- ATF6 (Activating Transcription Factor 6) is one of the three principal sensors of the unfolded protein response (UPR), alongside IRE1 and PERK. Unlike the other two branches, which rely on kinase and RNase signalling, ATF6 functions primarily as a transcription factor that enhances the protein‑folding capacity of the endoplasmic reticulum (ER). Its activation represents a central adaptive mechanism that helps cells recover from ER stress by increasing chaperone levels, expanding ER size and strengthening ER‑associated degradation (ERAD).
- ATF6 is a type II transmembrane protein located in the ER. Under non‑stress conditions, its luminal domain is bound by the chaperone BiP/GRP78, keeping it inactive. When misfolded proteins accumulate, BiP dissociates, allowing ATF6 to traffic from the ER to the Golgi apparatus. This translocation is a defining feature of ATF6 signalling and distinguishes it from the other UPR branches.
- In the Golgi, ATF6 undergoes regulated intramembrane proteolysis by Site‑1 and Site‑2 proteases (S1P and S2P). These cleavage events release the cytosolic fragment ATF6(N), a potent transcription factor that translocates to the nucleus. ATF6(N) binds ER stress‑responsive elements (ERSEs) in target gene promoters, activating a transcriptional programme that enhances ER folding capacity and proteostasis.
- ATF6 induces a wide range of genes, including molecular chaperones such as BiP/GRP78, GRP94, calreticulin and protein disulfide isomerases. These chaperones stabilise folding intermediates, prevent aggregation and support proper maturation of secretory and membrane proteins. ATF6 also upregulates components of ER‑associated degradation (ERAD), enabling efficient removal of irreversibly misfolded proteins. Through these actions, ATF6 directly strengthens the ER’s ability to manage proteotoxic stress.
- ATF6 signalling is tightly integrated with the other UPR branches. Crosstalk with IRE1–XBP1 ensures balanced induction of ERAD components and lipid‑biosynthesis genes. Interaction with PERK–ATF4 coordinates chaperone induction with translational attenuation and antioxidant defence. Together, these pathways form a multilayered network that restores ER homeostasis.
- Beyond the ER, ATF6 influences lipid metabolism, calcium signalling and mitochondrial function. It supports ER membrane expansion, enabling increased folding capacity during stress. ATF6 also contributes to cellular adaptation during physiological processes such as differentiation of secretory cells, immune activation and metabolic regulation.
- When ER stress becomes chronic or overwhelming, ATF6 alone cannot restore homeostasis. Under such conditions, the UPR shifts toward apoptotic signalling driven primarily by PERK–CHOP and IRE1–JNK pathways. However, ATF6 remains essential for delaying apoptosis and providing cells with the opportunity to recover.
- Dysregulation of ATF6 contributes to numerous diseases. In neurodegenerative disorders, impaired ATF6 signalling reduces chaperone availability, exacerbating protein misfolding. In metabolic diseases such as diabetes, ATF6 influences insulin production and β‑cell survival. In cancer, tumour cells often exploit ATF6 to tolerate high secretory demand and proteotoxic stress, making ATF6 an attractive therapeutic target.
- In summary, ATF6 is a central ER stress‑responsive transcription factor that enhances protein‑folding capacity, strengthens ERAD and supports ER membrane expansion. Through regulated trafficking, proteolytic activation and transcriptional reprogramming, ATF6 orchestrates adaptive responses that restore proteostasis and maintain cellular homeostasis. Its role in stress biology, metabolism and disease makes ATF6 a key focus of modern cell‑stress research.
