DIFFERENT ASSEMBLY SPECIES OF IGM ARE DIRECTED TO DISTINCT DEGRADATION SITES ALONG THE SECRETORY PATHWAY

In 38C B lymphocytes the membrane form of IgM is displayed on the cell surface whereas the secretory form of IgM is degraded. In the EH cell line, a light chain-deficient variant of 38C cells, the mu heavy chai ns are partially assembled with surrogate light chains characteristic of pre-B cells. In these cells neither the membrane (mum) nor the secr etory (mus) forms of the mu heavy chain reach their final destination, and both are rapidly degraded. The degradation of mu chains in EH cel ls, like that of mus in 38C cells, is nonlysosomal and occurs prior to the trans-Golgi. However, while As degradation in 38C cells is inhibi ted by brefeldin A, in EH cells mus and mum are retained and degraded by a brefeldin A-insensitive mechanism. These results indicate that de gradation in EH cells occurs within the endoplasmic reticulum, whereas degradation in 38C cells requires exit from this compartment. Thus, m u heavy chains can be degraded in multiple sites along the secretory p athway. The location of the degradation process is determined by the d evelopmentally regulated assembly species of the mu chains with either ''classical'' or surrogate light chains.