Characterization of recombinant human interleukin 4 receptor from CHO cells: role of N-linked oligosaccharides

Interleukin 4 (IL-4) mediates its biological activities through interaction with its receptor on the cell surface. A recombinant extracellular domain of the alpha subunit of human interleukin 4 receptor was expressed in CHO cells and purified to homogeneity by a combination of ion exchange and immunoaffinity chromatography. Analysis of the purified protein by MALDI MS provided an average mass of 38,241 Da while microsequencing identified the site of the signal sequence processing to be Ser23-Gly24. The receptor was highly glycosylated, containing N-linked complex oligosaccharides with bi-, tri-, and tetraantennary structures. Five of the six potential glycosylation sites could be assigned to Asn residues 53, 98, 128, 134 and 176. N-deglycosylation increased aggregation and reduced solubility of the receptor but did not affect its IL-4 binding activity. These observations provide preliminary insights into the role of N-linked oligosaccharides in IL-4 receptor biosynthesis and function at the cell surface.

An enzyme-linked immunosorbent assay (ELISA) for quantitation of adducts of granulocyte-macrophage colony stimulating factor (GM-CSF) and human serum albumin (HSA) in stressed solution mixtures

HPLC analyses of GM-CSF in solution mixtures containing both GM-CSF and HSA showed losses of GM-CSF which could not be accounted for using conventional electrophoretic and/or RP-HPLC techniques. Further investigation of these mixtures by immunoblotting and by immunoaffinity chromatography demonstrated the presence of high molecular weight (> 67,000 GM-CSF related species. No such species was detectable in solutions of GM-CSF alone. This experiment pointed to the formation of an adduct between GM-CSF and HSA in the solution mixtures. To probe further the hypothesis of a GM-CSF/HSA adduct, an immunologically based test was conceived which could react only with this type of hybrid molecule. A sandwich enzyme-linked immunosorbent assay (ELISA) was developed using two antibodies, anti-GM-CSF (capture antibody) and anti-HSA (detection antibody), as part of the quantitation of GM-CSF/HSA adducts. After confirming its existence by ELISA, a GM-CSF/HSA adduct was isolated from the solution mixture containing both GM-CSF and HSA. This isolate served as a primary reference standard in the ELISA assay. The immunoassay has a subnanogram sensitivity and is highly specific for GM-CSF/HSA adducts in the presence of either free GM-CSF or free HSA. As a verification, conjugates of GM-CSF/HSA were synthesized using a cross-linking reagent. These covalent conjugates reacted positively in the ELISA and are employed as a convenient alternative reference standard.

New suggestions for the physiological role of secretory protein-I

A major endocrine secretory protein of unknown function termed secretory protein-I or, alternatively, chromogranin A, is attracting increasing attention. Because of its abundance and apparently universal presence in endocrine and neuroendocrine cells, many suggestions for a physiological role have been advanced. Recent data provide the first firm bases for some proposed functions that include a regulator of a novel protease capable of converting prohormones to hormones, a hormone precursor, and through its calcium-binding and aggregating properties, a secretory granule condensing protein.

Sulfation of porcine parathyroid secretory protein I. Detection of tyrosine sulfate

Secretory Protein I (SP-I) is an acidic glycoprotein that is stored and co-secreted with parathormone by parathyroid glands. It has been found to be chemically similar, if not identical, to chromogranin A of the adrenal medulla and to be present in most endocrine cells. In the present study, 35SO4 was shown to be incorporated into SP-I and several other proteins of porcine parathyroid tissue incubated in vitro. The predominant sulfated species secreted to the medium was SP-I. Up to 20% of the tyrosine residues in secreted SP-I were labeled with 35SO4. Both the cellular and secreted forms migrated on sodium dodecyl sulfate gels as a pair of proteins with apparent molecular weights of 82,000 and 78,000. The 82-kDa protein could be converted to the 78-kDa species by treatment with neuraminidase. Sulfate exists in SP-I as tyrosine sulfate based on the identification of this amino acid by thin layer electrophoresis following alkaline hydrolysis. Extracellular Ca2+ (3 mM) greatly suppressed the secretion of 35SO4-labeled SP-I without affecting the intracellular sulfation of the molecule or the secretion of a minor sulfated protein unrelated to SP-I. The ratio of incorporated 35SO4 to 3H-amino-acid was greater in secreted SP-I than in tissue SP-I, suggesting that much sulfation of this protein occurred during or just before secretion.

Galactose-rich glycoproteins are on the cell surface of herpes virus-infected cells. 1. Surface labeling and serial lectin binding studies of Asn-linked oligosaccharides of glycoprotein gC

Cell-surface glycoproteins of mock-infected and herpes simplex virus type 1 (HSV-1)-infected BHK-21 and HEp-2 cells were radiolabeled by incubation with galactose oxidase followed by reduction with NaB3H4. The incorporation of radiolabel into glycoconjugates in both BHK-21 and HEp-2 cells was increased several fold following infection with HSV, showing an increase in surface-exposed Gal residues in the infected cells. This was further confirmed by an increase in binding of cell-surface-labeled glycoproteins gC and gB from HSV-infected BHK-21 cells to Ricinus communis agglutinin I, which is specific for beta-D-Gal residues. Prior treatment of cells with Clostridium perfringens neuraminidase enhanced the surface radiolabeling by the galactose oxidase/NaB3H4 method: HEp-2 cells exhibited over sixfold enhancement in labeling, while BHK-21 cells showed only a slight increase. HSV glycoprotein gC was the predominant cell-surface glycoprotein radiolabeled by the galactose oxidase/NaB3H4 method in virus-infected BHK-21 cells. The glycoprotein gC was purified by immunoaffinity column chromatography on monoclonal anti-gC-antibody-Sepharose. The radiolabel in the glycopeptides of gC was resistant to beta elimination, showing that it was associated only with Asn-linked oligosaccharides. A serial lectin affinity chromatography of glycopeptides on columns of concanavalin A-Sepharose, lentil (Lens culinaris) lectin-Sepharose, and Ricin I-agarose allowed the assignment of minimal oligosaccharide structures bearing terminal Gal residues in gC.

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

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