Ceapins inhibit ATF6α signaling by selectively preventing transport of ATF6α to the Golgi apparatus during ER stress

The membrane-bound transcription factor ATF6 alpha is activated by proteolysis during endoplasmic reticulum (ER) stress. ATF6 alpha target genes encode foldases, chaperones, and lipid biosynthesis enzymes that increase protein-folding capacity in response to demand. The off-state of ATF6 alpha is maintained by its spatial separation in the ER from Golgi-resident proteases that activate it. ER stress induces trafficking of ATF6 alpha. We discovered Ceapins, a class of pyrazole amides, as selective inhibitors of ATF6 alpha signaling that do not inhibit the Golgi proteases or other UPR branches. We show that Ceapins block ATF6 alpha signaling by trapping it in ER-resident foci that are excluded from ER exit sites. Removing the requirement for trafficking by pharmacological elimination of the spatial separation of the ER and Golgi apparatus restored cleavage of ATF6 alpha in the presence of Ceapins. Washout of Ceapins resensitized ATF6 alpha to ER stress. These results suggest that trafficking of ATF6 alpha is regulated by its oligomeric state.