Structural differences between heavy chains of secreted and membrane-bound IgM of a human lymphoblastoid cell line

The human lymphoblastoid cell line BL was shown to synthesise three distinct molecular species of immunoglobulin M heavy chains: membrane-bound (micrometer). intracellular (micro i) and secreted (microseconds) micro-chains. Only the membrane-bound form could be labeled with a lipophilic photoactivatable nitrene reagent. Analysis of their constituent CNBr fragments and carboxypeptidase A and B digestions of their C-terminal tails suggest that the CNBr peptide pattern of microseconds and micrometer, though similar, is not identical, and that amino acids released at the C-termini of the chains are different. The data confirm recent observations in human and murine systems be showing that the membranes-associated human micro-chain contains a hydrophobic segment, consistent with its anchorage into the lipid bilayer of the plasma membrane and a C-terminal amino acid sequence different from that of the secretory micro-chain.