DEGRADATION OF DISTINCT ASSEMBLY FORMS OF IMMUNOGLOBULIN-M OCCURS IN MULTIPLE SITES IN PERMEABILIZED B-CELLS

Protein degradation is essential for quality control which retains and eliminates abnormal, unfolded, or partially assembled subunits of oli gomeric proteins, The localization of this nonlysosomal pre-Golgi degr adation 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 s elective degradation of secretory IgM (sIgM) in B lymphocytes is trans port-dependent, We show that, upon permeabilization of the plasma memb rane with either streptolysin O or digitonin, sIgM is not degraded unl ess transport is allowed, Nevertheless, upon complete reduction of int erchain disulfide bonds with thiols, the free mu heavy chains are degr aded by a transport-independent quality control mechanism within the E R. This latter degradation is nonselective to the secretory heavy chai n mu s, and the membrane heavy chain mu m, which is normally displayed on the surface of the B cell, is also eliminated. Moreover, the degra dation of free mu s is no longer restricted to B lymphocytes, and it t akes place also in the ER of plasma cells which normally secrete polym ers of sIgM, Conversely, when assembled with the light chain, the degr adation is selective to sIgM, is restricted to B lymphocytes, and is a transport-dependent post-ER event.