Degradation of IgM mu heavy chains in light chain-negative pre-B cells is independent of vesicular transport, as is evident by its insensitivity to brefeldin A or cell permeabilization. Conversely, by the same criteria, degradation of the secretory mu heavy chain in light chain-expressing B cells depends on vesicular transport. To investigate whether the presence of conventional light chains or the developmental stage of the B-lymphocytes dictates the degradative route taken by mu, we express in 70Z/3 pre-B cells either lambda ectopically or kappa by lipopolysaccharides-stimulated differentiation into B cells and show their assembly with mu heavy chains. The resulting sensitivity of mu degradation to brefeldin A and cell permeabilization demonstrates that conventional light chains, a hallmark of B cell differentiation, are necessary and sufficient to divert mu from a vesicular transport-independent to a vesicular transport-dependent degradative route. Although both routes converge at the ubiquitin-proteasome degradation pathway, only in light chain-expressing cells is vesicular transport a prerequisite for mu ubiquitination.

Protein degradation is essential for quality control which retains and eliminates abnormal, unfolded, or partially assembled subunits of oligomeric proteins. The localization of this nonlysosomal pre-Golgi degradation to the endoplasmic reticulum (ER) has been mostly deduced from kinetic studies and carbohydrate analyses, while direct evidence for degradation within the ER has been provided by in vitro reconstitution of this process. In this article, we took advantage of the transport incompetence of permeabilized cells to directly demonstrate that the selective degradation of secretory IgM (sIgM) in B lymphocytes is transport-dependent. We show that, upon permeabilization of the plasma membrane with either streptolysin O or digitonin, sIgM is not degraded unless transport is allowed. Nevertheless, upon complete reduction of interchain disulfide bonds with thiols, the free heavy chains are degraded by a transport-independent quality control mechanism within the ER. This latter degradation is nonselective to the secretory heavy chain s, and the membrane heavy chain m, which is normally displayed on the surface of the B cell, is also eliminated. Moreover, the degradation of free s is no longer restricted to B lymphocytes, and it takes place also in the ER of plasma cells which normally secrete polymers of sIgM. Conversely, when assembled with the light chain, the degradation is selective to sIgM, is restricted to B lymphocytes, and is a transport-dependent post-ER event.Protein degradation plays a key role in a myriad of regulatory processes (1). A quality control mechanism is responsible for retention and elimination of abnormal, unfolded, or partially assembled subunits of oligomeric proteins (2-4). Metabolically controlled turnover is reported for various proteins, such as HMG-CoA 1 reductase (5, 6), apolipoprotein B (apoB) (7-10), and ornithine decarboxylase (11). The quality control mechanism operates within the endoplasmic reticulum (ER) and is probably assisted by the variety of ER chaperones (12, 13). Degradation in a nonlysosomal pre-Golgi compartment is reported for numerous proteins, including the ␣ and ␤ subunits of the T cell receptor (14 -16), the H2a subunit of the asialoglycoprotein receptor (17-19), a truncated form of ribophorin (20), the PiZ variant of ␣ 1 -antitrypsin (21), unassembled immunoglobulin light chain (22), and two mutated yeast vacuolar proteins (23), as well as HMG-CoA reductase (5, 6) and apoB (7-10). Localization of these processes to the ER has been directly demonstrated only for the yeast proteins (23). In most cases, however, it has been deduced from temperature and ATP requirements, kinetic studies, structure of carbohydrate moieties, and insensitivity to drugs that block export of proteins from the ER. Secretory IgM (sIgM) is an oligomeric protein that is rapidly degraded in B lymphocytes but is efficiently secreted from plasma cells (24,25). We have shown that, in the 38C B lymphocytes, sIgM degradation is also nonlysosomal and occurs prior to the trans-Golgi (26, 27). More...