Regulation of protein expression in differentiation by subunit assembly. Human membrane and secreted IgM.

Newly synthesized IgM heavy chains are either expressed as functional IgM or catabolized, depending on the stage of differentiation of the B cell. Heavy chains of the membrane type are rapidly degraded in pre-B cells, but expressed on the cell surface as monomeric IgM in resting and secreting B cells. Secreted-type heavy chains are catabolized in pre-B and resting B cells, but secreted as pentameric IgM by secreting B cells. The differences between the heavy chains that are expressed and those that are catabolized are post-translational. Stable membrane and secreted heavy chains have been covalently assembled with light chains, terminally glycosylated, and removed from intracellular proteases by insertion into the plasma membrane or by secretion. The carboxylic ionophores monensin and nigericin have been used here to determine the relative importance of these post-translational events in stabilizing newly synthesized heavy chains. Monensin and nigericin inhibited both the rates and extents of terminal glycosylation and of intracellular transport of these proteins, without affecting the covalent assembly processes. These ionophores did not affect the rate of catabolism of heavy chains in either a resting or a secreting B cell line. IgM heavy chains thus appear to be stabilized against intracellular proteolysis by full covalent assembly to monomeric membrane IgM and to secreted pentameric IgM. The fully assembled IgM can then be terminally glycosylated and transported to the cell surface.

Regulation of catabolism of IgM heavy chains in a B lymphoma cell line.

The human lymphoma cell line Daudi has the phenotype of a nonsecreting B cell. This cell line synthesizes both the membrane and secreted forms of the IgM heavy chain, but only expresses functional membrane IgM. We have found that secreted type heavy chains (mus) are rapidly degraded in these cells, with a half-life of 1.3 h. Some of the membrane type heavy chains (mum) are also rapidly catabolized but some are expressed in a stable form with a half-life of 13 h. Inhibiting the initial glycosylation of heavy chains with tunicamycin has differential effects on the catabolic rates of mus and mum chains. The turnover of mus chains is not affected by this inhibitor, but the degradation of mum chains is much more rapid after tunicamycin treatment. In comparison with their glycosylated counterparts, nonglycosylated mu chains do not covalently assemble to a significant degree with light chains. Tunicamycin treatment of Daudi cells thus seems to inhibit formation of stable mum protein, possibly by altering mu chain conformation and inhibiting its interaction with light chains. We conclude from these results that some mum chains are specifically protected from proteolysis by post-translational events. These processing events include covalent assembly with light chains, terminal glycosylation, and insertion into the plasma membrane.

The uptake of amines by polymorphonuclear leukocytes.

Muscarinic and beta-adrenergic ligands associate with polymorphonuclear leukocytes to show high-affinity, saturable accumulation. This association can be distinguished from specific receptor binding by its temperature dependence, sensitivity to pH, requirement of an energy source, inhibition by ionophores, and inhibition by a variety of permeable basic amines. Our results suggest that these amines accumulate in acidic lysosomes which are plentiful in these cells. This permeable amine effect can be inhibited without affecting specific receptor binding.