Intracellular processing of membrane and secreted immunoglobulin delta-chains

Membrane-bound and secreted immunoglobulin delta-chains are synthesized by the mouse hybridoma B1-8 delta.1 as two primary translation products (45,000 and 42,000, respectively) and are converted into three N-glycosylated forms. In addition to N-glycosylation, another modification, reflected in a size increase of 2000 to 3000, occurs within 8 min of synthesis and may be O-glycosylation. After these initial modifications, the N-linked carbohydrates of all three forms are partially trimmed, apparently in the endoplasmic reticulum. The secreted delta-chains acquire galactose and sialic acids less than 10 min before they are secreted. Monensin and CCCP, which are potent inhibitors of IgD secretion, inhibit the terminal glycosylation but not the other modifications of delta-chains. CCCP blocks the intracellular transport of IgD at an earlier stage of trimming than monensin. Within 10 min of the removal of CCCP, some of the accumulated IgD is terminally glycosylated and secreted. Membrane IgD is processed similarly to secreted IgD up to the stage blocked by CCCP. These observations suggest that final trimming and terminal glycosylation of IgD occurs in the Golgi complex, and that the other modifications occur in pre-Golgi compartments, where secreted IgD spends most of its transit time. We suggest that the rate-limiting step in the intracellular transport of both types of IgD is the passage from the ER to the Golgi complex.