Stress granule fusion is a mitochondria-coordinated process for stress adaptation
Stress granules are cytoplasmic membraneless organelles assembled during stress to maintain cellular homeostasis. Although fusion is a hallmark of liquid-like behavior of these condensates, whether this process carries functional significance beyond its physical coalescence remains unclear. Here, we show that stress granule fusion is facilitated by mitochondrial dynamics and membrane potential. Intact mitochondria actively associate with stress granules, facilitating fusion and maturation. In contrast, loss of mitochondrial membrane potential, along with disrupted mitochondrial structure or motility, weakens these interactions and reduces fusion frequency. We find that impaired fusion leads to the accumulation of immature granules that retain fewer sequestered components, which correlates with premature cell death. Remarkably, restoring mitochondrial membrane potential rescues granule fusion and enlargement, and is accompanied by increased cell viability and a corresponding increase in granule-associated apoptotic factors. These results demonstrate that stress granule fusion is actively coordinated by mitochondria rather than driven solely by passive coalescence, reshaping how condensate dynamics are understood to integrate with organelle function during cellular stress response.