Secretory kinase Fam20C tunes endoplasmic reticulum redox state via phosphorylation of Ero1α

Family with sequence similarity 20C (Fam20C), the physiological Golgi casein kinase, phosphorylates numerous secreted proteins that are involved in a wide variety of biological processes. However, the role of Fam20C in regulating proteins in the endoplasmic reticulum (ER) lumen is largely unknown. Here, we report that Fam20C interacts with various luminal proteins and that its depletion results in a more reduced ER lumen. We further show that ER oxidoreductin 1α (Ero1α), the pivotal sulfhydryl oxidase that catalyzes disulfide formation in the ER, is phosphorylated by Fam20C in the Golgi apparatus and retrograde‐transported to the ER mediated by ERp44. The phosphorylation of Ser145 greatly enhances Ero1α oxidase activity and is critical for maintaining ER redox homeostasis and promoting oxidative protein folding. Notably, phosphorylation of Ero1α is induced under hypoxia, reductive stress, and secretion‐demanding conditions such as mammalian lactation. Collectively, our findings open a door to uncover how oxidative protein folding is regulated by phosphorylation in the secretory pathway. Synopsis The ER sulfhydryl oxidase Ero1α is phosphorylated at Ser145 by Golgi kinase Fam20C during mammalian lactation, hypoxia and reductive stress. Phosphorylated Ero1α with higher oxidase activity is critical for maintaining ER redox homeostasis and promoting oxidative protein folding. Fam20C interacts with a wide range of luminal proteins in the secretory pathway. Ero1α is phosphorylated by Fam20C in the Golgi and relocated to the ER by ERp44. Phosphorylation of Ero1α is induced during mammalian lactation, hypoxia and reductive stress. Phosphorylated Ero1α with higher oxidase activity is critical for ER redox balance. Graphical Abstract The ER‐resident oxidase Ero1α is phosphorylated during mammalian lactation, hypoxia and reductive stress, leading to higher oxidase activity and the maintenance of ER redox homeostasis.