ER stress drives ER-to-Golgi trafficking of ATF6 by blocking its membrane insertion

Regulated ER-to-Golgi trafficking is a fundamental cellular process that enables ER resident transcription factors to sense perturbations in the ER environment and activates transcriptional programs that restore ER and cellular homeostasis. Current models suggest sensor activation is initiated by dissociation from its ER-resident binding partners. Here we challenge this model by demonstrating that the unfolded protein sensor ATF6 is sorted to the Golgi as a newly synthesized peripheral membrane protein beyond the reach of its ER retainer, and translocon inhibition alone is sufficient to drive ATF6 activation. We identify ATF6 transmembrane domain and its C-terminus as the intrinsic factors that control membrane insertion efficiency and stress sensing capacity, and the BAG6 complex as the receptor that triages ATF6 between membrane insertion and Golgi sorting. Besides ATF6, we show that translocon inhibition expedites the activation of the cholesterol sensor SREBP2, and the catalytic domain of Golgi-bound S1P that processes both ATF6 and SREBP2 resides in the cytosol. Therefore, we propose an alternative, sensing-by-synthesis model, in which transcription factors are continuously synthesized, and perturbations to the ER environment are quantitatively sensed by the fraction of sensors that fail to be properly incorporated into ER membrane and sorted to Golgi for activation.