Biosynthesis of two forms of IgM heavy chains by normal mouse B lymphocytes. Membrane and secretory IgM.

A study of the biosynthesis of IgM by purified mouse spleen lymphocytes showed that these cells synthesize both 8 S membrane IgM and 19S secretory IgM, which is identical with plasma cell IgM except in its kinetics of processing, assembly, and secretion. The heavy (mu) chains of these two types of lymphocyte IgM differ in their ultimate fate, in processing, isoelectric point, and peptide composition. The separate precursors of the two mu chains have very similar mobilities in sodium dodecyl sulfate polyacrylamide gel electrophoresis, but they can be distinguished by the use of endoglucosaminidase H (endo-H) to remove core sugars, by two-dimensional electrophoresis, and by one-dimensional gel analysis in pulse-chase experiments. CNBr peptide patterns of intracellular "secretory" mu chains of lymphocytes and plasma cells were similar, but membrane mu chains had a COOH-terminal peptide different in structure from that of secreted mu chains, with a higher apparent molecular weight.