Immunoglobulin G Biosynthesis in a Human Lymphoblastoid Cell Line

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Abstract
The cultured human B lymphoblastoid cell line Maja synthesises two forms of the ‐γ heavy chain of immunoglobulin G (IgG) that differ in apparent molecular weight. The lower‐molecular‐weight form is secreted into the culture medium as a water‐soluble product in association with light chains and comigrates on dodecyl sulphate polyacrylamide gels with serum IgG γ chains. The higher‐molecular‐weight form is not detected in culture supernatants. In distinction to the secreted form, the higher‐molecular‐weight form is labelled by a lipophilic, photoactivatable nitrene and is inserted asymmetrically in a transmembrane orientation into rough microsomes. It is concluded that Maja cells synthesise secretory (γs) and membrane‐associated (γm) forms of IgG heavy chains. Both forms of the heavy chain are glycosylated, and can contain one or two asparagine‐linked glycan units. The γm and γs heavy chains differ by about 10000 in apparent molecular weight. This difference resides exclusively in the polypeptide moiety. Although part of the difference comprises a transmembrane peptide and a cytoplasmic tail of apparent molecular weight about 2000 for γm chains, a substantial segment of unique peptide is most probably present on the non‐cytoplasmic side of the bilayer. The ionophore monensin inhibits the intracellular transport of γs and γm chains at a stage when they are sensitive to the enzyme endo‐β‐N‐acetylglucosaminidase H. In contrast, HLA‐A and HLA‐B antigens reach a stage at which they are insensitive to this enzyme in the presence of monensin, although their surface expression is inhibited by the ionophore. The implications of these results for the intracellular transport of membrane‐associated glycoproteins are discussed.