Title : Underglycosylation of
ATF6 as a novel sensing mechanism for activation of the unfolded
protein response
Abstract :
- ATF6 is a key transcriptional activator of the unfolded protein response (UPR), which allows mammalian cells to maintain cellular homeostasis when they are subjected to a variety of environmental and physiological stresses that target the endoplasmic reticulum (ER)
- ATF6 , a 90-kDa ER transmembrane protein , contains three evolutionarily conserved N-linked glycosylation sites within its carboxyl luminal domain
- Although it is well established that p90ATF6 activation requires transit from the ER to the Golgi, where it is cleaved by the S1P/ S2P protease system to generate a nuclear form p60ATF6 that acts as a transcriptional activator, the functional significance of p90ATF6 N-linked glycosylation is unknown
- Here we show that ER Ca(2 +) depletion stress, a triggering mechanism for the UPR, induces the formation of ATF6 (f), which represents de novo partial glycosylation of newly synthesized p90ATF6
- By mutating a single amino acid within the N-linked glycosylation site closest to the carboxyl terminus of p90ATF6 , we recreated ATF6 (f)
- This mutation sharply reduces p90 ATF6 association with calreticulin , a major Ca(2 +)-binding chaperone for N-glycoprotein
- We further determined that ATF6 (f) exhibits a faster rate of constitutive transport to the Golgi, resulting in a higher level of p60ATF6 in the nucleus and stronger transactivating activity in the absence of ER stress
- Additional analysis of p90ATF6 mutants targeting single or multiple N-glycosylation sites also showed higher constitutive transactivating activity than wild type ATF6
- Because accumulation of underglycosylated proteins in the ER is a potent inducer for the UPR, these studies uncover a novel mechanism whereby the glycosylation status of p90ATF6 can serve as a sensor for ER homeostasis, resulting in ATF6 activation to trigger the UPR