Disulfide bonds are formed in the endoplasmic reticulum (ER) by sequential interchange reactions: Ero1α and Ero1β transfer oxidative equivalents to protein disulfide isomerase (PDI), which in turn oxidizes cargo proteins. Neither Ero1α nor Ero1β contains known ER localization motif (s), raising the question of how they are retained in this organelle. Here the authors show that, unlike endogenous molecules, overexpressed Ero1α and Ero1β are secreted by HeLa transfectants, suggesting saturation of their normal retention mechanism(s). Co-expression of either PDI or ERp44 prevents Ero1 secretion in a KDEL/RDEL dependent way. Covalent interactions between ERp44 and Ero1 are essential for retention. In contrast, a mutant PDI lacking the four cysteines in the two active sites still inhibits secretion, albeit less efficiently. PDI and ERp44 compete for Ero1 binding. PDI also prevents Ero1 aggregation and dimerization, thus chaperoning its own oxidase. This dynamic retention mechanism of Ero1 may be important for fine-tuning the regulation of ER redox homeostasis and quality control.
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