Towards New Insights in Epithelial Ion Channels Regulation by the ER Luminal Chaperone ERp29

Abstract

ERp29 is an endoplasmic reticulum (ER) 29 kD thioredoxin‐homologous protein that displays chaperone‐like properties. We have previously demonstrated that ERp29 promotes CFTR biogenesis. As the biogenesis of CFTR and ENaC share similar features, we here tested the hypothesis that ERp29 would also regulate ENaC biogenesis and functional expression, as well as further probed ERp29 mechanism. Overexpression of wt ERp29 increased the abundance of the active form of γ‐ENaC, as well as ENaC functional expression. In contrast, ERp29 overexpression of a mutant ERp29 lacking its single Cysteine (C157S ERp29) decreased ENaC functional expression. These observations were not associated with altered expression of β‐ENaC at the apical surface, suggesting that ERp29 may modulate ENaC open probability at the apical surface. ERp29 overexpression promoted the interaction of both ENaC and CFTR with the coat complex II ER exit machinery, whereas C157S ERp29 overexpression decreased this interaction. These data suggest a model where ERp29 may promote ENaC cleavage by directing ENaC to the Golgi. Finally, we also tested the hypothesis that ERp29 subcellular localization is a crucial determinant for its action. We therefore designed a mutant ERp29 containing a KDEL retention motif (ERp29 KDEL) that should be better retained in the ER, and two mutants that would better escape the ER, by mutation and deletion (ERp29 KDEV and ΔKEEL, respectively). Together our findings suggest a key role for ERp29 in the biogenesis of ENaC and CFTR as well as emphasize ERp29 mechanism.