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 endoplasmic reticulum by a mechanism that is believed to monitor the oxidation status of one or more cysteines in their sequences. For example, a single cysteine residue critical for retention of secretory IgM assembly intermediates has been mapped to the C-terminal cysteine, Cys575, of the secretory mu chain. Little is known concerning the mechanism responsible for this system of quality control, which has been termed thiolmediated retention. In particular, it is not known if the mechanism 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 retention of secretory IgM and determined the redox status of cysteines in secretory mu proteins in polymers and polymer intermediates at various 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 assembly intermediates exit the endoplasmic reticulum as individual components, mature through the Golgi without undergoing further assembly, and most, if not all, are secreted. While the majority of IgM assembly intermediates have exposed thiols and are contained within the endoplasmic reticulum where they can be utilized for oligomerization, maturing assembly intermediates found in the Golgi and extracellular space are completely oxidized. Thus, while the retention of unpolymerized IgM is highly efficient, the retention system lacks the ability to distinguish fully oxidized assembly intermediates from fully oxidized completed polymers. The molecular mechanisms that may contribute to this aspect of IgM biogenesis and their implications for the concept of thiolmediated retention are discussed.