B lymphocytes contain three species of mu chains

The murine B cell line cloned from a single cell, 38C-13, synthesizes three species of mu chains, that of cell surface membrane IgM (m-mu), that of secreted IgM (s-mu) and that of intracellular IgM (i-mu). They differ in their mobility on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Sequence analysis of the different mu chains suggests that they are identical in the N-terminal as well as in their C-terminal positions. The ratio between incorporated radioactive monosaccharides to radioactive amino acids into the three different mu chains was higher in s-mu than in m-mu, but nevertheless m-mu migrated more slowly than s-mu on sodium dodecyl sulfate polyacrylamide gel electrophoresis. However, since this ratio may also be influenced by the rate of synthesis, it may not represent a real molar ratio of carbohydrate to protein. Studies with normal spleen cells clearly indicated the presence of the same three types of mu chains.