric acid, and (R)-3-hydroxybutyric acid in the native polysaccharide was demonstrated.

Purification and chemical characterization of beta-trace protein from human cerebrospinal fluid: its identification as prostaglandin D synthase

beta-Trace protein from pooled human CSF was purified to homogeneity. An apparent molecular mass of 23-29 kDa was determined for the polypeptide on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Amino-terminal sequencing of the polypeptide yielded the unique amino acid sequence APEAQVSVQPNFQQDKFLGRWFSA. Alignment of amino acid sequences obtained from tryptic peptides with the sequence previously deduced from a cDNA clone isolated by other investigators allowed the identification of beta-trace protein as prostaglandin D synthase [prostaglandin-H2 D-isomerase; (5Z, 13E)-(15S)-9 alpha, 11 alpha-epidioxy-15-hydroxyprosta-5,13-dienoate D-isomerase; EC 5.3.99.2]. A conservative amino acid exchange (Thr instead of Ser) was detected at amino acid position 154 of the beta-trace polypeptide chain in the corresponding tryptic peptide. The two N-glycosylation sites of the polypeptide were shown to be almost quantitatively occupied by carbohydrate. Carbohydrate compositional as well as methylation analysis indicated that Asn29 and Asn56 bear exclusively complex-type oligosaccharide structures (partially sialylated with alpha 2-3- and/or alpha 2-6-linked N-acetylneuraminic acid) that are almost quantitatively alpha 1-6 fucosylated at the proximal N-acetylglucosamine; approximately 70% of these molecules contain a bisecting N-acetylglucosamine. Agalacto structures as well as those with a peripheral fucose are also present.

Biosynthesis and secretion of human interleukin 2 glycoprotein variants from baculovirus-infected Sf21 cells. Characterization of polypeptides and posttranslational modifications

Human interleukin 2 (IL-2) and human IL-2 mutant proteins, with artificially introduced N-glycosylation or O-glycosylation sites, have been expressed in a lepidopteran cell line (Sf21, Spodoptera frugiperda) using recombinant baculovirus vectors. Only approximately 25% of the total recombinant IL-2 protein synthesized by Sf21 cells was secreted into the culture medium. Significant N-terminal truncations were detected in the secreted polypeptides (up to 85% of the molecules). Alanine and proline were absent in the major truncated forms; the first 3-5 amino acids were also absent in a small proportion of the purified proteins. The introduction of potential artificial O-glycosylation peptide sequences (..GGKAPTPPPK..), to the C-terminus or between positions 80 and 81 of the IL-2 polypeptide chain, resulted in the secretion of unglycosylated and O-glycosylated variant forms. Fast atom bombardment mass spectrometry, compositional analysis and methylation analysis, of the tryptic glycopeptide APTPPPK, revealed the presence of either GalNAc or the disaccharide Gal(beta 1-3)GalNAc as the only carbohydrate constituents attached exclusively to Thr in this peptide, in a specific ratio for each individual IL-2 mutant protein. The Gal(beta 1-3)GalNAc protein forms could be partially altered in vitro to mammalian-type glycoforms by porcine liver beta-galactoside alpha-2,3-sialyltransferase in the presence of CMP-N-acetylneuraminic acid. An IL-2 mutant form, with an 11-amino-acid peptide of human interferon-beta at position 4, which includes its only N-glycosylation site, had exclusively truncated proximally fucosylated oligomannosidic glycans; Man3GlcNAc[Fuc(alpha 1-6)]GlcNAc or Man2GlcNAc[Fuc(alpha 1-6)]GlcNAc structures, in a ratio of 3:1, were detected in the secreted proteins. No evidence was obtained for the presence of secreted proteins with complex oligosaccharide chains, irrespective of the cell-culture conditions used or the harvesting time, for infected cells with recombinant baculovirus constructs.

Structures of sialylated oligosaccharides of human erythropoietin expressed in recombinant BHK-21 cells

The native structures of the Asn-linked oligosaccharides and the O-glycans at Ser126 of human erythropoietin expressed from recombinant BHK cells have been elucidated. Enzymatically released N-glycans were studied by methylation analyses, fast-atom-bombardment mass spectrometry as well as one- and two-dimensional 1H-NMR spectrometry at 600 MHz. Many (82.7%) were found to be tetraantennary N-acetyllactosamine-type (22.8% with one, 3.6% with two and 0.4% with three N-acetyllactosamine repeats) being tetrasialylated (41%), trisialylated (29.6%) and disialylated (12.2%). A few (9.7%; 4.1% 2,4-branched, 5.6%, 2,6-branched) of the chains were triantennary (5.4% trisialyl, 4.3% disialyl) and 4.6% were of the disialyl diantennary type. Almost all of the innermost GlcNAc residues were alpha 1-6 fucosylated and NeuAc was exclusively alpha 2-3 linked to Gal beta 1-4GlcNAc-R; 60% of the protein was found to be O-glycosylated at Ser126; structures were monosialylated (70%) or disialylated (30%) forms of the Gal beta 1-3GalNAc core type. Glycosylation patterns at individual Asn-Xaa-Thr/Ser sites were determined by analytical high-pH anion-exchange chromatography with pulsed amperometric detection. Only tetraantennary chains with 0-3 N-acetyllactosamine repeats were detected at Asn38 and Asn83, while almost all of the di- and triantennary oligosaccharides were attached to Asn24. Batch analysis of different preparations of recombinant erythropoietin revealed the high reproducibility of the production procedure. Structures containing terminal GalNAc-GlcNAc were detected in small amounts in a few batches.

Primary structure of N-linked carbohydrate chains of a human chimeric plasminogen activator K2tu-PA expressed in Chinese hamster ovary cells

A recombinant human plasminogen activator hybrid variant K2tu-PA, expressed in Chinese hamster ovary cells, is partially glycosylated at Asn12 (A chain, kringle-2 domain) and completely glycosylated at Asn247 (B chain, protease domain). After release of the N-linked carbohydrate chains by peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F, the oligosaccharides were separated from the protein by gel permeation chromatography, then fractionated by FPLC on Mono Q, followed by HPLC on Lichrosorb-NH2, and analysed by 500-MHz 1H-NMR spectroscopy. The following types of carbohydrates occur: monosialylated diantennary (8%), disialylated diantennary (45%), disialylated tri- and tri'-antennary (1%), trisialylated tri- and tri'-antennary (28%), and tetrasialylated tetra-antennary (18%) structures, all having fucose in alpha(1-6)-linkage at the Asn-bound N-acetylglucosamine. Sialic acid occurred exclusively in alpha(2-3)-linkage to galactose, and consisted of N-acetylneuraminic acid (94%), N-glycolylneuraminic acid (3%), and N-acetyl-9-O-acetylneuraminic acid (3%). In addition, glycopeptide fragments corresponding with the A or B chain of K2tu-PA were analysed. The oligosaccharides attached to Asn12 are less processed than those attached to Asn247. Comparison of the glycosylation pattern of K2tu-PA with that of tissue-type plasminogen activator from different biological sources showed significant differences. Profiling studies on different K2tu-PA production batches demonstrated that the structures of N-linked oligosaccharides were identical, but that relative amounts vary with the applied isolation procedure of the chimeric glycoprotein.

Decreased affinity of recombinant antithrombin for heparin due to increased glycosylation

Recombinant antithrombin produced by baby hamster kidney (BHK) or Chinese hamster ovary (CHO) cells was separated into two fractions, containing comparable amounts of protein, by affinity chromatography on matrix-linked heparin. Fluorescence titrations showed that the more tightly binding fraction had a heparin affinity similar to that of plasma antithrombin (Kd approximately 20 nM), whereas the affinity of the more weakly binding fraction was nearly 10-fold lower (Kd approximately 175 nM). Analyses of the heparin-catalysed rate of inhibition of thrombin further showed that the fractions differed only in their affinity for heparin and not in the intrinsic rate constant of either the uncatalysed or the heparin-catalysed inactivation of thrombin. The recombinant antithrombin fraction with lower heparin affinity migrated more slowly than both the fraction with higher affinity and plasma antithrombin in SDS/PAGE under reducing conditions, consistent with a slightly higher apparent relative molecular mass. This apparent size difference was abolished by the enzymic removal of the carbohydrate side chains from the proteins. Such removal also increased the heparin affinity of the weakly binding fraction, so that it eluted from matrix-linked heparin at a similar position to the deglycosylated tightly binding fraction or plasma antithrombin. Analyses of N-linked carbohydrate side chains showed that the weakly binding fraction from CHO cells had a higher proportion of tetra-antennary and a lower proportion of biantennary oligosaccharides than the tightly binding fraction. We conclude that the recombinant antithrombin produced by the two cell lines is heterogeneously glycosylated and that the increased carbohydrate content of a large proportion of the molecules results in a substantial decrease in the affinity of these molecules for heparin. These findings are of particular relevance for studies aimed at characterizing the heparin-binding site of recombinant antithrombin by site-directed mutagenesis.

A strategy for the mapping of N-glycans by high-pH anion-exchange chromatography with pulsed amperometric detection

We have evaluated the high-pH anion-exchange chromatography with pulsed amperometric detection (HPAE-PAD) with respect to its suitability to establish a carbohydrate mapping database that would enable carbohydrate structural analysis by mere comparison of retention times. The suitability of HPAE-PAD for carbohydrate structural analysis was ascertained by validation experiments. The retention times of distinct N-glycans, prepared and measured on different days, were shown to be highly reproducible, with a coefficient of variation (CV) of less than 0.5%, requiring less than 100 pmol of N-glycan per injection for reliable measurements. Including appropriate internal chromatographic standards, such as (Neu5Ac)1, (Neu5Ac)2, (Neu5Ac)3, and Neu5Gc, the HPAE-PAD method fulfills the analytical requirements with respect to accuracy, precision, reproducibility, and sensitivity. The N-glycan mapping database was established, using two optimized linear gradients "S" and "A" for sialylated and asialo N-glycans, respectively. Approximately 100 different N-glycans of known structure, which have thus far been measured and characterized, have entered our Lotus 1-2-3 mapping database. The efficiency of the database for structural determinations was tested, using the N-linked carbohydrates isolated from rhuEPO, expressed in BHK cells. Nine different sialylated N-glycans of rhuEPO (BHK) could be assigned with a deviation of less than +/- 0.5%, using gradient S, and six of the eight asialo N-glycans of rhuEPO (BHK) detected with gradient A could be assigned with an accuracy of less than +/- 1%, three of them even with an accuracy of less than 0.1%, providing the reliability of the established HPAE-PAD mapping database.