Abstract

As secretory cells specialized in the production of mucins, intestinal goblet cells are challenged by the need for efficient protein folding. Goblet cells express Inositol-Requiring Enzyme 1{beta} (IRE1{beta}), a unique unfolded protein response (UPR) sensor that is part of an adaptive mechanism that regulates the demands of mucin production and secretion. However, how IRE1{beta} activity is tuned to mucus folding load remains unknown. We identified the disulfide isomerase and mucin chaperone AGR2 as a goblet cell specific protein that crucially regulates IRE1{beta}-, but not IRE1-mediated signaling. AGR2 binding to IRE1{beta} disrupts IRE1{beta} dimerization, thereby blocking its downstream endonuclease activity. Depletion of endogenous AGR2 from goblet cells induces spontaneous IRE1{beta} activation, suggesting that alterations in AGR2 availability in the endoplasmic reticulum sets the threshold for IRE1{beta} activation. We found that AGR2 mutants lacking their catalytic cysteine or displaying the disease-associated mutation H117Y were no longer able to dampen IRE1{beta} activity. Collectively, these results demonstrate that AGR2 is a central chaperone regulating the goblet cell UPR by acting as a rheostat of IRE1{beta} endonuclease activity.