α-chains of IgM and IgD antigen receptor complexes are differentially N-glycosylated MB-1-related molecules

The major B cell Ag receptors, membrane (m) IgM and mIgD, are noncovalently associated with disulfide-linked heterodimers of α, β, and γ glycoproteins. The β and γ chains have apparent molecular masses of 37 and 34 kDa, respectively, and are associated with both mIgM and mIgD. Receptor α chains, however, exhibit Ig isotype specificity. IgM-α and IgD-α have apparent molecular masses of 32 and 33 kDa, respectively. Recently, the α chain of the IgM Ag receptor complex was identified as the product of the mb-1 gene, and the β and γ chains were characterized as products of the B29 gene. The failure of mb-1 cDNA to hybridize with mRNA from J558δm2.6 plasmacytomas expressing surface mIgD in association with IgD-α has led to the conclusion that IgM-α and IgD-α are not closely related. In this report we have used protein biochemical methods to characterize differences in the mIgM- and mIgD-associated α chains. In addition to a slightly greater apparent m.w., IgD-α was slightly more acidic than IgM-α. The a chains had nearly identical proteolytic peptide maps, and were also noted to have multiple loci of identity with MB-1 based on amino terminal sequencing and immunoblotting. In an attempt to determine whether the α chains differed as a result of differential posttranslational modification, they were compared after deglycosylation with N-glycanase. The results indicate that the apparent m.w. as well as isoelectric point differences are primarily due to differential N-linked glycosylation. These studies indicate that IgM-α and IgD-α are products of the mb-1 gene or closely related genes.