Endoplasmic Reticulum Stress Induces Liquid–Liquid Phase Separation of GRP78 and Modulates Protein Aggregation Dynamics

Abnormal protein aggregation is a hallmark of neurodegenerative diseases, disrupting cellular homeostasis. Glucose-regulated protein 78 (GRP78), a key endoplasmic reticulum (ER) chaperone, plays a crucial role in protein folding and the ER stress response. Recent studies suggest that GRP78 undergoes liquid–liquid phase separation (LLPS) to form dynamic condensates; however, its functional implications under pathological conditions remain unclear. In this study, we designed and synthesized two fluorescent probes (ER-Pro and Agg-Pro) for specifically labeling GRP78 and monitoring microenvironmental polarity changes during protein phase transition. By integrating fluorescence lifetime imaging microscopy and confocal microscopy, we demonstrated that GRP78 undergoes LLPS under ER stress and recruits the amyotrophic lateral sclerosis-associated mutant protein SOD1(A4V), influencing its aggregation dynamics. Further investigations revealed that SOD1(A4V) aggregation is accompanied by local polarity changes, highlighting a potential role for GRP78 LLPS in protein quality control. Our findings provide new insights into ER homeostasis regulation and the pathogenesis of neurodegenerative diseases, offering potential strategies for early diagnosis and therapeutic intervention.