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

Abstract

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.

Phosphorylation switches protein disulfide isomerase activity to maintain proteostasis and attenuate ER stress

“…Similar results were observed when HepG2 cells were treated with another ER stress inducer tunicamycin (Tm), which inhibits protein N ‐glycosylation (Fig D and E). Again, we showed that Fam20C can also negatively regulate IRE1α activity in HeLa cells by using Fam20C knockout (KO) cells generated by CRISPR/Cas9 technology previously reported (Zhang et al , ; Fig EV1B and C).…”

“…In this study, we demonstrate that Fam20C plays an important role in regulating ER proteostasis and UPR signaling, in addition to its previously identified regulatory role in regulating redox homeostasis (Zhang et al , ) and Ca 2+ homeostasis (Pollak et al , ). Thus, Fam20C‐mediated phospho‐regulation could be a general mechanism to maintain ER homeostasis.…”

“…Approximately 90% of Fam20C is secreted to the extracellular space, and the intracellular protein is mainly localized in the cis ‐Golgi apparatus (Tagliabracci et al , ). Therefore, Fam20C‐catalyzed protein phosphorylation is believed to occur in the Golgi lumen or the extracellular space (Tagliabracci et al , ; Zhang et al , ). To decipher the mechanism of Fam20C‐catalyzed phosphorylation of PDI, we constructed PDI‐mEmerald‐KDEL and Fam20C‐mApple to detect the spatiotemporal interaction between PDI and Fam20C.…”

“…Functional mutations in Fam20C cause a rare and often lethal osteosclerotic bone dysplasia called Raine syndrome (Raine et al , ). Recently, the critical roles of Fam20C in fine‐tuning ER redox homeostasis (Zhang et al , ) and Ca 2+ homeostasis (Pollak et al , ) have been revealed. However, it is still an open question whether Fam20C is involved in ER proteostasis regulation and UPR signaling under ER stress.…”

Fam20C‐mediated phosphorylation of osteopontin is critical for its secretion but dispensable for its action as a cytokine in the activation of hepatic stellate cells in liver fibrogenesis

“…For instance, conditions like lactation, hypoxia and reductive stress have been shown to trigger the shuttling of factors, Ero1a in particular, involved in oxidative folding that requires interaction and phosphorylation by Fam20C in the Golgi apparatus to become fully active and favor the formation of disulfide bonds of nascent proteins in the ER. 49 Considering our aims and experimental approach as well as the action of OPN on cells other than HSCs that constitute the hepatic parenchyma, this study does not pretend to provide a comprehensive account of the role of OPN in liver fibrosis, which other authors have more extensively covered. 50 Nonetheless, our results shed light on the role of a novel player in liver fibrosis, Fam20C, as one of the principal factors taking part in liver fibrogenesis via phosphorylation of OPN, and raise the question whether Fam20C itself or mechanisms inducing its activity might be a potential target for therapeutic intervention in this disease.…”