Addition of glucosamine and mannose to nascent immunoglobulin heavy chains

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Abstract
The process of protein glycosylation was investigated to determine whether glucosamine and mannose are added to nascent chains prior to chain completion or only to completed chains after release from the ribosome. The MPC 11 mouse plasmacytoma cell line used in these studies synthesizes a glycosylated .gamma.2b H chain which accounts for 12% of the total protein synthesis. Nascent chains were separated from completed chains by ion-exchange chromatography of solubilized ribosomes on QAE-Sephadex. Glucosamine and mannose are apparently incorporated into nascent H chains prior to chain completion and release from the ribosome. Gel analysis of specifically immunoprecipitated nascent chains indicates that the carbohydrate moiety can be added to the nascent H chains very soon after the presumptive asparaginyl glycosylation site (CH2 [H chain constant region 2] domain) is synthesized on the ribosome.