Regulation of membrane IgM expression in secretory B cells: translational and post-translational events.

IgM secreting cells express little or no membrane IgM. This is not always due to absence of the relevant mRNA. To investigate the synthesis and processing of membrane (micron) and secreted (microseconds) polypeptides in secretory B cells, myeloma cells were transfected either with a plasmid containing an intact mu gene or with one only capable of directing micron (not microseconds) mRNA synthesis. Although myeloma transfectants could make abundant levels of micron mRNA, they did not express IgM on the cell surface. In the myeloma host, micron mRNA is translated some 5-fold less efficiently than microseconds mRNA. However, this translational control does not totally preclude micron synthesis, indicating post-translational regulatory events. No difference between micron and microseconds chains could be detected in their rate of assembly with light chains or in their stability, although both types of heavy chain were degraded more rapidly when synthesized in the absence of light chain, or when the hydrophobic nature of the leader sequence was destroyed by site-directed mutagenesis. However, whereas intracellular microseconds chains in IgM-secreting plasmacytoma were found to be concentrated in the Golgi, the micron chains were mainly located in the endoplasmic reticulum. Retention in the endoplasmic reticulum is also observed for both micron and microseconds when synthesized in the absence of light chain. We propose that it is the expansion of the endoplasmic reticulum that accompanies B cell to plasma cell differentiation which is in part responsible for the down-regulation of surface IgM expression. Such a mechanism may also affect the expression of other surface proteins.