- The unfolded protein response (UPR) is a highly conserved signalling pathway that protects cells when the protein‑folding capacity of the endoplasmic reticulum (ER) becomes overwhelmed. Because the ER is responsible for folding, modifying and trafficking one‑third of all cellular proteins, even small disturbances in its environment—such as oxidative stress, calcium imbalance, viral infection or increased secretory demand—can lead to the accumulation of misfolded or unfolded proteins. The UPR detects this imbalance and initiates a coordinated response that expands ER folding capacity, reduces protein load and restores cellular homeostasis.
- ER stress is sensed by three transmembrane proteins: IRE1, PERK and ATF6. Under non‑stress conditions, these sensors remain inactive through association with the chaperone BiP/GRP78. When misfolded proteins accumulate, BiP is recruited away from the sensors, allowing them to activate distinct but interconnected signalling pathways. Together, these pathways adjust transcription, translation and degradation to rebalance ER function.
- IRE1 is the most evolutionarily conserved UPR sensor. Upon activation, it oligomerises and initiates an unconventional splicing reaction that produces the transcription factor XBP1s. XBP1s upregulates genes involved in protein folding, ER‑associated degradation (ERAD), lipid synthesis and secretory pathway expansion. IRE1 also engages regulated IRE1‑dependent decay (RIDD), which degrades select mRNAs to reduce ER load. Through these mechanisms, IRE1 enhances ER capacity and promotes survival under moderate stress.
- PERK activation leads to phosphorylation of eIF2α, transiently reducing global protein synthesis and decreasing the influx of nascent polypeptides into the ER. This translational pause is essential for preventing further accumulation of misfolded proteins. Despite global translation attenuation, specific mRNAs such as ATF4 are preferentially translated. ATF4 induces genes involved in amino‑acid metabolism, antioxidant defence and autophagy, linking the UPR to broader cellular stress pathways including the oxidative stress response and autophagy.
- ATF6 activation involves transport to the Golgi, where it is cleaved to release a transcription factor that upregulates chaperones, ERAD components and lipid‑biosynthesis genes. ATF6 enhances folding capacity and supports ER membrane expansion, complementing the functions of IRE1 and PERK. Together, the three UPR branches form a multilayered network that restores ER proteostasis.
- When ER stress is prolonged or severe, the UPR shifts from adaptive to apoptotic signalling. Persistent PERK activation induces CHOP, a transcription factor that promotes cell death by downregulating anti‑apoptotic proteins and sensitising mitochondria to stress. This ensures that irreparably damaged cells do not compromise tissue function. In multicellular organisms, UPR‑mediated apoptosis is essential for preventing the accumulation of dysfunctional or potentially malignant cells.
- The UPR intersects with multiple cellular stress pathways. It cooperates with the heat‑shock response to increase chaperone availability, interfaces with protein quality control to eliminate misfolded proteins and communicates with mitochondria to regulate energy production and apoptosis. Crosstalk between the ER and other organelles ensures coordinated recovery from stress.
- Dysregulation of the UPR contributes to numerous diseases. Chronic ER stress underlies neurodegenerative disorders such as Alzheimer’s, Parkinson’s and ALS, where misfolded proteins overwhelm folding capacity. In metabolic diseases such as diabetes and fatty‑liver disease, UPR dysfunction impairs insulin production and lipid homeostasis. In cancer, tumour cells often exploit the UPR to tolerate high secretory demand and proteotoxic stress, making UPR components attractive therapeutic targets.
- In summary, the unfolded protein response is a central ER stress pathway that detects misfolded proteins, expands folding capacity, reduces protein load and restores cellular homeostasis. Through coordinated action of IRE1, PERK and ATF6, the UPR preserves proteostasis and protects cells from stress. Its failure contributes to ageing, neurodegeneration, metabolic disorders and cancer, making the UPR a major focus of modern cell biology.
