Biogenesis of membrane-bound and secreted immunoglobulins: two primary translation products of the human delta chain, differentially N-glycosylated to four discrete forms in vivo and in vitro.

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Abstract
Structural differences between the H chain of membrane IgD (.delta.m) and the H chain of secreted IgD (.delta.s) were investigated by using a human lymphoblastoid cell line that express idiotypically identical IgM and IgD. In a wheat germ cell-free system, mRNA from this cell line encoded 2 distinct .delta. chains that differed in MW. When translated in vitro in the presence of dog pancreatic microsomal membranes or when synthesized in vivo, these 2 .delta. chains were processed to 4 discrete glycosylated forms, all of which shared idiotypic determinants, C region determinants and L chain linkage. As shown by digestion with endo-.beta.-N-acetylglucosaminidase H, these 4 .delta. forms represent 2 .delta. polypeptide chains that are differentially N-glycosylated. Pulse-chase experiments demonstrated that, after endo-.beta.-N-acetylglucosaminidase H treatment, .delta.m has a higher MW than .delta.s. After integration into dog pancreatic microsomal membranes in vitro, .delta.m did not have a large cytoplasmic domain exposed to proteolytic digestion. The finding that .delta.m and .delta.s differ in primary structure is analogous to previous work with the corresponding H chains of IgM (.mu.m and .mu.s) from the same cell line. Thus, this cell line produces 4 Ig H chains (.mu.m, .mu.s, .delta.m, .delta.s), with the same idiotype. The observation of differential N-glycosylation, apparently unique for the .delta. class, is discussed.