Glycosylation of the chorionic gonadotropin alpha subunit synthesized by HeLa cells

Abstract

The glycoprotein hormone alpha subunit secreted by HeLa cells was retained by concanavalin A:Sepharose and by ricin:agarose, indicating, that the tumor protein has carbohydrate side chains containing both mannose and galactose residues. Lectin chromatography of the intracellular hormone suggests it is probably a precursor to the secreted protein, it was bound by concanavalin A but not by ricin, suggesting the presence of a high mannose core oligosaccharide but the absence of terminal sugar residues. The glycosylation inhibitors tunicamycin and 2-deoxy-D-glucose caused a reduction in alpha subunit secretion comparable to their reduction of general protein synthesis but considerably less than their inhibition of protein glycosylation. Various HeLa lines secreted alpha subunit at widely different rates, with HeLa CCL 2.2 having the highest rate of production, HeLa CCL 2.1 having the lowest, and HeLa CCL 2 being intermediate. Dose-response curves for alpha subunit from the different HeLa lines and from tunicamycin- and deoxyglucose-treated cells were sufficiently parallel to indicate similar immunological characteristics. The incorporation of radiolabeled glucosamine and N-acetylmannosamine into secreted proteins varied among the cell lines examined and was generally comparable to their hormone production rates. Concanavalin A:Sepharose chromatography of the alpha subunit secreted by HeLa CCL 2.2 and CCL 2.1 indicated that both proteins possess oligosaccharide side chains containing mannose while chromatography of these proteins on ricin:agarose suggested that less of the alpha subunit from CCL 2.1 contains galactose than that from CCL 2.2.