QUALITY-CONTROL IN PROTEIN BIOGENESIS - THIOL-MEDIATED RETENTION MONITORS THE REDOX STATE OF PROTEINS IN THE ENDOPLASMIC-RETICULUM

There is accumulating evidence that proteins can be retained in the en doplasmic reticulum by a mechanism that is believed to monitor the oxi dation status of one or more cysteines in their sequences, For example , a single cysteine residue critical for retention of secretory IgM as sembly intermediates has been mapped to the C-terminal cysteine, Cys57 5, of the secretory mu chain, Little is known concerning the mechanism responsible for this system of quality control, which has been termed thiol-mediated retention, In particular, it is not known if the mecha nism monitors the redox state of the important cysteine residue in the secretory mu protein itself or within the context of higher-order IgM complexes, To address this question, we evaluated the fidelity of ret ention of secretory IgM and determined the redox status of cysteines i n secretory mu proteins in polymers and polymer intermediates at vario us stages of maturation, We demonstrate that all secreting B cells and B cell lines secrete assembly intermediates in addition to completed, covalent pentameric and hexameric IgM polymers, A fraction of assembl y intermediates exit the endoplasmic reticulum as individual component s, mature through the Golgi without undergoing further assembly, and m ost, if not all, are secreted, While the majority of IgM assembly inte rmediates have exposed thiols and are contained within the endoplasmic reticulum where they can be utilized for oligomerization, maturing as sembly intermediates found in the Golgi and extracellular space are co mpletely oxidized, Thus, while the retention of unpolymerized IgM is h ighly efficient, the retention system lacks the ability to distinguish fully oxidized assembly intermediates from fully oxidized completed p olymers. The molecular mechanisms that may contribute to this aspect o f IgM biogenesis and their implications for the concept of thiol-media ted retention are discussed.