Tools to cleave glycoproteins

Pre-B acute lymphoblastic leukemia expresses cell surface nucleolin as a 9-O-acetylated sialoglycoprotein

Precursor B acute lymphoblastic leukemias (pre-B ALLs) abnormally express a specific glycan structure, 9-O-acetylated sialic acid (9-O-Ac-Sia), on their cell surface, but glycoproteins that carry this modification have not been identified. Using three different lectins that specifically recognize this structure, we establish that nucleolin (NCL), a protein implicated in cancer, contains 9-O-Ac-Sia. Surprisingly, antibodies against the glycolipid 9-O-Ac-Sia GD3 also detected 9-O-Ac-Sia NCL. NCL is present on the surface of pre-B ALL cells as a sialoglycoprotein that is partly 9-O-acetylated and conversely, 9-O-Ac-Sia-containing structures other than NCL are present on these cells as well. Interestingly, NCL and the 9-O-Ac-Sia signal had less co-localization on normal pre-B cells. We also investigated regulation of NCL on the cell surface and found that sialidase treatment increased the percentage of cells positive for cell surface NCL, suggesting that sialylation of NCL promotes internalization. Treatment of pre-B ALL cells with the chemotherapy drug vincristine also increased the percentage of cells with surface NCL and correlated with increased 9-O-Ac-Sia expression. All tested leukemia cells including primary samples expressed NCL, suggesting it as a possible therapeutic target. We confirmed this by showing inhibition of cell proliferation in some pre-B ALLs by exposure to a NCL-specific aptamer AS1411.

Modulation of glycan recognition by clustered saccharide patches

All cells in nature are covered with a dense and complex array of glycan chains. Specific recognition and binding of glycans is a critical aspect of cellular interactions, both within and between species. Glycan-protein interactions tend to be of low affinity but high specificity, typically utilizing multivalency to generate the affinity required for biologically relevant binding. This review focuses on a higher level of glycan organization, the formation of clustered saccharide patches (CSPs), which can constitute unique ligands for highly specific interactions. Due to technical challenges, this aspect of glycan recognition remains poorly understood. We present a wealth of evidence for CSPs-mediated interactions, and discuss recent advances in experimental tools that are beginning to provide new insights into the composition and organization of CSPs. The examples presented here are likely the tip of the iceberg, and much further work is needed to elucidate fully this higher level of glycan organization.

Episialin acts as an antiadhesive factor in an in vitro model of human endometrial-blastocyst attachment

Episialin, which is found on the apical membrane of human endometrial epithelium, has been postulated to act as an antiadhesive factor through the steric hindrance generated by its extensively glycosylated structure. The present studies were designed to test this hypothesis in an in vitro model of endometrial-blastocyst attachment. Episialin was expressed in human endometrial carcinoma cells (HEC-1A > RL95-2), and attachment of JAr choriocarcinoma cells to the endometrial cell monolayers was inversely related to episialin expression. Treatment of endometrial monolayers with type III sialidase increased JAr binding, and this increase was suppressed by HMFG1, a monoclonal antibody specific for episialin. The effects of sialidase appear to have resulted from a contaminant protease rather than from a loss of sialic acid residues, because sialidase preparations other than type III were ineffective. After sialidase treatment, conditioned medium from cells treated with type III sialidase contained more episialin than medium from cells treated with other sialidase preparations. Similar attachment-assay results were obtained using O-sialoglycoprotein endopeptidase; after treatment, the increase in JAr binding (>50%) was suppressed by the antiepisialin antibody. These results demonstrate for the first time that episialin acts as an antiadhesive agent in a model of human endometrial-blastocyst attachment.

Protein glycosylation: implications for in vivo functions and therapeutic applications

The glycosylation machinery in eukaryotic cells is available to all proteins that enter the secretory pathway. There is a growing interest in diseases caused by defective glycosylation, and in therapeutic glycoproteins produced through recombinant DNA technology route. The choice of a bioprocess for commercial production of recombinant glycoprotein is determined by a variety of factors, such as intrinsic biological properties of the protein being expressed and the purpose for which it is intended, and also the economic target. This review summarizes recent development and understanding related to synthesis of glycans, their functions, diseases, and various expression systems and characterization of glycans. The second section covers processing of N- and O-glycans and the factors that regulate protein glycosylation. The third section deals with in vivo functions of protein glycosylation, which includes protein folding and stability, receptor functioning, cell adhesion and signal transduction. Malfunctioning of glycosylation machinery and the resultant diseases are the subject of the fourth section. The next section covers the various expression systems exploited for the glycoproteins: it includes yeasts, mammalian cells, insect cells, plants and an amoeboid organism. Biopharmaceutical properties of therapeutic proteins are discussed in the sixth section. In vitro protein glycosylation and the characterization of glycan structures are the subject matters for the last two sections, respectively.

Membrane protein proteolysis assayed by fluorescence quenching: assay of O-sialoglycoprotein endopeptidase

The assay of the O-sialoglycoprotein endopeptidase of Pasteurella haemolytica has previously used the cleavage of 125I-labeled glycophorin A, measured by SDS-PAGE, autoradiography, gel-slicing, and scintillation counting. A new assay is based on the increased fluorescence which results from proteolytic cleavage of a fluorescence-quenched micellar substrate, 4,4-difluor-5,7-dimethyl-4-bora-3 alpha, 4 alpha-diaza-s-indacene-3-propionic acid conjugated to glycophorin A (BODIPY-FL-glycophorin A). Micellar association of glycophorin A molecules results in 97% fluorescence quenching despite a low molar ratio of BODIPY-FL-glycophorin A. Proteolysis of the membrane protein causes greatly enhanced fluorescence which is used for a rapid one-step proteolysis assay. Direct monitoring of proteolysis in microcuvettes, or routine assay in microtiter plates can be used. Reproducibility is higher than with the radiolabeled substrate and the K(m) values for the two substrates are similar. The assay is suitable for the O-sialoglycoprotein endopeptidase activity of chromatographically purified enzyme or unpurified bacterial culture supernatants and can be used to monitor inhibition of the O-sialoglycoprotein endopeptidase by neutralizing antibodies. The O-sialoglycoprotein endopeptidase assay employing BODIPY-FL-glycophorin A provides a rapid and nonradioactive method for the assay of this highly specific enzyme.

Characterization of densin-180, a new brain-specific synaptic protein of the O-sialoglycoprotein family

We purified an abundant protein of apparent molecular mass 180 kDa from the postsynaptic density fraction of rat forebrain and obtained amino acid sequences of three tryptic peptides generated from the protein. The sequences were used to design a strategy for cloning the cDNA encoding the protein by polymerase chain reaction. The open reading frame of the cDNA encodes a novel protein of predicted molecular mass 167 kDa. We have named the protein densin-180. Antibodies raised against the predicted amino and carboxyl sequences of densin-180 recognize a 180 kDa band on immunoblots that is enriched in the postsynaptic density fraction. Immunocytochemical localization of densin-180 in dissociated hippocampal neuronal cultures shows that the protein is highly concentrated at synapses along dendrites. The message encoding densin-180 is brain specific and is more abundant in forebrain than in cerebellum. The sequence of densin-180 contains 17 leucine-rich repeats, a sialomucin domain, an apparent transmembrane domain, and a PDZ domain. This arrangement of domains is similar to that of several adhesion molecules, in particular GPIbalpha, which mediates binding of platelets to von Willebrand factor. We propose that densin-180 participates in specific adhesion between presynaptic and postsynaptic membranes at glutamatergic synapses.

Influence of the carbohydrate moiety on the stability of glycoproteins

To study the role of oligosaccharides on the properties of glycoproteins, five glycoproteins (yeast external invertase, bovine serum fetuin, glucoamylase from Aspergillus niger, and chicken egg white ovotransferrin and avidin) of previously established glycan patterns were purified to homogeneity and deglycosylated with endo- and exo-glycosidases in native conditions. Thermal stability and conformational changes were measured by high-resolution differential scanning microcalorimetry and circular dicroism spectroscopy before and after they were deglycosylated. It was found that deglycosylation decreases protein thermal stability, as judged by the decrease in denaturation temperature and denaturation enthalpy, while it does not affect substantially the conformation as indicated by the CD spectra in the far UV range. The destabilization effect of deglycosylation seems to depend on the carbohydrate content, i.e., the maximum effect was observed for the most heavily glycosylated protein, irrespective of the types (N-linked or O-linked) or patterns (mono- or multi-branched) of the covalently attached carbohydrate chains. In addition, studies of the reversibility to heat denaturation revealed that deglycosylated proteins have a poorer thermal reversibility in calorimetric scans than their native counterparts and tend to aggregate during thermal inactivation at acidic pH. These results suggest that carbohydrate moieties, in addition to the apparent stabilizing effect, may prevent the unfolded or partially folded protein molecules from aggregation. Our results support the hypothesis that the general function of protein glycosylation is to aid in folding of the nascent polypeptide chain and in stabilization of the conformation of the mature glycoprotein.

Expression in Escherichia coli, purification and characterization of heparinase I from Flavobacterium heparinum

The use of heparin for extracorporeal therapies has been problematical due to haemorrhagic complications; as a consequence, heparinase I from Flavobacterium heparinum is used for the determination of plasma heparin and for elimination of heparin from circulation. Here we report the expression of recombinant heparinase I in Escherichia coli, purification to homogeneity and characterization of the purified enzyme. Heparinase I was expressed with an N-terminal histidine tag. The enzyme was insoluble and inactive, but could be refolded, and was purified to homogeneity by nickel-chelate chromatography. The cumulative yield was 43%, and the recovery of purified heparinase I was 14.4 mg/l of culture. The N-terminal sequence and the molecular mass as analysed by matrix-assisted laser desorption MS were consistent with predictions from the heparinase I gene structure. The reverse-phase HPLC profile of the tryptic digest, the Michaelis-Menten constant Km (47 micrograms/ml) and the specific activity (117 units/mg) of purified recombinant heparinase I were similar to those of the native enzyme. Degradation of heparin by heparinase I results in a characteristic product distribution, which is different from those obtained by degradation with heparinase II or III from F. heparinum. We developed a rapid anion-exchange HPLC method to separate the products of enzymic heparin degradation, using POROS perfusion chromatography media. Separation of characteristic di-, tetra- and hexa-saccharide products is performed in 10 min. These methods for the expression, purification and analysis of recombinant heparinase I may facilitate further development of heparinase I-based medical therapies as well as further investigation of the structures of heparin and heparan sulphate and their role in the extracellular matrix.